Photocatalytic degradation of methylene blue in aqueous solution by using ZnO-SnO2 nanocomposites

Photocatalytic degradation of methylene blue in aqueous solution by using ZnO-SnO2 nanocomposites

Materials Science in Semiconductor Processing 87 (2018) 24–31 Contents lists available at ScienceDirect Materials Science in Semiconductor Processin...

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Materials Science in Semiconductor Processing 87 (2018) 24–31

Contents lists available at ScienceDirect

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Photocatalytic degradation of methylene blue in aqueous solution by using ZnO-SnO2 nanocomposites Jiaojiao Lina, Zhanzhou Luob, Jiaojiao Liua, Ping Lia, a b


Key Laboratory of Inorganic Nanomaterials of Hebei Province, School of Chemistry and Materials Science, Hebei Normal University, Shijiazhuang 050024, China School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China



Keywords: ZnO–SnO2 Composite Chemical synthesis Photocatalytic performance Degradation mechanism

ZnO–SnO2 (ZS) nanocomposites with different compositions were prepared using a simple coprecipitation method with zinc chloride, stannic chloride pentahydrate, and ammonium hydroxide as raw materials. The morphology and structure of the samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Results showed that the synthesized ZS nanocomposites consisted of hexagonal wurtzite ZnO and tetragonal rutile SnO2. The particle size of ZS decreased evidently with increasing content of SnO2 in the sample. The photocatalytic activity of ZS was explored by analyzing the degradation of methylene blue (MB). After 60 min of UV irradiation, the degradation rate of MB with ZS0.25 reached 96.53%, which was 28.75% higher than that with pure ZnO. Catalytic activity remained at 87.90% after three recycles. A plausible MB degradation mechanism was proposed by testing using UV–vis absorption spectra, Fourier-transform infrared spectra, total organic carbon, high-performance liquid chromatography, and electrospray ionization mass spectrometry.

1. Introduction Along with scientific and technological development, environmental pollution, especially water pollution, has become an increasingly serious concern. A large amount of untreated industrial wastewater is discharged arbitrarily into rivers, threatening the survival of aquatic life [1]. Given water toxicity, human consumption of contaminated aquatic products may cause a variety of diseases, such as cancer [2–4]. Printing and dyeing wastewater, which accounts for a large proportion of industrial wastewater, is difficult to degrade due to its complex composition and high organic concentration [5,6]. In recent years, photocatalytic technology is widely used to manage organic pollutants [7–9]. Several studies indicated that under UV light irradiation, photogenerated electron–hole pairs of ZnO react with O2, H2O, and OH− adsorbed on its surface and produce ·OH and ·O2− radicals with strong oxidation [10]. Therefore, ZnO can catalyze the degradation of numerous organic pollutants. However, the electrons and holes generated by ZnO easily recombine and consequently inhibit the photocatalytic activity of the catalyst [11]. For solving this problem, the integration of ZnO with other semiconductors with different bandgap energy can effectively separate the photogenerated electrons and holes and thus improve photocatalytic activity [12–15]. Swaminathan et al. [16] synthesized AgBr–ZnO by using simultaneous precipitation of zinc

oxalate and AgBr followed by calcination at 400 °C for 12 h. AgBr–ZnO was more efficient than commercial ZnO for the mineralization of Acid Black 1. Karuthapandian et al. [17] synthesized CuO–ZnO nanorods by using a simple chemical synthesis. The potential performance of CuO–ZnO was evaluated by analyzing the photocatalytic degradation of Congo red and rhodamine B. CuO–ZnO exhibited significantly superior photocatalytic activity compared with CuO and ZnO. Lin et al. [18] prepared coupled ZnO/SnO2 photocatalyst in the RPB with a co-precipitation method. The decolorization efficiency of methylene blue with this photocatalyst was 98% after 120 min UV irradiation. The effects of addition of oxygen, catalyst dosage, and pH on this process were also studied. The degradation of organic pollutants in wastewater is complicated because numerous intermediates are produced in the reaction [19,20]. Research on the mechanism of degradation to date is limited. Therefore, the degradation mechanism of organic pollutants in the presence of a photocatalyst should be studied. In the current work, ZS nanocomposites with different compositions were prepared using a coprecipitation method. The photocatalytic activity of ZS was studied using methylene blue (MB) as a simulated organic pollutant. A possible MB degradation mechanism was proposed through various testing approaches.

