**Treatment of Mixture Pollutants with Combined Plasma Photocatalysis in Continuous Tubular Reactors with Atmospheric-Pressure Environment: Understanding Synergetic Effect Sources**

**Lotfi Khezami 1,\* and Aymen Amin Assadi 2,3,\***


**Abstract:** This study investigates the pilot-scale combination of nonthermal plasma and photocatalysis for removing Toluene and dimethyl sulfur (DMDS), examining the influence of plasma energy and initial pollutant concentration on the performance and by-product formation in both pure compounds and mixtures. The results indicate a consistent 15% synergy effect, improving Toluene conversion rates compared to single systems. Ozone reduction and enhanced CO2 selectivity were observed when combining plasma and photocatalysis. This process effectively treats pollutant mixtures, even those containing sulfur compounds. Furthermore, tests confirm nonthermal plasma's in-situ regeneration of the photocatalytic surface, providing a constant synergy effect.

**Keywords:** mixture of pollutants; coupling system; plasma; photocatalysis; synergetic effect; mineralization

#### **1. Introduction**

Outdoor air pollutants originate from familiar anthropogenic sources, including industry, transportation, heating, and agriculture [1]. These pollutants have two types of effects: (i) local effects on health and the environment, necessitating short- and medium-term actions, and (ii) global effects on the planet and climate, manifesting in the long term. In response to these concerns, the European directive 2016/2284/EC was published, aiming to reduce national emissions of air pollutants, including NH3, SO2, NOx, and volatile organic compounds (VOCs) (excluding CH4), with aromatic compounds posing the highest health risks [2]. Recognizing the urgency, France pledged 2005 to reduce its NMVOC, NOx, and SO2 emissions by 52 to 77% and ammonia by 13% [1–3]. Meeting this challenge necessitates the development of advanced treatment processes for industrial effluents, specifically at the emission step, to restrict the concentrations and fluxes released into the environment [4]. Current processes employing a liquid phase and oxido-reduction mechanisms, such as Stretford, Ferrifloc, Sulfurex, Burner-Scrubber, Catalyst-Scrubber, and Ozone processes, rely on large-volume equipment prone to corrosion caused by aggressive solutions [4].

Consequently, these processes incur high investments and maintenance costs, substantial chemical reagent consumption, and pose environmental issues during waste effluent disposal [5]. Thus, treatment processes often fail to meet industry requirements due to cost concerns. Therefore, an alternative process that does not necessitate the use of reagents but only relies on a power supply while generating only mineralizable inorganic by-products

**Citation:** Khezami, L.; Assadi, A.A. Treatment of Mixture Pollutants with Combined Plasma Photocatalysis in Continuous Tubular Reactors with Atmospheric-Pressure Environment: Understanding Synergetic Effect Sources. *Materials* **2023**, *16*, 6857. https://doi.org/10.3390/ma16216857

Academic Editors: Xingwang Zhu and Tongming Su

Received: 2 September 2023 Revised: 17 October 2023 Accepted: 19 October 2023 Published: 25 October 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

(CO2, H2O, etc.) would align better with industry needs, especially if space requirements and energy consumption are manageable [6].

In recent years, investigations have demonstrated the effectiveness of dielectric barrier discharge (DBD) reactors for hazardous pollutant removal from gas streams with low VOC concentrations at ambient temperatures [7]. The simultaneous reduction in coexisting pollutants has also been studied [8]. Despite its good attributes, DBD plasma has some drawbacks, such as the formation of toxic by-products like CO, NO, NO2, and O3. Achieving the desired total oxidation of CO2 and H2O is often challenging [9–12]. To address these challenges, coupling DBD plasma with photocatalysis presents a 'zero waste and zero reagent' technology with the potential for synergy and low energy consumption [13,14]. Several studies have investigated the coupling of DBD plasma and photocatalysis using different reactor types and targeting various odorous compounds, including isovaleraldehyde, isovaleric acid, trimethylamine, ammonia, and dimethyl disulfide (DMDS) [15]. It is known that the presence of a catalyst in the plasma enhances performance [16–19]. To avoid catalytic surface poisoning and maintain energetic efficiency, producing a higher concentration of reactive species through the controlled adjustment of the pulsed discharge conditions is crucial, as these species have very short lifetimes [18–21].

This study aims to investigate VOC removal by coupling plasma and photocatalysis, focusing on new electrode configurations, on the performance in handling pollutant mixtures, and on understanding the catalytic surface poisoning mechanisms through tests on the reuse part of the combined system. In fact, the regeneration effect of plasma on the material surface and the understanding of the reactional mechanism in order to lift the scientific barriers was studied in detail in this paper.

#### **2. Materials and Methods**

#### *2.1. Experimental Setup*

The oxidation (photocatalysis and plasma) runs were conducted using the experimental setup shown in Figure 1. It consists of a tubular cylindrical reactor formed by two concentric Pyrex tubes (100 cm long), one outer tub of 76 mm, and an inner tube of 58 mm. Their wall thickness was about 4 mm. The reactor can be combined with (i) photocatalysis by using an external UV lamp (Philips TL 40W/05 (Philips, Canton of Flayosc, France)) and/or (ii) DBD plasma (Dielectric Barrier Discharge system) by applying high voltage power. Experiments were realized at ambient temperature (25 ◦C) and atmospheric pressure. A TESTO sensor is used to measure the temperature and relative humidity. Before photocatalytic experiments, the UV lamp (100 cm long, in the inner concentric cylinder) is activated for homogeneous irradiation. The flow rate (a maximum of 10 m3.h−1) at the system's inlet is controlled using a mass flow meter (Bronkhorst In-Flow). For humidity experiments (from 5 to 90 ± 5%), a variable part of the airflow is derived through a packed column where water flows in the counter current (Figure 1). Two syringe/syringe driver systems (KD Scientific Model 100) were used continuously for liquid Toluene and DMDS injecting into the gas stream. A heating band was used in the injection zone to achieve a good evaporation of pollutants (Figure 1).

In the case of DBD plasma equipment, the different experimental parts used to create the plasma are illustrated in Figure 1. Plasma discharge was generated by applying high voltage using a signal generator (BFi OPTILAS (SRS) reference DS 335/1)-USA. The applied tension with sinusoidal waveform was amplified 3000 *v*/*v* using a TREK 30A/40 amplifier- (Denver, CO, USA) [1–5]. The outer and inner electrodes were connected to the amplifier (Figure 2). The voltages applied in the plasma reactor are measured using high-voltage probes and recorded with a digital oscilloscope (Lecroy wave surfer 24Xs, 200 MHz).

**Figure 1.** General schematic view of the photocatalysis and non-thermal plasma pilot flowsheeting.

**Figure 2.** Scheme (**a**) and sectional drawing (**b**) of the cylindrical reactor. (**c**) XRD diffractogram of pristine GFT and GFT coated with TiO2.

The operating parameter and their ranges are summarized in Table 1.



A commercial Glass Fiber Tissue (BET surface of 300 m2.g−1, 5–10 nm of diameter and 100% Anatase), produced by Ahlstrom Research and Services, was used as a photocatalyst which contains (i) 13 g.m−<sup>2</sup> of colloidal silica, (ii) 13 g.m−<sup>2</sup> of titanium dioxide nanoparticles, and (iii) inorganic fibers. To achieve this catalyst, Ahlstrom Research and Services starts with impregnating glass fibers using SiO2 and TiO2 nanoparticles suspension in pure water using an industrial-sized press (PC500 Millennium). The second step is a drying step of impregnated fibers [6]. The crystalline phase of the coated photocatalyst on the GFT support was examined using the X-ray diffractometer. The optical band gap of TiO2 nanoparticles has a value of 3.2 eV.

