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Photoresist and electron beam lithography techniques were used to fabricate embedded Ag bowtie and diabolo nanostructures with various apex angles on the surface of a TiO2 film. The reinforced localized surface plasmon resonance (LSPR) and electric field generated at both the Ag/TiO2 and air/TiO2 interfaces enabled high light absorbance in the TiO2 nanostructure. Results for both the bowtie and diabolo nanostructures showed that a reduction in the apex angle enhances both LSPR and Raman intensity. The maximum electric current density observed at the apex indicates that the strongest SPR confines at the tip gap of the bowtie and corners of the diabolo. In a long-wavelength region, as the apex angle increases, the resonant peak wavelength of the standing wave matches the increased length of the prism edges of the bowtie and diabolo to create a redshift. In a short-wavelength region, as the apex angle increases, the blueshift of the resonant peak wavelength is presumably attributable to the increase in the effective index of the local surface plasmon polariton standing wave mainly residing along both the bowtie and diabolo axes. The redshift and blueshift trend in the simulation results for the resonant peak wavelength agrees well with the experimental results. The fastest photocatalytic rate was obtained by placing the Ag/TiO2 bowtie at an apex angle of 30° in the methylene blue solution, revealing that the plasmonic photocatalysis causes the highest degradation efficiency. This is because the Schottky junction and LSPR can stimulate many valid radicals for the environmental improvement.
© 2016 Optical Society of America
1. Introduction
Environmental pollution caused by the release of toxic chemicals from industries has been a major concern in recent years [1]. Nontoxic TiO2 has been used in environmental treatments, such as water and air purification, disinfection, and sterilization, to decompose various toxic organic and inorganic contaminants because of its strong photocatalytic activity and chemical stability [2‐5]. Electron transit from the valence band to the conduction band to form many electron-hole pairs is the most widely accepted photocatalytic mechanism. These pairs react with the adsorbed oxygen and water on the photocatalyst surface and produce free radicals that decompose organics into water and carbon dioxide, resulting in the degradation of the adsorbates [6].
When light irradiates a metal/dielectric nanostructure, the surface plasmon resonance (SPR) at the interface causes surface plasmon polariton (SPP) propagation. If the metal nanostructure has a small defect, the SPR is confined in the vicinity of this defect; this phenomenon is called localized SPR (LSPR) [7–11]. Thus, the induced high-intensity local electric field creates more electron-hole pairs because of the strong light absorbance. This phenomenon is utilized in our proposed photocatalyst device with Ag bowtie and diabolo nanostructures. The electric field generated between the two metallic bowtie tips can be enhanced tremendously through plasmonic resonance and the curvature effect of the bowtie tips. The enhanced optical field is confined in a volume that is only a small fraction of the cube of the resonant wavelength [12]. The factors enhancing the electric field include the gap between the tips [13,14] and the thickness [14] and apex angle [12] of the bowtie nano antenna. First, as the gap decreases, the electric field increases. This phenomenon is analogous to that of a metal-dielectric-metal-plasmonic slab waveguide, where a more significant field confinement occurs when the dielectric region becomes narrower. Second, as the bowtie’s thickness decreases, the maximum electric field region approaches the bottom dielectric/Ag interface because of the characteristic of the surface wave [14]. Third, as the apex angle increases, the lateral dimension of the bowtie increases. The two Ag/air interfaces are so apart that the LSPP is confined at one of the Ag/air interfaces. The bowtie dominates by using isolated SPPs, which propagate back and forth along the prism edges, interfere, and generate a standing wave whose resonant wavelength scales with the length of the prism edges and thus redshifts [12]. Similar to a bowtie structure, a diabolo nanobar structure generates a standing wave by connecting two tips to form a waist width. The electric field accumulates on both sides to form a rectangle. However, as the waist width decreases, the electric field gradually approaches the waist, where considerable electronic mass is accumulated [15]. Thus, the diabolo electric field enhancing effect depends on the waist width [15]. The difference in boundary conditions can explain the more modest resonance enhancement offered by the diabolo nanoantenna compared with the bowtie nanoantenna [16].
