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Abstract
Au nanoplates with tunable in-plane dipolar localized surface plasmon resonance peaks in a broad range from the visible to near-infrared region were obtained in high yield using a seedless wet chemical growth method after purification. Cetyltrimethylammonium chloride was used as a surfactant, while hydrogen peroxide and sodium borohydride were used as the weak and strong reducing agents, respectively. The edge length and in-plane dipolar localized surface plasmon resonance peak of the Au nanoplates could be adjusted by varying the amounts of hydrogen peroxide and sodium borohydride. The Au nanoplates were further used as the saturable absorber to generate pulsed laser output in a passively Q-switched solid-state laser at approximately 2 µm. Our study offers a new method for obtaining Au nanoplates with tunable plasmonic peaks over a broad range.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Micro/nanophotonics have been extensively investigated in the past few decades because of their promising applications in many areas. [1–4] Au nanostructures exhibit unique optical properties and have potential applications in micro/nanophotonics. Owing to the shape- and size- dependent plasmonic properties, [5–9] Au nanostructures have attracted considerable interest in recent years. [10–14] Among them, Au nanoplates have particularly received much attention because of their excellent plasmonic properties [15,16] and potential applications in fields, such as sensing, [17] biomedicine, [18] catalysis, [19] and optoelectronics. [20] Driven by the applications, the preparation of Au nanoplates with tunable localized surface plasmon resonance (LSPR) peaks in a broad wavelength range is desirable.
Wet-chemical synthesis is often used to prepare Au nanoplates. [21,22] Cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) are common surfactants. Using CTAB as a surfactant, three-step seed-mediated, [23,24] one-step seed-mediated, [25,26] and one-step seedless syntheses [27] have been reported for the preparation of Au nanoplates. Au nanoplates with an in-plane dipolar LSPR peak tunable in a broad range, from approximately 700 [26,28] to over 1600 nm, [27] were obtained in the presence of CTAB. Although Au nanoplates are usually prepared using CTAB, centrifugation at temperatures below 25 °C may be difficult as CTAB crystallizes at low temperatures. When using CTAC as a surfactant, both seed-mediated and seedless syntheses of Au nanoplates have been achieved. [29,30] Au nanoplates with in-plane dipolar LSPR peaks tunable from approximately 600 to 810 nm were prepared using a seed-mediated method with CTAC. [31,32] Au nanoplates with in-plane dipolar LSPR wavelengths ranging from 630 to 1170 nm were also prepared using CTAC and KI3 via the two-step seed-mediated synthesis. [33] Nevertheless, some problems still remain, such as complex steps (KI3 and seeds are prepared first) and the reproducible preparation of seeds when using the seed-mediated method. Au nanoplates with in-plane dipolar LSPR peaks tunable in the visible region (620–700 nm) were prepared using a seedless synthetic route in the presence of CTAC. [30] However, to the best of our knowledge, no studies have been reported on the seedless wet-chemical synthesis of Au nanoplates with tunable in-plane dipolar LSPR peaks in a broad range from the visible to near-infrared region in the presence of CTAC.
Recently, many weak reducing agents have been used for the preparation of Au nanostructures. [34–38] As a common oxidant, hydrogen peroxide was also applied as the reducing agent for synthesis of Au nanostructures under ambient conditions. [39–41] For instance, hydrogen peroxide has been employed as a weak reducing agent instead of the commonly used ascorbic acid for the synthesis of Au nanorods under basic conditions with the addition of NaOH instead of HCl. [39] The increased pH value decreases the potential of the reaction; thus, hydrogen peroxide mainly functions as a reducing agent for Au(III) ions. [39] These results suggest that a weaker reducing agent other than ascorbic acid can be used to synthesize Au nanostructures with tunable plasmonic peaks.
Therefore, in this study, we prepared Au nanoplates using hydrogen peroxide as a weak reducing agent instead of the commonly used ascorbic acid and sodium borohydride as a strong reducing agent in the presence of the CTAC surfactant through a one-step seedless wet-chemical growth route. After purification, high-yield Au nanoplates with an in-plane dipolar LSPR peak tunable over a broad range from the visible to near-infrared region were obtained by varying the amounts of the added chemicals. To the best of our knowledge, the use of purified Au nanoplates as a saturable absorber for the generation of a pulsed laser at 2 µm in the passively Q-switched Tm:YAG solid-state laser was experimentally demonstrated for the first time.
