pubs.acs.org/NanoLett
Plasmon Resonant Enhancement of Carbon Monoxide Catalysis Wei Hsuan Hung,†,| Mehmet Aykol,‡,| David Valley,§,| Wenbo Hou,§ and Stephen B. Cronin*,‡ †
Department of Materials Science, ‡ Department of Electrical Engineering, and § Department of Chemistry, University of Southern California, Los Angeles, California 90089 ABSTRACT Irradiating gold nanoparticles at their plasmon resonance frequency creates immense plasmonic charge and high temperatures, which can be used to drive catalytic reactions. By integrating strongly plasmonic nanoparticles with strongly catalytic metal oxides, significant enhancements in the catalytic activity can be achieved. Here, we study the plasmonically driven catalytic conversion of CO to CO2 by irradiating Au nanoparticle/Fe2O3 composites. The reaction rate of this composite greatly exceeds that of the Au nanoparticles or Fe2O3 alone, indicating that this reaction is not driven solely by the thermal (plasmonic) heating of the gold nanoparticles but relies intimately on the interaction of these two materials. A comparison of the plasmonically driven catalytic reaction rate with that obtained under uniform heating shows an enhancement of at least 2 orders of magnitude. KEYWORDS Plasmon, plasmonic, catalysis, nanoparticle, oxidation
M
Metal oxides (e.g., TiO2, Fe2O3, PbO) are very promising photocatalysts for a number of applications, including solar fuel production, oxidation of pollutants, and antifogging/selfcleaning coatings for windows and lenses. When gold is dispersed as fine particles (2-5 nm) over select metal oxides, it has been found to exhibit exceptionally high catalytic activity,1,2,16 far exceeding that of the metal oxide and gold catalysts separately.17,18 The catalytic oxidation of carbon monoxide has become an important field of study in and of itself.19-27 For gold nanoparticles on transition metal oxides, the oxidation of CO is an exothermic reaction with extremely low catalytic activation barriers.19 In this work, we explore a fundamentally different mechanism of catalytic enhancement, achieved with plasmonic excitation. By monitoring the temperature of the reaction (using infrared and Raman spectroscopies), we separate the effects of plasmonic heating from exothermic chemical heating. To determine the catalytic enhancement factor, we compare the CO reaction rate achieved under laser irradiation with that observed under uniform heating. We also compare the reaction rate of the Au nanoparticle/Fe2O3 composite with that of its constituent materials, in order to further establish the uniqueness of this catalytic enhancement process. Experiments are carried out in a microreactor system consisting of a gas delivery system, an optical microscope, and an excitation laser source, as illustrated schematically in Figure 1a. A 532 nm 5 W Spectra Physics solid-state laser is collimated and focused through a Leica DMLM microscope. A 50× long-working distance microscope objective lens with NA ) 0.5 and spot size ) 1.25 µm is used to irradiate the sample while carbon monoxide (CO) is flowed through the microreactor system. The byproducts of these plasmon-induced reactions are monitored using a Pfieffer Omnistar quadrupole mass spectrometer (QMS). Since the
acroscopic catalysts are based on traditional chemical pathways. At the nanoscale, the catalytic properties of materials are often quite different from their bulk counterparts. For example, gold is considered to be a poor catalyst for most applications. However, gold nanoparticles on metal oxide supports demonstrate high catalytic activity even at room temperature.1,2 This work and others have established the great potential for new chemical pathways at the nanoscale. In this work, we investigate the additional catalytic enhancement produced by the surface plasmon resonance phenomenon. Surface plasmons have been studied by many research groups across many fields.3-5 The intense electric fields produced near plasmon resonant metallic nanoparticles have been utilized in surface-enhanced Raman spectroscopy (SERS) for over 30 years, yielding enhancement factors as high as 1014.6-8 Numerical simulations have predicted SERS enhancement factors up to 1010.9,10 Surface plasmon resonance is used by biochemists to study the mechanisms and kinetics of ligands binding to receptors. Plasmonic phenomena have also been explored for electromagnetic energy transport below the diffraction limit,11 subwavelength optical devices,12 and lasers.13,14 Because the imaginary part of the dielectric function is dominant, much of the energy in the plasmon resonance is dissipated as heat. This has been utilized for noninvasive cancer treatment therapy.15 Both plasmonic heating and plasmon-induced charge are advantageous when coupled to catalysts.
