Copper Ion Assisted Reshaping and Etching of Gold Nanorods

Article pubs.acs.org/JPCC

Copper Ion Assisted Reshaping and Etching of Gold Nanorods: Mechanism Studies and Applications Tao Wen,†,‡ Hui Zhang,†,‡ Xiaoping Tang,§ Weiguo Chu,∥ Wenqi Liu,†,‡ Yinglu Ji,† Zhijian Hu,† Shuai Hou,†,‡ Xiaona Hu,†,‡ and Xiaochun Wu*,† †

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, Beijing 100190, P. R. China ‡ University of the Chinese Academy of Sciences, Beijing 100190, P. R. China § Beijing Entry-Exit Inspection and Quarantine Bureau, Beijing 100094, P. R. China ∥ National Center for Nanoscience and Technology, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: This article describes copper ion assisted end-cap morphology transformation and anisotropic etching of gold nanorods (GNRs) under mild conditions. The copper ions are proposed to catalyze the oxidation of the GNRs by dissolved oxygen. We suggest that the surface Au atoms with high surface energy are stabilized by forming Au−O complexes (termed as static adsorbed oxygen). At low concentration of copper ions, the Au−O complexes are removed by oxidative etching, thus leaving a “clean” Au surface. Such a clean surface can relax to a more stable state via surface atom diffusion, leading to the GNR end-cap morphology variation. This is the first time that the existence of adsorbed oxygen species on the surface of the GNRs is probed and demonstrated. At high concentration of copper ions, anisotropic etching of the GNRs is initiated by dissolved oxygen (termed as dynamic adsorbed oxygen) and leads to shorter GNRs. For other etch agents such as H2O2 and Fe3+, addition of copper ions produces a synergistic effect. Due to the high affinity of oxygen to silver and palladium, such synergistic etching is observed in Au@Ag NRs and Au@Pd NRs as well. These interesting findings provide a new way to probe the surface reactivity of noble metal nanocrystals.

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INTRODUCTION Currently, seed-mediated growth is the most widely used synthesis method for gold nanorods (GNRs), first proposed by Murphy et al.1 and later developed by the El-Sayed group.2 In comparison with other noble metal nanocrystals, significant advancements in the controlled synthesis, including aspect ratio (AR), detailed morphology (especially for end-cap), size and size distribution, and rod yield, have been achieved for GNRs.3 The effects of various growth parameters, such as ascorbic acid or other reducing agents, surfactant, HAuCl4, halogen ions, and shape-regulating additives, have been extensively studied and optimized.2,4−12 For instance, GNRs with various end-caps, such as dog-bone structures, arrow, and spherical-ended dumbbells, have been obtained by controlling growth conditions.13−20 For metal ion additives, the most studied one is silver ions, and Ag underpotential deposition is suggested to be responsible for the unique structure of the GNRs.2,8,21 On the basis of the similar idea, the role of copper ions in morphology control has been explored recently. Keul et al. obtained predominantly {111} faceted gold particles in the presence of Cu2+ and proposed that the difference in Cu underpotential deposition on crystallographic surfaces is dominant.22 Similarly, Sun et al. also observed that increasing Cu2+ concentrations, the main shapes of the gold nanoparticles © 2013 American Chemical Society

can be changed from rods, cuboids, to decahedra, respectively. They proposed that Cu2+ ions can selectively retard the growth of the {111} lattice plane.23 In seed-mediated synthesis of Pd nanorods and branched Pd nanocrystals, Chen et al. also suggested that Cu underpotential deposition plays a critical role in shape control.24 Quite differently, we found another role of copper ions in the seed-mediated growth of the GNRs.25 The size distribution of the GNRs can be greatly improved by adding copper ions in the growth solution. We suggest that size-focused etching of the Au seeds is responsible for narrowing the size distribution of the GNRs, and copper ions catalyze the Au seed oxidation by dissolved oxygen. Furthermore, involvement of copper ions significantly accelerates the growth kinetics, leading to dumbbell GNRs just after growth. Very interestingly, overnight storage changes such morphology to cylinder shape with half-sphere heads. The role of dissolved oxygen in the growth of GNRs was first demonstrated and revealed with the help of copper ions. As is known, etching is also a useful way to fabricate nanostructures. It thus inspires us to explore the role of copper Received: August 3, 2013 Revised: October 17, 2013 Published: November 19, 2013 25769

