Hierarchical CuO Colloidosomes and Their Structure Enhanced

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Hierarchical CuO Colloidosomes and Their Structure Enhanced Photothermal Catalytic Activity Jie Xu,†,⊥ Xiaoman Li,§,⊥ Xuan Wu,† Wenzhong Wang,*,§ Rong Fan,‡ Xiaokong Liu,† and Haolan Xu*,† †

Future Industries Institute and ‡School of Natural and Built Environments, University of South Australia, SA 5095, Australia Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China

§

S Supporting Information *

ABSTRACT: Hierarchical CuO colloidosomes (hollow spheres) constructed by subunit of open-mouthed hollow spheres that consist of 2D nanoleaves are synthesized via a Pickering emulsion strategy. Cu2O particles were served as emulsifier particles to form the Pickering emulsions. Self-oxidation of Cu2O gave rise to the formation of CuO nanoleaves which spontaneously cross-link the neighboring particles on the emulsion droplet surfaces, forming the robust colloidosomes. Concurrently, asymmetric Kirkendall effect produced the open-mouthed hollow structure in the subunit CuO particles, resulting in the hierarchical hollow structures of the colloidosomes. Because of the complex and multilevel hollow structure, the CuO colloidosomes significantly enhanced light harvest and photothermal conversion, leading to high local temperature (∼200 °C) on the surfaces under light irradiation at room temperature. This structure enhanced photothermal effect realized the photothermal catalytic CO oxidation at room temperature with a reaction rate 20 times higher than that of thermal catalysis at 240 °C.

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INTRODUCTION Hollow particle is one of the most attractive functional materials for wide applications in catalysis, drug delivery, and energy storage due to its high surface area and large interior space.1,2 Recently, as a new class of materials, complex hollow particle with special shell structures has stimulated intense research interests because these structures can further optimize the functionality of the materials. For instance, compared with typical/simple hollow particles, the complex ones with yolk− shell,3−5 multishells,6−11 porous shell,12−14 and open shell15−21 bring larger surface area and load capacity, adjustable permeability, and mechanical strength which significantly boost their applications in many fields. However, synthesis of these complex hollow structures is challenging. Among the various pathways, Pickering emulsion strategy has shown its unique advantage in constructing hollow shells (colloidosomes) composed of small particles with controllable compositions and sizes.22−30 Pickering emulsions are emulsions stabilized by emulsifier particles.31−33 The oil−water interfaces in Pickering emulsions provide the emulsifier particles with unique asymmetric platform for special structure design and sculpture. Herein, with a facile Pickering emulsion strategy, for the first time we successfully synthesize CuO hierarchical hollow particles/colloidosomes (CuO HCs) composed of subunit of smaller open-mouthed CuO hollow spheres that are constructed by CuO nanoleaves. Such a hierarchical hollow structure is realized by a spontaneous oxidation process of emulsifier Cu2O particles at the oil−water interfaces in Pickering emulsion system, which involves (1) self-oxidation of emulsifier particles Cu2O to CuO, (2) self-cross-link of the CuO particles for robust hollow shells (colloidosomes), and (3) © XXXX American Chemical Society

self-sculpture of opened hollow structure in the subunit CuO particles via the asymmetric Kirkendall effect. As one of the most important p-type semiconductors, CuO has been intensively studied and applied in catalysis, lithium ion batteries, and gas sensors.34,35 Recently, CuO attracted increasing research interests due to its capability of catalytic oxidation of carbon monoxide (CO) to carbon dioxide (CO2).36−40 CO emitted from industry plants, vehicle, cooking equipment, and heater, etc., is harmful to human health and environment.37 Currently, catalytic oxidation of CO to CO2 is mainly conducted over noble metals such as Au, Pt, Pd, and Rh at high temperature to cleanse the exhaust from the vehicles and industries.41−45 Nonetheless, catalytic oxidation of CO at room temperature is also highly required to prevent indoor CO poisoning. Lowering the cost of the catalysts is another key point to facilitate the indoor application. CuO is one of the promising catalysts for CO oxidation mainly due to its abundance and low cost compared with the noble metal counterparts.36−40 However, generally the catalytic activity of CuO is extremely low at room temperature; high reaction temperature (>200 °C) and catalyst supporter are required to activate and enhance the catalytic performance, which causes additional energy consumption and materials cost. Bringing down the reaction temperature to room temperature and using unsupported catalysts are crucial for the practical application of CuO as an ideal catalyst for CO oxidation. Herein, for the first time, it is found that the as-prepared CuO HCs were able to Received: April 13, 2016 Revised: May 29, 2016

