Laser-induced modification of dog-bone-like Au nanorods for accurate growth of well-defined cylindrical structures

Abstract

Gold nanorods (Au NRs) with regular cylindrical structures can be prepared by the conventional seed-mediated growth method. But for every success, there are dozens of failures. In most cases, a significant fraction of irregular Au NRs with dog-bone-like structures are also frequently formed. Therefore, a complementary way (the reshaping of irregular rods, etc.) to reduce the defective rate in the fabrication of regular Au NRs is urgently desired. In this paper, a novel and effective strategy for accurate growth of well-defined cylindrical Au NR structures with fairly good uniformity is developed by laser-induced shape-modification of dog-bone-like Au NRs. In the first step, large-scale irregular shaped Au NRs with dog-bone-like structures were prepared by the standard seed-mediated growth approach. Subsequently, highly mono-dispersed perfect Au NRs with ultra-smooth surfaces can be obtained by non-focused laser irradiation of the irregular Au NR solution. The irradiation parameters such as laser power density, irradiation time and laser wavelength play an important role in the accurate growth of Au NRs. At optimum conditions (∼5 W cm⁻², <100 s, 1064 nm), a laser-induced reshaping process in the growth solution can also be employed to grow the cylindrical Au NRs into longer structures with aspect ratios ranging from about 2.3 to 5.6. After laser irradiation, the absorption spectra of the Au NRs show that longitudinal localized surface plasmon resonance (LSPR) peaks can be effectively modulated from about 750 nm to 810 nm. On the other hand, by adding some H₂PtCl₆ in the as-prepared Au NR growth solution, laser irradiation will lead to a guided growth behavior of a Pt–Ag structure, resulting in the fabrication of Pt–Ag nano-islands on Au NRs. Thus, it can be seen that the laser-induced modification is a simple, low-cost and high-throughput strategy to prepare noble metal perfectly cylindrical nano-structures with multiple structural controlling capabilities.

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Introduction

Well-defined cylindrical gold nano-rods (Au NRs) with fairly good uniformity and ultra-smooth surfaces have received increasing attention because of their unique optoelectronic properties, and tunable localized surface plasmon resonance (LSPR) in the visible and near-infrared (NIR) region.^1–7 Increasing evidence has shown that the excellent properties of Au NRs are strongly dependent on their size and shape. Compared with the size of Au NRs, the perfectly regular shape plays a crucial role in tailoring the properties. For example, for roughly irregular Au NRs such as dog-bone-like or dumbbell-shaped structures,⁷ etc., their longitudinal LSPR is characterized by disorder or uncontrolled surface plasma resonance (SPR), which will limit the LSPR from being effectively tuned from the visible to NIR region by changing the aspect ratio. Compared with irregular shaped nano-rods, the perfectly cylindrical Au NRs with shaped- and size-directed LSPR properties have interesting properties in photochemical/physical catalysis, biomedical sensors, novel-photo-degradation, etc.^1–8 Moreover, due to the clearly and appreciably longitudinal SPR, the enhanced electromagnetic field near Au NRs can be employed in side surface-enhanced Raman spectroscopy, which has opened up the possibilities of using the perfect Au NRs for in vitro and in vivo anatomical imaging and early stage tumor diagnosis with deep tissue penetration.^7–9 On the other hand, hybrid Au NRs functionalized by controlled overgrowth of Pd,^10,11 AgCdSe,¹² Pt–Ag¹³ on the as-prepared Au NRs exhibit improved methanol electro-oxidation, distinct photoluminescence properties, enhanced photochemical properties and dielectric sensitivity in visible and NIR region, etc. It has been proved that the convenient synthesis of perfectly regular shaped-Au NRs is of great significance for the specific applications, since the unique properties of the Au NRs are highly dependent on their structures.

