Growth Of Bimetallic Pt-au Nanoplates And Their Optical Properties
The preparation process of Pt-Au nanoplates is shown in Fig. 1a. In this case, a seed-mediated growth method was used to prepare Au nanotriangles [39], with CTAB as structure-directing agent, HAuCl4 as gold source, and AA as reductant. Through selective sedimentation to the bottom of the reaction vessel, Au nanotriangles with high dimensional uniformity were obtained at first step. Figure 1b shows the typical SEM image of purified Au nanotriangles. The average edge of Au nanotriangles is 180 nm. The energy-dispersive X-ray spectroscopic (EDS) analysis in Fig. S1a confirms the presence of Au. Under the mild reduction condition, the abundant AuCl4− ions were added to etch the tips of these nanotriangles, and these Au nanotriangles transform to hexagonal Au nanoplates gradually, as exhibited in Fig. 1c. The transformation process is dependent on the concentration of HAuCl4. The overall transformation processes with different amount of HAuCl4 were monitored by SEM and extinction spectra (Fig. S2). As shown in Fig. 1i and Fig. S2e-f, the main peak of in-plane dipolar plasmon resonance for the purified Au nanotriangles is presented at around 1188 nm. Accompanying with the increase of HAuCl4, the corresponding main peak is blue-shifted gradually. When the amount of HAuCl4 solution is reached to 70 µL, highly uniform hexagonal Au nanoplates with average edge size of 86 nm are obtained (Fig. 1c). The corresponding main peak blue-shifted to 799 nm.
For the following growth of Pt, in the presence of CTAB, NaI, and AgNO3, potassium chloroplatinate and ascorbic acid were used as platinum source and reductant, respectively. Figure 1d shows the SEM image of Pt-Au nanoplates with dipolar LSPR at 1036 nm. It is clearly that Pt atoms are preferentially deposited on the rim of hexagonal Au nanoplates. There also have some Pt nanoparticles deposited onto the flat surface of Au nanoplates. The EDS mapping shown in Fig. 1f-h display the element distributions of an individual Pt-Au nanoplate, which matches well with the morphology observation in SEM. In the reaction of Pt deposition, AgNO3 and NaI were added into the reaction solution. AgI may be precipitated, and Ag atoms may be deposited onto the Au nanoplates [25]. Halide ions strongly affects the anisotropic growth due to the selective binding of iodide ions onto specific surfaces of metal. Due to the existence of I- ions, Ag layer, and AgI precipitation, Pt atoms are preferentially deposited on the rim of hexagonal Au nanoplates [40–41]. After the deposition of Pt, the main peak of LSPRs is redshifted from 799 nm to 1036 nm, as shown in Fig. 1i. The corresponding photograph image of Au and Pt-Au nanoplates is exhibited in Fig. 1e.
Morphology And Optical Properties Of Pt-au Nanoplates With Varied Amounts Of Pt
As for the Pt deposition onto the Au nanoplates, the variation of Pt amount would influence the morphology and optical properties of bimetallic Pt-Au nanoplates. By varying the amount of potassium chloroplatinate (5 mM K2PtCl6) from 40 µL to 480 µL, the deposition position and diameter of Pt could be well controlled, as shown in Fig. 2. In this case, the initial hexagonal Au nanoplates have a dipolar LSPR at 799 nm. When 40 µL of K2PtCl6 solution is added, Pt is preferentially deposited on the rim of hexagonal Au nanoplates. As the volume of K2PtCl6 solution is increased, the thickness of Pt is increased obviously. Gradually, Pt is also deposited both on the top and bottom flat surfaces of nanoplates. The sample shows erythrocyte-like when the amount of K2PtCl6 solution reaches to 480 µL.
The extinction spectra of Pt-Au nanoplates with different volumes of K2PtCl6 solution are shown in Fig. 2g. For better illustrating the spectral evolution, the wavelength and linewidth of main peak are plotted in Fig. 2h. With the increase of Pt precursor amount, the main peak is red-shifted from 799 nm to 1036 nm. Meanwhile, the width of main peak is broadened from 196 nm to 368 nm. Interestingly, the high-order mode near 600 nm is gradually disappeared. The evolution of extinction spectra demonstrates the influence of Pt deposition on the LSPRs of Au nanoplates. The damping effect of Pt causes the peak red-shift and broadening of dipolar plasmon mode [26].
