Molecular Orientation and Energy Transfer Dynamics of a Metal Oxide Bound Self-Assembled Trilayer.

Self-assembly of molecular multilayers via metal ion linkages has become an important strategy for interfacial engineering of metalloid and metal oxide (MOx) substrates, with applications in numerous areas, including energy harvesting, catalysis, and chemical sensing. An important aspect for the rational design of these multilayers is knowledge of the molecular structure-function relationships. For example, in a multilayer composed of different chromophores in each layer, the molecular orientation of each layer, both relative to the adjacent layers and the substrate, influences the efficiency of vectorial energy and electron transfer. Here, we describe an approach using UV-vis attenuated total reflection (ATR) spectroscopy to determine the mean dipole tilt angle of chromophores in each layer in a metal ion-linked trilayer self-assembled on indium-tin oxide. To our knowledge, this is the first report demonstrating the measurement of the orientation of three different chromophores in a single assembly. The ATR approach allows the adsorption of each layer to be monitored in real-time, and any changes in the orientation of an underlying layer arising from the adsorption of an overlying layer can be detected. We also performed transient absorption spectroscopy to monitor interlayer energy transfer dynamics in order to relate structure to function. We found that near unity efficiency, sub-nanosecond energy transfer between the third and second layer was primarily dictated by the distance between the chromophores. Thus, in this case, the orientation had minimal impact at such proximity.

Electrochemical Analysis of the Thermal Stability of 0.9-4.1 nm Diameter Gold Nanoclusters.

Here we report the thermal properties of weakly stabilized 0.9, 1.6, and 4.1 nm Au nanoparticles (NPs)/nanoclusters (NCs) attached to indium-tin-oxide- or fluorine-doped-tin-oxide-coated glass electrodes (glass/ITO or glass/FTO). The peak oxidation potential (Ep) for Au measured by anodic stripping voltammetry (ASV) is indicative of the NP/NC size. Heating leads to a positive shift in Ep due to an increase in NP/NC size from thermal ripening. The size transition temperature (Tt) decreases with decreasing NP/NC size following the order of 4.1 nm (509 °C) > 1.6 nm (132 °C) > 0.9 nm (90 °C/109 °C, two transitions) as compared to the bulk melting point (Tm,b) for Au of 1064 °C. The Tt generally agrees with models describing the size-dependent melting point of Au NPs (Tm,NP) for 4.1 and 1.6 nm diameter Au NPs but is higher than the models for 0.9 nm Au NCs. Scanning electron microscopy (SEM) and UV-vis size analysis confirm the electrochemical results. The thermal stability of electrode-supported metal NPs/NCs is important for their effective use in catalysis, sensing, nanoelectronics, photovoltaics, and other applications.

Molecular Orientation of -PO3H2 and -COOH Functionalized Dyes on TiO2, Al2O3, ZrO2, and ITO: A Comparative Study.

Modification of transparent metal oxide (MOx) surfaces with organic monolayers is widely employed to tailor the properties of interfaces in organic electronic devices, and MOx substrates modified with light-absorbing chromophores are a key component of dye-sensitized solar cells (DSSCs). The effects of an organic modifier on the performance of a MOx-based device are frequently assessed by performing experiments on model monolayer|MOx interfaces, where an "inert" MOx (e.g., Al2O3) is used as a control for an "active" MOx (e.g., TiO2). An underlying assumption in these studies is that the structure of the MOx-monolayer complex is similar between different metal oxides. The validity of this assumption was examined in the present study. Using UV-Vis attenuated total reflection spectroscopy, we measured the mean dipole tilt angle of 4,4'-(anthracene-9,10-diyl)bis(4,1-phenylene)diphosphonic acid (A1P) adsorbed on indium tin oxide (ITO), TiO2, ZrO2, and Al2O3. When the surface roughness of the MOx substrate and the surface coverage (𝛤) of the A1P film were constant, the molecular orientation of A1P was the same on these substrates. The study was extended to 4,4'-(anthracene-9,10-diyl)bis(4,1-phenylene)dicarboxylic acid (A1C) adsorbed on the same group of MOx substrates. The mean tilt angle of A1C and A1P films on ITO was the same, which is likely due the intermolecular interactions resulting from the high and approximately equal 𝛤 of both films. Comparing A1C films at the same 𝛤 on TiO2 and Al2O3 having the same surface roughness, there was no difference in the mean tilt angle. MD simulations of A1C and A1P on TiO2 produced nearly identical tilt angle distributions, which supports the experimental findings. This study provides first experimental support for the assumption that the structure of the MOx-modifer film is the same on an "active" substrate vs. a "inert" control substrate.