Typical approaches of preparing the Au NRs mainly include the various wet chemical techniques, electrochemistry, sonochemical synthesis, solvothermal process, and photochemical reduction techniques, etc.^1–9 Among the various ways, seed-mediated growth method represents an interesting and well-developed synthesis technique. However, it has been believed that the nucleation and successive growth of rod-shaped Au nano-structures are extremely sensitive to many parameters including the gold seed structure, reactants content, environment temperature, time of growth and the amount of silver ions addition, etc. There is no doubt that Au NRs with fairly good uniformity can be successfully fabricated by carefully and systematically controlling the seed content, reducing agent, silver ion addition and stabilizer or micellar template during seed-mediated growth approach.^2,3,5–7 But for every success, there are dozens of failures. For example, the conventional synthesis does not repeatedly produce 100% regular Au NRs every time; rather, a significant fraction of thermodynamically favorable irregular structures such as dog-bone-like or dumbbell-shaped structures, etc. are frequently formed.¹⁴ A common reason for the failure is the slightly change in the surrounding temperature or duration of growth, which is almost inevitable in seed-mediated multiple process. In most cases, these inevitable and irregular Au NRs will be regarded as defective products and abandoned finally, since they won't be further modulated by the same growth approach. In order to improve the yield of Au NRs, therefore, a novel and effective strategy for direct conversion of these defective Au NRs to perfectly cylindrical structures has become increasingly important. This will be a complementary way to reduce the defective rate in the fabrication of Au NRs.

In recent years, a novel strategy based on low-power laser irradiation induced shape-conversion has tremendous potential ability to finely control the morphology of the nano-materials.^4,15–17 Laser irradiation in liquid has high no-equilibrium process with unique temperature and pressure, which is a new green approach to the synthesis of novel metal-stable phases of material. Many interesting works such as ZnO, TiO₂ and CuO sub-micrometer spheres,¹⁵ rapid synthesis of perfectly spherical Au nano-particles¹⁶ and size-tailored mono-disperse Cd nano-crystals,¹⁷ etc. have been realized by laser-induced reshaping process. Moreover, Yang et al. illustrated that gold nano-rods with controlled aspect ratio can be fabricated by using a 254 nm low-power (420 μW cm⁻²) UV light irradiation for about 30 h in the presence of silver ions.⁴ While, the relatively long irradiation time results in slow growth rate of nano-rod by using too low-power UV laser beam. It is noteworthy that because Au metal usually has relatively high adsorption efficiency in visible-UV region, the visible or UV laser beam with higher power usually leads to the unpredictable size manipulation owing to complete vaporization of Au nano-materials. Considering these previous works, the major focus is whether the laser irradiation strategy can be used for the conversion of irregular Au NRs to perfectly cylindrical structures. It is very important to be able to finely control the growth of irregular Au NRs by laser irradiation.

Herein, for the first time, we report on the accurate growth of well-defined cylindrical Au NRs structures with fairly good uniformity, which is based on laser-induced shape modification. In this paper, large-scale irregular shaped Au NRs with dog-bone-like structures were prepared by conventional seed-mediated growth approach. Subsequently, they were carefully transformed to well-defined cylindrical Au NRs by non-focused and low-power laser irradiation in very short time (∼5 W cm⁻², <100 s, 1064 nm). We found that the perfect cylindrical Au NRs with aspect ratios ranging from about 2.4 to 5.6 can be effectively modulated by laser irradiation time, resulting in the longitudinal LSPR peak red-shifted from about 750 nm to 810 nm. Finally, by adding H₂PtCl₆ in as-prepared Au NRs growth solution, laser irradiation can also lead to guide the growth of Pt–Ag nano-islands on Au NRs. Meanwhile, a detailed discussion of the relevant mechanism is addressed. There is no doubt that the present results in this work will provide a novel approach to reduce the defective rate in the fabrication of Au NRs, which is highly promising for the reconstruction of other complex nano-structures with compelling functionalities.

