Effect of the Functionalization of Porous Silicon/WO₃ Nanorods with Pd Nanoparticles and Their Enhanced NO₂-Sensing Performance at Room Temperature

Abstract

The decoration of noble metal nanoparticles (NPs) on the surface of metal oxide semiconductors to enhance material characteristics and gas-sensing performance has recently attracted increasing attention from researchers worldwide. Here, we have synthesized porous silicon (PS)/WO₃ nanorods (NRs) functionalized with Pd NPs to enhance NO₂ gas-sensing performance. PS was first prepared using electrochemical methods and worked as a substrate. WO₃ NRs were synthesized by thermally oxidizing W film on the PS substrate. Pd NPs were decorated on the surface of WO₃ NRs via in-situ reduction of the Pd complex solution by using Pluronic P123 as the reducing agent. The gas-sensing characteristics were tested at different gas concentrations and different temperatures ranging from room temperature to 200 °C. Results revealed that, compared with bare PS/WO₃ NRs and Si/WO₃ NRs functionalized with Pd NPs, the Pd-decorated PS/WO₃ NRs exhibited higher and quicker responses to NO₂, with a detection concentration as low as 0.25 ppm and a maximum response at room temperature. The gas-sensing mechanism was also investigated and is discussed in detail. The high surface area to volume ratio of PS and the reaction-absorption mechanism can be explained the enhanced sensing performance.

Keywords: Pd nanoparticles; WO3 nanorods; gas sensor; porous silicon; room temperature.

Figure 1

Schematic diagram of the Teflon double-tank cell.

Figure 2

(a) Schematic illustration of the fabrication process of porous silicon(PS)/WO₃ nanorods (NRs)–Pd nanoparticles (NPs); SEM images: top view of (b) PS (inset: cross-section view); (c) PS/W film; (d) PS/WO₃ NRs; and (e) PS/WO₃ NRs–Pd NPs (inset: magnification).

Figure 3

High-magnification SEM images of (a) PS/WO₃ NRs: top view (inset: cross-section view); (b) PS/WO_3–Pd20 (inset: single PS/WO₃ NR–Pd20); (c) PS/WO_3–Pd40 (inset: single PS/WO₃ NR–Pd40); (d) PS/WO_3–Pd60 (inset: single PS/WO₃ NR–Pd60); and (e) Si/WO_3–Pd20; EDS analyses of (f) PS/WO₃ NRs; and (g) PS/WO_3–Pd20; (h) XRD patterns of PS/WO₃ NRs; PS/WO_3–Pd20, PS/WO_3–Pd40, and PS/WO_3–Pd60.

Figure 4

TEM images of (a) PS/WO₃ NRs (inset: single WO₃ NR); (b) PS/WO₃–Pd60 (inset: single PS/WO₃–Pd60); (c) HRTEM image of PS/WO₃–Pd60 (inset: SAED image of WO₃ NR); (d) STEM image and EDS mapping images of PS/WO₃–Pd60.

Figure 5

Dynamic gas response curves of (a) the PS/WO₃ sensor; (b) PS/WO₃–Pd20 sensor; (c) PS/WO₃–Pd40 sensor; and (d) PS/WO₃–Pd60 sensor to different concentrations of NO₂ with varying times at RT.

Figure 6

(a) Gas response of the Si/WO₃–Pd20 sensor, PS/WO₃ sensor, PS/WO₃–Pd20 sensor, PS/WO₃–Pd40 sensor, and PS/WO₃–Pd60 sensor to 0.25, 0.5, 0.75, 1, 1.5, and 2 ppm NO₂ at RT; (b) response–recovery time of the PS/WO₃ sensor, PS/WO₃–Pd20 sensor, PS/WO₃–Pd40 sensor, and PS/WO₃–Pd60 sensor to 0.25, 0.5, 0.75, 1, 1.5, and 2 ppm NO₂ at RT; (c) the relationship between the response to 2 ppmNO₂ and the operating temperature for the Si/WO₃–Pd20 sensor, PS/WO₃ sensor, PS/WO₃–Pd20 sensor, PS/WO₃–Pd40 sensor, and PS/WO₃–Pd60 sensor.

Figure 7

(a) The cyclic response curve of the PS/WO₃–Pd20 sensor to 2 ppm NO₂ at RT; (b) the response of the PS/WO₃ sensor, PS/WO₃–Pd20 sensor, PS/WO₃–Pd40 sensor, and PS/WO₃–Pd60 sensor to different gases at RT.

Figure 8

(a) Schematic energy-level diagram of the PS/WO₃ NRs sensor: from n-type semiconductor with flat band structure to n-type semiconductor with depletion layer, p-type semiconductor with inversion layer, and p-type semiconductor with increased inversion layer; (b) schematic adsorption-reaction gas-sensing mechanism of PS/WO₃ NRs–Pd NPs sensor: in air and in NO₂.