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Enhancing Photochemical Activity of Semiconductor Nanoparticles with Optically Active Ag Nanostructures: Photochemistry Mediated by Ag Surface Plasmons Phillip Christopher, David B. Ingram, and Suljo Linic* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-2316 ReceiVed: February 23, 2010; ReVised Manuscript ReceiVed: April 6, 2010
Composite materials composed of optically active Ag nanostructures and TiO2 photocatalysts show enhanced photoactivity compared to the pure TiO2 in the decomposition of methylene blue. The enhanced photochemical activity is attributed to radiative transfer of energy, mediated by surface plasmons, from Ag particles to the semiconductor leading to higher concentrations of charge carriers (e-/h+ pairs) in the semiconductor and therefore to higher photochemical activity. We demonstrate that the performance of the composite photocatalysts is a strong function of the size and shape of Ag nanostructures. This can be explained by the size- and shape-specific optical activity of Ag nanostructures. We show that by rationally changing the size and shape of Ag nanostructures it is possible to maximize photochemical activity of a semiconductor at a given excitation wavelength. Introduction In addition to abundance and robustness, effective photocatalysts need to (i) efficiently absorb photons across the UV-visible region of the solar spectrum and convert these into electron hole (e-/h+) pairs, (ii) have surface electronic structure tailored so that the oxidation/reduction half reactions are thermodynamically feasible, and (iii) have surface chemical properties that allow elementary surface chemical transformations associated with the oxidation/reduction half reactions to be performed with modest activation barriers. These strict constraints have made it very difficult to identify promising photocatalysts, as most single component materials do not satisfy all the requirements.1-3 For example, the most investigated photocatalytic material, TiO2, has a number of drawbacks including the large band gap, ∼3.0 eV, which limits photoabsorption to the UV region of the solar spectrum.4,5 The photoabsorption of TiO2 can be extended to the visible region by doping with nitrogen, carbon, and a few other elements.6,7 However, the doped materials suffer from lower absorbance than undoped TiO2 and fast e-/h+ recombination rates, which ultimately limits their photochemical activity. In this contribution we show that composites of semiconductor nanoparticles and optically active metallic nanostructures represent a promising alternative to conventional photocatalysts. The main feature of these photocatalysts is that the interaction between semiconductor and metallic building blocks results in very efficient conversion of incident photons into e-/h+ pairs in the semiconductor. As a consequence, these materials exhibit photochemical reactivities that are significantly larger than a simple superposition of the reactivities of the building block materials. We also show that the photochemical reactivity of the composite materials can be rationally manipulated by changing the shape, size, and geometric arrangement of the building blocks. These findings suggest a large degree of design flexibility where specific combinations of semiconductors and * To whom correspondence should be addressed. E-mail: linic@ umich.edu.
optically active metallic particles can be tailored to exhibit optimal performance for a particular photochemical transformation. Our conclusions were obtained in a case study showing that the photochemical reactivity of TiO2, quantified by the photodecomposition of methylene blue (MB),8 is enhanced significantly when TiO2 is combined with optically active metallic Ag nanostructures. Our analysis showed that the enhanced photochemical reactivity is a result of the efficient creation of resonant Ag surface plasmons in response to a photon flux. The decay of plasmons results in energy transfer from the Ag nanostructure to TiO2 ultimately yielding a higher concentration of TiO2 e-/h+ pairs in the Ag/TiO2 samples compared to the TiO2-only samples. Controlling the size and shape of Ag nanostructures allows for the control of the properties of Ag surface plasmons (mainly the resonant energy and the mechanism of the plasmon decay), making it possible to tailor a particular metallic nanostructure to maximize the photochemical reactivity of specific semiconductors at a given excitation wavelength. Experimental Methods The Ag nanoparticles were prepared by using a modified procedure based on the polyol synthesis9 method that we have previously reported.10 The particles were washed with acetone once via centrifugation and redispersed in 200 proof ethanol. The particles were drop coated onto precleaned 1 cm2 SiO2 substrates and allowed to dry in a stagnant atmosphere. Au nanoparticles were synthesized via the citrate reduction method,11 concentrated by centrifugation, and dispered in a 10:1 ethanol:H2O solution prior to drop coating on SiO2 substrates. A solution of 2.3 mg of P25 TiO2 particles in 2 mL of 200 proof ethanol was prepared and sonicated for 2 h. A 20 uL sample of this solution was deposited on the Ag nanoparticle covered or clean SiO2 substrates and allowed to dry in a stagnant atmosphere. This procedure produced well-dispersed films with fairly constant optical properties across the entire surface. Methylene blue was deposited by submerging the substrates in a 0.65 mM methylene blue in ethanol solution for 2 h, followed
