• For Contributors +
• Journal Search +
Journal Search Engine
ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.30 No.12 pp.641-649
DOI : https://doi.org/10.3740/MRSK.2020.30.12.641

# Microwave Assisted Synthesis of SnS Decorated Graphene Nanocomposite with Efficient Visible-Light-Driven Photocatalytic Applications

Jun-Hui Wang1, Yi-Kai Zeng1, Hao Gu1, Lei Zhu1, Won-Chun Oh2
1Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng, 224051, P.R. China
2Department of Advanced Materials Science & Engineering, Hanseo University, Seosan 31962, Korea
Corresponding author E-Mail : leizhu2016@ycit.edu.cn (L. Zhu, Yancheng Inst.Technol.) wc_oh@hanseo.ac.kr (W.-C. Oh, Hanseo Univ.)
October 13, 2020 October 31, 2020 November 2, 2020

## Abstract

A facile microwave assisted solvothermal process is designed for fabricating SnS nanoparticles decorated on graphene nanosheet, which used as visible light driven photocatalyst. Some typical characterization techniques such as XRD, FT-IR, SEM with EDX analysis, and TEM and BET analysis are used to analyse the physical characteristics of as-prepared samples. Spherical SnS nanoparticles are uniformly dispersed on the surface of graphene nanosheet due to ammonia, which can prevent the aggregation of graphene oxide. Meanwhile, microwave radiation provides fast energy that promotes the formation of spherical SnS nanoparticles within a short time. The visible light photocatalytic activity of as-prepared SnS-GR nanocomposites is analysed through photodegradation efficiency of methylene blue with high concentration. According to the higher photocatalytic property, the as-prepared SnS-GR nanocomposites can be expected to be an efficient visible light driven photocatalyst. After five cycles for decolorization, the rate decreases from 87 % to 78 % (about 9 %). It is obvious that the photocatalytic activity of SnS-GR nanocomposite has good repeatability.

## 초록

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

Graphene is one kind of semi-metal material which has been attracted many attention for most researcher since 2004.1) Meanwhile, as a new two-dimensional material with large specific surface area it exhibits outstanding electrical, optical and mechanical properties.2,3) Recently, Peak et al. demonstrated that the electrochemical performance of SnO2 was enhanced via adding graphene nanosheet as one kind of electronic conductive channel. 4) Seger and Kamat proved that graphene nanosheet is more benefit for the dispersing of Pt nanoparticles.5) Based on the previous studies, graphene could be used as a support material for synthesis of nanoscale photocatalysts and acts as a photosensitizer for enhancing their photocatalytic activity under visible light irradiation.

Semiconduction photocatalysts are widely used in the destruction of organic pollutants. Among the semiconduction photocatalysts, TiO2 is one kind of focus material for photodegradation dye. However, its photo efficiency and photo response activity are limited by its high recombination rate of electrons and holes.6) In addition, it can only be responded under UV light irradiation because of its large bandgap energy (3.0 ~ 3.2 eV).7)Therefore, make it necessary to design new photocatalysts for visible light responded photocatalysis application. Metal sulfides as important semiconductors, have been used in many application areas such as laser communication and LED, hydrogenation processes, CO-shift reaction8-10) due to its quantum size effect, non-linear optical properties. The photocatalytic properties of metal sulphides also have been attracted more attention in recent years.11)

For preparing the graphene-metal sulfide composites, graphene oxide (GO) was generally used as substrate material due to its oxygen containing groups could facilitate the decoration of nanoparticles without aggregation. According to the previous study of Graphene-CdS composites,12,13) the photo-excited electrons transfer from CdS to graphene due to its fluorescence quenching effect. The decreased recombination of photo-generated electronhole pairs promotes the enhancement of visible light induced photodegradation activity. Huating H. et al. have synthesized graphene nanosheets-zinc sulfide nanocomposites via microwave-assisted method and used as photocatalyst.14) The graphene- zinc sulfide nanocomposite exhibits an efficient photocatalytic property for photodegradation of methylene blue.

