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One-dimensional plasmonic nano-photocatalysts: Synthesis, characterization and photocatalytic activity Simona E. Hunyadi Murph, PhD Savannah River National Laboratory, Aiken, SC
[email protected] ABSTRACT This study describes a simple two-step approach to coat gold nanorods with a silica/titania shell. Gold nanorods with an aspect ratio of 2.5 (L=48±2 and d=19±1) are synthesized by a silver-seed mediated growth approach according to our previously reported procedure (Hunyadi Murph ACS Symposium Series, Volume 1064, Chapter 8, 2011, 127-163 and reference herein). Gold nanorods are grown on pre-formed gold nano-seeds in the presence of surfactant, cetyltrimethylammonium bromide (CTAB), and a small amount of silver ions. A bifunctional linker molecule which has a thiol group at one end and a silane group at the other is used to derivatize gold nanorods. The silane group is subsequently reacted with both sodium silicate and titanium isopropoxide to a silica/titania shell around the gold nanorods. By fine tuning the reaction conditions, the silica/titania shell thickness can be controlled from ~5 to ~40nm. The resulting nanomaterials are stable, amenable to scale up and can be isolated without core aggregation or decomposition. These new materials have been characterized by scanning electron microscopy, energy dispersive X-ray analysis, UV-Vis spectroscopy and dynamic light scattering analysis. Photocatalytic activity of Au-silica/titania nanomaterials under visible and UV illumination is measured via degradation of a model dye, methyl orange (MO) under visible and UV illumination. The results indicate a 3 fold improvement in the photocatalytic decomposition rate of MO under visible illumination vs. UV illumination. Keywords: nanophotocatalysts, core-shell, gold-titania, anisotropic nano-photocatalysts, titania.
1. INTRODUCTION One of the most promising and environmentally friendly technologies for energy generation is the ability to produce chemical fuels using sunlight energy. Efficient harnessment, conversion, and manipulation of energy from sunlight have proven to be a difficult and complicated task. In recent years, however, significant advances have been made with the emergence of nanoscale materials which show higher selectivity and efficiency toward catalytic processes. Nanomaterials are particularly attractive due to their unique electronic, optical, and catalytic properties which are highly tunable by controlling the size, shape, composition, and assembly of nanoparticles.1,2 Furthermore, nanomaterials are ideal for catalytic applications as they possess much higher reactive surface areas than their bulk counterparts. This allows reduced catalyst load requirements and thus overall device cost without compromising performance.1,2,3,4 Titanium dioxide photocatalysis has received widespread attention as a means for harnessing solar energy to treat polluted water.5,6 Much of the emphasis is being placed on destroying volatile organic compounds that are found in contaminated groundwater. TiO2 it is also well known as an efficient photocatalyst at ultraviolet wavelengths for use in hydrogen production,7 self-cleaning,8 and solar cells.9 Titania has been considered one of the most appropriate candidates for photocatalytic processes due its powerful oxidation capability, superior charge transport, and corrosion resistance.7 However, titania does not absorb light in the visible region of the electromagnetic spectrum. With a wide band gap of ~3.2 eV, TiO2 can only be utilized in the UV region, which accounts for only a small fraction (~5%) of the solar spectrum.10 Thus, it is more desirable to use visible light as the energy source for photovoltaics applications.