Growth of bimetallic Au@Pt nanorings and their optical properties
Theoretical and experimental investigations show hollow plasmonic nanostructures have attractive optical properties in terms of field enhancement and polarization-dependent LSPR bands [42–44]. In this work, the Pt-Au nanoplates can be easily transformed to hollow ring-shaped nanostructures through etching and regrowth progresses. Figure 3a exhibits the schematic progresses of etching and regrowth, which lead to the formation of Pt nanorings and Au@Pt nanorings with high uniformity. Monodisperse Pt-Au nanoplates are used as templates (Fig. 3b). In the presence of iodide ions, the etching process is obtained after adding HAuCl4 and AA. As previously reported, HAuCl4 could be used to etch Au nanocrystals, and halide ions were often used as Au etchants [25]. As shown in Fig. 3c, the central region of Pt-Au nanoplates were selectively etched, resulting in Pt nanorings. The average width of Pt nanorings is 36 nm. Subsequently, due to the reduction HAuCl4 by AA, Au is regrown onto the Pt nanorings, forming Au@Pt nanorings. As shown in Fig. 3d, the Au@Pt nanorings with average side length of 135 nm are observed. The side width of nanorings is around 50 nm. The EDS analysis Fig. S1c-d confirms the component materials of these two nanorings.
The optical properties AuPt bimetallic nanostructures are closely related to the morphology, and therefore tuned by the etching and regrowth processes. As shown in Fig. 3e, the Pt-Au nanoplates before etching exhibit an in-plane dipolar plasmon peak located at 1036 nm, with a weak side band around 718 nm. The plasmon band is disappeared when Au is removed after the etching reaction. However, the LSPR is back after the regrowth of Au as shell onto the Pt nanorings. The final Au@Pt nanorings have a main LSPR peak located at 1010 nm with a side band around 606 nm.
Photothermal Performance
The afore-mentioned discussion indicates that the Pt deposition would damp the LSPR of Au. Due to the efficient light-harvesting of LSPRs, the plasmon damping by Pt deposition would possibly lead to an improvement of the photothermal conversion. The photothermal conversion efficiency is important for further successful photothermal therapy applications of plasmonic metal nanostructures. For investigating the photothermal effect of these bimetallic AuPt nanostructures, the LSPRs of Pt-Au nanoplates and Au@Pt nanorings were tuned to match 1064 nm laser in NIR-II window. Under 5 min laser irradiation with wavelength of 1064 nm and power intensity of 1 W/cm2, the temperature of various samples was monitored and recorded by NIR image camera every 30 seconds. During the measurements, the concentration of all the samples were kept same as 300 µg/mL. Figure 4a exhibits the temperature curves along time of purified Au nanotriangles, hexagonal Au nanoplates, Pt-Au nanoplates and Au@Pt nanorings, respectively. The temperature increment ΔT of purified Au nanotriangles, hexagonal Au nanoplates, Pt-Au nanoplates and Au@Pt nanorings is 23.4°C, 13.7°C, 30.4°C, 25.6°C, respectively. A remarkable maximum temperature of 60.1°C for the Pt-Au nanoplates is obtained.
To systematically illustrate the photothermal conversion performance of Pt-Au nanoplates, the sample prepared with different amounts of Pt source were compared. As shown in Fig. 4b, with the increase of platinum source, the deposited Pt shells on the rim of hexagonal Au nanoplates are grown thicker, and the maximum temperature increment ΔT is increased. For the Pt-Au nanoplates prepared with 5 mM K2PtCl6, the dependence of ΔT on sample concentration is shown in Fig. 4c. With the increase of sample concentration, the maximum temperature increment ΔT is also increased. When the sample concentration is reached to 300 µg/mL, the maximum temperature increment ΔT is 1.3 times that with concentration of 150 µg/mL. In additional, for clarifying the photothermal stability, six repeated cycles of laser heating were measured and shown in Fig. 4d. Owing to the strong NIR light absorption feature and the resonance coupling with 1064 nm laser, any noticeable decrease is not observed in the temperature elevation for the typical Pt-Au nanoplates, confirmed their excellent photothermal stability during laser irradiation.
The photothermal conversion efficiency of these nanostructures under same experimental condition is summarized in Table 1. Compared to the hexagonal Au nanoplates, the photothermal conversion performance of Pt-Au nanoplates is enhanced. The strong LSPRs of Au nanoplates leads to an efficient light-harvesting in NIR-II window. The enhanced photothermal conversion performance of Pt-Au nanoplates is attributed to the damping effect of Pt deposition, which promotes the photothermal conversion and heat generation. Furthermore, the Pt deposition produces rough surface and brings abundant active sites for catalytic reactions. It is expected that these bimetallic nanoplates and nanorings would show excellent performance in many photocatalytic applications.
Table 1
Photothermal conversion efficiencies of the samples
| Absorbance at 1064 nm | ΔT | 𝜂 |
Au nanotriangles | 1.41 | 23.4°C | 35.4% |
Hexagonal Au nanoplates | 0.57 | 13.7°C | 20.8% |
Pt-Au nanoplates | 2.05 | 30.4°C | 39.9% |
Au@Pt nanorings | 1.99 | 25.6°C | 36.8% |