3. LAL-induced oxides wrapped metal NPS

assisted methods usually results in bigger oxides or its precursor colloids (most of them are no less than 5 nm). Generally, metal cations (Fe3+, Al3+, Cu2+, Ti4+, Sn4+, etc.) are hydrolyzed in solution to form corresponding hydroxide colloids. The size of these hydroxide colloids is generally at several nanometers [31, 32], which is suitable for such a colloidal electrostatic self-assembly. And most of them are positively charged. On the other hand, many plasmonic metal NPs tend to adsorb anions on the surface and carry negative charges [33, 34]. Strong electrostatic attraction between the two kinds of colloids will occur when they are close enough to each other. The small hydroxide colloids will be attached on the surface of the plasmonic metal NPs and a monolayer hydroxide wrapping layer would be formed on the metal NPs due to the colloidal self-assembly, as schematically shown in Figure 1a and b. After dehydration treatment by annealing or heating, the hydroxides shell will be transformed to corresponding

Obviously, such a self-assembly process should be a flexible and universal, which is suitable to fabricate a series of core-shell NPs. And the thickness of the shell is highly relied on the size of the colloids produced by the hydrolysis, which could be simply controlled by the pH value and temperature of the colloidal precursor. This has been confirmed by a one-step laser

In order to avoid and remove interferences from other substances (such as surfactants), the laser ablation in liquid method, typically been accepted as a chemical green approach, has

In the laser-based synthesis procedure, metal foils are usually utilized as the ablation target while metal salt solutions are used as the liquid medium, as shown in Figure 2. When a pulsed laser is focused onto the surface of metal target, a localized high-temperature and high-pressure plasma involving atoms, ions, electrons, and clusters is generated. From the moment on formation, the

Figure 1. Schematic illustration for the fabrication strategy of ultrathin oxide layer-wrapped metal NPs based 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

ablation of plasmonic metal target in hydrolyses induced hydroxides sol solutions.

been adopted to verify the colloidal electrostatic self-assembly strategy.

NP. (c) Formation of ultrathin oxide shell layer on the metal NP by dehydration [24].

2.3. Laser ablation in hydroxides sol solutions

oxides (Figure 1c).

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Based on the aforementioned formation process, pulsed laser ablation in colloidal solution will one-pot fabricate the oxides wrapped metal NPs.

### 3.1. Typical morphology and structure of the NPs

First, let us take the TiO2-wrapped Au NPs as an example. The Ti(OH)4 sol solution shows obvious Tyndall effect, as shown in the (1–2) of the inset in the Figure 3a. The Zeta potential is about +28.9 mV, indicating they are positively charged. And after ablation of a gold target in the sol solution for 15 min, the Zeta potential drops to +26.2 mV, and the pH value was slightly decreased from 1.77 to about 1.65. The Zeta potential of the Au colloidal solution obtained by ablation in pure water was 24.2 mV. After centrifugation for three times of cleaning, the products were redispersed in water to form an aqueous colloidal solution, as shown in inset (3) of Figure 3a, and the Zeta potential was almost unchanged and nearly the same as curve (II) in Figure 3a. Optical absorption measurement shows that the well-known absorption peak of the Au NPs has an obvious red shift of about 23 nm, which means that the change of the dielectric environment around the Au NPs' surface [7, 37].

Figure 4 shows the typical microstructure and morphology of the electrostatically assembled core-shell Au@TiO2 NPs. The field emission scanning electron microscope (FESEM) micrographs reveal that the products consist of nearly spherical particles with diameters ranging from 10 to 60 nm with a mean size of 35 nm, as shown in Figure 4a. The energy dispersion spectrum (EDS) shows that the product contains the elements of Au, O, Ti, C, and Si, in which Si and C are from the silicon substrate and cleaning reagent, respectively. The inset of Figure 4b shows the EDS mapping from a transmission electron microscopy (TEM) of an isolated NP, which reveals that the elements of Ti and O are preferentially distributed on the surface of the spherical Au NP. Corresponding microstructural examination was carried on a TEM (Figure 4c), which shows that the surface of these NPs is obviously wrapped with ultrathin (few nanometers) shell layer. The well-defined core-shell structure can be vividly

observed. However, the corresponding selected area electron diffraction (SAED) pattern only shows the rings of polycrystalline gold, without diffraction pattern belonging to other crystalline substances. High-resolution TEM (HRTEM) photograph (Figure 4d) shows clean lattice fringes with an interplanar spacing of 0.24 nm in the core part, which corresponds to the (111) of Au. The shell was measured to be about 2.5 nm in thickness and reveals amorphous nature. Considering the Ti(OH)4 precursors in ablation process and existence of Ti and O elements in

Figure 4. 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

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In order to further confirm the components of the shell, X-ray diffraction (XRD) measurement was carried out for the products after dropping it on a cleaned amorphous silicon wafer and the subsequent natural drying. There are only three diffraction peaks at 2θ = 38.2, 44.4, and 64.6, corresponding to crystal planes {111}, {200}, and {220} of the Au crystal with the facecentered cubic structure (PDF, No. 00-001-1172), respectively. No other phase was detected, as illustrated in Figure 5a. This confirms the amorphous properties of the shell. The X-ray

the wrapping layer, we proposed that the shell might be amorphous TiO2.

inset is the corresponding SAED pattern. (d) HRTEM image of a partial particle [24].

Figure 3. (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 [24].

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3.1. Typical morphology and structure of the NPs

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dielectric environment around the Au NPs' surface [7, 37].

