Nanoparticle-Modified Electrode with Size- and Shape-Dependent

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Nanoparticle-Modified Electrode with Size- and Shape-Dependent Electrocatalytic Activities Yue Tang†,‡ and Wenlong Cheng*,†,‡ †

Department of Chemical Engineering, Faculty of Engineering, Monash University, Clayton, VIC 3800, Australia Melbourne Centre for Nanofabrication, Clayton, VIC 3168, Australia

‡

S Supporting Information *

ABSTRACT: The size, shape, composition, and crystalline structures of noble metal nanoparticles are the key parameters in determining their electrocatalytic performance. Here, we report on a robust chemical-tethering approach to immobilizing gold nanoparticles onto transparent indium tin oxide (ITO) glass electrode surfaces to systematically investigate their size- and shape-dependent electrocatalysis toward a methanol oxidation reaction (MOR) and an oxygen reduction reaction (ORR). Monodisperse 20 nm nanospheres (NS20s), 45 nm nanospheres (NS45s), and 20 nm × 63 nm nanorods (NRs), which could be chemically tethered to ITO-surface-forming submonolayers without any aggregation, were synthesized. These nanoparticle-modified ITO electrodes exhibited strong electrocatalytic activities toward MOR and ORR, but their mass current densities were highly dependent on the particle sizes and shapes. For particles with similar shapes, the size determined the mass current densities: smaller particle sizes led to greater catalytic current densities per unit mass because of the greater surface-to-volume ratio (NS20s > NS45s). For particles with comparable sizes, the shape or crystalline structure governed the selectivity of the electrocatalytic reactions: NS45 exhibited a higher mass current density in MOR than did NRs because its dominant (111) facets were exposed, whereas NRs exhibited a higher mass current density in ORR because its dominant (100) facets were exposed.

1. INTRODUCTION The development of high-performance nanoparticle electrocatalysts plays a critical role in methanol and hydrogel fuel cells.1,2 The particle size,3 shape,4 composition,5 and structures, including crystalline features, atomic steps, kinks, and ledges,6−8 are key factors influencing the electrocatalytic performance. Recent advances in wet chemistry synthesis offer an exciting avenue for tailoring both the composition and morphology of noble nanoparticles precisely. These tailor-made particles, such as Au-,9 Pt-,4 Pd-,10 and Pt-based alloy metals,11 have exhibited excellent electrocatalytic activities toward various types of species.12 To understand further the fundamental nature of catalysis by these novel nanomaterials, research efforts have been directed toward investigating the effects of their morphological10,13,14 and structural15 parameters on their catalytic performance. For example, it was found that size played a critical role in the electrocatalytic performance of Pt nanoparticles toward the oxygen reduction reaction (ORR) and that ∼3.5 nm Pt nanoparticles possessed the maximum mass current density owing to their highest surface fraction.3 The electrocatalytic activity is also dependent on the crystalline structures of nanoparticles. It was reported that Pt nanoparticles with a preferential (111) orientation exhibited a remarkably high catalytic activity toward methanol as well as formic acid electrooxidation.4 Cubic gold nanoparticles with dominant © XXXX American Chemical Society

(100) facets exposed exhibited superior electrocatalytic activity toward glucose oxidation.6 Branched metallic alloy nanostructures with a high surface-to-volume ratio could enhance the electrocatalytic efficiency and durability toward ORR,11,15,16 in some cases even better than the state-of-the-art Pt/C catalyst.5 In addition, the ORR activity was also found to be dependent on the crystalline surfaces of Au cores inside bimetallic nanostructures. Auoctahedron@Pt with the (111)-orientation-rich Pt surface showed a very high activity toward ORR.17 Wet chemical synthesis is a cost-effective route to metal nanoparticles with adjustable size, shape, and crystalline structures,18 therefore emerging as a viable strategy to tailor electrocatalysis with high accuracy.6,11,19−21 A prerequisite step to integrating wet chemically synthesized nanoparticles into an electrocatalytic system is to immobilize them onto a bulk conductive electrode surface.11,22−25 Physical deposition such as drop-casting11 has proven to be a simple immobilization strategy for such purposes, which unfortunately often leads to inhomogeneous deposition with severe particle aggregation. Consequently, nanoparticle surfaces are in principle only partially accessible to fuels in electrocatalytic reactions; therefore, there is a lower efficacy in material usage. Received: November 20, 2012 Revised: February 1, 2013

