Abstract
Hollow and heterostructured architectures are recognized as an effective approach to improve photocatalytic performance. In this work, ternary TiO2/CdTe/BiOI with hollow structure was constructed via a step-by-step method. In addition, the effect of TiO2 structural regulation and the energy band alignment of BiOI and CdTe quantum dots (CdTe QDs) with TiO2 in TiO2/CdTe/BiOI on photocatalytic dye removal were also studied. The results reveal that the TiO2/CdTe/BiOI heterostructures with hollow substrates exhibit much higher photocatalytic activities than pure TiO2, P25, TiO2/CdTe, and TiO2/BiOI and ternary TiO2/CdTe/BiOI with solid substrates. For TiO2(H)/CdTe/BiOI, several synergistic factors may be responsible for the remarkable visible-light photodegradation performance, such as strong visible-light absorption by BiOI and larger specific surface area.
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Background
Owing to the energy saving and environmentally friendly merits, semiconductor photocatalysis has drawn increasing interests in environmental conservation. Photocatalysts can be used in various aspects, such as self-cleaning, water treatment, air purification, and anti-bacteria [1, 2]. Among them, due to the advantages of low cost, excellent stability, and nontoxicity [3], titanium dioxide (TiO2) has been extensively investigated. However, it can only utilize small portion of solar spectrum because of its wide bandgap and relatively rapid charge recombination, limiting the photo-conversion efficiency [4].
In order to improve the visible-light photocatalytic efficiency of titania, various strategies have been adopted including ion do**, noble metal loading, heterojunction constructing, and sensitization [5,6,8 shows the degradation efficiency of MO during these photocatalytic experiments in the presence of the selected scavengers. It can be found that the photocatalytic process is suppressed than without any scavenger and the degradation efficiency is almost none in the presence of EDTA-2Na. However, the e− scavenger could accelerate the degradation, which demonstrated that the holes (h+) are the main active species for the MO degradation. The BQ as •O−2 scavenger just influenced the degradation in a small degree, suggesting that •O−2 are partially responsible for the photocatalytic oxidation process. In addition, the influence of IPA on the TiO2/CdTe/BiOI sample removing MO is hardly observed, indicating that •OH radicals are hardly useful in the current photocatalytic system.
The plots of photodegraded performance of MO under trap** the different photogenerated active species of TiO2(S)/CdTe/BiOI sample
The photoluminescence emission spectroscopy (PL) is conducted to further study the transfer behavior of photogenerated charge carriers. As shown in Fig. 9, all samples show a broad PL emission peak at about 450–500 nm with the excitation at 365 nm. The bare TiO2(S) has a strong emission peak, while TiO2(S)/CdTe/BiOI sample displays lower intensity than that of TiO2(S). This phenomenon indicates that the recombination rate of photogenerated charge carriers was efficiently restrained after decorating CdTe and BiOI on the surface of TiO2. Furthermore, the TiO2(H) /CdTe /BiOI exhibits significantly diminished PL intensity in comparison with the other samples, which is cause by the faster transfer of electrons and holes from CdTe QDs or BiOI nanosheets to the surface of TiO2. The PL results are consistent with the result from photodegradation experiment.
Photoluminescence spectra of the bare TiO2(S), TiO2(S)/CdTe/BiOI, and TiO2(H)/CdTe/BiOI samples (λ excitation = 365 nm)
The photocurrent of samples is shown in Fig. 10. It was worth noting that the TiO2(S)/CdTe composites exhibited higher photocurrent response than that of pure TiO2(S) and TiO2(S)/CdTe/BiOI composites. Therefore, the increased photocurrent could be mainly attributed to the efficient photogenerated separation and migration, which benefits for the photocatalytic performance.
The transient photocurrent response of TiO2(S), TiO2(S)/CdTe/BiOI, and TiO2(H)/CdTe/BiOI
Based on the results and discussions above, we propose a synergistic CdTe QDs/BiOI sensitization mechanism of the exciton transfer in TiO2/CdTe/BiOI to explain the enhanced photo activity, as illustrated in Scheme 2. It is well known that TiO2 with a wide bandgap (3.02 eV) could only utilize the UV region in solar light, while the narrow bandgap CdTe QDs (~ 1.5 eV) [32] and BiOI nanostructures (~ 1.8 eV) [33] can be excited by photons in the visible range. In addition, a p-n junction is formed between p-type BiOI and n-type TiO2 when Fermi levels reached equilibrium, which facilitate photo-induced electrons to migrate from CB of BiOI to that of TiO2 [17, 34]. Similarly, a type II heterojunction is formed between p-type CdTe [18] and TiO2 microspheres; thus, electrons in the CB of CdTe QDs can transfer to TiO2 [35]. Therefore, the lifetime of the photogenerated electron and hole is prolonged, which is beneficial for the degradation towards MO.
Illustration of photo-induced charge transfer in ternary TiO2/CdTe/BiOI photocatalytic system
Conclusions
In summary, a series of TiO2-based photocatalysts were synthesized by a facile hydrothermal method. Modifications by BiOI and CdTe QDs were carried out to fabricate binary and ternary heterostructures, and the narrow bandgap semiconductors extended light response for the hybrid photocatalysts. In the case of ternary TiO2/CdTe/BiOI heterostructured photocatalyst, the BiOI flakes and CdTe QDs act as sensitizers on one hand, which are excited by simulated solar light and transfer electrons to TiO2. Meanwhile, the TiO2 microspheres serve as separation centers for the photo-induced charges on the other hand; thus, the synergistic effect among TiO2, CdTe, and BiOI enhances the photocatalytic removal of MO. In addition, hollow TiO2 precursors were also employed to fabricate TiO2/CdTe/BiOI heterostructures, and the improved photocatalytic performance towards MO degradation is attributed to a higher surface area and dispersion of BiOI components. The strategy of material regulation and incorporation will provide possibilities for the design of the multi-component semiconductor photocatalysts.
Abbreviations
- TiO2(H):
-
TiO2 hollow microspheres
- TiO2(S):
-
TiO2 solid microspheres
- TiO2/BiOI:
-
TiO2 spheres modified with BiOI
- TiO2/CdTe:
-
TiO2 spheres modified with CdTe QDs
- TiO2/CdTe/BiOI:
-
TiO2, CdTe, and BiOI ternary composites
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This work was mainly funded by the Nation Natural Science Foundation of China (NSFC, Grant No. 61504073) and A Project of Shandong Province Higher Educational Science and Technology Program (No. J18KA011).
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XF and SL conceived and designed the experiments and were major contributors in performing the analysis with constructive discussions. MH, LF, and CH performed the experiments and analyzed the data. CQ and MH wrote the manuscript. JH and FL contributed the materials and analysis tools. All authors read and approved the final manuscript.
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Qu, X., Liu, M., Li, L. et al. Fabrication of CdTe QDs/BiOI-Promoted TiO2 Hollow Microspheres with Superior Photocatalytic Performance Under Simulated Sunlight. Nanoscale Res Lett 14, 50 (2019). https://doi.org/10.1186/s11671-019-2878-1
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DOI: https://doi.org/10.1186/s11671-019-2878-1