2.1 Morphology of ordered Ag nanowires and [email protected] hybrid nanotubes
2.1.1 Scanning electron microscopy
Fig. 2-a show the ordered Ag NWs prepared by the three-phase interface method, and fig. 2-b is an enlarged view. Through the three-phase interface method, the disordered Ag NWs can be self-assembled to large-area ordered Ag NWs, with mooth surface, nanowires parallel and closely connected to each other. The silver wire is the electron source and template for gold nanotube. Fig. 2c -l are scanning electron microscope images of the [email protected] hybrid nanotubes by galvanic replacement reaction. During the replacement reaction, the gray silver nanowire substrare turns brown, indicating the formation of Au nanoparticles. Fig. 2c-g shows the morphology of ordered [email protected] nanotubes were prepared by ordered Ag NWs reacting with 1 mM HAuCl4 solution for different times (2.5, 5, 10, 30min) respectively. As shown in Fig. 2, Au NPs formed on the surfaces of Ag NWs by the galvanic displacement reaction between Ag and AuCl4- for different times. With time going on, the layer of Au nanoparticles grow thicker. The incorporation of Au NPs onto the surface of Ag NWs forms mesoporous and hollow tube-like structure. The surface of the nanowires becomes rough, and a lot of nanoparticles are gradually formed. It was observed that the size and density of Au nanopaticles increases with the extension of reaction time, SEM showed that the gold layer on the surface of Ag NWs was synthesized, forming continuously and the porous gold nanotubes. The reason for the formation of gold nanoparticles was that the standard reduction potential of the Ag+/Ag pair (0.80 V vs. standard hydrogen electrode, SHE) is lower than that of the AuCl4-/Au pair (0.99 V vs. SHE).
2.1.2 Transmission electron microscopy of Ag NWs and [email protected] hybrid nanotubes
Fig. 3 shows TEM images of Ag NWs reacting with 1 mM HAuCl4 solution for different times. It can be seen that the surface of Ag NWs is smooth and uniform, the diameter is about 120 nm. After reacting with 1 mM HAuCl4, the surface of Ag NWs is corroded with the extension of reaction time. Fig 3b clearly shows that there are a few cavities on the surface of Ag NWs. With the increase of reaction time, particles form on the edge of cavities. These small particles agglomerate together to form larger clusters, and cavities are formed simultaneously. The porous hollow structure gradually formed, with the diameter increased slightly, and the surface becomes rougher.
2.1.3 Process analysis and mechanism discussion of galvanic replacement reaction
Because the standard electrode potential of Ag+/Ag（0.80V） is lower than that of AuCl4-/Au (1.00V), when Ag NWs were added into HAuCl4 solution, the displacement reaction between AuCl4- and Ag take place spontaneously. The process can be regarded as the corrosion process of Ag NWs, in which Ag is oxidized to Ag+ and AuCl4- is reduced to Au atoms. The chemical equation is as follows:
In the reaction, Ag NWs are electron sources and templates of gold nanotubes. Selective corrosion of the surface of Ag NWs plays an important role in the synthesis of [email protected] hybrid nanotubes. The as-synthesized Ag NWs are coated by (100) longitudinally and (111) at the end. PVP covers the (100) surface, passivating the surface, and hinders the deposition of silver atoms on the surface. In the displacement reaction, the uncoated (111) crystal surface first reacts with AuCl4- ions because of its high energy compared to (100). With the replacement reaction going on, the corroded surface of Ag NWs will be converted into surfaces with higher energy, which makes AuCl4- ions more likely to react on these new surfaces, resulting in more AuCl4- ions reducing to Au atoms at the sites where displacement reactions have taken place. The aggregation of these Au atoms leads to the grow of the particle size of gold nanoparticles. With the increase of reaction time, more surface silver atoms react with AuCl4- further, and more Au atoms are formed to form new crystal nuclei. These nuclei grow up with the reaction to form gold nanoparticles, which leads to the increase of the density of gold nanoparticles. At the same time, due to the higher surface energy of the Ag NWs near the formed gold nanoparticles, more AuCl4- ions are reduced to Au atoms near the original gold nanoparticles to form new crystal nuclei. With the further reaction, these new crystal nuclei and the original gold nanoparticles will grow up, leading to the aggregation of gold nanoparticles. From replacement reaction between Ag NWs and AuCl4-, only one Au atom can be replaced by three Ag atoms. Therefore, with the progress of the reaction, the corrosion degree of Ag NWs is far greater than that of gold nanoparticles, and the hollow structure is gradually formed.
