Article pubs.acs.org/JPCC
The Role of Photon Energy and Semiconductor Substrate in the Plasmon-Mediated Photooxidation of Citrate by Silver Nanoparticles Elizabeth S. Thrall,*,† Asher Preska Steinberg,‡ Xiaomu Wu, and Louis E. Brus Department of Chemistry, Columbia University, New York, New York 10027, United States S Supporting Information *
ABSTRACT: The plasmon-mediated photooxidation of citrate ions adsorbed on silver (Ag) nanoparticle−semiconductor electrodes is studied in a photoelectrochemical cell. Consistent with previous reports, a negative photovoltage and an anodic photocurrent arise from citrate photooxidation under weak visible light illumination. We measure the wavelength dependence of this reaction for three different types of Ag nanoparticles and find that both the photovoltage and photocurrent increase with photon energy over the visible spectral range. The electrode photoresponse does not closely track the localized surface plasmon resonance of the Ag nanoparticles. We also explore the role of the semiconductor substrate in this reaction, and we find a similar electrode photoresponse for several different substrates. The strong dependence of reaction rate on photon energy is consistent with a hot-carrier photochemical process where photoexcited hot holes generated in the Ag nanoparticles are responsible for the oxidation of adsorbed citrate.
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INTRODUCTION The absorption of light by plasmonic metal nanoparticles generates “hot” electrons and holes above and below the Fermi level, respectively.1 Although these energetic carriers should be available to participate in charge-transfer photochemistry, such processes must compete with the short 1−100 fs time scale of electronic relaxation.2,3 Nonetheless, photochemistry mediated by hot metallic carriers has been extensively studied in high vacuum surface science experiments,2,4−6 and hot carrier photochemistry at metal electrodes has also been reported.7,8 Additional examples of hot carrier photochemistry come from surface-enhanced Raman spectroscopy (SERS). In SERS, Raman scattering from molecules adsorbed on noble metal nanoparticles or roughened surfaces is enhanced by orders of magnitude.9 Changes in molecular Raman spectra in SERS experiments have been attributed to the photodestruction of the original species and the formation of degradation products with distinct Raman modes.10−15 The initial photoexcited electron transfer to an adsorbed molecule can be either oxidizing or reducing. A hot metallic electron can transfer onto an adsorbate lowest unoccupied molecular orbital (LUMO), or an electron from an adsorbate highest occupied molecular orbital (HOMO) can transfer into a hot hole below the metal Fermi level. In general, the division of photon energy between initial hot electrons and holes is poorly understood. Additional effects are possible in hybrid systems composed of plasmonic metal nanoparticles and semiconductors, either bulk or nanoscale. Knight et al.16 and Mubeen et al.17 have exploited Schottky barrier physics in solid state devices to collect hot carriers excited in plasmonic metal nanoparticles. Several recent studies have demonstrated enhanced photochemistry of composite materials based on a semiconductor, commonly TiO2, and Au or Ag nanoparticles. Enhancements have been © 2013 American Chemical Society
reported for reactions including the photooxidation of organic acids18,19 and other small molecules,20,21 the epoxidation of ethylene,22 the photodegradation of dye molecules,23−26 and the photoreduction of nitroaromatic compounds,27 carbon dioxide,28 and molecular oxygen.29 There have also been numerous reports of improved photoelectrochemical performance upon addition of plasmonic metal nanoparticles to semiconductor electrodes.30−37 Several types of mechanisms involving the semiconductor may operate in these systems, in addition to direct optical excitation of the metal.38 Here, we focus on an example of hot hole photochemistry the photooxidation of sodium citrate by Ag nanoparticles. In 2001, Jin et al. reported the photochemical conversion of small spherical Ag nanoparticles to flat triangular Ag disks, termed nanoprisms, under weak white-light illumination.39 Further mechanistic studies revealed that a key step in the synthesis was the irreversible photooxidation of the colloidal stabilizer sodium citrate by the Ag nanoparticles.40−46 Redmond et al. proposed that hot holes were excited in the Ag nanoparticles by light absorption and that these energetic carriers were responsible for citrate photooxidation.44 As photooxidation proceeds, the Ag nanoparticles accumulate electrons and charge negatively, developing a cathodic photovoltage. Such charged Ag particles reduce aqueous Ag+, leading to particle growth. Growth occurred at the visible plasmon wavelengths of the Ag prisms, although the photochemical excitation spectra were not directly measured. On the basis of the rate of nanoparticle growth, they estimated a citrate photooxidation quantum yield of 0.5% per absorbed photon for 488 nm excitation, assuming a unity Received: September 25, 2013 Revised: November 15, 2013 Published: November 18, 2013 26238