Although bare TiO2 exhibit high photocatalytic activity under UV irradiation, they are inactive under visible light irradiation. The motivation of this study is to use LSPR enhancing the visible light absorbance for promoting a high functional photocatalyst by making the nano silver bowties and diabolos with varied tip angles embedded on the anatase TiO2 film as a new device. The different apex angles resulted in varying SPRs, and the electric field enhancement was investigated for exploring the most effective photocatalyst that can be applied to visible light irradiation. The intensified electric field can generate several superoxide and hydroxide ion radicals for bactericidal action and disinfection. Methylene blue (MB) was used to examine the efficiency of degradation caused by these photocatalytic bowtie and diabolo nanostructures.
2. Experimental procedure
A 1500-nm-thick TiO2 film was deposited on a silicon wafer through sputtering (ULVAC SBH-3308RDE) with a pure titanium target under a vacuum pressure of 2 × 10−5 torr and mixed with oxygen/argon at a gas flow ratio of 3:1. To remove contaminants from the target surface, presputtering was performed for 10 min with a shutter covering the silicon wafer. Next, a uniformly thick anatase TiO2 film was deposited on a silicon substrate at 250 °C and a high power of 1500 W for 30 min.
A positive photoresist (TDUR-P015) was coated on the TiO2 film by using an 800-nm-thick spin coater. Electron beam (LEICA WEPRINT 200 E-Beam stepper) lithography with an exposure dose of 5 μC/cm2 was used to create bowtie or diabolo pattern cavities with apex angles of 30°, 60°, and 90°, as shown in Figs. 1-3, thus creating prisms of different lateral lengths. The nano Ag bowtie was 1130 nm long and 30 nm thick with a gap width of approximately 30 nm at the apex angles of 30°, 60°, and 90°, as shown in the left column in Figs. 1-3. The nano Ag diabolo was 1100 nm long and 30 nm thick with a waist width of approximately 35 nm connecting two tips at angles of 30°, 60°, and 90°, as shown in the right column in Figs. 1-3.
Fig. 1 Schematic dimension of nano Ag bowtie (a–c) and diabolo (d–f) with a apex angle at 30°, 60° and 90° respectively.
Prior to Ag evaporation, a 15-nm-thick Cr film was deposited as an adhesive layer. Subsequently, a 30-nm-thick Ag film was vapor-deposited at a current of 500 mA and a bias voltage of 500 V in an Ar atmosphere at a flow rate of 30 sccm into the cavities of the bowtie and diabolo pattern. Finally, the photoresist was lifted using pure acetone with ultrasonic assistance for 10 min.
The surface morphology (top view) of the samples was observed through scanning field-electron microscopy (SEM; Hitachi, FE-SEM 4800) at an accelerated voltage of 15 KV. The ultraviolet-visible-near-infrared (UV-VIS-NIR) absorbance spectra were recorded at wavelengths of 350–850 nm by using a Jasco v650 spectrophotometer. The Raman spectra (HORIBA iHR550) were recorded at wavelengths of 500–2500 nm under a 633 nm He-Ne laser at a power of 2 mW. To measure surface-enhanced Raman scattering (SERS), the photocatalytic nanostructures were dipped into a Rhodamine 6G (R6G) solution with a concentration of 10−5 M (in ethanol) (10−3 M for the pure TiO2 film without nano Ag embedment) and dried before the measurement.
Both Ag/TiO2 bowtie and diabolo nanostructures were cut into 1 cm × 1 cm samples by using a diamond cutter. The nano TiO2 bowtie/TiO2 and diabolo photocatalysts were placed into a 150 ml MB solution with a concentration of 10 ppm (in water) and irradiated for 4 h under a light emitting diode (LED) at wavelengths of 365 and 850 nm. For evaluating the photocatalytic degradation of the MB solution containing the various photocatalysts after the irradiation, the irradiated solution was sampled at 150 c.c. every 30 min and its optical density (incident intensity/transmitted intensity) was examined using Elisa (Bio-Tek uQuant) with a Xenon flash and an incident light of wavelength 665 nm.