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DOI: 10.1021/acs.jpcc.6b03750 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the formation of CuO colloidosomes: (a) mixing water, Cu2O cubic-like particles, and hexadecane; (b) emulsification for Cu2O stabilized Pickering emulsions; (c) self-oxidation of Cu2O in Pickering emulsion for CuO colloidosomes.

suspension was kept shaking for 24 h. The obtained black particles were collected and washed by ethanol and water for three times. To remove the possible residual Cu2O, the particles were stirred in diluted HCl solution (0.01 M) for 1 h for three times. Thermal Catalytic Oxidation of CO. The thermal catalysis activities of CuO for CO oxidation were tested in a quartz tube reactor with an inner diameter of 10 mm at atmospheric pressure. The reagent gas consisted of 50 ppm of CO + 10% O2 + the balance made up of N2 at a flow rate of 200 mL/min. 200 mg of CuO samples (commercial CuO, CuO HCs, and CuO SHSs) was used as catalyst. The rising rate of temperature in the oven is 5 °C/min. The CO conversion was analyzed online by a gas chromatograph (GC) equipped with a TCD. Photothermal Driven Catalytic Oxidation of CO at Room Temperature. The gas-phase CO oxidation was operated in a sealed Pyrex cell (600 mL, CuO 50 mg), equipped with a quartz window. Prior to the irradiation by a Xe lamp (500 W), CO was injected into the cell to reach a concentration of 30 ppm. The CO conversion was monitored by the decrease of CO in the reactor by GC analysis (GC 7900, Techcomp) equipped with a TDX-01, 80−100 mesh packed column followed by a methane conversion furnace and a flame ionization detector (FID). Characterization. Transmission electron microscope (TEM) images were implemented with JEOL JEM-2100F field-emission transmission electron microscope with an operation voltage of 200 kV. Field emission scanning electron microscopy (SEM) analysis was conducted with a Zeiss Merlin SEM. The powder X-ray diffraction (XRD) patterns were recorded with a ScintagARL X’tra diffractonmeter. The specific surface area of the samples was characterized using a Micromeritics Tristar 2000 instrument at 77 K and calculated by the Brunauer−Emmett−Teller (BET) method.

catalyze CO oxidation at room temperature under visible light irradiation. The hierarchical hollow structure enhanced the light trap, absorption, and photothermal conversion which rendered the surfaces with high local temperature (∼200 °C) under light irradiation, thus leading to catalytic CO oxidation at room temperature with a reaction rate 20 times higher than that of thermal catalysis at 240 °C.

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EXPERIMENTAL SECTION Materials. CuSO4·5H2O, glucose, hexadecane, ethanol, HCl, and NaOH were purchased from Sigma-Aldrich and used without further purification. Commercial CuO was purchased from Sinopharm Chemical Reagent. Milli-Q water with the resistance of 18.2 MΩ cm−1 was used for all experiments. Synthesis of Precursor Cu2O Cubic-like Particles. 1.25 g of CuSO4·5H2O was dissolved into 50 mL of H2O. The obtained CuSO4 solution was stirred at 300 rpm and kept at 55 °C for 2 min. Then, 30 mL of NaOH solution (3 M) was poured into the system rapidly, followed by quick heating to 70 °C. Blue Cu(OH)2 was immediately produced. After 5 min, 0.3 g of glucose was added into the suspension. The mixture solution was kept at 70 °C for about 20 min. The color of the suspension gradually turned to red, indicating the formation of Cu2O. The obtained red precipitates were centrifuged and washed with water and ethanol for three times. Synthesis of CuO Hierarchical Colloidosomes (CuO HCs). 0.08 g of Cu2O cubic-like particle was dispersed into 3 mL of H2O assisted by ultrasonication, followed by adding 0.5 mL of hexadecane. Then, the mixture was shaken with hands to form Cu2O stabilized Pickering emulsions. The as-formed Pickering emulsions were transferred to 0.1 M NaOH solution and stored for different times for self-oxidation of Cu2O (6, 12, and 24 h). Then the obtained particles were collected and washed with ethanol and water for several times. To remove the possible residual Cu2O, the particles were stirred in diluted HCl solution (0.01 M) for 1 h for three times. Synthesis of CuO Simple Hollow Spheres (CuO SHSs). 0.08 g of Cu2O cubic-like particle was dispersed into 3 mL of 0.1 M NaOH solution assisted by ultrasonication. The resulting