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Nanoparticle Electrode Fabrication. ITO and FTO slides were cleaned by sonication in ethanol and water for 15 min each before Ag nanoparticle electrode preparation. Ag nanoparticles were primarily deposited on ITO of sheet resistance 5−15 Ω/sq (Delta Technologies, Ltd.), but ITO slides obtained from Sigma-Aldrich (15−25 Ω/sq) and FTO (6−9 Ω/sq) were used in control experiments. The singlecrystal n-TiO2 electrode was cleaned before each experiment in several steps. First the electrode was wiped with a Kimwipe or lens tissue wetted with ethanol. The electrode was then sonicated in ethanol followed by water and dried with N2 gas. To remove any residual Ag, the electrode was gently polished with 0.05 μm alumina polish and polishing pads (Bioanalytical Systems, Inc.). Finally, the electrode was rinsed thoroughly with water from a wash bottle and sonicated in water for 5 min to remove alumina particles. To form quasi-spherical Ag nanoparticles, a thin Ag film (nominal thickness 2−4 nm) was deposited on the substrate by thermal (Edwards BOC/Auto 306) or e-beam (Semicore SC2000) evaporation, followed by annealing in air at 120 °C for 30 min. Colloidal Ag nanoparticles were deposited on ITO by immersion. First, the ITO surface was coated with a thin film of poly(allylamine hydrochloride) (PAH) to increase the density of adsorbed Ag particles.52 To deposit the PAH film, a clean ITO slide was soaked for 30 min in 1 M KOH and then immersed for 30 min in an aqueous 0.2 mg/mL PAH solution. The slide was rinsed with water and then immersed in the Ag nanoparticle colloid overnight. Characterization. Ag nanoparticles colloids and electrodes were imaged by TEM (JEOL JEM-100CX) and SEM (Hitachi 4700), respectively. Evaporated Ag nanoparticle electrodes were characterized by XPS (PHI 5500 ESCA (XPS)/ISS). UV−vis spectrophotometry (Hewlett-Packard 8453) was used to record the extinction spectra of nanoparticle colloids and electrodes. Photoelectrochemical Measurements. The evaporated or colloidal Ag nanoparticle electrode served as the working electrode in a three-electrode photoelectrochemical cell with a Pt wire counter electrode and an Au wire quasi-reference electrode. Before electrochemical measurements, the electrode was “ripened” by immersion in the electrolyte solution for at least 20 min during N2 or Ar purging. The standard electrolyte contained 500 μM trisodium citrate and 100 mM KNO3. Electrolyte solutions were bubbled with N2 or Ar for at least 20 min prior to use, and Ar gas was bubbled slowly through the electrolyte during electrochemical measurements. A 300 W tungsten halogen lamp was used as the light source for photoelectrochemical measurements. Interference filters with 10 nm bandwidth and neutral density filters were used to select the excitation wavelength and the light intensity. The incident power was measured with a Coherent LM-2 VIS power meter. The light spot illuminated an area of approximately 1.44 cm2 on the working electrode, although a somewhat larger area of the electrode was in contact with the electrolyte solution. The active area of n-TiO2 electrodes was approximately 0.12 cm2. The counter and reference electrodes were shielded from direct light. Two types of photoelectrochemical measurements were performed using a Parstat 2263 potentiostat (Princeton Applied Research). The photovoltage was measured under open-circuit conditions and was calculated as VOC,light − VOC,dark, where VOC is the steady-state open-circuit potential. Photovoltages were
quantum yield for the subsequent aqueous electrochemical reduction of Ag+. To explore this photooxidation more deeply, Redmond et al. then demonstrated that an electrode composed of Ag nanoparticles deposited on an indium tin oxide (ITO) film generated an anodic photoresponse in the presence of aqueous sodium citrate.44,47 Here, aqueous Ag+ ions are not present; rather, the photoinjected electrons in Ag transfer into ITO and are measured in the external circuit The measured citrate photooxidation quantum yield, based upon electrical current, in this system, however, was approximately 3 × 10−5 per absorbed photon at 488 nm, 2 orders of magnitude lower than the estimate for the colloidal Ag particles.47 While both experiments involve citrate photooxidation, colloidal growth and photoelectrochemical current generation proceed with different efficiencies. The photoelectrochemical experiment involves electron transfer into a doped oxide substrate rather than Ag+ ion reduction at an aqueous interface. In this paper we try to understand and relate these two hot carrier experiments, focusing on the dependence of the reaction on photon energy, on the Ag nanoparticle properties, and on the identity of the semiconductor substrate. We find that the citrate photooxidation quantum yield increases with photon energy for different types of Ag nanoparticles and that the photoresponse does not track the nanoparticle plasmon resonance. Additionally, we find a similar photoresponse for Ag nanoparticles deposited on different doped semiconductor substrates.