Figure 2c shows the XRD diffraction pattern of the GFT coated with TiO2 photocatalyst and the pristine GFT.

The average size of the TiO2 nanoparticles was calculated following the Scherrer equation considering the intense (101) plane peak, and the obtained value is 16.33 nm.

#### *2.2. Analytical Methods*


$$\rm O\_3 + 2I^- \rightarrow I\_2 + O\_2 + O^- + e^- \tag{1}$$

$$\text{Al}\_2 + 2\text{ S}\_2\text{O}\_3^{\cdot 2-} \rightarrow 2\text{I}^- + \text{S}\_4\text{O}\_6^{\cdot 2-} \tag{2}$$

#### **3. Results and Discussion**

The experimental parameters are defined as follows:


SE = REcombined process/[REplasma + REphotocatalysis].


The degradation studies of (i) Toluene alone (100% C7H8), (ii) DMDS alone (100% C2H6S2) on a continuous annular reactor, and (iii) their binary mixture (Toluene 50%-DMDS 50%) were investigated. The experiments with the photocatalysis process were carried out under different operating conditions. The plasma process performance was also monitored separately from the photocatalysis (without external UV-lamp), and then the association of the photocatalysis/plasma was studied.

#### *3.1. Photocatalysis Treatment: (i) Effects of Initial Pollutant Concentration and Air Flow on Degradation and (ii) Effect of Water Vapor*

This study systematically investigated the impact of varying initial toluene concentrations (10 and 20 mg.m−3) and airflow rates (1–4 m3.h−1) on the efficiency of the photocatalytic removal of toluene. The experiments were conducted with and without a light source (UV-lamp OFF) during an initial adsorption step to ensure stable inlet toluene concentrations. It was observed that, as the gas flow supplying the photocatalytic reactor was increased, the efficiency of toluene degradation was decreased. This trend can be attributed to the shorter contact time between toluene molecules, active sites on the catalyst, and oxidation species at higher flow rates, resulting in reduced degradation efficiency. The experimental data indicated that toluene degradation was approximately 38.5% at 1 m3.h−<sup>1</sup> for an initial concentration of 10 mg.m−<sup>3</sup> but decreased to around 13% at 4 m3.h−1. Moreover, an increase in the initial toluene concentration also led to reduced oxidation performance, with degradation rates of 38.5% and 21.9% observed at 1 m3.h−<sup>1</sup> for initial concentrations of 10 and 20 mg.m−3, respectively. This behavior aligned with previous research on TiO2-based catalysts [1,6–9].

The influence of humidity levels was also investigated by maintaining a constant inlet toluene concentration (10 mg.m<sup>−</sup>3) and an airflow rate of 2 m3.h−<sup>1</sup> while varying humidity levels at approximately 5%, 60% ± 5, and 90% ± 5 using a humidification column. Two distinct behaviors in toluene removal were revealed in Figure 3b. At lower humidity levels (<60% ± 5), an improvement in toluene removal was observed due to active intermediate species generated under these conditions, enhancing the oxidation step and overall photocatalytic toluene removal. Favorable toluene removal efficiency was demonstrated in the experiments at approximately 60% ± 5 humidity levels, increasing efficiency from 19 to 33.6%. However, at higher humidity levels (>60–90%), competitive adsorption between water vapor and toluene molecules on active sites became more pronounced, decreasing toluene removal efficiency. At high humidity levels (90% ± 5), a slight decrease in toluene removal efficiency to 21.5% was observed. The significance of humidity as an experimental parameter in photocatalytic oxidation processes is highlighted by our comprehensive

analysis, with lower humidity favoring toluene removal and higher humidity exerting a negative impact. Thus, increasing the relative humidity inside the reactor results in a net presence of water molecules. The water molecules adsorbed on the surface of the photocatalyst result in photogenerated holes following oxidation leading to the formation of OH radicals known as reactive species in the photocatalytic air treatment. On the other hand, the significant presence of water vapor molecules at high relative humidity levels reverses the trend and reduces the conversion of the pollutant due to the phenomenon of competition between the water molecule and the adsorption of ethylbenzene on the active sites of the photocatalyst [5].

**Figure 3.** (**a**) Toluene performance degradation at different [C7H8] and flow rates in the photocatalysis process. [T = 20 ◦C, UVintensity = 20 W m<sup>−</sup>2]. (**b**) Toluene performance degradation at different rates of humidity in the photocatalysis process. [Flow rate = 2 m3 h−1, [C7H8] = 10 mg m−3, T = 20 ◦C, UVintensity = 20 W m<sup>−</sup>2].

#### *3.2. DBD Direct Plasma Treatment: Effect of Plasma Energy*

To study the performance of pollutants' (toluene and DMDS) degradation via a plasma reactor, the degradation study of toluene was performed with (i) humid airflow (2 m3.h−1, 55% of humidity), (ii) an inlet concentration of 14 ppm, and (iii) a plasma energy of 4.5 J.L−<sup>1</sup> and 9 J.L<sup>−</sup>1. The same methodology was applied to DMDS, where the reactor contained a similar pollutant concentration. Figure 4 shows the degradation rate of toluene/DMDS studied separately at different plasma energies. The experimental data via DBD plasma configuration indicate that, with the two pollutants (aromatic and sulfuric compounds), the increase in specific energy (plasma power) leads to an increase in the removal efficiency of contaminants [7–9]. In our previous work, a similar trend of removal efficiency was displayed in fatty acids [10], aldehydes [15], and amines [21], either on a pilot or on an industrial scale. We reported that the removal efficiencies of these molecules strongly depend on the applied voltage. It was observed that increasing energy enhances the level of electrons, which improves the reactive oxygen species formation and consequently leads to greater removal efficiency. In our study, the toluene removal improved from 13% to 25.1% with 4.5 J.L−<sup>1</sup> and 9 J.L−<sup>1</sup> of plasma power, respectively. As for DMDS, when the specific energy amount is more and more important (9 J.L−1), the DMDS rate (27.51%) is slightly higher than for toluene (25.1%).

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**Figure 4.** Toluene/DMDS performance degradation at different plasma specific energy SEs in the plasma process. [Q=2m3 h<sup>−</sup>1, [COV] = 14 ppm, RH = 55%, T = 20 ◦C].

