Giant Enhancement of Near-Ultraviolet Light Absorption by TiO2

Sep 14, 2012 - significantly red shift its maximum absorption to the near-UV band ... embedded TiO2 nanoparticle, and results in giant enhancement of...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Giant Enhancement of Near-Ultraviolet Light Absorption by TiO2 via a Three-Dimensional Aluminum Plasmonic Nano Funnel-Antenna Xiao-Lan Zhong and Zhi-Yuan Li* Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Science, P.O. Box 603, Beijing 100190, China ABSTRACT: We propose a 3D aluminum funnel-antenna (FA) to enhance near-ultraviolet (UV) band absorption of a rutile TiO2 nanoparticle. The 3D finite-difference time-domain simulations show that rutile TiO2 nanoparticle placed within the antenna can significantly red shift its maximum absorption to the near-UV band around the mercury lamp 365 nm line, with a giant absorption cross section enhancement factor of more than two orders of magnitude. The field analysis indicates a significant 3D light harvesting effect, where the incident light energy impinging upon the wide-area antenna is efficiently collected, flows into the embedded TiO2 nanoparticle, and results in giant enhancement of local near-UV field intensity and effective absorption cross section. The design of the 3D aluminum FA opens up a promising way to boost the photocatalytic activity of TiO2 by using mercury lamp light.

I. INTRODUCTION In recent years, the photocatalytic property of TiO2 nanoparticle has received considerable attention in areas such as solar cells,1,2 sanitation,3 water purification,4 and cancer therapy.5,6 The photocatalytic efficiency is therefore an important performance factor for their practical applications. However, pure TiO2 usually has low photocatalytic activity because of its insufficient UV light absorption. As is reported by refs 7 and 8, rutile TiO2 has a wide bandgap of ∼3.0 eV, and thus photoexcitation occurs only in irradiation by light of a wavelength of ∼400 nm [near-ultraviolet (UV) region] or below. In general, the usual excitation wavelength is from 300 to 400 nm, for instance, the convenient mercury lamp with an emission line at 365 nm, yet the absorption maximum of pure rutile nanoparticles appears far from this range in shorter wavelengths.9 To activate the photocatalytic effect using UVlight with longer wavelengths and enhance this efficiency, many works have demonstrated that doping/mixing of other sulfides, metals, or oxides such as CdS, Fe2O3, Au, Ag, or Ag2O nanoparticles can be helpful.9−11 Most of these works focus on sample structure8,12,13 and preparation,14−16 or chemical interaction among particles17,18 to improve electron transition involved in the photocatalytic process. In this article, we focus our attention on the physical role of surface plasmon in increasing the photocatalytic efficiency of TiO2 in the UV region. Surface plasmon resonance can occur when light interacts with metal nanoparticles and nanostructures, and this can have many interesting applications.19,20 Previous study9 showed that gold nanoparticles can indeed improve the near-UV absorption of TiO2 nanoparticles via a plasmon-related light-harvesting effect, however, the maximum absorption band red shifts only to around 260 nm, which is still some distance below the mercury lamp 365 nm line. In © 2012 American Chemical Society

addition, the maximum enhancement factor is only several times. This light harvesting effect is closely related to the local field enhancement of plasmonic nanoantennae, which can create “hot spots” of light field within the vicinity, for example, the gap of plasmonic structures.21−24 Aluminum as one of the most abundant metallic elements in the earth has been widely used in aviation, building, and automobile. Compared with gold and silver, which are commonly used in most plasmonic experiments, aluminum has a higher plasma frequency due to its very large values of free electron density.25,26 Therefore it should have better plasmonic effects in the UV region than gold and silver,27 and it has been reported to be a substrate for surface-enhanced fluorescence28 and surface-enhanced Raman spectroscopy.27 It is well known that aluminum usually builds up a thin oxide surface of several nanometers. Yet, it has been reported that the presence of the oxide layer on the surface of Al triangular nanoparticles, especially on the tips of the nanotriangles, result in a significant red shift in the LSPR λmax;29 however, it does not have much influence on the plasmon excitation in the aluminum surface, in particular, the local field enhancement.27 Although 2D nanostructures with various shapes can enhance the local electromagnetic field by controlling the particle size and the interparticle distance,30 investigations also indicated that 3D nanostructures exhibit more active sites and favorable electrochemical properties, which can produce more “hotspots” and enhanced local electromagnetic field compared with lower dimensional nanostructures.31,32 We will show that when specially designed aluminum 3D nanostructures, which are Received: July 3, 2012 Revised: September 13, 2012 Published: September 14, 2012 21547