SnS is an orthogonal IV–VI group semiconductor material and exhibits excellent properties for photovoltaic material15) and anode of lithium ion batteries16) etc. Jayalakshmi M. et al.17) have investigated a hydrothermal method to synthesize nanoscale SnS and used as an electrode active material for supercapacitor. Nanosized SnS electrode has good cycling stability and higher capacitance under condition of alkali and neutral. Peisong T. et al.18) reported a method to synthesize SnS nanomaterials and demonstrated its high photocatalytic activity under visible light, which may be ascribed to the strong visible-light absorption and its efficient photo-electronic effect. However, up to date, very few literatures investigate the microwave assisted preparation of graphene based SnS nanocomposite and applications for visible light photodecomposition of MB solution.

In the research, graphene nanosheets decorated with SnS spherical nanoparticles were obtained directly via a facile microwave assisted solvothermal method. The intrinsic characteristics of resulting composite were studied by XRD, FT-IR, SEM with EDX analysis, TEM and BET analysis. Its visible light photocatalytic activity was evaluated by photodegradation organic dye methylene blue (MB) with high concentration in aqueous solution.

## 2. Experimental

### 2.1 Materials

Graphene oxide (GO) used as substrate material was obtained from Suzhou Tanfeng Science and Technology Ltd. (China). Ethylene glycol, anhydrous ethanol, tin (II) chloride dihydrate (SnCl2·2H2O) and sodium thiosulfate anhydrous (Na2S2O3), MB (C16H18N3S·Cl, 99.9 %) and ammonium hydroxide (NH3H2O) were purchased from Aladdin Chemical Co. (China). Anatase type titanium oxide with particle size below 25 nm as control sample was purchased from Sigma-Aldrich Chemistry, USA. All chemicals were used without purification and all experiments were carried out using ultra pure water (18.2 MΩ).

### 2.2 Synthesis of SnS-GR nanocomposites

A typical experiment for synthesis of SnS-GR nanocomposites was shown in Schematic 1, about 20 mg GO were dispersed in 100 mL ethylene glycol and followed by ultrasonic processed for 1 h to form graphene oxide nanosheet dispersion solution. Subsequently, 0.29 g of SnCl2·2H2O powders were added, followed by ultrasonic processed for 20 min. Afterwards, sodium thiosulfate anhydrous (Na2S2O3) and 1mL ammonium hydroxide (NH3H2O) were added under various stirring for 30min. After mixing together stirred for another 20 min, it was transferred into a 120 mL reaction vessel and placed in a conventional microwave oven (Midea, M1-L202B 700 W). The solution is then irradiated by microwave at full power for 5 sec on and 5 sec off for 300 seconds. The final product was washed with ultra pure water and ethanol for several times, and dried at 60 °C for 12 h. Pure SnS nanoparticle was prapared by the similar method without graphene oxide dispersion solution was used.

### 2.3 Characterization

X-ray diffraction (X'Pert3 Powder 3 kW) was selected to investigate the crystallinity with monochromatic high-intensity CuKα radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM, EVO18) was used to analysis its morphology about surface state and structure of the as-prepared samples. The element constitutes and weight ratios of the as-prepared nanocomposite were characterized by an energy dispersive X-ray (EDX) analysis. UV-vis diffuse reflectance spectra (DRS) were obtained using an UV-vis spectrophotometer (Varian - Gary 5000) by using BaSO4 as a reference at room temperature and were converted from reflection to absorbance by the Kubelka- Munk method.

### 2.4 Photocatalytic studies

For photocatalytic test process, the as-prepared nanocomposites were added into organic dye MB solution (3.00 × 10−5 mol/L, 100 mL). In order to achieve adsorption/ desorption equilibrium, it should be maintained in the dark for 2 h. A solar simulator (500 W Xe Lamp, NBeT Company) with a UV-cutoff filter (>420 nm) was used as the light source. The intensity of the light source was controlled as 100 mW/cm2 and detected by optical power meter (FZ400, NBeT Company). A certain volume of solution was extracted from the reactor by an order of 30 min, 60 min, 90 min, 120 min. The supernatant of the suspension after centrifugation (12,000 rpm, 5 min) was determined by a spectrophotometer (UV-2450, SHIMADZU) at λmax = 664 nm.

### 2.5 Reactive oxygen species analysis

In the process of reactive oxygen species analysis, 10 mL of 1,5-diphenyl carbazide solution with 1.00 × 10−2 mol/L was preliminary prepared and transferred into volumetric flask. 50 mg SnS-GR photocatalyst was added to the solution and then diluted to 50 mL with double distilled water. The final DPCI concentration and SnSGR photocatalyst amount were 1.00 × 10−3 mol/L and 1.00 g/L. Benzene as an extract solvent was added to 10 mL of above solution which was irradiated by visible light for 2 h. And then, all extracted solutions were diluted to 10 mL with benzene solution and their UV-vis spectra were determined.