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EXPERIMENTAL METHODS Materials. Sodium borohydride (≥98.5%), poly(vinylpyrrolidone) (average MW ∼ 29 000), and poly(allylamine hydrochloride) (average MW ∼ 56 000) were obtained from Aldrich. Silver nitrate (≥99.8%) (Riedel-de Haën), trisodium citrate dihydrate (≥99.5%), sodium hydroxide (≥97%), and hydrogen peroxide (30 wt %) were purchased from Sigma-Aldrich. Potassium nitrate (≥99.999%) was supplied by Strem Chemicals. Deionized water (resistivity 17.8−18.2 MΩ·cm) was used in all experiments. Indium tin oxide (ITO) with sheet resistance of 5−15 Ω/sq was purchased from Delta Technologies, Limited (CB-50IN). ITO with sheet resistance of 15−25 Ω/sq was also obtained from Sigma-Aldrich. Fluorine tin oxide (FTO) with sheet resistance of 6−9 Ω/sq was acquired from Pilkington (TEC-8). Single-crystal n-type TiO2 substrates were provided by Bruce Parkinson (University of Wyoming).48,49 A rutile (110) TiO2 crystal with doping density on the order of 1 × 1020 electrons/ cm3 was used in this study. Colloidal Nanoparticle Synthesis. Colloidal Ag nanoprisms were synthesized by a thermal method.50 Briefly, 12.5 mL of 0.1 mM AgNO3, 0.75 mL of 30 mM citrate, 0.75 mL of 0.7 mM PVP (average MW ∼ 29 000 g/mol), and 30 μL of H2O2 (30 wt %) were combined in a 50 mL Erlenmeyer flask and stirred vigorously. Then 100 μL of ice-cold 100 mM NaBH4 was injected, and the colloid was allowed to stir for 20− 30 min until a series of color changes occurred. Colloidal Ag nanospheres were synthesized following a modified Lee−Meisel procedure.51 A solution of 18 mg of AgNO3 in 100 mL of deionized water was brought to a boil in a 100 mL round-bottom flask equipped with a condenser, and then 2 mL of a 2 wt % solution of citrate was added. The solution was refluxed for an additional 2 h. 26239
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Figure 1. UV−vis extinction spectra (a−c) and SEM images (d−f) of typical evaporated Ag nanoparticle, colloidal Ag nanoprism, and colloidal Ag nanosphere electrodes. The electrodes were immersed in the 100 mM KNO3/500 μM citrate electrolyte solution when the spectra were recorded. The dashed lines show the extinction spectrum of the dry electrodes in air. The dotted lines in (b) and (c) show the extinction spectra of the nanoparticle colloids before use (divided by 3 and 5, respectively). All scale bars are 100 nm.
nanoprisms on ITO, but the in-plane dipole resonance can still be observed at 610 nm in air or 665 nm in the electrolyte (Figure 1b). The Ag colloid synthesized by reflux in citrate was dominated by quasi-spherical particles, with an average radius of 10−20 nm, although some rods and other shapes were also observed (Figure 1f). The electrode particle density, approximately 1.3 × 102 particles/μm2, is low enough that the particles do not couple strongly and the plasmon peak measured in the electrolyte solution, at 405 nm, is closer to the position expected for an isolated spherical Ag particle (Figure 1c). Evaporated Ag Nanoparticle Electrode. Figure 2a shows the photocurrent response of an evaporated Ag nanoparticle electrode under irradiation at 500 nm in solutions containing 500 μM citrate or 500 μM tricarballylate. Citric acid is a tricarboxylic acid with an α-hydroxyl group (Figure 3a); tricarballylic acid lacks the hydroxyl group but is otherwise identical (Figure 3b). As previously reported, there is a reproducible anodic photocurrent of approximately 50 nA/ cm2 in the presence of citrate. The photocurrent is on the order of 1.5 nA/cm2 in the tricarballylate solution, comparable to the value measured in 100 mM KNO3 alone. The change in the electrode open-circuit potential under the same conditions is shown in Figure 2b. In the presence of citrate, the open-circuit potential shifts −105 mV on irradiation. The photovoltage in tricarballylate is approximately −2.5 mV, and there is almost no response when only the supporting electrolyte, KNO3, is present. The dependence of both photocurrent and photovoltage on light intensity was previously investigated for an evaporated Ag nanoparticle electrode in the presence of citrate.47 The photocurrent was found to be linear with light intensity, suggesting that the photooxidation of citrate is a one-photon process. From the slope of the photocurrent vs irradiance curve (Figure S2a), a quantum yield at 500 nm of 1.1 × 10−4 e−/ absorbed photon can be calculated, in good agreement with the previous report. As shown in Figure S2b, the photovoltage magnitude is found to increase linearly with the logarithm of the light intensity.
typically measured in cycles starting at the highest wavelength to be used and moving to lower wavelengths.53 Photocurrents were measured in chronoamperometric mode with the potential held at VOC,dark. Photocurrents were calculated by taking the average of Ilight − Idark over three on/off cycles of 10 s each. The potentiostat’s internal 5.00 Hz filter was applied during current measurements to eliminate high-frequency noise. Unless otherwise stated, neutral density filters were used to equalize the incident photon flux at each wavelength for all photovoltage and photocurrent measurements. For n-TiO2 electrodes, the photocurrent was typically calculated by taking the average of Ilight − Idark over two on/off cycles, with the light on for 30 s and off for 20 s. The photocurrent was measured at three different light intensities for each wavelength, and the quantum yield was calculated from a linear fit to the photocurrent vs irradiance plot.