First, let us take the TiO2-wrapped Au NPs as an example. The Ti(OH)4 sol solution shows obvious Tyndall effect, as shown in the (1–2) of the inset in the Figure 3a. The Zeta potential is about +28.9 mV, indicating they are positively charged. And after ablation of a gold target in the sol solution for 15 min, the Zeta potential drops to +26.2 mV, and the pH value was slightly decreased from 1.77 to about 1.65. The Zeta potential of the Au colloidal solution obtained by ablation in pure water was 24.2 mV. After centrifugation for three times of cleaning, the products were redispersed in water to form an aqueous colloidal solution, as shown in inset (3) of Figure 3a, and the Zeta potential was almost unchanged and nearly the same as curve (II) in Figure 3a. Optical absorption measurement shows that the well-known absorption peak of the Au NPs has an obvious red shift of about 23 nm, which means that the change of the

Figure 4 shows the typical microstructure and morphology of the electrostatically assembled core-shell Au@TiO2 NPs. The field emission scanning electron microscope (FESEM) micrographs reveal that the products consist of nearly spherical particles with diameters ranging from 10 to 60 nm with a mean size of 35 nm, as shown in Figure 4a. The energy dispersion spectrum (EDS) shows that the product contains the elements of Au, O, Ti, C, and Si, in which Si and C are from the silicon substrate and cleaning reagent, respectively. The inset of Figure 4b shows the EDS mapping from a transmission electron microscopy (TEM) of an isolated NP, which reveals that the elements of Ti and O are preferentially distributed on the surface of the spherical Au NP. Corresponding microstructural examination was carried on a TEM (Figure 4c), which shows that the surface of these NPs is obviously wrapped with ultrathin (few nanometers) shell layer. The well-defined core-shell structure can be vividly

Figure 3. (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 [24].

Figure 4. 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 [24].

observed. However, the corresponding selected area electron diffraction (SAED) pattern only shows the rings of polycrystalline gold, without diffraction pattern belonging to other crystalline substances. High-resolution TEM (HRTEM) photograph (Figure 4d) shows clean lattice fringes with an interplanar spacing of 0.24 nm in the core part, which corresponds to the (111) of Au. The shell was measured to be about 2.5 nm in thickness and reveals amorphous nature. Considering the Ti(OH)4 precursors in ablation process and existence of Ti and O elements in the wrapping layer, we proposed that the shell might be amorphous TiO2.

In order to further confirm the components of the shell, X-ray diffraction (XRD) measurement was carried out for the products after dropping it on a cleaned amorphous silicon wafer and the subsequent natural drying. There are only three diffraction peaks at 2θ = 38.2, 44.4, and 64.6, corresponding to crystal planes {111}, {200}, and {220} of the Au crystal with the facecentered cubic structure (PDF, No. 00-001-1172), respectively. No other phase was detected, as illustrated in Figure 5a. This confirms the amorphous properties of the shell. The X-ray

atomic ratio of Ti to O in the lattice of wrapping layer should be about 6.34: (34.44 39%), or 1:2.1, which is in good agreement with the stoichiometric ratio of TiO2. From the results mentioned previously, the wrapping layer could be confirmed as the amorphous titanium oxide. As an example, it demonstrates that the LAL approach can facilely obtain ultrathin

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Furthermore, such LAL synthesis route based on colloidal electrostatic self-assembly has been confirmed as a universal approach, and it is suitable for fabricating many other ultrathin oxide layer-wrapped plasmonic metal NPs, such as Au@SnO2, Au@ ZnO, Au@CuO, Au@Al2O3, Au@In2O3, and Au@Fe2O3 NPs. All the products consist of spherical particles, with about

Besides the oxides wrapped Au NPs, the Pt@TiO2 and Pd@TiO2 NPs can also be prepared via laser ablation in Ti(OH)4 sol solution but using the Pt and Pd as ablation targets, respectively.

It should be pointed out that the method of electrostatic assembly should be suitable for all colloidal systems, only requiring two kinds of colloids with opposite electrical properties and big size gap. Therefore, the plasmonic metal NPs prepared by the traditional wet chemical

The electrostatic self-assembly of colloids has been confirmed by further experiments. First, laser-induced Au NPs from the pure water are slowly (1 mL per min) added into the stirred SnCl4 aqueous solution. According to the proposed electrostatic self-assembly mechanism, it should also obtain the ultrathin SnO2 layer-wrapped Au NPs, which is confirmed in Figure 6a. Similarly, by using the same two-step fabrication, we could also obtain the ultrathin TiO2 shellwrapped Au NPs, as illustrated in Figure 6b. The only thing that deserves a bit of attention is that the laser ablation-formed Au NPs should be slowly added into the corresponding hydrox-

To confirm the opposite charge-induced electrostatic self-assembly mechanism, we ablated Au target in Na2WO4 solution, which induces both negatively charged Au NPs and H2WO4

The thickness of the wrapping oxide layer was measured as a function of the Au core's size, as illustrated in Figure 7. For the Au@TiO2 NPs, the TiO2 shell is estimated approximately 2.5 nm in thickness for all NPs regardless of the Au core size. Similarly, the SnO2 shell also shows a similar tendency, indicating that the shell is around 2.5 nm for all Au NPs. The shell layer could

10–50 nm in mean size. The wrapping layer of these NPs is measured in 2–3 nm [24].

All the core-shell NPs are wrapped with ca. 1–3 nm shell layers [24].

method can also be wrapped by oxide layers via this strategy.