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Figure 1. TEM images of (A) Au nanospheres (d = 20 nm), (B) Au nanospheres (d = 45 nm), and (C) Au nanorods. M sodium citrate. At the same time, a mixture of 2.5 × 10−4 M HAuCl4 and 0.08 M CTAB was prepared as a growth solution. After aging the seeds for 2−5 h, 2.5 mL of seed solution and 0.05 mL of freshly prepared 0.1 M L-AA solution were injected into 7.5 mL of growth solution. Within 10 min of stirring, Au NSs with a diameter of 5.5 ± 0.6 nm were obtained. By adding 1.0 mL of the 5.5 nm Au NSs as seeds to the mixture of 9 mL of growth solution and 0.05 mL of 0.1 M L-AA solution, 8.0 ± 0.8 nm Au NSs were produced. Finally, the Au NS20s were generated through adding 1.0 mL of the 8.0 nm Au NSs to the combination of 9 mL of growth solution and 0.05 mL of 0.1 M L-AA solution. The final products were washed by centrifugation at 14 000 rpm for 10 min three times. As for the synthesis of Au NS45s, a protocol similar to the one reported by Mirkin et al. was used.32,33 The seeds were identical to those used to prepare Au NS20s. The growth solution containing 3 mL of 10 mM HAuCl4, 108 mL of 0.05 M CTAB solution with 50 μM KI, 0.6 mL of 0.1 M L-AA, and 0.6 mL of 0.1 M NaOH was divided into three parts: 9.35 mL for both growth solution 1 (S1) and growth solution 2 (S2) and the remainder for growth solution 3 (S3). In the first step, 1 mL of the seed solution was added to S1, followed by slight shaking for approximate 1.5 s. Next, 1 mL of the resulting S1 was injected into S2. After 1.5 s of strong shaking, all of the S2 was poured into S3. The growth of Au NS45s would be completed during 30 min of stirring. Then, the products were left in a 30 °C water bath overnight to precipitate larger particles. Au NS45s in the supernatant were purified by three rounds of centrifugation at a speed of 10 000 rpm for 10 min. Au NRs were synthesized by following previously reported methods.34 First, Au seeds were synthesized by injecting 0.60 mL of ice-cold 0.010 M NaBH4 into a mixture of 5.0 mL of 0.20 M CTAB and 5.0 mL of 0.50 mM HAuCl4. After 2 min of vigorous stirring, the seed solution was aged at 25 °C. Second, a growth solution containing 200 μL of 4.0 mM AgNO3, 5.0 mL of 0.20 M CTAB, 5.0 mL of 1.0 mM HAuCl4, and 70 μL of 0.0788 M L-AA was prepared. Finally, 12 μL of the seed solution was injected into the growth solution, and after one slight inversion and 2 h of growth in a 30 °C water bath, Au NRs were obtained. The products were purified by three rounds of centrifugation at 12 000 rpm. 2.3. Fabrication of Au-Nanoparticle-Modified ITO Electrodes. ITO-coated glass slides with a resistance of 15−20 Ω/square were purchased from Sigma. Before the modification, ITO slides were sonicated for 20 min in each of the following solvents: soapy water, deionized water, acetone, and methanol in an ultrasonic bath. Then they were left in an ethanol solution of 1% (v/v) APTMS for more than 12 h. To obtain the ITO/APTMS/PSS slide, APTMS-modified ITO slides were immersed in a 3 mM PSS aqueous solution in the presence of 2 M NaCl. Then the coated ITO electrodes were immersed in highly concentrated Au nanoparticle solutions overnight. Then nanoparticle-modified ITO electrodes were treated with UV−O3 cleaning for 5 min to eliminate superfluous organic ligands and polymers. 2.4. Instrument. The UV spectra of the samples were acquired by an Agilent 8453 UV−vis spectrophotometer in the range of 300−1100 nm. The morphologies and size distributions of Au nanoparticles on the ITO slide were checked on a JEOL 7001 field-emission SEM.