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RESULTS AND DISCUSSION To synthesize CuO HCs, cubic-like Cu2O particles with edge lengths of 596 ± 70 nm were first synthesized and used as starting materials (Figure 1a). Via simply hand shaking the mixture of Cu2O particles, water, and hexadecane, Pickering B

DOI: 10.1021/acs.jpcc.6b03750 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C emulsions stabilized by Cu2O particles were prepared (Figure 1b). Once the Pickering emulsions are formed, the Cu2O particles were firmly trapped at the oil−water interfaces (emulsion droplet surfaces). Because of high detachment energy, the removal of Cu2O particles from the oil−water interfaces is unfavorable, which renders a long-term stability of the as-formed Pickering emulsion. This long-term stability allows chemical conversion and structure evolution of the emulsifier Cu2O particles at the oil−water interfaces. After 24 h self-oxidation, robust CuO colloidosomes with black color were obtained (Figure 1C). The chemical conversion from Cu2O to CuO is confirmed by the XRD analysis. As shown in Figure S1a, the XRD pattern of the emulsifier Cu2O particles clearly presents the characteristic diffraction peaks of cubic phase Cu2O [JCPDS 65-3288]. After 24 h of self-oxidation at oil− water interfaces, the intensity of the characteristic diffraction peaks of Cu2O dramatically decreased while the peaks for monoclinic CuO are dominant [JCPDS 48-1548] (Figure S1b), indicating the formation of CuO. After removing the residual Cu2O by diluted HCl solution, pure CuO colloidosomes were obtained (Figure S1c). The resulting CuO colloidosomes were demonstrated by SEM and TEM images (Figure 2). As shown in Figure 2a, CuO colloidosomes with sizes of 254 ± 72 μm were produced. From Figure 2b one can see that the colloidosomes are composed of subunit small CuO particles. A cracked colloidosome clearly depicts its hollow structure (Figure 2c). Magnified SEM image of the shell wall shows that the subunit of the colloidosomes is microparticle with size of about 1.7 ± 0.2 μm (Figure 2d). The CuO microparticle is composed of 2D CuO nanoleaves with lateral size of 892 ± 54 nm and thickness of 8.7 ± 0.8 nm. These nanoleaves cross-link the neighboring particles and render a robust shell wall. Surprisingly, a close view of the inner side of shell wall reveals that these subunit CuO microparticles are hollow particles with single holes (Figure 2e), which is also known as open-mouthed hollow sphere. A TEM image further confirms the building block is hollow microparticles composed of nanoleaves (Figure 2f). According to the TEM and SEM images, it is clear that the CuO HCs were obtained via the selfoxidation of Cu2O in the Pickering emulsions. The CuO HCs are composed of the subunit of open-mouthed CuO hollow spheres while the subunit CuO hollow spheres are consisted of CuO nanoleaves. Figure 2g is a cartoon that demonstrates the detailed structure of the obtained CuO HCs. The formation of hierarchical colloidosomes via selfoxidation of emulsifier Cu2O particles in Pickering emulsions is of great interest. It is supposed that the oil−water interfaces in the Pickering emulsions system play a key role. It is wellknown that in Pickering emulsions the emulsifier particles (Cu2O) reside at the oil−water interfaces (emulsion droplet surfaces) with one part locating in water phase and the counterpart dwelling in oil phase (Figure 3a1). In our case, the Cu2O surfaces locating in water phase were quickly oxidized to form the CuO nanoleaves on the surfaces (Figure 3a2), while the counterpart of Cu2O in oil phase was protected (Figure 3a2). This asymmetric oxidation at the oil−water interfaces led to asymmetric structure of the oxidized emulsifier (CuO) particles. Concurrently, the Kirkendall effect which generally induces hollow voids in particles46−51 asymmetrically took place at the oil−water interfaces during Cu2O oxidation, leading to the open-mouthed hollow structure (Figure 3a3). When the initial shell of CuO is formed on the surfaces of Cu2O particles, direct reaction between Cu2O, oxygen, and

