Tellurium quantum dots (Te QDs) were prepared using bulk tellurium as the precursor. Te QDs can be a highly active photocatalyst for boosting the photocatalytic degradation of rhodamine B (RhB) under visible light irradiation. The morphology and composition of Te QDs were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results showed that in the presence of H2O2, the photocatalytic efficiency of Te QDs on RhB could achieve a good degradation effect within a very short time (30 min). The effects of initial dye concentration, pH value, light intensity, catalyst dosage and H2O2 concentration on dye degradation were successively studied. The effects of inorganic ions (NO3−, Cl−, SO42−, Ca2+, Mg2+ and Fe3+) on photocatalytic degradation were also discussed. Experimental results of free radical capture showed that OH• and O2•− played important roles in photocatalytic degradation. More importantly, Te QDs efficiency still remained above 85% after four cycles of use, indicating good stability, recyclability and utility. This work may inspire further design of other semiconductor QDs for highly efficient dye degradation.
Te QDs was prepared by a simple method and used for the efficient degradation of RhB.
OH· and O2•− formed in the presence of H2O2 effectively contributed to the degradation of RhB.
Te QDs possess good stability, commendable repeatability and high catalytic activity.
With rapid industrial and economic development, environmental issues, particularly the pollution of water, has become an important obstacle to the healthy development of the economy and society. Industrial wastewater contains large amounts of refractory and potentially carcinogenic organic dyes, such as rhodamine B (RhB). Photocatalytic oxidation has been demonstrated to be an efficient, advanced and eco-friendly technology for removing organic pollutants from wastewater (Gusain et al. 2019; Paździor et al. 2019). Semiconductors, as polyphase photocatalytic materials, have been widely recognized by researchers for their low cost and high activity in degrading organic pollutants (Cai et al. 2018; Wang et al. 2018; Anwer et al. 2019; Trinh et al. 2019). Tellurium (Te), an important semiconductor with a narrow band gap, has been widely researched for its utility in the photo-response, electrochemical, optoelectronic, biomedical fields and so on. Recently, the development of Te-based monoelemental nanomaterials, such as Te nanowires, Te nanorods, Te nanodots, Te nanosheets and Te nanoneedles have been documented (Liu et al. 2010; Huang et al. 2017; Kim et al. 2018; Xie et al. 2018; Shi et al. 2020). In addition, Te-based materials, such as cadmium telluride (CdTe) quantum dots, have been widely studied for dye removal and degradation (Hua et al. 2018; Ding et al. 2019). However, CdTe has certain limitations in application due to the heavy metal content. Therefore, it is important to explore an eco-friendly, efficient, low-cost and simple material to eliminate harmful organic dyes.
In this work, we studied the effective photocatalytic performance monoelemental Te quantum dots (Te QDs), which was prepared by ultrasonic exfoliation from bulk Te. The photocatalytic activity of Te QDs was assessed by the degradation of RhB. The results could expand the application of Te-based nanomaterials, particularly in wastewater treatment.
Chemicals and materials
None of the reagents were further purified before use. Tellurium blocks (Te, 99.999%) were purchased from Sinopac Chemical Reagents Co., Ltd. N-methyl pyrrolidone (NMP) was purchased from Shanghai Minrell Chemical Technology Co., Ltd. Ethylenediaminetetraacetic acid (EDTA), benzoquinone (BQ) and isopropanol (IPA) were purchased from Shanghai McLean Biochemical Technology Co., Ltd. RhB (RhB, ≥99.0%) was purchased from Aladdin Reagent (Shanghai) Co., Ltd. All experiments were conducted with ultrapure water.
Preparation of Te QDs
Te QDs were prepared by a simple, highly effective and low-cost method (Lu et al. 2017). The specific preparation process was as follows: first, 0.5 g Te powder was ground into a fine powder in a mortar and transferred to a 100 mL centrifuge tube. Then, 50 mL of NMP was added and the centrifuge tube was placed in the ultrasonic bath at room temperature for 4 hours. The suspension obtained was centrifuged at 2,000 rpm for 15 min, and the transparent supernatant was collected as a solution of Te QDs. This was centrifuged for a certain time to remove the solvent NMP, and the resulting precipitation was washed several times with ultrapure water to obtain a pure Te QDs precipitation. Finally, it was dried in a vacuum drying oven at 40 °C to obtain pure solid Te QDs.
