Ultrathin Oxide Layer-Wrapped Noble Metal Nanoparticles via

Oct 23, 2017 - Ultrathin Oxide Layer-Wrapped Noble Metal Nanoparticles via Colloidal Electrostatic Self-Assembly for Efficient and Reusable Surface En...
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Ultrathin Oxide Layer-Wrapped Noble Metal Nanoparticles via Colloidal Electrostatic Self-Assembly for Efficient and Reusable Surface Enhanced Raman Scattering Substrates Haoming Bao,†,‡ Hongwen Zhang,*,† Le Zhou,†,‡ Guangqiang Liu, Yue Li, and Weiping Cai*,†,‡ †

Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China ‡ University of Science and Technology of China, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: Controllable and flexible fabrication of ultrathin and uniform oxide layer-wrapped noble metal nanoparticles (NPs) has been expected. Here a new strategy is presented for them based on colloidal electrostatic attraction and selfassembly on the metal NPs via one-step laser ablation of noble metal targets in the hydrolysis-induced hydroxide sol solutions at room temperature. The Au NPs, with several tens of nanometers in size, are taken as core part and TiO2 as shelllayer to demonstrate the validity of the presented strategy. It has been shown that the TiO2 shell-wrapped Au NPs are obtained after laser ablation of Au target in the hydrolysis-induced Ti(OH)4 sol solution. The Au NPs are about 35 nm in mean size, and the TiO2 shell layers are amorphous in structure and about 2.5 nm in thickness. The shell thickness is nearly independent of the Au NPs’ size. Further experiments have shown that the thickness and crystallinity of the shell-layer can be tuned and controlled via changing the temperature or pH value of the Ti(OH)4 sol solution or prolonging the laser ablation duration. The formation of the TiO2 shell-wrapped Au NPs is attributed to attachment and self-assembly of Ti(OH)4 colloids on the laser-induced Au NPs due to the electrostatic attraction between them. Importantly, the presented strategy is universal and suitable for fabrication of many other ultrathin oxide-wrapped noble metal NPs. A series of oxide shell-wrapped noble metal NPs have been successfully fabricated, such as Au@oxides (Fe2O3, Al2O3, CuO, and ZnO) as well as Pt@TiO2 and Pd@TiO2, etc. Further, compared with the pure gold NPs-built film, the TiO2-wrapped Au NPs-built film has exhibited much stronger surface enhanced Raman scattering (SERS) performance to the anions NO3−, which weakly interact with noble metals, and the good reusability for the SERS-based detection of 4-nitrophenol, which could be photodegraded by xenon lamp irradiation. This work provides a flexible and universal route to the ultrathin and uniform oxide layer-wrapped noble metal NPs. weakly interact with noble metals.12 However, due to the shortrange effect of SERS for the noble metal NPs, the shell is expected to be uniform and thin enough in thickness (in nanoscale), and normally, it should be thinner than 5−10 nm at most.10,13,14 Traditionally, the syntheses of CS-NPs are mainly based on wet chemical methods, which include the Stöber method,15 the hydrothermal method,16 and the sol−gel method,17,18 and so on. For instance, Yoshio et al. fabricated the silver NPs coated with silica shell of several tens of nanometers in thickness through the Stö ber method.15 Kim et al. adopted the microwave assisted hydrothermal method to synthesize gold NPs wrapped with 60 nm TiO2 shell.16 Sakai et al. obtained 5− 10 nm TiO2 shell-wrapped Ag NPs via a sol−gel reaction of titanium tetraisopropoxide by using cetyltrimethylammonium