Experimental section

Irregular shaped Au NRs with dog-bone-like structures were prepared by conventional seed-mediated growth approach.^1–14 Briefly, 50 μL of 50 mM HAuCl₄ as gold source and 5 mL of 0.2 M hexadecyl trimethyl ammonium bromide (CTAB) as surfactant were introduced into 5 mL distilled water solution in a conical flask. Then, 0.06 mL of 0.01 M NaBH₄ was filled in the above solution under vigorous stir. The pink solution immediately formed after adding NaBH₄. The Au particles were used as seeds within one hour after preparation. Then the growth solution including 5 mL of 0.2 M CTAB, 100 μL of 50 mM HAuCl₄, 55 μL of 0.1 M freshly prepared ascorbic acid (AA), 5 mL distilled water and 50 μL of 0.01 M AgNO₃ was prepared in a conical flask. Next, 0.05 mL of the Au seed solution was added in the growth solution. Within 3–7 min, the solution color changed to reddish brown. The mixture solution was placed at 28 °C for about 2 hours in an oil bath without stir. The final color of the solution appears to light purple red, reflecting the formation of Au NRs structures. The dog-bone-like Au structures were examined by transmission electron microscopy (TEM, JEOL-JEM-2100F). Then, the irregular Au NRs solution was added in a rating glass dish (X–Y stage) with a speed of ∼600 rpm. The solution with 3 mm depth was irradiated by a non-focused Q-switched Nd–YAG (yttrium aluminum garnet) laser (Quanta Ray, Spectra Physics) beam. The low-power (∼5 W cm⁻²) laser beam operated at 10 Hz with a wavelength of 1064 nm and pulse duration of 6 ns vertically irradiated into the Au NRs solution. After laser irradiation at different time durations, the products were separately collected by centrifugation at 18 000 rpm for 10 min and washed repeatedly with distilled water for further characterizations and optical properties studies. The sediments were separately dropped on copper meshes and dried in an oven at room temperature for observation by transmission electron microscopy. Finally, the UV-visible absorption spectra of as-prepared Au NRs solutions were measured by UV-vis-IR spectrometer (Shimadzu, UV-1800).

Results and discussion

Pristine irregular shaped Au NRs prepared by conventional seed-mediated growth approach are analyzed with transmission electron microscopy (TEM), as shown in Fig. 1(a). The low-magnification TEM image in Fig. 1(a) clearly shows that numerous liquid-dispersed Au NRs with the average length of about 52 nm are dog-bone-like structures. The enlarged TEM image (inset in Fig. 1(a)) provides some typical dog-bone-like Au NRs with relatively flat heads and concave structures in middle region. The main reason for the formation of these dog-bone-like Au NRs is the CTAB binding strength on the surface of the Au NRs. It has been recognized that CTAB molecules will preferentially bind to the middle of the nano-rods, resulting in more Au atoms depositing at the ends.⁸ If the excessive CTAB molecules covered at middle region of the Au NRs can be carefully removed from the rods, the concave structures in dog-bone-like Au NRs will be reshaped and then smoothed by isotropic overgrowth. In this way, the irregular Au NRs solution (5 mg L⁻¹, 10 mL) was irradiated by non-focused and low-power laser irradiation in very short time (∼5 W cm⁻², <100 s, 1064 nm). The LSPR spectra of the Au NRs obtained by laser irradiation for irradiation times varied from 0 to 100 s are shown in Fig. 1(b). The absorption spectrum (cyan color line in Fig. 1(b)) of the original irregular Au NRs shows that three surface plasma peaks can be detected in visible and NIR region. The transversal and longitudinal peaks are separately located at about 520 nm and 750 nm, respectively, which is consistent with previous works.^1–9 Meanwhile, it should be noted that another surface plasma peak centered at about 606 nm is clearly detected in Fig. 1(b), which should be originated from the concave structures in dog-bone-like Au NRs.^7,8 When increasing the irradiation time (0–100 s), the direct photographs of the solutions (inset in Fig. 1(b)) illustrate that the color of colloidal Au NRs significantly changed from light purple-red to brick purple, implying the photo-chemical process occurred during laser irradiation. Most importantly, the relative intensity of the LSPR peak at 606 nm originated from concave structures in irregular Au NRs significantly decreases and gradually disappears as the irradiation times varied from 0 s to 100 s. The well-defined transversal and longitudinal peaks (red color line in Fig. 1(b)) of the Au NRs by laser irradiation for 100 s reveal that the original irregular Au NRs should be smoothed and reshaped by laser-induced modification. In order to verify the reshaped Au NRs, the products by laser irradiation for 100 s are examined by TEM investigation. As shown in Fig. 1(c), the low-magnification TEM image of the reshaped nano-materials clearly shows that well-defined cylindrical Au NRs can be completely developed by laser induced shape conversion. The enlarged TEM image (inset in Fig. 1(c)) provides that the typical Au NRs with fairly good uniformity and ultra-smooth surface possesses spherical heads, which is distinctly different from the dog-bone-like structure in Fig. 1(a). The above results in Fig. 1 clearly confirm that the original irregular Au NRs can be fairly smoothed and conversed into well-defined cylindrical Au NRs by non-focused laser irradiation in very short time. The possible mechanism of the laser-induced conversion is discussed in the following section. At the moment of laser beam arriving at Au NRs, the laser energy absorbed by the Au NRs is eventually transformed into heat, leading to a rise of the temperature of nano-materials and the surrounding CTAB medium. Because the Au NRs usually have relatively low absorption efficiency in NIR (>1000 nm), the adoption of 1064 nm low-power nano-second laser irradiation guarantees moderately photo-thermal heating for Au nano-materials, which is coincident with previous work.¹⁷ Compared to the inner Au NRs, the excess CTAB molecules capped on the middle of the Au NRs surfaces will preferentially to be heated and escaped from the original concave region. Meanwhile, the chemical reduction of Au ions (HAuCl₄ in the solution) enables the overgrowth of Au on the original concave region by the laser-induced modification, resulting in the conversion of dog-bone-like Au NRs to ultra-perfect cylindrical rods.