3.3. Confirmation of the electrostatic self-assembly mechanism

ides sol solutions to avoid a possible colloidal coagulation process.

3.4. The dependence of the shell thickness

be the oxides colloidal monolayer formed on Au NPs.

colloids. For this case, no obvious shell can be observed around the Au NPs.

oxides wrapped metal NPs.

3.2. The universality of LAL fabrication

Figure 5. (a) XRD pattern of the obtained products. The line spectrum corresponds to the standard pattern of Au powders (JCPDS No. 00-001-1172). XPS spectra of the as-prepared Au@TiO2 NPs. (b) the full spectrum. (c) Binding energy spectrum of Ti 2p. (d) Binding energy spectrum of O 1 s [24].

photoelectron spectrum (XPS) full spectrum presented in Figure 5b was also conducted to analyze the surface composition and chemical bonds of the core-shell structured NPs. It shows that the existence of elements Ti, O, and Au. And the atomic ratio of Ti to O is determined to be about 6.34:34.44. The Ti 2p spectrum presented in Figure 5c shows two 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 [38]. Figure 5d presents the spectrum of O 1 s, in which strongly overlapping peaks could be found. By a peak differentiation imitating analysis, three peaks at 530.3, 531.8, and 532.9 eV can be parsed out. The 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 [38, 39]. Furthermore, the integral area of the peak at 530.3 eV takes 39% of the whole integral area under O 1 s spectrum. So the atomic ratio of Ti to O in the lattice of wrapping layer should be about 6.34: (34.44 39%), or 1:2.1, which is in good agreement with the stoichiometric ratio of TiO2. From the results mentioned previously, the wrapping layer could be confirmed as the amorphous titanium oxide. As an example, it demonstrates that the LAL approach can facilely obtain ultrathin oxides wrapped metal NPs.

### 3.2. The universality of LAL fabrication

Furthermore, such LAL synthesis route based on colloidal electrostatic self-assembly has been confirmed as a universal approach, and it is suitable for fabricating many other ultrathin oxide layer-wrapped plasmonic metal NPs, such as Au@SnO2, Au@ ZnO, Au@CuO, Au@Al2O3, Au@In2O3, and Au@Fe2O3 NPs. All the products consist of spherical particles, with about 10–50 nm in mean size. The wrapping layer of these NPs is measured in 2–3 nm [24].

Besides the oxides wrapped Au NPs, the Pt@TiO2 and Pd@TiO2 NPs can also be prepared via laser ablation in Ti(OH)4 sol solution but using the Pt and Pd as ablation targets, respectively. All the core-shell NPs are wrapped with ca. 1–3 nm shell layers [24].

It should be pointed out that the method of electrostatic assembly should be suitable for all colloidal systems, only requiring two kinds of colloids with opposite electrical properties and big size gap. Therefore, the plasmonic metal NPs prepared by the traditional wet chemical method can also be wrapped by oxide layers via this strategy.

### 3.3. Confirmation of the electrostatic self-assembly mechanism

The electrostatic self-assembly of colloids has been confirmed by further experiments. First, laser-induced Au NPs from the pure water are slowly (1 mL per min) added into the stirred SnCl4 aqueous solution. According to the proposed electrostatic self-assembly mechanism, it should also obtain the ultrathin SnO2 layer-wrapped Au NPs, which is confirmed in Figure 6a. Similarly, by using the same two-step fabrication, we could also obtain the ultrathin TiO2 shellwrapped Au NPs, as illustrated in Figure 6b. The only thing that deserves a bit of attention is that the laser ablation-formed Au NPs should be slowly added into the corresponding hydroxides sol solutions to avoid a possible colloidal coagulation process.

To confirm the opposite charge-induced electrostatic self-assembly mechanism, we ablated Au target in Na2WO4 solution, which induces both negatively charged Au NPs and H2WO4 colloids. For this case, no obvious shell can be observed around the Au NPs.

### 3.4. The dependence of the shell thickness

photoelectron spectrum (XPS) full spectrum presented in Figure 5b was also conducted to analyze the surface composition and chemical bonds of the core-shell structured NPs. It shows that the existence of elements Ti, O, and Au. And the atomic ratio of Ti to O is determined to be about 6.34:34.44. The Ti 2p spectrum presented in Figure 5c shows two 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 [38]. Figure 5d presents the spectrum of O 1 s, in which strongly overlapping peaks could be found. By a peak differentiation imitating analysis, three peaks at 530.3, 531.8, and 532.9 eV can be parsed out. The 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 [38, 39]. Furthermore, the integral area of the peak at 530.3 eV takes 39% of the whole integral area under O 1 s spectrum. So the

Figure 5. (a) XRD pattern of the obtained products. The line spectrum corresponds to the standard pattern of Au powders (JCPDS No. 00-001-1172). XPS spectra of the as-prepared Au@TiO2 NPs. (b) the full spectrum. (c) Binding energy

spectrum of Ti 2p. (d) Binding energy spectrum of O 1 s [24].