3 UV spectra, fluorescence excitation and emission spectra
Silver and gold nanoparticles exhibit surface plasmon resonance with a specific wavelength in the ultraviolet–visible region, and the plasma resonance wavelengths are different . The surface plasmon resonance of [email protected] hybrid nanotubes is characterized by the UV-Vis spectrum. Fig.5 shows the UV Vis spectra of the AgNWs substrate, and different [email protected] hybrid nanotubes substrates. There are two characteristic peaks at 320 nm and 327 nm of AgNWs, and they corresponded to the quadrupole resonance and transverse plasmon resonance of nanowires, respectively. After the formation of Au nanoparticles, the characteristic peaks of UV Vis spectra of silver particles weaken or even disappear, and the broad peaks of gold nanoparticles appear at 520 nm, with the prolonging of reaction time, the UV-Vis spectrum the absorbance peaks redshift and become wider due to the increase of the number and volume of gold nanoparticles and the increase of the amount of gold of [email protected] hybrid nanotubes.
One of the most important factors affecting the fluorescence enhancement effect is related to the coincidence degree of the excitation/ emission spectra of photoluminescent materials. The higher the coincidence degree is, the stronger the enhancement effect is. The ordered [email protected] hybrid nanotubes have scattering or absorption characteristics will form their own plasma resonance peaks. Fig.6 shows the resonance spectra of ordered [email protected] hybrid nanotubes, UV absorption spectrum, excitation spectrum and emission spectra of P3HT. The excitation peaks appear at 557 nm and 596 nm of P3HT on the ordered [email protected] hybrid nanotubes. The maximum excitation wavelength of P3HT standard is 557nm; the emission spectrum of P3HT standard has two characteristic peaks at 665 nm and 726 nm respectively. The [email protected] hybrid nanotubes have lager overlapped plasma resonance peaks than that of Ag nanowires at 557 nm. Therefore, 557 nm is selected as the excitation source for P3HT steady-state fluorescence experiment. The overlap part of plasma resonance peak at 520nm of the hybrid nanotubes with excitation spectrum P3HT is much larger than that of plasmon resonance peak at 320 nm of the hybrid nanotubes with emission spectrum P3HT, and the overlap part of plasmon resonance peak of the hybrid nanotubes with P3HT standard emission spectrum curve is very small, which indicates that it is ordered [email protected] hybrid nanotubes. It is shown that the fluorescence enhancement effect of surface plasmon resonance on P3HT is mainly excitation enhancement.
4 Fluorescence effect of P3HT on different ordered [email protected] hybrid nanotubes arrays
Fig. 7 shows fluorescence spectra of P3HT on different ordered [email protected] hybrid nanotubes arrays. It can be seen that the fluorescence intensity of P3HT decreases with the increase of displacement reaction time. However, the fluorescence intensity of P3HT film on the ordered [email protected] hybrid nanotubes arrays is still greater than that on the glass substrate, which indicates that the ordered [email protected] hybrid nanotubes arrays can enhance the fluorescence effect of P3HT. After Ag NWs was replaced by gold nanoparticles on the surface of the ordered [email protected] hybrid nanotubes arrays, the enhancement of the fluorescence of P3HT on t hybrid nanotubes arrays was weakened. The fluorescence intensity is reduced because three Ag atoms are consumed by the generation of one Au atom; the Ag atoms will be significantly lost. Moreover, the order of the Ag NWs arrays is destroyed with the displacement reaction, resulting in the further decrease of the proportion of SPPs produced by the ordered Ag NWs. Moreover, with the displacement reaction, the order decreases with the displacement reaction, and the size and density of gold nanoparticles on the surface of [email protected] hybrid nanotubes increase. The local plasmon resonance magnetic field generated by the gold nanoparticles and the ordered Ag NWs in [email protected] hybrid nanotubes arrays is not matched, which leads to the poor coupling and propagation, and thus further reduced the surface local electromagnetic field of the hybrid nanotubes.
5 Raman effect of P3HT on different ordered [email protected] hybrid nanotubes arrays
The change of morphology or chemical structure of fluorescence molecular on the metal surface will cause the change of Raman peak position. Fig. 8 shows the Raman spectra of P3HT on different ordered [email protected] hybrid nanotubes arrays. There is no obvious change in the position of the Raman peaks of the P3HT on the hybrid nanotubes, which indicates that the existence of ordered [email protected] hybrid nanotubes arrays does not change the structure or chemistry properties of P3HT film. The two strong Raman peaks at 1380 cm-1 and 1444 cm-1 are the stretching vibration of C-C skeleton and the stretching vibration of thiophene ring C-C of P3HT, respectively. The results show that the local electromagnetic field has a great influence on the Raman signal, the Raman signal of P3HT weakened, followed by the local electromagnetic field. The intensity of Raman signal of P3HT film on different ordered [email protected] hybrid nanotubes arrays is obviously higher than that on the blank glass wafer, but the intensity of P3HT Raman signal decreases with the increase of reaction time, which indicates that the surface local electromagnetic field of the ordered Ag NWs is weakened after the displacement reaction, which is consistent with the above steady-state fluorescence spectra experimental results.