Metal/TiO2 hybrid nanostructures offer more efficient charge separation and a broader range of working wavelengths for photocatalytic reactions. The sizes and shapes of such hybrid nanostructures can affect the charge separation performance when the structures interact with light, but assessments of the interaction of light with these metal-TiO2 nanostructures have only been carried out on ensemble averages, hindering both systematic descriptions of such hybrid structures and the design of new ones. Here, we fabricated TiO2 nanotubes (NTs) with and without core Au nanowires (NWs), and used spectroscopy and calculations to assess their scattering and absorption of light at the single NW level. According to the results of spectral imaging and numerical calculations, the Au/TiO2 NWs scattered and absorbed light substantially more strongly than did the plain TiO2 NTs. Measurements of the degradation of the AO7 dye to assess the photocatalytic performance of the Au/TiO2 NWs were consistent with optical measurements demonstrating a two-fold improvement over plain TiO2 NTs under 360-nm-wavelength UV illumination. Our results suggests that nanoscale optical imaging can be used to visualize the performance of the photocatalytic reaction at the single nano-object level.
© 2016 Optical Society of America
TiO2 has been widely used for solar cells and photocatalysts because of its highly efficient charge separation induced by ultraviolet (UV) illumination . Recently, various kinds of TiO2 nanostructures have been hybridized with metals to increase photocatalytic efficiency [2–8]. The enhancement of photocatalytic efficiency is related to two factors: effective separation of charge between the metal and the semiconductor, and field enhancement with increased absorption of light by the photocatalyst resulting from surface plasmon resonance. For metal nanoparticles smaller than the wavelength of the absorbed light, surface plasmons can be excited when the frequency of the incident photon is resonant with the coherent oscillation of the conduction electrons in the metal nanoparticle, and this phenomenon is known as localized surface plasmon resonance (LSPR). There have been efforts to improve the charge-separation performance of TiO2 for the visible-light wavelength range using doping of metals [9–11] and nonmetal ions [12–14], ion implantation  and electron beam irradiation [16–18]. Recently, hybridization with gold (Au) has been used to improve the performance of TiO2 under visible light conditions by LSPR [19,20].
Metal/TiO2 hybrid nanostructures, like most nanomaterials produced in large quantities, are not homogeneous, differing in size, shape and composition. Analysis of the resulting ensemble average of nanostructures can provide only averaged values and information on overall properties, making an exact description of the properties of the material difficult to achieve and hence hindering both systematic descriptions of such hybrid nanostructures and the design of new ones. So far, no attempt has been made to assess the photocatalytic performance of a metal/TiO2 hybrid at the single nano-object level. Scattering spectroscopy and absorption spectroscopy are effective ways to characterize the plasmonic nanostructure because the scattering and absorption spectra are directly related to the surface plasmon excitation . These optical features strongly depend on the composition, size, shape, and the surrounding local dielectric environment of the particles .
In this study, we fabricated TiO2 nanotubes (NTs) and Au/TiO2 hybrid nanowires (NWs) and investigated their optical properties at the single NW level using nanoscale dark-field scattering and confocal absorption spectroscopic imaging. We obtained the scattering and absorption image and spectra of single Au/TiO2 NWs under UV and visible light illumination and showed those optical responses were significantly stronger than by plain TiO2 NWs. The optical measurement results were consistent with numerical calculations and photocatalytic performance measurements. Our results suggest that the assessment of photocatalytic efficiency of metal-TiO2 hybrid system can be carried out at the individual nano object level, using nanoscale optical imaging and spectroscopy.
2. Experimental details
Two different types of sample were prepared: TiO2 nanotubes (NTs) and Au/TiO2 hybrid nanowires (NWs). The TiO2 NTs were prepared by atomic layer deposition (ALD) onto anodic aluminum oxides (AAOs) as a template. The TiO2 NT wall thickness was determined by the number of deposition cycles, and was fixed at 10 nm. After the ALD deposition, TiO2 was annealed at 400°C for 1 h under ambient conditions to form the anatase phase. Then the AAO was removed by using a 1 M aqueous solution of NaOH. Au NWs were produced inside of TiO2 NTs by using the photo-reduction method. TiO2 NTs were immersed in a 5 mM aqueous solution of HAuCl4 and were irradiated with a UV lamp.
The single TiO2 NW and Au/TiO2 NW strands grown on a patterned Si substrate were located by using a scanning electron microscope (SEM) (JSM-7600F, JEOL). We acquired an energy dispersive X-ray spectroscopy (EDS) spectrum (X-Max, Oxford instruments) of the center of the NW to verify the presence of an Au core inside the TiO2 NT. Optical imaging was performed with a modified optical microscope (TE2000-U, Nikon). The TiO2 NW and Au/TiO2 NW were illuminated using a dark-field lens (100X, N/A0.9, Nikon) and polarized light from a UV (365 nm) LED with a short-pass filter (390 nm). A dark-field condenser was used to shine an annular beam onto the samples. For absorption measurements we used a bright-field objective lens (100X, N/A0.9, Nikon) in the same configuration. The scattered light was collected with the same dark-field lens and focused onto a charge-coupled device (CCD) for imaging or guided to the spectrometer for spectral analysis.