Fig. 1 (a) The typical low-magnification TEM image of pristine irregular Au NRs prepared by standard seed-mediated growth approach. The inset shows the representative enlarged TEM image of the Au NRs with dog-bone-like structures. (b) The UV-visible absorption spectra of Au NRs obtained by non-focused (1064 nm, ∼5 W cm⁻²) laser modification for irradiation times varied from 0 s to 100 s. (c) The TEM image of the Au NRs obtained by non-focused laser irradiation of dog-bone-like Au NRs, irradiation time: 100 s. The inset shows the typical enlarged TEM image of the ultra-perfect cylindrical Au NRs.

The controllable accurate growth of the fairly regular cylindrical structure is highly related to the relatively low-power NIR laser beam and the irradiation in very short time. Yang et al. believed that the photon energy of the lasers with wavelengths in the visible or UV region should be highly absorbed by the metals, resulting in complete vaporization of metal nano-materials, thus giving rise to uncontrollable/unpredictable poly-dispersed particles, which is also coincident with our case.¹⁷ We found that numerous poly-dispersed Au nano-spheres instead of Au NRs will be generated by shorter wavelength (532 nm) and low-power (∼5 W cm⁻²) laser irradiation of the irregular Au NRs (Fig. S1†). Moreover, 532 nm laser irradiation with higher power will further improve the formation of uncontrollable/unpredictable poly-dispersed particles, which is also coincident with the case of 1064 nm higher power (>5 W cm⁻²) laser irradiation in this paper. As for too low laser power (<4.8 W cm⁻²) using in the 1064 nm laser irradiation, we found that the dog-bone-like Au NRs cannot be effectively translated into regular nano-rod structures since the temperatures of Au NRs and surrounding CTAB medium are insufficient. Meanwhile, it should be noted that the concentration of the Au NRs in the solution is a key issue during the controllable accurate growth process. As shown in Fig. S2,† the dog-bone-like Au NRs will overlap with each other, because of the higher content (10 mg L⁻¹, 10 mL) of Au NRs in the solution. Then, the uncontrollable/unpredictable Au nano-structures are severely agglomerated by non-focused 1064 nm laser (∼5 W cm⁻²) irradiation of dog-bone-like Au NRs with irradiation time of 100 s. The high concentration tends to increase the possibility of collision and fusion among Au NRs during laser irradiation, which has been confirmed in previous work.¹⁶ Compared with concentrated Au NRs solution, the well diluted Au NRs are help for the controllable accurate growth. On the other hand, CTAB is well-known as a dispersing and stabilizing agent during the fabrication of Au NRs.^1–8 Without the dispersing and stabilizing agent, the Au NRs are accreted with each other and agglomerated/aggregated, which are very similar to the structures with high Au NR concentration in the Fig. S2† It is bad for the generation of ultra-prefect cylindrical and fairly good uniformity Au NRs. Besides the laser power and wavelength, it should be pointed out that the laser irradiation in very short time (<100 s) also plays an important role for the accurate growth. Further prolonging the irradiation time (120–400 s) will result in a reshaped-procedure of the ultra-perfect Au NRs to form spherical Au nano-particles. The TEM image of the Au NRs obtained by 1064 nm laser irradiation for 120 s is shown in Fig. 2(a). The morphology clearly shows that some large-sized Au nano-particles with quasi-spherical structures can be formed due to the further laser irradiation. We also studied the corresponding absorption spectra of Au NRs at different irradiation time, as shown in Fig. 2(b). One can observe that the longitudinal LSPR peak at about 606 nm gradually decreases from about 0.62 a.u. to 0.1 a.u. as the irradiation time increases from 120 s to 360 s. Moreover, the longitudinal LSPR peak significantly drops to 0.003 a.u. and seems to disappear completely in Fig. 2(c) as the irradiation time further increases to 400 s. After laser irradiation of 400 s, the corresponding absorption spectrum in Fig. 2(c) only contains the transversal peak at about 520 nm originated from Au nano-particles. We can deduce that an increasing proportion of spherical Au nano-particles transformed from regular Au NRs was formed, when increasing the irradiation time gradually. The TEM image of the final nano-materials in Fig. 2(c) clearly shows that numerous uncontrollable dispersed Au nano-spheres with size varying from about 20 to 50 nm are formed by laser irradiation for 400 s. To reveal the possible mechanism behind the morphological conversion of regular Au NRs into dispersed Au nano-spheres, the photo-thermal evaporation has been proposed, which was clearly illustrated in previous work.¹⁶ Compared with coulomb explosion model, the photo-thermal melting model is more favorable for Au nano-structure morphological conversion due the application of the 1064 nm nanosecond laser irradiation in this paper. As for the regular Au NRs with transversal and longitudinal peaks separately located at about 520 nm and 750 nm, the longitudinal surfaces will be easier heated by 1064 nm laser irradiation. Moreover, the increasing laser irradiation time (120–400 s) can bring about the removal and decomposition of stabilizing agents on the surface of the Au NRs. In this way, the Au NRs reach the boiling temperature and start to evaporation during cumulative pulses laser irradiation. The nano-rod-structures would be inevitably destroyed by the evaporation process. Then, the boiled Au materials driven by the surface free energy minimization will preferentially assemble into spherical shapes instead of rods or other asymmetric structures, because of the relative lower surface energy in the nature for the spherical structure. The further increasing irradiation time will significantly improve the rate of the morphological conversion in this paper. Liu and Li et al. illustrated that the redundant laser irradiation time will further increase the fusion possibility and induce the fusion of the agglomerated structures, finally result in the formation of large-sized nano-spheres, which is also coincident with our case.¹⁶ As shown in the Fig. 2(c), the Au nano-spheres with size varying from about 20 to 50 nm are the best evidence for the laser fusing process. This observation is correlated well with the evolution of the absorption spectra in Fig. 2(b). In short, a gentle/moderate (∼5 W cm⁻²) laser (1064 nm) irradiation operated in short time (<100 s) can induce completely shape-conversed from well-diluted irregular Au NRs with dog-bone-like structures to ultra-prefect cylindrical and fairly good uniformity Au NRs.