Indeed, a sparser, uniform deposition can be achieved by a chemical modification strategy in which nanoparticles could be chemically tethered to an electrode surface with reliable control over the particle number densities.26−30 Here, we extended such a chemical modification method to immobilizing presynthesized gold nanoparticles to transparent indium tin oxide (ITO) electrode surfaces to form an aggregation-free submonolayer for the investigation of size- and shapedependent electrocatalysis toward MOR and ORR. To exclude the possible effects of surface-capping ligands on electrocatalytic activity, we synthesized three types of gold nanoparticles (NS20s, NS45s, and NRs), which were all protected by the same cationic surfactanthexadecyltrimethylammonium bromide (CTAB). Then these positively charged particles were immobilized onto negatively charged polymer-modified ITO glass electrodes via electrostatic attraction forces. High transparency and conductivity of the ITO electrode enabled facile optical, electrochemical, and morphological characterizations without any additional processing steps. Our results show that the nanoparticle-modified electrodes were very stable, exhibiting high electrocatalytic activities toward both MOR and ORR. Nevertheless, the catalytic current density per unit mass was highly dependent on the particle sizes and shapes. A smaller particle size led to a higher electrocatalytic performance because of the high surface-to-volume ratio; particles with high-fraction (111) facets exposed favor MOR catalysis, and particles with high-fraction (100) facets exposed favor ORR catalysis. This work clearly demonstrates that chemical immobilization is a viable and robust strategy for immobilizing wet chemically synthesized metallic nanoparticles onto an electrode surface, representing a new route for investigating electrocatalytic reactions under aggregation-free and desorption-free conditions.

2. EXPERIMENTAL METHODS 2.1. Chemicals. Gold(III) chloride trihydrate (≥99.9%, HAuCl4), sodium borohydride (≥98%, NaBH4), sodium citrate (≥99%), hexadecyltrimethylammonium bromide (≥98%, CTAB), L-ascorbic acid (99%, L-AA), (3-aminopropyl)trimethoxysilane (97%, APTMS), poly(sodium 4-styrenesulfonate) (average Mw ∼70 000, PSS), sodium phosphate monobasic dehydrate (≥99%, NaH2PO4·2H2O), and sodium phosphate dibasic dodecahydrate (≥99%, Na2HPO4·12H2O) were purchased from Sigma. Silver nitrate (99.8−100.5%, AgNO3), potassium hydroxide (85%, KOH), potassium iodide (99.5%, KI), and methanol (GR) were obtained from Merck. All chemicals were used as received. 2.2. Preparation of Au Nanoparticles with Various Sizes and Shapes. All Au nanoparticles used in this study were synthesized via the seed-mediated method. As for Au NS20s,31 Au seeds were first produced by adding 0.6 mL of ice-cold, freshly prepared 0.1 M NaBH4 to a 20 mL solution containing 2.5 × 10−4 M HAuCl4 and 2.5 × 10−4 B

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TEM images were obtained with a Philips CM20 electron microscope. Wide-angle X-ray diffraction (WAXD) patterns were obtained at room temperature on a Philips PW1140/90 diffractometer with a Cu Kα radiation source (40 kV, 25 mA, λ = 1.54 Å). Samples were exposed at a scan rate of 2θ = 1° min−1 between 2θ = 30 and 110°. All electrochemical measurements were performed in a conventional three-electrode cell at ambient temperature by using a PARSTAT 2273 advanced electrochemical system (Princeton Applied Research). An Ag/AgCl electrode (in 3.0 M KCl) and a Pt wire were employed as a reference electrode and counter electrode, respectively. As for electrocatalysis toward ORR, the electrolyte solution was 0.1 M PBS solution (pH 7.2), the scan range was from −0.60 to 0.15 V, and the scan rate was 50 mV/s. As for electrocatalysis toward MOR, Aunanoparticle-modified ITO electrodes were scanned in an aqueous solution of 0.1 M KOH with 10 M methanol in the range from −0.30 to 0.45 V, and the scan rate was 50 mV/s.

