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Jun 16, 2015 - Si Woo Lee , Jong Wook Hong , Hyunhwa Lee , Dae Han Wi , Sun Mi ... Sun Mi Kim , Changhwan Lee , Kalyan C. Goddeti , Jeong Young Park...
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Photon-Induced Hot Electron Effect on the Catalytic Activity of CeriaSupported Gold Nanoparticles Sun Mi Kim,†,‡ Hyosun Lee,†,‡ Kalyan C. Goddeti,†,‡ Sang Hoon Kim,§ and Jeong Young Park*,†,‡ †

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon 305-701, Republic of Korea Graduate School of EEWS, KAIST, Daejeon 305-701, Republic of Korea § Center for Materials Architecturing, KIST, Seoul 136-791, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The role of charge transfer at the metal−oxide interface is a long-standing issue in surface chemistry and heterogeneous catalysis. Previous studies have shown that the flow of hot electrons crossing metal−oxide interfaces correlates with catalytic activity. In this study, we employed ceria-supported gold nanoparticles to identify a correlation between the catalytic activity of CO oxidation and hot electrons generated via light irradiation. We tuned the size of the Au nanoparticles by changing the discharge voltages used in the arc plasma deposition process, thus allowing us to investigate the influence of Au nanoparticle size on changes in catalytic activity. CO oxidation over the Au/CeOX catalysts was carried out, and we found that the activity of the Au nanoparticles increased as the size of the nanoparticles decreased, which is associated with the cationic character of the Au nanoparticles, as demonstrated by X-ray photoelectron spectroscopy analysis. We also show that the activity of the Au nanoparticles decreases under light irradiation and that smaller nanoparticles show a higher change of turnover frequency compared with larger ones, presumably due to the mean free path of the hot electrons. From these results, we conclude that the cationic property of the gold species, induced by interaction with the CeO2 support, and the flow of hot electrons generated on the interface during light irradiation are mainly responsible for the change in catalytic activity on the Au nanoparticles. inspiring result that ∼3.5 nm diameter Au nanoparticles supported on TiO2 thin films are the most active for the CO oxidation reaction, which is associated with the beginning of a metal-to-nonmetal transition observed to occur at that diameter.9 Haruta et al. also reported that the turnover frequencies for CO oxidation per surface gold atom increased sharply as the diameter of the gold particles decreased below 4 nm, almost independent of the kind of support oxide used.10 However, the high catalytic activity of Au nanoparticles is not solely explained on the basis of size. Recent studies significantly suggest the role of the metal−support interface that is assumed to be the origin of the unexpected synergetic effect between the support (typically metal oxides) and supported phases (typically metal nanoparticles) as another factor affecting nanoparticle catalytic activity. This phenomenon is referred to as the strong metal−support interaction (SMSI) effect, and it was first investigated by Schwab11,12 and Solymosi.13 They revealed that the catalytic nature of metal nanoparticles is newly created or tuned by metal oxide supports. Schwab et al. found that for methane oxidation on metal−semiconductor mixed catalysts ZnO/Ag displays a higher catalytic activity than other

1. INTRODUCTION The chemical reactivity of the noble metal gold has been regarded as intrinsically very low because of the lack of interaction between the orbitals of adsorbates and the filled d states of gold.1 In 1989, Haruta et al. found that gold nanoparticles (NPs) showed “superior catalytic activity” for low-temperature oxidation of hydrogen and carbon monoxide when it was highly dispersed on transition metal oxides, such as α-Fe2O3, Co3O4, and NiO.2 Subsequent theoretical calculations supported this discovery by revealing an increased reactivity of low-coordinated gold atoms, which are particularly abundant on the smallest (i.e., nanometer-sized) nanoparticles.3,4 Since then, the interest for gold-based catalysts has grown rapidly due to their potential applications in many reactions of both industrial and environmental importance.5−8 Many studies have reported that this unusual reactivity of supported gold catalysts appears to depend strongly on the method of synthesis, redox properties of the metal oxide support, the size and dispersion of the Au nanoparticles, and the contact structure of the Au−metal oxide interfaces. Among these, the size of the Au nanoparticle is considered to be one of the most forceful factors affecting the catalytic activity. When the size of metallic nanoparticles becomes smaller and smaller, it is well-known that the nanoparticles show new and different size-dependent catalytic properties. Valden et al. obtained the © 2015 American Chemical Society