208 Plasmonics

The thickness of the wrapping oxide layer was measured as a function of the Au core's size, as illustrated in Figure 7. For the Au@TiO2 NPs, the TiO2 shell is estimated approximately 2.5 nm in thickness for all NPs regardless of the Au core size. Similarly, the SnO2 shell also shows a similar tendency, indicating that the shell is around 2.5 nm for all Au NPs. The shell layer could be the oxides colloidal monolayer formed on Au NPs.

lead to formation of NPs with large size distribution. Furthermore, a secondary irradiation technique was proposed as an efficient way to fabricate more uniform NPs. Generally, the gold NPs turn more round and bigger after secondary irradiation treatment [40]. For our Au@SnO2 NPs, secondary irradiation with wavelength of 532 nm and frequency of 10 Hz for 10 min (yttrium aluminum garnet, and pulse width of 10 ns, 40 mJ per pulse) was also conducted. It has been found that both products show the typical ultrathin wrapped core-shell structure. However, the number of the small NPs is obviously decreased after the secondary irradiation. And this is confirmed by their size distributions. After the secondary irradiation, the distribution becomes narrower and the relative standard deviation (RSD) decreased from 46 to 21%.

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For some oxides that are easier to crystallize, such as tin oxide, the crystalline shell layer can be usually achieved by a simple drying process. Figure 8a and b shows TEM and corresponding SAED pattern of the initially prepared Au@SnO2 NPs. Besides the diffraction rings of Au crystal, some rings of the crystalline SnO2 also can be observed. The HRTEM image shows the well-defined interplanar spacing of 0.24 and 0.34 nm in the core and shell, corresponding to the (111) and (110) of Au and SnO2, respectively. However, for Au@TiO2 NPs (Figure 8c and d), most oxides layer is amorphous. Such an amorphous layer may influence or even decrease their performances in optical and electrical applications. Here, we found that the crystallinity of the shell layer could be significantly improved just by prolonging the ablation time. TEM examination has shown that only few areas in the shell layers were crystallized for the sample with 1-h ablation (Figure 8c), and for the sample obtained after ablation for 2 h (Figure 8d), the shell layers were almost completely crystallized. The lattice fringes with about 0.35 nm in spacing correspond to (101) planes of anatase TiO2 (PDF, No. 001-0562). Such crystallization

Figure 8. (a–d) the TEM observations of the Au@oxides NPs. (a, b) Au NPs wrapped with crystalline SnO2 shell. (c, d) the Au@TiO2 NPs prepared by prolonging the laser duration to 1 h (a) and 2 h (b). And (e), the XRD spectra of the Au@TiO2

Additionally, the mean size increased from 12 to 16 nm.

3.5.2. The shell crystalline

NPs corresponding to (c) and (d).

Figure 6. TEM image of the products obtained by adding the laser-induced gold colloidal solution into (a) Sn(OH)4 and (b) Ti(OH)4 sol solution. The inset is the magnified image of a single particle.

Figure 7. Shell thickness versus the core sizes of (a) Au@TiO2 and (b) Au@SnO2 NPs.

### 3.5. The controllability

The laser-based colloidal electrostatic self-assembly strategy shows good flexibility in the fabrication. Here, the controllability in the shell's crystallinity and thickness as well as core's uniformity will be discussed.

### 3.5.1. Homogenizing the NPs size with secondary irradiation

It is well known that a narrow size distribution is critical to the stable performance of NPs in applications. However, plenty of reports have demonstrated that the laser ablation commonly lead to formation of NPs with large size distribution. Furthermore, a secondary irradiation technique was proposed as an efficient way to fabricate more uniform NPs. Generally, the gold NPs turn more round and bigger after secondary irradiation treatment [40]. For our Au@SnO2 NPs, secondary irradiation with wavelength of 532 nm and frequency of 10 Hz for 10 min (yttrium aluminum garnet, and pulse width of 10 ns, 40 mJ per pulse) was also conducted. It has been found that both products show the typical ultrathin wrapped core-shell structure. However, the number of the small NPs is obviously decreased after the secondary irradiation. And this is confirmed by their size distributions. After the secondary irradiation, the distribution becomes narrower and the relative standard deviation (RSD) decreased from 46 to 21%. Additionally, the mean size increased from 12 to 16 nm.

### 3.5.2. The shell crystalline

3.5. The controllability

210 Plasmonics

uniformity will be discussed.

3.5.1. Homogenizing the NPs size with secondary irradiation

Figure 7. Shell thickness versus the core sizes of (a) Au@TiO2 and (b) Au@SnO2 NPs.

(b) Ti(OH)4 sol solution. The inset is the magnified image of a single particle.

The laser-based colloidal electrostatic self-assembly strategy shows good flexibility in the fabrication. Here, the controllability in the shell's crystallinity and thickness as well as core's

Figure 6. TEM image of the products obtained by adding the laser-induced gold colloidal solution into (a) Sn(OH)4 and

It is well known that a narrow size distribution is critical to the stable performance of NPs in applications. However, plenty of reports have demonstrated that the laser ablation commonly For some oxides that are easier to crystallize, such as tin oxide, the crystalline shell layer can be usually achieved by a simple drying process. Figure 8a and b shows TEM and corresponding SAED pattern of the initially prepared Au@SnO2 NPs. Besides the diffraction rings of Au crystal, some rings of the crystalline SnO2 also can be observed. The HRTEM image shows the well-defined interplanar spacing of 0.24 and 0.34 nm in the core and shell, corresponding to the (111) and (110) of Au and SnO2, respectively. However, for Au@TiO2 NPs (Figure 8c and d), most oxides layer is amorphous. Such an amorphous layer may influence or even decrease their performances in optical and electrical applications. Here, we found that the crystallinity of the shell layer could be significantly improved just by prolonging the ablation time. TEM examination has shown that only few areas in the shell layers were crystallized for the sample with 1-h ablation (Figure 8c), and for the sample obtained after ablation for 2 h (Figure 8d), the shell layers were almost completely crystallized. The lattice fringes with about 0.35 nm in spacing correspond to (101) planes of anatase TiO2 (PDF, No. 001-0562). Such crystallization

Figure 8. (a–d) the TEM observations of the Au@oxides NPs. (a, b) Au NPs wrapped with crystalline SnO2 shell. (c, d) the Au@TiO2 NPs prepared by prolonging the laser duration to 1 h (a) and 2 h (b). And (e), the XRD spectra of the Au@TiO2 NPs corresponding to (c) and (d).

should be attributed to laser-induced thermal effect. The XRD measurements reveal that for products with 1-h ablation, only Au diffraction peaks were detected, while with the ablation duration reached 2-h, additional peaks at 25 and 37� were observed, which correspond to crystal planes (101) and (103) of anatase TiO2 (Figure 8e).

