Introduction

Photo-driven chemical reactions, such as solar water splitting1,2, photodegradation3, solar conversion to electricity for photodetector4, solar cells5 and ultrafast optical data storage6,7 are widely considered for future applications towards renewable energy resources8,9,10,11,12. The underlying principle of photo-induced energy transformation is mainly based on the formation and separation of electron-hole pairs in semiconductor materials13,14. However, the limited absorption cross-section and narrow spectrum of conventional photoactive semiconductors (such as TiO2) due to their relatively large band gaps, could impede their further development for commercial value. Over the past decade, plasmonic metallic nanoparticles were extensively scrutinized due to their fascinating optical properties through an excitation of surface plasmon resonance (SPR)15. Surface plasmon is an incompressible electron cloud around finite metal, which exhibits a light-induced collective oscillation of these free electrons with incident photons as long as the collective coherent oscillation matches the frequency of photons. When localized at the surface of metallic nanoparticles this light-excited collective electron charge oscillation allows new strategies to provide platforms for conventional semiconductors, susceptible to cover the entire solar spectrum. Indeed, the tunability of the plasmon wavelength can be accurately controlled through the metallic nanoparticles properties such as their size, shape, and chemical composition16. The highly active photo-induced hot carriers (electrons with energies higher than the metal Fermi level) located on the metal are assumed to transfer to the conduction band of the semiconductor through a plasmon decay provided desirable nanostructures contact is achieved, leaving a “hot” hole on the metal17.

However, the transfer of hot carrier from metal to semiconductor derived from the surface plasmons decay might take place through multiple channels, and detailed mechanisms are still to be fully understood18. In brief, the transfer channels are categorized into either a radiative process, which could combine with far-field scattering and near-field electromagnetic enhancement effect, or a non-radiative process via SPR-mediated hot carrier injection10,19. In order to thoroughly investigate these mechanisms, it is necessary to elucidate the plasmon photo-physics process at various timescales, relating the energetic carrier dynamics and the transfer. The ultrafast measurement is a powerful approach to understand the dynamics of photo-induced electrons and holes in hetero-nanostructures20,21. The ultrafast dynamics of pure plasmonic metallic NPs have been investigated for more than two decades22,23. Many authors focused on nanospheres, nanorods, nanoprisms under transient absorption spectroscopy (TAS) measurements24,25,26, and better clarified the time scale of electron-phonon coupling and phonon-phonon coupling according to a two-temperature model27,28, and such a technique has even been extended to novel plasmonic oxides such as indium tin oxide (ITO) nanowires29 and indium-doped cadmium oxide (ICO) nanocrystals30. Nevertheless, compared with individual metals or semiconductor materials, hybrid systems, typically plasmon-semiconductor nanostructures, have rarely been reported so far. To date, M. Sun et al., for example, have investigated the plasmon-exciton interactions of metal and semiconductor hybrids for surface catalytic reactions31,32,33,34 via ultrafast pump-probe TAS in the Vis-NIR region. They demonstrated the probability and the enhanced efficiency of the surface Raman scattering of these hybrids to be co-driven by graphene-AgNWs hybridization35. Further, N. Wu and co-workers studied metal@Cu2O interactions such as Ag@Cu2O, Au@Cu2O and Au@SiO2@Cu2O core-shell nanostructures via TAS and showed that the photocatalytic activities were raised by simultaneously a hot electron transfer (HET) and a plasmon-induced resonant energy transfer (PIRET)36,37,38. These works demonstrate that (i) plasmon mediated charge and energy transfer can overcome the band edge constraints of single semiconductors, and (ii) the metal-semiconductor core-shell interactions have promising potentials for enhancing solar-light harvesting and energy-conversion efficiency. However, detailed explanations on the mechanisms concerning how metal-semiconductor interactions take place, and a universally applicable physical modelling considering size, shape and compositions of the hybrids still remain challenging and need more experimental and theoretical supports11.