#### *3.3. Treatment by Coupling Process (Photocatalysis/Plasma): Comparison of Process Performance*

The oxidation of toluene was monitored via three processes: (i) photocatalysis, (ii) DBD plasma, and (iii) a combination of photocatalysis and plasma at different plasma energies (4.5 and 9 J.L−1). In this section of the study, these two methods ((i) and (ii)) were used separately and simultaneously (iii) to enhance the degradation performance of the process. Comparing the treatment with the simultaneous application (photocatalysis/plasma) to that with photocatalysis and plasma applied separately, the experimental data (Figure 5a) indicate that the coupling exhibits a higher performance than the sum of photocatalysis alone and plasma alone. At a plasma energy of 9 J.L−1, the combined process achieved a toluene degradation rate of 61%, while the sum of the degradation rate for photocatalysis and plasma separately was 46.2%. Similarly, at low energy (4.5 J.L<sup>−</sup>1), the toluene degradation reached 40.23% when the combined application was used, compared to 34.5% for the sum of both processes used alone. In this case, an enhancement of 10% ± 3 in the removal efficiency was observed. The same methodology was applied to DMDS (see

Figure 5b). The results indicate a strong synergy between both processes for any pollutant used. Plasma significantly contributes to the desorption of the degraded by-products adsorbed on the TiO2 surface through the active species (O2 ◦−, O◦, HO◦), leading to the increased catalytic activity of the photocatalyst (in our case, TiO2 coated on Glass Fiber Tissue) and improved photocatalytic degradation [6,21–27]. For DMDS, the degradation rate during the combined application of photocatalysis and plasma (53%) surpasses the rates achieved with plasma or photocatalysis alone (45.4%). The same behavior has been found by Qi and his coworkers, with toluene removal in a plasma–catalytic hybrid system over Mn-TiO2 and Fe-TiO2 [28]. Moreover, Wang and his collaborators highlighted the synergetic effect on CO2 reduction in the presence of Dual-plasma enhanced 2D/2D/2D g-C3N4/Pd/MoO3 [11].

**Figure 5.** (**a**) Toluene performance degradation at different plasma specific energy SE via photocatalysis, plasma, and their combination: Evidence for synergetic effect. [Q=2m3 h−1, [toluene] = 14 ppm, RH = 50%, T = 20 ◦C, UVintensity = 20 W m−2]. (**b**) Pollutant performance degradation when applying photocatalysis, plasma, and their combination: Evidence for synergetic effect. [Q=2m3 h−1, [pollutant alone: 100% toluene or DMDS] = 14 ppm, ES=9JL−1, RH = 50%, T = 20 ◦C, SE= 9 J/L, UVintensity = 20 W m<sup>−</sup>2].

The combination of these processes demonstrates the synergy between DBD plasma and photocatalytic oxidation [13,16–20], which can be attributed to the following:


#### *3.4. Treatment by Coupling Process (Photocatalysis/Plasma): Effect of Mixture (Toluene/DMDS) and Plasma Power*

The same methodology was applied to the mixture of toluene and DMDS. Figure 6 illustrates the removal efficiency of the toluene/DMDS mixture using three processes: (i) photocatalysis, (ii) DBD plasma, and (iii) photocatalysis/plasma at different plasma energies. For the mixture studied, the reactor was supplied with: a humid air flow (2 m3.h−1, 55% humidity), an inlet concentration of 14 ppm ([Toluene] = [DMDS] = 7 ppm), and plasma energies of 4.5 and 9 J.L−1. As depicted in Figure 6, plasma energy is a crucial parameter for monitoring plasma application and it can significantly impact degradation efficiency.

**Figure 6.** Evolution of toluene/DMDS degradation rate in mixtures at different plasma specific energy SE via photocatalysis, plasma, and their combination: Evidence for synergetic effect. [Q = 2 m3 h<sup>−</sup>1, [Mixture of pollutant: 50% toluene + 50% DMDS] = 14 ppm, RH = 50%, T = 20 ◦C, UV intensity = 20 W m<sup>−</sup>2].

In the case of the mixture with high energy (9 J.L−1), the results indicate that (i) toluene degradation reached 42.57% when both applications were combined, compared to 35.69% when the two applications were applied separately, and (ii) DMDS degradation reached 53.92% when both applications were combined, compared to 49.31% for separate applications. Comparing the treatment of the toluene/DMDS mixture to the separate treatment of toluene and DMDS, the experimental data (Figures 6 and 5b, respectively) demonstrate that the coupling exhibits higher removal efficiency, surpassing the sum of photocatalysis and plasma treatments when applied separately.

The toluene/DMDS mixture results indicate a more significant synergetic effect for DMDS degradation than for toluene. When photocatalysis/plasma were used together, 42.57% of the toluene in the mixture and 53.92% of the DMDS in the mixture were decomposed. In contrast, during single treatments (Figure 5b), toluene removal reached 61%, and 52% of the DMDS was decomposed. This different behavior can be attributed to the competitive adsorption/oxidation of the two pollutants in the mixture, which becomes more pronounced [22–25]. The molecular chain structure of DMDS (C2H6S2) hampers the adsorption/oxidation and, thus, the photocatalytic oxidation of toluene (C7H8). This observation is consistent with the findings reported by Assadi et al. [21], who demonstrated a decrease in oxidation performance (in the case of a VOC mixture) due to the competitive interactions between pollutants, by-products, and active sites.

#### *3.5. Study of By-Products Generation: Ozone Selectivity of CO/CO2/SO2* 3.5.1. Monitoring of the Ozone Formed

Ozone formation, a strong oxidizing by-product, occurs during the operation of the DBD-plasma/photocatalytic reactor. Extensive experiments on oxidation under (i) a DBDplasma reactor or (ii) coupling plasma/photocatalysis technology have been conducted and detailed in our previous research studies [1,4,6,7,9,23–27]. These studies have shown that increasing the energy of plasma leads to generating a significant amount of ozone in the exhaust. However, experiments with the DBD plasma/photocatalysis combination have been performed to mitigate the excessive ozone production at low plasma energies (4.5 and 9 J.L−1). The humid air flow was fixed at 2 m3.h−<sup>1</sup> with 55% humidity, and the concentration of toluene/DMDS was maintained at 14 ppm. UV irradiation tests (with lamp ON/OFF) were conducted to monitor ozone formation at the reactor outlet at various plasma energies. The results, depicted in Figure 7, illustrate the behavior of ozone during the operation of DBD plasma technology and the coupling of plasma/photocatalysis.

**Figure 7.** Evolution of ozone concentration (ppm) with plasma specific energy SE. [Q = 2 m3 h<sup>−</sup>1, [Mixture of pollutant: 50% toluene + 50% DMDS] = 14 ppm, RH = 50%, T = 20 ◦C, UV intensity = 20 W m<sup>−</sup>2].

A slight decrease in ozone concentration was observed (Figure 7) with values of 36 and 32 ppm for (i) plasma alone and (ii) plasma/photocatalysis, respectively. This decrease can be attributed to the decomposition of ozone through reactions (3), (4), and (5), facilitated by the photo-generated radicals (H◦, HO◦) [26]. It is important to note that adding external UV light to the DBD-plasma system (coupling plasma with UV lamp ON) can play a crucial role in promoting ozone degradation into highly reactive species (O2◦− and HO2 ◦), thereby significantly enhancing the oxidation step.

$$\mathrm{H}\_{2}\mathrm{O} + \mathrm{e}^{-} \to \mathrm{H}^{\circ} + \mathrm{OH}^{\circ} + \mathrm{e}^{-} \tag{3}$$

$$\text{O}\_3 + \text{OH}^\circ \rightarrow \text{O}\_2 + \text{HO}\_2^\circ \tag{4}$$

$$\mathrm{O}\_{3} + \mathrm{H}^{\circ} \rightarrow \mathrm{O}\_{2} + \mathrm{OH}^{\circ} \tag{5}$$

#### 3.5.2. CO2, SO2, CO Selectivity

In this investigation, the selectivity rates of carbon dioxide (CO2), sulfur dioxide (SO2), and carbon monoxide (CO) under different oxidation conditions were analyzed. Figure 8 presents the experimental data, showcasing distinct selectivity rates for these pollutants. Carbon dioxide (CO2) exhibited the highest selectivity rate among the three, achieving 69.81% through photocatalysis alone, 43.75% through plasma alone, and a further increase to 58.98% when plasma and photocatalysis were combined. On the other hand, sulfur dioxide (SO2) showed a selectivity rate of 17.35% through photocatalysis, while plasma alone achieved 46%. However, the combination of plasma and photocatalysis resulted in a decreased selectivity rate compared to CO2. For carbon monoxide (CO), selectivity rates were negligible, with only 7% of the CO mineralization rate observed through photocatalysis, plasma, and plasma/photocatalysis.