Correspondence to: No. 20 Road East of 2nd Ring South Road, Shijiazhuang, Hebei 050024, China. E-mail address: [email protected] (P. Li). Received 17 January 2018; Received in revised form 7 June 2018; Accepted 1 July 2018 1369-8001/ © 2018 Elsevier Ltd. All rights reserved.

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2. Experimental 2.1. Synthesis and characterization of ZS nanocomposites Zinc chloride (ZnCl2), stannic chloride pentahydrate (SnCl4·5H2O), and ammonium hydroxide (NH4OH) were of analytical grade and used without further purification. ZnCl2 and SnCl4·5H2O were separately dissolved in deionized water at room temperature to obtain 1.0 mol L−1 standard solutions. ZS nanocomposites were prepared using a coprecipitation method described as follows: 20 mL of the ZnCl2 solution was mixed with a predesigned volume (0, 1, 5, and 10 mL) of the SnCl4 solution in a beaker. Then, deionized water was added to obtain a total solution volume of 100 mL. A small amount of concentrated NH4OH was used to adjust the pH level to 7. These precursors were maintained with continuous stirring at room temperature for 3 h. The resulting precipitate was filtered, washed, and then dried. Finally, the precipitate was calcined in a muffle furnace at 600 °C for 2 h, and ZS nanocomposites were harvested. For convenience, the samples prepared with Zn/Sn molar ratios of 1:0, 1:0.05, 1:0.25, and 1:0.5 were denoted by ZnO, ZS0.05, ZS0.25, and ZS0.5, respectively, in the following discussion. The crystal structure of the samples was studied using an X-ray diffractometer (Bruker-AXS D8 Advance). Electron micrographs were obtained using field-emission scanning electron microscopy (FESEM; Hitachi S-4800). Particle size distribution was obtained using laser particle size and ZETA potential analyzer (Malvern ZS90). The composition of the prepared ZS composites was determined using an energy-dispersive X-ray spectrometer (EDS; Oxford INCA Energy 350).

Fig. 1. XRD patterns of ZnO and ZS photocatalysts. Table 1 Phase composition of the samples. Composition of phases (wt%)

ZnO SnO2

Samples ZnO




100% 0%

97.2% 2.8%

89.1% 10.9%

86.9% 13.1%

were ZS composites. The peak intensity related to ZnO was evidently higher than that of SnO2 in the patterns. The data presented in Table 1 show that ZnO is a predominant phase in the ZS composites. Fig. 2 shows the FESEM images of the ZnO and ZS photocatalysts. Pure ZnO contained two kinds of particles with different sizes. The small particles were quasispherical with a mean diameter of 150 nm, and the large particles presented irregular morphology with an approximate average size of 300 nm. The particle size of the ZS composites decreased evidently with increasing SnO2 content in the samples. The average sizes of ZS0.05, ZS0.25, and ZS0.5 were 120, 100, and 10 nm, respectively. The particle size distribution of ZS0.25 is shown in Fig. 3. The sample presented a narrow size distribution and average size of 100.5 nm, which agreed well with the FESEM results. Fig. 4 shows the EDS spectra of the ZnO and ZS0.25 composites. Only the characteristic peaks of Zn and O were observed in the spectrum of pure ZnO (Fig. 4a). The characteristic peaks of Sn were present in ZS0.25 in addition to the Zn and O peaks (Fig. 4b). The average atomic ratios of O, Zn, and Sn were confirmed using INCA software (Table 2).

