Results and Discussions The prepared NUs possessed an ultrahigh density of sharp tips and abundant nanogaps. This superior structure exhibits broad absorption from 500 to 1000 nm, which facilitates the utilization of broadband solar energy (Fig. 1). To utilize the hot carrier generated by surface plasmon excitation better, the second step of the design is to construct a plasmonic-semiconductor nanoarchitecture that enables the hot carrier to be injected efficiently into the semiconductor before relaxation. The TiO₂ thin film prepared by the sol-gel method is a good electron acceptor, and it only displays distinct absorption below a wavelength of 450 nm; the absorption in the visible region is negligible. Compared with those of TiO₂, the steady-state optical spectra of NU-TiO₂ show that the addition of NUs can broaden the absorption spectral range to the visible region—see Fig. 2(f). As a result, the nanoarchitecture could achieve light absorption over a broad wavelength range. To investigate the physical or chemical processes driven by hot carriers further, the photocurrent response from the microscale reaction region on the photoanode was investigated. The results show that the prepared NU-TiO₂ nanoarchitecture has excellent photocatalytic activity—see Fig. 4(a). The superior performance is ascribed to the multiple hot spots between the spikes of the NUs that boost the generation efficiency of hot carriers, the abundant interface between metal and semiconductor that increases the chances of hot carrier injection, and the formation of a Schottky barrier that facilitates charge separation. Other factors affecting the hot carrier transfer efficiency were investigated, including the applied bias and excitation source power. The photocurrent increases significantly when the applied bias increases from 0 to 0.04 V—see Fig. 4(b). This drastic change can be attributed to the lowering of the Schottky barrier when a reverse bias is applied, benefiting the hot electron transfer. Meanwhile, the photocurrent response increases significantly with increase in excitation source power—see Fig. 4(c). With a higher kinetic energy of electrons at a higher power density, more hot carriers can cross the Schottky barrier to reach the conduction bands of TiO₂, which further improves the efficiency of hot carrier generation and injection. Under the combined action of the above factors, the hot carrier generation and injection efficiency can be further improved.

Hot carriers induced by surface plasmon excitation can trigger chemical reactions, realizing efficient utilization of solar energy. The rational design of plasmonic metal nanostructures is considered an effective strategy for improving the efficiency of hot carrier generation and injection for ultrabroadband light absorption and efficient energy conversion. In this study, plasmonic nanourchins (NUs) with high-density tips were synthesized, and plasmonic-semiconductor nanoarchitectures were constructed as photoanodes. The photocurrent response in the microscale reaction region on the photoanode was used to evaluate the hot carrier generation and injection efficiency. The excellent performance of plasmonic NUs is ascribed to the multiple hot spots between the spikes, boosting the generation efficiency of the hot carrier, and the abundant interface between the metal and semiconductor increases the hot carrier injection opportunities. These results may pave the way for efficient excitation and extraction of hot carriers.

In this study, NUs with ultrahigh-density sharp tips were synthesized through a seed-mediated growth route using L-Dopa as a reduction agent. Nanocrystalline TiO₂ thin films were prepared using sol-gel and spin-coating methods. Then, the metal-semiconductor nanoarchitecture was constructed using a simple drop-casting method as a photoanode for photoelectrochemical (PEC) reactions. Subsequently, the photocurrent response in the microscale reaction region of the photoanode was measured in a homemade PEC cell to assess the transfer efficiency of the hot carrier. The photocurrent-time curves were measured under periodically switched laser illumination by focusing a 514 nm laser with a micrometer-sized light spot closely on the sample particles through a water immersion objective. In addition, photocurrent measurements at different applied biases and excitation source powers were performed for the prepared NU-TiO₂ samples to investigate other factors affecting the hot carrier generation and injection efficiency. The morphologies of the NUs and NU-TiO₂ were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ensemble extinction spectra of the NUs and steady-state optical spectra of NU-TiO₂ were acquired using a Perkin-Elmer Lambda 750 spectrometer equipped with an integrating sphere. The crystal structure of NU-TiO₂ was studied by X-ray diffraction (XRD) using a Bruker D8 Advanced diffractometer.

NUs with high-density tips and broad-spectrum absorption properties were prepared through seed-mediated growth. An NU-TiO₂ nanoarchitecture was designed to steer the migration of hot carriers to the conduction bands of TiO₂, making it possible to fully use plasmonic hot carriers to realize broad-spectrum photocatalysis. The photocurrent response in the microscale reaction region on the photoanode was used to evaluate the hot carrier generation and injection efficiency. The “lightning rod effect” at the tips of the NUs makes a large number of randomly distributed hot spots possible, which significantly increases the efficiency of hot carrier generation. The abundant interface between the metal and semiconductor also increases the chances of hot carrier injection. Other factors affecting energy efficiency were investigated, including the applied bias and excitation source power. This study advances the understanding of the hot carrier transfer mechanism for promoting the utilization of solar energy in diverse photochemistry.