dx.doi.org/10.1021/jp306562u | J. Phys. Chem. C 2012, 116, 21547−21555

The Journal of Physical Chemistry C

Article

propagates along the z axis and polarizes along the x axis. The calculated absorption cross section spectra are plotted in Figure 1b. From Figure 1b we can clearly see that the absorption cross section rises with the particle size increasing, and there are three peaks on each absorption curve, which are located at 150, 210, and 250 nm, respectively. With increasing the size of the TiO2 nanoparticle, the 210 and 250 nm peaks obviously enhance. However, due to the material nature of the rutile TiO2, the absorption cross section is close to zero from 380 nm to longer wavelengths. (See the shadow area.) The arrow in Figure 1b corresponds to the mercury lamp 365 nm line. We can find that the absorption cross section is very low around this wavelength. As is mentioned above, extension to 3D morphologies would significantly broaden the category of nanoantenna design, so we turn our vision to 3D aluminum nanostructures with a wish to find out an appropriate nanoantenna structure with prominent plasmonic effects to improve the pure TiO2 absorption cross section in the near-UV region. The geometry of the successful designed Al nanostructure is shown in Figure 2a,b for 3D view

called funnel-antenna (FA) in this article, are used and incorporated with TiO2 nanoparticles, giant enhancement of light harvesting and local field magnitude will occur and help to promote rutile TiO2 absorption in the near-UV wavelength region around 365 nm by more than two orders of magnitude. The rest of this article is arranged as follows. In Section II, we present the geometric configuration of the 3D FA that we design and analyze the enhanced absorption cross section of TiO2 nanoparticle embedded into the antenna. In Section III, we further analyze the local field distributions and their evolution with time around the 3D FA to better understand the light-harvesting physics of the antenna. In Section IV, we compare the near-UV absorption enhancement performance of the aluminum 3D FA with conventional aluminum 2D planar antenna and with Ag and Au 3D FA. The result will show the advantage of the aluminum 3D FA in enhancing the near-UV absorption by TiO2 nanoparticles. Finally, in Section V, we present a brief summary to this article.

II. DESIGN OF FUNNEL-ANTENNA AND ANALYSIS OF OPTICAL SPECTRA Our design is drastically different from conventional design scheme of plasmonic nanoantenna33−35 and focusing lens.36−39 In the nanoantenna design, for example, the bow-tie antenna,21−23 the gap must be very tiny (on the order of 10 nm) to achieve a sufficiently high local field enhancement factor. Obviously this space is too limited to embed a TiO2 nanoparticle within the gap. In addition, the overall volume of “hot spots” in these nanoantennae is also very limited for meaningful photocatalytic use. A plasmonic lens can focus traveling surface plasmonic wave into a “hot spot”; however, it is not a near-field effect, and thus the field enhancement factor is also limited. Almost all plasmonic nanoantenna and lens are designed and fabricated in metal thin films and thus have 2D morphology. It is expected that extension to 3D morphologies would significantly broaden the category of our thinking over this problem and bring insightful new schemes of nanoantenna design. We first have a close look at the optical properties of pure TiO2 by calculating the absorption cross section of TiO2 nanoparticles embedded in air with different sizes via the discrete dipole approximation (DDA) method.40−42 The dispersion data of rutile TiO2 are acquired through linear interpolation according to the optical constants given in Palik43,44 As is schematically illustrated in Figure 1a, the studied pure TiO2 nanocuboid has the edge lengths of eS, eL, and eS along the x, y, and z axes, respectively. The incident light

Figure 2. Designed 3D Al funnel-antenna nanostructure embedded with a TiO2 particle. (a) 3D view and panel and (b) lateral view of the structure where y = 0.