2.2. Photocatalytic experiments The photocatalytic activity of ZS nanocomposites was evaluated using MB. A 250 W Hg lamp (λmax = 365 nm) was used in the degradation experiment. The photocatalyst and the MB solution were placed in a 250 mL beaker and ultrasonically dispersed for 10 min. The solutions were pretreated by magnetic stirring in the dark for 30 min to achieve adsorption–desorption equilibrium of MB on the catalyst surface. Then, the solutions were irradiated by Hg lamps for different time lengths and further analyzed by a 752 N UV–vis spectrophotometer. A catalyst concentration of 0.2 g L−1 was used to investigate MB degradation mechanism. Sample solutions degraded at different stages were analyzed using UV–vis absorption spectra with a Hitachi U-3010 UV–vis spectrophotometer, Fourier-transform infrared (FT-IR) spectra with a Nicolet iS50 spectrometer at room temperature, high-performance liquid chromatogram (HPLC) with an Agilent HPLC-1260 chromatograph, and electrospray ionization mass spectrometry (ESI-MS) with an AB SCIEX 3200 QTRAP mass analyzer. The total organic carbon (TOC) of the final solution after 100 min of degradation was determined using an Elementar Liquid TOC II analyzer.

3.2. Photocatalytic degradation of MB The photocatalytic degradation of MB was studied using ZS nanocomposites as photocatalysts. The initial concentration of MB was 20 mg L−1 in total, and the catalyst dosage was 0.4 g L−1. The curves of the relative degradation (Ct/C0) of MB versus time are shown in Fig. 5a. All ZS samples appeared to be efficient photocatalysts and showed superior photocatalytic activity to that of pure ZnO. ZS0.25 exhibited the highest photocatalytic activity among composites. MB degradation efficiency with ZS0.25 reached 96.53% after 60 min of UV irradiation and was 28.75% higher than that with pure ZnO. As shown in Fig. 5a, the curves of ln (C0/Ct) versus time were plotted and then linearly fitted (Fig. 5b). The calculated kinetic parameters of MB degradation are shown in Table 3. All curves exhibited good linear correlation, and the values of correlation coefficient (R2) were extremely close to 1. This result indicated that the photodegradation of MB with ZS catalysts followed pseudo-first-order kinetics [ln (C0/Ct) = kt]. ZS0.25 exhibited the highest reaction rate constant (k) and hence the highest photocatalytic activity.

3. Results and discussion 3.1. Morphological and structural characterizations of the photocatalyst Fig. 1 shows the X-ray diffraction (XRD) patterns of ZnO and ZS photocatalysts. The phase composition of the samples was analyzed using EVA software as listed in Table 1. For the pure ZnO sample, the diffraction peaks appearing at 2θ = 31.73°, 34.42°, 36.21°, 47.56°, 56.52°, 62.81°, and 68.03° were designated as (100), (002), (101), (102), (110), (103), and (112) crystal planes, respectively, which were in good agreement with the hexagonal wurtzite structure of ZnO (JCPDS No. 36-1451). In the XRD patterns of ZS composites, in addition to the characteristic peaks of ZnO, the peaks located at 2θ = 26.8°, 33.8°, and 52.1° were assigned to the crystal planes of (110), (101), and (211), respectively, of tetragonal rutile SnO2 (JCPDS No.41-1445). No other characteristic peaks were observed, suggesting that the samples 25

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Fig. 2. FESEM images of the photocatalysts: (a) ZnO, (b) ZS0.05, (c) ZS0.25, and (d) ZS0.5.