1. INTRODUCTION The core−shell structured nanoparticles (CS-NPs), consisting of the core part (inner material) and the shell layer (outer layer material), have received considerable attention recently due to their unique peculiarities, such as the functionalized modification of shell,1 tailorable properties,2 improved stability and dispersion,3 and synergistic properties of shell and core.4,5 Especially, the CS-NPs with noble metal (Au, Ag, etc.) cores and semiconducting oxide shells possess the excellent surface plasmon resonance (SPR) effect of noble metal cores and the unique modulated band structure of semiconducting shells, and hence have the massive potential applications in photocatalysis,6 electrocatalysis7 and photoelectric conversion,8 and surface enhanced Raman scattering (SERS) substrates.9,10 For instance, if such NPs are used for the SERS substrate, the shell layer not only avoids the direct contact between the analyte molecules and the noble metal surface which may lead to the photodegradation of the analytes during Raman measurement,11 but also could enrich some analyte molecules which © XXXX American Chemical Society

Received: July 26, 2017 Revised: September 18, 2017 Published: October 23, 2017 A

DOI: 10.1021/acs.langmuir.7b02610 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration for the fabrication strategy of ultrathin oxide layer-wrapped metal NPs base on the electrostatic colloidal attraction and self-assembly. (a) Adsorption or attachment of hydroxide colloids on a metal NP due to the electrostatic attraction. (b) Formation of monolayer hydroxide colloidal shell by colloidal self-assembly on the metal NP. (c) Formation of ultrathin oxide shell layer on the metal NP by dehydration.

bromide as protective agent.18 Generally, the wet chemical methods are multistep and tedious, and need various chemical reagents. The products are often encapsulated with surfactants which are difficult to be removed. In addition, an ultrathin and uniform shell is also unavailable. Recently, a facile and surfactant-free method based on laser ablation in solutions has been used to fabricate nanomaterials with various structures, 12, 19−23 including ultrathin layer-wrapped NPs.12,22,23 For example, the carbon shell-wrapped inorganic NPs were fabricated by pulsed laser ablation of the inorganic targets in hydrocarbon solutions.22 Silver NPs wrapped with graphitic carbon were obtained via laser ablation of a Ag plate in toluene solution.23 Mostly, the research focused on the fabrication of carbon-wrapped noble metal NPs, while the reports on semiconducting oxide-coated noble metal NPs are very limited.12 The controllable and flexible method to prepare the ultrathin and uniform oxide-coated noble metal NPs is still expected. Here, a flexible strategy to fabricate ultrathin and uniform oxide layer-wrapped noble metal NPs is presented based on colloidal electrostatic self-assembly. Au NPs, with several tens of nanometers in size, are taken as core parts and TiO2 as shelllayers to demonstrate the validity of this strategy. The strategy is realized just by one step laser ablation of Au target in the hydrolysis-induced titanium hydroxide [Ti(OH)4] sol solution. The laser ablation was employed for the clean surface of the core parts.19 It has been shown that the amorphous TiO2 shellwrapped Au NPs can be acquired after laser ablation. The mean thickness of the shell is about 2.5 nm and nearly independent of the Au NPs’ size. The thickness and crystallinity of the shelllayer can be tuned and controlled via adjusting the temperature or pH value of the sol solutions or prolonging the laser ablation duration. Importantly, the presented strategy is universal and suitable for preparation of the other ultrathin oxides-wrapped noble metal NPs, such as Au@oxides (Fe2O3, Al2O3, CuO, and ZnO) as well as Pt@TiO2 and Pd@TiO2, etc. Further, compared with the pure gold NPs-built film, such ultrathin TiO2-wrapped Au NPs-built ones have exhibited much stronger SERS performance to the anions NO3−, which weakly interact with noble metals, and good reusability for the SERS-based detection of 4-nitrophenol (4-NP) molecules, which can be photodegraded by xenon lamp irradiation. This work not only provides a flexible and universal fabrication route for the ultrathin and uniform oxide layer-wrapped noble metal NPs, but also presents a series of such NPs-built substrates with strong SERS performances and good reusability for SERS-based detection of some special molecules. The details are reported in this article.