### 3.5.3. The thickness of the shell

Regulation of the wrapping layer thickness for core-shell NPs is a key issue in gas sensing, SERS detection, and catalysis applications. Accordingly, it is critical to realize the flexible regulation of shell thickness. In present colloidal electrostatic self-assembly strategy, we demonstrate that the shell thickness depend only on the size of the hydrolysis-induced hydroxide colloids. As a result, to modulate the wrapping thickness, the hydroxide colloidal size should be precisely controlled by changing experimental parameters such as the concentration, temperature, and pH value of the colloidal solution [41]. Thus, the shell thickness would be simply tuned by changing the related conditions during laser ablation process.

### 3.5.3.1. The concentrations

If the content of metal ions is too low, most of the Au NPs could not be wrapped with the oxides shell due to insufficient oxide source in the solution. However, if the content of metal ions is too high, the wrapped Au NPs are not obviously increased and even decreased. For example, as illustrated in Figure 9, most of the Au@SnO2 NPs fabricated by ablating Au target in 0.5 M Sn4+ solution shows that the NPs are fused together. Meanwhile, the wrapping layer thickness is much thinner (approximately 1 nm) than those shown in Figure 8a and b, and even some NPs are not completely wrapped. In this case, only when the Sn4+ concentration ranges from 0.01 to 0.1 M, the shell layer of the NPs can be uniform and the Au cores are spherical with nearly same dimensions, as shown in Figures 8a, b and 9b. Although the concentration is difficult to accurately modulate the thickness of the shell, it does obviously affect the thickness of the shell to a certain extent.

3.5.3.2. The temperature

3.5.3.3. The pH value

line in Figure 10e).

interaction.

4. Enhanced performance

4.1. High performance SERS detection

shell was about 5.5 nm in thickness.

The shell thickness would increase with the temperature increasing 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

Figure 10. The evolution of the shell thickness and size of NPs with the treatment of different pH regulator. (a–d) TEM observations of the NPs obtained with the pH values of 2, 5, 8, and 9, respectively. The scale bars are 50, and 10 nm in the

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insets. (e) The statistical plots of the shell thickness and mean size versus the pH value of the regulator.

Furthermore, the pH value of precursor solution also dramatically influences the shell thickness. By simply dropping regulator solution (NaOH or HCl solution) in the laser ablation process with a dropping rate of 10 μL per min, Au@TiO2 NPs with diverse shell thicknesses are achieved. Figure 10a–d shows typical TEM photographs of isolated nanoparticles obtained under different pH regulators. It is worth noting that when the pH of the regulator is too low (pH = 2), the oxide shell disappears (Figure 10a). And with the pH increase, the shell thickness increases obviously. However, the sizes of Au NPs have no obvious changes (30–35 nm, red

As mentioned previously, ultrathin oxide layer–wrapped plasmonic metal NPs have potential applications in many fields. Among them, SERS substrate, which consisted of the coreshell NPs, has huge applications in the detection of some special target molecules that are difficult to be efficiently detected by pure plasmonic metal substrate due to their weak

Figure 9. TEM observations of the Au@SnO2 NPs prepared by using 0.5 M (a) and 0.01 M (b) SnCl4 solutions.

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Figure 10. The evolution of the shell thickness and size of NPs with the treatment of different pH regulator. (a–d) TEM observations of the NPs obtained with the pH values of 2, 5, 8, and 9, respectively. The scale bars are 50, and 10 nm in the insets. (e) The statistical plots of the shell thickness and mean size versus the pH value of the regulator.

### 3.5.3.2. The temperature

should be attributed to laser-induced thermal effect. The XRD measurements reveal that for products with 1-h ablation, only Au diffraction peaks were detected, while with the ablation duration reached 2-h, additional peaks at 25 and 37� were observed, which correspond to

Regulation of the wrapping layer thickness for core-shell NPs is a key issue in gas sensing, SERS detection, and catalysis applications. Accordingly, it is critical to realize the flexible regulation of shell thickness. In present colloidal electrostatic self-assembly strategy, we demonstrate that the shell thickness depend only on the size of the hydrolysis-induced hydroxide colloids. As a result, to modulate the wrapping thickness, the hydroxide colloidal size should be precisely controlled by changing experimental parameters such as the concentration, temperature, and pH value of the colloidal solution [41]. Thus, the shell thickness would be simply

If the content of metal ions is too low, most of the Au NPs could not be wrapped with the oxides shell due to insufficient oxide source in the solution. However, if the content of metal ions is too high, the wrapped Au NPs are not obviously increased and even decreased. For example, as illustrated in Figure 9, most of the Au@SnO2 NPs fabricated by ablating Au target in 0.5 M Sn4+ solution shows that the NPs are fused together. Meanwhile, the wrapping layer thickness is much thinner (approximately 1 nm) than those shown in Figure 8a and b, and even some NPs are not completely wrapped. In this case, only when the Sn4+ concentration ranges from 0.01 to 0.1 M, the shell layer of the NPs can be uniform and the Au cores are spherical with nearly same dimensions, as shown in Figures 8a, b and 9b. Although the concentration is difficult to accurately modulate the thickness of the shell, it does obviously

Figure 9. TEM observations of the Au@SnO2 NPs prepared by using 0.5 M (a) and 0.01 M (b) SnCl4 solutions.

crystal planes (101) and (103) of anatase TiO2 (Figure 8e).

affect the thickness of the shell to a certain extent.

tuned by changing the related conditions during laser ablation process.