The photocatalytic performance of each sample was evaluated by measuring the decomposition of AO7 (acid orange 7, C16H12N2O4S), which was performed by monitoring the change of the absorption peak intensity at a wavelength of 485 nm after illuminating the sample with a 355-nm-wavelength UV laser for 30 min. The laser beam was guided into the bottom of a quartz cuvette filled with 35 uL of an aqueous solution of AO7 with the TiO2 hybrid structure. The decomposition of the solute was measured by determining the decolorization rate of the AO7 in solution with a UV-Vis spectrometer (JASCO UV-670) and recording the spectra between 400 nm and 700 nm. The absorption properties of the TiO2 hybrid structures were numerically calculated using commercial finite-difference time-domain (FDTD) software (Lumerical). The TiO2 nanostructures were modeled into cylindrical shapes and their dimensions were determined as the same size of the samples.
3. Results and discussion
The structures and dimensions of the TiO2 nanotube (NT) and Au/TiO2 nanowire (NW) were characterized by using a SEM and transmission electron microscope (TEM). Figure 1(a) shows schematic diagrams depicting the TiO2 NT with and without the Au core. Figure 1(b) displays a TEM image of an Au/TiO2 NW, which showed the crystalline TiO2 NT and filled Au core to have an outer tube diameter of ~45 nm and a TiO2 NT wall thickness of ~10 nm. Figures 1(c) and 1(d) show the SEM images and EDS spectra of a TiO2 NT and Au/TiO2 NW, respectively, which confirmed the presence of the Au nanocore in Au/TiO2 NW, and its absence in TiO2 NT. The XPS measurement (data not shown) confirmed the existence of metallic-state Au in the Au/TiO2 NW sample which displayed Au 4f7/2 and Au 4f5/2 lines at 84.1eV and 87.8eV binding energies, respectively.
Figure 2 shows absorption images of single NW strands with polarized 365 nm-wavelength illumination. These NWs were identified as single TiO2 NW and Au/TiO2 NW strands according to the SEM images of the same samples (shown in Figs. 1(c) and 1(d)). Figures 2(a) and 2(b) show the absorption images of a TiO2 NT and Figs. 2(c) and 2(d) show the absorption images of an Au/TiO2 hybrid NW. We performed the absorption imaging with two perpendicular polarization with respect to the NW orientation, as the direction of polarization are marked in the images. The NWs appeared dark compared to the substrate due to the absorption of illuminated light by NWs. In Figs. 2(a) and 2(b), we found that the absorption of light by the TiO2 NT was greater when oriented parallel than perpendicular to the polarization direction of the light. Such stronger absorption of semiconductor NWs in the parallel orientation has been frequently observed, being attributed to the small NW diameter and the relatively low dielectric constant values of semiconductors .
The absorption by the Au/TiO2 hybrid NW was overall more intense than that by the plain TiO2 NT, regardless of the direction of the illumination polarization, which we attribute to the enhanced absorption by surface plasmon excitation occurring on Au/TiO2 interfaces. In addition to the LSPR, propagating plasmons (PP) can also be excited along the Au core in Au/TiO2 hybrid NW. Illumination parallel to the NW orientation can cause the excitation of PP, while perpendicular polarization of illumination will mostly excited the LSPR. In our samples, due to small diameter of 30 nm Au core, we expect the excitation of PP is minimal. Indeed we note that the absorption of the light by Au/TiO2 hybrid NW increased a lot more compared to the case of the plain TiO2 NTs when the NW long axis was oriented perpendicular to the polarization direction than when it was oriented parallel, indicating that the enhancement of absorption in the Au/TiO2 hybrid NW was mostly due to LSPRs of the Au core.
Figure 3 displays UV dark-field scattering images obtained using polarized 365 nm-wavelength illumination of the same set of TiO2 NT and Au/TiO2 NWs shown in Fig. 2. Here we again observed a significant increase of scattering by the Au/TiO2 hybrid NW, due to the LSPR excitation. As in the absorption measurement, the perpendicular orientation of the Au/TiO2 hybrid NW long axis relative to the polarization direction was more effective in scattering than was the parallel orientation.