2. STRATEGY It is well-known that many metal salts or cations (Fe3+, Al3+, Cu2+, Ti4+, Sn4+, etc.) can be hydrolyzed to form the corresponding hydroxide sol solutions with the evident Tyndall effect. Such hydroxide colloids are normally about few nanometers in size.24−26 According to the theory of electrical double layer of colloidal particles,27 the many hydroxyl colloids, such as aluminum hydroxide and iron hydroxide, tend to be positively charged due to adsorption or attachment of cations on them,28,29 while noble metal (Au, Ag, Pt, Pd, etc.) colloids are generally negatively charged.30,31 It could thus be expected that if a colloidal solution, containing noble metal NPs with several tens of nanometers in size, is mixed with the hydroxide sol solution, there would be strong electrostatic attraction between the metal NPs and hydroxide colloids. The smaller hydroxide colloids would thus be adsorbed on the surface of the much bigger noble metal NPs, and a monolayer hydroxide colloidal shell would be formed on the metal NPs due to the colloidal self-assembly, as schematically shown in Figure 1a,b. Subsequent dehydration would lead to formation of oxide shell layer-wrapped metal NPs (Figure 1c). Obviously, in this case, the shell thickness depends on the hydroxide colloidal size, which could be controlled by tuning the pH value and temperature of the solution, but should be independent of the metal NPs’size. Such idea has been confirmed in this work. The strategy shown in Figure 1 is general and suitable for fabrication of noble metal NPs@oxides. Here we first take Au@TiO2 as an example to demonstrate the validity of this strategy, via one step laser ablation of Au target in the titanium hydroxide [Ti(OH)4] sol solution, as illustrated in Figure S1. 3. EXPERIMENTAL SECTION 3.1. Materials. The metal salts [TiCl4, Zn(NO3)2·6H2O, FeCl3· 6H2O, KAlSO4·12H2O, and Cu(AC)2·H2O] and the ethanol were analytical reagents and purchased from Alfa Aesar Corporation. The noble metal targets Au (25 mm × 25 mm × 3 mm in size), Pt (25 mm × 25 mm × 3 mm in size), and Pd (ϕ15 mm × 3 mm in size) were bought from Hefei Kejing Materials Technology Co., Ltd. with purities >98%. Deionized water with a resistance of 18.2 MΩ (25 °C) was prepared by a Millipore Milli-Q system. 3.2. Fabrication of Au@TiO2 NPs. First, 10 μL TiCl4 liquid was added into 10 mL deionized water with 10 °C and stirred gently for 5 min. The Ti(OH)4 sol solution was thus obtained with a pH value about 1.77. The cleaned gold target was then put in the bottom of a beaker filled with the Ti(OH)4 sol solution. A Nd:YAG laser, with 1064 nm in wavelength, 10 Hz in frequency, and 10 ns in pulse duration was used to vertically irradiate the gold target in the Ti(OH)4 sol solution for 15 min, as schematically shown in Figure S1. The height between the solution surface and the target was about 10 mm. B

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Figure 2. (a): Zeta potentials of different colloidal solutions. Curve (I): TiCl4 aqueous solution (or Ti(OH)4 colloidal solution); Curve (II): the colloidal solution obtained by laser ablation of Au target in the TiCl4 aqueous solution without or with centrifugation for cleaning; Curve (III): the pure Au colloidal solution induced by laser ablation of Au target in water. The insets (1) and (2) are the photos of the Ti(OH)4 colloidal solution without and with an incident laser beam (532 nm), respectively; (3) is the photo of the colloidal solution of curve (II) in (a). (b): Optical absorbance spectra of the different colloidal solutions. Curves (I), (II), and (III) correspond to the samples (II), (III), and (I) in (a), respectively.

