Nitrogen Photoelectrochemical Reduction on TiB2 Surface Plasmon Coupling Allows Us to Reach Enhanced Efficiency of Ammonia Production

Abstract

Ammonia is one of the most widely produced chemicals worldwide, which is consumed in the fertilizer industry and is also considered an interesting alternative in energy storage. However, common ammonia production is energy-demanding and leads to high CO₂ emissions. Thus, the development of alternative ammonia production methods based on available raw materials (air, for example) and renewable energy sources is highly demanding. In this work, we demonstrated the utilization of TiB₂ nanostructures sandwiched between coupled plasmonic nanostructures (gold nanoparticles and gold grating) for photoelectrochemical (PEC) nitrogen reduction and selective ammonia production. The utilization of the coupled plasmon structure allows us to reach efficient sunlight capture with a subdiffraction concentration of light energy in the space, where the catalytically active TiB₂ flakes were placed. As a result, PEC experiments performed at -0.2 V (vs. RHE) and simulated sunlight illumination give the 535.2 and 491.3 μg h^-1 mg_cat^-1 ammonia yields, respectively, with the utilization of pure nitrogen and air as a nitrogen source. In addition, a number of control experiments confirm the key role of plasmon coupling in increasing the ammonia yield, the selectivity of ammonia production, and the durability of the proposed system. Finally, we have performed a series of numerical and quantum mechanical calculations to evaluate the plasmonic contribution to the activation of nitrogen on the TiB₂ surface, indicating an increase in the catalytic activity under the plasmon-generated electric field.

Figure 1

Schematic concept of the Au grating/TiB₂@AuNP preparation: preparation of LSP excitation-supported AuNPs on the surface of redox-active TiB₂ flakes and subsequent deposition on Au grating able to support SPP excitation. The resulting structure will ensure the LSP–SPP coupling in place of TiB₂ flakes.

Figure 2

(A) AFM studied the morphology of TiB₂ and TiB₂@AuNP flakes deposited on the Si substrate; (B) TEM image of the TiB₂@AuNP flake and corresponding EDX mapping of Ti, B, and Au; (C) XRD patterns of TiB₂ and TiB₂@AuNP flakes; (D) SEM–EDX measured spatial distribution of TiB₂@AuNP flakes on the Au grating surface; and (E) UV–vis and reflection spectroscopy of Au grating/TiB₂@AuNPs.

Figure 3

(A) Schematic illustration of the ammonium photoelectrochemical production in the H-type cell under simulated sunlight illumination; (B) LSV plots measured on the Au grating/TiB₂@AuNP photoelectrode in the dark and under irradiation in Ar, air, and N₂-saturated solution; (C) current densities measured in the chronoamperometry regime at various potentials and simulated sunlight illumination on the Au grating/TiB₂@AuNP photoelectrode in N₂-saturated solution under sunlight illumination.

Figure 4

(A) UV–vis absorption spectra of ammonia photometric solutions demonstrating the difference between the characteristic band intensities obtained with the utilization of the Au grating/TiB₂@AuNP photoelectrode (−0.2 V vs. RHE) in the dark or under simulated sunlight illumination; (B) ammonia yields and faradaic efficiencies as a function of the applied potential in PEC regime with the Au grating/TiB₂@AuNP photoelectrode; (C) ammonia yields as a function of illumination and PEC electrodes: Au grating vs. Au grating/TiB₂@AuNPs (−0.2 V vs. RHE); (D) ¹H NMR spectra of isotope labeling experiments performed with the utilization of the Au grating/TiB₂@AuNP photoelectrode and reaction mixture saturation with ¹⁵N₂ or ¹⁴N₂; (E) stability of Au grating/TiB₂@AuNPs, estimated in chronoamperometry mode (3 × 7 h cycles) at −0.2 V vs. RHE and under sunlight illumination. Inserts show the color of photometric test solutions and amounts of NH₃ produced after each cycle; (F) comparison of NH₃ and H₂ production rate on the Au grating/TiB₂@AuNP surface in the PEC regime.

Figure 5

(A) FDTD-calculated distribution of the plasmon-related volumetric energy density under the illumination of the coupled AuNP–Au grating system with TiB₂ spacer; (B) calculated free-energy diagrams for the NRR on the Ti-(001) surface under external (or zero) potential and plasmon-related electric field 54 MV m^–1 through the enzymatic pathway; and (C) atomic structures of the corresponding intermediates.