Realization of Tunable Localized Surface Plasmon Resonance of Cu

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Realization of Tunable Localized Surface Plasmon Resonance of Cu@Cu2O Core−Shell Nanoparticles by the Pulse Laser Deposition Method Hongbu Yin,† Yan Zhao,*,† Xibin Xu,† Jie Chen,† Xuemin Wang,† Jian Yu,† Jin Wang,† and Weidong Wu†,‡

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†

Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, Sichuan, People’s Republic of China ‡ Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China S Supporting Information *

ABSTRACT: Cu@Cu2O core−shell nanoparticles (NPs) not only possess a stabilized structure but also exhibit better photocatalytic performance as compared to pure Cu2O. Therefore, preparation of Cu@Cu2O core−shell NPs is key toward efficient photocatalysis applications. In this paper, the fabrication of Cu@Cu2O core−shell NPs on single-crystal MgO(100) substrates has been studied systematically by pulse laser deposition. Scanning electron microscopy (SEM) images show that the average diameter of NPs is enlarged from 89.9 to 150.3 nm with the increasing of oxygen pressure. Transmission electron microscopy (TEM) images indicate that the NPs have elongated hexagons and a core−shell structure with a shell thickness of about 10 nm. UV−vis absorption spectra show that the position of the localized surface plasmon resonance (LSPR) peaks shifts from 648 to 858 nm and the full width at half-maximum (fwhm) of the LSPR peaks broadens from 226.7 to 436.5 nm with increasing average diameter of NPs. According to the analysis, the red shift of the LSPR peaks is caused by enlargement of the core diameter; higher fwhm is a result of broadened particle size distribution and the elongated morphology of NPs. Therefore, the width and range of LSPR peaks of the absorption spectrum can be tuned using this method, which is beneficial for enhancing the light absorption and improving the photocatalytic efficiency of Cu@Cu2O core−shell NPs.

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(fwhm) and particle size distribution3,12,18,19 can increase the absorption range to the visible light, which can increase the utilization of solar energy. The pulse laser deposition (PLD) method is utilized to overcome the drawbacks of wet chemical methods. It is well known that PLD is an elementary and convenient method for the fabrication of metal NPs. Metal NPs prepared by the PLD method are mostly polygonal,18 which can effectively enhance the LSPR effect,20 allowing the NPs to exhibit a broad light absorption range in the spectrum. Meanwhile, compared with chemical methods,6,7,16 the fabrication process via the PLD method can not only avoid the generation of complex intermediates but also minimize impurities. Therefore, it is feasible to prepare core−shell NPs films which achieve the two requirements above by the PLD method.21 Meanwhile, this approach is rarely studied. Hence, the effort to prepare core− shell NPs films by the PLD method will be a valuable model

INTRODUCTION Cu@Cu2O core−shell nanoparticles (NPs) are a type of photocatalysts that possess a stable structure and high surface to volume ratios,1 which are essential for photocatalytic applications.2−4 Up to now, many methods have been proposed to fabricate Cu@Cu2O core−shell NPs, including microwave irradiation, electrochemical methods, solid-state reactions, chemical reduction, and so on.3,5−11 Usually, wet chemical methods are used for preparing Cu@Cu2O core− shell NPs, which suffered from two drawbacks. First, the agglomeration tends to occur in the photocatalytic reaction,12,13 which reduces the effective specific surface area and leads to the decrease of photocatalytic efficiency. Second, the particle size of Cu@Cu2O core−shell NPs is unitary,14−16 exhibiting a localized surface plasmon resonance (LSPR) peak with relatively narrow width, which reduces the utilization of solar energy. This work attempts to address these difficulties using two approaches. First, the preparation method for NP films17 allows the NPs to be uniformly dispersed on the substrate, thereby effectively preventing agglomeration. Second, preparing NPs with broad full width at half-maximum © XXXX American Chemical Society

Received: May 1, 2019 Accepted: July 24, 2019

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Figure 1. In situ XPS measurements of all these five samples: (a) XPS survey spectra; (b) Cu 2p core-level XPS spectra; (c) XAES spectra of Cu L3VV; (d,e) AES spectra of sample 1#; (f,g) AES spectra of sample 3#; (h) Cu L3VV XAES spectra of sample 3#; (i) variation of Cu+ content in each sample as a function of oxygen pressure.

