Michela Cittadini

Low-temperature processed Ga-doped ZnO coatings from colloidal inks.

We present a new colloidal synthesis of gallium-doped zinc oxide nanocrystals that are transparent in the visible and absorb in the near-infrared. Thermal decomposition of zinc stearate and gallium nitrate after hot injection of the precursors in a mixture of organic amines leads to nanocrystals with tunable properties according to gallium amount. Substitutional Ga(3+) ions trigger a plasmonic resonance in the infrared region resulting from an increase in the free electrons concentration. These nanocrystals can be deposited by spin coating, drop casting, and spray coating resulting in homogeneous and high-quality thin films. The optical transmission of the Ga-ZnO nanoparticle assemblies in the visible is greater than 90%, and at the same time, the near-infrared absorption of the nanocrystals is maintained in the films as well. Several strategies to improve the films electrical and optical properties have been presented, such as UV treatments to remove the organic compounds responsible for the observed interparticle resistance and reducing atmosphere treatments on both colloidal solutions and thin films to increase the free carriers concentration, enhancing electrical conductivity and infrared absorption. The electrical resistance of the nanoparticle assemblies is about 30 kΩ/sq for the as-deposited, UV-exposed films, and it drops down to 300 Ω/sq after annealing in forming gas at 450 °C, comparable with state of the art tin-doped indium oxide coatings deposited from nanocrystal inks.

Hydrogen Spillover between Single Gold Nanorods and Metal Oxide Supports: A Surface Plasmon Spectroscopy Study

We used dark field spectroscopy to monitor the dissociation of hydrogen on single gold nanoparticles embedded in metal oxide supports. Individual gold nanorods were monitored in real time to reveal the peak position, the full width at half-maximum, and the relative intensity of the surface plasmon resonances during repeated N2-H2-N2 and air-H2-air cycles. Shifts in the spectra are shown to be due to changes in electron density and not to refractive index shifts in the environment. We demonstrate that hydrogen does not dissociate on gold nanorods (13 nm × 40 nm) at room temperature when in contact with silica and that electrons or hydrogen atoms migrate from Pt nanoparticles to Au nanoparticles through the supporting metal oxide at room temperature. However, this spillover mechanism only occurs for semiconducting oxides (anatase TiO2 and ZnO) and does not occur for Au and Pt nanoparticles embedded in silica. Finally, we show that hydrogen does dissociate directly on anatase surfaces at room temperature during air-H2-air cycles. Our results show that hydrogen spillover, surface dissociation of reactants, and surface migration of chemical intermediates can be detected and monitored in real time at the single particle level.

Au and Pt Nanoparticles Effects on the Optical and Electrical Gas Sensing Properties of Sol–Gel-Based ZnO Thin-Film Sensors

The optical and conductometric responses of sol-gel-based ZnO thin films with embedded 5% mol. Au or Pt nanoparticles (NPs) have been compared at different operating temperatures, to assess the effect of noble metal additions on to H2, CO and NO2 gases sensors response. The scanning electron microscopy and X-ray diffraction techniques have been utilized for morphological characterization of the crystal structures of the films after annealing at 400°C for 30 min and the effect of Au and Pt NPs additions on crystallite growth is discussed. Au NPs additions is effective to enhance the optical response to H2 and CO at 390 nm and in the 500-700-nm wavelength range at 300- °C temperature. No optical response to NO2 gas, even at low concentrations has been measured in the optical mode. The catalytic action of Pt NPs additions powerfully improves both the optical to H2 and CO at 390 nm, and the conductometric gas responses. Detection limits as low as 20-ppb NO2 and 50-ppm H2, in line with the best performances reported to the best of our knowledge in literature for ZnO-based sensors, have been measured in the conduction mode. Response times have been also compared, highlighting the positive effects played by Pt additions.

Xylene sensing properties of aryl-bridged polysilsesquioxane thin films coupled to gold nanoparticles

Surface plasmon resonance gas sensors based on organic–inorganic hybrid thin films coupled to gold nanoparticles were fabricated and tested against the detection of xylene at the concentration of 30 ppm. Such nanocomposites are prepared either by dispersing Au nanoparticles inside an aryl-bridged polysilsesquioxane system, synthesized via a sol–gel process, or by depositing an aryl-bridged polysilsesquioxane film on Au nanoparticle sub-monolayers. Ultra-high-vacuum temperature programmed desorption of xylene on both the aryl-bridged polysilsesquioxane films and the nanocomposite Au/hybrid system was investigated, resulting in an interaction energy between the sensitive film and the gas molecules in the 38–139 kJ mol−1 range. The functional activity of the nanostructured composites as xylene gas optical sensors was tested monitoring gold localized surface plasmon resonance, and was shown to be reversible. The detection sensitivity was calculated in 0.1 ppb through a calibration procedure in the 16–30 ppm range, and a threshold limit of detection of 265 ppb xylene was estimated as three standard deviations of the baseline noise. Typical response and regeneration times are of one min and about one ten of minutes, respectively.

Fabrication of Tunable, High‐Refractive‐Index Titanate–Silk Nanocomposites on the Micro‐ and Nanoscale

The combination of water-based titanate nanosheets dispersion and silk fibroin solution allows the realization of a versatile nanocomposite. Different fabrication techniques can be easily applied on these nanocomposites to manipulate the end form of these materials on the micro- and nanoscale. Easy tunability of the refractive index from n = 1.55 up to n = 1.97 is achieved, making it attractive for flexible, biopolymer-based optical devices.