Fig. 2 (a) The typical low-magnification TEM image of Au NRs prepared by non-focused laser irradiation of dog-bone-like Au NRs, irradiation time: 120 s. (b) The UV-visible absorption spectra of Au NRs obtained by non-focused (1064 nm, ∼5 W cm⁻²) for irradiation times varied from 120 s to 400 s. (c) The TEM image of the Au NRs obtained by non-focused laser irradiation of dog-bone-like Au NRs, irradiation time: 400 s.

Meanwhile, we also noted that it is not feasible to make varied aspect ratio Au NRs only by controlling the laser parameters. After seed-mediated growth process, the growth solution reused in above laser irradiation experiment has relatively low growth rate since the metal ions in the solution has been largely consumed in the initial seed growth. It is well known that Au NRs with high aspect ratio can be medicated by carefully controlling the metal ions contents in the growth solution.^1–9 Inspired by these previous works, we provide laser irradiation of irregular Au NRs in a new growth solution including 5 mL of 0.2 M CTAB, 100 μL of 50 mM HAuCl₄, 55 μL of 0.1 M freshly prepared ascorbic acid (AA), 5 mL distilled water and 50 μL of 0.01 M AgNO₃. After seed-mediated growth process, the irregular Au NRs were collected by centrifugation at 18 000 rpm for 10 min, and washed repeatedly with distilled water. Then the sediments were added in the new growth solution. The morphologies examined by low- and enlarged TEM images in Fig. 3(a) show that the large-scale liquid-dispersed dog-bone-like Au NRs with the average length of about 50 nm are consistent with the structures in Fig. 1(a). Roughly, the average aspect ratio for the irregular Au NRs is about 2.3, based on TEM images, and more than 300 Au NRs were measured with the aid of the graphics-editing program of Macromedia Fireworks 8. After laser irradiation (1064 nm, ∼5 W cm⁻²) for 105 s, the expected perfect-cylindrical Au NRs with average length of 150 nm are obtained, as shown in Fig. 3(b). The length of the perfect Au NRs is about three times longer than that of original irregular Au NRs. Based on the Au NRs with fairly good uniformity structures in Fig. 3(b), the corresponding average aspect ratio is about 5.6, which is obviously higher (about 2.4 times) than that of the pristine irregular shaped Au NRs in Fig. 3(a). To get clearly information of the length-controlled Au NRs by laser irradiation, the length-distribution histogram is shown in Fig. 3(c). Each distribution is obtained by measuring the length of more than 300 nano-structures in sight on the TEM images. As shown in Fig. 3(c), compared with the poly-dispersed irregular rods, the elongated Au NRs obtained by laser irradiation for 105 s have obviously narrow length dispersion. The result also reveals that the optimal laser irradiation will enable the irregular Au NRs to be converted into perfectly cylindrical rods structures, and then finely control the aspect ratio of Au NRs in appropriate growth solution by laser induced accurate growth process. The inset in Fig. 3(c) illustrates the schematic growth of Au NRs by laser irradiation of irregular rods in growth solution. Firstly, laser-induced conversion of irregular Au NRs to well-defined cylindrical structures will dominate the reshaping processes. The concave region in the dog-bone-like Au NRs can be smoothed by over growth of Au ions due to the excess CTAB molecules capped on the concave space can be released by laser-induced heating. Subsequently, the gentle laser irradiation in growth solution guarantees moderately photo-thermal heating for all Au NRs, providing the accurate growth of higher aspect ratio cylindrical Au NRs. Yang et al. believed that enough Au and Ag ions contents can play a critical role in the regulating the shapes during the laser-induced photochemical synthesis, which is coincident with our case.