* To whom correspondence should be addressed,
[email protected]. |
Authors contributed equally to the work. Received for review: 12/13/2009 Published on Web: 03/30/2010 © 2010 American Chemical Society
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FIGURE 1. (a) Schematic diagram of the microreactor system. (b) Silicon microchannels with glass cover and metal eyelets.
sensitivity of the mass spectrometer is limited by the background vapors, namely, the unreacted reactants, micrometer-sized channels (similar to microfluidic channels) etched in silicon28 are used to reduce the volume of gas flowing through the system to a 20 × 20 µm2 cross-sectional area. A photograph of these microchannels is shown in Figure 1b. This is a critical step in realizing the full sensitivity of the mass spectrometer for measuring small reactions on the order of the focused laser spot size (∼1 µm2). Gold nanoparticles are deposited in the bottom of the microchannels by electron-beam evaporation of 5 nm thick films of Au. This is known to produce island-like growth of strongly plasmonic nanoparticles separated by only a few nanometers.8 Raman and near-infrared spectra are collected with the same objective lens used to irradiate the sample in a Renishaw inVia micro-Raman spectrometer. The catalyst temperatures are determined from the downshifts observed in the Fe2O3 Raman modes. The calibrations of these temperature-induced downshifts were performed in a Linkam THMS temperaturecontrolled stage. The catalyst temperatures are also determined from the blackbody radiation intensity observed at a wavelength of 980 nm, which increases with the temperature to the fourth power (I ) σT4).29 When Au nanoparticles are irradiated with light at their plasmon resonance frequency in a dilute Fe(CO)5/CO gas environment, crystalline Fe2O3 (in the hematite phase) is deposited. This process is called plasmon resonant chemical vapor deposition (PRCVD) and is described in a previous publication.30 Figure 2a shows the mass spectrometer signals of CO2 and O2 plotted during a 1300 s laser exposure (48 mW at 532 nm). Figure 2b shows the corresponding temperature data (as determined from the calibrated Raman and infrared emission spectra) taken during the same 1300 s laser exposure. During the first 300 s, the temperature remains constant at a value of 330 °C, due to the plasmonic heating of the nanoparticles. During this time, Fe2O3 nanocrystals are deposited.30 Throughout the first 300 s of laser exposure, the CO2 and O2 concentrations (Figure 2a) remain unchanged. After 300 s, the CO2 concentration increases and the O2 concentration decreases as they are produced and consumed in the reaction 2CO + O2 f 2CO2, respectively. The sudden increase in temperature observed after 300 s is caused by the exothermic CO oxidation, which becomes strongly catalyzed by the newly formed Fe2O3-Au nano© 2010 American Chemical Society
FIGURE 2. (a) Quadropole mass spectrometer data and (b) infrared temperature data taken during laser irradiation in CO.
particle composite. This highly exothermic reaction (∆H ) -532 kJ/mol) causes the temperature to increase rapidly, exceeding 1000 °C after 550 s. The drop in temperature observed after 770 s is caused by blockage of the microchannel, which restricts the flow of reactants and is, therefore, not related to the reaction kinetics. If, at any time, the laser is turned off, the reaction stops and the temperature drops to room temperature. As described in our previous work,30 we observe the formation of carbon nanotubes at these elevated temperatures (above 700 °C). The precipitation of carbonaceous material occurs by the reaction 2CO f C + CO2. While this reaction has a lower negative heat of formation (∆H ) -173 kJ/mol) than the oxygenation reaction described above, it becomes thermodynamically (entropically) favorable at these high temperatures. The catalytic activity of this Au nanoparticle/Fe2O3 composite is then evaluated in purified CO, in which the Fe(CO)5 has been removed, to prevent further deposition of Fe2O3. Figure 3 shows the mass spectrometer signals of CO2 plotted during a 5 s laser exposure (48 mW at a wavelength of 532 nm). Here, we observe a rapid increase in the CO2 concentration when the laser is incident on the Fe2O3-Au nanoparticle composite. However, when Au nanoparticles are irradiated in a region with no Fe2O3, no change in the CO2 signal is observed, as shown in Figure 3. Similarly, when we irradiate Fe2O3 alone with no Au nanoparticles, we observe 1315
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FIGURE 4. Mass spectrometer signals of CO2 taken after uniformly heating the Au nanoparticle sample to 350 °C. FIGURE 3. Quadropole mass spectrometer signals for CO2 during 5 s laser-induced reactions on various catalytic substrates.