of the band structure and photogenerated electron–hole (e−–h+) separation in the ZS composite. According to the literature [23], the CB potential of ZnO (−0.36 eV) is lower than that of SnO2 (−0.11 eV), and the VB potential of SnO2 (+ 3.44 eV) is higher than that of ZnO (+2.84 eV). When a ZS composite is irradiated using UV light, the photogenerated electrons with negative charges move from low potential to high potential, whereas positive holes move in the opposite direction; in other words, electrons move from the CB of ZnO to the CB of SnO2, whereas the holes move from the VB of SnO2 to the VB of ZnO. As a result, photogenerated electrons gather in the CB of SnO2, and the holes gather in the VB of ZnO. The separation of the electrons and holes prevents recombination. Therefore, the photocatalytic activity of ZS is markedly improved relative to that of pure ZnO. For evaluating the stability of the prepared ZS photocatalysts, recycling experiments were conducted under similar conditions. Photodegradation experiments were repeated for three times with the same catalyst. The catalyst was filtered and thoroughly washed after each experiment. The results from the recycling runs are shown in Fig. 7. Although degradation efficiency decreased after three cycling runs, the catalyst still exhibited more than 85% activity toward MB. Therefore, ZS nanocomposites are efficient and stable photocatalysts with reuse potential in degradation. Hence, the ZS photocatalyst exhibits potential practical application in wastewater treatment.

Fig. 3. Particle size distribution of ZS0.25.

ZnO is an n-type semiconductor material [21]. When the material is irradiated using UV light, the electrons in the valence band (VB) are excited to the conduction band (CB) and form photogenerated electrons. Meanwhile, the same amount of photogenerated holes form in the VB. The electrons on the CB react with O2 and form ·O2−, while the holes on the VB react with OH− and form ·OH. Both ·O2− and ·OH radicals exhibit strong oxidation and can degrade and mineralize organic pollutants [22]. ZnO constantly shows relatively low photocatalytic efficiency because of its large bandgap energy and photogenerated electrons and holes, which are prone to recombination. The integration of ZnO with another semiconductor is an effective approach to solve the previously mentioned problem. Fig. 6 shows the schematic

3.3. Degradation mechanism of MB MB degradation was studied using ZS0.25 as photocatalyst. The initial concentration of MB was 20 mg L−1 in total, and the catalyst dosage was 0.2 g L−1. Fig. 8 shows the UV–vis spectra of the MB solution with different irradiation times. The inserted image illustrates the colors of the solutions during degradation. Four absorption bands were observed within the wavelength range of 200–800 nm. The bands at 26

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Fig. 4. EDS spectra of the photocatalysts: (a) ZnO and (b) ZS0.25. Table 2 EDS semiquantitative analysis of the samples. Atom %

O Zn Sn

Table 3 Pseudo-first-order kinetic parameters of MB degradation.

Samples ZnO




56.49 43.51 0

58.70 39.24 2.06

73.26 18.28 8.46

70.81 19.22 9.97


Linear regression equation

ZnO ZS0.05 ZS0.25 ZS0.5


= −0.04808 + 0.01967X = −0.03893 + 0.02839X = −0.00654 + 0.05353X = 0.01415 + 0.0339X

k (min−1)


0.01967 0.02839 0.05353 0.03390

0.9937 0.9967 0.9933 0.9970

debris. With ZS0.25 as a catalyst, the TOC contents of influent and effluent were 11.38 and 7.12 mg L−1, respectively. The removal rate of MB was 37.43%, indicating that the dye was not fully mineralized. With pure ZnO as a catalyst, the TOC contents of influent and effluent were 11.46 and 10.51 mg L−1, respectively, and the removal rate was 8.29%. Comparing these results, we can see that the composite photocatalyst can better mineralize the dye. The degradation products of MB were characterized using FT-IR (Fig. 9). The FT-IR spectrum of MB before degradation is shown in Fig. 9a, and the assignments of spectral peaks are listed in Table 4. The FT-IR spectra of the degraded products collected at 30, 60, and 100 min are shown in Fig. 9b. In the comparison of Fig. 9a and b, the number of absorption peaks after degradation markedly decreased. The C–S–C skeletal vibration absorption (662 cm−1) related to the chromophoric groups of MB disappeared. The stretching vibration absorption of C˭N (1635 cm−1) and C˭S (1134 cm−1) also disappeared. This result indicated that the sulfur–nitrogen conjugated system in the MB molecule was destroyed during degradation. The C–N stretching vibration absorption (1324 cm−1) on the aromatic ring disappeared; hence, a deamination or denitration reaction occurred during degradation. The C˭C skeletal vibration absorption (1592 and 1482 cm−1) and C–H deformation vibration absorption (827 cm−1) of the aromatic ring

