Surprising Effects of Al₂O₃ Coating on Tribocatalytic Degradation of Organic Dyes by CdS Nanoparticles

Abstract

With a band gap of 2.4 eV, CdS has been extensively explored for photocatalytic applications under visible light irradiation. In this study, CdS nanoparticles have been investigated for the tribocatalytic degradation of concentrated Rhodamine B (RhB) and methyl orange (MO) solutions. For CdS nanoparticles in a glass beaker, 78.9% of 50 mg/L RhB and 69.8% of 20 mg/L MO solutions were degraded after 8 h and 24 h of magnetic stirring using Teflon magnetic rotary disks, respectively. While for CdS nanoparticles in a beaker with Al₂O₃ coated on its bottom, 99.8% of the RhB solution was degraded after 8 h of magnetic stirring and 95.6% of the MO solution was degraded after 12 h of magnetic stirring. Moreover, another contrast was observed between the two beaker bottoms—a new peak at 250 nm in UV–visible absorption spectra was only observed for the MO degradation by CdS in the as-received glass beaker, which indicates that MO molecules were only broken into smaller organic molecules in that case. These findings are meaningful for expanding the catalytic applications of CdS and for achieving a better understanding of tribocatalysis as well.

1. Introduction

With the development of science and technology, the discharge of pollutants from all walks of life is constantly damaging the water resources on the earth [1]. The composition of polluted water is very complicated, often containing plastics, antibiotics, organic dyes, etc. [2,3,4,5,6]. Organic dyes not only destroy the ecosystem, but also are toxic to humans, leading to skin diseases, such as dermatitis and psoriasis, and even inducing malignant lesions, causing great negative effects [7]. Therefore, it is urgent to find simple, effective, and low-cost methods to treat polluted water and improve the living environment.

There exists a huge amount of clean energy in nature in the forms of solar energy [8], wind energy [9], geothermal energy [10], ocean energy [11], and so on. Presently, these forms of clean energy are mainly obtained from the environment and are converted into chemical energy and electricity [12]. Photocatalysis is a mainstream method for water pollution control by using solar energy as clean energy [13]. Electrons and holes are generated in photocatalysts by light irradiation, and the electrons and holes further participate in the REDOX reaction to decompose harmful substances in wastewater [14]. Although photocatalysis is a mature water remediation method, which has undergone several decades of development, it still faces some challenges and obstacles in practical application, including a high photogenic carrier recombination rate and a low visible light utilization rate [15,16,17]. Obviously, other forms of clean energy should be harnessed more to fill the gap of photocatalytic reactions in environmental remediation.

As mechanical energy is abundant and widely available in the ambient environment, it has received more and more attention to be collected through some catalytic technologies in recent years. In this context, tribocatalysis has emerged as an appealing technology in environmental remediation in recent years. In fact, the terminology of tribocatalysis was proposed decades ago. Heinicke et al. first defined tribocatalysis as a branch of tribochemistry, whose subject is the change in catalytic properties of solids under the action of mechanical energy [18]. Since then, tribocatalysis has been mostly studied for the promotion of tribochemical reactions, reducing friction, and achieving super lubrication [19,20,21]. In 2019, Li et al. first reported the tribocatalytic degradation of organic pollutant dyes by Ba_0.75Sr_0.25TiO₃ nanoparticles [22], in which Ba_0.75Sr_0.25TiO₃ nanoparticles degraded organic pollutants into pollution-free small molecules by collecting mechanical energy under the condition of magnetic stirring. Very quickly, tribocatalysis has also been reported for some materials to absorb mechanical energy via friction for the conversion of H₂O and CO₂ into flammable gasses [23,24]. Obviously, the scope of tribocatalysis has been extended from tribochemical reactions to the collection and conversion of mechanical energy.