Figure 2. TEM images of the fabricated NPs: (a−d) TEM images of sample 1#; (e,f) are the SAED image of samples 1# and 3#, respectively; (g−l) TEM images of sample 3#.

calculation and UV−vis absorption spectra, the LSPR properties of all the samples from the visible to the nearinfrared light region are discussed.

and novel method for the fabrication of homogenous core− shell NPs. In this work, Cu@ Cu2O core−shell NPs films are fabricated by the PLD method. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) measurements are used to characterize the microstructure and morphology of all the prepared samples. In situ X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) measurements are used to investigate the chemical state of all the samples; combined with theoretical

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RESULTS AND DISCUSSION In situ XPS measurements were carried out in order to investigate the chemical states of the Cu element in all the samples, as shown in Figure 1. Figure 1a presents the survey scan of the entire binding energy from −20 to 1100 eV, which focuses on the existence of B

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Figure 3. (a−e) SEM images of all these five samples; (f−j) corresponding particle size distribution histogram.

and S3). Figure 2a−d is TEM images of sample 1#. A crystalized Cu NP is evident in Figure 2b. Figure 2c,d is the inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) images obtained from high-resolution TEM (HRTEM) image of region I in Figure 2b. The lattice constant of Cu(111) face are measured to be 2.09 Å. Although the total size distribution of NPs is widely spread and the shape of the NPs are mostly elongated hexagonally, as shown in Figure 2g, the thickness of the Cu2O shell is consistently about 10 nm [measured in TEM images of other samples as well (Figure S1)]. Figure 2h is the HRTEM image of a single NP which provided the detailed structural information of the Cu core, Cu2O shell, and the interface between the core and the shell. Figure 2i,j is the IFFT and FFT images obtained from the HRTEM image of the Cu core in Figure 2h. Figure 2k,l is the IFFT and FFT images obtained from the HRTEM image of the Cu2O shell in Figure 2h. Both Cu and Cu2O have the facecentered cubic crystalline structure; the lattice constants of the (111) face are measured to be 2.07 and 2.42 Å, respectively. Figure 2e,f is the selected area electron diffraction (SAED) images of samples 1# and 3#; only the diffraction information of Cu can be distinguished in Figure 2e; meanwhile, both the diffraction information of Cu and Cu2O can be found in Figure 2f. It can be concluded that the Cu@Cu2O core−shell NP films have been fabricated by the PLD method successfully. In order to further investigate the morphology of NPs, SEM measurements of a series of samples were carried out. Figure 3a−e shows the SEM images of samples 1# to 5#, and the respective Gaussian fits of their particle size distribution histograms are shown in Figure 3f−j. The average particle diameters of samples 1#, 2#, 3#, 4#, 5# were determined to be 89.9, 103.1, 118.1, 135.4, 150.3 nm, respectively. The fwhm of Gaussian peaks were determined to be 84.6, 81.8, 85.0, 97.2, 120.7 nm. Therefore, it can be pointed out that the average particle diameters of NPs increases with the increase of oxygen pressure. The mechanism of the formation of Cu@Cu2O core−shell NPs can be described as follows. After annealing for 1 h, the surface of the MgO(100) substrate facilitated the nucleation of Cu atoms and clusters. Atoms’ and clusters’ aggregation enhanced to form NPs. The interior temperature of the NPs was higher than the exterior temperature, which facilitated the migration of oxygen atoms from the interior to the exterior.26 This primarily resulted in the oxidization of the