and lateral view, respectively. The designed structure looks like a square-shaped funnel but is composed of several Al walls of different heights. The geometric parameters are explicitly marked in Figure 2. The shortest height and the thickness of the wall are h and t, respectively. The distance between two adjacent walls is w, and the height difference is Δh. Obviously this structure has a 3D configuration and is very different from the usual planar plasmonic nanoantenna and focusing lens nanostructures constructed in a metal film. A rectangular slot is located at the center of the designed Al funnel nanostructure to harbor TiO2 nanoparticle, and it is a bit bigger than the TiO2 particle with 8 nm longer in each edge. As is shown in Figure 2b, the origin of coordinates is chosen such that x = 0 and y = 0 are located at the center of the TiO2 particle, whereas z = 0 is 10 nm below the bottom surface of the TiO2 particle. The x and y axes are parallel to the square edges of the funnel antenna. We employ the 3D finite-difference time-domain (FDTD) method to simulate the optical properties of rutile TiO2 brought into the designed Al nanostructure. We first consider the situation with eS = eL, namely, a nanocube. The simulation results are shown in Figure 3a with different color solid and dashed lines for TiO2 nanocubes with and without the Al nanostructure (called embedded TiO2 nanoparticle and pure TiO2 nanoparticle), respectively. In this article, we fix the geometric configuration of the FA and choose parameters as h = 10 nm, w = 30 nm, t = 20 nm, and Δh = 10 nm. The FA includes four square rings, and the inner edge length of the smallest ring is 80 nm × 80 nm. It can be clearly seen that the absorption cross section of rutile TiO 2

Figure 1. (a) Schematic geometry of pure rutile TiO2 particle and (b) the absorption cross section of pure rutile TiO2 particle with different sizes. 21548

dx.doi.org/10.1021/jp306562u | J. Phys. Chem. C 2012, 116, 21547−21555

The Journal of Physical Chemistry C

Article

Figure 3. (a) Absorption cross section of embedded (denoted as FA, solid lines) and pure (denoted as No FA, dashed lines) TiO2 nanocubes with different sizes. (b) Corresponding enhancement coefficient of TiO2 absorption cross section. The shadow area in panel b denotes the position where the enhancement coefficient is larger than 20. (c−e) Calculated electric field intensity patterns at 362 nm with the TiO2 particle size of 40 nm × 40 nm. These parameters correspond to the peak magnitude of panel b. (c) Top view of the slot within the horizontal plane of z = 40 nm. (d) Lateral view of the field patterns in the vertical plane of y = 0. The enlarged picture of a local region in panel d is drawn in panel e to have a clear show to better understand the local field enhancement effect.

Figure 3d,e. We can find that the local field intensity enhances greatly in the slot with a maximum quantity of ∼300. As a result, the rutile TiO2 particle has much more opportunity to interact with the incident optical field, and the absorption cross section of rutile TiO2 greatly increases. Besides the factor of designed FA nanostructure, the geometric parameters of TiO2 nanoparticles also have a large influence on the absorption properties. We change the size of the rutile TiO2 nanocuboid with other parameters of the Al FA nanostructure kept unchanged. The calculated absorption cross-section spectra of TiO2 are plotted in Figure 4a−c. We need to emphasize that the size of the particle is now eS × eL. From these curves we can find that for the same eS, TiO2 nanoparticles with a larger eL will lead to a much larger red shift of spectrum. The enhancement coefficients with different sizes of TiO2 particles are plotted in Figure 4d−f. We also use shadow areas in Figure 4d−f to denote the position where the enhancement coefficient is larger than 40, 100, and 200, respectively. Obviously, the maximum absorption cross section of rutile TiO2 appears at some certain appropriate sizes rather than at the largest size. The reason is that when the size of TiO2 particle becomes too large, the confinement capability of the near-UV optical field becomes lower in the large-size slot of FA. To make a clear show of how the FA enhances the near-UV local field encountered by the TiO2 particles, we consider the field pattern evolution with time when near-UV light shines on the FA nanostructures. For this purpose, we take the TiO2