Figure 2. SEM images of (a) CuO colloidosomes, (b) an individual CuO colloidosome, (c) a cracked CuO colloidosome, (d) outside shell wall of a CuO colloidosome, (e) inside shell wall of CuO colloidosome, (f) a TEM image of the subunit of CuO hollow sphere, and (g) a cartoon illustration of the structure of CuO hierarchical colloidosomes.

water was hindered. Then further oxidation of Cu2O continued by diffusion of atoms and ions through the CuO shell. When the diffusion rate of the core materials (Cu2O) is faster than that of shell materials (CuO), a net outward mass diffusion to the shell will take place, leading to the formation of hollow voids. To shed light on the structure evolution of the CuO HCs, especially the formation of open-mouthed hollow structure, the particles at different oxidation stages were collected and monitored. It is noticed that 6 h self-oxidation failed to form robust CuO colloidosomes. After drying the Pickering emulsions, most of the hollow structures collapsed, leaving behind small pieces of the shell wall (Figure 3b). The C

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robust CuO colloidosomes were obtained (Figure 3d). CuO nanoleaves formed on the outside surfaces of the particles can strongly bind the neighboring particles (Figure S4). At this stage, part of the Cu2O cores have been consumed due to the Kirkendall effect during the oxidation, leading to the hollow CuO particles composed of CuO nanoleaves (Figure 3e and Figure S5). SEM image of the inside shell wall demonstrates the appearance of the open-mouthed hollow structure. After 24 h oxidation, almost all the Cu2O were consumed, and the CuO HCs were formed (Figure 2). The chemical conversion from Cu2O to CuO, the open-mouthed hollow structure creation, and the cross-link of the subunit CuO hollow spheres were simultaneously accomplished. It is noticed that the synthesized black color CuO HCs possess excellent photothermal effect which is reminiscent of the recently developed concept of photothermal catalysis (PTC). The PTC takes advantage of photothermal effect of the catalysts or catalyst supporters to covert solar light into local heat on the surface of catalysts, thus significantly enhancing the catalytic activity and reducing the required temperature for catalysis.52−55 Herein, we investigate and compare the photothermal catalytic CO oxidation over the CuO HCs and commercial CuO. The BET surface area is 0.03 ± 0.002 m2/g for commercial CuO and 14.48 ± 0.72 m2/g for CuO HCs. Generally, without special treatment, CuO has no catalytic activity for CO oxidation at room temperature. To activate the catalysts, high temperature is required. To figure out the critical temperature for catalytic CO oxidation over the CuO, thermal catalysis at different temperature was conducted. As shown in Figure 4a, the commercial CuO only catalyzes CO oxidation when the temperature is above 200 °C. The CO conversion is only about 35 ± 1.8% at 240 °C. The calculated reaction rates at different temperatures are listed in Table S1. As a comparison, our CuO HCs begin to catalyze the CO oxidation at 70 °C. The catalytic activity is low in the temperature range of 70−160 °C and is remarkable enhanced when the temperature is higher than 160 °C. The CO conversion reaches 98.5 ± 0.4% at 240 °C, and the reaction rate is about 24.8 ± 0.10 μmolCO g−1cat h−1 (Table S1). The thermal catalysis confirms that both commercial CuO and CuO HCs are inactive at room temperature. To activate CuO HCs for complete CO oxidation, a temperature >240 °C is required, while an even higher temperature is essential for commercial CuO. However, when the catalytic reaction over CuO HCs was conducted under the visible light irradiation at room temperature, surprisingly, outstanding performance of catalytic CO

Figure 3. (a) Schematic illustration of the asymmetric structure evolution of the emulsifier particles at oil−water interfaces (emulsion droplet surfaces): SEM images of (b) collapsed CuO colloidosomes and (c) their inside shell wall obtained after 6 h oxidation in Pickering emulsion; SEM images of (d) CuO colloidosomes and (e) their inside shell wall obtained after 12 h oxidation.