Cu and O. As shown in Figure 1b, the Cu 2p spectra exhibit doublets (resulted from the spin−orbit splitting) with the binding energy at 932.5 and 952.4 eV corresponding to the Cu 2p3/2 and Cu 2p1/2, respectively. Meanwhile, the weak peaks (marked by black rhombus) located at the lower binding energy side of the main peaks are the satellite lines22 of 2p3/2 and 2p1/2 levels, which were excited by the Mg Kα X-ray radiation. Notably, the absence of sharp characteristic shake-up satellites of Cu2+, which are generally observed at a binding energy of about 9 eV higher than the main peaks, indicates the existence of Cu+ or Cu0 (Cu metal). The absence of a weak hump23 (marked by the hollow rhombus) at around 965 eV also suggests that Cu in the samples should be Cu0, which is further corroborated by the X-ray excited Auger electron spectroscopy (XAES) spectrum of Cu L3VV. Comparing with XPS spectra of the Cu 2p core level, the XAES spectrum of Cu L3VV can be used to distinguish between Cu metal and Cu2O. The major Cu L3VV Auger peak (marked by the red arrow) with the kinetic energy of 918.8 eV is shown in Figure 1c. It has been reported that the Cu0 Auger spectrum has a distinct satellite feature22−24 at a kinetic energy of about 2.5 eV higher than that of Cu+ (marked by the black arrow), which is located at 916.4 eV. Therefore, the appearance of the weak peak lower than the major peak implies the existence of Cu+ in sample 2#−5#. Figure 1d−g are AES spectrums of samples 1# and 3#, respectively. These results further support the conclusion that only Cu0 is present in sample 1#, whereas both Cu0 and Cu+ are found in sample 3#. Figure 1h,i presents the molar composition of Cu+ in each sample, which is determined using the area under the peaks of the elements in the spectra using the following equation25 nCu+ I +/ S I + = Cu Cu = Cu nCu ICu/SCu ICu

(1)

where I represents the intensity of the atomic peak and S is the atomic sensitivity factor. From eq 1, the content of Cu+ in each sample increases with the increase of oxygen pressure. However, the formation of the Cu@Cu2O core−shell structure needs to be further probed by TEM measurements. To investigate the microstructure of the fabricated NPs, TEM images of sample 1# and sample 3# are shown in Figure 2, providing direct evidence of the core and the shell (TEM images with detailed information are presented in Figures S2 C

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surface of NPs. When the formation of Cu@Cu2O core−shell NPs was stabilized, the Cu2O shell prevented the Cu core from being further oxidized. The mean free path of oxygen molecules can be calculated by using the formula λ = kBT/ 2πd2P, where λ is the mean free path, kB is the Boltzmann constant, T is the Kelvin temperature, d is the molecular effective diameter, and P is the pressure of gas. It can be estimated that the mean free path of oxygen molecules is about 10 to 103 m, which is far greater than the distance between the target and the substrate. Therefore, the main effect of oxygen molecules is adsorbed to the surface of the substrate, which contributes to decrease the migration and diffusion of Cu atoms and clusters, resulting in the formation of larger Cu@ Cu2O core−shell NPs. The optical properties of the Cu@Cu2O NPs were investigated both experimentally and theoretically, as shown in Figure 4. In Figure 4a, the spectral features from 300 to 550 nm of sample 2# to sample 5# are attributed to the interband transitions in Cu2O and the scattering from the Cu2O shell.27 The absorption feature above 550 nm is attributed to LSPR for all samples.28 Sample 1# (magenta curve) exhibits an LSPR peak located at 648 nm, which is from bare Cu NPs.15 The position of the LSPR peaks for sample 2#, 3# 4#, 5# are 701, 717, 772, 858 nm, respectively. Interestingly, a weak hump is obtained at 912 nm, which remains unmoved with the increase of the Cu core diameter. This peak can be attributed to the interaction between nearby NPs.29−31 As the position and area of this weak peak (the fit peak 1 in Figure 4f−h) do not change, it suggests that the distance between NPs in each sample was nearly unchanged [measured in SEM images of all the samples as well (Figure S5)]. A commercial finite-difference time-domain package32,33 is used to simulate the absorption coefficient spectra of the Cu@ Cu2O core−shell NPs with different geometric parameters. As the particle size distribution of the NPs was broad, only the geometric parameters of NPs close to the determined average particle diameter are used for the simulation of absorption coefficient spectra. Spherical shape is adopted for the simulation of NPs. Dielectric constants for MgO, Cu, and Cu2O are adapted from Palik’s experimental data. In Figure 4b, for each calculated absorption coefficient spectrum, the model of two NPs with the same geometric parameter set on MgO substrates is established, and the distance between the two NPs is adjusted to 10 nm. The corresponding geometric parameters of the NPs used in the calculation of Figure 4b are summarized in Table 1. Figure 4c is the calculated absorption coefficient spectra of two Cu@Cu2O core−shell NPs with the same geometric parameter but different distances between each other. With the distance of two Cu@Cu2O core−shell NPs decreasing, the interaction between the two NPs increased, which resulted in the 850 nm peak becoming stronger. This supports the previous deduction that NPs in each sample maintain the same distance from each other. As shown in Figure 4e, the LSPR peak positions of the calculated absorption coefficient spectra are in good agreement with the experimental spectra, except for some minor deviations. However, the peaks in Figure 4b are more narrow and symmetric than those in Figure 4a. According to the electric dipole approximation and Mie theory, the absorption coefficient α34 can be obtained by

