High photocatalytic activity over starfish-like La-doped ZnO/SiO2 photocatalyst for malachite green degradation under visible light

https://doi.org/10.1016/j.jre.2020.04.009Get rights and content

Highlights

  • A novel starfish-like La–ZnO/SiO2 photocatalyst was fabricated.

  • The La–ZnO/SiO2 exhibits superior photocatalysis performance for MG degradation.

  • DFT calculations confirm that La doping reduces the band gap of ZnO.

  • ·OH, h+ and ·O2 are confirmed as the main reactive species.

Abstract

In this work, the unique starlike La-doped ZnO–SiO2 photocatalysts were constructed by an evaporation and calcination method and characterized in detail. UV–vis reflectance and DFT calculation confirm that the doping with La allows to obtain a decrease of band gap of ZnO/SiO2, which enhances visible light absorbance and oxidizing ability. The photoluminescence intensity reduces greatly, indicating more effective separation of the photogenerated carriers of La-doped ZnO-SiO2. Photocatalytic activities of La-doped ZnO-SiO2 with different doping ratios under simulated visible light irradiation were evaluated with malachite green (MG) as a model pollutant. Under optimized conditions including solution pH of 8, 15 mg/L of MG solution and 15 mg of catalyst dosage, 0.2% La-ZnO-SiO2 exhibits the best catalytic activity in photodegradations of MG in water. The removal and mineralization efficiency of MG can reach 96.1% and 70.9% in 140 min, respectively. The as-prepared catalysts present superior stability and recyclability after four times reuse. Moreover, selective quenching experiments indicate that hydroxyl radical (·OH), hole (h+) and superoxide radical (·O2-) are the main reactive species responsible for MG degradation. Possible mechanism for photocatalytic elimination of MG over La-doped ZnO/SiO2 photocatalyst is finally proposed.

Graphical abstract

This work reports a unique starfish-like La–ZnO/SiO2 photocatalyst, which shows obviously excellent photocatalytic activity and stability for MG degradation. This work highlights using lanthanum as a promising dopant to design efficient photocatalyst with collaborative optimizing light utilization and charge separation.

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Introduction

Malachite green (MG) is a triphenylmethane compound, which is used as a clothing colorant, food additive, medical disinfectant and insect repellent. However, the MG solution is under a certain degree of mutagenicity and teratogenicity in the body of a mammal. The traditional biodegradation, adsorption, filtration, coagulation and other methods have defects of a long time and secondary pollution.1, 2, 3

Photocatalysis is a form of advanced oxidation technology for the treatment of printing and dyeing wastewater at present. Nano-semiconductor photocatalysis materials have been developed rapidly in environmental protection, health and other fields. Most of the widely studied semiconductor photocatalysts belong to n-type semiconductor compounds with wide bandgap, such as CdS, SnO2, TiO2, ZnO, ZnS, PbS, SrTiO3, V2O5, WO3.4, 5, 6, 7, 8, 9, 10, 11, 12 Among these semiconductors, ZnO is one of the most widely used catalysts. When the photocatalyst ZnO is illuminated, it can fully mineralize the organic matter into water and inorganic matter and will not lead to secondary pollution. ZnO is non-toxic, harmless and reusable. Compared with the defects of traditional catalysts, ZnO has a broad application background. Although the advantages of nano-zinc oxide are prominent, nano-zinc oxide itself is easy to agglomerate, has a strong surface polarity, and is not easily dispersed uniformly in an organic medium, which greatly limits its nano-effect. Rare earth metals are famous for their ability of trapping the electrons, which can effectively reduce the recombination of photosensitized electron–hole pairs. Yayapao et al. found the 3%Ce-doped ZnO was the most efficient in degradation of MB solution under UV light irradiation.13 Khataee et al. investigated the high photocatalytic activity of Eu doped ZnO nanostructures for AR17 degradation under visible light irradiation.14 The La-doped ZnO has appealed to the photoelectric process owing to its high photochemical activity for the degradation of organic contaminants.15,16 In this work, we designed and synthesized a ZnO/SiO2 material with a starfish-like core–shell structure. Doping of metal lanthanum improves the weak point of photocatalytic degradation of ZnO. The valence electron orbital of La is 5d16s2, contains 1d electron, and there is no electron in 4f orbital. The electron energy level of ZnO after La doping is placed in the forbidden band. The doping of La3+ seems to make the valence band of ZnO produce a new valence band. It not only accepts valence band electrons but also absorbs energy to cause photons to transition to the conduction band, broadening the absorption wavelength to visible light. Therefore, La-doped ZnO/SiO2 catalyst is highly efficient. Fig. 1 shows schematic illustration of the preparation of La-doped ZnO/SiO2 photocatalyst. DFT calculations also confirm the effect of lanthanum doping on the ZnO forbidden bandwidth.

In the previous report, the photocatalytic degradation of the MG solution is attributed to a large amount of hydroxyl radicals (·OH), and ·OH can be combined with the organic matter efficiently and quickly, and the organic matter can be degraded. However, there are only a few papers to confirm this.17,18 In this paper, the principle of using isopropyl alcohol as the ·OH elimination agent was tested. It is established that ·OH plays a key role in degrading dye solution. In this experiment, the practicability of the catalyst was investigated from four aspects: metal doping amount, initial solution pH, catalyst dosage and initial MG concentration. The maximum photocatalytic degradation rate of 0.2%La–ZnO/SiO2 to MG is 96.1%, which will benefit the printing and dyeing industry.

Section snippets

Chemicals

Zn(NO3)2·6H2O was purchased from Tianjin Jinbei Fine Chemical Co., Ltd. 2-Methylimidazole was purchased from Aladdin Reagent Network. NaOH was acquired from Tianjin Kaitong Chemical Reagent Co., Ltd. Chloroacetic acid was purchased from the Chinese Medicine Reagent Network. The TEOS, methanol, ammonia and MG used in the experiment were purchased from Tianjin Fuchen Reagent Factory. Lanthanum nitrate was purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were commercially available

Characterization

Fig. 2 shows the XRD patterns of SiO2, ZnO-SiO2 and different La-doped ZnO/SiO2 catalysts, respectively. The small broad peaks in the region of 20°–30° verified the formation of amorphous SiO2. All characteristic peaks of ZnO-SiO2 were indexed as the hexagonal wurtzite crystal structure of ZnO (No. 36–1451)19,20, indicating that ZnO was highly crystallized in ZnO–SiO2. No peaks of lanthanum oxide appeared in the composite 0.1% La–ZnO/SiO2 and 0.2% La–ZnO/SiO2, suggesting that there was no

Conclusions

In summary, a novel starfish-like La-doped ZnO/SiO2 photocatalyst was successfully constructed though an evaporation and calcination method. The obtained La-doped ZnO-SiO2 with different doping ratios exhibits obviously enhanced photocatalytic activity for MG degradation under visible light irradiation in comparison to pure ZnO sample. 0.2%La-ZnO-SiO2 exhibits the best catalytic activity, the removal and mineralization efficiency of MG can reach 96.1% and 70.9%. This significant improvement is

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