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RESULTS Nanoparticle Electrode Characterization. Ag nanoparticles fabricated by evaporation and annealing were quasispherical with an average radius of 5−10 nm and a particle density of approximately 9.7 × 102 particles/μm2 (Figure 1d). The electrode UV−vis extinction spectrum shows a primary peak at 495 nm from the localized surface plasmon absorbance of the particles (Figure 1a). The peak is red-shifted from the expected position for spherical Ag nanoparticles in air due to particle−particle interactions and the higher dielectric constant of the ITO.54 When the electrode is immersed in the electrolyte, the plasmon peak red-shifts by about 10 nm. XPS measurements indicate little change in the oxidation state of the Ag after annealing in air (Figure S1). Colloidal Ag nanoprisms were larger, with edge lengths ranging from 30 to 70 nm and thickness on the order of 10 nm (Figure 1e). The particle density of the colloidal Ag nanoprism electrode was approximately 1.0 × 102 particles/μm2, smaller than that of the evaporated particle electrode. The anisotropic Ag nanoprisms support both in-plane and out-of-plane dipolar and quadrupolar plasmon modes, which have been previously assigned.39 The plasmon peaks are less distinct for the 26240
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Figure 2. Current vs time (a) and open-circuit potential vs time (b) of an evaporated Ag nanoparticle electrode under irradiation at 500 nm (irradiance = 3.5 mW/cm2). The electrolyte contained 100 mM KNO3 and either 500 μM sodium citrate (red) or sodium tricarballylate (blue). The shaded rectangles indicate when the light was on.
Figure 4. Photocurrent action spectrum (a) and photovoltage action spectrum (b) of an evaporated Ag nanoparticle electrode. The electrode UV−vis extinction spectrum is plotted against the right axis. The photon flux at each wavelength was normalized to approximately 1.1 × 1016 photons/(cm2 s).
The photocurrent action spectrum for an evaporated Ag nanoparticle electrode is shown in Figure 4a. The electrode absorbance, measured in the electrolyte solution to account for a slight red-shift, is also plotted. The photocurrent increases by approximately 25-fold going from 650 to 460 nm, without any sign of saturation. Notably, the photocurrent does not appear to track the electrode plasmon resonance, which peaks near 500 nm. The photovoltage magnitude also increases monotonically with decreasing wavelength (Figure 4b). In contrast to the photocurrent, however, it does not rise as steeply below 560 nm.
Colloidal Ag Nanoparticle Electrodes. The photovoltage action spectra of the colloidal Ag nanoprism and nanosphere electrodes are shown in Figures 5a and 5b, respectively. The maximum photovoltage for the colloidal nanoparticle electrodes is weaker by a factor of 2−3 in comparison to the evaporated nanoparticle electrode, but the spectral response is similar. For both electrodes, despite their different absorption spectra, the photovoltage rises with decreasing wavelength without any sign of leveling off. The difference between the plasmon and photovoltage spectra is striking for the nanoprisms, which have
Figure 3. (a) Molecular structure of citric acid. (b) Molecular structure of tricarballylic acid. (c) Proposed photooxidation mechanism of citrate adsorbed on Ag nanoparticles. 26241
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and photovoltage action spectra are similar for particles on the different substrates (Figure S6a,b), although the nanoparticle plasmon resonance is blue-shifted on FTO, and this is reflected in the spectral photoresponse. The quantum yield per absorbed photon is essentially the same on the two ITO substrates and is lower by a factor of 2 on FTO. Finally, the photoresponse was measured for evaporated Ag nanoparticles on a highly reduced single-crystal n-TiO2 electrode. Figure 6a shows a typical photocurrent trace at a
Figure 5. Photovoltage action spectra of a colloidal Ag nanoprism electrode (a) and a colloidal Ag nanosphere electrode (b). The electrode UV−vis extinction spectrum is plotted against the right axis. The photon flux at each wavelength was normalized to approximately 1.2 × 1016 photons/(cm2 s).