dx.doi.org/10.1021/jp407774s | J. Phys. Chem. C 2013, 117, 25769−25777

The Journal of Physical Chemistry C

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ions in other etching processes. Tsung et al.26 reported an effective method for shortening GNRs by making use of dissolved oxygen. Usually, the etching conditions are somehow harsh, requiring a high acid concentration (about 1 M HCl) and increased temperature (70 °C) with continuous stirring and bubbling of O2. It finally led to their modification of the original procedure by changing to a stronger oxidant (H2O2).27,28 In addition, other etchants (cyanide29 or Au(III) ions30 and Fe(III)31) are also employed to initiate anisotropic oxidation, sometimes with extra assistance of heating32 or laser heating.33 So, can the catalytic role of copper ions be transferred from very reactive small Au seeds to much larger nanocrystals and make etching processes happen in much milder conditions or even more by endowing more tunability and controllability for the etching process? First, we studied the role of copper ions in detail using dogbone GNRs as examples. We suggest the following mechanism for copper ions: copper ions can catalyze the oxidation of the GNRs by dissolved oxygen. Low pH and high concentration of CTAB both accelerate the oxidation. Surface Au atoms, especially those with high reactivity, are easily attacked by oxygen via the formation of the Au−O complex. In the presence of copper ions, the complex is decomposed to produce Au+ and H2O, thus leaving a “clean” Au surface. The GNR can change to a more stable morphology via further surface Au atom diffusion. Both static and dynamic adsorbed oxygen are demonstrated. Therefore, copper ions can be used as a probe to sense the reactivity of surface Au atoms. At very high copper ions, Ostwald ripening and the fusion process can be observed especially on gold nanocrystals with smaller sizes. On the basis of the similar role, copper ions exhibit a synergistic role in other etchants, such as hydrogen peroxide and Fe(III), via scavenging adsorbed oxygen species and providing more Au atoms for etchants. Under optimized conditions, accelerating etching and narrowing the size distribution can be achieved under mild conditions. Such a role of copper ions can be extended to other noble metals, such as Ag and Pd.

For a typical preparation of the GNRs, in a growth solution consisting of a mixture of CTAB (10 mL, 0.1 M), HAuCl4 (200 μL, 25 mM), AgNO3 (70 μL, 10 mM), H2SO4 (100 μL, 1 M), and AA (80 μL, 0.1 M) was added seed solution (24 μL) to initiate the growth of GNRs. The growth temperature was kept at 30 °C. After 12 h, the GNRs were purified by centrifuging the solution at 12 000 rpm for 5 min twice. The precipitate was collected and redispersed in deionized water. The GNR concentration is estimated using ICP-MS and TEM. ICP-MS is employed to determine the total concentration of Au atoms in a given GNR suspension. According to the molar volume of a gold atom (10.2 cm3/mol), we obtain the total volume (V) of all GNRs in the suspension. From the TEM images, we measure the diameters and lengths of GNRs and then calculate the mean volume of a single GNR (V0) by assuming a cylindrical shape with two half-sphere end-caps. The GNR molar concentration in the suspension can be estimated as V/ AV0, where A is Avogadro’s constant. Flat or spherical end-caps of GNRs are obtained with the same method except different concentration of AgNO3. The end-caps of GNRs tend to be spherical with low AgNO3, and they tend to be flat at a high concentration of AgNO3. Typical Synthesis of Arrow-Headed GNRs. Arrowheaded GNRs were prepared as reported previously.19 In a typical synthesis of the arrow-headed GNRs, HAuCl4 (24 mM, 10 μL) was added to a solution containing the GNRs (1 mL), CTAB (2 mL, 0.1 M), AA (0.1 M, 24 μL), and AgNO3 (10 mM, 24 μL). The mixture was then diluted with deionized water to give a final volume of 6 mL and was kept in a water bath at 30 °C for 12 h. The sample was purified by centrifuging the solution at 12 000 rpm for 5 min. Typical Synthesis of Au@Ag NRs. Au@Ag NRs were prepared as reported previously.34 Typically, the purified GNR solutions (1 mL) were mixed with CTAB (1 mL, 0.1 M), H2O (1 mL), AgNO3 (10 mM, 40 μL), NaOH (0.2 M, 40 μL), and AA (0.1 M, 40 μL). The mixture was kept in a water bath at 30 °C for at least 2 h. The sample was purified by centrifuging the solution at 12 000 rpm for 5 min. Typical Synthesis of Au@Pd NRs. Au@Pd NRs were prepared as reported previously.35 Typically, the purified GNR solutions (1 mL) were mixed with CTAB (300 μL, 0.1 M), H2O (1 mL), K2PdCl4 (2 mM, 75 μL), and AA (0.1 M, 15 μL). The mixture was kept in a water bath at 30 °C for 12 h. The sample was purified by centrifuging the solution at 12 000 rpm for 5 min. Reshaping. Dogbone-like GNRs, synthesized with a [AA]/ [Au3+] ratio of 5, were employed to conduct reshaping experiments. Purified dogbone-like GNRs were dispersed in a CTAB aqueous solution. Different concentration combinations of CTAB, Cu2+ ions, and H2SO4 were used for reshaping at different temperatures. All the other experiments were carried out in a 30 °C water bath except the research on the temperature effect. Characterizations. UV−vis−NIR spectra were recorded on a Varian Cary 50. Transmission electron microscopy (TEM) images were obtained with a Tecnai G2 20 S-TWIN operating at an acceleration voltage of 200 kV. The values of elements were from an inductively coupled plasma mass spectrometer (ICP-MS) (NexION300D).