Figure 3. Morphological and microstructural observations of the as-prepared products. (a) FESEM image. The inset is the size contribution of the particles. (b) EDS spectrum. The inset is the EDS mapping of a single particle. (c) TEM image. The inset is the corresponding SAED pattern. (d) HRTEM image of a partial particle. The laser power was 80 mJ per pulse and the laser spot on the target was about 2 mm in size by a focusing lens with a focal length of 150 mm. After laser ablation, the products were formed and dispersed in the solution. 3.3. Characterization. The morphology of the products was observed on a field emission scanning electron microscope (FESEM, FEI Sirion 200) equipped with an Oxford IE250X-Max50 energy dispersion spectroscope (EDS). Microstructural examination, selected area electron diffraction (SAED), and high resolution transmission electron microscopic (HRTEM) observation were performed on a transmission electron microscope (TEM, JEOL JEM-2100) operated at 120 kV. X-ray photoelectron spectrum (XPS) analysis was conducted using an Al Kα X-ray source on a Thermo-VG ESCALAB MKII spectrometer. X-ray diffraction (XRD) patterns were

recorded on an X’Pert Philips Diffractometer using Cu Kα radiation (0.15406 nm). The optical absorbance spectra were measured on a Shimadzu UV-2600 spectrometer. Zeta potential measurements were conducted on a Nano ZS instrument (Model ZEN 3600, Malvern Instruments) and the curve of the zeta potential is the average result after measurement for 15 times.

4. RESULTS AND DISCUSSION After TiCl4 liquid was added into the deionized water, the colorless aqueous solution was prepared and shows strong Tyndall effect, indicating formation of the Ti(OH)4 sol solution, as shown in insets (1, 2) in Figure 2a. The zeta potential of the Ti(OH)4 sol solution was measured to be +28.9 C

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Figure 4. XPS spectra of the as-prepared Au@TiO2 NPs. (a) Binding energy spectrum of Ti 2p. (b) Binding energy spectrum of O 1s.

gold (the inset of Figure 3c), which is in agreement with the XRD results. Further, HRTEM observation was conducted, as typically illustrated in Figure 3d. The lattice fringes with an interplanar spacing of 0.24 nm in the core part can be observed, which match well with Au (111). The shell layer was measured to be about 2.5 nm in thickness but no obvious lattice fringe was observed. Considering the existence of Ti and O elements in the particles’ surface layer (Figure 3b), the shell layer could be the amorphous TiO2. XPS was thus used to analyze the surface composition and chemical bonds of the core−shell structured NPs. The corresponding binding energy spectrum has shown the existence of Ti and O, in addition to Au, in the core−shell structured NPs, as shown in Figure S4. The atomic ratio of Ti to O in the sample was determined about 6.34:34.44. Figure 4a gives the Ti 2p spectrum. The peaks at 464.6 and 458.9 eV correspond to Ti 2p3/2 and 2p1/2, respectively. The splitting with 5.7 eV is in good agreement with the standard value for Ti in TiO2.32,33 Figure 4b presents the spectrum of O 1s, which consist of three peaks at 530.3, 531.8, and 532.9 eV. The main peak at 530.3 eV corresponds to the oxygen in TiO2, and the peaks at 531.8 and 532.9 eV originate from hydroxyl groups (OH) and adsorbed H2O, respectively.32−35 Further, the integral area of the peak at 530.3 eV takes 39% of the whole integral area under the O 1s spectrum. So, the atomic ratio of Ti to O in the shell-layer should be about 6.34: (34.44 × 39%), or 1:2.1, which is in agreement with the EDS analysis. The results mentioned above have indicated that the shell-layer is the amorphous titanium oxide. Finally, the shell thickness versus the size of the Au NPs was measured, as illustrated in Figure 5. The thickness of the shell is around 2.5 nm for all Au NPs’ sizes, indicating near