3. RESULTS AND DISCUSSION To avoid possible effects of the surface-capping ligands on catalytic activities, we synthesized nanoparticles protected by the same surface-capping ligands, namely, CTAB. Using the seed-mediated approaches, CTAB-capped gold NS20s, NS45s, and NRs could be synthesized in large quantities (details in Expenimental Methods). TEM characterizations confirmed the high monodispersity (Figure 1). Furthermore, powder X-ray diffraction (XRD) was used to characterize the crystalline structures of these high-quality nanoparticles. As shown in Figure 2, all three types of nanoparticles showed the pronounced Bragg peaks corresponding to the reflections from (111), (200), (220), (311), and (222) facets, clearly demonstrating a face-centered cubic (FCC) structure.35 Nevertheless, relative peak intensities for these characteristic FCC planes exhibited a strong dependence on particle morphology. The intensity ratios of (111) versus (200) planes for bulk gold, NS20, NS45, and NR are 3 (JCPDS), 5.89, 6.75, and 1.20, respectively. This indicates that NS45 and NS20 have more dominant (111) planes than does bulk gold, whereas NRs have the smallest fraction of (111) planes but with predominant (200) planes, in agreement with the previous report on the rodshaped nanostructure.36 CTAB-capped gold NS20s, NS45s, and NRs could be tethered to the ITO glass surface by a chemical modification procedure (Scheme 1). First, a thoroughly cleaned ITO glass slide was immersed in an APTMS ethanol solution. APTMS is a dual-functionalized molecule with silane and amine groups on each end. The ITO surface is rich in hydroxyl groups that attack and displace the alkoxy groups on the silane end of APTMS, thus forming a covalent −Si−O−Si− bond. Such a silanization process resulted in the formation of a self-assembled monolayer of APTMS molecules on ITO surfaces, leaving positively charged amino group exposed. Second, an APTMS-modified ITO slide was immersed in a solution of negatively charged PSS polymer, leading to the formation of an additional polymeric layer and turning surface charges negative. Finally, the ITO/ APTMS/PSS slides were immersed in solutions of NS20s, NS45s, and NRs (all of these particles are covered with positively charged CTAB ligands), leading to a submonolayer of nanoparticles via electrostatic bonding. The high transparency and conductivity of ITO electrodes enabled direct morphological and spectral characterization without any additional sample-processing steps by SEM and UV−visible spectrophotometry, respectively. Figure 3 shows that Au NS20s, Au NS45s, and Au NRs were all distributed on the ITO surface uniformly, forming submonolayers; no evident

Figure 2. XRD patterns of (A) Au nanospheres (d = 20 nm), (B) Au nanospheres (d = 45 nm), and (C) Au nanorods.

particle aggregation or morphological transformation was observed during nanoparticle modifications or UV−O3 treatment. We further characterized the plasmonic properties of nanoparticle assemblies on ITO electrodes (Figure 4). Notably, the main characteristic plasmonic peaks for NS20s (∼527 nm), NS45s (∼531 nm), and Au NRs (∼511 and ∼698 nm) remained before (Figure 4, solid lines) and after (Figure 4, dashed lines and dotted lines) their immobilization on the ITO surface. Slight shifts in the characteristic peaks were observed, which might be caused by the changes in the surrounding dielectric environments from water to ITO.37,38 There were no evident plasmonic coupling peaks observed, indicating the absence of particle aggregation in agreement with the above SEM results (Figure 3). Nevertheless, there were two additional shoulder peaks shown for NS45s. We speculate that a possible C

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Scheme 1. Modification Procedures of ITO/APTMS/PSS/Au Nanoparticle (NP) Electrodesa

a In the first step, clean ITO slides were immersed in a 1% APTMS ethanolic solution for 2 h. In the second step, ITO/APTMS slides were left in the mixture of 3 mM PSS and a 2 M NaCl aqueous solution for 20 min. In the last step, Au NS20s, Au NS45s, and Au NRs were modified on ITO/ APTMS/PSS slides through overnight immersion.

Figure 3. SEM images of (A) Au NS20-, (B) Au NS45-, and (C) Au NR-modified ITO electrodes. The scale bar is 200 nm.

possibility of the excitation of transverse plasmonic modes. Further UV−O3 treatment did not lead to evident changes in the plasmonic spectra (dashed lines versus dotted lines). The crystalline features of immobilized nanoparticle surfaces could be probed by electrochemical methods such as cyclic voltammetry (CV). It has been known that CV curves for the electrochemical oxidation of gold in concentrated acid solutions are characteristic of specific lattice structures, which can serve as

reason is that impurities such as triangular and plateletlike particles have a stronger tendency to adsorb onto ITO than NS45s. Therefore, the two shoulder peaks might be due to the presence of a larger fraction of impurity particles (Figure 3B). In addition, the transverse plasmonic peak for NRs at ∼511 nm appeared to be weaker than that in aqueous solution. This may be due to the fact that all NRs were lying on ITO surfaces perpendicular to the incident light beam, lowering the D

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Figure 4. UV spectra of (A) Au NS20s, (B) Au NS45s, and (C) Au NRs dispersed in aqueous solution (−−−) after a modification of the ITO electrode (---) and after cleaning by UV−O3 cleansing (···).