3.5.3. The thickness of the shell

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3.5.3.1. The concentrations

The shell thickness would increase with the temperature increasing 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 thickness.

### 3.5.3.3. The pH value

Furthermore, the pH value of precursor solution also dramatically influences the shell thickness. By simply dropping regulator solution (NaOH or HCl solution) in the laser ablation process with a dropping rate of 10 μL per min, Au@TiO2 NPs with diverse shell thicknesses are achieved. Figure 10a–d shows typical TEM photographs of isolated nanoparticles obtained under different pH regulators. It is worth noting that when the pH of the regulator is too low (pH = 2), the oxide shell disappears (Figure 10a). And with the pH increase, the shell thickness increases obviously. However, the sizes of Au NPs have no obvious changes (30–35 nm, red line in Figure 10e).

### 4. Enhanced performance

### 4.1. High performance SERS detection

As mentioned previously, ultrathin oxide layer–wrapped plasmonic metal NPs have potential applications in many fields. Among them, SERS substrate, which consisted of the coreshell NPs, has huge applications in the detection of some special target molecules that are difficult to be efficiently detected by pure plasmonic metal substrate due to their weak interaction.

### 4.1.1. SERS effect of the oxide-wrapped plasmonic metal NPs

Due to the strong LSPR effect of gold cores, strong local electromagnetic field thus can be formed near the surface of the NPs with the incident light irradiating. It is reasonable that such core-shell NPs illustrate SERS effect to specific target molecules. However, considering that the SERS is a short-range effect, the distance it can effectively act on is generally considered not to exceed 10 nm on the plasmonic particle surface. As previously reported [15], the enhancement effect of SERS decreases exponentially with the shell thickness increasing. So, the SERS activity of the coated Au NPs should be much lower than that of pure Au NPs due to the existence of the wrapping layer.

all the peaks should originate from nitrate NO<sup>3</sup> [42]. In contrast, the signal from the Au NPbuilt substrate was much lower. The intensity of the main peak at 1048 cm<sup>1</sup> is five times higher for the Au@ TiO2 substrate than that from the Au NPs' substrate. Furthermore, the Raman spectral dependence on the NO<sup>3</sup> concentration was measured for the Au@TiO2 substrate, as demonstrated in Figure 11b. It shows the plot of the peak intensity at

Ultrathin Oxide Wrapping of Plasmonic Nanoparticles via Colloidal Electrostatic Self-Assembly and their…

Most SERS substrates are often disposable. As a result, the reusability of them is significantly important from the view of economics and complexity in the repeated fabrication of substrate. As we known, the oxides have shown excellent photocatalyst properties in many previous works. The oxide wrapping layer is thus expected to photocatalyze the target molecules after a SERS detection. Indeed, the reusability of the Au@TiO2 NP substrate has been confirmed, as previously reported [24]. Typically, the 4-Nitrophenol (4-NP), which can be photocatalyzed by TiO2, [43] was used as the target molecules for SERS detection. The Raman spectrum of the Au@TiO2 NP-built film after soaking in the 4-NP solution with 20 ppm is well shown. All Raman peaks are from 4-NP molecules [44]. If the soaked film was subsequently irradiated for 5 min by a xenon lamp before Raman spectral measurement, the Raman signals are significantly decreased. A 10-min irradiation led to complete disappearance of the Raman peaks. After resoaking the substrate in 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

It is well known that metal oxides are typical gas-sensing materials and are sensitive to many gas molecules such as NO2, NH3, H2S, etc., exhibiting huge application prospect in this field. Here, the colloidal electrostatic self-assembly formed oxides wrapped gold NPs also has shown excel-

Usually, the significant responses to the gas molecules generally occur at a high temperature (200–450C). Thus, the response to gas molecules at room temperature is very important due to low power consumption and safety purpose (especially for some inflammable gases). It has been found that such colloidal electrostatic self-assembly formed Au@SnO2 NPs are much better in the gas sensing to H2S gas at room temperature than the pure SnO2 NPs' film [45].

Typically, the responses to H2S gas (1–3 ppm) at room temperature for these two NPs build films are illustrated in Figure 12a. For the pure SnO2 NPs' film, the response to H2S at room temperature was not recoverable [25]. As a result, the pure SnO2 NPs' film cannot be used at room temperature for detection of H2S gas. While for Au@SnO2 sensor, the response was quickly recovered to the baseline upon gas off. Besides, Au@SnO2 exhibits much faster

relation between them. The SERS detection limit is less than 1 ppm.

concentration, which exhibits a good linear double logarithmic

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1048 cm<sup>1</sup> versus the NO3

4.1.3. Reusability of SERS substrate

detection of such molecules.

4.2. Enhanced gas sensing

lent performance in the gas sensing.