2. Experiments
2.1 Preparation and purification of Au nanoplates
In a typical synthesis, HAuCl4 solution (0.32 mL, 5 mM) and NaOH solution (0.016 mL, 0.1 M) were first mixed and then added into the mixed solution containing water (6.4 mL), CTAC solution (1.28 mL, 0.1 M) and KI solution (0.06 mL, 0.01 M). Different amount of hydrogen peroxide (0.01-0.12 mL, 30%) was added into the above solution. After the solution was mixed, freshly made sodium borohydride solution (0.025-0.175 mL, 0.005 M) was added immediately into the mixed solution. The resulting solution was left undisturbed at room temperature overnight.
The separation of Au nanoplates can be achieved by the depletion interactions between particles in the presence of surfactant micelles such as CTAB and CTAC. [29,42,43] The mixture was first centrifuged, and the precipitate was redispersed in water. For the Au nanoplates with in-plane dipolar LSPR wavelengths <1100 nm, a CTAC solution was added to the dispersion solution to precipitate the Au nanoplates. Different volumes of the CTAC solution (0.3 M) were added to the dispersion solution to determine the optimal purification conditions. The final CTAC concentration was changed from 0.02 to 0.2 M and depended on the size of the Au nanoplates, as estimated from the LSPR peaks. A lower final CTAC concentration was used for Au nanoplates with longer in-plane dipolar LSPR wavelengths. For Au nanoplates with in-plane dipolar LSPR wavelengths >1100 nm, a final CTAB concentration of 0.02–0.04 M was used. After incubating at room temperature for approximately 10 h, a precipitate was observed at the bottom of the tube. If no precipitate was observed at the bottom of the tube, centrifugation was performed at a low velocity (1000, 1500, 2000, or 2500 rpm) to obtain precipitates. A lower centrifugation velocity was applied to the Au nanoplates with a larger in-plane dipolar LSPR wavelength. The precipitate was redispersed in the colloidal solution for further characterization.
2.2 Characterization
Scanning electron microscopy (SEM) images were obtained by a Hitachi SU8010 scanning electron microscopy. The absorption spectra were measured by a 723N Visible spectrophotometer (Shanghai Yoke Instrument Co. Ltd, China) and Lambda 950 ultraviolet-visible-near infrared spectroscopy (UV-Vis-NIR) spectrophotometer (PerkinElmer, USA).
3. Results and discussion
3.1 Preparation and purification of Au nanoplates
In this study, Au nanoplates were prepared by a one-step seedless wet-chemical growth method using hydrogen peroxide and sodium borohydride as reducing agents in the presence of a CTAC surfactant. CTAC and KI were used as the surfactant and shape-directing agent, respectively. [30] Sodium tetrachloroaurate was obtained by mixing HAuCl4 and NaOH solutions. [30] Hydrogen peroxide was used as a weak reducing agent to replace the commonly used ascorbic acid in the reduction of Au(III) to Au(I) ions. [39] Sodium borohydride was used as a strong reducing agent for the reduction of Au(I) ions to Au atoms, resulting in the nucleation of small Au nanoparticles. [44] The change in solution color indicates the growth of Au nanostructures. After preparation, the purification step was performed, which was based on the depletion interactions between the Au particles induced by surfactant micelles CTAB and CTAC in the solution. [29,42,43] After purification, the Au nanoplates were obtained in high yield.
Figure 1(a) shows the optical absorption spectrum of the colloidal solution of purified Au nanoplates. The amounts of hydrogen peroxide and sodium borohydride solution used were 0.01 and 0.1 mL, respectively. The peak at longer wavelength was larger than 1100 nm (a rising trend at 1100 nm), which was assigned to the in-plane dipolar LSPR wavelength of the Au nanoplates. [23] The shoulder peak at approximately 750 nm is assigned to the in-plane quadrupole mode of the Au nanoplates. [23] The peak at approximately 600 nm may be attributed to pseudospherical nanoparticles and small nanoplates. The high intensity ratio of the absorption peak at wavelengths larger than 1100 nm to that of the nanoparticles suggests a relatively high yield of Au nanoplates. The SEM images of the samples are shown in Figs. 1(b) and (c). According to the size analysis from the SEM images, the edge length of the Au nanoplates was approximately 220 ± 36 nm. The thickness of the Au nanoplates was estimated to be approximately 18 nm based on the tilted Au nanoplates shown in Fig. 1(c).
Fig. 1. (a) Optical absorption spectrum of the colloidal solution of the purified Au nanoplates. (b) and (c) SEM images of the purified Au nanoplates.
