Plasmonic systems, which convert light into electrical charges and heat, are increasingly explored for mediating catalytic transformations. A key area of debate centers on the role of hot carriers (high-energy electrons and holes) generated during plasmon decay in driving these reactions. While some studies suggest hot carriers directly participate in catalysis, others attribute catalytic activity primarily to photothermal effects, due to the ultrafast relaxation time of hot carriers (around 1 picosecond). Accurately measuring surface temperatures of plasmonic materials is challenging, leading to uncertainties in quantifying the thermal contribution to the catalytic process. This difficulty in distinguishing between charge carrier catalysis and photothermal effects hinders a complete understanding of plasmon-mediated catalysis. This study aims to directly demonstrate the involvement of plasmon hot electrons in HER by carefully designing a system that minimizes thermal effects and maximizes the contribution from hot carriers. The successful demonstration of hot electron-driven HER would significantly advance the field of plasmonic photocatalysis and provide insights for designing more efficient and selective catalysts.

Previous research has explored the use of plasmonic systems in various catalytic processes, including solar-to-chemical energy conversion, epoxidations, dehydrogenations, and ammonia electrosynthesis. However, the debate about the primary role of hot carriers versus photothermal effects persists. Studies utilizing cobalt porphyrins supported on plasmonic nanoparticles have shown hydrogen production upon illumination. However, the interpretation of these results remains contested; some suggest a combination of plasmon hot carriers and localized thermal effects, while others propose a photothermal effect as the key driver. The ultrafast relaxation of hot carriers poses a significant challenge in differentiating between these mechanisms, highlighting the need for innovative experimental designs capable of minimizing thermal effects and directly probing hot carrier involvement.

This study employed a novel plasmonic nanohybrid system composed of NiO/Au/[CoII(phen-NH2)2(H2O)2], where NiO serves as a hole acceptor, and the cobalt complex acts as an electron acceptor. The cobalt complex, a modified version of a previously reported HER catalyst, was designed to decompose at a temperature well below that required for water thermolysis (500-1000 °C). This design ensures that any observed hydrogen evolution is primarily driven by hot carriers rather than thermal effects. The synthesis of the components involved established procedures for gold nanoparticle synthesis (Turkevich method) and a novel approach for preparing the cobalt complex. Detailed characterization of the materials involved UV-Vis spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), both under ultra-high vacuum (UHV) and near-ambient pressure (NAP) conditions, dynamic light scattering (DLS), and atomic force microscopy (AFM). Electrochemical studies included bulk electrolysis, cyclic voltammetry, and chronoamperometry, both in the dark and under illumination with a 532 nm laser (resonant excitation) to selectively excite the Au plasmon. Off-resonance excitation (650 nm) experiments were also performed to probe potential near-field effects. Gas production during photoelectrochemical reactions was analyzed with online quadrupole mass spectrometry (QMS). To investigate the ultrafast charge dynamics, unbiased transient absorption spectroscopy (TAS) and transient infrared absorption spectroscopy (TIRAS) were employed. In situ NAP-XPS measurements were conducted using a dip-and-pull approach to analyze the electrode surface under different applied potentials, providing insights into the reaction mechanism.

The fabricated NiO/Au/[CoII(phen-NH2)2(H2O)2] system showed significant light-driven HER activity, with a hydrogen production rate of 3.1 nmol/(min·cm²). The enhancement in photocurrent under resonant illumination (532 nm) was attributed to hot electron-driven catalysis, supported by the absence of significant activity under off-resonant excitation (650 nm). The observed response to light modulation exhibited characteristics consistent with a hot carrier-mediated process, unlike the gradual response typical of heat-driven processes. The TAS data revealed that both electrons and holes are rapidly transferred to their respective acceptors, NiO and the cobalt complex, upon plasmon excitation. The e-ph lifetime decreased substantially in the complete system, indicating efficient charge transfer. Unbiased TIRAS data confirmed the transfer of hot carriers to the acceptors, producing a free carrier absorption signal. In situ NAP-XPS studies showed that the cobalt oxidation state remained unchanged during the catalytic process, suggesting that the reduction and protonation steps involving the cobalt center are very fast and not rate-limiting. This contrasts with previously reported mechanisms for similar cobalt complexes which involve sequential reduction and protonation steps at the metal center. The analysis suggests a mechanism where hot electrons are transferred to the phenanthroline ligands, followed by concerted proton-electron transfer (CPET) steps, enabling efficient hydrogen evolution. The presence of NiO significantly enhanced the current response to light modulation, compared to a system lacking NiO, further indicating a hot carrier-dominated mechanism with heat playing a secondary role.

This study successfully resolves the longstanding ambiguity regarding the role of hot carriers in plasmon-driven catalysis. The carefully designed system, which minimizes the heat contribution, provided conclusive evidence for the direct involvement of plasmon hot electrons in driving HER. The observed fast charge transfer from the Au nanoparticles to the acceptors, as evidenced by ultrafast spectroscopy, supports the proposed reaction mechanism. The in situ NAP-XPS results further elucidated the mechanism, showing no change in the cobalt oxidation state, implying that the rate-limiting step occurs after the electron transfer to the ligand and before hydrogen evolution. This study establishes a robust methodology for disentangling the contributions of hot carriers and heat in plasmon-driven catalysis. The proposed mechanism, involving ligand-mediated CPET steps, offers valuable insights for the design of future plasmonic catalysts with enhanced efficiency and selectivity. The results significantly advance the understanding of plasmon-mediated catalysis and open new avenues for exploring the potential of hot carriers in driving various chemical transformations.

This research conclusively demonstrates the direct involvement of plasmon hot electrons in driving the hydrogen evolution reaction. The strategic design of the NiO/Au/[Co(1,10-Phenanthrolin-5-amine)2(H2O)2] system, coupled with advanced in situ spectroscopies, provided compelling evidence for a hot carrier-mediated mechanism. The findings resolve a long-standing debate in the field and offer valuable insights for future development of plasmonic photocatalysts. Future research could focus on optimizing the catalyst structure and exploring other plasmonic materials to further improve the efficiency of hot electron-driven catalysis.

The relatively low cobalt loading in the catalyst resulted in a lower signal-to-noise ratio in some spectroscopic measurements, potentially affecting the accuracy of the quantitative analysis. The study focused primarily on the 532nm excitation wavelength; investigations with other wavelengths might offer broader insights into the wavelength-dependent catalytic activity. While the study effectively minimizes the heat contribution, it does not completely eliminate it; the exact quantification of the heat contribution remains challenging.