no production of CO2. We, therefore, conclude that this reaction is not driven solely by the thermal (plasmonic) heating of the gold nanoparticles but relies intimately on the interaction of these two materials. Metal nanoclusters on metal oxide supports are known to produce catalytic efficiencies that exceed those of the two constituent materials by themselves.1,2 However, in these prior studies, nanocluster sizes less than 5 nm were required in order for significant catalytic enhancement to be achieved.1 The nanoparticle sizes used in this plasmonic work are 1 order of magnitude larger than this. Furthermore, these previous studies were carried out under uniform heating, rather than laser irradiation. As shown below, these Au nanoparticle/Fe2O3 composites show no detectible catalysis under uniform heating. Therefore, the CO gas reaction kinetics we observe in this work originate from a fundamentally different catalytic mechanism. In order to distinguish this plasmonically driven catalysis from standard thermally driven catalysis, we uniformly heated the same gold nanoparticles in the same gas environment to 350 °C without laser irradiation. This is slightly higher than the temperature achieved by plasmonic heating (see Figure 2). The CO2 mass spectrometer signal of this reaction is plotted in Figure 4. Once the CO gas flow is turned on, there is an increase in the CO2 signal, since the Au nanoparticles themselves are slightly catalytic. The CO2 production rate begins to decrease after 3 min, as the catalytic surface becomes coked with amorphous carbon and Fe2O3. This is a well-known problem in catalysis, limiting catalytic performance and lifetime. The same uniform heating reaction was repeated at temperatures of 550 and 900 °C, resulting in more severe coking of the sample surface that could be clearly seen by scanning electron microscopy. These results are very different from those observed under laser irradiation, where the CO2 signal, and hence reaction rate, increased monotonically for 15 min. Here, we observe a reaction rate that is comparable to that achieved under laser irradiation (picoamperes); however, the reaction area is 2-3 orders of magnitude larger than that of the plasmonically driv© 2010 American Chemical Society
FIGURE 5. (a) QMS signal for CO2 during a series of 10 30-s laser irradiations with a base substrate temperature of 300 °C. (b) Comparison of the average CO2 QMS signal for laser irradiation and uniform heating.
en reactions in Figures 2 and 3, indicating that superior catalytic performance is obtained with plasmonic excitation. In order to quantify the plasmonic enhancement in the catalytic activity, we compared the CO2 production rate of these Au nanoparticle/Fe2O3 composites under laser irradiation with that under uniform heating. Figure 5a shows the mass spectrometer signal for CO2 plotted versus time at a base substrate temperature of 300 °C. The laser is pulsed on and off 10 times with 30 s pulse durations. With the laser on, the CO2 signal rises significantly above the baseline. This 1316
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Acknowledgment. This research was supported in part by ONR Award No. N00014-08-1-0132, AFOSR Award No. FA955008-1-0019, and ARO Award No. W911NF-09-1-0240. W.H. was supported as part of the Center for Energy Nanoscience, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001013.
was repeated at several different base substrate temperatures. Figure 5b shows the average CO2 quadropole mass spectrometer (QMS) signal under laser irradiation compared with that under uniform heating over a wide range of base substrate temperatures. The data show that the laserinduced catalysis is at least 2-3 orders of magnitude higher than that of uniform heating. We see no detectible change in the baseline CO2 signal, even after 25 depositions of Fe2O3 on the Au nanoparticles. This 2-3 order-of-magnitude enhancement is a lower limit for the plasmonic enhancement, since the uniform heating data are limited by the noise in our system. As a final control experiment, we performed the same experiment with 785 nm light, which is below the plasmon resonance energy of the Au nanoparticles. Here, we observed no decomposition of Fe(CO)5 or CO and no deposition of material on the substrate, even at laser powers 10 times higher than those used with 532 nm. Therefore, in order for sufficient electric fields to be achieved and significant heating to occur, the laser energy must match the plasmon resonance frequency of the nanoparticles. Also, regions on the substrate without nanoparticles were irradiated, and no decomposition of CO or deposition of Fe2O3 was observed. While the amount of CO is known and the amount of CO2 produced can be quantified from the mass spectrometry data, there are several difficulties associated with determining the rate constant of these plasmonically driven reactions. As mentioned above, the Au nanoparticle/Fe2O3 composites are formed during an early stage of the process and continue to grow and change their configuration during the reaction process. These changes can also vary the activation energy of the reaction with the time, which in turn affects the rate constant of the reaction. Another difficulty is the undifferentiated CO2 byproduct from the two oxidation reactions: 2CO + O2 f 2CO2 and 2CO f C + CO2 at high temperatures. The main difficulty, however, in quantifying the rate constant of this reaction is that the temperature changes dramatically over the course of the reaction and is nonuniform over the catalytic surface area. In conclusion, we demonstrate plasmonically driven catalysis of CO on Au nanoparticle/Fe2O3 composites. The catalytic performance of these composites greatly exceeds that of the Au nanoparticles and Fe2O3 alone, when irradiated with visible light. Control experiments indicate that the plasmonically driven catalytic reaction rate is several orders of magnitude higher than that obtained under uniform heating, without irradiation. We conclude that this enhanced catalytic reaction is not driven solely by the thermal (plasmonic) heating of the gold nanoparticles but relies on the interaction between the plasmonic nanoparticles and the metal oxide catalyst. The improved catalytic processes described in this work will likely be extended to other applications such as solar energy conversion and storage. © 2010 American Chemical Society
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