⁴ The detailed descriptions of the mechanism to drive the Au NRs grow into one direction will be illustrated in the following section. The above descriptions will be verified by the absorption spectra of the Au NRs obtained by non-focused (1064 nm, ∼5 W cm⁻²) laser irradiation with times varied from 0 s to 105 s. As shown in Fig. 3(d), after laser irradiation for 35 s, the absorption spectrum of the Au NRs shows that the LSPR peak at about 606 nm originated from concave structures in irregular Au NRs gradually disappears, implying the end of completely reshaped process. Meanwhile, the longitudinal LSPR peaks show there are no red-shifted spectra after laser-induced conversion process. Prolonging the irradiation time (70–105 s) enable the longitudinal LSPR peak to distinctly red-shifted from about 750 nm to 810 nm, which is the best confirmation of the formation of higher aspect ratio cylindrical Au NRs by laser-induced photochemical synthesis. As for the Au NRs obtained by seed-mediated growth process, it is well known that the CTAB and AgNO₃ in the growth solution will play an import role in the Au nano-rod formation when the ascorbic acid (AA) is used.^1–7 Increasing evidences have shown that the cylindrical CTAB micelle structures will more readily approach the tips/spherical heads of the Au NRs than the longitudinal sides/longitudinal surfaces because the surface potential decays of Au NRs more rapidly at the tips/spherical heads than at the sides/longitudinal surfaces.^1,5 The unique model can improve the deposition of the Au metal at the tips/spherical heads of Au NRs that should be surrounded by CTAB stabilizing agent.⁵ When the Au NRs originally formed, meanwhile, it should be noted that the further growth reaction for higher aspect ratio NRs should be inevitably stopped, since the Ag ions adsorb at the Au NRs surface in the form of AgBr (Br ions from the CTAB) structures and restrict further rod-growth.^1–5 Recently, Patra et al. illustrated that higher aspect ratio cylindrical Au NRs can also obtained by using curcumin as the secondary reducing agent during seed-mediated growth approach.⁵ The prominent work demonstrated that the presence of curcumin have favored the cylindrical CTAB micelle growth, which enable the Au NRs to grow in one direction giving higher aspect ratio NRs structures. In this paper, the formation mechanism of the higher aspect ratio Au NRs by laser irradiation in the Fig. 3 should be related the fact that laser-induced-removal and decomposition of the AgBr byproduct adsorbed at the Au NRs surface. After the Au NRs originally formation, the AgBr byproduct will be significantly heated by subsequent laser-induced thermal-heating process, which can be efficiently dissolved and removed from the Au NRs. The introducing subsequent and moderate laser irradiation could modify the Au NRs surface, and enable the CTAB micelle to come back to the Au NRs surface, which also will facilitate cylindrical CTAB micelle growth. In this way, the CTAB micelle-structure will further promote the deposition of the Au metal at the tips/spherical heads of Au NRs and enhance the aspect ratio of the Au NRs structure. In summary, the secondary growth of higher aspect ratio Au NRs structure is based on the removal and decomposition of the AgBr byproduct by subsequent laser irradiation, which is different from the prominent work by using curcumin as secondary reducing agent.⁵ The present results have opened up a new strategy for the facile synthesis of higher aspect ratio Au NRs. It can be deduced that Au NRs with further higher aspect ratio should be prepared by carefully controlling the laser-induced accurate growth condition, which is a challenge in the ongoing research.