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EXPERIMENTAL SECTION Materials. Sodium borohydride (NaBH4), chlorauric acid (HAuCl4·3H2O), cetyltrimethylammonium bromide (CTAB), hexadecyl trimethyl ammonium chloride (CTAC), sodium citrate, silver nitrate (AgNO3), L-ascorbic acid (AA), 2,6pyridinedicarboxylic acid (PDCA), and potassium tetrachloropalladate(II) (K2PdCl4) were purchased from Alfa Aesar and used as received. Sodium sulfate (Na2SO4), sodium sulfite (Na2SO3), sodium hydroxide (NaOH), sulfuric acid (H2SO4), copper(II) chloride dihydrate (CuCl2·2H2O), copper(II) nitrate trihydrate (Cu(NO3)2·3H2O), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O), and hydrogen peroxide (H2O2, 30%) were at least analytical reagent grade and purchased from Beijing Chemical Reagent Company (Beijing, China). Milli-Q water (18 MΩ cm) was used for all solution preparation. Typical Synthesis of GNRs. GNRs were synthesized using the well-developed seed-mediated growth method. Briefly, CTAB-capped Au seeds were synthesized by chemical reduction of HAuCl4 with NaBH4: CTAB (7.5 mL, 0.1 M) was mixed with HAuCl4 (100 μL, 25 mM) and diluted with water to 9.4 mL. Then, ice-cold NaBH4 (0.6 mL, 0.01 M) was added under magnetic stirring. The solution color turned immediately from bright yellow to brown, indicating the formation of Au seeds. The Au seeds were used within 2−5 h.

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RESULTS AND DISCUSSION Mechanism of Copper Ion Assisted End-Cap Morphology Conversion for the GNRs. To demonstrate the role of 25770



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(see red circle in Figure 1A). In the extinction spectra of GNR suspensions, the relatively smooth region from 350 to 450 nm reflects the interband transitions which are independent of particle shapes.36,37 Therefore, the extinction values at this range can be used to evaluate the total amount of Au atoms. At a given GNR concentration, the extinction values at a chosen position can describe the size (volume) change of the GNRs. Herein, we use the extinction values at 400 nm (E400nm) to reflect the rod volume variation. From extinction spectra, the evolution process can be divided into two stages (Figure S1, Supporting Information): a fast end-cap morphology conversion and a subsequent slow etching. During the first 10 h, we observe a 14% decrease in E400nm with a large blue shift of 145 nm in the LSPR maximum (blue-shift rate of 17 nm/h). The great blue shift reflects the shape change, and the reduction in E400nm indicates that the reshape process is accompanied by etching. We suggest that some surface Au atoms with high surface energy are bound with “static” adsorbed oxygen species. In this way, on one hand, their surface energy is decreased; on the other hand, the less stable morphology of the dog-bone is also stabilized. The full width at half-maximum (fwhm, 0.25 eV) of the LSPR band first increases a little and then decreases to around 0.22 eV, reflecting the variation in end-cap shape distribution. In the second stage, the LSPR shows quite a small change (peak blue-shift from 632 to 629 nm and 7% decrease in E400nm in 38 h), which means a quite slow etching process. We suggested that conversion of the end-cap shape includes two steps: first, copper ions catalyze surface Au atom oxidation and then subsequent surface atom diffusion. The oxidation reaction can be described as

copper ions more clearly, we use dog-bone GNRs as the example (Figure 1A). After incubation with CTAB, acid, and