Great efforts are being made to develop efficient visible light active photocatalysts to utilize a larger proportion of the solar spectrum. Numerous attempts have been made to expand the wavelength limitation of titania nano-photocatalyst, including doping with anions and cations, coupling with other oxides, dye sensitization, and defect creation among others.11,12 ,13,14, 15,16 ,17 However, because the visible light activity of the TiO2 is highly dependent on the concentration of the adsorbed dye, these types of photocatalysts are not reusable once the dye molecules have degraded.18,19 In addition, over concentration of sensitizer can inhibit the redox reactions occurring on the TiO2 surface.15,20,21 Another major concern with these procedures is their low energy conversion efficiency. Doping of titania nanoparticles with metal nanoparticles has also been investigated as a means for improving the photocatalytic properties of titania and extending the response of the material into the visible region.2,10,16,22,23 Noble metallic nanoparticles, particularly gold and silver, are of special interest in this area because they efficiently absorb and scatter light in the visible region of the electromagnetic (EM) spectrum. This is due to localized surface plasmon resonance (LSPR).2,3,24,25 Plasmons are a collective oscillation of electrons produced in response to external electromagnetic radiation. 24,25 This property has been widely explored in surface-enhanced spectroscopies as a way of detecting molecules on metallic substrates. 24,25,26 Recently, the enhanced local field properties of plasmonic NPs have been used to design plasmon enhanced dye sensitized solar cell.27,28,29 Several groups have recently reported an enhancement of charge carrier generation and photocurrent generation in dye-sensitized solar cells (DSSC) for dye monolayers on flat TiO2 films incorporating silver islands or sparse coatings of metallic nanoparticles.30,31 Enhanced light absorption has also been demonstrated in dye-sensitized solar cells on flat TiO2 films when silver islands are thermally evaporated upon the TiO2.32 A successful strategy for incorporating Au-SiO2 nanoparticles with strong surface plasmon has been reported by Snaith’s group.27 This new plasmonic photovoltaic system has led to enhanced light absorption and photocurrent generation in dye-sensitized solar cells. Another study explores the influence of gold nanoparticles on α-Fe2O3 photoanodes for photoelectrochemical water splitting.33 A relative enhancement in the water splitting efficiency at photon frequencies corresponding to the plasmon resonance in gold is observed. A number of architecture strategies have been employed to sensitize TiO2 to visible light. These include decorating TiO2 surface with metal nanoparticles and encapsulation of metal in TiO2. While effective catalytic activities can be achieved with these geometries, agglomeration, sintering, corrosion or dissolution of the exposed metallic occurs. One way to avoid these processes is encapsulation in porous materials that produce core-shell architectures. The pore channels of the coating must be large enough to allow effective transfer of the reacting molecules, while small enough to prevent the cores from unwanted reactions. By increasing the surface area of TiO2 via porous structures, more photogenerated electrons or holes can be in contact with reactants, and thus the photocatalytic efficiency will increase. Several studies show that incorporating “bare” metal nanoparticles into the bulk of dye sensitized solar cells is detrimental due to both corrosion and the metal acting as a charge recombination center.27,28 However, Au-silica nanoparticles incorporated into the TiO2 mesoporous paste are capable of sustaining the sintering process of the TiO2 with negligible influence to optical properties. Furthermore, they are corrosion resistant in the iodide/triiodie electrolyte and have no apparent detrimental electronic consequence in the solar cells, as far as charge recombination is concerned.27,28 To further improve photocatalytic performance, TiO2 can be combined with another semiconductor with a similar position in their conduction bands or metal nanoparticles. This can produce a so-called charge separation effect to extend the lifetime of the electron-hole pairs.34 This is possible because the photogenerated electrons are excited to the conduction band in the first material and can move to the conduction band of the second material, which can delay the recombination with photogenerated holes. Two-layer oxides can improve photocatalytic performance by 2-3 times as compared with a single TiO2 layer.35 TiO2 is also thermally unstable and readily loses its surface area due to sintering at high temperature. In contrast, titania/silica materials have higher thermal stability, surface acidity, and mechanical properties than bulk TiO2. In addition, the thermal expansion of these oxides does not change to a significant extent over a wide temperature range. 36 While there is a large amount of literature on TiO2-coupled with metallic nanoparticles, most of the research is focused on spherical metallic nanoparticles. For catalytic applications, the activity of a nanomaterial is not only dependent on its size but also on the distribution and exposure of its crystal faces. Depending on which crystal faces are exposed to the environment, different catalytic activities may be obtainable.2,3,37 For photocatalytic applications, one dimensional nanostructures are advantageous because they can offer direct electrical pathways for photogenerated charges. This results in superior charge transport properties.38 Light scattering from a small metal nanoparticle embedded in a homogeneous medium is nearly symmetric in all directions. However, this situation changes when the particle is placed
close to the interface between two dielectrics, in which case light will scatter preferentially into the dielectric with the larger permittivity. The scattered light will then acquire an angular spread in the dielectric that effectively increases the optical path length.39 On the other hand, light scattering and trapping is also very sensitive to particle shape. The distribution of the electromagnetic field around the particles is highly dependent upon the size and shape of the nanoparticles, the interparticle distance, and the dielectric function of the surrounding medium. The light-trapping effects are most pronounced at the peak of the plasmon resonance spectrum, and can be tuned by engineering the dielectric constant of the surrounding media.40 In essence, designing an efficient photocatalysts requires a careful selection of the nanoparticle size, geometry, composition, and local dielectric environment. Despite these unique properties of noble metals, there are no studies exploring the photocatalytic activity of anisotropic metallic nanoparticles. In this report, we describe a straightforward approach to the synthesis and characterization of Au nanorods with wellcontrolled dimensions encapsulated in a tunable silica-titania shell. The photocatalytic activity of the catalysts is measured via degradation of model dye methyl orange (MO), after exposure to visible or UV illumination.