The crystallographic structure of the Te QDs was investigated using power X-ray diffractometer (XRD) with a diffraction angle range from 10° to 80°. X-ray photoelectron spectra (XPS) showed the chemical states of Te QDs. The transmission electron microscopy (TEM) images showed the morphology and size of the QDs. The optical characteristics of Te QDs suspension were studied using a UV-visible spectrophotometer (Perkin Elmer UV-Vis-NIR model Lambda 950).
The photocatalytic performance of Te QDs was studied in a photocatalytic reactor with RhB as a typical organic pollutant for degradation. A xenon lamp source system (300 W, no filter) was used as the irradiation source to simulate sunlight (300–1,100 nm). The distances between the xenon lamp and the fluid levels of the sample were 5, 10, 15, 20 cm, and the light intensities were 4.29, 3.08, 2.38 and 1.77 W/cm2, respectively. The reaction temperature was kept at 25 ± 1 °C. The experimental device is shown in Figure S1 (Supplementary Information).
Semiconductor materials transfer positive holes and electrons when absorbing light, and further promote the degradation of organic pollutants by generating a series of active free radicals. In photocatalysis, either of these may dominate, or all of the free radicals may co-exist. Isopropanol (IPA, hydroxyl radical scavenging agent), EDTA (hole scavenging agent) and benzoquinone (BQ, superoxide anion radical scavenging agent) were used to detect the active radical during RhB degradation (Zhou et al. 2018; Wan et al. 2019). 3 mM scavenger, 20 mg Te QDs and 0.5% H2O2 were added to 40 mL RhB (10 mg/L). The experiment then followed the same steps as the degradation experiment.
RESULTS AND DISCUSSION
Characterization of catalyst
where α is the absorption coefficient, A, is a constant, hν is the photon energy, h is the Planck's constant, ν is the incident photon frequency, Eg is the semiconductor band-gap width (band gap). The calculated band gap value of Te QDs is 1.32 eV, which is consistent with a previous report (Lu et al. 2017). The narrow band gap is advantageous to the generation of photogenic electron hole pairs, thus improving the photocatalytic performance.
Photocatalytic activity of Te QDs on RhB
The photocatalytic performance of Te QDs was thoroughly evaluated by the photocatalytic degradation of RhB in different systems. Eight groups with different treatments were assigned: (1) RhB + light; (2) RhB + dark; (3) RhB + H2O2 + light; (4) RhB + H2O2 + dark; (5) RhB + Te QDs + light; (6) RhB + Te QDs + dark; (7) RhB + Te QDs + H2O2 + light; (8) RhB + Te QDs + H2O2 + dark.
As shown in Figure 2(a), a negligible change in the concentration of RhB was found under simulated sunlight, indicating that RhB is resistant to sunlight. In any case, the degradation of RhB could hardly happen in a dark room. The UV-visible absorption peak was basically unchanged, indicating that physical adsorption could not take place between Te QDs and dye molecules. The degradation may be attributed to the electronic transition of catalyst when exposed to the light (Cai et al. 2018; Fernandes et al. 2019). The positive holes (h+) in the valence band and hoto-induced negative electrons (e−) in the conduction band are generated simultaneously from the catalyst under light radiation.
H2O and the abundant OH− in the system could react with h+ to generate hydroxyl radicals (OH•), resulting in the decolorization of RhB. Unfortunately, the decolorization efficiency was not ideal in the treatment with catalyst alone under light due to the rapid recombination of h+ and e−. In order to further improve the decolorization of RhB, a simple and green strategy was adopted by adding H2O2 into the reaction system.
As anticipated, a significant improvement was found in the synergistically treated group, Te QDs and H2O2 under irradiation, as shown in Figure 2(b). The decolorization of RhB was only 33.8% within 20 min in the presence of H2O2 alone, while the decolorization was as high as 79.5% in the presence of both H2O2 and Te QDs. The degradation rate was up to 96.6% within 30 min after Te QDs and H2O2 had been added, which was much higher than in H2O2 only treatment group (54.7%). Importantly, the degradation ratio (96.6%) was superior to that of numerous conventional titanium dioxide-based photocatalyst in similar conditions, including TiO2/C (71.7%) and TiO2 nanowires (28.6%) (Chen et al. 2020; Yao et al. 2020). And the degradation products were almost transparent, resulting in no light absorption, analogous to that of pure water. This indicated the critical role of Te QDs in catalytic process.