Herein, in the present work, we prepared AgNWs@TiO2@AuNPs heterostructures through a facile wet chemical approach. As illustrated in Fig. 1, the as-synthesized AgNWs (depicted as the white pentagonal prism) are firstly coated with a TiO2 shell with well-defined thickness (blue) via typical sol-gel chemistry, and then such nanostructures are further grafted with AuNPs (yellow) to obtain AgNWs@TiO2@AuNPs heterostructures. These tunable nanostructures allowed focusing on the plasmon decay lifetime observation under the influence of various parameters. We finally studied two effects: (i) TiO2 shell thickness vs. silver nanowires photobleaching lifetime resulted from plasmon mode. (ii) The emerging transfer channel raised by AuNPs decorated on the TiO2 surface. These new observations may provide valuable strategies to design better SPR-mediated carrier transfer nanostructures towards future promising photochemical transformation devices.

Figure 1
figure 1

Schematic representation synthesis of the AgNWs@TiO2@AuNPs heterostructures with thin and thick layers of TiO2. White pentagonal prism stands for Ag, blue coating stands for TiO2 layer, and light-yellow particles are AuNPs.

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Results and Discussion

Morphology and spectral properties of plasmonic TiO2 nanostructures

PVP (poly(vinyl pyrrolidone)) coated silver nanowires (40 ± 2 nm) were functionalized via MUA (mercaptoundecanoic acid) in pure ethanol. Then, a thickness-controlled layer of titania was deposited on the surface of AgNWs by using a proper sol-gel chemistry. Since it was well-known that the reactivity of titanium alkoxides is much higher than that of silicon alkoxides, e.g. TEOS the hydrolysis-condensation rate of the titanium precursor, i.e. TTIP (titanium (IV) tetraisopropoxide), needed to be slowed down via chemical retardants. Acetyl acetone (Acac) is such a complexing ligand which by coordinating titanium makes the Ti-Acac bond much less hydrolysable39. The reactivity of TTIP is then generally controlled through the molar hydrolysis ratio: H2O/Ti and the complexation ratio: Acac/Ti. Basically, when Acac/Ti ≥ 2, the resulting complex is stable enough to slow down the hydrolysis of TTIP for coating on AgNWs. (Data not shown here).

Then, the molar hydrolysis ratio of H2O/Ti and the total concentration of Ti precursor are crucial for the thickness-controlled coating of TiO2 onto the AgNWs. As an example, at a fixed concentration of 14 mM TTIP (Acac/Ti ratio: 2/1), when H2O/Ti ≤ 1.5, almost no clear coating of TiO2 (1.8 ± 0.3 nm) is observed indicating that the hydrolysis-polycondensation process was mainly inhibited probably because there was not enough water (Fig. 2a,b). When H2O/Ti = 4.5, a thick layer (140 ± 9 nm) of TiO2 and no free titania nanoparticles were observed (Fig. 2c,d), which suggests that at this molar hydrolysis ratio, the TTIP could be fully hydrolyzed and polycondensed on the AgNWs surface.

Figure 2
figure 2

TEM images of AgNWs@TiO2 nanostructures synthesized with (a,b) H2O/Ti = 1.5, (c,d) H2O/Ti = 4.5. Inserts are magnified TEM image. The estimated thickness of TiO2 coating in (b) is 1.8 ± 0.3 nm and in (d) is 140 ± 9 nm. All the statistics are performed with more than 100 times measurements of the objects. 1 mL 14 mM TTIP mixed with Acac (1:2 molar ratio) was prepared for each run.