Figure 4. UV−vis absorption coefficient spectra of the NPs. (a) Experimentally measured UV−vis absorption coefficient spectra of samples 1−5#; (b) calculated absorption coefficient spectra of NPs with different geometric parameters; (c) calculated absorption coefficient spectra of two Cu@Cu2O core−shell NPs with different distances between each other; (d) calculated absorption coefficient spectra by using the spectra in (b); (e) experimentally measured and calculated LSPR wavelength as a function of the overall diameter of NPs; (f) UV−vis subpeaks fitting the figure of sample 2#; (g) UV−vis subpeaks fitting the figure of sample 4#; (h) UV−vis subpeaks fitting figure of sample 5#.

Table 1. Geometric Parameter of NPs Used in the Calculation of Absorption Coefficient Spectra sample name Cu2O shell thickness (nm) Cu core diameter (nm)

α=

1# 0 80

2# 10 80

18πnd 3/2 Pε2 λ (ε1 + 2εd)2 + ε2 2

3# 10 100

4# 10 120

5# 10 140

(2)

where εd is the refractive index of the matrix. λ is the wavelength of the incident radiation. ε1 and ε2 are the real and the imaginary parts of the permittivity of the metal particles, respectively, and P is the volume fraction occupied by the metal particles. Equation 2 shows the absorption peak D

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NPs.40,41 As it reported in the literature, the {100} facets remain most red-shifted than others.41 In our experiment, most of the crystal faces of the Cu2O shells are {100}, which contributes to the occurrence of the red shift. According to Figure 4d, the green curve named with the sum is the weighted average result of three calculated extinction spectra from different core diameters. Obviously, the peak in the green curve is broader than that in the other curves. The position of the peak in the green curve is located at 746 nm, which is within the value of the three other curves. From the above analysis, the red shift of LSPR peaks in Figure 4a is owing to the enlargement of the core diameter, and the broadening of the LSPR peaks is a result of the broadened particle size distribution and the elongated hexagonal shape of the NPs.

attributed to the LSPR effect of the free electron in metal particles when ε1 + 2εd = 0, and the absorption coefficient reaches a maximum. When nd is a constant over the measured range of the absorption spectrum, the position of the absorption peak is mainly dependent on ε1, and the fwhm of the absorption peak is mainly dependent on ε2. According to the Drude−Lorentz−Sommerfeld free electron model, the permittivity35 of the metal particles can be described as εm = ε1 + iε2 = 1 −