4.2.1. Fast response at room temperature

### 4.1.2. Enhanced interaction between analytes and SERS substrate

Improvement of the interaction between the analytes and the SERS substrates is important in SERS-based detection. For the analytes which weakly interact with plasmonic metal NPs but can strongly be adsorbed on oxides, the ultrathin oxide layer-wrapped plasmonic metal NPs could be the better SERS substrate than the pure noble metal NPs.

For example, the explosive raw material KNO3, which weakly interacts with the Au NPs, was utilized as the analyte. Because of the positively charged Au@TiO2 NPs in KNO3 solution, the surface of NPs could enrich more nitrate anions than the pure negatively charged Au NPs. Thus, the Au@TiO2 NPs-built film as the SERS substrate can absorb the molecules well for further SERS detection. Figure 11a presents the Raman spectra for the Au@TiO2 NPs and the pure Au NPs substrates after soaking in the KNO3 solution with 1000 ppm and drying. For the Au@TiO2 NPs substrate, there exists a strong main peak at 1048 cm<sup>1</sup> together with a relatively weak peak at 712 cm<sup>1</sup> and a very weak peak at 1345 cm<sup>1</sup> . Such a spectral pattern is in good agreement with that of pure solid KNO3, and

Figure 11. (a) Raman spectra of KNO3 on different substrates of Au@TiO2 NPs and pure Au NPs, respectively, after soaking in the KNO3 solution with 1000 ppm. And the Raman spectrum of pure solid KNO3. (b) Plot of the peak intensity I at 1048 cm<sup>1</sup> versus the KNO3 concentration C for the soaked Au@TiO2 substrate [24].

all the peaks should originate from nitrate NO<sup>3</sup> [42]. In contrast, the signal from the Au NPbuilt substrate was much lower. The intensity of the main peak at 1048 cm<sup>1</sup> is five times higher for the Au@ TiO2 substrate than that from the Au NPs' substrate. Furthermore, the Raman spectral dependence on the NO<sup>3</sup> concentration was measured for the Au@TiO2 substrate, as demonstrated in Figure 11b. It shows the plot of the peak intensity at 1048 cm<sup>1</sup> versus the NO3 concentration, which exhibits a good linear double logarithmic relation between them. The SERS detection limit is less than 1 ppm.

### 4.1.3. Reusability of SERS substrate

4.1.1. SERS effect of the oxide-wrapped plasmonic metal NPs

4.1.2. Enhanced interaction between analytes and SERS substrate

could be the better SERS substrate than the pure noble metal NPs.

the wrapping layer.

214 Plasmonics

1345 cm<sup>1</sup>

Due to the strong LSPR effect of gold cores, strong local electromagnetic field thus can be formed near the surface of the NPs with the incident light irradiating. It is reasonable that such core-shell NPs illustrate SERS effect to specific target molecules. However, considering that the SERS is a short-range effect, the distance it can effectively act on is generally considered not to exceed 10 nm on the plasmonic particle surface. As previously reported [15], the enhancement effect of SERS decreases exponentially with the shell thickness increasing. So, the SERS activity of the coated Au NPs should be much lower than that of pure Au NPs due to the existence of

Improvement of the interaction between the analytes and the SERS substrates is important in SERS-based detection. For the analytes which weakly interact with plasmonic metal NPs but can strongly be adsorbed on oxides, the ultrathin oxide layer-wrapped plasmonic metal NPs

For example, the explosive raw material KNO3, which weakly interacts with the Au NPs, was utilized as the analyte. Because of the positively charged Au@TiO2 NPs in KNO3 solution, the surface of NPs could enrich more nitrate anions than the pure negatively charged Au NPs. Thus, the Au@TiO2 NPs-built film as the SERS substrate can absorb the molecules well for further SERS detection. Figure 11a presents the Raman spectra for the Au@TiO2 NPs and the pure Au NPs substrates after soaking in the KNO3 solution with 1000 ppm and drying. For the Au@TiO2 NPs substrate, there exists a strong main peak at 1048 cm<sup>1</sup> together with a relatively weak peak at 712 cm<sup>1</sup> and a very weak peak at

Figure 11. (a) Raman spectra of KNO3 on different substrates of Au@TiO2 NPs and pure Au NPs, respectively, after soaking in the KNO3 solution with 1000 ppm. And the Raman spectrum of pure solid KNO3. (b) Plot of the peak intensity

I at 1048 cm<sup>1</sup> versus the KNO3 concentration C for the soaked Au@TiO2 substrate [24].

. Such a spectral pattern is in good agreement with that of pure solid KNO3, and

Most SERS substrates are often disposable. As a result, the reusability of them is significantly important from the view of economics and complexity in the repeated fabrication of substrate. As we known, the oxides have shown excellent photocatalyst properties in many previous works. The oxide wrapping layer is thus expected to photocatalyze the target molecules after a SERS detection. Indeed, the reusability of the Au@TiO2 NP substrate has been confirmed, as previously reported [24]. Typically, the 4-Nitrophenol (4-NP), which can be photocatalyzed by TiO2, [43] was used as the target molecules for SERS detection. The Raman spectrum of the Au@TiO2 NP-built film after soaking in the 4-NP solution with 20 ppm is well shown. All Raman peaks are from 4-NP molecules [44]. If the soaked film was subsequently irradiated for 5 min by a xenon lamp before Raman spectral measurement, the Raman signals are significantly decreased. A 10-min irradiation led to complete disappearance of the Raman peaks. After resoaking the substrate in 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.

