Rational design of plasmonic catalysts: matching the surface plasmon resonance with lamp emission spectra for improved performance in AgAu nanorings

Abstract

In order to enable practical applications of SPR-excitation in heterogeneous catalysis, facile procedures for the synthesis of plasmonic catalysts as well as the use of commercially available and inexpensive lamps as the excitation source are highly desirable. In this context, the development of catalysts displaying SPR extinction that matches, as much as possible, the emission spectra of commercially available lamps represent an intuitive strategy to maximize performance. We report the design and facile synthesis of AgAu nanorings displaying SPR extinction that closely matches the emission spectra of a commercial halogen–tungsten lamp. The AgAu nanorings were employed as catalysts for the SPR-mediated oxidation of methylene blue in the liquid phase (water as the solvent), under ambient conditions, and using a halogen–tungsten lamp as the only energy input. The activity of the nanorings was benchmarked against Ag and Au nanospheres. We found that the activity of the nanorings was higher relative to the nanospheres, and that both hot electrons and holes generated as a result of the SPR excitation participated in the methylene blue oxidation reaction with similar relative contributions. Our results show that the rational design of metallic nanostructures plays an important role for enabling practical applications in the field of plasmonic catalysis, in which facile procedures can be employed for the synthesis of the catalysts (attractive for large-scale production) and commercial lamps may be used as the only energy input.

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Silver (Ag) and gold (Au) nanostructures display remarkable optical properties in the visible range as a result of their localized surface plasmon resonance excitation (SPR).^1–6 Specially, Ag and Au nanorings have attracted interest in the field of plasmonics as their SPR can be tuned as a function of their sizes (along longitudinal and transverse directions), wall thickness, hollow interiors, and composition (in the case of AgAu nanorings).^7–9 However, many strategies for the synthesis of Ag and Au nanorings still rely on lithography techniques, which require special facilities and complex procedures.^10–12 Albeit chemical approaches have also been developed, they often rely on the use of 2D nanocrystals (nanoplates) as seeds and/or require the utilization of metals such as palladium (Pd) and platinum (Pt), making them unattractive for large-scale production.^9,13–15 Therefore, despite the progress, facile procedures for the synthesis of Ag and Au nanorings remain challenging.

It has been demonstrated that the SPR excitation in Ag and Au nanomaterials can be put to work to mediate and/or enhance a variety of catalytic transformations, such as oxidations,^16–18 reductions,^18–20 and coupling reactions.^17,18,20,21 Regarding practical applications in the field of plasmonic catalysis, the utilization of commercially available and inexpensive lamps as the SPR excitation source would be highly desirable. Consequently, the design of Ag and Au nanoparticles having SPR extinction that closely matches the emission spectra of commercial lamps represent an intuitive strategy to maximize performance towards SPR-mediated transformations.^22,23 However, most studies employing white light as the excitation sources (instead of lasers or LEDs) still focus on the utilization of conventional nanoparticles as catalysts, such as Ag and Au nanospheres, whose SPR extinction do not necessarily match the lamp emission. Thus, in principle, a large amount of light power may be wasted is these systems.^{18,20,22–24}

We report the rational design of AgAu nanorings displaying SPR extinction that closely matches the emission spectra of a commercial and inexpensive halogen–tungsten lamp by a facile strategy using Ag nanospheres as starting materials. The AgAu nanorings were employed as catalysts for the SPR-mediated oxidation of methylene blue in the liquid phase (water as the solvent), under ambient conditions, and using a halogen–tungsten lamp as the only energy input. We found that the catalytic activity for the AgAu nanorings was significantly higher as compared to its Ag and Au nanospheres counterparts. We also performed a series of control experiments to unravel the role of hot electrons and holes over the SPR-mediated oxidation of methylene blue.

We started our studies with the synthesis of AgAu nanorings having SPR positions that could match, as much as possible, the emission spectra of a commercial halogen–tungsten lamp which is shown in Fig. 1 (yellow trace). Although the halogen–tungsten lamp emission covers the entire visible range, the emission becomes increasingly larger as a function of the wavelength, i.e., it steadily increases from 400 to 800 nm. It can be observed that the SPR extinction of conventional Ag and Au nanospheres does not match the lamp emission spectra (Fig. 1, green and red traces respectively). In this case, Ag and Au nanospheres display SPR bands centered at 405 and ∼522 nm, respectively.