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In order to obtain a tunable thickness of the TiO2 coating down to several nanometers, the concentration of the Ti precursor was then studied. To investigate this effect, the H2O/Ti ratio was set as 4.5 to make sure all the Ti precursor was hydrolysed as previously indicated, and the Ti precursor solution (14 mM TTIP, Acac/Ti = 2) was diluted 2 to 10 times as described in the experimental part. In this way, the thickness of the TiO2 coating was strongly related to the concentration of TTIP as shown in Fig. 3. When the initial TTIP concentration is increased from 1.4 mM (ten times dilution) to 7 mM (two times dilution) and 14 mM (no dilution), the thickness of TiO2 coating varied from approximately 5 nm, to 70 nm and 140 nm respectively as confirmed by TEM observations (Fig. 3b–f) and EDX-mapping (Fig. 3g–j) of the sample presented in Fig. 2b. The absorption spectra of the silver sols change during the deposition process, as shown in Fig. 4a, where the position of the surface plasmon band is shown to be dependent on the titania thickness. The peak position prior to titania deposition was 383 nm; it red-shifted for silver sols modified with an increasing amount of titania. The red-shift is caused by the increased refractive index around the colloid particles after the titania deposition and the scattering from the shell and the thicker the semiconductor coating around the particles, the larger the shifts. For a shell thickness beyond 70 nm, almost no clear plasmon band was observed (Data not shown), indicating that a sufficiently large titania shell promotes significant scattering which makes the plasmon band of AgNWs hard to be detected. Theoretical extinction spectra were calculated to quantify the effects of the different titania shell thicknesses on the optical response of the silver nanowires (Fig. 4b)40. The simulation considered model nanowires with a pentagonal symmetry coated with uniform titania shell. Johnson and Christy’s data41 were used for the silver dielectric function, while a uniform refractive index of 1.33 was used for the nanoparticle environment as this value typically leads to a good reproduction of resonance positions of chemically synthesized nano-objects in aqueous suspension with noting that the small shoulders are the higher ordered multipole effects such as quadrupole. These simulations do confirm that the main effects of increasing the thickness of the titania layer are a red-shift and a broadening of the plasmon resonances. Figure 4c shows the position of the extinction peak predicted through calculations using the Finite-Difference Time-Domain (FDTD) method to calculate the fields and the associated absorption and scattering cross-sections., which were basically in qualitative agreement with the measurements.

Figure 3
figure 3

Controlled synthesis of AgNWs@TiO2 core-shell nanostructures. (a) typical image of a silver nanowire; (b–f): typical TEM images showing the variation of the average thickness of the TiO2 shell as a function of the concentration of TTIP precursor (from 5 to 70 nm, with TTIP concentration from 1.4 mM to 7 mM, respectively). More than 50 measurements were done for each sample for the thickness estimation and circa values were obtained by rounding off; (g–j) Dark field TEM image (g) of a AgNWs@TiO2 nano-object with a 5 nm shell thickness as in (b) and its corresponding EDX-mapping of (h) Ag, (i) Ti and (j) overlay of Ag, Ti and O.

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Figure 4
figure 4

(a) Effects of titania thickness on the UV spectra of titania-coated silver colloids. There is a red-shift of the peak position with increasing TiO2 thickness. (b) Calculated extinction spectra of Ag nanowires without and with different TiO2 shell thicknesses via FDTD method. The small shoulders are the higher ordered multipole effects. (c) Plot of transverse plasmon peak position of AgNWs@TiO2 vs. the TiO2 shell thickness: black dots: theoretical results and red dots: experimental ones. All AgNWs have a diameter of 40 ± 2 nm and lengths of tens of µm with narrow size distribution.

