Direct Z-Scheme Cs2O-Bi2O3-ZnO Heterostructures as Efficient Sunlight-Driven Photocatalysts

Abstract

Limited light absorption, inefficient electron-hole separation, and unsuitable positions of conduction band bottom and/or valence band top are three major critical issues associated with high-efficiency photocatalytic water treatment. An attempt has been carried out here to address these issues through the synthesis of direct Z-scheme Cs₂O-Bi₂O₃-ZnO heterostructures via a facile, fast, and economic method: solution combustions synthesis. The photocatalytic performances are examined by the 4-chlorophenol degradation test under simulated sunlight irradiation. UV-vis diffuse reflectance spectroscopy analysis, electrochemical impedance test, and the observed transient photocurrent responses prove not only the significant role of Cs₂O in extending light absorption to visible and near-infrared regions but also its involvement in charge carrier separation. Radical-trapping experiments verify the direct Z-scheme approach followed by the charge carriers in heterostructured Cs₂O-Bi₂O₃-ZnO photocatalysts. The Z-scheme charge carrier pathway induced by the presence of Cs₂O has emerged as the reason behind the efficient charge carrier separation and high photocatalytic activity.

Figure 1

(a) XRD patterns of the samples CBZ0 (Bi₂O₃–ZnO) and CBZ15 (Cs₂O–Bi₂O₃–ZnO), (b) and (c) SEM, (d) TEM, and (e) HRTEM images of the sample CBZ15.

Figure 2

(a) XPS survey of Cs₂O–Bi₂O₃–ZnO (sample CBZ15) and high- resolution XPS of (b) O 1s, (c) Zn 2p, (d) Cs 3d, and (e) Bi 4f of CBZ15.

Figure 3

(a) Degradation curves, (b) kinetics of the photodegradation, (c) catalyst dosage effect, and (d) effect of the 4-CP concentration on photocatalytic efficiency.

Figure 4

(a) DRS UV–vis spectra, (b) photocurrent response, (c) and electrochemical impedance measurements of the prepared samples.

Figure 5

(a) UV–vis DRS spectra of Cs₂O, Bi₂O₃, and ZnO and the Tauc’s plot of (b) Cs₂O and ZnO, and (c) Bi₂O₃.

Scheme 1. Comparison of the Charge Carrier Migration Pathway in Cs₂O–Bi₂O₃–ZnO Based on Heterojunction and Z-Scheme Approaches

Figure 6

(a) Scavengers’ experiments, (b) PL spectral changes observed during the irradiation of the CBZ15 sample in the presence of 5 × 10^–4 M TA, (c) DMPO spin-trapping ESR signals for DMPO–^•O^–₂ of the CBZ15 sample in methanol dispersion upon simulated sunlight illumination, and (d) schematic diagram showing the energy band structure and the proposed photocatalytic mechanism of the Cs₂O–Bi₂O₃–ZnO heterostructure.