## 3. Results and Discussion

### 3.1 Characterization

Fig. 1(a) shows the FT-IR results of graphene oxide (GO) and SnS-GR synthesized via solvothermal reaction. It is well known that some carbon double bonds easily generate functional groups such as -COOH, and –OH, while it was processed by acidic intercalation and thermal treatment.19,20) It is obvious, the stretching vibration of –OH groups exhibits clear resonance band which is around 3,400 cm−1 , and the vibrations of the adsorbed water molecules exhibits its band at 1,619 cm−1. Meanwhile, carbonyl stretching mode of C=O arises at 1,728 cm−1, the stretching mode of C-OH arises at 1,226 cm−1, and the stretching mode of C-O arises at 1,050 cm−1.

Meanwhile, the amount and distribution of the carboxylic groups have great influence on the further positive-selective modification of graphene nanosheets with nanoparticles. Between these functional groups and metal ions, the existence of electrostatic attractions and chemical interactions may generate selective penetration efficency of graphene oxide sheet.21) In this study, some tin ions may absorbed on some particular positions of graphene which has high density of carboxyl and easily transformed into SnS nanoparticles after chemical reaction process. Finally, a nanostructure of graphene nanosheet based SnS composite was formed in the process of simultaneous reduction by ethylene glycol.22) The aggregation of graphene oxide was a certain extent prevented due to the used ammonia has salt effect,23) and the generated maximal charge accelerate uniform distribution of SnS nanoparticles on graphene nanosheet. At the same time, some oxygencontaining functional groups of graphene oxide obviously were disappeared in the whole solvothermal process, confirming a formation of graphene-based materials.

Fig. 1(b) shows the XRD pattern of the as-prepared materials. The peaks appeared at 25.9° and 42.7° coule be assigned to the graphite (002) and (100) reflections (JCPDS No. 01-0646),24) shown us the typical diffraction image of graphene. For SnS-GR composite, all the diffraction peaks can be assigned to cubic lattice with cell constant a = 4.3284 Å, that is agreed to the previous report about SnS.25) Meanwhile, there were no peaks related to GO (001) or graphene (002) were detected for as-prepared SnS-GR nanocomposite. According to Ref.,26) Graphene oxide was reduced to graphene due to the disappearance of some oxygen-containing functional groups. According to earlier studies, diffraction peaks may also become weak or even disappear example by exfoliation which may cause the regular stack of GO or graphite broken.27)

The surface state of graphene and SnS-GR nanocomposites was analyzed by SEM. In Fig. 2(a, b), it can be clearly seen naturally aggregated and stacked to multilayers graphene exhibits numerous edges. As shown in Fig. 2(c, d), the morphology of SnS-GR composite is different from graphene nanosheets. Some spherical SnS nanoparticles can be clearly seen and tightly distributed on the surface of graphene sheets. Hereby, microwave assisted synthesize technique is facilitate to obtain small SnS nanoparticles within short time. Comparing with common synthesis methods, microwave assisted synthesis techniques own many outstanding advantages such as it can reduce the solvent consumption and supply a clear reaction process with small waste.28) Reaction time was significantly reduced thus saves electricity and extra cooling reflux water. High frequency electric fields can be supplied by microwaves. And any material such as solid state conducting ions or polar molecules can be heated due to its movable electric charges in a solvent.

More detailed information of the surface state and particle size can be confirmed by the TEM. In Fig. 3, it could be seen that SnS nanospheres were uniformly distributed on a single layered graphene sheets which may play a support material role in helping SnS crystals growth, no apparent aggregation of the SnS spheres was discerned. According to result of TEM, some SnS spheres exhibit small particle size around 10 to 30 nm. As compared to common synthesize methods, the microwave synthesis technique provides advantages such as a very short reaction time, small particle size, excellent distribution state and it is high purity and suitable method for synthesis of polycrystalline products.29)

The elemental analysis of composite was carried out by acquiring EDX spectra. Fig. 4 shows the EDX microanalysis and element weight % of SnS-GR composite. It proves the existence of these main elements such as C, Sn and S. Carbon signal with high intensity could be assigned to graphene. Another two peaks such as Sn and S was assigned to the precursor materials SnCl2·2H2O and Na2S2O3.