Received: April 5, 2015 Revised: June 5, 2015 Published: June 16, 2015 16020

DOI: 10.1021/acs.jpcc.5b03287 J. Phys. Chem. C 2015, 119, 16020−16025

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Figure 1. SEM images of (a) the CeO2 thin-film surface and (b) CeO2 thin-film cross section. (c−e) TEM images and (f−h) size distribution histograms of Au nanoparticles deposited by five shots of arc plasma on carbon substrates under different arc discharge voltages: (c, f) 90 V, (d, g) 180 V, and (e, h) 270 V.

investigated the effect of hot electrons on surface chemical reactions (e.g., CO oxidation) as a function of particle size.

catalysts (i.e., ZnO and Ag individually), giving an extremely high yield of products at even lower temperatures. For this catalytic promoter effect, they assumed that it originates from electron transfer between the metal catalyst and the semiconductor support. At present, a great deal of research is devoted to verifying this hypothesis and develop strategies to study the origin of the SMSI effect. The flow of charge during the exothermic catalytic reaction was measured directly with metal−semiconductor nanodiodes, and a correlation between the catalytic activity and hot electron flow was found.14−16 Another approach involves the creation of additional charge at the support by illuminating light on or changing the doping of the support, thus influencing the catalytic reactions occurring on the metal nanoparticle surfaces.17 For example, Schäfer et al. recently observed that the type of gallium nitride (GaN) doping affected the reactivity of the Pt nanoparticles.18 They suggested that this is attributed to the strong electronic interaction between the nanoparticles and the n- or p-type GaN substrates, showing a large influence on the chemical composition and oxygen affinity of the supported nanoparticles under X-ray irradiation. Kim et al. showed that the catalytic activity of metal−semiconductors, including Pt−CdSe−Pt nanodumbbells19 and Pt nanoparticles on GaN,20 is affected by light irradiation, which was attributed to the flow of photongenerated hot electrons. In spite of the increasing popularity of gold as a topic in heterogeneous catalysis, the microscopic origin of the catalytic activity of Au nanoparticles remains controversial. In this paper, we carried out CO oxidation on Au nanoparticles supported on a CeO2 thin film to identify a correlation between the catalytic activity and the size of the Au nanoparticles. We also

2. EXPERIMENTAL DETAILS 2.1. Preparation of Cerium Oxide (CeO2) Thin Films via the Sol−Gel Method. Chemicals. Cerium chloride heptahydrate (CeCl3·7H2O, Sigma-Aldrich) as the precursor and ethanol (C2H5OH, Merck Chemicals) as the solvent were used to make CeO2 sols. Nitric acid (HNO3, Daejung Chemicals) was used to help the CeO2 sols to form gels, and myristyltrimethylammonium bromide ((CH3(CH2)13N(Br)(CH3)3, Sigma-Aldrich) was used as a surfactant to make uniform CeO2 thin films. All chemicals were used without further purification. The CeO2 thin films were synthesized via the sol−gel process as follows. Briefly, 0.252 g of cerium chloride heptahydrate and 0.126 g of myristyltrimethylammonium bromide were added to 20 mL of ethanol and then mixed with 168 μL of nitric acid while stirring. This solution was sprinkled on silicon substrates that were previously cleaned in acetone and absolute ethanol for 5 min in turn and then rinsed with distilled water. The substrate was subsequently rotated at a rate of 3000 rpm for 30 s in a spin coater, and this process was repeated 6 times to make the CeO2 thin films with a uniform thickness. Finally, it was preheated at 323 K for 1 h in an air-conditioned oven and then annealed at 723 K for 1 h in a furnace to crystallize the thin films. The morphology and the thickness of the CeO2 thin film were characterized using scanning electron microscopy (SEM, Magellan 400), as shown in Figure 1a,b. Measured from the cross-sectional SEM images, the CeO2 thin film was 20 ± 1 nm thick. The crystal structure and the oxidation state of the 16021

DOI: 10.1021/acs.jpcc.5b03287 J. Phys. Chem. C 2015, 119, 16020−16025

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Figure 2. Catalytic activity of the CO oxidation reaction on Au/CeO2 catalysts prepared by arc plasma deposition with and without light. Turnover frequency (TOF) of (a) 1, (b) 1.5, and (c) 2.7 nm Au nanoparticles deposited on CeO2 thin films.