Fig. 1 UV-VIS extinction spectra recorded from aqueous suspensions containing Ag nanospheres (green trace), Au nanospheres (red trace), and AgAu nanorings (blue trace). The yellow trace shows the emission spectrum for the commercial tungsten–halogen lamp employed as the excitation source in our SPR-mediated catalytic experiments.

It has been established that the galvanic replacement reaction between Ag nanospheres and AuCl₄⁻_(aq) represents an effective strategy for tuning the SPR position in the entire visible range.^25–27 In this case, the control over the Ag to AuCl₄⁻_(aq) molar ratios enables one to control the composition, structure (solid vs. hollow interiors), optical properties, and catalytic activity of the obtained AgAu nanomaterials.^25–28 Rather than using 2D nanocrystals as seeds, AgAu nanorings having a SPR spectrum that closely resembles the emission spectra of the halogen–tungsten lamp could be obtained from the aforementioned galvanic replacement reaction using “conventional” Ag nanospheres as seeds as shown in Fig. 1 (blue trace). Specifically, the SPR spectra for the AgAu nanorings displayed two SPR bands: one shoulder located at 536 nm and a broad signal centered at 725 nm. The detected red-shift and broadening of the SPR in the nanorings relative to the Ag and Au nanospheres are in agreement with the Au deposition as well as the formation of the hollow interiors in during the galvanic reaction (Fig. 2A).^29–31

Fig. 2 (A) Synthesis of AgAu nanorings by galvanic replacement reaction between Ag nanospheres and AuCl₄⁻_(aq). TEM (B) and HRTEM (C and D) images for Ag nanospheres. SEM (E) and HRTEM (F and G) images for AgAu nanorings. The image in (D) and (G) correspond to phase contrast images of the areas highlighted by the white squares in (C) and (F).

Fig. 2B–D shows TEM and HRTEM images of the Ag nanospheres employed as templates (whose spectra is shown in Fig. 1). They were relatively uniform, polycrystalline, and 38 ± 3 nm in diameter. The 0.255 nm 〈111〉 lattice spacing characteristic of fcc Ag could be identified in the HRTEM image. The SEM and HRTEM images of AgAu nanorings (Fig. 2E–G) confirmed that they were also uniform. Moreover, they displayed well-defined shapes comprised of hollow interiors and ultrathin walls. The AgAu nanorings were 38 ± 4 nm in outer diameter and 8 ± 1 nm in wall-thickness. HRTEM images confirmed the presence of 0.255 nm 〈111〉 lattice spacing that can be assigned to the presence of fcc Ag and Au. ICP-OES results showed that the nanorings contained 81 at% in terms of Au, which is also in agreement with EDX spectra obtained from an individual nanoring (Fig. S1,† 80 at% in terms of Au). Fig. S2A and B† show the histograms of size distribution for Ag nanoparticles and AgAu nanorings. The XRD patterns (Fig. S3†) also confirmed the formation of Ag and AgAu nanostructures without the presence of any significant crystalline impurities.

The obtained AuAu nanorings presents two very attractive features for SPR-mediated catalytic applications: (i) their SPR extinction that matches the emission spectra of the halogen–tungsten lamp; and (ii) the ultrathin walls/hollow interiors that enable one to get higher surface to volume ratios, and thus surface areas, relative to their solid counterparts. In the next step, we employed the SPR-mediated oxidation of methylene blue (Fig. 3A) as a model transformation to benchmark the performance of the AgAu nanorings against its Ag and Au nanosphere counterparts. This reaction was carried out under ambient conditions, in the liquid phase (water as the solvent), and employing a commercial halogen–tungsten lamp (300 W) as the only energy input. For comparison, we also synthesized Au nanoparticles 39 ± 2 nm in diameter by a seeded growth approach as shown in Fig. S4.† ³² Their extinction spectra is shown in Fig. 1 (red trace).