10.1021/jp101633u 2010 American Chemical Society Published on Web 04/29/2010
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by rinsing with ethanol. The films were allowed to dry for 2 h prior to photocatalytic testing. The photocatalytic experiments were performed with a 100 W source with an emission wavelength of 365 nm and a fwhm of 4 nm. The degradation of MB was monitored by using reflection UV-vis spectroscopy every 1 min for the course of the experiment. Scanning electron micrographs were obtained by using an FEI Nova 200 Nanolab. The accelerating voltage was 5 kV for all images. Transmission UV-vis absorbance spectroscopy was performed with an 8453 UV-vis Chem Station spectrophotometer (Agilent Technologies). Reflection absorbance UV-vis measurements were recorded with an AvaSpec 2048 spectrometer and an AvaLight DHc compact deuterium halogen light source (Avantes). Photoluminescence measurements were made with a Fluoromax-3 spectrofluorometer (Jobin Yvon/Spex Horiba, NJ). The interaction of light with the Ag nanoparticles was simulated by using the finite-difference time-domain (FDTD) method. FDTD is a computational electrodynamics modeling technique, which solves discretized Maxwell equations in space and time subject to the input geometry, material properties, and boundary conditions.12 The optical properties of Ag are represented by using a Drude-Lorentz model13 with empirical optical constants,14 which has been shown to give accurate results for Ag nanostructures.15 Extinction, scattering, and absorbance cross sections were calculated by using the total-field/scattered-field (TFSF) formalism.12 The scattering efficiency at 365 nm (see Figure 4b) was calculated by dividing the simulated scattering cross section at a wavelength of 365 nm by the particle’s geometric cross-sectional area.
Christopher et al.
Figure 1. (a) Kinetic data for the decomposition of methylene blue over pure TiO2, pure Ag nanocubes, 15 wt % composite Ag/TiO2 systems containing Ag spheres, cubes, and wires, and an Au/TiO2 composite system. (b-d) SEM micrographs of Ag nanocube/ TiO2 composite, Ag nanosphere/TiO2 composite, and Ag nanowire/ TiO2 composite. The scale bar is 250 nm.
Results and Discussion The MB decomposition studies were performed in a batch reactor system at room temperature. It has been proposed previously that the decomposition of MB on TiO2 is an oxidative process initiated by formation of holes in the valence band of TiO2. Under atmospheric conditions extra electrons are scavenged by O2.8 Three different composite Ag/TiO2 photocatalysts as well as pure TiO2 and Ag samples were tested. The three composite materials contained P25 TiO2 nanoparticles (equivalent to 2-3 close-packed monolayers) deposited on SiO2 substrates that contained submonolayer coverages of Ag nanostructures. The TiO2 weight loading was kept constant at 0.023 mg for all samples and all experiments. The weight loading of Ag in different experiments ranged from 5 to 25 wt % with respect to TiO2. We explored Ag/TiO2 composite materials with three different Ag nanostructures: 75 ( 13 nm diameter nanospheres, 79 ( 12 nm edge length nanocubes, and 90 ( 12 nm diameter nanowires with an aspect ratio >30. All Ag particles were synthesized with a modified polyol process, and the polyvinylpyrrolidone (PVP) stabilizer was not removed after synthesis.9,10 Representative scanning electron micrographs (SEM) of Ag/TiO2 composites are shown in Figure 1b-d. The composite system is characterized by Ag nanostructures partially covered by P25 TiO2. The building blocks are separated only by the nonconductive PVP stabilizer that surrounds the Ag particles. We note for this configuration the resonant energy or spectral shape of the Ag UV-vis extinction spectra, see Figure 2a, is not affected significantly by TiO2 due to the small direct contact area between Ag and TiO2 particles and the incomplete coverage of TiO2 on Ag. MB was deposited on the TiO2 surface and its degradation under UV light, 365 nm source with fwhm of 4 nm and a source power of ∼15 mW/cm2 at the sample, was monitored by reflectance UV-vis spectroscopy by following the 650 mn absorption peak.