the “fingerprint” of specific lattice planes.39 In particular, formal potentials for (100) and (111) in 0.01 M aqueous H2SO4 were reported to be 1.3 and 1.1 V (vs Ag/AgCl, saturated KCl), respectively.35 For our nanoparticle-modified electrodes, the line shapes of CV curves in 0.5 M H2SO4 were different from NSs to NRs (Figure 5). For NS45s, a pronounced peak was

that X-rays could penetrate planes, whereas electrochemical oxidation reactions occurred only on surfaces. It appears that the CV results demonstrated that NS20s contain balanced (100) and (111) facets on surfaces. Moreover, on the basis of the scanning results in Figure 5, we determined that the electrochemically active areas of Au NS20-, Au NS45-, and Au NR-modified ITO electrodes were 1014.5, 361.2, and 796.0 cm2/mg, respectively. All nanoparticle-modified ITO electrodes exhibited high electrocatalytic activities toward MOR in alkaline media as well as ORR in PBS buffer (Figure 6). Nevertheless, the catalytic mass current densities were dependent on particle sizes and shapes (Tables 1 and 2). We first examined size effects by comparing the mass current densities of NS20s and NS45s because they are both spherelike shapes with similar crystalline structures. As for MOR, NS20s gave a mass current density of 11 mA/mg, whereas NS45s gave a mass current density of 5.59 mA/mg. As for ORR, NS20s gave a mass current density of −1.63 mA/mg, whereas NS45s gave a mass current density of −0.87 mA/mg. Clearly, this demonstrated that NS20s exhibited better electrocatalytic activities than NS45s for both MOR and ORR. Therefore, we conclude that smaller nanoparticles provide higher mass current densities. Next, we examined shape effects by comparing the mass current densities of NS45s and NRs because they have comparable dimensions. For MOR (Table 1), NS45s gave a mass current density of 5.59 mA/mg, which is about 6 times that for NRs (only 0.91 mA/mg). However, for ORR (Table 2), NS45s exhibited a mass current density of −0.87 mA/mg, which is smaller than that for NRs (−1.00 mA/mg). The electrocatalytic selectivity of NS45s and NRs toward MOR and ORR could originate from their crystalline structures. It is known that MOR and ORR discriminate crystalline planes:40 MOR is generally favored on (111) facets, and ORR is favored on (100) facets.35 Hence, (111)-facet-dominant NS45s exhibited higher MOR yet lower ORR activity than did (100)-facet-dominant NRs. Among different nanoparticles, the overall catalytic mass current densities are a combined effect of size and shape. For MOR, both NS20s and NS45s are dominated by (111) facets; therefore, their mass current densities are greater than those in NRs. NS20s have a greater surface area than NS45s; therefore, they exhibited greater catalytic activities. This was proven by our experimental results that the order of mass current densities

Figure 5. CV curves of Au NS20-, Au NS45-, and Au NR-modified ITO electrodes electrochemically scanned in 0.5 M H2SO4. The scan rate was 50 mV/s.

shown at around +1.4 V (versus Ag/AgCl, 3 M KCl), confirming the dominant (111) facets exposed to acid solution in agreement with our XRD results (Figure 2B). In the case of Au NRs, a clear oxidation peak was shown at around 1.2 V (vs Ag/AgCl, 3 M KCl), suggesting the presence of dominant (100) facets on NR surfaces in agreement with the XRD results (Figure 2C). As for NS20s, there were no characteristic CV peaks from either (111) or (100) planes. Apparently, this is against the XRD characterization (Figure 2A), which indicated the dominance of (111) planes. However, it has to be noted E

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Figure 6. CV curves of Au NS20- (−−−), Au NS45- (---), and Au NR-modified (···) ITO electrodes electrochemically scanned in (A) 0.1 M KOH with 10 M methanol and (B) 0.1 M PBS (pH 7.2). The scan rate was 50 mV/s.