mV, as is illustrated in curve (I) of Figure 2a, indicating the positively charged electrical property. After laser ablation, the colorless liquid turned into a purple red colloidal solution, and the zeta potential was decreased to +26.2 mV, as shown in curve (II) of Figure 2a, and the pH value was slightly decreased from 1.77 to about 1.65. After centrifugation for three times for cleaning and the products were dispersed in water to form an aqueous colloidal solution, as shown in inset (3) of Figure 2a, the zeta potential was almost unchanged and nearly the same as curve (II) in Figure 2a. The corresponding optical absorbance spectrum is shown in curve (I) of Figure 2b. There exists an obvious peak at 548 nm which should be attributed to the surface plasmon resonance (SPR) of Au NPs.7,15 Correspondingly, for the pure gold colloidal solution, obtained by laser ablation of the gold target in water, the SPR is located at 525 nm, as shown in curve (II) of Figure 2b. The red shift with 23 nm could be attributed to the different dielectric constants of the medium around Au NPs.7,15 This indicates that the products corresponding to curve (I) in Figure 2b could be of core−shell structure. As for the Ti(OH)4 sol solution, no absorption peak was observed except an absorption edge at about 300 nm (curve (III) in Figure 2b). 4.1. Morphology and Structure. XRD analysis was carried out for the products in the as-prepared colloidal solution after dropping it on a cleaned amorphous silicon wafer and drying, as illustrated in Figure S2. The diffraction pattern shows four well-defined peaks at 2θ = 38.2°, 44.4°, 64.6°, and 77.5° corresponding to the crystal planes of the Au (111), (200), (220), and (311) (PDF, No. 4-0784), respectively. No other phase was detected. The FESEM micrographs were also taken via dropping the as-prepared colloidal solution on a cleaned silicon wafer and drying. It has been revealed that the products consist of the nearly spherical particles with diameters mostly falling into the range 10 to 60 and 35 nm in mean size, as shown in Figure 3a and its inset. Figure 3b presents the corresponding EDS spectrum, indicating the existence of the elements Au, Ti, and O in the products. The atomic ratio of O:Ti was determined to be about 2:1, which is close to the stoichiometry of TiO2. As for the signals of Si and C, they should be from the silicon substrate and cleaning reagent, respectively. Further, the EDS mapping was performed, as typically shown in Figure S3. The inset of Figure 3b gives the results for a single particle, and shows that the elements Ti and O are distributed in the surface layer of the spherical particle. Correspondingly microstructural examination was carried out, as typically demonstrated in Figure 3c. All the nanoparticles are wrapped with an ultrathin (few nanometers) shell layer, showing the core/shell structure. But the SAED pattern shows the diffraction rings only from

Figure 5. Shell thickness versus the core part Au NPs’ size. D

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37° were observed, which correspond, respectively, to crystal planes (101) and (103) of anatase TiO2 (PDF, No. 001-0562), indicating crystallization of the shell layer. Further, TEM examination has shown that only few areas in the shell layers were crystallized for the sample with 1 h ablation, and for the sample obtained after ablation for 2 h, the shell layers were almost completely crystallized, as typically shown in Figure 7b,c. The lattice fringes with about 0.35 nm in spacing correspond to (101) planes of anatase TiO2, Such crystallization should be attributed to laser-induced thermal effect. 4.4. Electrostatic Attraction and Colloidal SelfAssembly on Au NPs. Now let us discuss the formation of the ultrathin TiO2 layer-wrapped gold NPs or Au@TiO2 NPs. It could be attributed to Ti(OH)4 colloidal attachment on the laser-induced Au NPs due to the electrostatic attraction, and the colloidal self-assembly on Au NPs. When TiCl4 was dropped into water, the hydrolysis reaction would take place,38,39 and Ti(OH)4 sol solution would thus be formed according to the reaction40,41

independence of the Au NPs’ size. The shell layer could be the TiO2 colloidal monolayer formed on Au NPs. 4.2. Control of the Shell Thickness. As mentioned in section 2, the shell thickness should depend only on the size of the hydrolysis-induced Ti(OH)4 colloids. Actually, Ti(OH)4 colloidal size could be controlled by changing the temperature and pH value of the colloidal solution.36 Therefore, the shell thickness could be tuned simply by changing the temperature and pH value of Ti(OH)4 sol solution during laser ablation. These have been confirmed by further experiments. The shell thickness would increase with the rising temperature of the colloidal solution during laser ablation. Typically, when the temperature of Ti(OH)4 sol solution was increased to 50 °C, the mean thickness was about 3.8 nm. If the temperature was increased up to 95 °C, the shell was about 5.5 nm in mean thickness, as shown in Figure 6. Similarly, increasing pH value of the colloidal solution could increase shell thickness.