Figure 2. (a) Experimental transmission UV-visible spectra for a ∼60nm Ag cube sample, a P25 TiO2 sample, a composite sample, and a difference spectrum calculated by subtracting the contribution of TiO2 from the composite spectrum. (b) Normalized (on a weight basis) UV-vis absorbance spectra for Ag nanowire, nanosphere, and nanocube films on a SiO2 substrate. (c-e) SEM micrographs of the nanowire, nanocube, and nanosphere samples used in the study.
Figure 1 shows the result of the MB decomposition experiments for the pure TiO2, pure Ag, and the Ag/TiO2 composite materials. The Ag/TiO2 composite systems shown in Figure 1 contained a 15% weight loading of Ag particles with respect to the TiO2 loading. Figure 1 shows that (1) the decomposition rate is first order in MB, (2) the pure Ag systems do not exhibit measurable photoactivity, (3) the composite materials exhibit enhanced rates of MB decomposition compared to pure TiO2, and (4) the magnitude of the rate enhancement on the composite materials compared to pure TiO2 is a function of the shape of Ag nanostructure. The measured rate enhancement factors for the Ag/TiO2 composite systems compared to pure TiO2 were 2.1 for the Ag nanowires composite, 2.4 for the Ag spheres composite, and 3.8 for the Ag nanocubes composite. There are a number of physical mechanisms previously observed that could potentially explain the enhanced photochemical reactivity of the composite systems compared to pure TiO2, including (i) electron transfer from TiO2 to Ag nanoparticles effectively increasing the lifetime of e-/h+ pairs,16,17 (ii) electron transfer from Ag to TiO2 mediated by Ag surface plasmons,18,19 (iii) the localized heating of Ag nanostructures due to the nonradiative decay of Ag surface plasmons into phonon modes in the Ag particles,20,21 and (iv) transfer of
Enhancing Photochemical Activity of Nanoparticles photons from Ag nanoparticles to TiO2 via radiative decay of surface plasmon states to excite electrons in TiO2 and increase the concentration of e-/h- pairs.22 The proposed mechanism labeled i is motivated by previous reports16,17,23 that by impregnating particles of noble metals on TiO2, the lifetime of e-/h+ pairs and the photochemical reactivity of the system are enhanced. The enhancement in the lifetime of e-/h+ pairs is attributed to the transfer of excited electrons from TiO2 to metal particles.23 To test this hypothesis we performed MB decomposition measurements using Au/TiO2 composite materials. In these materials, Au nanoparticles, stabilized by citrate ligands, were deposited in a similar geometric arrangement as Ag nanostructures in the Ag/TiO2 composites. The Au nanoparticles are spherical in shape with diameters ∼30 nm and the weight loading was 5% with respect to TiO2. We choose Au because its electronic structure, characterized by a filled d-band and valence s-band, is similar to that of Ag. In addition, the work function of Au is larger than that of Ag suggesting that there should be a larger driving force to transfer electrons from TiO2 to Au than Ag.24 The results for the decomposition of MB on the Au/TiO2 sample are shown in Figure 1. The figure shows that the Au/TiO2 samples showed very little enhancement in photochemical reactivity compared to that of pure TiO2 materials. These measurements suggest that the metal-facilitated separation of charge in the TiO2 semiconductor particles is not the driving force for the observed Aginduced enhancement in the photochemical reactivity of TiO2. The insignificant contribution of this mechanism to the rate enhancement is not surprising considering that in our composite systems the direct contact between metallic nanoparticles (Au or Ag) and TiO2, required for an efficient electron transfer, is limited by the geometric arrangements of the particles (small direct contact area) and the presence of nonconductive polymeric stabilizers on the metallic particles. It is also important to point out that although Au nanoparticles exhibit SPR, the resonant energy is lower that the energy of the source (2.2 eV vs 3.4 eV) used in the experiment. This indicates that the SPR of Au was not excited during these experiments and any mechanism involving the interaction of the Au surface Plasmon states and TiO2 is not operational in this system.11 Since the other proposed mechanisms, ii-iv, are mediated by Ag surface plasmons it is important to discuss the ways by which surface