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AgNWs@TiO2@AuNPs hybrid nanostructures were synthesized thanks to electrostatic interactions acting as the driving force between negatively charged AgNWs@TiO2 nanowires in pure water (IEPTiO2 close to 6.2) and the positively charged APTES (APTES: 3-Aminopropyl) triethoxysilane)-modified gold nanoparticles introduced at pH~8. A typical grafting of the AuNPs on the AgNWs@TiO2 nanowires with 5 nm TiO2 shell is illustrated in Fig. 5. The overall surface of the AgNWs@TiO2 can be tagged with gold nanoseeds (4–5 nm). Further, as the TiO2 thickness increased from 5 nm to 20 nm, a red-shifted from 383 nm to 400 nm of the characteristic transverse SPR mode of the AgNWs is observed and a weak absorption peak (except in the 20 nm TiO2 sample) at 538 nm which stands for the absorption of the grafted AuNPs on the AgNWs@TiO2 surface is generated (Fig. S2 and Table 1). These observations are probably due to both the effects of refractive index increase and the plasmonic coupling of AuNPs-AgNWs and closely packed AuNPs (the AuNPs alone have a SPR peak at 519 nm, see Fig. S3), since the size of grafted AuNPs does not change and no clear agglomerates form during the synthesis. Notably, the same amounts of AuNPs were used in each AgNWs@TiO2@AuNPs synthesis suggesting that for 10 nm and 20 nm TiO2 samples, the grafting of AuNPs would be less efficient than for the thinner 5 nm TiO2 coating sample due to the increased surface area, which leads to a less densely packed surface of AuNPs (Fig. S2).

Figure 5
figure 5

Typical TEM images of AgNWs@TiO2 nanostructures with 5 nm thickness of TiO2 shell before and after AuNPs grafting.

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Table 1 Summary of the UV absorption peak positions of AgNWs@TiO2@AuNPs nanostructures.
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Non-radiative carrier relaxation dynamics of AgNWs@TiO2

In metallic nanoparticles, the coherent plasmon oscillation induced relaxation dynamics can undergo electron-electron scattering followed by electron-phonon scattering processes27. Time-resolved pump-probe spectroscopy measurement is then a powerful tool to examine these processes due to their desirable time scale down to the picosecond. The transient absorption contour map of pure AgNWs is displayed in Fig. S4a. The photo-bleaching signal evidences the decay of the plasmon-induced electron relaxation due to the coincidence of the bleaching and the steady-state absorption peaks. The bleaching maxima locates around 349 nm and 371 nm resulting from the plasmonic modes of the pentagonal silver nanowires similar to those of bulk Ag at 350 nm42,43,44,45,46. Initially, the nanowires produce a non-Fermi electron distribution after the dephasing resulting from elastic and inelastic electron scattering (about 10 fs). The ultrafast LSPR dephasing time is usually estimated through the line widths measurements of single nanoparticles. The distribution can quickly bring thermalization via electron-electron scattering, which results in a high temperature electron distribution. After this process, the electron gas, which has plasmon energy cools down to equilibrate the temperature between electron and lattice, which is attributed to electron-phonon interactions. These established photo-physics mechanisms explain the bleaching peak around the plasmon wavelength and the subsequent decay of bleaching over a period of time within picoseconds. The corresponding electron-phonon coupling time τe-ph can be calculated as γT0/G under low excitation powers, where T0 is the ambient temperature and γ is a damping constant27,47. The corresponding electron-phonon coupling constant G directly determines the electron relaxation lifetime; the stronger the interaction between electrons gas and phonon in the lattice and the faster the electron gas cooling as well as the shorter the bleaching decay in TAS spectra27.