TiCl4(l) + 4H 2O(l) → Ti(OH)4 (colloids) + 4HCl(aq) (1)

As mentioned above (Figure 2a), the zeta potential of the asprepared Ti(OH)4 sol solution was measured as +28.9 mV while the Au colloidal aqueous solution, obtained by laser ablation of gold target in pure water, was measured to be −24.2 mV, as illustrated in curve (III) of Figure 2a. Such negatively charged gold colloids could be attributed to adsorption or attachment of the hydroxyl groups on the Au NPs in water. When the laser beam shot on the gold target in the Ti(OH)4 sol solution, the Au plasma plume with high temperature and pressure would be instantly formed, due to absorption of light energy, on the target and expand into the solution. Au NPs would thus be formed in the solution with the extinction of the expanding plasma, as extensively reported.20,21 The Ti(OH)4 colloids with few nanometers in size in the solution would then be adsorbed or attached on the laser-induced Au NPs with much bigger size due to the electrostatic attraction, as schematically shown in Figure 1a. The adsorbed Ti(OH)4 colloids would self-assemble on the Au NPs and form the Ti(OH)4 colloidal monolayer wrapping the Au NPs (Figure 1b). The Ti(OH)4 colloids are easily dehydrated to form TiO2 after heating,42 or the reaction

Figure 6. FESEM images of the products obtained by laser ablation of the Au target in the Ti(OH)4 colloidal solution with different temperatures. (a) 50 °C, (b) 90 °C and the scale bars are 50 nm. The insets are the corresponding magnified images of the single isolated NPs and the scale bars are 5 nm.

4.3. Improvement of the Shell’s Crystallinity. As mentioned above, the shell layer is amorphous for the Au@ TiO2 NPs shown in Figure 3. Such an amorphous layer may influence or even decrease their performances in optical and electrical applications.37 Here, we found that the crystallinity of the shell layer could be significantly improved just by prolonging the ablation time. Figure 7 shows the XRD patterns for the samples obtained after laser ablation for different durations. When the laser ablation was for 1 h or less, only Au diffraction peaks were detected for the products. However, if the ablation was for 2 h or longer, additional peaks at 25° and

Δ

Ti(OH)4 → TiO2 + 2H 2O

(2)

would take place. In our case, the Ti(OH)4 colloids adsorbed on Au NPs would subsequently be dehydrated due to the thermal effect from the Au NPs which absorb the laser energy during ablation, and the ultrathin TiO2 shell-wrapped Au NPs were finally obtained after the laser ablation (Figure 1c or Figure 2). Obviously, the long laser ablation would lead to the thermal effect enough to completely crystallize the amorphous TiO2 shell, as shown in Figure 7c. Since the shell layer is actually the Ti(OH)4 colloidal monolayer formed by self-assembly, its thickness depends only on the hydrolysis-induced Ti(OH)4 colloidal size and is nearly independent of the Au NPs’ size, as shown in Figure 5. Finally, from reaction 1, increasing the temperature or pH value of the solution would enhance the hydrolysis reaction and hence increase the Ti(OH)4 colloidal size, leading to increase in shell thickness, as typically shown in Figure 6.

Figure 7. (a) XRD patterns of the as-prepared products obtained after laser ablation for different durations. Curve (I): 1 h. Curve (II): 2 h. The line spectrum is the standard diffraction of anatase TiO2. (b,c) Typical HRTEM images of the single Au@TiO2 NPs corresponding to curves (I) and (II), respectively. E