664 and 612 nm in the visible region corresponded to the sulfur–nitrogen conjugated system that acted as the chromophoric group in the MB molecules [24]. The intensity of the two bands gradually decreased as the degradation reaction progressed. This outcome indicated that the sulfur–nitrogen conjugated system was destroyed. As a result, the color of the MB solution gradually lightened. In addition, blue shifts of the two bands were observed with the prolonging of illumination time possibly because the MB molecules were demethylated and converted to other intermediates [25]. The bands at 292 and 246 nm in the UV region corresponded to the phenothiazine structure in the MB molecules [26]. The intensity of the two bands also weakened with the degradation progress. This result indicated the occurrence of oxidative decomposition, including the ring-opening reaction of the phenothiazine species. The absorbance of the solution was approaching zero after 100 min of degradation, and the solution color changed from bright blue to colorless. This outcome indicated that the MB molecules were completely destroyed. For further determining whether the decrease in absorbance was a result of a simple decolorization process, the degraded products were tested using a TOC analyzer during 100 min degradation time. The TOC value represented the content of organics in the solution and thus can determine the degradation degree of organic

Fig. 5. (a) Photocatalytic activity of ZS nanocomposites in MB degradation. (b) Kinetic linear simulation curves of MB degradation. 27

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Fig. 6. Schematic of the band structure and e−–h+ separation in the ZS composite.

prolonged degradation time, the intensity of the three peaks gradually increased. This finding indicated that MB was possibly decomposed into small organic molecules, such as hydrocarbons. In addition, several new absorption peaks appeared in the degraded spectra. The absorption peak at 3348 cm−1 can be attributed to the stretching vibration of –OH in the associated alcoholic polymer. Absorption peaks at 1092 and 1047 cm−1 can be attributed to the stretching vibration of C–O in the alcoholic molecule. Therefore, MB produced alcohols and other substances during degradation. For further exploring the degradation mechanism of MB, products with different degradation times were analyzed using HPLC. A volume ratio of V (100 mmol L−1 ammonium acetate):V (acetonitrile) = 70:30 was used as mobile phase. The flow rate was 1 mL min−1, and the detection wavelength was 246 nm. Fig. 10 shows the HPLC chromatogram of the products obtained with different degradation times. Peaks f and g were characteristic of the MB molecule before degradation. As degradation progressed, the intensity of the two peaks decreased gradually and disappeared entirely at 100 min. This result indicated that the molecular structure of MB was completely destroyed. Peaks d and e, which did not exist before degradation, first increased and eventually decreased during degradation. This phenomenon indicated that several intermediates formed and then continued to degrade into other substances. The intensity of peaks a, b, and c, which were similarly nonexistent before degradation, gradually increased during the reaction. When the time was extended to 100 min, only the three previously mentioned peaks remained. This result indicated that several new substances corresponding to these peaks were ultimately generated. Therefore, under this experimental condition, a part of MB was oxidized and mineralized into small molecules, such as H2O and CO2, and the rest was degraded into three new substances. The products at different degradation stages were further characterized using mass spectrometry (Fig. 11). The experiment was conducted using ESI-MS in the positive ion scanning mode to determine the mass-to-charge ratio (m/z) of the products. The solution before degradation was detected as MB cations that removed Cl− ions because the strong characteristic peak at 284 (m/z) was only the molecular weight of MB. After 30 min of degradation, two new substances with m/ z = 215 and 136 were detected in addition to MB. The molecular formulas corresponded respectively to C8H11N2SO3 and C8H12N2. After 60 min of degradation, the three substances disappeared, and five other substances were detected. The m/z values of the five newly apparent substances were 202, 158, 114, 137, and 110, corresponding to the molecular formulas of C6H4NSO5, C6H6SO3, C6H10O2, C8H11NO, and C6H6O2. After 100 min of degradation, only substance C6H10O2 with m/ z = 114 remained, and two other substances were detected. The m/z values of 74 and 104 corresponded to the molecular formulas of C3H6O2 and C3H4O4, respectively.

