Realization of Oriented and Nanoporous Bismuth Chalcogenide Layers via Topochemical Heteroepitaxy for Flexible Gas Sensors

Abstract

Most van der Waals two-dimensional (2D) materials without surface dangling bonds show limited surface activities except for their edge sites. Ultrathin Bi₂Se₃, a topological insulator that behaves metal-like under ambient conditions, has been overlooked on its surface activities. Herein, through a topochemical conversion process, ultrathin nanoporous Bi₂Se₃ layers were epitaxially deposited on BiOCl nanosheets with strong electronic coupling, leading to hybrid electronic states with further bandgap narrowing. Such oriented nanoporous Bi₂Se₃ layers possessed largely exposed active edge sites, along with improved surface roughness and film forming ability even on inkjet-printed flexible electrodes. Superior room-temperature NO₂ sensing performance was achieved compared to other 2D materials under bent conditions. Our work demonstrates that creating nanoscale features in 2D materials through topochemical heteroepitaxy is promising to achieve both favorable electronic properties and surface activity toward practical applications.

Figure 1

TEM images of (a) BiOCl nanosheets and (b) Bi₂Se₃/BiOCl heterostructures. (c) STEM image of a Bi₂Se₃/BiOCl heterostructure, revealing a nanoporous Bi₂Se₃ layer with a pore size of 1-3 nm. (d) Side-view STEM image of Bi₂Se₃/BiOCl heterostructures. (e) STEM image and EDX mapping of Bi₂Se₃/BiOCl heterostructures. (f) XRD patterns of BiOCl nanosheets and Bi₂Se₃/BiOCl heterostructures. (g) Schematic illustration of the formation process of the Bi₂Se₃/BiOCl heterostructure.

Figure 2

(a) SAED pattern of a Bi₂Se₃/BiOCl heterostructure. The pattern for BiOCl with a fourfold symmetry is indicated by a yellow square; two patterns for Bi₂Se₃ with a sixfold symmetry are indicated by red and green hexagons. (b) HRTEM image of an area with overlapping lattices of BiOCl and Bi₂Se₃. Inset: the corresponding FFT diffraction pattern. (c) BiOCl [001]-zone and (d) Bi₂Se₃ [001]-zone lattice patterns generated by performing inverse-FFT of the spots forming the red hexagon and the yellow square in (b), respectively. Insets in (c and d): the selected spots. (e) Schematic top-view and side-view models indicating the BiOCl (001) || Bi₂Se₃ (001) and BiOCl [100] || Bi₂Se₃ [110] epitaxial relationship. Side-view HRTEM images of typical Bi₂Se₃/BiOCl heterostructures with (f) 1 QL and (g) 2 QLs Bi₂Se₃ grown on a BiOCl nanosheet.

Figure 3

(a) Calculated PLDOS of Bi₂Se₃ (the top 1^st panel) and BiOCl (bottom panel) before and after (the 2^nd and 3^rd panels) contact. (b) Calculated structure model of the Bi₂Se₃-BiOCl interface superimposed with the charge density difference (cyan: positive; pink: negative). (c) Raman spectra of BiOCl, Bi₂Se₃, and Bi₂Se₃/BiOCl heterostructures.

Figure 4

(a) Dynamic response-recovery curve of the sensor fabricated from Bi₂Se₃/BiOCl heterostructures in response to NO₂ gas with increasing gas concentration at room temperature. (b) Sensing response vs. NO₂ concentration for BiOCl-, Bi₂Se₃-, and Bi₂Se₃/BiOCl-based sensors. (c) Response of Bi₂Se₃/BiOCl heterostructures upon exposure to 10 ppm NO₂, H₂S, C₇H₈, C₂H₅OH, NH₃, (CH₃)₂CO, CO₂, and HCHO at room temperature. (d) Sensing response vs. NO₂ concentration for Bi₂Se₃/BiOCl-based sensor with and without a 365 nm light irradiation (8.31 μW).

Figure 5

(a) Calculated structural models of NO₂ molecules absorbed on the BiOCl (001), Bi₂Se₃ (001), (100), and (110) surfaces. (b) Calculated adsorption energy (eV) and electron transferred (e) toward NO₂ on different sensing surfaces. HRTEM images showing the (c) (110) edge plane and (d) (100) edge plane of Bi₂Se₃.

Figure 6

(a) Schematic diagram of the Bi₂Se₃/BiOCl heterostructures on flat and bent substrates. (b) Responses of Bi₂Se₃/BiOCl-based sensors under flat and bent states at various NO₂ concentrations. Inset: a photograph of the printed electrode array. (c) Comparison of the sensing performance between our flexible gas sensor and other 2D material-based flexible sensors under bending conditions.