Figure 1. Typical TEM images of original dog-bone GNRs (A) and after 10 h shape conversion (B) and evolution of UV−vis−NIR spectra of the GNRs during shape conversion (C). Reaction conditions: [GNRs] = 0.5 nM, [CTAB] = 0.1 M, [H2SO4] = 10 mM, [Cu2+] = 100 μM.

copper ions for 10 h, the dog-bone GNRs change their end-cap morphology to half-sphere (Figure 1B). Notice that the dogbone cuboids (red circle in Figure 1) also become rounded at the corners. Such end-cap conversion affects their localized surface plasmon resonance (LSPR) features greatly. The original dog-bone GNRs have a LSPR maximum at 778 nm and a transverse SPR (TSPR) at 510 nm. A third shoulder peak at around 586 nm (Figure 1C) originates from dog-bone cubes

Cu 2 +

4Au 0 + 8Br − + O2 + 4H+ HooooI 4AuBr2− + 2H 2O

(1)

For this mechanism, CTAB, pH, dissolved oxygen, and copper ions should all play a role in this process. We first investigated

Figure 2. Effect of copper ions, CTAB, and acid concentrations: LSPR maximum shift versus time for copper ions (A), CTAB (B), and H2SO4 (C). LSPR blue shift rate versus all the concentration of reagents (D). Reaction conditions: (A) [CTAB] = 0.1 M, [H2SO4] = 10 mM, [Cu2+] = 0 (a), 50 (b), 100 (c), 500 μM (d) and 1 mM (e); (B) [Cu2+] = 100 μM, [H2SO4] = 10 mM, [CTAB] = 10 (a), 20 (b), 50 (c), 100 (d), and 200 mM (e); (C) [Cu2+] = 100 μM, [CTAB] = 0.1 M, [H2SO4] = 0 (a), 5 (b), 10 (c), 25 (d), 50 (e), and 100 mM (f). All with dog-bone GNRs, [GNRs] = 0.5 nM. 25771



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the effect of the concentration of copper ions. In the absence of copper ions, the LSPR maximum shows negligible change after 20 h incubation at a 30 °C water bath. By increasing copper ions, the LSPR blue-shifts faster, and a linear relationship is observed between the blue-shift rate and copper ion amount (Figure 2D). Higher copper ions lead to faster end-cap variation and enhanced etching thereafter. During the conversion process, fwhm often increases a little first and then decreases to a lower value close to the end point of conversion, possibly due to the existence of different end-caps. According to eq 1, the effects of CTAB and pH are as predicted: more CTAB or lower pH accelerates the shape conversion process (Figure 2B and C). CTAB molecules provide Br− ions to bind produced gold ions,31,38 thus stabilizing the product and accelerating the etching process.39 At [CTAB] = 200 mM (Figure S2A, Supporting Information), the fast LSPR blue-shift is finished in 5 h with 20% reduction at E400nm. Note that, at such high CTAB concentration, etching of the GNRs is greatly enhanced. For [CTAB] = 50 mM, the blueshift needs more than 20 h. For shape conversion, the optimal CTAB concentration is ca. 100 mM, with a fast reshape rate, slight etching, and obvious improvement in fwhm. The reaction rate is proportional to the concentration of CTAB (Figure 2D). The reshaping process is very sensitive to acid concentration and exhibits an exponential dependence. Addition of acid increases the oxidation potential of the half-reaction involving oxygen.26 A higher acid concentration (>10 mM) can also enhance the etching of GNRs (E400nm decreases more), so the optimal acid concentration for the end-cap reshaping is chosen to be 10 mM H2SO4 considering a proper reshaping rate. To prove the effect of dissolved oxygen, we use Na2SO3 as an oxygen scavenger40 in the etching of half-sphere head GNRs. Higher copper ions were employed to show the effect more obviously. As Figure S3 (Supporting Information) shows, the LSPR maximum has nearly no change in Na2SO3 solution (Figure S3B, Supporting Information) after adding copper ions, while it exhibits a 56 nm buleshift (from 720 to 664 nm) within one hour in the control solution (Figure S3A, Supporting Information). The reaction rate can be easily regulated by tailoring the concentrations of CTAB, acid, and copper ions. For instance, for a 0.5 nM dog-bone GNR suspension, an optimized reshaping condition is: [CTAB] = 0.1 M, [H2SO4] = 10 mM, and [Cu2+] = 100 μM. The removal of active surface Au atoms via oxidative etching can be justified by adding reducing agent ascorbic acid (AA) after rehsaping. After 12 h reshaping, the obtained Au+ ions can be reduced back to the GNRs by AA. The increase in E400nm and LSPR intensity indicates the growth of the GNRs (Figure S4, Supporting Information). To demonstrate the role of copper ions better, we further measured the apparent activation energy (Eaapp) of the GNR reshaping. The reshaping rates, described as the shift of the LSPR maximum per minute, are obtained from time evolutions of extinction curves at different temperatures (Figure S5, Supporting Information). Using an Arrhenius plot, the activiation energy in the presence of copper ions (Ea(Cu)) is estimated to be (64.3 ± 3.6) kJ/mol (R2 = 0.9873) from Figure 3B. In the absence of copper ions, we cannot obtain Ea due to negligible reshaping reaction at this temperature range (even at 80 °C within the same period, Figure S5A, Supporting Information). The corresponding mechanism is shown in Figure 3C: dissolved oxygen has a high affinity for the surface Au atoms at high curvature parts of the rod due to their high