3. Results
In Fig. 1, the morphologies of the top view of the Ag/TiO2 bowtie and diabolo nanostructures illustrate their well-defined shape, precise tip gap, and precise waist width with various apex angles. The gap width between the bowtie tips and the waist width of the diabolo are both approximately 30 nm (Fig. 1).
3.1 Simulation
All simulated intensity spectra and calculations of the fields surrounding our devices were obtained using the finite-element method (FEM) from the commercial software COMSOL, which was used for modeling the 3D Ag/TiO2 bowtie and diabolo nanostructures illustrated in Fig. 2; the specifications of these devices are close to those of the fabricated devices. A 30-nm-thick Ag film was placed directly on the TiO2 layer and all nano triangles had 20 nm radii of curvature at the tip and corners with apex angles of 30°, 60°, and 90°. Both the gap width between the bowtie tips and the waist width of the diabolo were approximately 30 nm. A 15-nm-thick Cr film was used as an adhesive layer between the Ag nano triangle and the TiO2 layer. The substrate was a 0.525 mm-thick silicon wafer, but using a 1000-nm-thick top layer of air. The computation domain is enclosed by perfectly-matched layers to avoid reflection. The wavelength-dependent dielectric constants of Ag, TiO2, and Si reported by Palik et al. [17] were used. Plane wave continuous excitation was incident from the top with electric field component amplitudes of Ex = 1, Ey = 0, and Ez = 0.
All calculated field distribution images are plots of normalized electric field amplitude in a plane with z set to 15 nm above the Ag/Cr interface. The calculated resonance intensity spectra of the absorption cross-section for the bowtie and diabolo at apex angles of 30°, 60°, and 90° shown in Fig. 4 are plots of the maximum normalized electric field (normalized |E|2) amplitude within this plane at each wavelength. For experimental application of photocatalysis, the relevant fields are those located directly inside or just outside the bowtie gap and diabolo corner regions; this necessitated the resonance spectral and spatial distribution information. The maximum resonance intensity and associated values of absorption cross-section of the bowtie occurred at double wavelengths of λres = 462 nm/678 nm, 459 nm/685 nm, and 453 nm/691 nm for the apex angles of 30°, 60°, and 90°, respectively [Figs. 4(a)–4(c)]. The maximum resonance intensity and associated values of absorption cross-section of the diabolo [Figs. 4(d)–4(f)] appeared at double wavelengths of λres = 462/680 nm, 455/685 nm, and 453/687 nm for the apex angles of 30°, 60°, and 90°, respectively. The maximum resonance intensity was found to redshift in the longer-wavelength region, and the magnitude intensity decreased from 678 nm, 685 nm to 691 nm for the bowtie nanostructure [Fig. 4(b)] and from 680 nm, 685 nm to 687 nm for the diabolo nanostructure [Fig. 4(e)] as the apex angle increased. However, the maximum resonance intensity blueshifted and the magnitude of intensity reduced at the short-wavelength region from 462 nm, 459 nm to 453 nm in the bowtie and from 462 nm, 455 nm to 453 nm in the diabolo [Figs. 4(b) and 4(e)] as the apex angle increased.
Fig. 4 Calculated (a–c) bowtie and (d–f) diabolo images of the electric field distribution, normalized electric field intensity, and absorption cross-section spectra.