significantly increases when it is placed within the Al funnel nanostructure compared with the pure TiO2 nanocube, and the absorption peak moves to the longer wavelength region. What is more, with increasing the edge length, the peak red shifts more significantly. To give more clarified physical images and insightful understandings on the relationship between the size of the TiO2 nanocube and the absorption cross section, we define a parameter called enhancement coefficient to describe the enhanced absorption effect by the designed structure, which is equal to the ratio of the absorption cross section of an embedded TiO2 nanoparticle over that of a pure TiO2 nanoparticle. The results are shown in Figure 3b with solid lines of different colors. From Figure 3b, we can see that the highest peak appears at 362 nm when the size is 40 nm × 40 nm, and the magnification factor of absorption cross section is near 40 times. We use shadow area in Figure 3b to denote the position where the enhancement coefficient is larger than 20.

III. FIELD ANALYSIS OF FUNNEL ANTENNA To make a clear show of the physical image, we calculate the field intensity distribution of the designed structure for the TiO2 nanocube with edge length of 40 nm × 40 nm at 362 nm. The results are plotted in Figure 3c−e. From Figure 3c,d, we can find that in the slot the local field meets its strongest value and it looks like the funnel nanostructure makes the energy flow into the slot. To see the local field enhancement effect clearly, we plot the enlarged picture of the concerned region in 21549

dx.doi.org/10.1021/jp306562u | J. Phys. Chem. C 2012, 116, 21547−21555

The Journal of Physical Chemistry C

Article

Figure 4. (a−c) Absorption cross section of embedded TiO2 nanocuboid particles (denoted as FA, solid lines) with different sizes in comparison with pure TiO2 nanoparticles (denoted as No FA, dashed lines). (d−f) Enhancement coefficient of TiO2 particle corresponding to panels a−c, respectively. The shadow areas correspond to different enhancement coefficients which are marked in panels d−f.

nanoparticle with a size 40 nm × 60 nm as an example. As can be found in Figure 4f, the maximum absorption cross section of the embedded TiO2 particle occurs at the wavelength of 368 nm. We then use a 368 nm continuous-wave (CW) incident light to illuminate the FA embedded with the TiO2 particle. In our 3D FDTD calculation, we set a time monitor on the top of the FA (where z = 50 nm, it corresponds to the top plane of TiO2 nanocuboid) to record the electric field intensity with the change of time. We snapshot six cross-sectional pictures in the horizontal plane of z = 50 nm at different time points, which are displayed in Figure 5. From the plotting scale of Figure 5 we can see clearly that the light energy is collected and flows into the slot when it impinges upon the designed FA. Figure 5a is the snapshot at 0.55 fs, and we can find that the whole FA is irradiated at the same time. The field intensity is small and comparable to that of the incident light. Because the direction of the incident light polarization is along the x axis, only those vertical walls that are perpendicular to the x axis can excite surface plasmon polariton (SPP). As a result, the light

energy is mainly collected and gathered in the y direction, and the local field intensity is much stronger around the y-axis walls than around the x-axis walls. These features can be seen clearly in Figure 5. As time goes on, most of the energy begins to flow from the outer part of the FA and concentrate into the slot. The E-field intensity increases rapidly in the slot. As can be seen from the snapshot pictures evolving from 3.87 to 12.39 fs (from Figure 5b−f), the local field intensity gradually grows at the slot where the TiO2 particle is located and finally reaches a stable state of significantly enhanced field intensity, which can be more than 700 times larger than the incident CW wave. As a result, giant enhancement of the effective near-UV absorption cross section for TiO2 nanoparticles is achieved. To see the funnel effect more clearly and to give a more clarified insight of the underlying physics, we plot the distribution of electric field intensity along the x-axis direction, where y = 0 and z = 50 nm, which corresponds to the dashed lines in Figure 5. The results are displayed in Figure 6. Here we use the log scale to see clearly the full information. From Figure 21550

dx.doi.org/10.1021/jp306562u | J. Phys. Chem. C 2012, 116, 21547−21555

The Journal of Physical Chemistry C

Article

Figure 5. E-field intensity snapshot of the movie monitored at different time points at (a) t = 0.55 fs, (b) t = 3.87 fs, (c) t = 6.97 fs, (d) t = 9.95 fs, (e) t = 12.39 fs, and (f) t = 15.48 fs. The TiO2 nanoparticle has a size of 40 nm × 60 nm, and the contour line of the funnel-antenna morphology is shown by white lines for the convenience of viewing. The dashed lines denote the position (with y = 0) where the contour plots of E-field intensity are shown in Figure 6.