Furthermore, the degradation effect under realistic sunlight was tested under the same conditions with the sample in Group 7 with a pH of 3. As shown in Figure S5, the degradation rate was about 78.5% in 30 min, which was inferior to that under simulated illumination using the xenon lamp (95.3%).
An assessment of the degradation effect in real industrial wastewater was also studied, as shown in Figure S6. Degreasing wastewater and electrophoresis wastewater were chosen as the real industrial wastewater samples, which were obtained from the Sichuan FAW Toyota Motor Co., Ltd. The pH of the degreasing wastewater is 7.60, and that of electrophoresis wastewater was 5.20. The results suggested that the decolorization of RhB in electrophoresis wastewater (71.9%) was better than that of degreasing wastewater (33.6%) under simulated light. The reason may be that the low basic water environment was unfavourable to the degradation reaction. Generally speaking, the degradation effect of Te QDs in real industrial wastewater was poor due to the complexity of industrial wastewater.
Effect of initial concentration of RhB and catalyst
RhB solutions with concentrations of 5, 10, 15 and 20 mg/L were selected to study the effect of different initial concentrations of the RhB solution on its degradation rate. As shown in Figure 3(a), the lower the initial concentration of RhB, the higher the photodegradation rate. The catalyst might be coated by the intermediate products, resulting in many active sites being covered (Zhang et al. 2016). Moreover, the photodegradation rate changes little by increasing the concentration of RhB from 10 to 20 mg/L. Therefore, the 10 mg/L concentration of RhB was adopted in subsequent experiments.
The influence of the Te QDs (5, 10, 20 and 40 mg) dosage was studied, with other conditions unchanged. With the increase in the amount of Te QDs, the dye removal rate increased gradually, showing the dose-dependent as illustrated in Figure 3(b). The maximum degradation rate was about 86.2%, which was much high than that of a pucherite-based photocatalyst (less than 40%) under the same conditions (Lin et al. 2019). It should be noted that highest removal rate of RhB was obtained at the Te QDs concentration of 20 mg, which can be attributed to the reduced light transmittance caused by the high concentration of Te QDs (Zangeneh et al. 2015). Given this, a Te QDs dose of 20 mg was chosen to study the photocatalysis properties.
Effects of light intensity
The light intensity is directly related to the efficiency of photocatalytic degradation of pollutants (Tan et al. 2015). The light intensity values used were 4.29, 3.08, 2.38 and 1.77 W/cm2. As can be seen from Figure 3(c), with the increase in light intensity, the RhB degradation rate increased accordingly as the distance decreased. As shown in Figure 3(c), the optimum distance between light source and the system was about 10 cm, resulting in a high degradation rate of 91.4% within 30 min. The greater the light intensity, the more OH• (which has very strong oxidizing power) was generated, which could make organic pollutants degraded in an indirect way. However, the degradation rate sinks by over 13% when the distance is reduced to 5 cm. One possible reason is that the excessive light may cause a rise in the temperature, additional loss of energy and the adverse competition for light between intermediates and dyestuff (Anwer et al. 2019).
Influence of H2O2 dosage
As shown in Figure 3(d), the presence of H2O2 increases the sensitivity of the catalyst to sunlight. The results strongly indicate a synergistic effect of the catalyst and H2O2 in the photocatalytic reaction. The removal rate of RhB gradually increases when the volume ratio of H2O2 to dye solution changes from 0.1% to 0.5% due to the increasing OH•. For example, the removal rate of RhB is 50.2% (H2O2, 0.1%), 67.1% (H2O2, 0.3%) within 30 min. The degradation rate is about 95.6% at the concentration of 0.3% H2O2 by introducing Te QDs, while it was reported as being about 70% in the same conditions by introducing LaFeO3-doped mesoporous silica or traditional NaBiS2 (Guo et al. 2018; Phan et al. 2019).