3.2 UV-VIS-NIR analysis
The absorbance spectra of the nano Ag/TiO2 bowtie and diabolo with various apex angles were measured in the range of UV to NIR light by using a UV-VIS-NIR spectrophotometer. For bowtie, the resonant wavelength appear at 385/780, 382/800, and 372/809 nm, with the absorbance decreasing from 0.77/0.71, 0.64/0.65 to 0.58/0.54 following the increase in the angle from 30°, 60° to 90° [Fig. 5(a)]. Similarly, for diabolo, the resonant wavelength appears at 392/784, 382/796, and 375/811 nm with the absorbance reducing from 0.68/0.57, 0.61/0.53 to 0.53/0.44 as the apex angle increases from 30°, 60° to 90° [Fig. 5(b)]. The results reveal that the maximum absorbance peak occurs at approximately 382 nm and that it originates from the bare TiO2 film (Fig. 5). For the Ag/TiO2 bowtie nanostructure, the maximum absorbance peaks occurs at 385, 382, and 372 nm in the near-UV region and other coupled maximum peaks appear at 780, 800, and 809 nm at the longer-wavelength region for the apex angles 30°, 60°, and 90°, respectively. Similarly, for the Ag/TiO2 diabolo nanostructure, the maximum absorbance peaks occur at 392, 382, and 375 nm in the near-UV region and other coupled maximum absorbance peaks occur at 784, 796, and 811 nm in the longer-wavelength region for the apex angles of 30°, 60°, and 90°, respectively [Fig. 5 (b)]. However, the resonant peaks blueshift within the UV short-wavelength region as the apex angle increases for both the bowtie and diabolo nanostructures. The resonant peaks in a long-wavelength region redshift as the apex angle of the bowtie and diabolo nanostructures increases (Fig. 5).
Although the wavelengths of absorbance peaks obtained from the experimental results (Fig. 5) differ from the simulated (Fig. 4), the trend and order are identical. The resonant peaks of bowtie and diabolo nanostructures in Fig. 4 are far from the LED with wavelengths of 365 nm and 850 nm might be caused objectively by both the demension error and degree error of tip angle in fabricated bowtie and diabolo nanostructure with ± 10 nm and ± 2° respectively at the very sharp tips and corners during the fabrication process. In contrast, the maximum resonance peaks and associated values of absorption cross-section of bare TiO2 spectrum occured at 388 nm wavelength depicted in Fig. 4, which is nearly identical to the absorption spectrum for bare TiO2 film in Fig. 5 at 382 nm wavelength. The absorbance efficiency trend is shown in Fig. 5 as the apex angle decreases, not only does the absorbance intensity reach the maximum, but the two wavelengths at the maximum absorbance peak from both sides also shift toward to the visible light region.
3.3 Surface enhanced Raman spectroscopy
When the incident light strikes and excites surface plasmons on the metal surface, the electric field is enhanced. The Raman spectra of R6G on various nano Ag-embedded bowtie and diabolo nanostructures under a 633 nm wavelength laser are shown in Fig. 6. The Raman peaks labelled with stars correspond well with the vibration modes of R6G [18]. The higher the Raman peak intensity, the smaller the apex angle for both the bowtie and diabolo Ag/TiO2 nanostructures. Either the nano Ag/TiO2 bowtie or the diabolo with the smallest apex angle of 30° exhibits the highest Raman peak intensity in Fig. 6. This is because when the apex angle is small, the tip curvature and the electric charge density at the sharp apex increases, resulting in more SPP accumulation at the apex. The highly confined electric field between the two tips can be seen clearly from the mode volume M (i.e., the electric field volume), which can be expressed as M = Etotal/Emax. The electric field volume M is inversely proportion to the electric field Emax [18]. Thus, the LSPR intensity of the nano Ag/TiO2 bowtie is stronger than that of the nano Ag/TiO2 diabolo because the electric field is confined in the tip gap of the bowtie.
Fig. 6 Raman spectra of nano Ag/TiO2 (a) bowtie and (b) diabolo with various apex angles. Asterisks represent the vibration modes of R6G.