Figure 6. E-field intensity distribution along the x-axis direction with y = 0 and z = 50 nm, which corresponds to the dashed lines in Figure 5. Panels a−f correspond to different time points at (a) t = 0.55 fs, (b) t = 3.87 fs, (c) t = 6.97 fs, (d) t = 9.95 fs, (e) t = 12.39 fs, and (f) t = 15.48 fs, respectively.

6, we can see the accumulation of energy in the slot. In the early stage of the near-UV light excitation, that is, t = 0.55 fs, the light energy well-disperses in the whole area of the FA slits. As time goes on, the energy in the slot increases very fast, which is shown from Figure 6b−d. Eventually it arrives at the steady state, as depicted in Figure 6e,f.

We also calculate the cross-section pattern of E-field intensity in the horizontal plane of y = 0 at different time points, and several typical results are shown in Figure 7 in correspondence with the patterns shown in Figure 5. From the plotting scale of Figure 7, we can see the same phenomena as in Figure 5 but with a more clarified insight. In the early stage of the near-UV 21551

dx.doi.org/10.1021/jp306562u | J. Phys. Chem. C 2012, 116, 21547−21555

The Journal of Physical Chemistry C

Article

Figure 7. Cross-section of E-field intensity snapshot of the movie monitored at different time points at (a) t = 0.55 fs, (b) t = 3.87 fs, (c) t = 6.97 fs, (d) t = 9.95 fs, (e) t = 12.39 fs, and (f) t = 15.48 fs. The TiO2 nanoparticle has a size of 40 nm × 60 nm, and the contour line of the funnel-antenna is shown by white lines in each panel.

local field, similar to the case for the central slot. It is expected that when TiO2 nanoparticles are embedded into these regions, they also greatly contribute to the overall photocatalytic efficiency. A typical example to support the above assumptions is illustrated schematically in Figure 8a, where three TiO2 nanoparticles are placed within the trenches in the direction away from the central hole besides the particle already within the hole. The sizes of these TiO2 nanoparticles are 26 nm × 26 nm × 8 nm, 26 nm × 26 nm × 18 nm, and 26 nm × 26 nm × 28 nm, respectively. The calculated absorption cross sections and enhancement coefficients of these TiO2 particles are displayed in Figure 8b,c. It can be clearly seen that the absorption cross section of rutile TiO2 significantly increased when it is placed within the Al funnel nanostructure compared with the pure TiO 2 nanocube, even though the TiO 2 nanoparticle is not embedded in the central hole of the FA. In this regard, the FA can work as a good photocatalytic substrate for a dense array of TiO2 nanoparticles. To improve the efficiency further, one can fabricate a periodic array of FA on a single substrate.

light excitation, the whole FA is illuminated by the incident light, and the distribution of E-field intensity in the xoz plane is nearly the same as the incident E-field intensity, which is set as unity. What is more, the local field enhancement has already appeared around the corner of the walls, although it is not very obvious in this early interaction time between the incident light and the FA. The reason is that SPP can be excited at the sharp corner much easier than in other places. As time goes on, that is, from 3.87 to 9.95 fs, the energy in the slot around the TiO2 increases very rapidly (on a time scale of 4 to 5 fs, which is only several temporal periods of the incident light with wavelength of 368 nm) and is enhanced obviously. The local field around the corner of the walls still has a high value, although it is weaker than around the slot. It seems like most of the energy flows into the slot when time passes. Eventually the system arrives at the steady state and the maximum E-field intensity tends to reach a constant value after t = 12.39 fs, which can be seen in Figure 7f. The pictures indicate that the excitation of SPPs and their transfer between adjacent metal walls play a key role in facilitating the energy collection, focusing, and accumulation in the slot. All of these factors sum and lead to giant enhancement of the local field intensity and effective absorption cross section of near-UV light by the TiO2 nanoparticle. In the above, we consider only a single TiO2 nanoparticle placed in the central “hot spot” of the FA and find a giant enhancement of the absorption cross section, yet according to our numerical calculations, significant field enhancement of near-UV light can also take place in other regions of the FA, in addition to the largest one at the central slot. When TiO2 nanoparticles are brought into the outer empty trenches and grooves of the FA, the interaction between these TiO2 nanoparticles and the FA will lead to great enhancement of