plasmons could affect the photoactivity of TiO2. When exposed to UV-vis radiation, nanoparticles of coinage metals (mainly Ag and Au) exhibit large extinction cross sections. This extinction cross section is the consequence of the formation of surface plasmons. The spectral shape and intensity of the extinction spectra are a function of the particle size, shape, and local environment.11,25,26 The impact of particle shape on the extinctions spectra is illustrated in Figure 2a where we show the UV-vis extinction spectra, normalized on a per weight basis, for as-synthesized Ag nanowires, nanocubes, and nanospheres deposited on SiO2 substrates. The SEM images of the Ag nanostructures are shown in Figure 2b-d. The excitation and subsequent decay of surface plasmon states is manifested through three resonant processes: (a) nonradiative decay, the absorption of photons which excite phonon modes, lattice vibrations, that locally heat the particle; (b) radiative decay, a Rayleigh scattering process where photons are concentrated at the surface of the particle and scattered in high intensity in the vicinity of the particle without being degraded into heat; and (c) direct transfer of electrons from plasmon states to surrounding, electron-acceptor medium.27,28
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Figure 3. (a) Change in UV-vis absorbance of Ag nanospheres in a composite Ag/TiO2 film as a function of the irradiation time. The contribution of TiO2 to the spectra was subtracted from the composite spectra to obtain the Ag absorbance. (b) (Left axis, corresponds to symbols) PL emission enhancement at the 466 nm peak as a function of excitation wavelength for three nanosphere/TiO2 composite systems. The systems contain three different Ag nanosphere loadings. Data labels denote the volume of Ag nanoparticle solution deposited on the substrate. (Right axis, corresponds to data lines) Corresponding UV-vis absorbance spectra for the Ag particles used to make the composites.
Figure 4. (a) Rate constant measured for composite systems is plotted as a function of the Ag plasmon intensity at the source wavelength for films containing only Ag particles of different shapes. We found that the concentration of Ag spheres and wires required to achieve a given plasmon intensity was twice as high as the concentration of Ag cubes. (b) Scattering efficiencies calculated at 365 nm, using FDTD simulations for Ag cubes and spheres of different sizes.
The proposed mechanism labeled ii above is based on the direct injection of electrons from the excited Ag surface plasmon states into TiO2. To test whether this mechanism is significant we measured the absorption spectra for Ag/TiO2 composites illuminated by the 365 nm source as a function of illumination time. It has been shown previously that charged TiO2 particles exhibit an absorption peak at 680 nm, and that electron-deficient Ag nanostructures show red-shifted surface plasmon resonance.18 Figure 3a shows that upon irradiation of the composite sample in a deaerated ethanol solution, there was no appearance of the 680 nm TiO2 band and we observed no shift in the Ag surface plasmon resonance. The deaerated solution was used only in this set of experiments to avoid a possible transfer of electrons from the conduction band of TiO2 to O2. These measurements suggest that there is no significant charge transfer from Ag to TiO2. The lack of change in the plasmon resonance of Ag further substantiates the conclusions from the Au/TiO2 experiments showing that there is no significant charge transfer from TiO2 to Ag; the SPR band of a charged Ag particle would show a blue shift. We have also hypothesized above, mechanism iii, that the surface plasmon induced localized heating of Ag nanoparticles can lead to thermochemcial degradation of MB on the Ag nanostructures. To test this hypothesis we measured the degradation of MB on illuminated samples that contained only Ag without any TiO2. The results of these measurements are shown in Figure 1. The figure shows that samples containing
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only Ag nanostructures did not activate the decomposition of MB, therefore indicating that the local Ag heating is not responsible for the degradation of MB. These findings are consistent with previous reports which showed that local heating of metallic particles is only important for fairly small particles (