The analytical enhancement factor (AEF) of SERS is defined as AEF = (ISERS/CSERS)/(IRS/CRS) [19,20], where ISERS represents the integrated Raman intensity obtained for the SERS substrate under a certain concentration CSERS = 10−5 M, IRS indicates the integrated Raman intensity obtained under the non-SERS condition at an analyte concentration of CRS = 10−3 M. The AEF values of the nano Ag/TiO2 bowtie, nano Ag/TiO2 diabolo, and the reference substrate (TiO2/Si) were estimated by considering the R6G Raman peak at 609 cm−1 and 1357 cm−1, because these are the strongest peaks of all bands in the spectra. In Fig. 7, the AEF values in Table 1 obtained by integrating the Raman intensity of the R6G peaks at 609 cm−1 are 11.6 × 108, 7.8 × 108, and 5.8 × 108 for the bowtie structure and 8.9 × 108, 5.9 × 108, and 2.7 × 108 for the diabolo structure at apex angles of 30°, 60°, and 90°, respectively. A similar trend is observed for R6G Raman intensity peaks at 1357 cm−1 in Fig. 7: the AEF values listed in Table 1 are 14.2 × 108, 10.1 × 108, and 6.1 × 108 for the bowtie structure and 10.2 × 108, 6.3 × 108, and 3.9 × 108 for the diabolo structure at apex angles of 30°, 60°, and 90°, respectively. The extremely high AEF values decrease as the apex angle increases, and the AEF values for bowties is superior to those for diabolos. All values of the measured SERS AEFs shown in Fig. 7 are on the order of 108, which is much better than that reported in the literature (on the order of 106) [18,20]. Under the 609 cm−1 and 1357 cm−1 Raman bands, the AEF values show the following trend: 30° Ag/TiO2 bowtie > 30° Ag/TiO2 diabolo > 60° Ag/TiO2 bowtie > 60° Ag/TiO2 diabolo > 90° Ag/TiO2 bowtie > 90° Ag/TiO2 diabolo. This demonstrates that various apex angles and structures cause different LSPR enhancements. For both nanostructures, a higher AEF value occurs at the 609 cm−1 Raman band than at 1357 cm−1.
Table 1. AEF Calculation Process
3.4 Degradation of MB
The photocatalytic activities of the Ag/TiO2 bowtie and diabolo nanostructures with various apex angles placed in an aqueous solution were evaluated through the photodegradation of the MB organic dye under LED irradiation at 365 nm UV light combined with an 850 nm NIR light. The time profile of MB degradation was determined by monitoring the changes in MB concentrations (C/C0) derived from the evolution of characteristic absorbance (A/A0) of MB at 665 nm, where C0 and A0 are the initial concentration and corresponding initial absorbance values of the MB solution, respectively. The “dark condition” represents the bare solution without light irradiation, under neither nano Ag/TiO2 bowtie nor nano Ag/TiO2 diabolo placed into the MB aqueous solution, and the “control condition” represents the bare solution with light irradiation. The photocatalytic degradation rate constant, k, values of various bowtie and diabolo nanostructures were evaluated and plotted in Fig. 8(b). The highest value of k, 6.206 × 10−3 min−1, occurred at the Ag/TiO2 bowtie nanostructure with an apex angle of 30° and 4.719 × 10−3 min−1 at the Ag/TiO2 diabolo with an apex angle of 30°. The most effective photodegradation in Fig. 8 obtained at the nano Ag/TiO2 bowtie at 30°, where the degradation rate is nearly 6.206 × 10−3 min−1, is approximately four times that at bare TiO2, where the degradation rate is nearly 1.553 × 10−3 min−1. The photodegradation rate for these photocalyst nanostructures follows the following trend: 30° Ag/TiO2 bowtie > 30° Ag/TiO2 diabolo > 60° Ag/TiO2 bowtie > 60° Ag/TiO2 diabolo > 90° Ag/TiO2 bowtie > 90° Ag/TiO2 diabolo. The MB degradation trend (Fig. 8) agrees well with that of the aforementioned AEF values of SERS (Fig. 7). Clearly, the efficiency of photocatalysis in nano Ag/TiO2 bowtie is always superior to that in Ag/TiO2 diabolo. A sharper angle is preferred in both bowtie or diabolo more than an obtuse angle for facilitating valid plasmonic photocatalysis through the SPR-enhanced local electric field.