plasmon resonances in the visible. Like the photovoltage, the photocurrent measured with these electrodes is lower, on the order of 5−10 nA/cm2 at most under typical experimental conditions. We note that the photocurrent and photovoltage depend on light intensity in the expected manner, as discussed above. Substrate Effects. Several control experiments were conducted to assess the effect of the ITO substrate on the Ag nanoparticle electrode photoresponse. First, the photoresponse of a bare ITO substrate was measured in the presence of citrate. Although an anodic photocurrent was observed at lower wavelengths, it was extremely weak; at 460 nm, the lowest wavelength used in this study, the photocurrent was less than 1 nA/cm2 (Figure S3). Further, the photocurrent is essentially the same in the presence or absence of citrate. The photovoltage response of the bare substrate was also weak. Figure S4 shows the open-circuit potential vs time measured for a bare ITO electrode under irradiation at 460 and 500 nm. After 20 min irradiation at 460 nm, the potential has dropped by approximately 13.5 mV without reaching a steady-state value. When 5 μM AgNO3 is added to the electrolyte solution, however, the system reaches a steady-state photovoltage of 14 mV. For additional confirmation that the measured photoresponse was independent of the substrate used, photocurrent and photovoltage action spectra were obtained for evaporated Ag nanoparticles fabricated on ITO supplied by Sigma-Aldrich and on TEC-8 FTO. The electrode extinction spectra on these two substrates are compared in Figure S5a. The photocurrent
Figure 6. (a) Current vs time of an evaporated Ag nanoparticle−TiO2 electrode under irradiation at 500 nm (irradiance = 6.8 mW/cm2) in a 500 μM sodium citrate/100 mM KNO3 electrolyte solution. The shaded rectangles indicate when the light was on. (b) Quantum yield action spectrum for the same electrode.
wavelength of 500 nm and irradiance of 6.8 mW/cm2. We observe a consistent decay in the photocurrent at the start of each illumination cycle. The photocurrent approaches a steadystate value after approximately 30 s. This transient decay has been observed in other nanoparticle−TiO2 systems and has been attributed to charging of an organic surfactant layer31 or to the release of trapped charge at the TiO2 surface.34 Although the photocurrent magnitude in Figure 6a is low, it should be noted that the electrode area is approximately 12-fold smaller than the illuminated area of the Ag nanoparticle−ITO electrodes. The quantum yield action spectrum for the same Ag nanoparticle−TiO2 electrode is plotted in Figure 6b. The spectral profile of the Ag nanoparticle response on the TiO2 substrate is similar to the response on ITO and FTO, with an increase in photocurrent at short wavelengths. The quantum yield at 500 nm, approximately 1.7 × 10−5 e−/incident photon, 26242
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irradiation;47 instead, light absorption generates nonthermalized (“hot”) electrons and holes in the Ag particles. The excited metallic carriers can transiently localize on chemisorbed molecules and then return to the metal; in some cases this reversible interaction can cause a significant broadening of the plasmon line width.57 If the transient localization leads to an irreversible chemical reaction, however, the excited carrier can be trapped on the molecule.58 Figure 7a illustrates this indirect
is the same order of magnitude as the value measured for the evaporated Ag nanoparticle−ITO electrode. We observed significant variability in the photoresponse of the Ag− nanoparticle TiO2 electrodes, with the quantum yield per incident photon an order of magnitude smaller in some cases. The heavily doped TiO2 crystal is dark blue in color due to the absorption of Ti3+ ions produced in the doping process.55 For this reason it was not possible to measure the UV−vis extinction spectrum of the nanoparticles deposited on TiO2 or to calculate the quantum yield per absorbed photon. Photovoltage measurements for Ag nanoparticle−TiO2 electrodes were not very reproducible, and the value of VOC,dark was somewhat unstable. Preliminary measurements, however, indicated that the Ag nanoparticle−TiO2 electrode photovoltage response was weaker than that of the evaporated Ag nanoparticle−ITO electrode.
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ANALYSIS AND DISCUSSION As previously proposed by our group and others, adsorbed citrate on aqueous colloidal Ag nanoparticles is photooxidized by low-intensity visible light.41,44−46 In this two-electron photoKolbe reaction, citrate decomposes to 1,3-acetonedicarboxylate and carbon dioxide. Figure 3c illustrates the proposed photoKolbe mechanism. The hydroxyl group oxygen donates a lone pair of electrons to the central carbon to form a carbon−oxygen double bond, accompanied by the heterolytic cleavage of the carbon−carbon bond between the central carbon and the shortarmed carboxylate group. Transient localization of a photogenerated hot hole on the citrate molecule initiates the irreversible decarboxylation, transferring an electron to the metal. This cathodically charges the Ag particle, making it a stronger reducing agent for residual Ag+ ions in solution. As a result, irradiated colloidal Ag particles grow larger by accumulating reduced Ag+. For Ag particles with adsorbed citrate on the photoelectrode surface, we collect photogenerated electrons in the external circuit in the absence of aqueous Ag+ ion in the electrolyte. The photocurrent and photovoltage with adsorbed tricarballylate are at least an order of magnitude lower than with adsorbed citrate. Experimental51 and theoretical56 studies have found that citrate binds to the silver surface through its two methylene carboxylate groups, suggesting that tricarballylate is capable of adsorbing in the same manner. Nonetheless, tricarballylate lacks a hydroxyl group (Figure 3b) and would be unable to decarboxylate following the photo-Kolbe mechanism above. This comparison with tricarballylate strongly suggests that the rate-limiting step in photocurrent generation on the electrode surface is citrate photooxidation, similar to the case for colloidal Ag particles not on the electrode. In addition, the observed logarithmic photovoltage dependence upon light intensity also implicates citrate photooxidation as the rate-limiting step: such logarithmic dependence is predicted by the Butler−Volmer activated rate model for molecular redox processes at a metallic electrode.47 Also, the fact that the data essentially do not depend upon the substrate (ITO, FTO, or highly reduced nTiO2) suggests that the rate-determining step in current generation does not involve the substrate−Ag particle interface. There are two possible microscopic photoprocesses that could generate a hot hole on adsorbed citrate. Citrate itself does not absorb visible light, but Ag nanoparticles have an enhanced absorption (and scattering) cross section at visible wavelengths due to their localized surface plasmon resonance. As previously noted, the coherent plasmon is not excited by our narrow-band
Figure 7. Models for citrate photooxidation by Ag nanoparticles. (a) Indirect hot hole transfer to citrate. (b) Direct photoinduced molecule-to-metal charge transfer.