2. EXPERIMENTAL 2.1. Materials: Chloroauric acid trihydrate, silver nitrate, trisodium citrate (99%), cetyltrimethylammonium bromide (CTAB), sodium borohydride, ascorbic acid, sodium silicate solution (27% SiO2 in water), 3mercaptopropyltrimethoxysilane (MPTMS, 95%) and titanium (IV) isopropoxide were purchased from Sigma Aldrich. All chemicals were used as received. All glassware was cleaned with aqua regia, and thoroughly rinsed with deionized water prior to use. 2.2. Synthesis of Au nanorods: In a typical experiment, a solution 2.5x10-4 M HAuCl4 was prepared in 0.1M CTAB. NaBH4 (600 mL, 10 mM) was added to the gold/CTAB solution (10 mL) with vigorous stirring for 10 min and used further as seeds. Briefly, the following aqueous solutions was prepared: CTAB solution (9.5 mL, 0.1 M), silver nitrate solution (50ul, 10 mM), chloroauric acid (0.5mL, 10 mM). To this solution ascorbic acid (0.55 uL, 0.1 M) was added with gentle mixing. Finally seed solution (12 uL) was added and the entire solution was mixed and left undisturbed overnight (14–16 hours). 2.3. Silica-titania coating: 25uL of mercaptopropyl trimetoxysilane (MPTMS) 0.5 mM -solution was added to a 1-mL aliquot of Au nanoparticles. The solutions were then stirred for 1hr. to ensure that MPTMS was bound to the Au nanoparticles. Then 10uL sodium silicate (Na2O(SiO2)3, a 0.252 M stock solution) followed by a 5-50 uL titanium isopropoxide of 0.5mM was added and the stirring was continued for another 20 min. The final solution was allowed to sit for 24 hrs. 2.4. Photocatalytic activity: The photocatalytic activity of synthesized Au-silica/titania nanorods was evaluated in the photodegradation of MO dye in water as a model. The test was carried out by using a 400 W mercury lamp as visible radiation source or UV light (365nm LED). The decomposition of MO of 0.1mM was conducted in a 1ml cylindrical quartz photo reactor. The Au-silica/titania nanorods were immersed into 1 ml of dye solution by continuously stirring. A blank solution was prepared by the same method without nano-photocatalyst. At regular intervals (30 minute), the MOnanophotocatalyst solution was sampled and its concentration measured by its absorbance at 460 nm by a UV-Visible spectrophotometer. During irradiation the solution was stirred continuously to assure a homogeneous process. The solution was equilibrated in the dark for 30-60 min before exposure to light source. Titania nanospheres with ~150nm diameter were prepared by a sol-gel procedure in ethanol, at room temperature, and were used as a baseline catalyst in the dye photodegradation studies. 2.5 Instrumentation: UV-vis spectroscopy was performed with Varian model Cary 500 Scan UV-Vis-NIR spectrophotometer. Light scattering and ξ-potential measurements were performed with a Brookhaven Zeta PALS instrument. Low-resolution transmission electron microscope (TEM) images were acquired with a 200kV Hitachi H8000 instrument. High-resolution TEM images and energy dispersive spectra (EDX) were acquired with a 200kV JEOL 2100F microscope equipped with a CEOS GmbH hexapole probe corrector. TEM samples were prepared by dropcasting the purified nanoparticle solution (10 uL) on holey-carbon Cu grids and allowing them to dry at ambient conditions.