ωp2

ω2 Γ ω 2 + Γ ω(ω 2 + Γ 2) i 2

(3)

where Γ is the relaxation constant. The relationship between Γ and the mean free path of the electron (l) is Γ = vF/l, where vF is the Fermi velocity. ωp = (nee2/ε0me)1/2 is the Drude plasmon resonance frequency, where ne and me are the density and mass of electron, respectively. In case the the value of ω is far greater than Γ35 ε1 = 1 −

ε2 =

ωp2 ω2 + Γ 2 ωp2 Γ

ω(ω 2 + Γ 2)

≈1−

≈1−

ωp2 ω2 ωp2 ω3

=1−

Γ=1−

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CONCLUSIONS In summary, a valuable model and novel method, the PLD method, was employed to fabricate core−shell NPs. Cu@ Cu2O core−shell NPs were successfully fabricated on MgO(100) substrates. By precisely raising the oxygen pressure, a series of Cu@Cu2O core−shell NPs with enlarged core diameter and broadened particle size distribution were fabricated and well dispersed on MgO substrates. The increase of oxygen pressure promotes the formation of enlarged Cu@ Cu2O core−shell NPs, which exhibit a red shift and a broadened LSPR peak in the UV−vis absorption spectra. Combined with the calculated results, the red shift of the LSPR peaks is a result of enlargement of the core diameter, and the broadening of LSPR peaks is due to the broadened particle size distribution and the elongated hexagonal shape of the NPs. Therefore, the width and the range of LSPR peaks in the absorption spectrum can be tuned using this method. This is beneficial for enhancing the light absorption and improving the photocatalytic efficiency of Cu@Cu2O core−shell NPs.

nee 2 ε0meω 2 nee 2 ε0meω3

(4)

Γ (5)

According to the resonance condition ε1 + 2εd = 0, the resonance absorption frequency is given as ÅÄÅ ÑÉÑ1/2 Å ÑÑ nee 2 ÑÑ ω = ÅÅÅÅ ÅÅÅ ε0me(1 + 2εd) ÑÑÑÑ (6) Ç Ö It can be concluded from eqs 2 and 6 that the frequency and peak intensity of the surface plasmon resonance absorption peak of the metal NPs are determined by the electron density of the surface.36 Electrons on the surface of the spherical NPs are evenly distributed, resulting in a single absorption peak that is sharp and symmetric. The establishment of eqs 2 and 6 are based on the prerequisite that the metal NPs are spherical. However, in our work, the shape of the NPs is elongated hexagonal because of the effect of the crystal substrate surface.37 It is known from electrodynamics that electrons are more concentrated at the angle of polygonal metal NPs, and thus the density distribution of electrons exhibits strong anisotropy. In elongated hexagonal NPs, two peaks are related to two types of nonequivalent angles that can be distinguished from the spectra, which lead to the asymmetrical shape of the peaks in Figure 4a. The results of fitted peaks (fit peak 2 and fit peak 3) are presented in Figure 4f−h (Figure S4). The surface plasmon resonance frequency is related to the surface electron density.38 The surface electron density is related to the surface area and the number of electrons. Obviously, the surface area of NPs is increased with increasing the average diameter of the NPs, leading to a decrease in the surface electron density.39 This gives rise to a decrease in the resonance frequency, resulting in the red shift of LSPR peak. Actually, there exists a little deviation between experimental value and theoretical calculated value in Figure 4e. In fact, there exists a diameter of NPs’ distribution for each sample, and each diameter size corresponds to a definite absorption peak. Thus, the experimentally measured UV−vis absorption coefficient spectra are broader than the theoretical calculated spectra. Meanwhile, it is noteworthy that the facet effect has great impact on the LSPR band position of Cu@Cu2O core−shell