charge-transfer process. An alternative mechanism is the direct photoinduced molecule-to-metal charge transfer depicted in Figure 7b. The rates of both processes in principle depend upon the initial position of the Fermi level in the metal and the photon energy. If photon absorption by Ag particles is the initial step, then photon absorption will peak at wavelengths near the nanoparticle plasmon resonance. However, it is not necessarily the case that the photochemical excitation spectrum will track the Ag plasmon spectrum. Hot Ag holes generated by higherenergy photons should be deeper below the Ag Fermi level and 26243
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Figure 8 compares the calculated quantum yield per absorbed photon for the three types of Ag nanoparticle electrodes
thus more strongly oxidizing, as shown in Figure 7a. For a difficult to oxidize species such as citrate, this increase in oxidizing power might offset a decrease in nanoparticle absorption at wavelengths shorter than the plasmon resonance maximum. This model predicts an increase in the photocurrent and photovoltage with decreasing wavelength, consistent with our results. Support for this effect comes from studies of hot electron photochemistry in graphite,59 on Pd surfaces,60 and in Ag colloids. In particular, Suh et al. found that the reaction rate for the photochemical degradation of phthalazine by Ag colloids increased with photon energy and did not track the nanoparticle plasmon resonance spectrum.11 This same behavior, increasing oxidizing power at shorter wavelengths, is likely true in the direct molecule-to-metal excitation mechanism. In this case, the photooxidation rate would be expected to track the charge-transfer transition cross section, which might peak at shorter wavelengths than the nanoparticle plasmon resonance. Petek has argued in detail that the direct, one-step excitation mechanism is actually dominant for small species on single-crystal metal electrodes in a vacuum.61 As seen in Figures 4 and 5, the photoresponse for the three types of Ag nanoparticle electrodes appears to be dominated by photon energy. We find that adsorbed citrate photooxidation on Ag on electrode surfaces does not track the plasmon resonance but is much stronger for shorter wavelengths. This behavior holds even for the colloidal Ag nanoprisms, which absorb weakly at shorter wavelengths. Should we expect this same excitation spectrum for colloidal Ag particles? The oxidizing power for a given photon energy will depend upon the Fermi level position. If the photostationary Fermi level is lower in the colloid than on the electrode surface, the oxidizing power will be different, likely higher. Conversely, for 488 nm laser irradiation, it was previously shown that photocurrent decreases as the electrode potential is scanned cathodically.47 This voltage dependence is not well understood either experimentally or theoretically. One possible source of this Fermi level shift is the lower concentration of Ag+ ions in the present photoelectrochemical electrolyte solution in comparison to the colloid. Etching of colloidal particles by oxygen creates a small steady-state concentration of Ag+ ions in the colloid; Xue et al. measured this concentration to be 4 and 20 μM in the absence and presence, respectively, of a stabilizing ligand.46 The Ag+ concentration should be significantly lower in the electrolyte solution used in our experiments because oxygen is excluded. Redmond et al. found that the Ag nanoparticle electrode opencircuit potential shifted by approximately +90 mV when 250 μM AgNO3 was added to the electrolyte.47 Thus, a lower solution concentration of Ag+ should translate to a more negative potential, or a higher Fermi level, for the Ag nanoparticle electrode. Several recent papers have reported wavelength-dependent photochemical conversion rates for citrate-stabilized Ag nanoparticle colloids that are consistent with our results.62−64 In all cases, the reaction rates increase with photon energy over a range of visible wavelengths. Disentangling the two effects of photon energy and light absorption is challenging in these studies, however, because the spherical seed colloids generally used in photochemical syntheses have a plasmon resonance near 400 nm. As a result, both photon energy and light absorption should favor an increased reaction rate at shorter wavelengths.
