ABSTRACT: Constrained by the strong Coulombic interaction of electron−hole pairs in semiconductor photocatalysts, the charge carrier separation and the resultant photocatalytic capability are greatly compromised. In this work, we rationally construct a built-in electric field (BEF) from the (111) facet of CdTe quantum dots (CdTeQDs) to the (200) facet of two-dimensional Bi2WO6 (2DBWO) nanosheets by the formation of a Te−Ox bond. We validate experimentally and theoretically that the BEF can profoundly promote the dissociation of a photoexcited exciton and separation of a charge carrier, resulting in the formation of a Zscheme electronic structure of the CdTeQDs/2DBWO photocatalyst. Benefifiting from the role of the BEF, the photoinduced generation of the superoxide anion radical and hydroxyl radical is signifificantly promoted, based on which photodegradation performances of the CdTeQDs/ 2DBWO photocatalyst are 6.64, 1.95, and 5.4 times those of pure 2DBWO for tetracycline, phenol, and rhodamine B, respectively. This work provides a mechanistic insight into the design and optimization of semiconductor heterojunction photocatalysts for efficient charge carrier separation and environmental remediation.
Semiconductor photocatalysts have attracted tremendous attention due to their great potential in promoting light-driven reactions associated with energy conversion1−4 and contaminant treatment.5−7 Unfortunately, when materials shrink from bulk to smaller size or dimensionality, they show strong excitonic effects (the Coulombic interaction of electron−hole pairs),8−10 causing the quick recombination and annihilation of electron−hole pairs,11 significantly restricting charge carrier (excited electron and hole) transfer, and impeding the generation of reactive oxygen species (ROS).12,13 Notably, recombination of electron−hole pairs typically finishes within a few picoseconds, much faster than photoinduced charge carrier separation and photocatalytic reactions.14 This is the dominant limiting step to achieve satisfactory photocatalytic performance.15 In order to tackle this challenge, researchers focused on the design of Z-scheme photocatalysts to overcome the recombination of electron−hole pairs16−18 and promote the dissociation of an exciton into a charge carrier.19,20
Two-dimensional (2D) semiconductor materials are prospective building blocks to construct Z-scheme photocatalysts due to the minimized resistance and fully exposed active sites.21−24 2D Bi2WO6 (2DBWO) nanosheets, with a band gap of 2.6−2.8 eV, are endowed with prominent photocatalytic properties and high stability.25−27 However, the energy band structure of BWO results in poor separation of the photoexcited charge carriers and solar energy absorption ability, thus resulting in low photocatalytic capability.28−30 In contrast to pure 2DBWO, it provides a viable strategy to enhance the photocatalytic activity by coupling with semiconductors of high optical absorption coefficients, which contributes to promoting the generation of ROS such as •O2− and • OH.9,30 Among semiconductors, cadmium telluride quantum dots (CdTeQDs) have been used to construct a Z-scheme heterojunction with 2DBWO nanosheets because of their high visible light absorption efficiency and quantum confinement effect. The fabrication of a Z-scheme CdTeQDs/ 2DBWO photocatalyst can achieve high separation efficiency of photogenerated charge carriers through transferring electrons from the conductor band (CB) of the BWO nanosheet to the valence band (VB) of CdTeQDs. The CB and VB of the CdTeQDs/2DBWO Z-scheme heterostructure with a higher band energy can be conducive to generating ROS.31,32
2. EXPERIMENTAL SECTION
The photocatalytic capability of samples was evaluated by photodegrading tetracycline, phenol, and rhodamine B (RhB). Typically, 50 mg of samples was dispersed into a 150 mL quartz vessel with 100 mL of tetracycline solution (20 mg/L) and then stirred for 30 min to ensure adsorption−desorption equilibrium in the dark. 1.5 mL of the reaction solution was regularly collected at 15 min intervals under visible light irradiation using a 300 W xenon lamp (CEL-HXF300, Beijing China Education Au-light Co., Ltd., China) with cutoff filters (λ > 420 nm), and the visible light intensity was measured to be 110.5 mW·cm−2 using an optical power meter (CEL-NP2000-2A, Beijing China Education Au-light Co., Ltd., China). For the degradation of phenol and RhB, the other steps are the same as the above process except the concentration of phenol (10 mg/L) and RhB (10 mg/L). In addition, radical scavenging tests and recyclability tests are recorded in the Supporting Information.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization of Photocatalysts. Powder X-ray diffraction (XRD) patterns reveal that the introduction of CdTeQDs does not disturb the crystallinity of the 2DBWO nanosheet (Figure S1 and Note S1). The Fouriertransformed infrared (FTIR) spectrum of CdTeQDs/2DBWO shows the CO asymmetric stretching vibration at 1628 cm−1 from the −COOH group of the TGA capping agent, indicating that CdTeQDs are successfully modified on 2DBWO (Figure S2a and Note S2). This is also confirmed from the Raman spectrum of CdTeQDs/2DBWO, in which the peak at 296.2 cm−1 is assigned to the CdS LO mode, indicating the existence of bonding between Cd2+ and −SH from the TGA capping agent (Figure S2b and Note S2).44 Transmission electron microscopy (TEM) clearly reveals the 2D morphology of BWO (Figures 2a and S3a,b). Figures 2b, S3c, S3d, and S4a show that the size of CdTeQDs mainly falls within 5−7 nm. After hybridization, the 2DBWO nanosheet is covered by a high density of CdTeQDs (Figures 2c and S3e,f) and has an average height thickness of ∼10 nm; the atomic force microscopy (AFM) image (Figure 2g) implies that CdTeQDs are dominantly attached on the surface of the nanosheets. The energy-dispersive spectroscopy (EDS) elemental mapping result further illustrates that Cd, Te, Bi, W, and O are evenly exhibited on the CdTeQDs/2DBWO photocatalyst (Figure 2f). The prominent dispersity of the samples in water was verifified by the Tyndall effect (Figure S4b). In addition, the zeta potential values are −21.4 and 15.3 mV for CdTeQDs and 2DBWO, respectively, revealing that CdTeQDs/2DBWO is fabricated by an electrostatic interaction. The high-resolution TEM (HRTEM) images reveal that the exposed crystal facets in the junction are CdTe(111) and BWO(200), respectively (Figure 2d,e). This facet preference was reproducible, as observed in repetitive experiments (Figure S3g,h). According to density functional theory (DFT) calculations, the formation energy between the CdTe(111) facet and the BWO(200) facet was calculated as −3.5 eV/nm2 , which was the lowest among those between other different crystal facets (Figure S5 and Note S3) and thus consistent with the microscopic observation. The actual mass percentage of CdTeQDs represented by Cd in samples is measured using an inductively coupled plasma-optical emission spectrometer, as shown in Figure 5. DFT calculations and Z-scheme formation mechanism. Top view and front view of the (a) BWO(220) and (b) CdTe(111) atomic structure model. (c) 3D view of Δρ for the CdTe(111)/BWO(200) interface (yellow and cyan areas indicate charge accumulation and charge depletion, respectively). (d) Calculated Δρ across the x−y plane layers for CdTe(111)/BWO(200) (violet dashed line represents the critical line of the gained and lost electron). (e) Bader charge analysis of CdTe(111) and BWO(200) facets. (f) Calculated DOS profiles of CdTe(111) and BWO(200) (the Fermi level is set to zero). (g) Energy level diagrams of the CdTeQD and 2DBWO facets before and after contact show the photoexcited carrier separation mechanism and the CdTeQDs/2DBWO Z-scheme formation. ESR spectra of (h) DMPO-•O2− (in methanol) and (i) DMPO-• OH (in water) under visible light irradiation.
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