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EXPERIMENTAL METHODS Experimental parameters are summarized in Table 2. To achieve the fabrication of Cu@Cu2O core−shell NPs, a copper Table 2. Experimental Parameters for the Cu@Cu2O Core− Shell NP Fabrication experiment conditions laser working parameters background vacuum working vacuum target substrate energy density the distance between the target and substrate annealing condition oxygen pressure substrate temperature pulse numbers

experiment parameters 248 nm, 2 Hz, 20 ns 5.0 × 10−7 Pa 1.0 × 10−6 Pa pure copper (purity > 99.999%) single crystal MgO(100) 1 J/cm2 50 mm 873.15 K; 1 h 1 × 10−3 to 5 × 10−5 Pa 873.15 K 14 400 pulses

target was chosen for the ablation process. Prior to the deposition process, the deposition chamber was evacuated down to the background vacuum, and the laser frequency and laser energy density were adjusted. The substrates were then annealed at working vacuum in order to obtain a clean and E

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Table 3. Experimental Parameters of Oxygen Pressure sample name oxygen pressure (Pa)

sample 1# background vacuum

sample 2# 5 × 10−5

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b01253.

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TEM images of sample 4#; TEM images with detailed information of regions I, II, and III in Figure 2; SAED images with detailed information of samples 1# and 3#; UV−vis absorption coefficient spectra of the NPs; UV− vis subpeaks fitting the figure of sample 1#; UV−vis subpeaks fitting figure of sample 4#; and distribution histogram of the distance between the NPs in each sample (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yan Zhao: 0000-0003-3931-5969 Author Contributions

W.W. supervised the project. H.Y., Y.Z., and J.C. performed the experiments and analyzed the data. X.X. contributed to the interpretation of the data and theoretical simulations. H.Y., Y.Z., and W.W. co-wrote the paper. All the authors discussed the result and commented on the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by the National Natural Science Foundation of China (no. 11404302) and the Laser Fusion Research Center Funds for Young Talents (grant no. RCFPD1-2017-9).