Up to now, many materials investigated in tribocatalytic environmental remediation possess a semiconductor-type band gap, such as CaCu₃Ti₄O₁₂ (CCTO) [25], BaTiO₃ [26], TiO₂ [27], Si [28], Bi₂WO₆ [29], ZnO [30], SrTiO₃ [31], and CoFe₂O₄ [32]. Based on the excitation of electron-hole pairs in semiconductors by mechanical energy absorbed through friction, a mechanism has been established for tribocatalysis [23,24]. This mechanism is not only very similar to that of photocatalysis, but also suggests that those materials with outstanding photocatalytic properties may also be promising for tribocatalytic environmental remediation. It is well known that CdS is an important semiconductor material with a band gap of 2.4 eV, which is resistant to optical and chemical corrosion, absorbs a wide range of electromagnetic waves, and has been widely studied as a visible light photocatalyst to convert toxic chemicals into nontoxic small molecules through photocatalysis [33,34,35]. As a matter of fact, CdS is also among the earliest semiconductors that were investigated for tribocatalytic environmental remediation [36]. Neverthless, CdS has only been investigated for the tribocatalytic degradation of rhodamine B (RhB) of a low concentration (5 mg/L) up to now; this is much easier to be degraded than many dye solutions that have appeared in tribocatalytic investigations. In this study, we have further explored the potential of CdS nanoparticles in the tribocatalytic degradation of some much more stubborn organic pollutants, incuding 50 mg/L RhB and 20 mg/L methyl orange (MO) solutions. Though it is quite challenging for CdS nanoparticles to degrade them through magnetic stirring in a normal way, degradation is found to be surprisingly enhanced through an Al₂O₃ coating on the beaker bottoms. Especially for the 20 mg/L MO solution, an Al₂O₃ coating not only dramatically increases the degradation speed, but also changes the degradation mode from a partial degradation to a full one. These results are important not only for tribocatalytic environmental remediation, but also for achieving a better understanding of tribocatalysis as a whole.

2. Materials and Methods

2.1. Materials and Characterization

Commercial CdS nanoparticles with a nominal purity of 99.99 wt% were used in this study, which were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). The crystal structure of the CdS powder was measured through an X-ray diffractometer (XRD, SmartLab SE, Rigaku, Tokyo, Japan) using Cu Kα radiation. The morphology of the CdS powder was observed through a scanning electron microscope (SEM, Zeiss GeminiSEM 500, Oberkochen, Germany), and the microstructure was analyzed. X-ray photoelectron spectroscopy (XPS, Thermo escalab 250XI, Waltham Massachusetts, USA) was used to analyze the elemental composition and chemical state of CdS nanoparticles, and electron binding energies were calibrated using the reference peak of C 1s (284.6 eV).

2.2. Coating Al₂O₃ Ceramic Disks on the Bottoms of Glass Beakers

Commercial flat-bottomed glass beakers, φ 45 mm × 60 mm, were divided into two groups in this study. In the first group, the glass beakers were used directly; in the other group, Al₂O₃ ceramic disks of φ 40 mm × 1 mm were pasted on their bottoms using a strong glue (deli super glue 502) before they were used. In this way, we had two kinds of glass beakers with glass and Al₂O₃ bottoms, respectively.

2.3. Tribocatalytic Degradation of RhB and MO Solutions

For solutions of organic dyes, the higher the concentration, the more difficult it is to be degraded. In this study, relatively concentrated RhB (50 mg/L) and MO (20 mg/L) solutions were adopted to increase their degradation difficulty. In a typical experiment, 0.30 g of CdS nanoparticles were dispersed in a glass beaker containing 30 mL of either 50 mg/L RhB or 20 mg/L MO solution. A homemade Teflon magnetic rotary disk, which was described in detail in a previous paper [7], was used to magnetically stir the suspension at 400 rpm. The room temperature was kept at 25 °C, and the beaker was kept in the dark. During the test, 1 mL of the solution was taken at regular intervals and was centrifuged at 8000 rpm for 5 min to remove CdS nanoparticles. The absorbance of the solutions was measured through a UV–visible spectrometer (UV-2550; Shimadzu, Kyoto, Japan) over the range of 200–650 nm.

2.4. Detection of Radical Species

For hydroxyl radical detection, 10 mL of deionized water, 50 μL of 5, 5-dimethyl-1-pyrrolin-n-oxide (DMPO), and 0.15 g of CdS nanoparticles were added to two glass beakers (φ 45 mm × 60 mm) with either a glass or Al₂O₃ bottom, respectively. For the detection of superoxide radicals, 10 mL of methanol, 50 μL of 5, 5-dimethyl-1-pyrrolin-n-oxide (DMPO), and 0.15 g of CdS nanoparticles were added to two glass beakers with either a glass or Al₂O₃ bottom, respectively. A Teflon magnetic rotary disk was used in every beaker to stir the suspension in it at 400 rpm for 15 min in the dark at room temperature. An electron paramagnetic resonance (EPR) spectrometer (A300-10/12, Bruker, Berlin, Germany) was used to separately detect hydroxyl radicals and superoxide radicals.

3. Results and Discussion

The XRD pattern of the CdS nanoparticles used in this study is shown in Figure 1. According to the standard diffraction card PDF# 41-1049 of wurtzite CdS, all significant peaks can be indexed as those of wurtzite CdS, as shown in the figure. However, there is a very weak peak at 30.6°, which could be from an impurity phase.

Figure 2 presents the XP spectrum of the CdS nanoparticles. Besides the lines for Cd and S [37], only two lines for C 1s and O 1s, separately, can be observed. C impurity is well known for XPS analyses, while O 1s at 532.5 eV is most probably representative of chemisorbed oxygen [38]. Generally speaking, the CdS powder used in this study was of a high purity.