Time-resolved pump-probe spectroscopy measurements of the core-shell nanostructures are illustrated in Figs 6a and S4a, which show the TAS 2D color map of AgNWs with 20 nm TiO2 shell as well as that of the pure AgNWs, respectively. We select 3.87 eV pump beam energy for the plasmon excitation in order to avoid the pump scatter disturbance or saturation onto the plasmonic peak (at 350 nm)48. The differential transmission at various time delays of the same sample (Fig. 6b) shows two bleaching peaks located at 357 nm and 388 nm resulting from the plasmon modes of AgNWs@TiO2. Besides, two weak and broad positive absorption peaks appear at lower and higher energies around bleaching plasmon band. The photo-induced absorption wings are consistent with previous reports involving silver nanospheres, gold nanospheres, and gold nanorods24, which is attribute to a broadening and blue-shift of the plasmon resonance as electron temperature rises after optical pumping49. The relaxation of carriers cooling is investigated by varying the TiO2 coating layer thickness. The normalized decay of the photobleaching at 357 nm in Fig. 6c shows that not only does the TiO2 coating induce a faster relaxation time as compared to pure AgNWs, but also the thinner the TiO2 layer the faster the decay time. The corresponding time constants of carrier lifetimes were best fitted with a mono exponential function and those derived from the mono exponential listed in Table 2 highlight the strong influence of the shell thickness on the downfall decay time from 5.9 ps of τe-ph in AgNWs to 0.87 ps in the AgNWs@TiO2 core-shell structure. Increasing the shell thickness leads to a subsequent increase of this life time up to a saturation value for the 20 nm and 50 nm samples of 2.55 ps (inserted curve in Fig. 6c), but still shorter than pure AgNWs.

Figure 6
figure 6

Transient absorption measurements of the AgNWs with different TiO2 shell thicknesses. (a) ΔOD spectral mapping of AgNWs@TiO2 with a 20 nm shell. (b) Transient absorption spectra at various time delays of AgNWs@TiO2 with 20 nm shell (c) Normalized curves of shell-thickness dependence of the AgNWs plasmon photobleaching decay dynamics. Probe wavelength is 349 nm. All the experiments were conducted under pump wavelength of 320 nm (3.87 eV) with average power of 3 mW.

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Table 2 Amplitudes (A) and time constants (τ) derived from exponential fit of the transient absorption of various TiO2 shell thicknesses at the photobleached band located at 357 nm (349 nm for pure AgNWs).
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The existence of a direct contact between the semiconductor and the metallic nanowires facilitates the carrier transfer between them. The photo-excited surface plasmon can decay through non-radiative channel generating electron-hole pairs as mentioned before. A previously established model was employed to study the size-dependent carrier decay time in silver nanoparticles and nanotriangles as28:

$$G\propto {\int }_{0}^{{q}_{D}}{q}^{3}{(\frac{{V}_{q}}{{\varepsilon }_{0,q}})}^{2}dq$$

Where the electron-phonon coupling constant, G, is related to the unscreened electron-phonon interaction potential Vq and inversely proportional to the dielectric function ε0,q, which is associated to the electron density ne. The spill-out of electron changes the ε, making less screening and improving the coupling rate. Hot electron injection could happen from the metal to the semiconductor conduction band to interrupt original electron-phonon coupling channel. The new decay channel through injection may provide similar effect and accelerate the relaxation of hot electrons in silver nanowires. Besides the hot electron transfer, other mechanisms such as defect-mediated recombination and PIRET process could also exist in this nanostructure system leading to shell thickness dependent decay lifetime. However, it is worth noting that it is difficult for our current pump-probe transient set-up to discern these mechanisms and it is apparently beyond our scope in the work.