Table 1. Mass Current Density of Au Nanoparticle-Modified ITO Electrodes towards MOR Au nanospheres (d = 20 nm) −2

number density of Au nanoparticles (cm ) surface-to-volume ratio (nm−1) mass coverage (mg/cm2) peak position (V) mass current density at peak position (mA/mg)

(1.54 ± 0.01) × 10 0.300 1.25 × 10−3 0.30 11.00

10

Au nanospheres (d = 45 nm)

Au nanorods (20 × 60 nm2)

(1.09 ± 0.06) × 10 0.133 1.00 × 10−3 0.33 5.59

(1.30 ± 0.06) × 1010 0.279 4.50 × 10−3 0.28 0.91

9

Table 2. Mass Current Density of Au Nanoparticle-Modified ITO Electrodes towards ORR Au nanospheres (d = 20 nm) ‑2

number density of Au nanoparticles (cm ) surface-to-volume ratio (nm‑1) mass coverage (mg/cm2) peak position (V) mass current density at peak position (mA/mg)

(2.55 ± 0.02) × 10 0.300 2.06 × 10−3 −0.25 −1.63

10

Au nanospheres (d = 45 nm)

Au nanorods (20 × 60 nm2)

(8.86 ± 0.12) × 10 0.133 8.16 × 10−3 −0.48 −0.87

(1.62 ± 0.10) × 1010 0.279 5.60 × 10−3 −0.28 −1.00

9

Figure 7. SEM images of (A) Au NS20-, (B) Au NS45-, and (C) Au NR-modified ITO electrodes after electrochemical scanning in a 0.1 M KOH solution containing 0, 1, 2, 3, and 5 M methanol for 50 cycles. (For each concentration, the scanning was conducted for 10 cycles.) The scale bar is 200 nm.

Figure 8. SEM images of (A) Au NS20-, (B) Au NS45-, and (C) Au NR-modified ITO electrodes after electrochemical scanning in 0.1 M PBS (pH 7.2) for 10 cycles. The scale bar is 200 nm.

F

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Langmuir is NS20s > NS45s > NRs (Table 1). In contrast, the order of mass catalytic current densities in ORR is NS20s > NRs > NS45s (Table 2). The second part of this inequality could be well explained by the shape argument as described above. The first part of the inequality reflects the combined size and shape effects. Although NS20s have a smaller fraction of (100) facets exposed than do NRs, they have greater surface areas. Overall, the size effect outweighs the shape effect, resulting in a greater mass current density for NS20s than for NRs. We also investigated the stability of our nanoparticlemodified electrodes during electrocatalytic reactions. After 50 cycles of MOR reactions in 0.1 M KOH, both NS20s and NS45s aggregated on ITO surfaces (Figure 7A,B). The aggregation might be caused by the presence of (111) facets, which were active in MOR. This argument was supported by the control experiments with (100)-dominant NRs. There were no observable NR aggregates forming before and after MOR reactions (Figure 7C). In contrast, all three types of nanoparticle-modified electrodes are virtually indefinitely stable during ORR reactions. No aggregation was observed for NS20s, NS45s, or NRs (Figure 8).

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REFERENCES

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We presented a robust chemical approach to tethering CTABcapped nanoparticles on ITO electrode surfaces in order to investigate systematically the particle size and shape effects on electrocatalytic performance in MOR and ORR under aggregation-free and desorption-free conditions. In particular, gold NS20s, NS45s, and NRs with a narrow size distribution were successfully synthesized and immobilized on ITO substrates. The order of the mass electrocatalytic activity toward MOR is Au NS20s > Au NS45s > Au NRs, whereas the order for ORR is Au NS20s > Au NRs > Au NS45s. These results clearly show that smaller sizes led to greater mass catalytic current densities if shapes or crystalline structures are the same, whereas shapes or crystalline structures determine catalytic selectivities toward specific electrocatalytic reactions. The overall mass current density is a combined effect of size and shape. Because a rich family of recipes were available for precisely controlling over fractions of crystalline facets on particle surfaces,18 our chemical-modification strategy may serve as a general route to fabricating highly stable, customizable electrocatalytic electrodes for a wide spectrum of applications in sensors and fuel cells.

ASSOCIATED CONTENT

S Supporting Information *

Calculations of the mass current density and the electrochemical active surface area. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work is financially supported by ARC discovery project DP120100170. This work is dedicated to celebrate 80th anniversary of Professor Erkang Wang.

4. CONCLUSIONS

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AUTHOR INFORMATION

Corresponding Author

*Tel: +61 3 9905 3147. Fax: +61 3 9905 5686. E-mail: [email protected]. Website: http://users.monash. edu.au/∼wenlongc/. Notes

The authors declare no competing financial interest. G

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dx.doi.org/10.1021/la304616k | Langmuir XXXX, XXX, XXX−XXX