Fig. 3 (a) The typical low-magnification TEM image of pristine irregular Au NRs prepared by standard seed-mediated growth approach. The inset shows the representative enlarged TEM image of the Au NRs with more clearly dog-bone-like structure. (b) The TEM image of the Au NRs obtained by non-focused laser irradiation (∼5 W cm⁻², 1064 nm) of dog-bone-like Au NRs, irradiation time: 105 s. (c) The average length distribution histograms of Au NRs before and after laser irradiation for 105 s. The inset shows the schematic growth of Au NRs by laser irradiation of irregular rods in growth solution. (d) The UV-visible absorption spectra of Au NRs obtained by non-focused (1064 nm, ∼5 W cm⁻²) laser irradiation for irradiation times varied from 0 s to 105 s.

Finally, the preliminary controllable synthesis of hybrid Pt–Ag nano-islands on as-prepared Au NRs by laser irradiation strategy is illustrated in Fig. 4. The low- and high magnification TEM images of the hybrid Pt–Ag@Au NRs are displayed in Fig. 4 (a) and (b), respectively. Compared with the ultra-perfect Au NRs in Fig. 1(c) or 3(b), the hybrid Pt–Ag@Au NRs have relatively rough surfaces with nano-island-like structures. The average size of the nano-islands is about 12 nm by measuring the diameters of more than 300 nano-dots in sight on the TEM images. Moreover, the elemental mapping images of the typical hybrid Pt–Ag@Au NRs show that the hybrid nano-rods are indeed composed of Au, Pt and Ag elements, and the corresponding relative ratio is about 173 : 3 : 25, respectively. It can be deduced that the nano-islands should be mainly composed of Pt elements, which is different from the Au@Ag₅₀Pt₅₀ NRs obtained by island growth mode in previous work.¹³ Laser-induced photochemistry plays an important role in the formation of Pt–Ag island-like structures. Without laser irradiation, Au NRs with Pt–Ag shell structures instead of nano-island-like shapes will be formed after adding Pt ions in solution. As shown in Fig. S3,† the regular Au NRs gradually change to poly-dispersed and irregular shaped core–shell structures (inset in Fig. S3(c)†). The more Pt–Ag shell structures gradually increase as the reaction time increases from 2 min to 42 min. By simply adding Pt ions, it is difficult to get Pt–Ag nano-islands structures, which is also extremely sensitive to co-reduction process of Pt and Ag ions with reductive agent (ascorbic acid, AA). The UV-visible absorption spectra of core–shell-shaped Pt–Ag@Au NRs at different reaction time (Fig. S3(d)†) also show that the absorption spectra become much broader in the visible region (500–900 nm). The transversal (520 nm) and longitudinal peaks (∼800 nm) of Au NRs are completely overlapped by the broader absorption spectra in in Fig. S3(d).† In order to get comparison results, the corresponding LSPR spectra of the hybrid Pt–Ag@Au NRs with nano-island-shaped structures are shown in Fig. 4(c). Compared with absorption spectra of core–shell-shaped hybrid NRs, the results in Fig. 4(c) illustrate that the broader absorption spectrum of hybrid NRs by laser irradiation for 1 min implies the formation of thin Pt–Ag layer on the Au NRs. Increasing irradiation time (1–12 min) can enable the new embossed absorption region (800–950 nm) to be formed on broader spectra (arrow mark in Fig. 4(c)), owing to the formation of nano-island-shaped Pt–Ag@Au NRs. It can be used to monitor the formation of Pt–Ag island-shaped structures. The nano-island growth mode has been extensively investigated in previous works.^10–13 Briefly, it has been believed that the formation of metal nano-islands is related to the fabrication of thin metal layers by co-reduction of metal ions with reductive agent (AA), which is coincident with our case by laser irradiation strategy. The subsequent step mainly includes carefully and systematically controlling the added amount of Ag and Pt ions. While, a significant fraction of core–shell-shaped structures are frequently formed, due to the exacting terms in the accurate growth. Instead of using the second step, the laser-induced photochemical synthesis in this paper will result in rapidly inhomogeneous metal overgrowth on the NRs, leading the preferential formation of nano-island-shaped structures (inset in Fig. 3(c)). The laser energy absorbed by the Au NRs is eventually transformed into heat, leading to a rise of the temperature of nano-materials and the surrounding Ag, Pt ions. The nucleation process of metal ions is exceedingly rapid since the laser quenching time is extremely short (tens of picoseconds).¹⁸ The laser irradiation approach is an attractive technique with a high non-equilibrium processing character, which has promising potentials for the fabrication of metal nano-island-structures. Thus, in our further investigations, it is worthwhile to illustrate the relationship between the laser irradiation parameters (laser wavelength, irradiation time, laser power and the ratio of metal ions, etc.) and the induced-island-like structures in more detailed to give better quantitative conclusion.