IV. FURTHER DISCUSSIONS The above design scheme of 3D FA is drastically different from the conventional design scheme of plasmonics nanoantenna and focusing lens. To prove explicitly the advantages of 3D FA, we also investigate the traditional 2D antenna structure which is usually used in many theoretical and experimental works. The schematic geometry of the aluminum antenna embedded with a TiO2 nanoparticle is shown in Figure 9a. The model structure consists of a TiO2 nanocube particle suspended between the gap of the Al antenna (made from two nanocubes). The size of each Al nanocube is 40 nm, and the size of TiO2 particle takes 21552

dx.doi.org/10.1021/jp306562u | J. Phys. Chem. C 2012, 116, 21547−21555

The Journal of Physical Chemistry C

Article

Figure 8. (a) Designed 3D Al funnel-antenna nanostructure embedded with three TiO2 particles (marked with 1, 2, and 3) in addition to the particle within the central hole. (b) Absorption cross section of embedded (denoted as FA, solid lines) and pure (denoted as No FA, dashed lines) TiO2 particles with different sizes and positions for the three particles. (c) Corresponding enhancement coefficient of TiO2 absorption cross section for the three particles. The shadow area in panel c denotes the position where the enhancement coefficient is larger than 50.

Figure 9. (a) Schematic geometry of TiO2 particle with aluminum antenna. (b) Calculated absorption cross section of TiO2 particle with different sizes embedded in the antenna (solid lines). The dashed lines are the absorption cross section of pure TiO2 particles for comparison.

TiO2. However, compared with the results of TiO2 embedded in 3D aluminum FA, which are shown in Figure 3a,b, we can find that the enhancement of absorption cross section by the 2D antennas is much lower than that by the 3D FA in the concerned wavelength band around 365 nm. This indicates that the 3D FA has a better near-UV light harvesting capability than the 2D antennas.

the value of 20, 30, and 40 nm, respectively. We keep the edgeto-edge distance between the Al and TiO2 particles fixed at 10 nm. The calculation results of the absorption cross section spectrum for these structures are shown in Figure 9b. From Figure 9b, we can find that when the aluminum antenna is introduced, the interaction between the aluminum antenna and TiO2 particles leads to enhanced absorption cross sections of 21553

dx.doi.org/10.1021/jp306562u | J. Phys. Chem. C 2012, 116, 21547−21555

The Journal of Physical Chemistry C

Article

Figure 10. Absorption cross section of embedded (solid lines) and pure (dashed lines) TiO2 nanocubes with different sizes: (a) gold FA and (b) silver FA.

■

To certify the advantages of aluminum as a plasmonic material in the near-UV regime, we also use gold and silver instead of aluminum to calculate the absorption cross section of TiO2 when the particle is embedded in 3D FA. It is well known that the imaginary part of the refractive index of Al is much larger than its real part, and this means that it has good plasmonic resonance effect. In contrast, for Au or Ag, the imaginary part is comparable to the real part; as a result, they have strong absorption of UV-light and poor plasmonic resonance effect. From these analyses, it can be understood easily why the aluminum FA is better than the gold FA or silver FA in plasmonic enhancement. Indeed, our calculation supports this assumption. The calculated absorption cross section of TiO2 embedded in gold FA and silver FA are shown in Figure 10. We can find that both of the absorption cross sections of TiO2 with gold and silver are even lower than those of the pure TiO2. This seems to be a very surprising result. Yet, it is also easy to explain. Because of the material nature of gold and silver, the plasmon resonance effect is poor in the near-UV region, but the absorption is very strong. The Ag and Au FA will strongly and directly absorb the incident UV light, and very little light can reach the TiO2 particle embedded in the FA, leading to lower absorption cross section as shown in Figure 10.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the 973 Program of China (Grant No. 2013CB632704) and the Knowledge Innovation Program of the Chinese Academy of Sciences (No. Y1 V2013L11).