Figure 3. UV−vis−NIR extinction spectra (A) of the dog-bone GNRs after reshaping at different temperatures (a: original GNRs, b: 30 °C, c: 40 °C, d: 50 °C, e: 60 °C, f and g: 80 °C) with 100 μM (b−f) or without Cu2+ ions (g). Arrhenius plot of the reshaping rate versus reciprocal of the temperature (B) and schematic (C) of the reshaping activation energy for the dog-bone GNRs in the presence of copper ions.

surface energy. They can occupy these active Au sites and stabilize them via forming gold−oxygen complexes. We believe that the dog-bone GNRs are stabilized by such oxygen adsorption, although such adsorption has not been considered previously. However, some recent reports suggest that surface oxides are responsible for singlet oxygen production in noble metal nanocrystals and GNRs.41−43 In the presence of copper ions, due to the strong affinity of copper ions to oxygen atoms, they can drag the adsorbed oxygen from the Au atoms via surface etching, leaving a “clean” Au surface. The new “surface” can reconstruct to a more stable morphology via surface atom diffusion or is available for other reactions (such as further etching). Often, the active surface Au atoms locate at the high tip curvature parts of the GNRs. We therefore employed flat-head and arrow-head GNRs to verify our idea. Indeed, these two GNRs also take place during end-cap transformation: both NRs change to half-sphere-like end-caps (Figure 4). For the flat-head GNRs, statistics from TEM images shows that the average length of the GNRs decreases from (52.5 ± 7.0) to (51.7 ± 6.9) nm, while the average diameter increases from (16.9 ± 25772



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concentrations higher than 1 mM, anisotropic etching of the GNRs can be initiated in conjunction with 0.1 M CTAB and 10 mM H2SO4 at 30 °C. The LSPR maximum changes more with higher concentration of Cu2+, and the size distribution can be improved to a certain degree by controlling reaction conditions (Figure S6, Supporting Information). With the help of copper ions, Tsung’s26 recipe can be realized at 30 °C (Figure S7, Supporting Information). In the presence of 100 μM copper ions, a 10% decrease in E400nm is achieved in one hour (Figure S7b, Supporting Information), while the change is less than 2% (Figure S7a, Supporting Information) in the absence of copper ions. So it can be a good way to obtain GNRs with different aspect ratios and an improved monodispersity. From the viewpoint of stability, spherical gold nanoparticles (GNPs) are thermodynamically more stable. It stimulates our curiosity to see what will happen for spherical GNPs. As a first try, we use two spherical GNPs with diameters of about 56 and 18 nm, respectively. Intriguingly, a particle size dependent phenomenon is observed. From TEM images of the 56 nm GNPs, there exist some elongated and irregualr GNPs in the original sample. They are responsible for the long-wavelength tailing of the extinction spectra (Figure S8A inset, Supporting Information). These particles are less stable and are more easily attacked in the etching process. After the exposure to etching conditions, they become more round as reflected from the obvious decrease in long-wavelength tailing of the extinction spectra and TEM images. Statistics from TEM images (Table S1, Supporting Information) show that the diameters of the GNPs do not show obvious change before and after etching. Therefore, etching makes 56 nm GNPs have a better sphericity. In the case of 18 nm GNPs, at copper ions lower than 1 mM, the sizes of the GNPs increase slightly (a slight red-shift of the SPR maximum is observed, Figure S9A and Table S2, Supporting Information). At 5 mM copper ions, particle fusion takes place and leads to the formation of some big and irregular

