In Operando Photoswitching of Cu Oxidation States in Cu-Based Plasmonic Heterogeneous Photocatalysis for Efficient H2 Evolution.

Metal nanoparticles (NP) supported on TiO2 are known to be efficient photocatalysts for solar-to-chemical energy conversion. While TiO2 decorated with copper NPs has the potential to become an attractive system, the poor oxidative stability of Cu severely limits its applicability. In this work, we demonstrate that, when Cu NPs supported on TiO2 nanobelts (NBs) are engaged in the photocatalytic generation of H2 from water under light illumination, Cu is not only oxidized in CuO but also dissolved under the form of Cu+/Cu2+ ions, leading to a continuous reconstruction of nanoparticles via Ostwald ripening. By nanoencapsulating the CuOx (Cu/CuO/Cu2O) NPs by a few layers of carbon supported on TiO2 (TC@C), Ostwald ripening can be suppressed. Simultaneously, the resulting CuOx@C NPs are photoreduced under light illumination to generate Cu@C NPs. This photoswitching strategy allows the preparation of a Cu plasmonic photocatalyst with enhanced activity for H2 production. Remarkably, the photocatalyst is even active when illuminated with visible light, indicating a clear plasmonic enhancement of photocatalytic activity from the surface plasmonic resonance (SPR) effect of Cu NPs. Three-dimensional electromagnetic wave-frequency domain (3D-EWFD) simulations were conducted to confirm the SPR enhancement. This advance bodes for the development of scalable multifunctional Cu-based plasmonic photocatalysts for solar energy transfer.

Catalytic Applications of Non-Centrosymmetric Oxide Nanomaterials.

Noble-metal-free catalytic nanoparticles hold the promise being abundant, low-cost materials having a small environmental footprint and excellent performance, albeit inferior to that of noble metal counterparts. Several materials have a long-standing history of success in photocatalysis, in particular titanium dioxide, and in recent years more complex oxides and added functionality have emerged with enhanced performance. We will discuss different approaches related to the use of non-centrosymmetric and polar oxide nanoparticles and how the bulk photovoltaic effect, piezoelectricity, and pyroelectricity add to photocatalysis and tribocatalysis. We pay special attention to discriminate between the role of free versus that of bound charges within the catalyst, which is crucial to disentangle the different contributions to the catalytic reaction for the benefit of the overall enhanced catalytic performance in e.g. wastewater treatment and ultimately water-splitting.

Sample induced intensity variations of localized surface plasmon resonance in tip-enhanced Raman spectroscopy.

Tip-enhanced spectroscopy techniques, in particular tip-enhanced Raman spectroscopy (TERS), rely on a localized surface plasmon resonance (LSPR). This LSPR depends on the near field antenna, its material and shape, and the surrounding medium with respect to its relative permittivity and the volume fraction of the optical near field occupied by the sample. Here, we investigate the effects of the surface composition and topography on the change of the LSPR intensity in tip-enhanced spectroscopy on SrTiO3 nanoislands by monitoring the LSPR enhanced luminescence of gold tips. Our experimental results and analytical estimates indicate that by affecting the effective permittivity of the dielectric environment at the tip apex, the material composition as well as topography of the studied sample induce a change in LSPR intensity. This result significantly helps the understanding of the evolution or origin of the LSPR intensity during a typical TERS measurement, which in turn leads to a more accurate assessment of the relative intensity of different Raman modes in TERS.

Topography-induced variations of localized surface plasmon resonance in tip-enhanced Raman configuration.

We report on topography-induced changes of the localized surface plasmon resonance (LSPR) enhanced luminescence of gold tip on SrTiO3 nanostructures with apertureless scanning near-field optical microscopy (aSNOM) in tip-enhanced Raman spectroscopy (TERS) configuration. Our experimental and simulated results indicate that the averaged refractive index of the dielectric environment of the tip apex containing both air and SrTiO3 in variable volume ratios, is dependent on the topography of the sample. This reveals that the local topography has to be taken into consideration as an additional contribution to the position of the LSPR.

Smooth pyroelectric luminescence in LiNbO3 single crystals.

We investigate the phenomenon of pyroelectric luminescence in LiNbO3 single crystals. This faint emission of light due to temperature-induced changes of permanent polarization is induced by different types of charge carrier recombination, outside and inside the crystal. With decreasing atmospheric pressure, the external discharge mechanism transitions from sparse intense gas discharge pulses at ambient pressure, to frequent faint discharges close to 1 mbar, to a continuous emission which is referred to as smooth pyroelectric luminescence. Our experimental setup exposes the crystal to constant positive and negative temperature changes in the range of 360-450 K under high vacuum while simultaneously measuring the surface charge density and the emitted intensity. A microscopic model of the luminescence allows the description of the time-dependent pyroelectric luminescence, in particular the determination of deep trap potentials that are otherwise inaccessible to thermal ionization. Using this model, we show that the behavior of this emission in LiNbO3 crystals is consistent with the release of trapped electrons by the Poole-Frenkel effect from a Dirac-well potential, while the commonly assumed coulombic trap shape is in clear disagreement with both the temporal evolution of the emission as well as the magnitude of the electric field obtained in our measurements.

Contrast Enhancement for the Recovery of Obliterated Serial Numbers in Different Polymers by Correlated Raman Imaging of Strain, Phonon Lifetime, and Strain-Induced Anisotropy.

In the field of forensic science, we have recently introduced Raman imaging as a promising nondestructive technique to efficiently recover obliterated serial numbers in polycarbonate. The present study is extending the investigation toward different polymers for the reconstruction of abraded information by Raman spectroscopy. Samples of polyethylene, nylon, and nylatron, which are mainly used in items such as firearms, banknotes, and package materials, are investigated by monitoring the vibrational modes which are most susceptible to peak shifts and changes in the full width at half-maximum (fwhm) and peak intensity ratios. In all cases, the most affected peak depends on the polymer's 3D structure and displays a ∼1 cm-1 shift and a broadening above ∼2 cm-1, as well as a relative intensity change of over 50%, more than enough for a successful recovery through confocal imaging. Depending on the polymer's structural arrangement, any of the three contributions prevails for the strongest contrast. The propagation of the plastic deformations is mainly affected by the Young's modulus of the material, due to a change in its elasticity. The shift, the width, and the relative intensity of the Raman peaks being three independent parameters, they can be correlated to enhance the contrast and thus to accelerate the image acquisition or to enhance statistical significance.