Effect of Metal Nanoparticle Aggregate Structure on the Thermodynamics of Oxidative Dissolution.

Here, we compare the electrochemical oxidation potential of 15 nm diameter citrate-stabilized Au NPs aggregated by acid (low pH) to those aggregated by tetrakis(hydroxymethyl) phosphonium chloride (THPC). For acid-induced aggregation, the solution changes to a blue-violet color, the localized surface plasmon resonance (LSPR) band of Au NPs at 520 nm decreases along with an increase in absorbance at higher wavelengths (600-800 nm), and the peak oxidation potential (Ep) in anodic stripping voltammetry (ASV) obtained in bromide has a positive shift by as large as 200 mV. For THPC-induced aggregation (Au/THPC mole ratio = 62.5), the solution changes to a blue color as the LSPR band at 520 nm decreases and a new distinct peak at 700 nm appears, but the Ep does not exhibit a positive shift. Scanning transmission electron microscopy (STEM) images reveal that acid-induced aggregates are three-dimensional with strongly fused Au NP-Au NP contacts, while THPC-induced aggregates are linear or two-dimensional with ∼1 nm separation between Au NPs. The surface area-to-volume ratio (SA/V) decreases for acid-aggregated Au NPs due to strong Au NP-Au NP contacts, which leads to lower surface free energy and a higher Ep. The SA/V does not change for THPC-aggregated Au NPs since space remains between them and their SA is fully accessible. These findings show that metal NP oxidative stability, as determined by ASV, is highly sensitive to the details of the aggregate structure.

Reversing the Thermodynamics of Galvanic Replacement Reactions by Decreasing the Size of Gold Nanoparticles.

Here, we describe the surprising reactivity between surface-attached (a) 0.9, 1.6, and 4.1 nm diameter weakly stabilized Au nanoparticles (NPs) and aqueous 1.0 × 10-4 M Ag+ solution, and (b) 1.6 and 4.1 nm diameter weakly stabilized Au NPs and aqueous 1.0 × 10-5 M PtCl42-, which are considered to be antigalvanic replacement (AGR) reactions because they are not thermodynamically favorable for bulk-sized Au under these conditions. Anodic Stripping Voltammetry (ASV) and Scanning Transmission Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (STEM-EDS) mapping provide quantitation of the extent of Ag and Pt replacement as a function of Au NP diameter. The extent of the reaction increases as the Au NP size decreases. The percentage of Ag in the AuAg alloy following AGR based on ASV is 17.8 ± 0.6% for 4.1 nm diameter Au NPs, 87.2 ± 2.9% for 1.6 nm Au NPs, and an unprecedented full 100% Ag for 0.9 nm diameter Au NPs. STEM-EDS mapping shows very close agreement with the ASV-determined compositions. In the case of PtCl42-, STEM-EDS mapping shows AuPt alloy NPs with 3.9 ± 1.3% and 41.1 ± 8.7% Pt following replacement with 4.1 and 1.6 nm diameter Au NPs, respectively, consistent with qualitative changes to the ASV. The size-dependent AGR correlates well with the negative shift in the standard potential (E0) for Au oxidation with decreasing NP size.

Effect of Size, Coverage, and Dispersity on the Potential-Controlled Ostwald Ripening of Metal Nanoparticles.