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sample 4# 5 × 10−4

sample 5# 1 × 10−3

their application as copper-based catalysts for dimethyldichlorosilane synthesis. Chem. Eng. J. 2012, 211−212, 421−431. (3) Radi, A.; Pradhan, D.; Sohn, Y.; leung, K. T. Nanoscale shape and size control of cubic, cuboctahedral, and octahedral Cu-Cu2O core-shell nanoparticles on Si(100) by one-step, templateless, capping-agent-free electrodeposition. ACS Nano 2010, 4, 1553−1560. (4) Ai, Z.; Zhang, L.; Lee, S.; Ho, W. Interfacial hydrothermal synthesis of Cu@Cu2O core-shell microspheres with enhanced visible-light-driven photocatalytic activity. J. Phys. Chem. C 2009, 113, 20896−20902. (5) Zhao, Y.; Zhu, J.-J.; Hong, J.-M.; Bian, N.; Chen, H.-Y. Microwave-induced polyol-process synthesis of copper and copper oxide nanocrystals with controllable morphology. Eur. J. Inorg. Chem. 2004, 4072−4080. (6) Giannousi, K.; Sarafidis, G.; Mourdikoudis, S.; Pantazaki, A.; Dendrinou-Samara, C. Selective synthesis of Cu2O and Cu/Cu2O NPs: antifungal activity to yeast Saccharomyces cerevisiae and DNA interaction. Inorg. Chem. 2014, 53, 9657−9666. (7) Kalidindi, S. B.; Sanyal, U.; Jagirdar, B. R. Nanostructured Cu and Cu@Cu2O core shell catalysts for hydrogen generation from ammonia-borane. Phys. Chem. Chem. Phys. 2008, 10, 5870−5874. (8) Kim, A.; Muthuchamy, N.; Yoon, C.; Joo, S.; Park, K. MOFderived Cu@Cu2O nanocatalyst for oxygen reduction reaction and cycloaddition reaction. Nanomaterials 2018, 8, 138. (9) Zheng, H. M.; Liu, X. H.; Yang, S. B.; Wang, X. New approach for preparation of ultrafine Cu particles and shell/core compounds of Cu/CuO and Cu/Cu2O. J. Mater. Sci. 2005, 40, 1039−1041. (10) Pászti, Z.; Petõ, G.; Horváth, Z. E.; Karacs, A. Laser ablation induced formation of nanoparticles and nanocrystal networks. Appl. Surf. Sci. 2000, 168, 114−117. (11) Radwan, N. R. E.; Ei-Shall, M. S.; Hma, H. Synthesis and characterization of nanoparticle Co3O4, CuO and NiO catalysts prepared by physical and chemical methods to minimize air pollution. Appl. Catal., A 2007, 331, 8−18. (12) Movahed, S. K.; Dabiri, M.; Bazgir, A. A one-step method for preparation of Cu@Cu2O nanoparticles onreduced graphene oxide and their catalytic activities in N-arylation of N-heterocycles. Appl. Catal., A 2014, 481, 79−88. (13) Kandjani, A. E.; Sabri, Y. M.; Periasamy, S. R.; Zohora, N.; Amin, M. H.; Nafady, A.; Bhargava, S. K. Controlling core/shell formation of nanocubic p-Cu2O/n-ZnO toward enhanced photocatalytic performance. Langmuir 2015, 31, 10922−10930. (14) Ghodselahi, T.; Vesaghi, M. A. Localized surface plasmon resonance of Cu@Cu2O core−shell nanoparticles: absorption, scattering and luminescence. Phys. B 2011, 406, 2678−2683. (15) Kou, T.; Wang, Y.; Zhang, C.; Sun, J.; Zhang, Z. Adsorption behavior of methyl orange onto nanoporous core−shell Cu@Cu2O nanocomposite. Chem. Eng. J. 2013, 223, 76−83. (16) Santillan, J. M. J.; Videla, F. A.; Schinca, D. C.; Scaffardi, L. B. Plasmonic properties and sizing of core-shell Cu-Cu2O nanoparticles fabricated by femtosecond laser ablation in liquids. Plasmonics: Metallic Nanostructures and Their Optical Properties X, 2012; Vol. 8457, pp 84572U−84581U. (17) Xiong, Z.; Cao, L. Interparticle spacing dependence of magnetic anisotropy and dipolar interaction of Ni nanocrystals embedded in epitaxial BaTiO3 matrix. Ceram. Int. 2018, 44, 8155−8160. (18) Yu, J.; Xiao, T.; Wang, X.; Zhao, Y.; Li, X.; Xu, X.; Xiong, Z.; Wang, X.; Peng, L.; Wang, J.; Yin, H.; Chen, J.; Meng, G.; Li, Y.; Wu, W. Splitting of the ultraviolet plasmon resonance from controlling FePt nanoparticles morphology. Appl. Surf. Sci. 2018, 435, 1−6. (19) Xiong, Z. W.; Cao, L. H. Red-ultraviolet photoluminescence tuning by Ni nanocrystals in epitaxial SrTiO3 matrix. Appl. Surf. Sci. 2018, 445, 65−70.

uniform surface. Meanwhile, high-purity oxygen was prepared for the deposition process. A series of experiments were carried out with the same temperature but different oxygen pressures. The samples are labeled as samples 1#, 2#, 3#, 4#, and 5# and the details are listed in Table 3. First, in situ XPS with an unmonochromatized Mg Kα source (1253.6 eV) and AES were used to investigate the chemical composition and state of NPs. During XPS and AES measurements, the pressure of the ultrahigh vacuum chamber was maintained at about 5 × 10−8 Pa. Then, the morphology and microstructure of those NPs were characterized by SEM and TEM. Finally, optical absorption spectra were captured in air by a double-beam UV−visible spectrophotometer with a wavelength resolution of 2 nm (UV360, Shimadzu).

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sample 3# 1 × 10−4

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DOI: 10.1021/acsomega.9b01253 ACS Omega XXXX, XXX, XXX−XXX