(Invited) Tuning Thermal– and Hot Electron–Driven H2 Generation with Plasmonic TiN/Pt Antenna–Reactor Nanohybrids
Authors: Rej Sourav , Kment Štěpán , Zbořil Radek , Naldoni Alberto 
Published: February 2020

Plasmonic nanostructures have recently shown the capability to greatly enhance photocatalytic processes through the generation of highly energetic hot carriers.[1–3] These species, generated upon nonradiative decay of surface plasmons, enable to reach high quantum yield in chemical conversion with increasing photon flux. Photocatalysts composed by plasmonic metals or, more recently, by antenna–reactor systems [4,5] have shown their potential mainly for gas phase reactions (e.g. CO, NH3, and propylene oxidation, CO2 reduction), while detailed investigations of liquid phase reactions have been lacking. [6] Furthermore, the catalytic contribution related to thermoplasmonic heat generation and hot electron creation is still under lively debate.[7] In this invited talk, I will start discussing the challenges that limit the present understanding of hot carrier–driven chemical transformations. Thereafter, I will show our recent efforts to understand thermal and electronic contributions in hydrogen generation with plasmonic TiN/Pt antenna–reactor nanohybrids, correlating catalytic data with Electron Energy Loss Spectroscopy (EELS) spectral images.

Uniform and highly dispersed Pt nanocrystals onto the TiN nanocrystals were prepared by reduction of appropriate amount of potassium tetrachloroplatinate (K2PtCl4) in the presence of formaldehyde (HCHO). The obtained plasmonic catalysts include TiN nanocubes with size of 50 nm homogeneously decorated with Pt nanocrystals (Figure 1a). Each plasmonic antenna (TiN nanocubes) is decorated with reactive catalytic center (Pt nanocrystals) to optimize the electromagnetic interactions and boost the photocatalytic efficiency.

Once dispersed in water solution, the TiN/Pt nanohybrids show a strong and broad localized plasmonic resonance (LSPR) around 700 nm. In this talk, I will also discuss our Electron Energy Loss Spectroscopy (EELS) results showing the nanometric spatial distribution of different plasmonic modes of TiN/Pt catalysts excited through a highly focused, accelerated electron beam (Figure 1b).

The hydrogen evolution experiments from ammonia borane (NH3BH3) hydrolysis were carried out by adding NH3BH3 solution to an aqueous dispersion of TiN/Pt hybrid nanocatalyst in a rounded glass flask under different experimental condition. The maximum temperature achieved by illuminating an aqueous dispersion of TiN/Pt hybrid nanocrystals under a particular light intensity is termed as plasmonic temperature. Different sets of experiments were conducted at plasmonic temperature in the dark with external thermal heating or under that particular light irradiation to distinguish the thermal and electronic contributions, respectively, in our plasmonic catalysis. All photocatalytic reactions were carried out under AM1.5G solar illumination at different light intensity.

Figure 1c shows the catalytic hydrogen production rate expressed as turnover frequency (TOF) values for TiN/Pt in the dark and under solar light illumination. The TOF values for TiN/Pt nanocrystals in the dark at RT (22 °C) and under 10 Suns illumination are 32.1 and 346.1 mol(H2) molPt
-1 min-1 respectively. Photocatalytic hydrogen evolution rates obtained under solar light show a drastic enhancement, reaching about 11 times TOF value in comparison to dark reaction when using 10 Suns illumination. The plasmonic nature of photocatalytic activity is demonstrated by a set of experiments carried out at different wavelength and revealing a TOF maximum at 700 nm, in well agreement with absorption spectrum of TiN/Pt hybrid nanocatalyst. Interestingly, Figure 1c clearly demonstrates that hot electron contributions in photocatalytic hydrogen production is enhanced under solar illumination, especially at high concentrated power (i.e. 10 Suns), where TOF value reaches 2.3 times the TOF obtained under plasmonic temperature in the dark.

To summarize, in this invited talk I will show our effort in plasmonic photocatalysis with TiN/Pt antenna–reactor systems for liquid phase hydrogen generation from ammonium borane. Photocatalytic data will be discussed in terms of thermal and hot electron contribution, electronic vs. multielectronic mechanism, and activation of rate determining step. Finally, these photocatalytic data will be related to nanometer EELS plasmon maps and hot electron generation capability of TiN/Pt hybrids.

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Figure 1