■

REFERENCES

(1) Regan, B. O.; Gratzel, M. Nature 1991, 353, 737−740. (2) Xu, C.; Wu, J.; Desai, U. V.; Gao, D. Nano Lett. 2012, 12, 2420− 2424. (3) Liu, Z. Y.; Zhang, X. T.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami, T.; Fujishima, A. J. Phys. Chem. C 2008, 112, 253−259. (4) Lazzeri, M.; Vittadini, A.; Selloni, A. Phys. Rev. B 2001, 63, 155409. (5) Ivankovic, S.; Gotic, M.; Jurin, M.; Music, S. J. Sol-Gel Sci.Technol. 2003, 27, 225−233. (6) Rozhkova, E. A.; Ulasov, I.; Lai, B.; Dimitrijevic, N. M.; Lesniak, M. S.; Rajh, T. Nano Lett. 2009, 9, 3337−3342. (7) Amtout, A.; Leonelli, R. Phys. Rev. B 1995, 51, 6842−6851. (8) Baizaee, S. M.; Mousavi, N. Physica B 2009, 404, 2111−2116. (9) Du, S. Y.; Li, Z. Y. Opt. Lett. 2010, 35, 3402−3404. (10) Wang, J.; Tafen, D. N.; Lewis, J. P.; Hong, Z. L.; Manivannan, A.; Zhi, M. J.; Wu, N. Q. J. Am. Chem. Soc. 2009, 131, 12290−12297. (11) Lindgren, T.; Lu, J.; Hoel, A.; Granqvist, C.; Torres, G. R.; Lindquist, S. Sol. Energy Mater. Sol. Cells 2004, 84, 145−157. (12) Pabisiak, T.; Kiejna, A. Phys. Rev. B 2009, 79, 085411. (13) Abdel-Aziz, M. M.; Yahia, I. S.; Wahab, L. A.; Fadel, M.; Afifi, M. A. Appl. Surf. Sci. 2006, 252, 8163−8170. (14) Yang, Y. F.; Sangeetha, P.; Chen, Y. W. Int. J. Hydrogen Energy 2009, 34, 8912−8920. (15) Chen, Z. Y.; Hu, Y.; Liu, T. C.; Huang, C. L.; Jeng, T. S. Thin Solid Films 2009, 517, 4998−5000. (16) Bian, L.; Song, M. X.; Zhou, T. L.; Zhao, X. Y.; Dai, Q. Q. J. Rare Earths 2009, 27, 461−468. (17) Pradhan, S.; Ghosh, D.; Chen, S. W. Appl. Mater. Interfaces 2009, 1, 2060−2065. (18) Shi, H. Q.; Kohyama, M.; Tanaka, S.; Takeda, S. Phys. Rev. B. 2009, 80, 155413. (19) Lezec, H. J.; Degiron, A.; Devaux, E.; Linke, R. A.; MartinMoreno, L.; Garcial-Vidal, F. J.; Ebbesen, T. W. Science 2002, 297, 820−822. (20) Ren, F. F.; Ang, K. W.; Ye, J.; Yu, M.; Lo, G. Q.; Kwong, D. L. Nano Lett. 2011, 11, 1289−1293.

V. SUMMARY In summary, our theoretical and numerical studies have shown that the designed 3D Al FA nanostructure can induce more than two orders of magnitude enhancement in the near-UV absorption cross section of an embedded rutile TiO 2 nanoparticle around 365 nm, the mercury lamp emission line. The key contribution to this effect is the excitation of SPPs at the near UV-band and their transfer between adjacent metal walls in the FA. These plasmonic effects greatly facilitate the energy collection, focusing, and accumulation in the slot and result in giant enhancement of the local field intensity and effective absorption cross section of near-UV light by the TiO2 nanoparticle. The great enhancement of near-UV light absorption efficiency of TiO2 by doping into the designed Al nanostructure suggests an effective method to boost the photocatalytic activity of rutile TiO2. The 3D FA nanostructure opens up a new route to design plasmonic antennae with application in photocatalysis, optical sensing, optoelectronic detection, and other areas. 21554