Figure 8. Quantum yield per absorbed photon plotted for 15 different Ag nanoparticle electrodes of three types: evaporated Ag nanoparticle electrodes (circles), colloidal Ag nanoprism electrodes (triangles), and colloidal Ag nanospheres (diamonds).
studied here. In general, even when accounting for electrode absorbance, the colloidal nanoparticle electrodes have a lower quantum yield than the evaporated nanoparticle electrode. Possibly the blue photochemical excitation spectrum and the low quantum yield for the electrode both somehow reflect a strongly shifted initial Fermi level compared with the colloid. Additionally, it may be that the polymer film impedes the charge transfer between the prism colloidal particles and the ITO. We do not observe a significant dependence of the Ag nanoparticle electrode photoresponse on the semiconductor substrate. Figure S6 shows that the photoresponse is essentially the same with ITO obtained from a different supplier and is also similar when FTO is used as the substrate. High hot electron charge injection yields have been reported for quantum dots65 and dye molecules66 adsorbed on highly doped single-crystal n-TiO2 substrates, where the semiconductor space charge region assists in sweeping photoexcited electrons away from the interface. In our present citrate experiments, we observe instead a hot hole photooxidation process, with subsequent static charging of the Ag particle. Does the substrate play a more direct role in the observed photoresponse? Even if charge transfer and storage occur on the Ag nanoparticles, it could be that ITO absorbs blue light and then transfers a hole to the Ag particles, which subsequently oxidize citrate. In a series of articles, Kamat et al. showed that the deposition of Au nanoparticles on TiO2 increased the photocurrent upon band gap excitation of the TiO2.30,31,55 In our case, however, the photocurrent response is not uniformly enhanced when Ag nanoparticles are deposited on ITO. Instead, the Ag nanoparticle electrode photoresponse is significant only when citrate is added to the electrolyte. The photocurrent measured in KNO3 alone is approximately an order of magnitude larger for the Ag NP-ITO electrode than for the bare ITO electrode, but in the presence of citrate the Ag NP-ITO electrode photocurrent is over 100-fold larger than that of the bare ITO electrode. As noted previously, the bare ITO photoresponse is essentially the same with or without citrate added to the 100 mM KNO3 supporting electrolyte (Figure S3). 26244
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assistance. We thank the DOE Basic Energy Sciences program for support of this work under DE-FG02-11ER16224.
Finally, we note that bare Ag nanoparticles without adsorbed citrate behave differently under irradiation when adsorbed on TiO2. Tatsuma has shown that nanoparticles excited in the plasmon bands transfer an electron to TiO2, as we also observe.67 However, the hole directly oxidizes the Ag lattice to make Ag+. As a result, the particle grows smaller; this process occurs with a higher quantum yield than we observe with adsorbed citrate on our substrates. In our experiments, adsorbed citrate shuts off Ag lattice dissolution under plasmon irradiation, presumably by preventing direct Ag metal access to water. Tatsuma and colleagues observe a similar effect for alkanethiol passivation of Ag nanoparticles on TiO2. Consistent with this idea, Redmond et al. reported a very low exchange current density for the Ag+/Ag redox couple in the presence of citrate.47 It is citrate that is photooxidized instead, and this requires a more oxidizing hot hole than Ag lattice oxidation.