Fig. 4 (a and b) The typical low- and enlarged magnification TEM images of hybrid Au NRs with Pt–Ag nano-islands on the surfaces obtained by laser irradiation (1064 nm, ∼5 W cm⁻²) in Au NRs solution by adding 2 mM H₂PtCl₆. The below pictures show the elemental mapping images of the typical hybrid nano-rods. (c) The UV-visible absorption spectra of hybrid Pt–Ag@Au NRs obtained by non-focused laser irradiation for irradiation times varied from 0 s to 12 min.

Conclusions

In summary, ultra-perfect cylindrical Au NRs with fairly good uniformity have been obtained by laser-induced shape-modification of irregular dog-bone-like Au NRs. During laser irradiation, the laser energy absorbed by the dog-bone-shaped Au NRs will lead to a rise of the temperature of nano-materials and the surrounding CTAB molecules. The excess CTAB capped on the middle of the Au NRs surfaces will preferentially to be heated and escaped from the original concave region. Meanwhile, the chemical reduction of Au ions (HAuCl₄) enables the overgrowth of Au elements on the original concave region by the laser-induced modification, resulting in the fabrication of the fairly regular cylindrical Au NRs. Moreover, the aspect ratio of the Au NRs can be controlled by the moderate laser irradiation in the growth solution, due to the accurate growth process by laser-induced photochemical model. After laser irradiation for 105 s, the average aspect ratio of the well-defined Au NRs is about 5.6, which is significantly higher (about 2.4 times) than that of the original irregular shaped Au NRs. The longitudinal LSPR peak of the Au NRs can be effectively modulated from about 750 nm to 810 nm as the irradiation time increases from 70 s to 105 s. Finally, we found that the laser irradiation in growth solution containing some H₂PtCl₆ will lead to rapidly inhomogeneous metal overgrowth on the Au NRs, resulting in the formation of hybrid Pt–Ag@Au NRs with nano-island-structures. The obtained results in this paper illustrate that the laser-induced modification strategy is an attractive technique with a unique non-equilibrium processing character, which has promising potentials for the reconstruction of other complex nano-structures with compelling functionalities in the further.

Acknowledgements

This work was supported by the Natural Science Foundation of China under Grant No. 11575102, 11105085, 11275116 and 11375108, the Fundamental Research Funds of Shandong University under Grant No. 2015JC007.

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Footnote

† Electronic supplementary information (ESI) available: The TEM image of the poly-dispersed Au nano-spheres generated by 532 nm low-power (∼5 W cm⁻²) laser irradiation. The TEM image of pristine irregular Au NRs with high concentration (10 mg L⁻¹, 10 mL) and TEM image of the Au NRs obtained by non-focused 1064 nm laser (∼5 W cm⁻²) irradiation of dog-bone-like Au NRs, irradiation time: 100 s. The TEM images of the irregular shaped hybrid Pt–Ag@Au NRs core–shell structures. UV-visible absorption spectra of core–shell-shaped Pt–Ag@Au NRs. See DOI: 10.1039/c6ra16547h

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