Figure 4. UV−vis−NIR extinction spectra for flat-head (a, b) and arrow-head (c, d) GNRs before (a, c) and after (b, d) reshaping (A) and TEM images (B, C, D, E) for lines (a, b, c, d) in (A). The scale bar is 100 nm. Reaction conditions: [GNRs] = 0.5 nM, [CTAB] = 0.1 M, [H2SO4] = 10 mM, [Cu2+] = 100 μM.

2.1) to (18.4 ± 2.2) nm in 5 h, verifying the existence of reshaping: the gold atoms migrate from the end-caps to the side facets of the rod. This process also results in fwhm decreasing from 0.41 to 0.33 eV. AR distribution from TEM measurements decreases slightly (from 3.2 ± 0.6 to 2.8 ± 0.5). We hence ascribe the fwhm reduction mainly to the decreased endcap shape distribution after reshaping. Note, for the flat-head NRs, due to less reactive surface Au atoms, that we observe a quite slight decrease in E400nm. In contrast, we observe a 10% decrease in E400nm for arrow-head GNRs. Apart from probing the reactive surface Au atoms, with the help of copper ions, we can even distinguish the stability of Au nanocrystals. At higher copper ions, we can visualize the etching of the GNRs by dissolved oxygen. With 1 mM copper ions, a 25% decrease in E400nm is observed within 16 h. Increasing copper ions to 10 mM, a 55% decrease is achieved at the same time period, indicating that the etching degree of the GNRs can be manipulated by the concentration of copper ions (Figure S6, Supporting Information). Indeed, at copper ion

Figure 5. Synergistic effect of Cu2+ ions on H2O2 etching GNRs: evolutions of extinction spectra of the GNRs in the presence of 30 μM Cu2+ (A), 10 mM H2O2 (B), 30 μM Cu2+ and 10 mM H2O2 (C), and LSPR shift vs etching time (D). Lines a, b, and c in (D) are for (A), (B), and (C), respectively. a+b is the linear addition of line a and b. Other reaction conditions: [CTAB] = 0.1 M, [H2SO4] = 5 mM. Inset in (C) is the schematic of anisotropic etching GNRs. 25773



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(R2 = 0.9845) in the presence of copper ions (Ea(H2O2+Cu)). The reaction process can be described as follows (Figure 6B): there is a dynamic equilibrium between dissolved oxygen in the suspension and adsorbed oxygen on the surface of GNRs. In the absence of copper ions, H2O2 directly oxidizes surface Au atoms unoccupied by adsorbed oxygen species and leads to the etching of the GNR. The equation is as follows

gold nanocrystals (Figure S9F, Supporting Information). Therefore, 18 nm GNPs are less stable in comparison with 56 nm GNPs. They tend to become more stable by growing larger via Ostwald ripening44 or even fuse together. Thus, we propose that a combination of copper ions and dissolved oxygen can be used to detect the reactivity of surface Au atoms and size-dependent stability. Application 1: Synergistic Effects of Copper Ions for Other Etchants (H2O2 and Ferric Ions). The next interesting question is: do adsorbed oxygen species play a role for other etchants? We choose two reported etchants for GNRs: H2O2 and Fe3+ ions. The GNRs with hemisphere end-caps are employed. As shown in Figure 5, for the GNRs incubated with 30 μM CuCl2 for 1 h, only an 18 nm blue-shift in LSPR maximum and 5.9% decrease in E450nm (here, we use E450nm instead of E400nm due to the influence of reaction product Au3+) are found. After incubation with 10 mM H2O2 for 1 h, we observe a 26 nm blue-shift and a 7.1% decrease in E450nm. In contrast, in the coexistence of 30 μM CuCl2 and 10 mM H2O2, much faster etching is achieved, as evidenced by an 86 nm blueshift and a 20.8% decrease in E450nm. Obviously, copper ions can also accelerate H2O2-induced etching for GNRs. Considering the difference in the concentration of dissolved oxygen (ca. 0.25 mM) and H2O2 (10 mM) in the etching solution, the affinity of the former to the Au surface is quite high. In the Fe3+ ion etching system, we use a low concentration of Fe3+ ions (1 mM) instead of 55 mM as in ref 31 and Cu(NO3)2 instead of CuCl2 to ignore the effect of Cl− mentioned in the reference. A similar synergy effect is observed as well (Figure S10, Supporting Information). Generally, addition of the catalyst decreases the activiation energy. Indeed, we observe an increased etching rate with a lowered Ea in the H2O2 etching system. To avoid interference from formed Au3+ ions around 400 nm, the LSPR shift per minute is used to denote etching rate (nm/min). Using an Arrhenius plot, the apparent activation energy of the GNR etching is obtained as shown in Figure 6A. In the absence of copper ions, Ea(H2O2) is estimated to be (53.8 ± 6.2) kJ/mol (R2 = 0.9482). In contrast, it decreases to (46.8 ± 2.9) kJ/mol