CeO2 thin film were revealed using X-ray diffraction (XRD, Rigaku D/MAX 2500) patterns with Cu Kα radiation and X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific Sigma Probe system equipped with an Al Kα X-ray source (1486.3 eV)), as shown in the Supporting Information in Figures S1 and S2, respectively. The XRD spectra show that the CeO2 thin film has the fluorite structure, and the XPS spectra show that the film has two oxidation states, Ce3+ and Ce4+, after annealing at 723 K. Hall effect measurements were also carried out to understand the electrical behavior of the CeO2 thin film. This is well-known as one of the most powerful methods for obtaining basic properties relating to electrical conduction (e.g., the sign of the carriers, carrier concentration, and mobility). Via these electrical measurements, we confirmed that the sign of the carrier, carrier concentration, and mobility of these films in turn have the corresponding values of the negative, 3.3 × 1015/cm3, and 4.5 cm2/(V s), demonstrating that the CeO2 thin film is an n-type semiconductor. 2.2. Deposition of Au Nanoparticles on CeO2 Thin Films Using the Arc Plasma Deposition Technique. Au nanoparticles were deposited on the CeO2 thin films using an arc plasma deposition (APD) system (ULVAC, ARL-300) at room temperature under 10−6 Torr vacuum. This system consists of three parts: a cathode, a trigger, and an anode electrode. Structurally, a cylindrical cathode rod is the source of the metal nanoparticles, and a trigger electrode is placed at the center with a cylindrical anode mounted coaxially to wrap the cathode. A trigger pulse induces the arc discharge on the surface of the target metal rod, generating a highly ionized Au plasma. A detailed explanation of this process is described elsewhere.21 Recently, the APD technique has drawn considerable interest for catalytic applications because it is possible not only to prepare the nanoparticles or thin films without using an organic capping agent but to also synthesize nanocatalysts in a largescale, simple, and dry process.7,22,23 The size of the nanoparticles can be tuned using the discharge voltage, discharge condenser capacitance, number of plasma pulse shots, and the nature of the evaporating materials and substrates.21,24 Among these, the discharge voltage is the most convenient parameter for controlling the deposition amount and the average particle size of the metal nanoparticles. For this reason, we deposited Au nanoparticles onto the CeO2 thin films under arc voltages of 90, 180, and 270 V to prepare the different particle sizes; we used five plasma pulses with the condenser capacitance at 1080 μF. We also deposited Au nanoparticles on carbon substrates that were characterized using transmission electron microscopy

(TEM) to confirm the size, shape, and size distribution of the as-prepared Au nanoparticles, as shown in Figure 1c−h. These results clearly indicate that the particle size and coverage increase as the voltage increases from 90 to 270 V. As shown in the histograms in Figure 1f−h, the average particle size for the 90 V deposition was 1.0 ± 0.2 nm, about 2 or 3 times smaller than that for the 270 V deposition (2.7 ± 0.6 nm). 2.3. Measurement of the Catalytic Activity of CO Oxidation on Au/CeO2 Model Catalysts. The CO oxidation reaction on the active Au metal nanoparticles/CeO2 thin film prepared using APD was carried out under 40 Torr of CO and 100 Torr of O2 as reactant gases and 620 Torr of He as the balancing gas in a batch reactor equipped with a recirculation pump, similar to that used for studies of single-crystal metal samples. The gases were circulated through the reaction line by a Metal Bellows recirculation pump. A gas chromatograph equipped with a thermal conductivity detector and 6 ft × 1/8 in. SS molecular sieve 5A was used to separate the products for analysis. The measured reaction rates are reported as turnover frequencies (TOF) and are measured in units of product molecules of CO2 produced per metal surface site per second of reaction time. The number of metal sites was calculated using geometrical considerations based on SEM measurements of the surface area of the nanoparticle arrays. A halogen lamp (Dolhan-Janner D150) was used as the light source during light irradiation, and its spectral profile is shown in the Supporting Information in Figure S3. With the distance between the sample and the halogen lamp set at ∼8 cm, the light intensity was measured to be 49.4 mW/cm2 using an optical power meter (ADCMT 8230).