**Figure 8.** Variation in CO2, SO2, and CO (%) and ozone (ppm) during toluene/DMDS removal with photocatalysis/plasma coupling. [Q = 2 m3 h−1, [Mixture] = 14 ppm, RH = 50%, T = 20 ◦C, UVintensity = 20 W m<sup>−</sup>2, SE = 4.5 J L−1].

It is worth noting that ozone (O3) concentrations decreased compared to plasma alone. Previous research has shown that incorporating external UV light into the DBD– plasma system (via coupling plasma with UV lamp ON) can significantly enhance the mineralization rate and improve the degradation of pollutants and by-products [16–20].

#### **4. Reusability and In Situ Regeneration**

A series of experiments consisting of four cycles was conducted to evaluate the photocatalytic stability of the catalyst after multiple cycles. These experiments involved alternating phases: (i) the oxidation step using photocatalysis, plasma, and the coupling of both, and (ii) the catalyst regeneration step. The photoactivity tests were conducted under

dry conditions with a continuous flow rate of 2 m3.h−1, using a toluene/DMDS mixture with a concentration of 14 ppm.

Figure 9 presents the degradation rate of toluene in the mixture and the synergetic effect (SE) value after four cycles (each cycle consisting of experiments with photocatalysis, plasma, and their combination). It can be observed that the degradation rate of toluene slightly decreases after four cycles of continuous oxidation, resulting in a 10% loss of the catalyst's photoactivity. However, a stable degradation rate is observed when the coupling of plasma/photocatalysis is applied. In our case, the regeneration step involved using photocatalysis/plasma.

**Figure 9.** Effect of catalysis reusability on synergetic effect in the case of mixture toluene/DMDS. [Q = 2 m3 h<sup>−</sup>1, [Mixture] = 14 ppm, RH = 50%, T = 20 ◦C, UVintensity = 20 W m<sup>−</sup>2, SE = 4.5 J L−1].

Previous studies have also observed the deactivation of photocatalysts during the oxidation of sulfur compounds [27]. These studies have shown that plasma can effectively regenerate poisoned catalysts. Therefore, the results indicate that the regeneration process can be enhanced by combining photocatalysis and plasma [28–34]. This finding confirms the in situ regeneration of the photocatalytic support in the presence of plasma [35–41].

#### **5. Conclusions**

In this comprehensive study, we conducted a thorough investigation into various parameters, including the synergetic effect (SE), inlet concentrations of toluene (TOL) and DMDS, and relative humidity (RH), to assess their influence on the performance of three distinct processes: DBD plasma, photocatalysis, and the combined DBD plasma/photocatalysis system. Our findings have illuminated the pivotal role of water vapor in VOC removal, revealing optimal RH values that enhance CO2 selectivity and diminish CO formation. Furthermore, RH was observed to have a mitigating effect on ozone formation.

Across all operational parameters explored, it is evident that coupling DBD plasma with a TiO2 catalyst under external UV irradiation can yield a synergetic effect, resulting in improved toluene and DMDS removal. The significant enhancement in CO2 selectivity during the coupling process is particularly noteworthy, a remarkable 11% improvement compared to DBD plasma alone. The observed reduction in ozone concentration during plasma–photocatalysis coupling can be attributed to the breakdown of ozone into more active oxidizing species facilitated by UV radiation.

These findings hold profound practical implications for pollutant removal and treatment strategies. As demonstrated in this study, the combination of DBD plasma and photocatalysis presents a promising avenue for more efficient and environmentally friendly approaches to addressing VOC pollution. By unraveling the intricate interplay of parameters and processes, we are better positioned to develop cleaner and more sustainable solutions for industrial effluent treatment.

Future research endeavors could delve deeper into optimizing the coupling process, explore additional parameters, and investigate its adaptability in diverse industrial settings. The pursuit of innovative technologies that reduce environmental impact while aligning with industry requirements remains of paramount importance.

**Author Contributions:** Writing—original draft, Investigation and conceptualization: L.K.; Writing review and editing and Visualization: A.A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research through the project number IFP-IMSIU-2023031. The authors also appreciate the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) for supporting and supervising this project.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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## *Article* **Hydrothermal Synthesis of MoS2/SnS2 Photocatalysts with Heterogeneous Structures Enhances Photocatalytic Activity**

**Guansheng Ma 1, Zhigang Pan 1,2, Yunfei Liu 1,2, Yinong Lu 1,2 and Yaqiu Tao 1,2,\***


**\*** Correspondence: taoyaqiu@njtech.edu.cn; Tel.: +86-137-7078-0496

**Abstract:** The use of solar photocatalysts to degrade organic pollutants is not only the most promising and efficient strategy to solve pollution problems today but also helps to alleviate the energy crisis. In this work, MoS2/SnS2 heterogeneous structure catalysts were prepared by a facile hydrothermal method, and the microstructures and morphologies of these catalysts were investigated using XRD, SEM, TEM, BET, XPS and EIS. Eventually, the optimal synthesis conditions of the catalysts were obtained as 180 ◦C for 14 h, with the molar ratio of molybdenum to tin atoms being 2:1 and the acidity and alkalinity of the solution adjusted by hydrochloric acid. TEM images of the composite catalysts synthesized under these conditions clearly show that the lamellar SnS2 grows on the surface of MoS2 at a smaller size; high-resolution TEM images show lattice stripe distances of 0.68 nm and 0.30 nm for the (002) plane of MoS2 and the (100) plane of SnS2, respectively. Thus, in terms of microstructure, it is confirmed that the MoS2 and SnS2 in the composite catalyst form a tight heterogeneous structure. The degradation efficiency of the best composite catalyst for methylene blue (MB) was 83.0%, which was 8.3 times higher than that of pure MoS2 and 16.6 times higher than that of pure SnS2. After four cycles, the degradation efficiency of the catalyst was 74.7%, indicating a relatively stable catalytic performance. The increase in activity could be attributed to the improved visible light absorption, the increase in active sites introduced at the exposed edges of MoS2 nanoparticles and the construction of heterojunctions opening up photogenerated carrier transfer pathways and effective charge separation and transfer. This unique heterostructure photocatalyst not only has excellent photocatalytic performance but also has good cycling stability, which provides a simple, convenient and low-cost method for the photocatalytic degradation of organic pollutants.

**Keywords:** MoS2; SnS2; photocatalysis; composite catalyst; visible light degradation

#### Academic Editors: Xingwang Zhu and Tongming Su

**Citation:** Ma, G.; Pan, Z.; Liu, Y.; Lu, Y.; Tao, Y. Hydrothermal Synthesis of MoS2/SnS2 Photocatalysts with Heterogeneous Structures Enhances Photocatalytic Activity. *Materials* **2023**, *16*, 4436. https://doi.org/

Received: 29 May 2023 Revised: 12 June 2023 Accepted: 14 June 2023 Published: 16 June 2023

10.3390/ma16124436

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### **1. Introduction**

Today's industrialized and urbanized world is facing severe energy shortages and environmental pollution problems. The excessive use of organic dye and the indiscriminate release of organic pollutants cause serious damage to ecosystems and have serious effects on future generations [1]. Moreover, the organic ingredients in our living environment are difficult to degrade and toxic in nature. There have been many methods to solve the problem of organic dye contamination in environment, such as adsorption [2,3], membrane separation [4,5], biological decomposition [6], chemical oxidation [7], electrocatalysis [8] and photocatalysis [9,10] decomposition. Among them, the use of solar photocatalysis for the degradation of organic pollutants is considered one of the most promising and efficient strategies [11,12].