3. RESULTS AND DISCUSSIONS 3.1. Synthesis and characterization of gold-silica/titania nano-photocatalysts Gold nanorods are grown on pre-formed gold nano-seeds in the presence of surfactant, CTAB, and a small amount of silver ions.2and reference herein,3,24 Initially, the reduction of Au3+ ions in the presence of a strong reducing agent, sodium borohydride, produced 1.5 nm-diameter gold nano-seed nanoparticles. These nano-seeds are further used to grow Au nanorods by adding more Au3+ ions, CTAB, additive Ag ions and a milder reducing agent, ascorbic acid. Silver ions are used as a way to control the shape, size and aspect ratio of the resulting Au nanorods. After preparation and purification, the anisotropic gold nanorods are coated with a silica-titania shell through an indirect coating method that involves the use of a bifunctional linker, 3-mercaptopropyltrimethoxysilane (HS(CH2)3Si(OCH3)3, MPTMS), which binds to the Au surface via the thiol group.2,41 The silane group is subsequently reacted with both sodium silicate and titanium isopropoxide to produce a silica/titania shell around the gold nanorods.2 SEM analysis of Au nanorods demonstrates that the silver assisted seed mediated approach produces nanomaterials with very good size and shape uniformity (Figure 1a). The Au nanoparticle library (L=48±2 and d=19±1) consists of tunable Au nanorods (~95%). High resolution TEM (HRTEM) is used to characterize the lattice arrangement and crystallinity of the Au nanorods. HRTEM images of the nanorods display well-defined, continuous and equally-spaced fringe patterns for their atomic lattice (Figure 1b). Previous reports have concluded that single-crystal nanorods are obtained when CTAB is used during silver seed-mediated growth process. 3 Growth of the silica-titania shell onto Au nanorods is monitored via ξ zeta potential measurements, energy dispersive Xray analysis, transmission electron microscopy (TEM), and UV-visible absorption spectroscopy. We are producing a silica dielectric network between the metallic nanoparticle and TiO2 semiconductor in order to increase physical separation, electrical isolation and reduce premature recombination of the electron-hole charges. Microscopy studies (Figure 1c, d) show that silica/titania shell is uniform along the entire length of Au nanorods and the rods are completely encapsulated within silica/titania shell. Discrete, non-agglomerated nanocomposite particles with homogeneous shells are produced. The thickness of the silica-titania shell can be easily controlled from approximately 5 to 40 nm by adjusting the growth conditions, including reactants concentrations.
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Figure 1. (a) SEM image of Au nanorods library, (b) HRTEM of Au nanorods, (c) Au-silica/titania nano-photocatalyst 20 nm shell thickness, (d) Au-silica/titania nano-photocatalyst 12 nm shell thickness and (e) titania nanospheres (150 nm).
Energy dispersive X-ray analysis was performed on the Au (data not shown) and Au-silica-titania nanoparticles to quantitatively identify the elements of interest. The results confirmed the presence of elements of interests, including Au, Ti, Si and O (Figure 2). Copper peaks in this spectrum are the result of copper grid used to drop cast the sample for TEM analysis.
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Figure 2. EDX of Au-silica/titania nano-photocatalyst. The effective surface charge of the Au nanorods before and after silica-titania coating changed from + 30mV to -47 mV, for example, for a 25nm silica/titania shell thickness. The original Au nanoparticles prepared by the seed mediated approach are stabilized by the CTAB bilayer and have an effective surface charge of ~ +30mV as determined by dynamic light scattering experiments. This is due to the positively charged quaternary nitrogen of the CTAB surfactant. 2,3,42 Silica and titania are well-known to bear a net negative charge in water due to deprotonated Ti-OH and Si-OH surface groups; thus, the ξ zeta potential measurements are in accord with the purported surface group.41 The influence of the silica/titania shell on the optical properties of the Au nanorods was studied with UV-vis spectroscopy (Figure 3). Gold nanorods exhibit two plasmon bands: longitudinal and transverse. This is due to the nanorods ability to absorb and scatter light along multiple axes: the long axis (longitudinal plasmon band) and the short axis (transverse plasmon band) at around 660nm and 520nm, respectively. 2,3 Both longitudinal plasmon band at ~660 nm and transverse plasmon bands at ~ 520nm appeared to red-shift with increasing silica/titania shell thickness deposition, consistent with theoretical predictions in the literature, which correlate this observation with an increase in the local dielectric constant surrounding the Au nanorods that is due to silica/titania shell.41
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Figure 3. UV-Vis spectra of (a) Au nanorods, (b) Au-silica/titania 10nm shell and (c) Au-silica/titania 25nm shell.