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targets, respectively, as typically demonstrated in Figure 9. All the NPs are wrapped with a 2−3 nm shell layer. 4.6. Application in SERS-Based Detection. As mentioned above, this kind of noble metal NPs coated with ultrathin oxide shell-layers has many applications. Among them is the SERS substrate for detection of some special target molecules which are difficult to be detected efficiently by pure noble metal NPs’ SERS substrate due to the weak interaction or surface catalysis. Here we take two examples to demonstrate the special applications of the ultrathin oxide layer-wrapped noble metal NPs in SERS-based detection. 4.6.1. Enhancement of Interaction between Analytes and SERS Substrate. Improvement of the interaction between the analytes and the SERS substrates is important in SERS-based detection of them. For the analytes which can only weakly interact with noble metal NPs but strongly be adsorbed on oxides, the above ultrathin oxide layer-wrapped noble metal NPs could be the better SERS-substrate than the pure noble metal NPs. Here, we take the explosive raw material KNO3, which weakly interacts with the Au NPs, as the analyte, and the Au@TiO2 NPs-built film as the SERS substrate. The details of the film preparation and SERS measurements are described in the Supporting Information. Figure 10a presents the Raman spectrum for the Au@TiO2 NPs-built film after soaking in the KNO3 solution with 1000 ppm and drying. There exists a strong main peak at 1048 cm−1 together with a relatively weak peak at 712 cm−1 and a very weak peak at 1345 cm−1. Such a spectral pattern is in good agreement with that of pure solid KNO3 [see curves (I) and (III)] and all the peaks should originate from nitrate NO3−.43 In contrast, the Raman signal from the soaked Au NPs-built film was much lower (curve (II) in Figure 10a). The intensity of the main peak at 1048 cm−1 is 5 times higher for the Au@ TiO2 NPs-built film than that for the Au NPs-built film. Such enhancement could be attributed to the fact that the Au@TiO2 NPs were positively charged in KNO3 solution and hence could enrich more nitrate anions than the pure Au NPs.

For confirmation of the above formation process, a two-step experiment was carried out. Briefly, the gold colloidal solution was first prepared by laser ablation of gold target in pure water, and then added to the Ti(OH)4 sol solution and stirred for 10 min. We also obtained the ultrathin layer-wrapped NPs, as typically shown in Figure 8.

Figure 8. TEM image of the products obtained by mixing the gold colloidal solution with Ti(OH)4 sol solution. The inset is the magnified image of a single particle.

4.5. Universality of the Method. Further, the synthesis route presented in this study is of universality and suitable for fabrication of many other ultrathin oxide layer-wrapped noble metal NPs. For example, we have successfully fabricated Au@ ZnO, Au@Fe2O3, Au@Al2O3, and Au@CuO NPs by laser ablation of the gold target, respectively, in the hydrolysisinduced Zn(OH)2, Fe(OH)3, Al(OH)3, and Cu(OH)2 sol solutions, and also obtained Pt@TiO2 and Pd@TiO2 NPs, via laser ablation in Ti(OH)2 sol solution but using the Pt and Pd

Figure 9. (a) Typical TEM images of the single ultrathin oxide layer-wrapped noble metal NPs prepared by laser ablation of the metal targets in the hydrolysis-induced hydroxide colloidal solutions, the scale bars are 5 nm. (b,c) Photos and optical absorbance spectra of the oxide-wrapped noble metal NPs colloidal solutions, respectively. F

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Figure 10. (a) Raman spectra of KNO3 on different substrates. Curves (I, II): on the Au@TiO2 NPs’ film and the pure Au NPs’ film, respectively, after soaking in the KNO3 solution with 1000 ppm. Curve (III): the Raman spectrum of pure solid KNO3. (b) Plot of the peak intensity I at 1048 cm−1 versus the KNO3 concentration C for the soaked Au@TiO2 NPs’ film (data from Figure S5).

the 4-NP solution before Raman measurement, we can obtain the Raman spectrum with the similar peak intensities to those of the initial one, showing good reusability of Au@TiO2 NPs for the SERS-based detection of such molecules, as shown in curves (I) and (IV) of Figure 11.

Further, the Raman spectral dependence on the NO3−concentration was measured for the Au@TiO2 NPsbuilt film, as demonstrated in Figure S5. Figure 10b shows the plot of the peak intensity at 1048 cm−1versus the NO3− concentration, which exhibits a good linear double logarithmic relation between them; the detection limit is