Fig. 8 Photocatalysis of nano Ag/TiO2 bowtie and diabolo with various apex angles on the MB solution under LED irradiation at the composite wavelength.
4. Discussion
As the apex angle of the Ag bowtie and daibolo decreases, the current density of electric charge at the sharp apex increases because of the higher tip curvature. The surface-enhanced Raman scattering shown in Fig. 6 occurs at the metal Ag/dielectric TiO2 interface and creates the highly confined electric field, as confirmed by the simulated results in Fig. 4, in the gap cavity between the two tips in the bowtie and at the corners in the diabolo. As a consequence of SERS (Figs. 4 and 6), the maximum light absorption intensity of UV-VIS-NIR spectra in Fig. 5 occurs simultaneously at both the short wavelength of near-UV region and the longer wavelength of the visible light region.
The numerical simulation results of the long-wavelength region in Fig. 4 indicated that the Raman resonance intensity decreases as the apex angle increases for both the bowtie and diabolo structures. The results agree with the trend of experimental absorbance in Fig. 5. In Figs. 4 and 5, the redshift of the simulated/experimental resonant wavelengths occurs at 678/780, 685/800, and 691/809 nm for bowtie structures and 680/784, 685/796, and 687/811 nm for diabolo structures with apex angles of 30°, 60°, and 90°, respectively. When the lateral length of the bowtie and diabolo structures increases because of the increase in the apex angle, couplings at the upper and lower edges are weak and the SPP propagating back and forth laterally dominates the structures. Thus, the lateral side can be regarded as a resonator. The resonant wavelength of the standing waves matches the increased lateral length of the prism edges and redshifts as the apex angle increases [12,21]. The resonator wave packet formula is = constant, where L is the lateral length of the bowtie or diabolo structure, neff is an effective refractive index, λ is the resonant wavelength, and k is the wave number. The effective refractive index neff changes slightly from 4.213468, 4.213483 to 4.213486 as the apex angle increases from 30°, 60° to 90° for the bowtie structure. The effective refractive index neff of the diabolo structure also decreases slightly from 3.682881, 3.682874 to 3.682843 as the apex angle increases from 30°, 60° to 90°. The effective refractive index neff is almost a constant, and λ is proportional to L. Therefore, the increase in L because of the increase in the apex angle could cause a resonant wavelength redshift in a long-wavelength region. On the contrary, the blueshift that occurred in the short-wavelength region can be attributed to the decrease in the effective refractive index. The effective refractive index neff of the bowtie structure decreases from 0.559869, 0.533871 to 0.522941 as the apex angle increases from 30°, 60° to 90°. The effective refractive index neff of the diabolo structure also decreases from 0.486689, 0.486064 to 0.485824 as the apex angle increases from 30°, 60° to 90°. Apparently, the blueshift of the wavelength of the resonant peak in the near-UV region is attributable to the decrease in the effective refractive index of the local SPP standing wave mainly residing along both the bowtie and diabolo axes [12]. The effective refractive index declines owing to the decrease in the curvature caused by the increase in the apex angle. Because neff is proportional to λ, the blueshift in the short-wavelength region matches the change in the effective refractive index.
The highest SERS AEF values of 14.2 × 108 and 11.6 × 108 (Fig. 7) lead to the highest photodegradation rate of 6.206 × 10−3 min−1 at Ag/TiO2 bowtie at 30° (Fig. 8), resulting from effectively photogenerated electrons and holes to inducing many radicals, in conducting the photocatalysis, specifically at the resonant wavelengths of 385 nm and 780 nm. As the apex angle increases for both bowtie and diabolo nanostructures, their AEF decreases, degrading the photodegradation rate (Fig. 8). In summary, the advantage of the smallest angle of 30° for bowtie and diabolo nanostructures can feasibly use the visible sunlight irradiation for photocatalysis, because two maximum absorbance peaks move together toward the visible light region from the left UV region and right NIR region, respectively.