Transition metal sulfides have attracted a lot of attention in wastewater treatment because of their high specific surface area, high surface activity and special microstructure. In recent years, MoS2 has been widely used in organic dye decomposition due to its low

cost, high abundance and noble-metal-like activities [13–15]. MoS2 has a graphene-like layered structure with three crystal phases: 1T, 2H and 3R [16]. In a natural state, MoS2 is usually present in the steady 2H phase, which exhibits semiconducting properties [17,18]. However, the low density of active sites and relatively poor conductivity of 2H-MoS2 lead to limited photocatalytic activity [19,20]. Compared with 2H-MoS2, the metallic 1T-MoS2 phase has the advantages of significant conductivity and a high density of marginal active sites at room temperature and shows better performance in photocatalysis [21–24]. So far, most 1T-MoS2 is fabricated as two-dimensional nanosheets to construct hybrid structures with nanoconjunctions [25–27].

Li et al. [28] prepared two-dimensional heterostructured MoS2/g-C3N4 (graphite-C3N4) photocatalysts using a facile impregnation–calcination method. The experimental results showed that surface MoS2 nanosheets were successfully loaded horizontally onto g-C3N4 nanosheets. Meanwhile, the two-dimensional heterojunction formed between g-C3N4 nanosheets and MoS2 nanosheets improved the separation efficiency and charge transfer rate of photogenerated electrons. One of the synthesized samples, MCNNs-3 (3 wt% MoS2 in MoS2/g-C3N4 heterojunction), with a catalyst content of 0.8 g/L, reduced the concentration of rhodamine B (RhB) by about 96% after 20 min of irradiation. Chen et al. [29] prepared MoS2/TaON (tantalum oxynitride) hybrid nanostructures by a hydrothermal method. This work showed that the photocatalytic degradation of rhodamine B (RhB) on Ta1Mo1 (mass ratio of TaON:MoS2 = 1:1) was about 65% after 2 h of visible light irradiation, which was about five times higher than that of pure TaON. In addition, MoS2/SiO2/TaON ternary photocatalysts were constructed to further improve the photocatalytic performance. When the mass ratio of Ta8Si1 (TaON:SiO2 = 8:1) to MoS2 was 1:1, the degradation rate of RhB reached 75% under 2 h of visible light irradiation. Yin et al. [30] synthesized two kinds of MoS2 and PbBiO2Cl nanosheets by the solvothermal method and then prepared a novel 2D/2D MoS2/PbBiO2Cl photocatalyst by mechanical stirring at room temperature. The resulting experiments showed that 1 wt% of MoS2/PbBiO2Cl showed stronger photocatalytic performance and 80% of rhodamine B (RhB) could be completely degraded within 120 min, whereas the photocatalytic activity decreased when the content of MoS2 was higher.

Composites containing MoS2 with other similar materials are an effective way to enhance the photocatalytic ability of the material. SnS2 has a narrow band gap of 2.0 to 2.3 eV and is a low-cost, non-toxic CdI2-type layered semiconductor [31–33]. According to the literature [34–36], SnS2 is a relatively stable visible-light-driven photocatalyst in the degradation of organic compounds. However, like most semiconductor photocatalysts, SnS2 also has the disadvantage of high recombination rates of photogenerated electrons and holes, resulting in low photocatalytic efficiency [37]. Among the modification strategies explored to improve the photocatalytic efficiency of SnS2, the combination with a suitable semiconductor or other components (e.g., graphene) facilitates the separation of photogenerated electrons and holes through interfacial charge transfer [38–40]. Zhang et al. [41] prepared 2D/2D-type SnS2/g-C3N4 (graphite–C3N4) heterojunction photocatalysts using an ultrasonic dispersion method. The electron microscopic characterization analysis showed that a large contact zone was induced at the heterojunction interface due to the lamellar structure of both the SnS2 and g-C3N4 materials. In the photoluminescence spectra, it can also be shown that the photo-coordination effect of the SnS2/g-C3N4 heterojunction effectively enhances the interfacial carrier transfer, leading to enhanced charge separation during the photocatalytic reaction.

Due to the outstanding reactivity of both MoS2 and SnS2 in the photocatalytic degradation of organic dyes, the structures of MoS2 and SnS2 were coupled to construct a heterogeneous structure to enhance the degradation of organic dyes. Compared with previous work, this experiment is further improved: firstly, by changing the synthesis method of the sample and the synthesis conditions, the high temperature and high energy consumption in the experiment as well as the shortened reaction time are avoided; secondly, the synthesis steps are simpler and less cumbersome; and finally, the reactants are easily

available throughout the experiment and the heterogeneous structure is binary, which can efficiently solve the cost problem in the application.

In this work, we used a convenient hydrothermal method to obtain MoS2/SnS2 composite catalysts with SnS2 nanosheets grown on MoS2 nanoparticles. By adjusting the hydrothermal time of the reaction (12 h, 14 h and 16 h) and changing the molar ratio of the substances (1:1, 2:1, 3:1 and 4:1 atomic molar ratio of molybdenum–tin), a heterogeneous structure was constructed between MoS2 and SnS2 after the hydrothermal reaction, resulting in a semiconducting composite photocatalyst with a narrow band gap. Theoretically, the narrowing of the band gap of the material can effectively improve the absorption of visible light and the catalyst material can produce a large number of electrons and holes when light can be irradiated. In addition, the heterostructure can effectively modulate the electronic structure of the complex system while promoting electron transport between the interfaces more effectively, thus improving the photocatalytic ability. This work highlights that the construction of heterojunctions between two substances may be an attractive method for the removal of pollutants from industrial wastewater.

#### **2. Materials and Methods**

#### *2.1. Raw Materials*

Thiourea (CH4N2S) was purchased from Shanghai Ling Feng Chemical Reagent Co. (Shanghai, China). Tin chloride pentahydrate (SnCl4·5H2O, 99%) was supplied by Shanghai Test Four Hervey Chemical Co. (Shanghai, China). Ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, 99%) was purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Hydrochloric acid (HCl) was supplied by Yonghua Chemical Co. (Changshu, China). Methylene blue (MB) was supplied by Tianjin Chemical Reagent Research Co. (Tianjin, China). Except for hydrochloric acid, which is superiorly pure, the rest of the chemical reagents are of analytical grade and were used without further purification.

#### *2.2. Synthesis of Photocatalysts*

The fabrication steps involved in the synthesis of the MoS2/SnS2 composite catalysts are schematically illustrated in Figure 1. First, the synthesis of MoS2 nanoparticles [42]: MoS2 nanoparticles were synthesized by the hydrothermal method. Typically, 1.0592 g of (NH4)6Mo7O24·4H2O and 1.828 g of CH4N2S were dissolved in 60 mL of deionized water at room temperature with continuous stirring until complete dissolution. The mixed solution was then transferred to a 100 mL Teflon-lined autoclave and kept at 180 ◦C for 16 h. Then, the obtained black precipitate was dried at 80 ◦C for 2 h.