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(1) Govorov, A. O.; Zhang, H.; Gun’ko, Y. K. Theory of Photoinjection of Hot Plasmonic Carriers from Metal Nanostructures into Semiconductors and Surface Molecules. J. Phys. Chem. C 2013, 117, 16616−16631. (2) Zhu, X. Y. Surface Photochemistry. Annu. Rev. Phys. Chem. 1994, 45, 113−144. (3) Link, S.; El-Sayed, M. A. Optical Properties and Ultrafast Dynamics of Metallic Nanocrystals. Annu. Rev. Phys. Chem. 2003, 54, 331−366. (4) Hasselbrink, E. Mechanisms in Photochemistry on MetalSurfaces. Appl. Surf. Sci. 1994, 80, 34−40. (5) Lindstrom, C. D.; Zhu, X. Y. Photoinduced Electron Transfer at Molecule-Metal Interfaces. Chem. Rev. 2006, 106, 4281−4300. (6) Watanabe, K.; Menzel, D.; Nilius, N.; Freund, H. J. Photochemistry on Metal Nanoparticles. Chem. Rev. 2006, 106, 4301−4320. (7) Diesing, D., Kritzler, G., Otto, A., Wandelt, K., Thurgate, S., Eds.; Surface Reactions of Hot Electrons at MetalLiquid Interfaces.Springer: Berlin, 2003; Vol. 85, pp 367−427. (8) Fedurco, M.; Shklover, V.; Augustynski, J. Effect of Halide Ion Adsorption upon Plasmon-Mediated Photoelectron Emission at the Silver/Solution Interface. J. Phys. Chem. B 1997, 101, 5158−5165. (9) Moskovits, M. Surface-Enhanced Raman Spectroscopy: A Brief Retrospective. J. Raman Spectrosc. 2005, 36, 485−496. (10) Suh, J. S.; Moskovits, M.; Shakhesemampour, J. Photochemical Decomposition at Colloid Surfaces. J. Phys. Chem. 1993, 97, 1678− 1683. (11) Suh, J. S.; Jang, N. H.; Jeong, D. H.; Moskovits, M. Adsorbate Photochemistry on a Colloid Surface: Phthalazine on Silver. J. Phys. Chem. 1996, 100, 805−813. (12) Bjerneld, E. J.; Svedberg, F.; Johansson, P.; Kall, M. Direct Observation of Heterogeneous Photochemistry on Aggregated Ag Nanocrystals Using Raman Spectroscopy: The Case of Photoinduced Degradation of Aromatic Amino Acids. J. Phys. Chem. A 2004, 108, 4187−4193. (13) Alaverdyan, Y.; Johansson, P.; Kall, M. Photo-Induced Transformations in 2,2′:5 ′,2″-Terthiophene Thin Films on Silver. Phys. Chem. Chem. Phys. 2006, 8, 1445−1450. (14) Huang, Y. F.; Zhu, H. P.; Liu, G. K.; Wu, D. Y.; Ren, B.; Tian, Z. Q. When the Signal Is Not from the Original Molecule To Be Detected: Chemical Transformation of para-Aminothiophenol on Ag during the SERS Measurement. J. Am. Chem. Soc. 2010, 132, 9244− 9246. (15) Vlckova, B.; Gu, X. J.; Moskovits, M. SERS Excitation Profiles of Phthalazine Adsorbed on Single Colloidal Silver Aggregates as a Function of Cluster Size. J. Phys. Chem. B 1997, 101, 1588−1593. (16) Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J. Photodetection with Active Optical Antennas. Science 2011, 332, 702− 704. (17) Mubeen, S.; Hernandez-Sosa, G.; Moses, D.; Lee, J.; Moskovits, M. Plasmonic Photosensitization of a Wide Band Gap Semiconductor: Converting Plasmons to Charge Carriers. Nano Lett. 2011, 11, 5548− 5552. (18) Kowalska, E.; Abe, R.; Ohtani, B. Visible Light-Induced Photocatalytic Reaction of Gold-Modified Titanium(IV) Oxide Particles: Action Spectrum Analysis. Chem. Commun. 2009, 241−243. (19) Naya, S.; Inoue, A.; Tada, H. Self-Assembled Heterosupramolecular Visible Light Photocatalyst Consisting of Gold NanoparticleLoaded Titanium(IV) Dioxide and Surfactant. J. Am. Chem. Soc. 2010, 132, 6292−6293. (20) Hung, W. H.; Aykol, M.; Valley, D.; Hou, W. B.; Cronin, S. B. Plasmon Resonant Enhancement of Carbon Monoxide Catalysis. Nano Lett. 2010, 10, 1314−1318. (21) Chen, X.; Zhu, H. Y.; Zhao, J. C.; Zheng, Z. T.; Gao, X. P. Visible-Light-Driven Oxidation of Organic Contaminants in Air with
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CONCLUSION We have measured the wavelength dependence of citrate photooxidation by Ag nanoparticle−semiconductor electrodes in a photoelectrochemical cell. We observe similar action spectra for three different types of Ag nanoparticles with different localized surface plasmon resonance spectra, and we also find that the response is broadly similar for different types of semiconductor substrates. We conclude that photon energy is the dominant factor in the citrate photooxidation rate. These findings are compatible with a photochemical mechanism involving the transfer of photoexcited hot holes to adsorbed citrate and consistent with the strong photon energy dependence that has been observed for hot-carrier photochemistry in other systems.
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ASSOCIATED CONTENT
S Supporting Information *
XPS spectra of an evaporated Ag nanoparticle electrode before and after annealing, plots of the irradiance dependence of the photocurrent and photovoltage of an evaporated Ag nanoparticle electrode, plots of the photocurrent and photovoltage response of an unmodified ITO electrode, a scanning electron micrograph of an evaporated Ag nanoparticle electrode on FTO, and the absorption and photovoltage action spectra of evaporated Ag nanoparticle electrodes on FTO and on a different ITO substrate. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (E.S.T.). Present Addresses †
E.S.T.: Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115. ‡ A.P.S.: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Bruce Parkinson and Kevin Watkins of the University of Wyoming for providing us with the single-crystal TiO2 electrode and for their helpful suggestions. We also acknowledge Steffen Jockusch for the loan of equipment and other 26245
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