3. RESULTS AND DISCUSSION Au/CeO2 model catalysts have been widely investigated in a variety of chemical reactions (e.g., CO oxidation,25,26 preferential oxidation of CO in rich hydrogen,27 and lowtemperature water-gas shift (WGS) reaction28) because of its high catalytic activity. Figure 2 shows the catalytic activity of CO oxidation on Au nanoparticles supported on CeO2 thin films with and without light. From these results, we can come to two distinct conclusions: First, the turnover frequency of 1 nm Au nanoparticles is 2 or 3 times higher than that of 2.7 nm Au nanoparticles on CeO2 thin films, showing that the activity of the Au nanoparticles is size dependent. Second, the catalytic activity of Au nanoparticles decreases regardless of Au nanoparticle size under light irradiation, implying the important role of metal−oxide interfaces. 16022

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Many studies have shown that cationic gold species affect the catalytic activity of gold for CO oxidation.34−38 Especially, Guzman et al. demonstrated that the specific rate of CO oxidation correlates with the concentration of the cationic character of the gold (e.g., Au3+ and Au1+) using in situ infrared (IR) spectroscopy.39 When the Au/CeO2 catalystspretreated with COwere exposed to oxygen, the intensity of the IR bands at 2148 cm−1 (representing Au3+−CO) and 2130 cm−1 (representing Au1+−CO) decreased at a faster rate than that of the band at 2104 cm−1 (representing Au0−CO), suggesting that the catalytically active sites incorporate cationic gold. Therefore, our finding that smaller Au nanoparticles at higher oxidation states showed higher catalytic activity for CO oxidation coincides very well with this spectroscopic evidence. Another appealing aspect of the catalytic activity of Au/CeO2 catalysts is that the percentage change of the activity upon light irradiation depends upon the size of the Au nanoparticles. Figure 4b shows the percentage change of the TOF for CO oxidation measured on Au nanoparticles deposited on CeO2 thin films at 513 K during light irradiation. It is clear that the activity of the Au nanoparticles decreases under light irradiation, and smaller nanoparticles showed a higher change in TOF compared with larger ones. The drop in catalytic activity of the Au nanoparticles during illumination is explained by the reduced net charge of the Au nanoparticles, which results from the recombination of electrons from the Au nanoparticles with holes in the n-type CeO2 semiconductor, as shown in Figure 4a. Therefore, these results suggest that the charge states of the Au nanoparticles play an important role in determining the catalytic activity. However, we note that the specific role of the charge states of Au nanoparticles is still being debated.40−44 To confirm the reversibility of the hot electron effect, we repeatedly carried out CO oxidation on 1 nm APD Au nanoparticles/n-type CeO2 semiconductor with and without light irradiation, as shown in Figure 5. Additionally, to identify the generality of hot electron effects in a catalytic chemical reaction, we carried out H2 oxidation on Au nanoparticles/ CeO2 semiconductor with and without light irradiation (Supporting Information, Figure S2). Regardless of the kind of chemical reaction, the repeated catalytic measurements for the oxidation of CO and H2 show a decrease in the activity of the Au nanoparticles under light irradiation. The catalytic activity of the oxidation of CO and H2 increased by 7−21% and 9−17%, respectively, without light irradiation (i.e., in the dark) compared with under light irradiation. This light-dependent catalytic activity clearly ensures that the hot charge carriers generated by light injection control the flow of hot electrons between the metal and semiconductor layer and cause a change in the CO oxidation and H2 oxidation reaction. The similar trends of CO and H2 oxidation indicate that the hot electron effect is a general phenomenon for exothermic chemical reactions on metal−semiconductor nanocatalysts.45 Furthermore, surface plasmon resonance and local heating can also be considered as possible origins of our observations because the reactions were carried out under light irradiation. However, because it is known that gold nanoparticles with diameters below 2 nm do not exhibit surface plasmon resonance owing to quantum size effects, the surface plasmon resonance can be ruled out as the possible origin of change of the catalytic activity under light irradiation. Local heating by excitation of electron− hole pairs and subsequent recombination can be another source, but it results in an increase in catalytic activity. In our

The effect of the particle size of gold on the catalytic activity has been reported for several chemical reactions. Earlier studies by Haruta et al. reported that ultrafine gold particlessmaller than 10 nmuniformly dispersed on transition metal oxides (e.g., Co3O4, α-Fe2O3, and NiO) were highly active for H2 and CO oxidation.2 Recently, they examined hydrogen dissociation via Au clusters deposited on TiO2 surfaces as a function of mean gold particle diameter and observed that the rate of HD formation increased as the Au particle size decreased, with a marked increase when the Au particle size fell below 2 nm.7 Tana et al. also found that the Au−CeO2 contact boundary, altered by the size of the gold particles, significantly affects the activity of Au/CeO2 catalysts for CO oxidation.29 In spite of significant efforts to find the nature of active sites in gold catalysts supported on various metal oxides, the origin of the catalytic activity found when reducing the size of gold particles is still controversial. The proposed causes include an increased fraction of edge or corner atoms, a change in the electronic properties of the Au nanoparticles, or the reactivity of the perimeter interface around the gold particles in contact with the oxide supports.30−33 Our XPS results indicate the presence of positively charged cationic gold species that play a role in affecting the catalytic activity of the Au nanoparticles. Figure 3 shows XPS spectra that reveal the oxidation states of

Figure 3. (a) XPS spectra and (b) a comparison of the oxidation states of Au 4f as a function of the size of the Au nanoparticles supported on CeO2 thin films.