**Figure 1.** Schematic diagram of the synthesis procedure of MoS2/SnS2 catalysts.

Preparation of MoS2/SnS2 composite catalysts [43] with different reaction times (12 h,14 h and 16 h): the black MoS2 powder was weighed according to the ratio and dispersed in 60 mL deionized water to form a suspension. Certain proportions of SnCl4·5H2O and CH4N2S were added to the suspension; then, a certain amount of 1 mol/L hydrochloric acid was added to adjust the mixed solution to acidity. Finally, the mixture was transferred to a 100 mL Teflon-lined autoclave and kept at 180 ◦C for a certain time. After the reaction was completed, the catalyst was collected by centrifugation and dried at 80 ◦C for 2 h to obtain the catalyst. The catalysts were named MS12-2-H, MS14-2-H and MS16-2-H according to the reaction time and the atomic molar ratio of molybdenum to tin.

Preparation of MoS2/SnS2 composite catalysts with different Mo/Sn atomic molar ratios (1:1, 2:1, 3:1 and 4:1): the reactants were weighed according to the ratios and the synthesis steps were the same as above; the final composite catalysts were named MS14-1-H, MS14-2-H, MS14-3-H and MS14-2-H according to the naming principle.

#### *2.3. Structural Characterization*

Powder X-ray diffraction patterns were obtained using a Rigaku Smart Lab diffractometer with Cu Kα (λ = 0.154178 nm) as the radiation source. The morphology of the samples was measured using a JSM-6510 scanning electron microscope. Nitrogen adsorption– desorption isotherms were measured at −196 ◦C using a specific surface area and pore-size analyzer, the V-Sorb 1800. The samples were all pretreated at 105 ◦C for 12 h prior to measurement. Electrochemical impedance was tested with a CH1660E electrochemical workstation. X-ray photoelectron spectra were obtained with a KRATOS AXIS SUPRA.

#### *2.4. Photocatalytic Degradation and Photoelectrochemical Test*

The catalytic performances of the composite catalysts were evaluated by their ability to degrade the target pollutant, MB, under visible light using a 300 W xenon lamp as a visible light source. In each test, 30 mg of catalyst was dispersed in 80 mL of MB solution (15 mg/L). The mixed solution was stirred in the dark for 30 min prior to the test to achieve an adsorption–desorption equilibrium between the catalyst and the solution. The 7 mL suspension was removed every 10 min under visible light and centrifuged at 3500 r/min for 5 min. The absorbance of the solution at different reaction times was measured by UV–visible spectrophotometer.

The determination of the concentration of organic dyes can be described by the Beer– Lambert law, and the amount of light absorbed by the solution follows the Beer–Lambert law. The specific equation is as follows [44]:

$$\mathbf{A} = \varepsilon \mathbf{b} \mathbf{c} \tag{1}$$

where A is the absorbance, ε is the light absorption coefficient, b is the solution thickness and c is the dye concentration solution at the time of sampling. According to the Beer– Lambert law, the relationship between dye concentration and absorbed light is linear.

The electrochemical impedance test was carried out in a CH1660E electrochemical workstation with a platinum electrode as the counter electrode and a saturated glycerol electrode as the reference electrode, and the corresponding open circuit voltage and frequency were set. In this experiment [43], catalyst-coated conductive glass was used as the working electrode, namely 20 mg of the catalyst dispersed into a mixture containing 40 uL of 5 wt% Nafion and 0.5 mL of anhydrous ethanol. After mixing well with ultrasound, 200 uL of the suspension was coated onto the surface of the conductive glass with a pipette gun; this was then dried naturally at room temperature. The working electrode for the electrochemical impedance was tested in 0.1 M Na2SO4 solution.

#### **3. Results and Discussion**

#### *3.1. Characterization and Properties of Composite Catalysts Synthesized for Different Reaction Times* 3.1.1. XRD Characterization

Figure 2 represents the XRD patterns of the MoS2/SnS2 composite catalysts prepared from 1T-MoS2 synthesized by a hydrothermal reaction at 180 ◦C for different reaction times. It can be seen from the figure that the characteristic peaks of the SnS2 component at 2θ equal to 14.9◦, 28.2◦, 32.1◦, 41.8◦, 49.9◦, 52.4◦ and 54.9◦ are clearly observed, which correspond to the (001), (100), (101), (102), (110), (111) and (103) SnS2 crystalline planes, respectively (JCPDS No. 23-0677) [45]. The SnS2 component is successfully synthesized in the MoS2/SnS2 composite catalysts. The characteristic peaks of MoS2 at 2θ equal to 10.9◦, 32.8◦ and 57.2◦ are not clearly shown in the figure because of the unique lamellar structure and small grain size of MoS2 [46]. In addition, the layer spacing of MoS2 becomes larger during

the reaction process, resulting in a shift of the (002) crystal plane to 10.9◦. The presence of both MoS2 and SnS2 components in the synthesized catalysts without spurious peaks in the XRD patterns indicate the successful synthesis of MoS2/SnS2 composite catalysts.

**Figure 2.** Powder X-ray diffraction patterns of MoS2/SnS2 composite catalysts with different reaction times.

With increasing reaction time, the characteristic peak of MoS2 gradually broadens and the characteristic peaks of SnS2 gradually narrow, indicating the stronger crystallinity of the SnS2 phase, which on the other hand also means that the structure of SnS2 in the reaction process is more complete.

#### 3.1.2. Morphology Analysis

Scanning electron micrographs of the catalysts MS12-2-H, MS14-2-H and MS16-2- H synthesized at different times are shown in Figure 2. In Figure 3a, the hexagonal SnS2 nanosheets are grown on MoS2 nanosphere flowers [1], whereas the hexagonal SnS2 nanosheets are not uniformly distributed and a large portion of the nanoflake particles are not in contact with the nanosheets.

The morphology of the catalysts in Figure 3b changed considerably. The growth of SnS2 nanosheets on the surface of the flower-like morphology of MoS2 was not only more uniformly distributed but also the agglomeration of SnS2 nanosheets was slight, which was obviously different from the other catalysts and could expose more active sites. The SnS2 nanosheets in Figure 3c also have better crystallinity of the grains, although they are more uniformly distributed than in (a), which is consistent with the results for the XRD experiments.

#### 3.1.3. BET Measurements

Table 1 shows the results of the specific surface area test results. The nitrogen adsorption–desorption isotherms were measured at −196 ◦C after all the samples were pretreated at 105 ◦C for 12 h prior to measurement. The specific surface area of the composite catalyst showed a trend of increasing and then decreasing with the increase in the reaction time. The reaction time did not have a great influence on the average pore diameter in the range 2.32–2.34 nm and total pore volume of 0.003 cm3/g of the composite catalysts, which were almost negligible. Combined with the analysis of the SEM images of the composite catalysts, the specific surface area of the MoS2 and SnS2 materials was greatly enhanced due to the uniform growth of lamellar SnS2 particles on the surface of the flower-like MoS2 particles; thus, effectively improving the catalytic performance [47].