Figure 3 shows two representative SEM images obtained for the CdS powder in this study. From Figure 3a, it can be seen that the CdS particles are quite non-uniform in size, with large ones around 300 nm and small ones smaller than 50 nm. In addition, the CdS particles are of quite irregular shapes with clear edges, as shown in Figure 3b, which indicates a high degree of crystallinity [30].

For the 50 mg/L RhB solution suspended with CdS in a glass-bottomed beaker, the absorption peak at 554 nm in the UV–VIS absorption spectrum decreased steadily with increasing stirring time, as shown in Figure 4a. The degradation efficiency of an organic dye is usually quantified using the formula D = 1 − A/A₀, where A₀ and A represent the initial and sustained intensity of the dye’s characteristic absorption peak. After 8 h of magnetic stirring, the degradation efficiency was 78.9%. Due to its rather high initial concentration, the solution still exhibited a bright color, though its absorption peak had substantially decreased after 8 h of magnetic stirring, as shown in the inset of Figure 4a. In contrast, for the 50 mg/L RhB solution suspended with CdS in an Al₂O₃-bottomed beaker, after 8 h of magnetic stirring, the solution became colorless and its absorption peak at 554 nm in the UV–VIS absorption spectrum almost disappeared, as shown in Figure 4b. Obviously, the Al₂O₃ coating had imposed a remarkable enhancement on the tribocatalytic degradation of RhB by the CdS nanoparticles. Figure 4c compares the degradation efficiency of the RhB solution by CdS nanoparticles over time between the two kinds of beakers. On this basis, the pseudo-primary kinetics fit to the degradation efficiency of the RhB solution was further carried out, as shown in Figure 4d. The degradation rate constants of the CdS nanoparticles rubbed on glass and Al₂O₃ bottoms were 0.176 h⁻¹ and 0.773 h⁻¹, respectively. The Al₂O₃ coating had increased the degradation rate constant of CdS nanoparticles by 4.38 times. It is worthy to mention that in a previous related investigation [36], 97.0% and 98.0% of 5 mg/L RhB were degraded by CdS nanowires after 14 h of magnetic stirring in a glass container and after 7 h of magnetic stirring in a polypropylene (PP) container, respectively. It is clear that PP also showed an enhancement on the tribocatalytic degradation of organic dyes by CdS, while its enhancement was smaller than that of Al₂O₃.

For the 20 mg/L MO solution suspended with CdS in a glass-bottomed beaker, the absorption peak at 464 nm in the UV–VIS absorption spectrum decreased rather slowly with increasing stirring time, as shown in Figure 5a. After 24 h of magnetic stirring, the degradation efficiency was only 69.8% and the solution was still yellowish, as shown in the inset of Figure 5a. Moreover, as the absorption peak at 464 nm decreased, a new absorption peak at 250 nm appeared in the ultraviolet region. The same result had been observed in previous studies [39,40], which actually indicates that MO molecules were only broken into smaller organic molecules like benzoic acid, succinic acid, and p-phenol, according to mass spectrometry tests [28]. As a matter of fact, due to the presence of high-energy bonds (C=N and N=N) in its molecules, MO is relatively rather difficult to degrade among various common organic dyes, and this relatively slow degradation of 20 mg/L MO by CdS is rather usual for MO. To our great surprise, a different result was observed for the 20 mg/L MO solution suspended with CdS in the Al₂O₃-bottomed beaker. As shown in Figure 5b, after 12 h of magnetic stirring, the solution became colorless and its absorption peak at 464 nm in the UV–VIS absorption spectrum almost disappeared, and no new absorption peak was generated at 250 nm. This suggests that for the tribocatalytic degradation of MO by CdS nanoparticles, the Al₂O₃ coating not only significantly accelerated the speed, but also upgraded the degradation mode from a partial one to a complete one. Figure 5c compares the degradation efficiency of the MO solution by CdS nanoparticles over time between the two kinds of beakers, and Figure 5d presents the pseudo-primary kinetic fit to the degradation efficiency of the MO solution. The degradation rate constants of the CdS nanoparticles rubbed on the glass and Al₂O₃ coatings were 0.046 h⁻¹ and 0.272 h⁻¹, respectively. The Al₂O₃ coating increased the degradation rate constant of CdS nanoparticles by 5.87 times.

