Plasmonic titanium nitride (TiN) nanoparticles on TiO2 nanowire photoanodes generate and collect hot carriers (energetic carriers produced by surface-plasmon decay) over a broad visible–near-infrared band, and were shown to outperform Au in photoelectrochemical water splitting. However, the link between the calculated internal generation rates of carriers energetic enough to cross the junction and the measured device photocurrents has never been quantitatively closed: the extraction efficiency, the atomic-scale TiN–TiO2 interface and its native oxide, the energy-resolved hot-carrier lifetimes and scattering lengths in TiN nanoparticles, and the energy-dependent interfacial injection transmission all remain unmeasured on the device geometry, and later first-principles work even contradicts the original reduced-inelastic-scattering phenomenology. This problem asks for a mechanism-resolved, experimentally validated accounting of hot-carrier extraction in TiN/TiO2 photoelectrodes — from plasmon decay to collected photocurrent — and for its systematic extension to other plasmonic transition-metal nitrides and highly doped transparent conducting oxides.

Background

When a metallic nanoparticle absorbs light through a localized surface plasmon resonance (a collective oscillation of conduction electrons confined to the particle), the plasmon can decay non-radiatively and produce 'hot' carriers — electrons with energies well above the Fermi level. If such an electron reaches a metal–semiconductor interface with enough energy to enter the semiconductor conduction band, it can be injected and drive a photochemical reaction; this is the basis of plasmonic hot-electron photoelectrochemistry, a proposed route to harvest sub-bandgap (visible and near-infrared) sunlight in wide-bandgap semiconductors such as TiO2. Whether injected carriers actually contribute to a photocurrent is governed by (i) the rate at which the nanoparticle internally generates carriers above the interfacial energy threshold, (ii) transport to the interface, which is limited by carrier cooling (inelastic electron–electron and electron–phonon scattering) over a mean free path, and (iii) the interfacial transmission probability, which depends on the junction band alignment and on the overlap of electronic wavefunctions across the interface. In a conventional plasmonic Schottky interface a potential barrier blocks carriers below a threshold energy, so collection is restricted to genuinely hot carriers; an Ohmic (barrierless) junction instead allows 'downhill' collection of even cooled, interband-excited carriers. Titanium nitride is a refractory (thermally and chemically robust) ceramic plasmonic material with resonances covering the red and near-infrared, compatibility with CMOS (complementary metal–oxide–semiconductor, the standard silicon integrated-circuit manufacturing technology) processing, and Ti 3d conduction states that overlap those of TiO2. Naldoni, Guler, Wang, Marelli, Malara, Meng, Besteiro, Govorov, Kildishev, Boltasseva and Shalaev (Adv. Opt. Mater. 5(15), 1601031, 2017, DOI 10.1002/adom.201601031) showed that TiO2 nanowire photoanodes decorated with TiN nanoparticles deliver about 25% higher photocurrent enhancement than Au-decorated ones, attributed to higher broadband absorption efficiency, an Ohmic TiN/TiO2 junction, a longer effective electron mean free path, and better wavefunction overlap. Crucially, the comparison of calculated internal generation rates of over-barrier carriers with the measured wavelength-dependent photocurrents (incident photon-to-current efficiency) relied on phenomenological transport parameters taken from the literature — mean free path, inelastic-scattering strength, wavefunction overlap — rather than on measurements of this system, and on an assumed idealized junction, while the microscopic TiN–TiO2 interface structure, the native oxide layer that forms on TiN, and the electrochemical depletion conditions in the operating cell were uncharacterized. Later first-principles work sharpened this gap: Dal Forno and Lischner (Phys. Rev. Materials 2019) found that electron–phonon coupling in TiN is much stronger than in gold, so hot carriers thermalize within roughly 200 fs — faster than in Au — which tensions the original assumption of reduced inelastic scattering in TiN. The unresolved question is thus a quantitative, mechanism-resolved reconciliation between theory and device measurement for non-noble refractory-nitride hot-carrier photoelectrodes.

Problem Statement

For TiO2 nanowire photoanodes decorated with plasmonic TiN nanoparticles — and, as the source paper envisions, for analogous systems built from other plasmonic transition-metal nitrides or highly doped transparent conducting oxides — establish a quantitative, experimentally validated account of hot-carrier extraction that connects the calculated spectrum-weighted internal generation rates of over-barrier carriers to the measured wavelength-dependent photocurrents. Concretely, this requires: (i) measuring and modeling the net carrier extraction efficiency, including the roles of nonideal junction behavior (local barrier-height inhomogeneity or, in the Ohmic-junction case, the absence of a barrier), nanoscale edge-induced tunneling, and electrochemical depletion conditions in the operating cell; (ii) determining the microscopic atomic structure, crystallographic facets, and native-oxide chemistry and thickness at the actual TiN–TiO2 nanocrystal interfaces, and their influence on the local barrier, wavefunction overlap, and tunneling transmission probabilities; (iii) replacing the phenomenological transport parameters (mean-free-path enhancement, reduced inelastic scattering, wavefunction overlap) with measured energy-resolved hot-carrier lifetimes and scattering lengths in TiN nanoparticles of realistic size and shape, together with energy-dependent injection transmission probabilities at the real nanocrystal–oxide interfaces; and (iv) demonstrating, through systematic material synthesis and size/shape control, stability under operating electrochemical conditions, and device-level optimization, whether the predicted generation-rate advantages of these materials translate into proportionate photocurrent improvements. The single overarching objective is a mechanism-resolved theory–measurement reconciliation for plasmonic hot-carrier photoelectrodes based on non-noble refractory nitrides: the interface, dynamics, and generalization results in (ii)–(iv) are the ingredients that make the quantitative reconciliation in (i) predictive rather than fitted. The reconciliation (i) for the TiN/TiO2 system, with the interface (ii) and the energy-resolved lifetimes, scattering lengths, and injection transmission (iii) determined for that system, is the core target, and the generalization (iv) to at least one further representative material — any other plasmonic transition-metal nitride or highly doped transparent conducting oxide — tests whether the generation-rate advantage is general, regardless of the sign of that outcome.

References

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