Au/TiO2 hybrid NW showed a stronger interaction than plain TiO2 NT with the visible light as well. In Fig. 4, we show the results of dark-field scattering spectroscopic imaging performed with the visible light illumination of broad-band wavelength ranging 450 nm – 750 nm. Figures 4(a) and 4(b) display the dark-field scattering images and representative spectra obtained from single TiO2 NT and Au/TiO2 NW strands, respectively. Au/TiO2 NW was observed, when comparing the images, to yield the stronger scattering. Moreover, this NW yielded the characteristic LSPR peak of Au at ~610 nm in its scattering spectrum, suggesting that its enhanced scattering resulted from the presence of the Au core and LSPR excitation . We found the results from visible light scattering imaging and spectroscopy to be highly sensitive to the presence of the Au core, providing an unambiguous way of monitoring in a non-destructive manner the presence of this metal. Figures 4(c) and 4(d) show such an example. As seen in the dark-field scattering image in Fig. 4(c), an Au/TiO2 NW strand yielded a very different scattering color in its top half (denoted by ① in the figure), which was devoid of Au, than in its bottom half (denoted by ②), which was full of Au. The local scattering spectra shown in Fig. 4(d) were obtained from positions in the top and bottom halves of this same Au/TiO2 NW, and the bottom position (②) clearly yielded the characteristic Au LSPR peak at ~600 nm. The presence of an Au core was not detected in the atomic force microscope image (inset of Fig. 4(c)) because of the uniform outer diameter of the Au/TiO2 NW.
Because of its nondestructive character and its high spatial resolution, dark-field scattering spectroscopic imaging can provide real-time monitoring of the growth of Au along the TiO2 NT. Figure 5 displays visible dark-field scattering imaging snapshots of the photodeposition process of Au/TiO2 NWs. We immersed TiO2 NTs in the precursor solution and irradiated UV light on a specific area to induce the local growth of an Au along the TiO2 NTs while performing dark-field scattering imaging. As expected, scattering became stronger in the area illuminated by UV light, indicating the local growth of Au cores inside the TiO2 NTs.
We performed the FDTD calculations to numerically simulate the absorption strength and the absorption spectra. Figures 6(a) and 6(b) show snapshots of electromagnetic field distributions around, respectively, the TiO2 NT and Au/TiO2 NW, both illuminated with 365-nm and 600-nm wavelength light. The stronger field enhancement results in the UV and visible wavelength ranges calculated for the sample containing the Au core is consistent with the absorption images shown in Fig. 2. We also show the calculated absorption spectra of the TiO2 NT and Au/TiO2 NW, oriented parallel (“longitudinal”) (Fig. 6(c)) and perpendicular (“transverse”) (Fig. 6(d)) to the polarization direction. In both orientations, the Au/TiO2 NW was calculated to display the stronger absorption in the UV and visible wavelength range. Note in Fig. 6(d) the characteristic absorption peak due to Au LSPR at a wavelength of ~600 nm, consistent with the experimental observation.
In order to investigate the effect of the enhanced absorption and scattering to the photocatalytic performance, we measured the photodegradation of the dye AO7 in water by the TiO2 NT and Au/TiO2 hybrid NW, performed with 355-nm-wavelength laser illuminations, as the result is shown in Fig. 7.
The change of the absorption rate of AO7 without the TiO2 samples was also provided as a reference. Decolorization of AO7 was used as the indicator of the photodegradation effect and thus the photocatalytic performance of the sample. This decolorization was found to be significantly faster when using the Au/TiO2 hybrid NW sample than when using the plain TiO2 NT sample. Based on comparing the amount of time it took for C/Co to reach e−1, a two-fold increase of the reaction time was achieved when including the Au. This result confirmed that the hybridization of the TiO2 NT with an Au core greatly increased the photocatalytic efficiency, and this increase resulted from the LSPR excitation of the Au core. We expect that with the larger size of Au core or addition of Au nanoparticles on Au/TiO2 hybrid NWs, the absorption at UV illumination can increase. Therefore we believe that photocatalytic performance of Au/TiO2 hybrid NWs can be more enhanced by optimizing the size and the structure.
We fabricated Au/TiO2 hybrid NWs and investigated their scattering and absorption properties at the single NW level. The results of nanoscale optical imaging and spectroscopy showed that the scattering and absorption of Au/TiO2 NWs were substantially stronger than those of plain TiO2 NWs. The results of the degradation of the AO7 dye to measure the photocatalytic performance of the Au/TiO2 NWs were consistent with optical measurements demonstrating a two-fold improvement over plain TiO2 NTs under UV illumination. Our results suggest that nanoscale optical imaging of scattering and absorption can provide in situ assessments of the photocatalytic performances of individual metal-TiO2 hybrid nanostructures.
This work was supported by IBS-R011-D1. S. Lee and H. Shin acknowledge the financial support in part from the NRF of Korea, funded by the Korean Government (MEST) (NRF-2012M3A7B4049986 and 2013R1A2A2A01068499) and the Human Resource Development program (No. 20124010203270) of KETEP grant. Portions of this work were presented at the International Conference on Nanophotonics in 2016, (IN-25).
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