Porous Hybrids Structure between Silver Nanoparticle and Layered Double Hydroxide for Surface-Enhanced Raman Spectroscopy

Abstract

Silver nanoparticle (AgNP), in terms of antibacterial, catalytic, electronic, and optical applications, is an attractive material. Especially, when prepared to furnish sharp edge and systematic particle orientation on the substrate, AgNPs can take advantage of surface-enhanced Raman spectroscopy (SERS). In this research, we suggested a synthetic method to immobilize the AgNP on metal oxide by utilizing Ag-thiolate and layered double hydroxide (LDH) as precursor and template, respectively. The layer-by-layer structure of LDH and Ag-thiolate transformed through reductive calcination to metal oxide and AgNP array. Physicochemical characterization, including powder X-ray diffraction, N₂ adsorption-desorption, microscopies, and X-ray photoelectron spectroscopy, revealed that the AgNP with sufficient crystallinity and particle gap was obtained at relatively high calcination temperature, ~600 °C. UV-vis diffusion reflectance spectroscopy showed that the calcination temperature affected particle size and electronic structure of AgNP. The prepared materials were subjected to SERS tests toward 4-nitrothiophenol (4-NTP). The sample obtained at 600 °C exhibited 50 times higher substrate enhancement factor (SEF) than the one obtained at 400 °C, suggesting that the calcination temperature was a determining parameter to enhance SERS activity in current synthetic condition.

Keywords: layered double hydroxide; porous structure; silver nanoparticle; surface-enhanced Raman spectroscopy.

Scheme 1

Schematic of the production of layered double oxide (LDO) and AgNP hybrid (Ag@LDO), starting from the layer-by-layer structure of layered double hydroxide (LDH) and Ag-thiolate complex with carboxylate moiety (Ag/3-MPA).

Figure 1

Powder X-ray diffraction (PXRD) patterns of (A) Ag/3-MPA, (B) Ag@LDH, (C) Ag@LDO400, (D) Ag@LDO500, and (E) Ag@LDO600.

Figure 2

(A) N₂ adsorption–desorption isotherms and (B) corresponding Barrett-Joyner-Halenda (BJH) pore size distribution for Ag@LDO400 (black), Ag@LDO500 (blue), and Ag@LDO600 (red). Solid and open symbols in isotherms represented adsorption and desorption branch, respectively.

Figure 3

Field-emission scanning electron microscopy (FE-SEM) and field-emission transmission electron microscopy (FE-TEM) images for (A,D) Ag@LDO400, (B,E) Ag@LDO500, and (C,F) Ag@LDO600 (Upper row: SEM images, lower row: TEM images).

Figure 4

Magnified FE-TEM images and corresponding fast Fourier transform (FFT) patterns for (A) Ag@LDO400, (B) Ag@LDO500, and (C) Ag@LDO600.

Figure 5

X-ray photoelectron spectroscopy (XPS) of (A) Ag 3d and (B) O 1s electrons for (a) Ag@LDO400, (b) Ag@LDO500, and (c) Ag@LDO600. Black line: observed spectrum, open circles: summation of separated peak, red line: Ag(0) or Al-bound O, blue line: oxidized Ag or Mg-bound O, green line: background for peak separation.

Figure 6

(A) UV-vis diffuse reflectance spectra of Ag powder (dot line) and Ag@LDOs and (B) Kubelka-Munk-transformed reflectance spectra of Ag@LDOs (black line: Ag@LDO400, blue line: Ag@LDO500, and red line: Ag@LDO600). No offset was set among graphs.

Figure 7

Raman spectra of (A) 4-NTP (10⁻⁴ mol/L) on (a) bare Si wafer, (b) Ag@LDO400, (c) Ag@LDO500, and (d) Ag@LDO600, and (B) 4-NTP with various concentrations on Ag@LDO600. All spectra were taken using a 514 nm Ar-ion laser, ~10 μW power, single 5 s accumulation. *: Raman signal for silicon substrate.