Here, we describe the size-dependent, electrochemically controlled Ostwald ripening of 1.6, 4, and 15 nm-diameter Au nanoparticles (NPs) attached to (3-aminopropyl)triethoxysilane (APTES)-modified glass/indium-tin-oxide electrodes. Holding the Au NP-coated electrodes at a constant negative potential of the dissolution potential in a bromide-containing electrolyte led to electrochemical Ostwald ripening of the different-sized Au NPs. The relative increase in the diameter of the NPs (Dfinal/Dinitial) during electrochemical Ostwald ripening increases with decreasing NP size, increasing applied potential, increasing NP population size dispersity, and increasing NP coverage on the electrodes. Monitoring the average size of the Au NPs as a function of time at a controlled potential allows the measurement of the Ostwald ripening rate. Anodic stripping voltammetry and electrochemical determination of the surface area-to-volume ratio provide fast and convenient size analysis for many different samples and conditions, with consistent sizes from scanning electron microscopy images for some samples. It is important to better understand electrochemical Ostwald ripening, especially under potential control, since it is a major process that occurs during the synthesis of metal NPs and leads to detrimental size instability during electrochemical applications.

Size Stability Study of Catalytically Active Sub-2 nm Diameter Gold Nanoparticles Synthesized with Weak Stabilizers.

Here we report on the very low size stability of electrocatalytically active 1.5 to 2.0 nm diameter tetrakis(hydroxymethyl)phosphonium chloride-stabilized Au nanoparticles (THPC Au2nm NPs) chemically attached to glass/indium tin oxide electrodes. The potential for oxidative dissolution of THPC Au2nm NPs in the presence of bromide is about 250 mV negative of 4 nm diameter citrate-stabilized Au NPs (Cit Au4nm NPs) and 450 mV negative of bulk Au, which provides us with an easy method to assess the size stability using anodic stripping voltammetry. The THPC Au2nm NPs show a strong CO2 reduction wave at about -0.40 V (vs RHE), which is nonexistent for the Cit Au4nm NPs or bulk Au. The THPC Au2nm NPs are also comparatively more electroactive for the hydrogen evolution reaction. In acid electrolyte, however, the potential for surface Au2O3 formation on THPC Au2nm NPs is significantly negative relative to bulk Au, and a single cycle through the surface oxide and reduction waves leads to an increase in the NP size to about 4 nm. Similarly, the THPC Au2nm NPs undergo Ostwald ripening in the presence of bromide within 5 min at potentials well before oxidation, which increases their size to 4-10 nm in diameter by 35 min. Exposure to ozone for only 1-2 min also causes the THPC Au2nm NPs to increase in size to about 4 nm. In comparison, Cit Au4nm NPs are stable under all of these conditions, requiring much longer times to change in size. These differences in reactivity and size stability are due to the different Au NP size. Sub-2 nm diameter NPs with weak stabilizers are potentially very useful for electrocatalysis, but their low oxidation potential and poor size stability are major issues of concern.

Size Determination of Metal Nanoparticles Based on Electrochemically Measured Surface-Area-to-Volume Ratios.

Here we report the electrochemical determination of the surface-area-to-volume ratio (SA/ V) of Au nanospheres (NSs) attached to electrode surfaces for size analysis. The SA is determined by electrochemically measuring the number of coulombs of charge passed during the reduction of surface Au2O3 following Au NS oxidation in HClO4, whereas V is determined by electrochemically measuring the coulombs of charge passed during the complete oxidative dissolution of all of the Au in the Au NSs in the presence of Br- to form aqueous soluble AuBr4-. Assuming a spherical geometry and taking into account the total number of Au NSs on the electrode surface, the SA/ V is theoretically equal to 3/radius. A plot of the electrochemically measured SA/ V versus 1/radius for five different-sized Au NSs is linear with a slope of 1.8 instead of the expected value of 3. Following attachment of the Au NSs to the electrode and ozone treatment, the plot of SA/ V versus 1/radius is linear with a slope of 3.5, and the size based on electrochemistry matches very closely with those measured by scanning electron microscopy. We believe the ozone cleans the Au NS surface, allowing a more accurate measurement of the SA.