surface feature of the outside shell wall is different from that of the inside shell wall. From Figure S2 one can see that the outside of shell wall are densely covered by CuO nanoleaves due to the oxidation of Cu2O in water phase. However, SEM images of the inside shell wall clearly show the cubic Cu2O particles are retained which indicates the neglectable oxidation on the surfaces in the oil phase (Figure 3c and Figure S3). This result confirms the asymmetric oxidation of Cu2O particles in the Pickering emulsions (Figure 3a2). After 12 h oxidation, the

Figure 4. (a) Thermal catalytic CO oxidation at different temperatures over commercial CuO (blue triangle), CuO HCs (red circle), and CuO SHSs (black square). (b) Photothermal catalytic CO oxidation over commercial CuO, CuO HCs, and CuO SHSs at room temperature under visible light irradiation. (c) Temperature curve of commercial CuO, CuO HCs, and CuO SHSs along with the irradiation time. D

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quently catalytic activity under light irradiation at room temperature. It is no doubt that when the light irradiates on the CuO HCs and SHSs, the photothermal effect leads to high local temperature on the surfaces of the hollow particles (Figure 5). A part of the photogenerated heat conducts outward to the

oxidation was achieved. As shown in Figure 4b, the CO conversion over CuO HCs reaches about 99.3 ± 0.5% after 20 min light irradiation. The reaction rate over CuO HCs at room temperature was calculated to be 482.1 ± 2.41 μmolCO g−1cat h−1, which is almost 20 times higher than that of thermal catalysis at 240 °C. Nonetheless, the commercial CuO shows no catalytic activity at room temperature under the same visible light irradiation. The outstanding catalytic activity of CuO HCs at room temperature under visible light irradiation stems from the photothermal effect of CuO. To roughly estimate the surface temperature, a thermocouple was buried in the CuO HCs powders. The temperature evolution under the visible light irradiation was recorded. As shown in Figure 4c, upon the light irradiation, the temperature of the CuO HCs increases quickly and eventually reaches about 198 ± 3.2 °C. It is believed that the real local temperature on the surfaces of CuO HCs should be even higher than 200 °C, thus bringing high catalytic activity at room temperature. However, the highest temperature for commercial CuO is only about 175 ± 2.4 °C, much lower than the critical temperature for catalytic CO oxidation as shown in Figure 4a. This explains why the commercial CuO has no catalytic activity at room temperature under visible light irradiation. The higher surface temperature of CuO HCs under light irradiation benefits from the complex and multilevel hollow structure. It has been well demonstrated that the hollow structures and the flower-like structure can effectively enhance the light harvest via increasing the chance of light trap and absorption in the structures.56,57 Herein our CuO HCs integrate these two structures for effective light absorption (Figure 2). Additionally, the open-mouthed hollow structure in the subunit CuO spheres is expected to further enhance the light absorption and photothermal conversion, thus leading to the high surface temperature under light irradiation. To further prove the merits of this specific hierarchical hollow structure for light absorption, photothermal conversion, and the consequent catalytic activity enhancement, CuO hollow spheres with relatively simple structure were synthesized, and their catalytic activity was studied and compared with that of CuO HCs. The same precursor Cu2O cubic-like particles were used as starting materials for self-oxidation in bulk water without the presence of Pickering emulsions. TEM and SEM images (Figure S6) reveal that the obtained product is flowerlike CuO hollow particles composed of nanoleaves, which is similar to the subunit of the CuO HCs. This CuO hollow sphere is denoted as CuO simple hollow sphere (CuO SHS). The BET surface area of CuO SHSs is 17.2 ± 0.86 m2/g. Thermal catalytic CO oxidation over the same amount of CuO SHSs showed that CO conversion reached about 98.2 ± 0.6% at temperature of 167 °C. The reaction rate is about 27.4 ± 0.16 μmolCO g−1cat h−1 (Table S1), which is about 5 times higher than that over CuO HCs (5.4 ± 0.53 μmolCO g−1cat h−1) at the same temperature. However, it is surprising that the catalytic reaction rate over CuO SHSs under visible light irradiation at room temperature is lower than that of CuO HCs (Figure 4b). It is noticed that the temperature of the CuO SHSs under visible light irradiation reached about 183 ± 3.8 °C (Figure 4c), which is lower than that of CuO HCs powder. These results confirm that although the thermal catalytic activity of CuO HCs is much lower than that of CuO SHSs, its photothermal catalytic activity is higher. The hierarchical hollow structure of the CuO HCs significantly contributes to the light absorption, photothermal conversion, and conse-