**Figure 3.** SEM images of MoS2/SnS2 composite catalysts: (**a**) MS12-2-H; (**b**) MS14-2-H; (**c**) MS16-2-H.


**Table 1.** BET analysis of composite catalysts for different reaction times.

#### 3.1.4. Electrochemical Impedance Measurement

Figure 4 shows the electrochemical impedance plots for the composite catalysts synthesized for different reaction times. In the electrochemical impedance diagram, the radius of the semicircle in the high-frequency region is positively correlated with the charge transfer resistance, reflecting the transfer characteristics of photogenerated electrons and holes in the catalysts under the light conditions. According to the test results, the impedance radius of the MoS2/SnS2 composite catalysts showed a trend of decreasing and then increasing as the reaction time increased, in which the MS14-2-H composite catalyst had the smallest impedance radius and presented a high charge transfer migration efficiency [48]. This also proves that a suitable reaction time has a great impact on the electronic structure of the two components in the catalysts, resulting in the improvement of their charge transfer capacity and thus the photocatalytic degradation performance.

**Figure 4.** Electrochemical impedance diagram of MoS2/SnS2 composite catalysts at different reaction times.

#### 3.1.5. Photocatalytic Performance

The degradation of methylene blue solution by MoS2/SnS2 composite catalysts formed at different reaction times is shown in Figure 5. From the figures, it can be observed that the composite catalyst degraded MB in visible light. Of the three catalysts, MS14-2-H exhibited the highest MB degradation rates. With the increasing of the synthesis time of the MoS2/SnS2 composite catalysts, a trend of enhancing and then weakening is observed, which is consistent with the test results of the electrochemical impedance of the composite catalysts. The length of the reaction time has an effect on the electronic structure of the synthesized composite catalyst, thus affecting the migration rate of photogenerated charges in visible light and producing an effect on the performance of the degradation of MB. From the degradation data, it was found that the best catalytic performance was achieved by sample MS14-2-H, which had an 83.0% degradation rate after 80 min of visible light irradiation. This was followed by sample MS12-2-H, which had a 55.9% degradation rate. This corroborates the previous results of the XRD, SEM and EIS analyses.

**Figure 5.** Degradation rate diagram of MoS2/SnS2 composite catalysts at different reaction times.

#### *3.2. Characterization and Properties of Composite Catalysts in Different Mo–Sn Molar Ratios* 3.2.1. XRD Characterization

Figure 6 represents the XRD patterns of the composite catalysts synthesized by varying the molybdenum–tin molar ratio in the reactants at a reaction temperature of 180 ◦C for 14 h. It can be seen from the figures that the characteristic peaks at 2θ equal to 14.9◦, 28.2◦, 32.1◦, 41.8◦, 49.9◦, 52.4◦ and 54.9◦ correspond to the (001), (100), (101), (102), (110), (111) and (103) crystal planes of SnS2 in the composite catalysts of different molybdenum–tin molar ratios, respectively [45]. In addition, no excess spurious peaks were found in the XRD patterns, indicating that the MoS2/SnS2 composite catalysts were successfully synthesized.

**Figure 6.** Powder X-ray diffraction patterns of MoS2/SnS2 composite catalysts synthesized at different molybdenum–tin molar ratios.

When increasing proportion of Mo in the reactants, the characteristic peaks of both MoS2 and SnS2 gradually broadened, especially the (100), (101) and (102) crystal planes in SnS2.

#### 3.2.2. Morphology Analysis

SEM images of the samples MS14-1-H, MS14-2-H, MS14-3-H and MS14-4-H are shown in Figure 7. From the figures, MoS2 particles are shown as having flower-like morphology composed of layered nanosheets. Increasing the proportion of Mo leads to more serious MoS2 agglomeration and the spherical flower particles of MoS2 become more regular. The growth of SnS2 nanosheets on the surface of the flower-like MoS2 morphology exhibits hexagonal morphology and the SnS2 nanosheets have a size of about 400 nm [1]. With an increase in the MoS2 fraction, the main change in the morphology is that the SnS2 nanosheets are not uniformly distributed on the MoS2 surface, as shown in Figure 7c,d. At lower Mo/Sn ratios, the scanning images of the catalyst changed more, and the MoS2 morphology was no longer a regular spherical flower shape but a nano-flake shape. In addition, the SnS2 nanosheets were closely distributed on the MoS2 surface, as shown in Figure 7a,b. The close distribution of the SnS2 nanosheets on the MoS2 surface contributes to the close bonding of the two materials, which can effectively change the electronic structure of the catalyst and increase the photocatalytic active sites.

**Figure 7.** SEM diagrams of composite catalysts. (**a**) MS14-1-H. (**b**) MS14-2-H. (**c**) MS14-3-H. (**d**) MS14-4-H.

Figure 8a shows TEM images of the prepared MS14-2-H catalyst, which further shows that the flake SnS2 grows on the surface of MoS2 with a small size. Lattice stripes with distances of 0.68 nm and 0.30 nm are shown in Figure 8b, which could correspond to the (002) plane of MoS2 and the (100) plane of SnS2, respectively [49]. These images objectively further explain the coexistence of SnS2 and MoS2 in the MS14-2-H composite catalyst. In addition to this, the lattice stripe in the (002) plane of MoS2 is larger than the standard spacing value (0.62 nm) according to the Bragg equation:

$$\text{2dsin}\theta = \text{n}\lambda \tag{2}$$

where d is the lattice spacing, θ is the angle between the incident ray, the reflection line and the reflected crystal plane, λ is the wavelength and n is the number of reflection levels. It is known that under specific conditions, the lattice spacing d is inversely proportional to the angle θ, i.e., at this time, the lattice spacing of the (002) plane of MoS2 is large and the corresponding diffraction peak angle becomes small, which is similar to the diffraction peak of the (002) corresponding to the MoS2 phase of the composite catalyst in the XRD analysis shifting to 10.9◦.

**Figure 8.** TEM images of MS14-2-H: (**a**) TEM image; (**b**) high-resolution TEM image.

#### 3.2.3. BET Measurements

Table 2 shows the results of the specific surface area tests of the MoS2/SnS2 composite catalysts with different molybdenum–tin molar ratios. From the data in the table, it can be observed that the specific surface area of the composite catalysts shows a trend of increasing and then decreasing when increasing the molybdenum–tin molar ratio from 1:1 to 4:1. In addition, the average pore diameter of the catalyst was 2.30~2.34 nm and the total pore volume was 0.003 cm3/g.


**Table 2.** BET analysis of composite catalysts with different Mo–Sn molar ratios.

The effect of molybdenum–tin molar ratio on the average pore diameter and total pore volume of the composite catalysts is almost negligible, and the effect on the catalyst performance is mainly due to the specific surface area, which is 17.10 m2/g for MS14-2-H, followed by 15.05 m2/g for MS14-1-H. Combined with the analysis of the SEM images of the composite catalysts, the reason for the large differences in the specific surface areas of the catalysts may be due to the size and shape of the particles of MoS2 and SnS2 and the difference in the shape of the particles.

#### 3.2.4. Electrochemical Impedance Measurement

The electrochemical impedance diagrams of the composite catalysts synthesized at different molybdenum–tin molar ratios are shown in Figure 9. According to the test results, the impedance radii of the MoS2/SnS2 composite catalysts showed a trend of decreasing and then increasing as the molybdenum–tin molar ratio increased; the MS14-2-H composite catalyst having the smallest impedance radius. Since the radius of the semicircle in the high-frequency region is positively correlated with the charge transfer resistance in the electrochemical impedance diagram, the sample MS14-2-H has the smallest charge transfer resistance [48].

**Figure 9.** Electrochemical impedance plot of MoS2/SnS2 composite catalysts synthesized with different molybdenum–tin molar ratios.