3.2. Photocatalytic activity under Vis and UV irradiation The majority of studies reporting metal-semiconductor nanocomposites for photovoltaic applications utilize metal nanoparticles as sinks for electrons that are photoexcited in the semiconductor, which acts to lengthen recombination times and expedite the transfer of electrons to the electrolyte solution.10,43 However, electrons can be injected into the conduction band of TiO2 by surface plasmons of the noble metals photoexcited by visible light. Under visible illumination, a charge transfer mechanism occurs whereby the plasmon resonance excites electrons in Au, which are then transferred to the conduction band of the adjacent TiO2.44,45,46,47,48 A similar charge transfer mechanism was proposed for dye-sensitized solar cells.49 The optical properties of control MO and MO in the presence of Au-silica/titania nanophotocatalyst are investigated by UV-Vis spectroscopy and are presented in figure 4. Incorporation of Au nanorods into the silica/titania shell in the presence of MO leads to the plasmonic amplification of light absorption. An amplitude enhancement of 2-3 fold is observed. This enhanced light absorption for MO recorded on Au-silica/titania nanorods will be used for subsequent plasmonic photocatalysis experiments.
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Figure 4. UV-Vis spectra of solutions of (a) Au nanorods, (b) MO, and (c) MO dye in the presence of nanophotocatalysts. The photocatalytic activity of as-prepared samples is evaluated by photocatalytic decolorization of MO aqueous solution at ambient temperature. The absorption spectra taken before and after illumination with both UV and Vis light at 30 minute intervals are used to quantify the relative MO concentration. A decrease in intensity of the MO absorption spectra is used to evaluate the photodecomposition rate. Photocatalytic efficiency is illustrated in figure 5. The concentration of MO decreases with illumination time for Au-silica/titania nanoparticles exposed under both UV and Vis illumination.
% Degradation MO
Figure 5 shows the percentage of MO that has decomposed over several hours in the presence of Au-silica/titania nanoparticles. As a control experiment, the photodegradation of MO aqueous solution under Vis and UV illumination in the absence of any photocatalysts is performed. Additionally, photodegradation studies of MO aqueous solution under Vis and UV illumination in the presence of non-coated Au nanorods is performed. As expected, no MO photodecomposition is observed (data not shown). This demonstrates clearly that titania nano-photocatalyst drives the photodecomposition process of MO. The exact concentration of titania nanoparticles responsible for this process is currently unknown, but is under investigation and will be reported at a later date. Titania spherical nanoparticles (~150 nm) (Figure 1e) are also used as a control experiment. The photodegradation studies on titania nanospheres are presented in figure 5. Titania spherical nanoparticles do not show significant MO degradation under Vis illumination (diamonds). The data also shows a 5-7 % photodegradation when titania spherical nanoparticles are exposed to UV illumination (squares).
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Figure 5. % MO photo-degradation analysis of MO in the presence of titania nano spheres under Vis illumination(diamonds), titania nano-spheres under UV illumination(squares), Au-silica/titania exposed under Vis illumination (triangles) and Au-silica/titania exposed under UV illumination (circles).