In our experiment, the maximum in-plane dipolar LSPR wavelength of the Au nanoplates was >1100 nm, which is longer than that observed for Au nanoplates prepared via seedless synthesis using ascorbic acid as the weak reducing agent in the presence of the CTAC surfactant (620–700 nm). [30] In Ref. [30], ascorbic acid was used as the reducing agent, and a second addition of NaOH was used to initiate the reaction. The main differences between our study and Ref. [30] are the replacement of ascorbic acid with hydrogen peroxide as the weak reducing agent and the use of sodium borohydride as the strong reducing agent instead of the second addition of NaOH. This method is based on slow reduction. [37,39] A slow growth rate reportedly promotes the formation of Au nanorods when using a reducing agent weaker than ascorbic acid. [37,39] In this study, we used hydrogen peroxide as a replacement of ascorbic acid, which resulted in the formation of Au nanoplates with edge lengths in the micrometer scale (data not shown). Sodium borohydride is usually used as the strong reducing agent for the synthesis of Au nanostructures. [44] Therefore, in this study, sodium borohydride was added instead of the second addition of NaOH. Using hydrogen peroxide and sodium borohydride, Au nanoplates with edge lengths in the nanometer to submicrometer scale and an in-plane dipolar LSPR peak in the near-infrared region can be obtained. In addition, the in-plane dipolar LSPR wavelength can be adjusted by changing the amounts of hydrogen peroxide and sodium borohydride (see Section 3.2 and 3.3). In another report, Au nanoplates can also be prepared in the presence of the CTAB surfactant by using the seedless wet chemical synthesis method. [27] The in-plane dipolar LSPR wavelength of the obtained Au nanoplates was in the near-infrared region (from 1180 to over 1600 nm) with the use of CTAB. [27] Our finding suggests that Au nanoplates with tunable in-plane dipolar LSPR wavelengths in a broad range from the visible to near-infrared region (from 650 to over 1100 nm) with high yield can be obtained using CTAC through a seedless synthesis route after purification.
3.2 Influence of hydrogen peroxide
Hydrogen peroxide can be used as a weak reducing agent for the synthesis of Au nanostructures. [39–41] Fig. 2 shows the absorption spectra of the purified Au nanoplates prepared by varying the amount of hydrogen peroxide and keeping the amounts of the other reagents constant. When the amount of hydrogen peroxide was 0.02 mL, a broad absorption band was observed from 550 to 680 nm, which was attributable to the Au nanoplates with large edge lengths. [45] When the amount of hydrogen peroxide was increased to 0.04 mL, the in-plane dipolar LSPR wavelength of the Au nanoplates was >1100 nm (an increasing trend at 1100 nm). [23] The shoulder peak at approximately 760 nm is assigned to the in-plane quadrupole mode of the Au nanoplates. [23] When the amount of hydrogen peroxide was increased from 0.04 to 0.12 mL, the in-plane dipolar LSPR wavelength was blue-shifted to approximately 805 nm. This change in blueshift upon increasing the amount of hydrogen peroxide is similar to those observed in previous reports that used hydrogen peroxide and hydroquinone as weak reducing agents in the seed-mediated synthesis of Au nanoplates. [40,46]
Fig. 2. Absorption spectra of the purified Au nanoplates prepared with different amounts of hydrogen peroxide: (1) 0.02 mL, (2) 0.04 mL, (3) 0.08 mL, and (4) 0.12 mL.
SEM images of purified Au nanoplates with varying amount of hydrogen peroxide (from 0.02 to 0.12 mL) are shown in Figs. 3(a)–(c). The mean edge lengths of the Au nanoplates were obtained by analyzing more than 100 particles in the SEM images. When the amount of hydrogen peroxide was increased from 0.02 to 0.12 mL, the average edge length of the Au nanoplates demonstrated a decrease from approximately 417 ± 109 nm to 108 ± 23 nm. The mean edge length of the Au nanoplates decreased with an increase in the amount of hydrogen peroxide, which is similar to that reported by Yin et al. [40] The optical absorption and SEM results suggest that the edge length and in-plane dipolar LSPR peak of the Au nanoplates can be changed by varying the amount of hydrogen peroxide.
Fig. 3. SEM images of the purified Au nanoplates synthesized using different amounts of hydrogen peroxide: (a) 0.12 mL, (b) 0.04 mL, and (c) 0.02 mL.
3.3 Influence of sodium borohydride
Sodium borohydride is often used as a strong reducing agent in the synthesis of Au nanostructures. [44] Fig. 4 shows the absorption spectra of the purified Au nanoplates synthesized by varying the amounts of sodium borohydride solution while the other chemicals were unchanged. When the amount of sodium borohydride solution was increased from 0.025 to 0.175 mL, the in-plane dipolar LSPR wavelength of the Au nanoplates was blue-shifted from approximately 970 to 650 nm. The tendency of the in-plane dipolar LSPR peak to blue-shift with an increase in the amount of sodium borohydride solution is similar to that reported by Straney et al. who used ascorbic acid and sodium borohydride as reducing agents with CTAB for the seedless synthesis of Au nanoplates. [27]
Fig. 4. Absorption spectra of the purified Au nanoplates prepared with different amounts of sodium borohydride solution: (1) 0.025 mL, (2) 0.04 mL, (3) 0.07 mL, (4) 0.085 mL, (5) 0.1 mL, and (6) 0.16 mL.