The generated LSPR at both solution/Ag and Ag/TiO2 interfaces produces a strong confined electric field to create many electron-hole (h+) and electron-electron (e-) pairs at the TiO2 surface, resulting in high light absorbance. These e- and h+ carriers produced in the gap cavity of the bowtie and at the corners of the diabolo generate many superoxide and hydroxide ion radicals for decomposing organic pollutants into carbon dioxide and water. The Ag may additionally function as electron traps [22] to inhibit the electron-hole recombination, leading to a significant improvement in the photocatalytic activity. The mechanism of LSPR enhanced photocatalysis effect is schematically illustrated in Fig. 9. Recently, the plasmonic photocatalysis [23] comes to the SPR charge and energy transfer mechanism in the noble metal-semiconductor configuration as depicted in Fig. 9. The plasmonic photocatalysis proposed by Tsai’s research group [23] explains that a Schottky junction and localized surface plasmon resonance (LSPR) benefits photocatalysis differently. The Schottky junction resulting from the contact of the noble metal and semiconductor enhances the separation of the photo-excited electrons/holes and suppresses the electron/hole recombination [23]. The LSPR created in response to the electromagnetic field of the incident light makes the noble Ag bowtie/diablo to drive a collective oscillation of the electrons, which excites more electrons and holes by energy transfer and/or charge carrier transfer. Then a space-charge region is created in the TiO2 in the vicinity of the Ag surface due to the electrons diffuse from the TiO2 side to the Ag side and create a positively charged region with no free carriers in the TiO2 [23]. When an electron–hole pair is excited in or near the space-charge region by an incident light, the internal electrical field forces the electron to move to the Ag region and the hole to the TiO2 region, preventing their recombination. The electrons and holes are then captured by the oxygen and water in the solution, respectively. Consequently, many valid radicals are generated from further redox reactions. Another possible channel of electron transfer is that the LSPR excited electrons in Ag have sufficient energy to go across the space-charge region and are fed into the conduction band of TiO2. Thus, the LSPR, the Schottky junction, charge carrier trapping and large contact surface would significantly enhance the creation and separation of active electrons/holes and thus raise the reaction rate.
Fig. 9 (a)Schematic for the SPR-enhanced photocatalytic mechanism (b) the gap cavity of Bowtie and (c) field confinement distribution in diabolo.
5. Conclusions
SERS intensity is significantly enhanced by Ag/TiO2 bowtie and diabolo nanostructures compared with bare TiO2, thus verifying the existence of surface plasmon in the proposed device. The simulation results of redshift and blueshift trend in the resonant peak wavelength agree well with that of the experimental results. Two of the strongest light absorbance values at 385 and 780 nm for nano Ag/TiO2 bowtie with an apex angle of 30° are attributed to the reinforced LSPR at the tip gap of the nano bowtie and the corners of nano diabolo. As the apex angle increases, the resonant wavelength at the NIR region of the standing wave matches the increased lateral length of the prism edges and redshifts. In the short-wavelength near-UV region, as the apex angle increases, the resonant peak wavelength blueshift presumably attributes to the decrease in the effective index. The fastest photocatalytic rate after LED irradiation under the composite wavelength is nearly 6.206 × 10−3 min−1 by using Ag/TiO2 bowtie photocatalyst with an apex angle of 30° placed in the MB solution. The fast degradation efficiency arises from a considerable plasmonic photocatalysis caused by a shottky junction to suppress the electron-hole recombination and LSPR enhancing the light absorption, the local electric field and the excitation of active electrons and holes, to produce many valid radicals.
Acknowledgments
The authors acknowledge the help of Jheng-Hong Shih, Yi-Cheng Chung, and Chih-Kai Chiang at National Taiwan Ocean University. This work is supported by Ministry of Science and Technology (MOST), Taiwan, under the Grant Nos. MOST 102-2221-E-019-008-MY3 and MOST 103-2221-E-019-028-MY3.
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