**1. Introduction**

Semiconductor photocatalysts have been widely investigated for use in environmental science and technology applications, especially toward the degradation of organic pollutants [1–4], the photocatalytic production of hydrogen [5–7], and the photocatalytic reduction of carbon dioxide [8–10]. However, conventional semiconductor photocatalysts exhibit poor photocatalytic performances under visible light illumination (λ > 400 nm) due to their wide band gaps. For example, titanium dioxide (TiO2) in the crystalline anatase phase has a band gap of 3.2 eV (λ = 388 nm), and TiO2 semiconductor nanomaterials are active under ultraviolet (UV) light (over only 7.5% of the full solar spectrum). Around 54% of solar power falls within the visible and near-infrared regions (from 400 nm to

800 nm); therefore, significant efforts have been applied toward increasing the photoresponses of semiconductors in this spectral region and enhancing photocatalytic performances.

Recently, hybrid nanostructures containing semiconductor and plasmonic metal nanomaterials have been used to enhance photocatalytic activities over the visible range. For example, integrating TiO2 nanoparticles (NPs) with plasmonic nanostructures can significantly improve photocatalysis performances by enhancing the localized surface plasmon resonance (LSPR) of plasmonic nanostructures, enabling efficient hot electron transfer to TiO2 NPs [11–16]. At the LSPR excitation, plasmonic nanostructures can absorb and concentrate visible light at nanoscale gaps (hot spots) between metallic nanostructures, and highly energetic hot electrons can be injected into nearby TiO2 NPs. As a result of this plasmonic sensitization process, a wide band gap TiO2 material that is inactive under visible light can become active under visible light [11–17].

Here, we report the development of 3D hybrid nanostructures through the compact integration of TiO2 NPs into the 3D cross-points of vertically stacked Ag NWs. The 3D hybrid nanostructures were prepared using a simple two-step vacuum filtration process, applied first to Ag NWs and second to TiO2 NPs, over microfiber filters. We observed that low concentrations of TiO2 NPs selectively localized at the small nanogap region of the 3D crossed Ag NWs. The resulting 3D composite nanomaterials exhibited a noticeable absorption across the entire UV and visible region due to a high density of hot spots in the 3D stacked Ag NWs [18]. At optimized concentrations of the TiO2 NPs and Ag NWs, the photocatalytic efficiencies were measured to be 49.8% over 10 min illumination and 91.3% over 60 min illumination under standard air mass (AM) 1.5G conditions. We performed theoretical simulations of the local field enhancements in the 3D crossed regions using different wavelengths, and we systematically investigated the Raman spectral changes of organic dyes as a function of the photodegradation conditions. The results clearly revealed the crucial contribution of the hot spots (3D cross-points) to the highly enhanced photocatalytic activities and surface-enhanced Raman spectroscopy (SERS).

#### **2. Materials and Methods**

#### *2.1. Fabrication of the 3D Hybrid Nanostructures*

The hybrid nanostructures were fabricated using a two-step vacuum filtration process over a glass microfiber filter (HG00047F, HyundaiMicro, Tokyo, Japan). First, 4 mL of a 0.5 wt% Ag NWs aqueous solution (N&B Co, Ltd., Daejeon, Korea) was poured onto the microfiber filter, then 4 mL of a colloidal solution containing TiO2 NPs was applied onto the 3D stacked Ag NWs without breaking the vacuum. The composite substrate was dried on a hot plate at 150 ◦C for 3 min. The TiO2 NPs aqueous solution (Sigma Aldrich, St. Louis, Missouri, MO, USA) used in our experiments was a mixture of the anatase (80%) and rutile (20%) phases.

#### *2.2. Photodegradation of Methylene Blue*

Each substrate was immersed into a petri dish containing 10 mL of a 0.05 mM MB aqueous solution and remained on the bottom of the petri dish under illumination generated from a Xenon solar simulator lamp (AM 1.5G, 100 mW/cm2) as a UV-visible light source. A UV-cutoff filter was placed between the lamp and the petri dish during photocatalytic activity testing in the visible range (λ > 400 nm). The photocatalytic activities of the TiO2 NPs themselves were tested by mixing a TiO2 colloidal solution with an MB aqueous solution and measuring photodegradation using a solar simulator.