The influence of the initial pH
As shown in Figure S3, the maximum absorption peak position and absorbance of RhB were basically the same under different pH values (Figure S3), indicating that RhB was hardly affected by pH. The progressive pH decrease (pH = 1 and 3) caused a rapid degradation of about 87% in the first 15 min, compared to about 57% under neutral or alkaline medium. RhB has a higher photocatalytic degradation efficiency (96.3%) at pH = 3 within 30 min compared with a conventional g-C3N4-based photocatalyst at the same condition (75%) (Yang et al. 2019). However, the decolorization of RhB under alkaline conditions (pH = 9) falls to 56.95% in the same amount of time. The increase in pH is unfavourable for degradation efficiency because the excess OH− results in the surface of the photocatalyst particles becoming negatively charged and the photocatalyst particles further agglomerating (Bao et al. 2020). Additionally, the color change of reaction system is shown in Figure S4.
Effects of inorganic ions
To study the possible photocatalytic mechanism of the decolorization of RhB under irradiation over Te QDs, the generation of the primary active species such as the superoxide radical (O2•−) and OH• during the photodegradation process was explored. The capture of the active species was carried out by BQ and IPA, respectively (Yu et al. 2019). As shown in Figure 5, the degradation rate of RhB was significantly reduced with the addition of 3 mM IPA, indicating that OH• is the main active radical in the degradation process. In the presence of BQ, the degradation rate of RhB solution decreased slightly, indicating that O2•− played a secondary role in the degradation of RhB solution, which is consistent with the above predictions. Furthermore, EDTA was used as a hole trap, and few holes were observed, as shown in Figure 5. Therefore, Te QDs with the help of H2O2 under irradiation could produce abundant OH• and a small amount of O2•− to degrade RhB.
However, a majority of photo-generated electron and hole pairs recombining decreased the decolorization efficiency. In order to promote the degradation efficiency, a simple and important strategy was to add H2O2 to the reaction system. The decolorization rate was significantly increased (to 95.6%) after the addition of H2O2. As an effective collector of e− in the conduction band, H2O2 can dramatically decrease the recombination between h+ and e−, resulting in the generation of OH• (Equations (13) and (14)). The O2•− and OH• generated in this process could further interact with RhB to degrade RhB completely (Equations (15) and (16)).
Recycling of Te QD catalyst
Whether a photocatalyst can be reused repeatedly is an important issue when treating sewage. In each test, the catalyst was collected and reused after washing with water and drying. The results of four cycles of reuse under simulated sunlight are shown in Figure 7(a). In each experiment, the initial concentration of RhB was 10 mg/L, the volume ratio of H2O2 was 0.5%, the amount of Te QDs was 20 mg, and the illumination distance was 10 cm. The pH value and reaction time were 3 and 30 min, respectively. As shown in Figure 7(a), the degradation rate of RhB still reached over 80%, indicating that the material can be utilized several times. In addition, the XRD and XPS spectra of the Te QDs photocatalyst remain unchanged after four cycles, further demonstrating its stability and recyclability.
In this paper, Te QDs was used as a novel photocatalyst to degrade RhB. A highly efficient decolorization of RhB was observed (more than 95%) in the presence of H2O2 under simulated sunlight. The experimental results revealed that Te QDs was highly effective for eliminating RhB, particularly under strongly acid conditions. The experiment to test free radical capture proved that both OH• and O2•− were the active free radicals, which effectively promoted the degradation of RhB. Several ions were selected to assess their competitive effect on the degradation of Te QDs, and the results indicated that anions such as NO3−, Cl− and SO42− had less influence than cations. Significantly, Fe3+ has a positive auxiliary effect, while Ca2+ and Mg2+ had an inhibitory effect. Furthermore, Te QDs still showed good photocatalytic efficiency after four cycles, showing the stability and recyclability of Te QDs. To sum up, Te QDs are proposed as an efficient and environmentally friendly photocatalyst for degrading dyes in the context of wastewater treatment.
This work was financially supported by National Natural Science Foundation of China (No. 21871246), the Grant of Jilin Province Science & Technology Committee (No. 20200201082JC), Jilin Province Education Department the Science & Technology development project (No. JJKH20200741KJ and JJKH20200803KJ). The authors thank Jilin University-Enterprise Joint Technology Innovation Laboratory of Green Chemistry and Environmentally Friendly Materials.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.