In this system, the more available hot carriers transferred from the metal they could more efficiently result in a faster energetic charge carrier injection into TiO2, leading to a shorter carrier lifetime as shown in TAS measurements which is similar to hot electron injection in MoS2 nanosheets50. Moreover, the performed absorption and scattering calculations for pure AgNWs and AgNWs with 20 nm TiO2 (Fig. S5) show that the scattering is more dominant in AgNWs than in the core-shell while the absorption fraction prevails in the core-shell nanowires. TiO2 coatings improve the generation of electron-hole pairs derived from plasmon decay because the radiation pathway is determined by scattering while the non-radiative electron-hole pairs pathway results from absorption. The simulation outcomes are similar to those presented in a recent report on Ag-Pt core-shell nanocubes51. The larger production of e-h pairs may strongly increase the carrier injection rate. The simulated electric field enhancement also demonstrated the thinner shell sample exhibiting much larger near-field hot spot enhancement than the thicker one, which is shown in Fig. S6. However, the easy injection from silver to TiO2 via overcoming Schottky barrier could also lead to reverse transfer due to the lack of insulator at the silver-TiO2 interface. The thicker TiO2 layers also create more photo-excited electrons in the conduction band due to the high energy of pump laser. Owing to the interband transition of TiO2, this provides more pathways for a back transfer from TiO2 to AgNWs, as a result of less hot electron extraction and spill out of electrons. In addition, the thicker shells could result in a slower back recombination rate of injected hot electrons in AgNWs52, which also leads to a longer carrier relaxation time for the 357 nm plasmon band. The saturation of the lifetime constant with the TiO2 shell thickness (up to 50 nm) is significant of a possible electron transfer equilibrium between the metal and the semiconductor. The final lifetime value still lower than that of the bare individual silver may be attributed to a larger absorption cross-section in plasmonic nanowires. In addition, Govorov’s recent simulation work also demonstrated that the thinner TiO2 shell could result in more energetic electrons in hot spots, which provides greater opportunities for hot electron injection processes due to the induced carriers with higher energy53. Meanwhile, the hot electrons injection is a competing process against electron-electron scattering, which reduces the excited energy of an electron preventing the excited electron from having enough energy to cross the interfacial energy barrier54. Besides, weaker e-e scattering indicates a lower initial temperature of electrons Te, leading to a faster e-ph coupling decay time according to the two-temperature models. The higher hot electron injection efficiency could induce faster decay of electron phonon coupling55. According to the above-mentioned proposal, we conclude the hot electron transfer process between metal and TiO2 could play an important role for their transient dynamics performance.

Relaxation dynamics after AuNPs decoration

The small APTES-modified AuNPs have been deposited onto AgNWs@TiO2 surface to form the desirable nanostructures. The concentration of gold NPs was fixed low enough so as it did not affect their surface plasmon behaviour apparently. The same kind of transient spectroscopic studies were performed after addition of AuNPs on 5 nm TiO2 layer (Fig. 7a) and 20 nm TiO2 layer (Fig. 7b), and we could easily observe that the bleaching decay for AgNWs plasmon got faster after AuNPs decoration. Table 3 gives a summary of their dynamics performance. With the presence of AuNPs onto the TiO2 surface, the lifetime of e-ph coupling decreases to 0.43 ps for the 5 nm thickness TiO2, which exhibits a 51% change. Unexpectedly the decrease is of only 7% for the 20 nm TiO2 sample, which decreases from 2.53 ps to 2.35 ps after AuNPs decoration. The detailed experimental results are shown in Supplementary Information (Fig. S4).

Figure 7
figure 7

Normalized decay kinetics at 357 nm for the reference AgNWs (black sphere), before (purple) and after (yellow) addition of AuNPs onto AgNWs@TiO2, with 5 nm (a) and 20 nm (b) TiO2 shell thickness.

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Table 3 Amplitudes (A), time constants (τ), derived from exponential fits of the transient absorption for the AuNPs decorated onto 5 nm and 20 nm TiO2 shells at the photobleaching band located at 357 nm.
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The small AuNPs could bring external plasmon-phonon coupling to assist the original plasmon induced electron-phonon interaction of silver nanowires and electron transfer, which is similar to the perovskites-TiO2 injection56. The result demonstrates that the fact to have two plasmon modes generates a synergy for a hot electron transfer from silver nanowires and a faster electron injection into TiO2. This is confirmed by the role of the shell thickness since the thinner the TiO2 layer exhibits the more efficient the bleaching decay. Nevertheless, the detailed mechanism of plasmon-assisted fast bleaching decay still needs to be further investigated. It is also worthwhile noting that the thinner TiO2 layer allows a much severe decrease in decay time after the same deposition of AuNPs, highlighting the stronger interaction between AuNPs and AgNWs plasmons because of shorter distance. Such studies of carrier relaxation dynamics provide a promising pathway to design nanostructures with a high efficiency for hot electron injection focusing on energy conversion devices. However, further fundamental studies are needed to clarify these hybrid nanostructures for practical catalysis applications. Among these works there is a need (i) to optimize desirable plasmon nanohybrids through preparation of nanoobjects with distinct contribution of interband/intraband transition under pump wavelength dependent TAS measurement57, (ii) to elucidate different plasmon-induced transfer mechanism such as charge transfer or energy transfer according to tunable plasmon/semiconductor spectra overlap58, (iii) to determine carrier transfer channels such as sequential process or direct process stem from chemical interface damping59 under line width observation60. The ultrafast and spatially resolved measurement technique could provide a promising approach to analysis the physical and chemical origins of these photo processes.

