(Invited) Enabling Full-Spectrum Solar Hydrogen Production via Au@Cu₇S₄-Decorated TiO₂ Nanowires

Developing photocatalysts capable of efficiently harvesting solar energy across the entire solar spectrum, covering ultraviolet (UV), visible, and near-infrared (NIR) regions, is a central challenge in advancing solar-to-hydrogen conversion technologies. While extensive efforts have been devoted to extending the photoresponse of TiO₂ into the visible region, the near-infrared portion of sunlight, which accounts for more than half of the solar energy flux, remains largely underutilized. The scarcity of photocatalysts that can simultaneously exhibit strong NIR absorption, efficient charge separation, and sufficient redox power underscores the need for new design strategies that enable full-spectrum solar hydrogen production. In this work, we report a full-spectrum-responsive photocatalytic system based on monoclinic-phase TiO₂ nanowires decorated with dual-plasmonic Au@Cu₇S₄ yolk@shell nanostructures. Monoclinic TiO₂, a relatively uncommon polymorph compared to anatase and rutile, was selected due to its more negative conduction band edge relative to these common phases and its narrower bandgap, both of which are advantageous for photocatalytic reduction reactions such as hydrogen evolution. By integrating Au nanoparticles encapsulated within hollow Cu₇S₄ shells onto the TiO₂ nanowire surface, a well-defined TiO₂–Au@Cu₇S₄ heterostructure was constructed, enabling synergistic optical and electronic functionalities. The formation of the Au@Cu₇S₄ yolk@shell architecture was achieved through sequential Au deposition, Cu₂O coating, and subsequent sulfidation, during which a nanoscale Kirkendall effect generated a porous and hollow Cu₇S₄ shell surrounding the Au core. Structural characterization by SEM, TEM, SAED, and EDS mapping confirmed the intimate interfacial contact among TiO₂, Au, and Cu₇S₄, as well as the preservation of the monoclinic TiO₂ framework. Importantly, the porous nature of the Cu₇S₄ shell facilitated molecular diffusion while maintaining effective electronic coupling between the plasmonic and semiconducting components. Optical characterization revealed that the TiO₂–Au@Cu₇S₄ heterostructure exhibited broadband absorption extending from the UV to the deep NIR region (up to ~2200 nm). This extended photoresponse arises from the combined contributions of intrinsic bandgap excitation of Cu₇S₄ and localized surface plasmon resonance (LSPR) effects from both Au and nonstoichiometric Cu₇S₄. In contrast to noble-metal-only plasmonic systems, Cu₇S₄ provides tunable and intense NIR plasmonic absorption originating from its high free-carrier concentration, thereby substantially enhancing light harvesting in the NIR region. Beyond optical enhancement, interfacial charge transfer and separation were critically improved through a Z-scheme charge transfer mechanism. Spectroscopic analyses, including steady-state and time-resolved photoluminescence (PL and TRPL) as well as X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), revealed favorable band alignment among TiO₂, Au, and Cu₇S₄. Under illumination, photogenerated electrons preferentially accumulate in the Cu₇S₄ domain, while holes are retained in the TiO₂ domain, with Au serving as an efficient electron mediator. This vectorial charge transport suppresses charge recombination while preserving strong redox potentials, a key advantage over conventional type-II heterojunctions. Site-selective photodeposition experiments further confirmed the Z-scheme mechanism by directly visualizing spatially separated reduction and oxidation sites. As a result of these synergistic effects, the TiO₂–Au@Cu₇S₄ heterostructure demonstrated outstanding photocatalytic hydrogen production across a wide spectral range. Apparent quantum yields (AQYs) reached 10.51% at 300 nm, 4.38% at 450 nm, 4.17% at 800 nm, and 3.66% at 1800 nm, representing the highest reported performance among NIR-responsive TiO₂-based photocatalysts to date. Notably, while some TiO₂-based systems exhibit higher AQYs under UV or visible illumination, they generally lack meaningful activity in the NIR region, highlighting the unique advantage of the present dual-plasmonic architecture for full-spectrum solar utilization. In addition to high activity, the TiO₂–Au@Cu₇S₄ photocatalyst exhibited excellent long-term stability under continuous irradiation, maintaining structural integrity and sustained hydrogen production over extended operation times. Surface wettability measurements further revealed a highly hydrophilic surface, which enhances water accessibility and facilitates interfacial reaction kinetics, contributing to the overall photocatalytic efficiency. Collectively, this study demonstrates that integrating monoclinic TiO₂ nanowires with dual-plasmonic Au@Cu₇S₄ yolk@shell nanostructures provides a powerful and versatile platform for full-spectrum solar hydrogen production. The combination of broadband light harvesting, Z-scheme charge separation, strong redox capability, and robust stability establishes a general design paradigm for next-generation TiO₂-based photocatalysts capable of effectively exploiting the largely untapped near-infrared portion of solar energy.