Fig. 3 (A) SPR-mediated oxidation of methylene blue. C/C₀ (B) and ln(C/C₀) (C) profiles as a function of time employing Ag nanospheres (red trace), Au nanospheres (blue trace), and AgAu nanorings (blue trace) as catalysts. A blank reaction without any catalyst was also performed (green trace). (D) Pseudo-first order rate constants (k) calculated from (C). (E) Stability tests employing AgAu nanorings as catalysts.

Fig. 3B and C shown the variations in C/C₀ and ln(C/C₀) for the methylene blue oxidation as a function of time employing AuAg nanorings, Ag nanospheres, and Au nanospheres as catalysts (blue, green, and red traces, respectively) under visible light illumination. In all photocatalytic experiments, the temperature of the reaction mixture was carefully controlled and corresponded to 40 °C. It can be observed that the utilization of AgAu nanorings as catalyst led to an increase in the reaction rate relative to the nanospheres as illustrated by the C/C₀ profiles as a function of time (Fig. 3B). The C/C₀ ratios after 5 h corresponded to 0 (total oxidation), 0.22, 0.24, and 0.53 for AgAu nanorings, Au nanospheres, Ag nanospheres, and for a blank reaction, respectively. This behavior is also supported by the ln(C/C₀) profiles as a function of time (Fig. 3C) and the calculated pseudo-first-order rate constants (k) (Fig. 3D). For instance, k corresponded to 0.98, 0.23, 0.21, and 0.14 h⁻¹ for AgAu nanorings, Au nanospheres, Ag nanospheres, and blank reaction, respectively. All catalytic experiments were performed under the same metal loading, in which the metal concentration in all catalyst suspensions was adjusted to 2.2 mM (as measured by ICP-OES) by centrifugation of the suspensions followed by the addition of a water to re-suspend the nanostructures. Our results showed that AgAu nanorings displayed improved performances relative to Ag and Au nanospheres of similar sizes. The activity for the AgAu nanorings was 4.3 and 4.7-fold higher relative to the Au and Ag nanospheres, respectively. In order to exclude the effect of the nanorings bimetallic composition over the catalytic performances, we also prepared AgAu alloy nanoparticles having with similar compositions relative to the nanorings as shown in Fig. S5† (81 mol% in terms of Au). No significant differences in the SPR-mediated methylene blue conversion was observed for the bimetallic AgAu alloys as compared to the monometallic Ag and Au nanospheres (Fig. S6†).

It is plausible that the higher activity of the nanorings relative to the nanospheres can be explained both by its increased surface-to-volume ratios and better matching between its SPR extinction and the lamp emission spectra. It is important to note that, considering the surface area of the nanostructures and the metal loading employed in the catalytic experiments, the calculated surface areas corresponded 1630, 1570, and 5100 m² mol⁻¹ for Ag nanospheres, Au nanospheres, and AgAu nanorings, respectively (details on these calculations are provided in the ESI†). This indicates that the available surface area for the nanorings was 3.1 and 3.2-fold higher relative to Ag and Au NPs, respectively. Encouragingly, no degradation products were identified in the GC-MS chromatogram from the reaction mixture after the reaction, indicating that the methylene blue was completely mineralized by the SPR-mediated oxidation process (Fig. S7†).

The AgAu nanorings could be recovered from the suspension after at the end of the oxidation reaction by centrifugation and re-used for at least ten reaction cycles without any loss of activity, indicating their good stability towards this SPR-mediated transformation (Fig. 3E). Fig. S8A–D† show the UV-VIS spectra collected as function of time during the methylene blue oxidation in presence of AgAu nanorings (Fig. S8A†), Ag nanospheres (Fig. S8B†), Au nanospheres (Fig. S8C†), and in the absence of any catalyst (Fig. S8D†).