Surface plasmon spectroscopy study of electron exchange between single gold nanorods and metal oxide matrix during hydrogen gas sensing (Presentation Recording)

The direct optical monitoring of electron exchange on single plasmonic nanoparticles, involved in chemical reactions with gas molecules, is one of the main challenges in the heterogeneous catalysis and gas sensing fields. Catalysts are substances that speed up reactions by providing an alternative pathway with lower activation energy than that required for the uncatalysed reaction. A lot of research, both fundamental and applied, has been carried out to investigate how catalysts work and to increase their efficiency. The present work shows how the use of Dark Field Microscopy (DFM) coupled with surface plasmon spectroscopy, enables the direct observation of the kinetics of H2 gas interaction with single gold nanorods (NR) coupled with Pt nanoparticles (NPs) and/or with metal oxide matrices. The plasmonic particles, gold NRs, act as optical probes, and enable the monitoring of the electron exchange through the measurement of their surface plasmon resonance (SPR) band shift. To improve the redox reaction kinetics, the Au NRs have been coupled with Pt NPs and embedded also into a TiO2 or ZnO low scattering matrix. The Au NRs, the Pt, TiO2 and ZnO NPs have been synthetized by colloid chemistry. Several samples made of bare Au NRs, or Au NRs coupled with only Pt NPs or with Pt and TiO2 NPs or with Pt and TiO2 have been deposited by spin coating on silica substrates. The longitudinal Au SPR band shift has been monitored by DFM looking at the variation of the scattering spectrum of a single Au NRs in the presence of H2. Time-resolved measurements have been also conducted at fixed wavelength in order to monitor the kinetics of the H2 reaction. With such measurements it was possible to elucidate the importance of the adsorbed oxygen and the TiO2 matrix on the H2 reaction with the Pt NPs.

Grating-coupled surface plasmon resonance gas sensing based on titania anatase nanoporous films

Nanoporous TiO2 anatase film has been investigated as sensitive layer in Surface Plasmon Resonance sensors for the detection of hydrogen and Volatile Organic Compounds, specifically methanol and isopropanol. The sensors consist of a TiO2 nanoporous matrix deposited above a metallic plasmonic grating, which can support propagating Surface Plasmon Polaritons. The spectral position of the plasmonic resonance dip in the reflectance spectra was monitored and correlated to the interaction with the target gases. Reversible blue-shifts of the resonance frequency, up to more than 2 THz, were recorded in response to the exposure to 10000 ppm of H2 in N2 at 300°C. This shift cannot be explained by the mere refractive index variation due to the target gas filling the pores, that is negligible. Reversible red-shifts were instead recorded in response to the exposure to 3000 ppm of methanol or isopropanol at room temperature, of magnitudes up to 14 THz and 9 THz, respectively. In contrast, if the only sensing mechanism was the mere pores filling, the shifts should have been larger during the isopropanol detection. We therefore suggest that other mechanisms intervene in the analyte/matrix interaction, capable to produce an injection of electrons into the sensitive matrix, which in turn induces a decrease of the refractive index.

Plasmonic Sensors for Aromatic Hydrocarbon Detection

The development of innovative materials for sensitive and selective gas sensing is a very relevant field for the current nanotechnology research. A strong effort is dedicated to the fabrication of low-cost and efficient nanoscale devices capable of a fast detection. Resistive electrical devices are the most adopted solutions for in-situ and real-time detection, but their main drawbacks are the low selectivity, response drift, electromagnetic noise dependence and need of contact measurements. Optical gas sensors allow to overcome such limits, and could moreover exhibit thermal and mechanical stability, operate at room temperature, and be integrated on-chip. Within this framework, plasmon-based optical devices are knowing an increasing development and diffusion. Herein plasmonic sensors for aromatic hydrocarbon detection are presented. These systems are based on aryl-bridged polysilsesquioxanes (aryl-PSQs), obtained either \(\textcircled{1}\) coupling such hybrid films with Au nanoparticles (NPs), aiming to the excitation of localized surface plasmon resonances (LSPRs), or \(\textcircled{2}\) depositing them onto metallic waveguiding layers, to form gratings supporting the propagation of surface plasmon polaritons (SPPs). Aryl-PSQs are sol-gel materials characterized by a native controlled porosity and other functionalities (Loy and Shea, Chem Rev 95:1431–1442, 1995; Dabrowski et al., Appl Surf Sci 253:5747–5751, 2007; Brigo et al., Nanotechnology 23:325302, 2012). Temperature programmed desorption investigations of xylene on phenyl-bridged (ph-PSQ) and diphenyl-bridged (diph-PSQ) PSQ films indicate a specific π −π interaction between the organic component of the films and xylene molecules: the interaction energy is quantified in 38 ± 14 kJ/mol and 115 ± 13 kJ/mol, respectively (Brigo et al., J Mater Chem C 1:4252, 2013). For type \(\textcircled{1}\) sensors, a thin film of aryl-PSQ was deposited on a submonolayer of Au NPs coating a fused silica substrate. These sensors were tested monitoring the variation of the LSPRs under cycles of exposure to N2 and to 30 ppm xylene in N2.