### 4.2. Enhanced gas sensing

It is well known that metal oxides are typical gas-sensing materials and are sensitive to many gas molecules such as NO2, NH3, H2S, etc., exhibiting huge application prospect in this field. Here, the colloidal electrostatic self-assembly formed oxides wrapped gold NPs also has shown excellent performance in the gas sensing.

### 4.2.1. Fast response at room temperature

Usually, the significant responses to the gas molecules generally occur at a high temperature (200–450C). Thus, the response to gas molecules at room temperature is very important due to low power consumption and safety purpose (especially for some inflammable gases). It has been found that such colloidal electrostatic self-assembly formed Au@SnO2 NPs are much better in the gas sensing to H2S gas at room temperature than the pure SnO2 NPs' film [45].

Typically, the responses to H2S gas (1–3 ppm) at room temperature for these two NPs build films are illustrated in Figure 12a. For the pure SnO2 NPs' film, the response to H2S at room temperature was not recoverable [25]. As a result, the pure SnO2 NPs' film cannot be used at room temperature for detection of H2S gas. While for Au@SnO2 sensor, the response was quickly recovered to the baseline upon gas off. Besides, Au@SnO2 exhibits much faster

opportunity for the quantitative gas detection of H2S at room temperature. In addition, such core-shell materials also show good selectivity to the H2S gas. Under the same test conditions and gas concentration, it shows no responses to a variety of gases, such as ammonia and benzene vapor, and only shows weak responses to alcohol and formaldehyde vapors. And the corresponding sensitivity was 24 and 38 times lower than that of substrate to H2S gas, as

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In short, since many metal oxides can be used as the gas-sensing materials, the colloidal electrostatic self-assembly fabrication can provide a general method to achieve the ultrathin

We have introduced a facile strategy for fabrication of ultrathin oxide layer-wrapped plasmonic metal NPs based on colloidal electrostatic attraction and self-assembly. In this approach, hydrolysis-induced small positively charged hydroxide colloids are wrapped on negatively charged plasmonic metal NPs via the electrostatic self-assembly. After dehydration process by annealing, the shell will be transformed to oxide, resulting in oxides wrapped metal NPs. Based on this strategy, one-step laser ablation of metal targets in the hydrolysis-induced hydroxide sol solutions have been conducted to fabricate the Au@oxides (Fe2O3, Al2O3, Al2O3, CuO, and ZnO) as well as Pt@TiO2 and Pd@TiO2 NPs. Furthermore, the thickness of these oxide layers are as thin as 13 nm and homogenous. And it also shows independence on the plasmonic metal NPs' size. Additionally, such a strategy shows excellent controllability to the shell in the fabrication. Typically, a secondary irradiation can homogenize the NPs' size. Prolonging the ablation duration can improve the shell's crystallinity hugely. And the shell thickness also could be tuned by the temperature, concentration, and pH value, simply by adjusting the hydrolysis of the metal ion. Finally, enhanced SERS and gas-sensing performances of such oxide layer-wrapped plasmonic metal NPs also have been demonstrated. It demonstrates that ultrathin TiO2-wrapped Au NPs can achieve a much stronger SERS performances in the detection of nitrates due to its positively charged composite NPs. In addition, such a SERS substrate can be recycled by irradiating the used substrate to photodegradate the target organic molecules. Also, significantly better gas-sensing performance of Au@SnO2 NPs has been studied, which demonstrates quickly and linearly respond to H2S gas at room

This work is financially supported by the National Key Research and Development Program of China (Grant No. 2017YFA0207101), Natural Science Foundation of China (Grant Nos. 51771182, 51531006, and 11574313), and the CAS/SAF International Partner-ship Program for

metal layer-wrapped plasmonic NPs, which are a class of new gas-sensing materials.

shown in Figure 13b.

5. A brief summary

temperature with excellent selectivity.

Acknowledgements

Creative Research Teams.

Figure 12. (a) Responses to H2S gas at room temperature for the Au@SnO2 NPs' film and the pure SnO2 NPs' film. (b) The response and recover part of the plot of Au@SnO2 NPs' film to 1 ppm H2S gas.

response (320 ms) and recovery (11 s) compared with pure SnO2 counterpart (Figure 12b). This could be associated with both electronic sensitization of Au metal and the ultrathin wrapping layer in such a core/shell structure. Further work is needed to understand and reveal the origin of this phenomenon.

### 4.2.2. Quantifiable sensing and selectivity

Quantitative detection makes sensing more reliable and scientific. The response of Au@SnO2 NPs to H2S here shows obvious concentration dependence. With the concentration increasing from 1 to 10 ppm, the sensitivity increases linearly, as shown in Figure 13a. This provides an

Figure 13. (a) The responses of the Au@SnO2 NPs' film to different concentrations of H2S gas. (b) The selectivity of the substrates to a variety of gases.

opportunity for the quantitative gas detection of H2S at room temperature. In addition, such core-shell materials also show good selectivity to the H2S gas. Under the same test conditions and gas concentration, it shows no responses to a variety of gases, such as ammonia and benzene vapor, and only shows weak responses to alcohol and formaldehyde vapors. And the corresponding sensitivity was 24 and 38 times lower than that of substrate to H2S gas, as shown in Figure 13b.

In short, since many metal oxides can be used as the gas-sensing materials, the colloidal electrostatic self-assembly fabrication can provide a general method to achieve the ultrathin metal layer-wrapped plasmonic NPs, which are a class of new gas-sensing materials.