Au 4f as a function of the size of the Au nanoparticles on the CeO2 thin films. All of the XPS spectra were calibrated using the C 1s peak at a binding energy of 284.6 eV. The Au 4f7/2 core level is visible at 84.0 eV, which is slightly shifted from the value of Au (83.8 eV) reported in the handbook of X-ray photoelectron spectroscopy. As shown in Figure 3a, all of the Au nanoparticles have two surface oxide peaks: Au(X+) and Au(X++) with energy levels of 85.0 and 86.7 eV, respectively. These two peaks are associated with Au atoms in two different oxidation states or coordinated to a different number of oxygen atoms. Another interesting feature is that the ratio of the peak area of Au(X+) and Au(X++) to Au(0) increased as the size of the gold particles decreased, as shown in Figure 3b. 16023

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Figure 4. (a) Energy band alignment of the Au/n-type CeO2 model catalysts showing a plausible route for hot carrier flow under light irradiation and (b) comparison of TOF of the CO oxidation reaction measured on Au nanoparticles deposited on CeO2 thin films at 513 K as a function of light.

The activity decreased as the size of the nanoparticles increased, which is thought to be associated with the cationic character of the Au nanoparticles, as demonstrated by XPS analysis. We also studied the effect of hot electrons on surface chemical reactions (i.e., CO oxidation) as a function of the particle size of the gold. It is clear that the activity of the Au nanoparticles decreased under light irradiation and that smaller nanoparticles showed a higher change of TOF compared with larger nanoparticles, presumably due to the mean free path of the hot electrons. On the basis of these results, we conclude that the cationic properties of the gold species, induced by interaction with the CeO2 support, and the flow of hot electrons generated on the interface during light irradiation are responsible for the changes in catalytic activity on the Au nanoparticles.

Figure 5. Catalytic activity of 1 nm APD Au nanoparticles/n-type CeO2 semiconductor with and without light measured under CO oxidation at various temperatures.

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study, we observed a reduction of catalytic activity during light irradiation, which cannot be explained by local heating, but exclusively attributed to the hot electron effect. This correlation between the interfacial charge transfer process and the activity of metal nanoparticles was previously reported by Kim et al.20 In this study, they showed that the catalytic activity of Pt nanoparticles depends on the doping type of the GaN semiconductor under light irradiation. Also, the percentage change of TOF according to the size of Au nanoparticles is presumably associated with the mean free path of hot electrons. Generally, after hot electrons are generated at the interface between the Au nanoparticles and the n-type CeO2 semiconductor, it is predicted that the smaller the particle size, the faster the hot electrons can recombine with holes in the n-type CeO2 semiconductor, which then cannot participate in the surface chemical reaction (e.g., CO oxidation). We can conclude, therefore, that hot electrons are generated at the metal−semiconductor (Au/n-type CeO2) interface under light irradiation and that it will affect catalytic activity.

ASSOCIATED CONTENT

S Supporting Information *

X-ray diffraction (XRD) pattern (Figure S1) and X-ray photoelectron (XPS) spectra (Figure S2) of the CeO2 thin film after annealing at 723 K, spectral profile of the halogen lamp used as the light source (Figure S3), and the results of hydrogen oxidation on Au nanoparticles/n-type CeO2 semiconductor (Figure S4). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03287.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.Y.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by IBS-R004-G4 and by a grant from the future R&D Program funded by KIST (2E25371).

4. CONCLUSIONS CO oxidation on Au nanoparticles supported on a CeO2 thin film was carried out to more clearly identify a correlation between the size of the Au nanoparticles and the catalytic activity. For the support, two-dimensional CeO2 thin films were prepared using the sol−gel method. The size of the deposited Au nanoparticles (i.e., the metal catalyst) was controlled by changing the discharge voltages used in the arc plasma deposition (APD) process. The catalytic activity of the Au/ CeO2 catalysts depended on the size of the Au nanoparticles.

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DOI: 10.1021/acs.jpcc.5b03287 J. Phys. Chem. C 2015, 119, 16020−16025