The decomposition of MO is similar for both Au-SiO2/TiO2 nanoparticles when exposed under either UV or Vis illumination in the first 4 hours of exposure (spheres and triangles). The decomposition of MO in the presence of AuSiO2/TiO2 particles is 12% and 16% after 4 hours of exposure under UV and Vis illumination, respectively. This is quite interesting since in each case, a different photodegradation mechanism occurs when particles are exposed under UV or Vis illumination. For example, UV light is absorbed by direct interband transitions in the TiO2 semiconductor. However, under visible light, the Au nanorod surface plasmons are excited and transferred to the defect states in the bandgap of the TiO2. These results suggests a plasmonic enhanced photocatalytic activity due to the large plasmonic enhancement of the incident electromagnetic fields, which increases the electron–hole pair generation rate at the TiO2 surface, and hence the photodecomposition rate of methyl orange. A significant degradation of MO is observed after a 20 hour exposure in Au-SiO2/TiO2 under both UV and Vis illumination (Figure 6). Preliminary data shows an enhanced photocatalytic decomposition of MO of ~56% under Vis illumination compared with 22% under UV illumination. Given that Au nanorods are completely encapsulated in the silica/titania shell, it is believed that the increased decomposition of MO is not the result of Au nanoparticles acting as an electron trap to assist electron-hole separation and is due to the localized surface plasmon resonance. Irradiating Au nanoparticles at their plasmon resonance frequency creates intense electric fields which can be used to increase electron– hole pair generation rates in semiconductors. Recently, Watanabe’s group50 reported a new type of plasmonic photocatalyst that employs the enhanced electric field amplitude on the surface of Ag nanoparticles in the spectral vicinity of their plasmon resonances. The measured photocatalytic activity under UV illumination of MO onto these plasmonic photocatalysts, consisting of titania deposits onto core-shell Au-silica nanorods, is enhanced by a factor of 7 and is highly dependent on the silica shell thickness.50 By integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO2, Cronnin’s group reports enhanced photocatalytic decomposition of methyl orange under visible illumination.47 5
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Figure 6. (a) Photodegradation data on Au-silica/titania nanoparticles under Vis illumination and (b) photodegradation studies of MO on Au-silica/titania nanoparticles after 20 hour exposure under UV and Vis illumination. There are many factors that influence the photocatalytic properties of titania, such as crystal structure, morphology, surface area and porosity as well as the shape and distribution of pores. It is worth mentioning that the photodegradation studies are conducted on non-annealed Au-silica/titania core-shell nanoparticles prepared at room temperature. While not investigated in detail (currently under investigation), results suggest that silica/titania core-shell may be amorphous. The literature agrees with this assessment.51 Amorphous phase has many structural defects which form trapped states that could reduce the lifetime of photogenerated electron-hole pairs, which may reduce their photocatalytic activity. However, annealing amorphous TiO2 nanoparticles at high temperatures >500oC leads to phase transitions to its crystalline forms, anatase and brookite. For example, some studies report that one of the most suitable phases of TiO2 for photocatalytic processes is anatase phase. This is due to both a reduced number of defect sites and greater surface areas produced after annealing experiments.7,52 More recently, studies reported by Hurum have shown that a mixture of different crystalline phases can provide even better photocatalytic performance.53,54 For example, the mixture of anatase and rutile TiO2 at an appropriate ratio can outperform either pure anatase or pure rutile.53,54
Additionally, earlier reports have shown that photocatalytic activity of the plasmonic photocatalysts is highly dependent on the silica shell thickness.28,50 For example, as the distance between a dye molecule and a plasmonic particle increases, the plasmon-enhanced extinction of the dye decreases due to the decaying electromagnetic field in the dye sensitized solar cells.28 Photocatalytic activity with 100 nm thick SiO2 is observed to be close to that of pure TiO2 on Ag-silica nanoparticles embedded in titania films.50 In light of these results, further studies are ongoing to fully understand and control the nature and charge generation in these plasmonic nano-photocatalysts. However, although still crude, these structures have the potential to be used in solar cells and photocatalytic applications. Because the method of making these is a wet-chemical one, it is amenable to scaling up. Nevertheless, the ease of preparation, unique size, shape, and geometry of these structures can be further refined and exploited for future applications. 4. CONCLUSIONS This study illustrates production of Au-silica/titania nanomaterials that are suited for enhancing the performance and cost of catalysts for solar cell applications. A straightforward syntheses approach is developed to create anisotropic Au nanoscale materials. Thiol chemistry is successfully employed to derivatize the Au surface for silica-titania shell deposition. The nanocomposite particles are monodisperse in size and shape, are stable, and non-agglomerated. A ~ 3 fold improvement in the photocatalytic decomposition rate of MO under visible vs. UV illumination is recorded for a model analyte. This enhancement is attributed to the electric field enhancement near the Au nanorods and subsequent energy transfer to the semiconductor. These results suggest that the photocatalytic activity of large bandgap semiconductors, like TiO2, may be extended into the visible region of the electromagnetic spectrum. ** We thank Savannah River National Laboratory LDRD-DOE for funding. References: 1
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