2Au 0 + 8Br − + 3H 2O2 + 6H+ → 2AuBr4 − + 6H 2O (2) 2+

2+

In the presence of Cu ions, Cu ions help remove adsorbed oxygen species from the GNR surface and leave more free Au atoms for H2O2 to oxidize. According to this mechanism, a competitive adsorption between H2O2 and dissolved oxygen onto the Au surface should exist. At higher concentration of H2O2, the effect of dissolved oxygen should be neglected. At 1 M H2O2, no synergistic effect is observed in the presence of copper ions as shown in Figure S11A (Supporting Information). Similar results are observed for Fe3+ ion etching (Figure S11B, Supporting Information). Furthermore, the dynamic adsorption of dissolved oxygen can be verfied by adding 2,6pyridinedicarboxylic acid (PDCA)45 to mask copper ions during the etching process as shown in the H2O2 etching system in Figure S12 (c and e) (Supporting Information). After etching for 10 min, addition of PDCA obviously suppresses etching. To our surprise, in the case of Fe3+ etching, we obtain an increased Eaapp in the presence of copper ions. The extinction change at 400 nm per minute is used to describe etching rate. As Figure S13 (Supporting Information) shows, in the absence of copper ions, Ea(Fe) is estimated to be (48.1 ± 4.2) kJ/mol (R2 = 0.9697), while it increases to (77.4 ± 3.8) kJ/mol (R2 = 0.9906) in the presence of copper ions (Ea(Fe+Cu)). At the moment, we have not found the explanation for this. By comparison of the pre-exponential factors (A) of the two etchants, we have found that in the Fe3+ system A increases significantly with LnA(Fe+Cu) and LnA(Fe) of (27.1 ± 1.4) min−1 and (14.8 ± 1.6) min−1, respectively. In contrast, in the H2O2 system, A has no obvious change with LnA(H2O2+Cu) and LnA(H2O2) of (19.3 ± 1.1) min−1 and (20.3 ± 2.4) min−1, respectively. We hypothesize that in the Fe3+ system the synergistic role of copper ions is more complex. Furthermore, in the case of Fe3+ etching, after preincubation of Cu2+, PDCA, and the GNRs for 10 min, involvement of PDCA enhances the etching (Figure S14f, Supporting Information). The PDCA can also chelate Fe3+ ions46 (Figure S14e, Supporting Information), which however does not explain the enhanced etching, and the mechanism is unclear at the moment. We suppose there are several complex reactions in the Fe3+ ion etching system, and they lead to these fancy and unusual results. Obviously, in the Cu2+-assisted etching system, due to more flexibility in choosing etching parameters, the size-focusing etching can be achieved more easily (Figure S15 and Table S3, Supporting Information). Application 2: Extension to Other Noble Metals. In comparison with Au, Ag and Pd are more easily oxidized. The tailoring role of copper ions should be more obvious. Indeed, we observe much milder etching conditions for them upon adding copper ions. The end-cap reshaping conditions of the GNRs are strong enough for the etching of Ag and Pd. As shown in Figure 7, addition of copper ions can lead to significant etching of the Ag and Pd shell by dissolved oxygen. The etching process can be divided into two parts. In the case

Figure 6. Arrhenius plot of the etch rate versus the reciprocal of temperature for H2O2 etching GNRs in the absence (dark square) and presence (red circle) of Cu2+ ions (A) and schematics of reaction processes (B). 25774