Discussion

In summary, we have prepared different shell thicknesses of TiO2 coating onto silver nanowires surface, and functionalized them with small AuNPs to form AgNWs@TiO2@AuNPs nanostructures via a facile wet chemical approach before proceeding to the study of their morphology and steady optical properties. The plasmon decay dynamics of silver nanowires were also measured thanks to ultrafast pump-probe technique. Our results indicate a faster decay time after TiO2 shell coating and its dependence on the shell thickness, showing that this decay time decreases as the shell thickness becomes thinner. The shorter carrier lifetimes obtained from TAS measurements reveal the strong electron-phonon interaction, which could be a result of a hot carrier injection process. The simulation of the absorption and the scattering fraction further supports the hypothesis that TiO2 coating facilitates the electron-hole pairs formation. Lifetimes obtained from photobleach recovery measurements suggest that the presence of gold nanoparticles did improve the hot carrier transfer via plasmonic coupling effects and the emerging transfer channel is greatly affected by the TiO2 shell thickness. We expect these multiple plasmon modes of such core-shell nanostructures to be a potential tool for enhancing the efficiencies of photo-driven energy conversion devices.

Methods

Synthesis of AgNWs and surface modification

The PVP capped AgNWs were prepared by a modified polyol method61. The synthesis yielded a large amount of silver nanowires with a typical diameter of 40 ± 2.1 nm and lengths of tens of µm with narrow size distribution. Then, these AgNWs were surface modified with mercaptoundecanoic acid (MUA) as follows: 1 mL of as-synthesized AgNWs (0.1 mg/mL) was added with 18 mL pure ethanol and mixed with 1 mL of a 10 mM MUA solution in pure ethanol. After that, the mixture was kept on a roller-mixer overnight at a rate of 250 rpm/min. Then, the mixture was washed with pure ethanol and centrifuged 3 times before eventually keeping them in 20 mL pure ethanol at a concentration of 0.08 mg/mL.

Synthesis of APTES-AuNPs

Around 4 nm APTES-AuNPs were synthesized by using X. Sun’s method with slightly modifications62. Typically, 400 μL of APTES were added to 0.84 mL of a 0.1 M HAuCl4 aqueous solution with 2.66 mL Milli-Q water under vigorous stirring at room temperature to obtain supramolecular microstructures of the mixture. Then the colloidal solution was heated up to 100 °C for 30 min and kept at room temperature overnight, followed by centrifugation to remove free APTES and other by-products. The synthesized AuNPs colloid has a concentration of 10.88 × 103 nM and an average diameter of 4.2 nm as observed by TEM measurement (not shown).