Figure 5. Cartoon illustration of the hot sites of the CuO HCs and CuO SHSs under visible light irradiation. The red areas represent high temperature sites induced by the photothermal effect.

ambient, resulting in temperature decrease away from the surfaces. For a CuO SHS, all of its surfaces conduct heat toward the ambient. However, compared with individual SHS, when they are arranged in the form of a HC, the heat conduction to the ambient only occurs on a maximum half space because of the close attachment between the neighboring particles. Therefore, the maximum temperature increase of CuO HCs at steady state should be higher than that of CuO SHSs. Furthermore, a part of heat conducts inward into the hollow chamber of the HCs and SHSs, producing hot sites inside the hollow structures. The reactant CO and O2 can diffuse into the hot hollow chambers and be heated quickly. When these preheated gases encounter the CuO hollow shells, accelerated catalytic CO oxidation takes place. Therefore, it is expected that larger hollow chamber can reserve more preheated CO and O2 for catalytic reaction. Compared with CuO SHSs, the CuO HCs possess much larger hollow chamber. Assuming a CuO HC are composed of number N of small CuO hollow spheres, and the size of subunit CuO hollow sphere is the same with that of CuO SHS, then the volume of hollow chamber for a CuO HC versus number N of CuO SHSs can be estimated as follows: NL2 ≈ 4πR2

VHC =

4 3 πR + NL3 3

VSHS = NL3

(1) (2) (3)

where R is the inradius of the CuO HC and L is the inner edge length of CuO SHS (cubic shape). According to the optical microscopy as well as SEM and TEM images, we set R = 95 ± 2.7 μm and L = 0.6 ± 0.035 μm. Then N is calculated to be about 318504 ± 46599, and VHC:VSHS is about 54 ± 3.4, which means the volume of hollow chamber in a CuO HC is much larger than that of N CuO SHSs. All these factors explain the higher surface temperature and catalytic activity of CuO HCs E

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under visible light irradiation at room temperature. The structure of the photothermal materials is of great importance to the PTC.

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CONCLUSION In summary, CuO HCs composed of the subunit openmouthed CuO hollow spheres which are constructed by CuO nanoleaves were synthesized via a Pickering emulsion strategy. Asymmetric oxidation of emulsifier Cu2O particle and Kirkendall effect at the oil−water interfaces in Pickering emulsion systems lead to the formation of CuO HCs. The photothermal catalytic oxidation of CO over the CuO HCs was studied. Because of the complex and multilevel hollow structure, the CuO HCs effectively harvest the visible light and convert it into heat on the surfaces which renders the CuO HCs outstanding catalytic activity for CO oxidation at room temperature. The photothermal catalytic reaction rate at room temperature is about 20 times higher than that of thermal catalytic reaction rate at 240 °C. Furthermore, due to the larger hollow chamber as heat reservoir and the limited heat conduction to the ambient, the CuO HCs provides more hot sites and higher local surface temperature than CuO SHSs, rendering the former a superior PTC activity than the latter. This work proves the significance of the structure of materials for the photothermal effect and the consequent PTC. This concept is expected to be extended to other PTC systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03750. XRD patterns of the particles, SEM images of the colloidosomes obtained by oxidation at different stages, TEM and SEM images of the simple hollow spheres, table of thermal catalytic reaction rate (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +86 21 52415295 (W.W.). *E-mail [email protected]; Tel +61 8 83023623 (H.X.). Author Contributions ⊥

J.X. and X.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by Australia Research Council (DE120100042) and University of South Australia (Foundation Fellow).

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REFERENCES

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.6b03750 J. Phys. Chem. C XXXX, XXX, XXX−XXX