Fig. 4 shows the proposed mechanism for the SPR-mediated oxidation of methylene blue. The SPR excitation leads to the formation of hot electrons (electrons that transiently occupy levels above the Fermi level in the metal) and holes. In this case, hot electrons can be charge-transferred to adsorbed O₂ molecules to generate ⁻O₂ (activated oxygen). Meanwhile, holes can also react with water to form the hydroxyl radical (˙OH).^1,33–40 Therefore, its is plausible that both the activated oxygen and hydroxyl radical species can participate in the methylene blue oxidation. In order to confirm this hypothesis, we performed a series of control experiments. When the reaction was performed in the absence of external illumination, no oxidation of methylene blue was detected. When acetonitrile was employed as the solvent and the reaction was carried out in N₂, no degradation was detected even after 30 min of reaction. This is in agreement with the fact that neither ⁻O₂ oxygen or ˙OH species could be generated in the absence of O₂ and water. When the reaction was carried out in air (and acetonitrile as the solvent), 25% conversion after 30 min was detected (as opposed to 56% for the reaction under O₂ atmosphere and employing water as the solvent). This decrease in the conversion occurred because only ⁻O₂ species can be generated when no water is present. When water was employed as the solvent and N₂ as the reaction atmosphere, only 34% conversion after was detected. Here, no ⁻O₂ species were generated, and only ˙OH can participate in the methylene blue oxidation. Therefore, these results demonstrate that hot electron and holes have similar contributions towards the SPR-mediated catalytic oxidation of methylene blue.

Fig. 4 Proposed mechanism for the SPR-mediated oxidation of methylene blue employing AgAu nanorings as catalysts, in which the generation of both activated oxygen and hydroxyl radicals following the SPR excitation contributed to the oxidation of methylene blue.

In order to estimate the contribution from external thermal effects, we compared the conversion (%) with and without the visible-light excitation (Fig. S9†). In the absence of visible-light excitation and using 40 °C as the reaction temperature, a low conversion of methylene blue was detected (Fig. S9A†). Specifically, only 5% of conversion was observed after 24 h as compared to 100% after only 3 h under visible-light excitation. We also performed a control experiment at 80 °C (without visible-light excitation, Fig. S9B†), and no improvement in the conversion (%) were observed as compared to the reaction performed at 40 °C, indicating that thermal effects should play a minor role over the detected activities as compared to the charge-transfer mechanism involving hot electrons and holes.

In order to further illustrate the versatility of the AgAu nanorings as catalyst, we also studied their activity as heterogeneous catalysts towards the 4-nitrophenol reduction relative to the Au and Ag nanospheres as shown in Fig. S10.† The AgAu nanorings displayed significantly improved performances relative to the nanospheres, which can be assigned to their higher surface to volume ratios as enabled by their hollow interiors and ultrathin walls. They also could be re-used for at least ten reaction cycles without any loss of activity.

In summary, we have demonstrated that the utilization of a plasmonic nanomaterial, the AgAu nanorings, displaying SPR extinction that closely matches the emission spectra of a commercial halogen–tungsten lamp as catalyst led to superior performances towards the SPR-mediated oxidation of methylene blue relative to conventional nanoparticles (Ag and Au nanospheres). Rather than relying on the utilization of 2D nanocrystals as templates, Pd or Pt, complicated procedures, and expensive facilities, the AgAu nanorings could obtained by a facile galvanic replacement reaction between Ag nanospheres and AuCl₄⁻_(aq), making it very attractive for large-scale production. The SPR-mediated catalytic activity of the AuAg nanorings was 4.3 and 4.7-fold higher than Au and Ag nanospheres, respectively. Moreover, by performing a series of control experiments, we found that both the generation of hot electrons and holes contributed to the SPR-mediated oxidation of methylene blue. While hot electrons led to the generation activated oxygen species, holes led to the formation of hydroxyl radicals. Both these species can subsequently participate in the methylene blue oxidation process with similar contributions. We believe that our results demonstrate that the rational design of metallic nanoparticles plays an important role for enabling practical applications in the field of plasmonic catalysis, in which facile procedures can be employed for the synthesis of the catalysts (attractive for large-scale production) and commercial lamps may be used as the only energy input.

Acknowledgements

This work was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (grant numbers 2013/19861-6 and 2015/21366-9). T. S. R. thanks the CAPES, A. G. M. S. thanks CNPq, and I. G. F. thanks FAPESP for the fellowships.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11362a

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