Controlled deposition of TiO2 on AgNWs

The deposition of TiO2 on the AgNWs surface was performed using acetylacetone as a retardant reagent to control the hydrolysis and polycondensation of the titanium precursor. In an optimized synthesis, a total 23 mL solution including: 20 mL of as-synthesized MUA functionalized AgNWs (0.08 mg/ml) in pure ethanol and a mixture of 0.25 mL of Milli-Q water and 2.75 mL absolute ethanol were introduced in a 50 mL flat-bottom flask with stirring as a template solution. In parallel, 1 mL of precursor solution at a concentration of 14 mM TTIP mixed with Acac (1:2 molar ratio) was prepared. The precursor solution was then diluted with various fractions of pure ethanol. The thickness of the TiO2 coating was effectively dependent on the degree of dilution, typically to obtain 5, 10, 20, 50 and 70 nm thickness of TiO2, the precursor solution must be diluted 10, 5, 4, 3 and 2 times respectively via addition of pure ethanol. 1 mL of either of these light yellow solutions was then injected into the template solution at a rate of 1 mL/h by a syringe pump. After the completion of the injection, the resulting colloidal suspension was heated up to 80 °C with stirring for 1.5 h to ensure the condensation procedure was completed. The colloidal suspension was then washed by 5 extensive centrifugation cycles with pure EtOH and eventually stored in 25 mL pure ethanol at a concentration of approximately 0.5, 0.8, 1.4, 1.9 and 2.1 mg/mL for ca. 5, 10, 20, 50 and 70 nm samples respectively. The obtained colloidal suspensions hereafter named as AgNWs@TiO2 are then ready for the next step. In any case, for large production, the reaction can simply be scaled up to 10–20 times.

Synthesis of AgNWs@TiO2@AuNPs

Typically, for AgNWs with 5 nm TiO2 shell, 0.1 mL, 10.88 × 103 nM of the as-synthesized APTES-AuNPs was diluted ten times with 0.9 mL pure water and sonicated for 10 min before use. Then 1 mL of 0.5 mg/mL AgNWs@TiO2 was added to the APTES-AuNPs solution with 15 min sonication. The mixture was kept at room temperature on a roller-mixer overnight at a rate of 250 rpm/min. Then, the mixture was washed 3 times with pure water and precipitated by centrifugation and kept eventually in 2 mL pure water at a concentration of 0.1 mg/mL. The obtained colloidal suspension hereafter named as AgNWs@TiO2@AuNPs is then ready for the next step. In any case, for large production, the reaction can be simply scaled up to 10–20 times.

Morphology characterization

Transmission electron microscope experiments were performed with a JEOL JEM-1400Plus microscope operating at 120 kV. The samples were prepared as follows: colloids were diluted in ethanol and one drop of the diluted suspension was deposited on a copper grid coated with a carbon membrane. Chemical analysis was carried out by STEM coupled to EDX were acquired with a JEOL 2200 FS equipped with a field emissive gun, operating at 200 kV and with a point resolution of 0.23 nm.

Optical simulation

The simulated extinction spectra were performed using a three-dimensional module on Finite-difference time-domain (FDTD) based software (Lumerical Solutions). The model systems include a background surrounding with water and the dielectric function data for silver taken from Johnson and Christy.

Visible femtosecond transient absorption measurements

In this study Spectra-Physics Solstice were utilized as the ultrafast laser beams with a regeneratively amplified Ti: sapphire laser system (Coherent Legend, 800 nm, 100 fs and 1 kHz repetition rate) and the signal acquisition was collected by optical fiber. The 800 nm output laser beams were firstly split at BS1, one part was through into Traveling Wave Optical Parametric Amplifier (TOPAS) define to generate tunable pulse for pump. A series of neutral-density filter wheels were used to adjust the power of the pump beam. The pump beam was focused at the sample with a beam waist of about 200 μm as following chopped by as synchronized chopper to 500 Hz. Another part from beam split was focusing the 800 nm probe, which went into a CaF2 window to create a white light continuum (WLC) from 350 nm to 800 nm. The probe beams were focused into a fiber-coupled multichannel spectrometer with complementary metal-oxide-semiconductor (CMOS) sensors and detected at a frequency of 1 kHz. The delay between the pump and probe pulses was controlled by a motorized delay stage. The samples were dispersed in ethanol and averaged over several spots to ensure uniformity. The collected data are fitted in Origin 8.5 software with proper exponential fittings with respect that the time range presented are as representative as possible.