Figures 5(a)–(d) show the SEM images of the purified Au nanoplates prepared using different amounts of sodium borohydride. The average edge length of the Au nanoplates decreased from 172 ± 38 to 63 ± 9 nm when the amount of sodium borohydride was increased from 0.025 to 0.175 mL. The mean edge length of the Au nanoplates decreased with an increase in the amount of sodium borohydride solution, which is similar to a previous study. [27] Sodium borohydride was used for the reduction of Au (I) ions to Au atoms. [44] When the amount of sodium borohydride is increased, the number of the small Au nanoparticle nuclei increases. Because the starting concentration of HAuCl4 is the same in the growth solution, smaller-sized Au nanoplates were obtained when more number of small Au nanoparticle nuclei were formed. These results indicate that the mean edge length and in-plane dipolar LSPR peak of the obtained Au nanoplates can be adjusted by varying the amount of sodium borohydride solution.
Fig. 5. SEM images of the purified Au nanoplates synthesized using different amounts of sodium borohydride solution: (a) 0.025 mL, (b) 0.07 mL, (c) 0.1 ml, and (d) 0.16 mL.
3.4 Applications
Au nanoplates have been utilized as a saturable absorber to generate pulsed laser output in passively Q-switched solid-state lasers at approximately 1 µm [20,47–50] and fiber laser at 1.56 µm. [51] In this study, the application of Au nanoplates as a saturable absorber for the generation of pulsed laser output at approximately 2 µm in the passively Q-switched Tm:YAG solid-state laser was experimentally demonstrated for the first time. Figure 6(a) shows the absorption spectrum of the film of purified Au nanoplates, which was prepared by depositing a solution of purified Au nanoplates with an in-plane dipolar LSPR wavelength >1100 nm on glass and then drying in air. A broad absorption band was observed near 2 µm, showcasing the potential applications of Au nanoplates in the infrared window. The inhomogeneously broadened absorption peak can be attributed to the plasmonic coupling when the Au nanoplates are in close proximity to each other in the film. [52] An Au nanoplate film was used as a saturable absorber in a passively Q-switched Tm:YAG laser. The experimental setup is similar to that used in our previous study. [52] Initially, the continuous-wave laser performance was observed using output mirror M2 without Au nanoplates. A solution of purified Au nanoplates was directly deposited onto the surface of output mirror M2, which was then inserted into the laser cavity and consequently generated pulsed laser output at approximately 2 µm. The output pulse parameters with maximum average output power of 300 mW, shortest pulse duration of 2.39 µs, and the repetition rate of 21.3 kHz were obtained. Figure 6(b) shows the pulse train with the pulse width of 2.39 µs at the repetition rate of 21.3 kHz. Optimized modulation depth and saturation intensity, and hence improved pulse energy, narrow pulse width, and high output power, should be possible with an optimized concentration or density of the sample. [53,54] Thus, the results demonstrate the potential application of Au nanoplates as the saturable absorber to generate pulsed laser output in a 2 µm solid-state laser.
Fig. 6. (a) Optical absorption spectrum of the film of the purified Au nanoplates. (b) Q-switched pulse train achieved from the Tm:YAG laser with the Au nanoplates film as the saturable absorber.
4. Conclusion
In this study, Au nanoplates in high yield with the in-plane dipolar LSPR peak tunable in a broad range from the visible to near-infrared region were obtained in the presence of CTAC using a one-step seedless wet-chemical growth method after purification. The effects of hydrogen peroxide and sodium borohydride on Au nanoplates were investigated. The Au nanoplates with the in-plane dipolar LSPR wavelength was tunable from approximately 650 nm to over 1100 nm. Moreover, the use of purified Au nanoplates as the saturable absorber for generation of pulsed laser at 2 µm in passively Q-switched solid-state laser was achieved, suggesting their potential applications in the infrared window. Our study provides a new method for synthesis of Au nanoplates with tunable plasmonic wavelengths over a broad range, which may have potential applications in many areas.
Funding
Training Program of Innovation and Entrepreneurship for Undergraduates of Jiangsu Province (Grant No. 202010320055Z); Priority Academic Program Development of Jiangsu Higher Education Institutions.
Acknowledgments
We thank Dr. C. Y. Ren and C. J. Shi for helpful discussions.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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