#### 3.2.5. Structural Composition Analysis of Materials

Figure 10 shows the XPS diagram of the catalyst MS14-2-H. The elemental composition and elemental chemical valence of the sample catalyst can be identified by analyzing the XPS. From Figure 10a, it can be seen that the catalyst MS14-2-H contains C, O, S, Mo and Sn, and the sample contains all the elements of the target substance by the elemental full spectrum.

**Figure 10.** XPS diagram of catalyst MS14-2-H: (**a**) survey spectra; (**b**) Mo 3d; (**c**) Sn 3d; (**d**) S 2p.

Figure 10b shows the XPS spectra of Mo 3d, in which five different characteristic peaks appear. It is known from the split peak fitting and literature review [50] that peaks at 229.1 eV and 232.2 eV correspond to Mo4+ 3d5/2 and Mo4+ 3d3/2, respectively, peaks at 233.3 eV and 235.5 eV correspond to Mo6+ 3d5/2 and Mo6+ 3d3/2, respectively, and the peak at 225.8 eV corresponds to S2<sup>−</sup> 2s. The Mo4+ species belong to MoS2 and Mo6+ signals and may be caused by slight oxidation in air. Only two characteristic peaks appear in Figure 10c, and a review of the literature shows that [51] signals at binding energies of 486.8 eV and 495.3 eV correspond to Sn4+ 3d5/2 and Sn4+ 3d3/2, respectively, whereas the Sn4+ species belong to SnS2. Signals of S<sup>2</sup><sup>−</sup> 2p3/2 and S2<sup>−</sup> 2p1/2 from the composite catalyst present at binding energies equal to 161.9 eV and 163.1 eV [52]. Therefore, the analysis shows that the composition of the synthesized samples is consistent with the target MoS2/SnS2 catalyst.

#### 3.2.6. Photocatalytic Performance

Figure 11a shows the performance of the composite catalysts synthesized at different molybdenum–tin molar ratios in the degradation of methylene blue solutions. From the degradation data in the figure, the visible light degradation performances in methylene blue solution of the composite catalysts synthesized at different molybdenum–tin molar ratios are better than that of either pure MoS2 or SnS2, indicating that the photocatalytic performance can be effectively improved by using a composite of these two materials. In addition, after visible light irradiation for 80 min, the best catalytic performance was achieved for sample MS14-2-H, which had a degradation rate of 83%, whereas the degradation rate of sample MS14-3-H was 44.6% and that of sample MS14-4-H was 24.7%, indicating that the optimal molybdenum–tin ratio that can effectively improve the photocatalytic performance is 2:1 and that the effect on the photocatalytic performance is limited by only increasing the content of MoS2 in the composite catalyst. Attention should be paid to the reasonable distribution of the two components in the composite catalysts, which is consistent with the previous results of the XRD, SEM and EIS analyses.

**Figure 11.** (**a**) Degradation rate diagram of MoS2/SnS2 composite catalysts synthesized with different molybdenum–tin molar ratios. (**b**) Cycling stability testing of the composite catalyst MS14-2-H.

Figure 11b shows the cycling stability test of the composite catalyst MS14-2-H. It can be seen from the figure that the catalytic degradation efficiency of the composite catalyst MS14- 2-H in the MB solution decreased from 83.0% to 74.7% after four visible photocatalytic cycle tests and that the loss of photocatalytic activity was 8.3%. This indicates that the stability and repeatability of the composite catalyst MS14-2-H are good, whereas the loss of photocatalytic activity may be caused by the loss of photocatalysis during the cycle test [1].

#### *3.3. Photocatalytic Mechanism*

The photocatalytic mechanism of the composite catalyst is shown in Figure 12. MoS2 is a p-type semiconductor material with a narrow band structure (e.g., =1.85 eV), whereas SnS2 is an n-type semiconductor material with a forbidden band width of 2.08 eV [48]. Because the two semiconductors have opposite conductivity types, the electrons and holes of these two semiconductor materials are transferred when they are in close contact to form a heterojunction until the Fermi energy levels of the two semiconductor materials are equal, at which point the p–n heterojunction is in thermal equilibrium and a stable built-in electric field is formed.

**Figure 12.** Possible photocatalytic mechanism for the degradation of methylene blue over a composite catalyst.

In the mechanism diagram of the composite catalyst, the CB and VB of MoS2 are higher than that of SnS2, the energy band structures of both are staggered and the heterogeneous structure of the composite catalyst is of type II. When irradiated by visible light, a large number of photogenerated electrons accumulate in the conduction band and a large number of photogenerated holes accumulate in the valence band of both semiconductor materials. Under the effect of potential difference, electrons in the conduction band of MoS2 are transferred to the conduction band of SnS2, whereas holes in the valence band of SnS2 are transferred to the valence band of MoS2. In this way, the electrons and holes can be separated to the maximum extent.

The photocatalytic degradation of MB by composite catalysts under visible light is mainly based on the chemical reaction of the photogenerated electron reduction transferred to the surface of the photocatalyst with dissolved oxygen, which produces strongly oxidizing superoxide radicals (·O2−), and the chemical reaction of the strongly oxidizing holes transferred to the surface of the photocatalyst with hydroxyl radicals (OH−) in water and aqueous solutions, which produces hydroxyl radicals (·OH) [1]. The photocatalytic reaction process is as follows:

MoS2/SnS2 + h<sup>ν</sup> <sup>→</sup> <sup>e</sup><sup>−</sup> + h<sup>+</sup> (3)

$$\text{e}^- + \text{O}\_2 \rightarrow \text{\textquotedblleft O}^{2-} \tag{4}$$

$$\cdot \text{h}^+ + \text{H}\_2\text{O} / \text{OH}^- \rightarrow \cdot \text{OH} \tag{5}$$

$$\cdot \text{O}^{2-} + \text{MB} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} \tag{6}$$

$$\cdot\text{OH} + \text{MB} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} \tag{7}$$

#### **4. Conclusions**

In summary, a novel MoS2/SnS2 heterostructure was successfully prepared by growing SnS2 nanosheets on MoS2 nanospheres by a facile multi-step hydrothermal method. Based on the measurements of the XRD, SEM, HRTEM and XPS analyses, the present composite sample was found to have high crystalline quality and excellent heterojunction formation. By constructing heterojunctions between the two sulfides, an improved photocatalytic performance was achieved, which greatly solved the problems of low visible light utilization and photogenerated charge recombination. Compared with pure MoS2 or SnS2, this easily

accessible and simple composition photocatalyst shows higher photocatalytic activity and good photostability; these effects are attributed to the constructed heterostructure, better light trapping and rapid separation and migration of light-induced electron and hole pairs with the assistance of the MoS2 metal phase. The optimal MoS2/SnS2 photocatalyst (i.e., the one that achieved the best photocatalytic performance) had a degradation efficiency of 83.0% for MB solution, which was 8.3 times higher than the degradation with pure MoS2 and 16.6 times higher than the degradation with pure SnS2. The experimental results indicate that this construction of heterojunctions between semiconductors can effectively improve the photocatalytic ability of MoS2/SnS2 catalysts in terms of MB degradation.

**Author Contributions:** Conceptualization, Y.T.; software, Y.L. (Yunfei Liu) and Y.L. (Yinong Lu); data curation, G.M.; writing—original draft preparation, G.M.; writing—review and editing, Y.T. and Z.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the priority academic program development of Jiangsu Higher Education Institution (PAPD).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors gratefully acknowledge the assistance from Yaqiu Tao and Zhigang Pan from NJTECH and the staff from State Key Laboratory of Materials-Oriented Chemical Engineering.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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