Fig. 7. Reusability of ZS0.25 catalyst in the photodegradation of MB.

Fig. 8. UV–vis spectra of the MB solutions at different irradiation times (the inserted image shows the color change of the MB solution).

disappeared, and the intensity of C–H deformation vibration absorption (883 cm−1) of the aromatic ring evidently decreased, indicating that the aromatic ring of the MB molecule was severely damaged. The stretching vibration (2970 and 2898 cm−1) and the deformation vibration (1385 cm−1) of C–H on methyl slightly shifted after degradation, respectively corresponding to 2973, 2887, and 1381 cm−1. With


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Fig. 9. FT-IR spectra of MB before (a) and after (b) degradation.

Table 4 Assignments of absorption peaks in the FT-IR spectrum of MB [27,28]. Band (cm−1)

Mode of vibration

Chemical environment

2970, 2898 1635 1592, 1482 1385 1324 1134 883, 827 662

υ(C–H) υ(C˭N) δ(C˭C) γ(C–H) υ(C–N) υ(C˭S) γ(C–H) δ(C–S-C)

Methyl Sulfur–nitrogen conjugate system Aromatic ring Methyl Aromatic tertiary amine Sulfur–nitrogen conjugate system Aromatic ring Sulfur-nitrogen conjugate system

Note: υ for the stretching vibration, γ for the deformation vibration, and δ for the skeletal vibration.

Fig. 11. ESI-MS analysis of the samples at different degradation stages of MB.

molecules g (propionic acid) and h (malonic acid). The remaining intermediates are oxidized to several smaller molecules, such as CO2 and H2O. 4. Conclusions In summary, ZS nanocomposites with different compositions were prepared using a simple coprecipitation method. The composite samples effectively separated the photogenerated electrons and holes and thus exhibited higher photocatalytic activity than pure ZnO. The plausible mechanism of MB degradation with the photocatalyst was deduced from a series of tests on the intermediates in the process. Results showed that 40% of MB was completely mineralized, and the remaining 60% was oxidized to small molecular substances, such as propionic acid and malonic acid. In addition, ZS maintained its high photocatalytic activity after being recycled for three times. Therefore, the ZS composite is a good candidate in the treatment of industrial wastewater to eliminate organic pollutants.

Fig. 10. HPLC chromatogram of the photocatalytic degradation of MB.

Finally, on the basis of the abovementioned analyses, this study presented a plausible pathway for MB degradation (Fig. 12). First, the sulfur–nitrogen conjugated system in the MB molecule is attacked by hydroxyl radicals and then degraded into b (2-amino-4-(N,N-dimethyl)benzenesulfonic acid) and c (4-(N,N-dimethyl)-aniline). Then, a portion of b undergoes a series of reactions to generate f (2,4-hexadiene-1,6diol), which cannot be further degraded, to the end of the reaction. The other part of b is oxidized and deaminized to form d (p-nitrobenzenesulfonic acid) and is oxidized further to form e (benzenesulfonic acid). Simultaneously, c is oxidized and deaminized to form i (4-(N,N-dimethyl)-phenol) and subsequently oxidized to j (hydroquinone). Part of the resulting intermediates e and j are attacked by hydroxyl radicals, and their rings are opened to form small organic

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21203052), and the Natural Science Foundation of Hebei Province of China (No. B2018205205). The authors would also like to thank EnPapers for its linguistic assistance during the manuscript preparation. 29

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Fig. 12. Photocatalytic degradation pathway of MB.

Declarations of interest

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