In order to examine the cycle stability of CdS, a cycle experiment of CdS nanoparticles was carried out, in which a 50 mg/L RhB solution suspended with CdS nanoparticles was degraded repeatedly via magnetic stirring. After 8 h of tribocatalysis, a small amount of a highly concentrated RhB dye solution was added to the degraded solution so that the solution concentration in the beaker was restored to 50 mg/L for a subsequent cycle. The degradation rate of the 50 mg/L RhB solution after 8 h of magnetic stirring was measured for every cycle, which was 99.9%, 100.0%, 99.9%, and 99.9% for the 1st, 2nd, 3rd, and 4th cycles, respectively. Obviously, CdS nanoparticles have an excellent cyclic stability and a great potential to degrade organic pollutants.

For both the tribocatalytic degradation of organic pollutants and the tribocatalytic conversion of H₂O and CO₂, coating materials on the bottoms of beakers/reactors has been found to be a convenient and effective method to enhance or regulate catalytic reactions [23,41,42,43]. The effects of the coatings are generally believed to result from the dynamic frictions between the coatings and the catalysts. As for the enhanced degradation of organic dyes observed for CdS nanoparticles in Al₂O₃-bottomed beakers in this study, the dynamic friction between Al₂O₃ and CdS nanoparticles must have played a vital role, as shown in a schematic drawing for the tribocatalytic process in Figure 6 [44].

According to the electronic transition mechanism for tribocatalysis [23], electron-hole pairs are excited in CdS by the mechanical energy absorbed through friction, which can be expressed as:

$$\mathrm{CdS}\stackrel{Frictionenergy}{\to }\mathrm{CdS}+e^{-}+h^{+}$$

(1)

Electrons further react with oxygen to form superoxide radicals, while holes react with hydroxide to form hydroxyl radicals, as follows:

$${OH}^{-}+h^{+}\to ·OH$$

(2)

$$O_2+e^{-}\to ·O_2^{-}$$

(3)

Hydroxyl radicals and superoxide radicals further react with organic dyestuffs, in which organic dyestuffs are degraded and become pollution-free small molecules:

$$OH\left(or{·O}_2^{-}\right)+Dye\to Decomposition$$

(4)

Hydroxyl radicals and superoxide radicals generated by CdS nanoparticles under magnetic stirring were detected through EPR, and the results are shown in Figure 7. Obviously, for CdS nanoparticles in beakers with both glass and Al₂O₃ bottoms, four distinct characteristic peaks of hydroxyl radicals with a ratio of (1:2:2:1) [45] were observed after the nanoparticles were stimulated via magnetic stirring for 15 min in deionized water (Figure 7a), and four characteristic peaks of superoxide radicals with a ratio of (1:1:1:1) [46] were detected after the nanoparticles were stimulated in methanol (Figure 7b). In addition, it can be seen that the peaks of both hydroxyl and superoxide radicals are stronger for the Al₂O₃-bottomed than for the glass-bottomed beaker, which would suggest that more radicals are generated for the Al₂O₃ ceramic bottom.

For catalysts in photocatalysis, various techniques have been adopted to modify them to improve their catalytic performances. For example, for titanium dioxide–bronze nanosheets, surface-exposed defect sites were formed through light illumination, which greatly enhanced their photocatalytic H₂ production rate [47]. For catalysts in tribocatalysis, similar techniques can also be adopted to reform them. For tribocatalysis, besides the modification of catalysts, coating materials on the bottoms of beakers is another convenient and effective method to enhance the catalytic effect. It is well known that Al₂O₃ ceramics are cheap, robust, and of extremely high chemical stability. Up to now, they have been employed as coatings in several tribocatalytic investigations and some highly surprising effects, including the ones observed in this study, have been revealed [24,43,48]. Presently, these effects cannot be satisfactorily understood. Further studies are highly desirable, which are important for the practical development and for achieving a better understanding of tribocatalysis.

4. Conclusions

CdS nanoparticles have been employed to degrade 50 mg/L RhB and 20 mg/L MO solutions through magnetic stirring. With a Teflon magnetic rotary disk used to magnetically stir CdS nanoparticles in a glass beaker, 78.9% of the RhB and 69.8% of the MO solutions were degraded after 8 h and 24 h of magnetic stirring, respectively. With other conditions unchanged, the degradations were surprisingly enhanced through placing an Al₂O₃ disk on the beaker bottom—as much as 99.8% of the RhB and 95.6% of the MO solutions were degraded after 8 h and 12 h of magnetic stirring, respectively. In addition, for the degradation of MO by CdS nanoparticles in the glass beaker, a new peak appeared in the UV–visible absorption spectrum at 250 nm, indicating that the MO molecules are only decomposed into smaller organic molecules, such as benzoic acid, succinate, and p-phenol. However, with the Al₂O₃ coating, such a peak did not appear, indicating that the MO molecules are degraded mostly into H₂O and CO₂. These findings indicate that CdS has a great potential in the field of tribocatalysis and demonstrates that coating materials on vessel bottoms is a convenient and effective method for tribocatalysis enhancement.