**4. Conclusions**

In this paper, we proposed the use of new composite nanomaterial into which TiO2 NPs were compactly integrated into the 3D cross-points of vertically stacked Ag NWs for plasmon-enhanced photocatalysis. The composite nanomaterials improved the photocatalytic activities under UV-visible and visible illumination due to the synergistic effects of visible light absorption by the 3D Ag NWs and efficient charge separation at the interface between the Ag NWs and TiO2 NPs. We performed theoretical simulations of the local field enhancements by the 3D stacked Ag NWs illuminated at different wavelengths, and we systematically investigated the Raman spectra during plasmon-enhanced photocatalysis. These results revealed that the organic dyes underwent photo-induced decomposition mainly at the cross-points of the 3D vertically stacked Ag NWs and 3D hybrid nanostructures. As the photocatalytic activities were highly localized at specific regions of the 3D nanostructures, it is important to enhance the mass transport of reagents to the hot spot regions to boost the photocatalytic performance.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/3/468/s1, Figure S1: SEM images of 3D hybrid nanostructures upon application of a 0.01 wt% TiO2 NPs solution onto 3D stacked Ag NWs. Small TiO2 NPs covered entire pores of the 3D stacked Ag NW substrate; Figure S2: Extinction spectra of different nanomaterials. The black and green dashed lines indicate the interband transitions of Ag and the band gap of TiO2, respectively; Figure S3: Simulation setup of the 3D stacked Ag NWs. The inset shows a region of interest for a simulation of the cross-points between Ag NWs. The TEM image clearly shows that the Ag NWs were pentagonal in shape; Figure S4: Local electric field intensities at the junctions of crossed Ag NWs. The FDTD method was used in the calculations, along with 450 nm incident. The maximum field enhancement was obtained by searching the maximum values in the domain of the figure. The maximum values were located at the left and right bottom corners of the central pentagon, as shown in (a) and (b). The average field enhancement along the central Ag NW surface was extracted, as indicated by the green lines (0.5 nm away from the Ag NW surface to avoid the staircase effect in the simulation); Figure S5: Absorbance changes in the MB solutions after 10 min illumination. 10 mL of 0.05 mM MB aqueous solution were mixed with 4 mL of 0.01 wt% TiO2 NPs aqueous solution.

**Author Contributions:** S.-G.P. designed and directed the research; V.T.N.L and H.S.J. fabricated the samples and performed the photocatalytic experiments; X.F.X., V.G., and S.A.M performed the plasmonic simulation; S.-G.P., D.-H. K and Y.-I.L. revised and discussed the results; S.-G.P. wrote the manuscript. All the authors read and commented on the manuscript.

**Funding:** This research was supported by the Fundamental Research Program (PNK 6070) of the Korean Institute of Materials Science (KIMS). This work was supported by the "Ministry of Trade, Industry and Energy". (Grant N0002310).

**Acknowledgments:** S. A. Maier acknowledges ONR Global (N00014-16-1-2288), the EPSRSC Reactive Plasmonics Programme (EP/M013812/1), the Lee-Lucas Chair in Physics, and the Bavarian Solar Technologies Go Hybrid (SOLTECH) Programme. X. Xiao is supported by Lee Family Scholars.

**Conflicts of Interest:** The authors declare no conflict of interest.