dx.doi.org/10.1021/jp306562u | J. Phys. Chem. C 2012, 116, 21547−21555

The Journal of Physical Chemistry C

Article

(21) Fromm, D. P.; Sundaramurthy, A.; Schuck, P. J.; Kino, G.; Moerner, W. E. Nano Lett. 2004, 4, 957−961. (22) Jin, E. X.; Xu, X. Appl. Phys. Lett. 2005, 86, 111106. (23) Hatab, N. A.; Hsueh, C. H.; Gaddis, A. L.; Retterer, S. T.; Li, J. H.; Eres, G.; Zhang, Z.; Gu, B. Nano Lett. 2010, 10, 4952−4955. (24) Feuz, L.; Jonsson, M. P.; Hook, F. Nano Lett. 2012, 12, 873− 879. (25) Fox, M. Optical Properties of Solids; Oxford University Press: New York, 2001. (26) Zoric, I.; Zach, M.; Kasemo, B.; Langhammer, C. ACS Nano 2011, 5, 2535−2546. (27) Dorfer, T.; Schmitt, M.; Popp, J. J. Raman Spectrosc. 2007, 38, 1379−1382. (28) Malicka, J.; Gryczynski, I.; Gryczynski, Z.; Lakowicz, J. R. J. Phys.Chem. B 2004, 108, 19114−19118. (29) Chan, G. H.; Zhao, J.; Schatz, G. C.; Duyne, R. P. V. J. Phys. Chem. C 2008, 112, 13958−13963. (30) Liu, N.; Tang, M. L.; Hentschel, M.; Giessen, H.; Alivisatos, A. P. Nat. Mater. 2011, 10, 631−636. (31) Dong, J.; Qu, S.; Zhang, Z.; Liu, M.; Liu, G. J. Appl. Phys. 2012, 111, 093101. (32) Davis, T. J.; Hentschel, M.; Liu, N.; Giessen, H. ACS Nano 2012, 6, 1291−1298. (33) Adato, R.; Yanik, A. A.; Altug, H. Nano Lett. 2011, 11, 5219− 5226. (34) Kang, T.; Choi, W.; Yoon, I.; Lee, H.; Seo, M. K.; Park, Q. H.; Kim, B. Nano Lett. 2012, 12, 2331−2336. (35) Seo, M. A.; Park, H. R.; Koo, S. M.; Park, D. J.; Kang, J. H.; Suwal, O. K.; Choi, S. S.; Planken, P. C. M.; Park, G. S.; Park, N. K.; Park, Q. H.; Kim, D. S. Nat. Photonics 2009, 3, 152−156. (36) Feng, L.; Tetz, K. A.; Slutsky, B.; Lomakin, V.; Fainman, Y. Appl. Phys. Lett. 2007, 91, 081101. (37) Ma, C.; Liu, Z. Appl. Phys. Lett. 2010, 96, 183103. (38) Ginzburg, P.; Nevet, A.; Berkovitch, N.; Normatov, A.; Lerman, G. M.; Yanai, A.; Levy, U.; Orenstein, M. Nano Lett. 2011, 11, 220− 224. (39) Shitrit, N.; Nechayev, S.; Kleiner, V.; Hasman, E. Nano Lett. 2012, 12, 1620−1623. (40) Goodman, J. J.; Draine, B. T. Opt. Lett. 1991, 16, 1198−1200. (41) Draine, B. T.; Flatau, P. J. Opt. Soc. Am. A 1994, 11, 1491−1499. (42) Zhou, F.; Li, Z. Y.; Liu, Y.; Xia, Y. J. Phys. Chem. C 2008, 112, 20233−20240. (43) Palik, E. D. Handbook of Optical Constants of Solids; Academic: Orlando, FL, 1985. (44) Bass, M. Handbook of Optics, 3rd ed.; McGraw-Hill: New York, 2009; Vol. 4.

21555

dx.doi.org/10.1021/jp306562u | J. Phys. Chem. C 2012, 116, 21547−21555