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Figure 7. UV−vis−NIR extinction spectra of Au@Ag NRs (A) and Au@Pd NRs (B) after etching for 12 h at different conditions: (b) 0.1 M CTAB, (c) 10 mM H2SO4 and 0.1 M CTAB, (d) 0.1 M CTAB and 100 μM Cu2+ ions, (e) 0.1 M CTAB, 10 mM H2SO4, and 100 μM Cu2+ ions. (a) is the original GNR core. Insets in (A) and (B) are the spectra normalized at E400nm of lines a and e. (C) TEM images before (a, c) and after etching (b, d) with the scale bars of 100 nm for Au@Ag (a, b) and Au@Pd (c, d) NRs. (e) is the schematic of isotropic etching Au@Ag and Au@Pd NRs. (D) LSPR maximum vs etching time for line (e) in (A) and (B). a, b, and c are original Au cores, Au@Ag, and Au@Pd NRs, respectively.

of Au@Ag NRs, the first part corresponds to the rapid etching of the Ag shell, as witnessed by the fast red-shift of the LSPR maximum and disappearance of Ag shell features at the 300− 450 nm region. The second part corresponds to the slow etching of the GNR cores. In the case of Au@Pd NRs, similar results are obtained. A blue-shift of the LSPR maximum and the appearance of Pd2+ ion complexes (complexing with CTAB) absorption bands at 252 and 307 nm indicate the rapid etching of the Pd shell (Figure 7B). After that, the etching of the inside Au core is initiated. In comparison with GNRs, two differences are observed for Au@Ag and Au@Pd NRs. The GNRs exhibit anisotropic etching, mainly along the long axial direction of the rod. In contrast, both Ag and Pd demonstrate mainly isotropic etching. Second, the inside GNR core seems to be etched more easily than pure GNRs (Figure 7D). As shown in Figure 7A and B insets, the Au core after the removal of the Ag shell exhibits an enhanced LSPR intensity compared to template GNRs. In contrast, the Au core after the removal of the Pd shell exhibits a damped LSPR intensity compared to template GNRs. It suggests the possible residual of Ag and Pd. Indeed, we observed the existence of Ag and Cu in the final Au@Ag NRs and Pd and Cu in the final Au@Pd NR samples via ICP-MS characterization, respectively. We suppose that there may exist an alloy interface, which leads to enhanced etching of Au cores.

etchants, as we demonstrated here for hydrogen peroxide and Fe3+. At optimized conditions, both accelerated etching and improved size distribution are obtained. Such a catalytic role of copper ions can be used to two levels: the first level is probing the surface reactivity. Less regular GNRs change their morphology via a reshape process to reach the minimum surface energy. The static adsorbed oxygen species are more important. The second level reflects the particle activity as we demonstrated for aspect ratio (GNRs) and particle size (GNPs) dependent etching, where dynamic adsorption of dissolved oxygen contributes dominantly. Small GNPs undergo obvious Ostwald ripening and fusion where larger ones remain stable at higher copper ions. For the first time, using copper ions, we have probed and demonstrated the existence of adsorbed oxygen species on the surface of the GNRs and extended this role to Au@Ag and Au@Pd NRs. Considering the unique role of copper ions, we expect many more interesting phenomena awaiting ahead.

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ASSOCIATED CONTENT

S Supporting Information *

Details for UV−vis−NIR spectra of different GNR solution, TEM images, tables of characteristic parameters of the GNRs with different conditions, and the used synthetic method for GNPs. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS In summary, using dog-bone GNRs, we give a complete picture of the role of copper ions in the reshaping and etching of GNRs: dissolved oxygen has a strong affinity to the surface Au atoms with high surface energies. They adsorb on these sites by forming Au−O complexes. Copper ions can catalyze the decomposition of Au−O complexes, possibly by assisting electron transfer from Au to O. At relatively low concentrations of copper ions, conversion of end-cap morphology of the GNRs is realized. At relatively high concentration of copper ions, etching of the GNRs along the long axis of the rod is achieved. The driving force is the minimization of surface energy. The synergistic role of copper ions can also be employed to other

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-10-82545577. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by National Key Basic Research Program of China (2012CB934001 and 2011CB932802) and National Natural Science Foundation of China (Grant No. 25775



Article

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Copper Ion Assisted Reshaping and Etching of Gold Nanorods

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