The mesopores on the surfaces of the samples original graphene and SnS-GR was analyzed through Nitrogen adsorption isotherms as shown in Fig. 5. Type IV adsorption isotherms confirmed the major mesopores was formed on the surfaces of the samples. A special hysteresis loop was detected from the Type IV isotherm, which may be attributed to capillary condensation generated in mesopore 0 and limiting uptake at high range of p/p°. The monolayer-multilayer adsorption comes into being the initial part of Type IV isotherm. It follows the same path as the corresponding part of a Type II isotherm. According to paper study, many mesoporous industrial adsorbents may exhibits Type IV isotherms.30,31) This proves that the SnS-GR composite studied were mainly mesoporous in character, with a minor presence of wider pores where capillary condensation occurred.

### 3.2 Photocatalytic activity of samples

Adsorption capacity of each photocatalyst is an important factor for obtaining enhanced photocatalytic performance. Because of that photo-oxidation reaction usually takes place at the catalyst surface. The dye adsorptive property was tested using MB solution and control sample TiO2, SnS and SnS-GR composite keep in dark for two hours. After achieving the adsorption– desorption equilibrium, some dye molecules were adsorbed on thesurface of photocatalyst. And a clear decreased concentration of methylene blue can be visualized in Fig. 6. The BET surface area of control sample TiO2, as well as as-prepared SnS and SnS-GR nanocomposites were 11.54 m2/g, 45.89 m2/g and 62.31 m2/g, respectively.

The effect of catalyst composition on MB (3.00 × 10−5 mol/L, 100 mL) degradation efficiency was investigated under visible light irradiation with a catalyst amount of 0.03 g. As shown in Fig. 6(a-c), the adsorption effect of SnS-GR was better than that of any other samples due to the relatively higher surface area. The adsorptive effect of control sample TiO2 was the lowest. SnS-GR has the largest BET surface area, which can enhance the adsorptive effect. In the degradation step, the control sample TiO2, SnS and SnS-GR composite showed a good degradation effect. From which we can obviously see that the SnSGR composites have a excellent photocatlytic activity, which may owe to the following reasons: (1) graphene nanosheets with wondrously high charge mobility16) acting as good electron acceptors, 12-14) is expected to improve the interfacial electron transfer and restrain the electron/ hole (e/h+) pair recombination of SnS; (2) graphene nanosheets with very high surface area-to-volume ratios and extremely high specific surface area24) not only enhance the dispersion of SnS nanoparticles but also enhance the photon absorption ability on the surface of photocatalysts.

After adsorption–desorption equilibrium was achieved, the photocatalytic activity obeys pseudo-first-order kinetics in relation to the concentration of MB:

Integration of equation (with the restriction of c=cads at t=0, with the cads being the initial concentration in the bulk solution after dark adsorption and t the reaction time) will lead to the following expected relation:

where c and cads are the reactant concentration at time t = t and t=0, respectively, kapp and t are the apparent reaction rate constant and time, respectively. According to the equation, a plot of -ln(c/cads) versus t will yield a slope of kapp. The results are displayed in Fig. 6(d). The linearity of plot suggests that the photodegradation reaction approximately follows the pseudofirst-order kinetics with kapp of 0.0115 min- for degradation MB over the SnS-GR composites under visible light.

SnS-GR composite was selected to investigate its photocatalytic activity for oxidizing1,5-diphenyl carbazide (DPCI) as shown in Fig. 7. Electron-hole pairs were formed on the surface or in the inner of SnS-GR sample, while some electrons are transferred from valence band (VB) to conduction band (CB). The electrons and holes can be generated superoxygen radical anions (•O2−) and hydroxyl radicals (•OH) if they react with O2 and H2O. 1,5-diphenyl carbazone (DPCO) was formed while DPCI was oxidized by •OH. DPCO can be easily extracted by benzene and exhibits a typical absorbance wavelength at 560 nm. Sequentially, the produce and output of •OH can be easily detected.32,33) Comparing with dark condition, the absorption intensity of DPCO around 560 nm shows an obvious increase according to different irradiation time.34)

### 3.3 The repeatability of photocatalystic activity

The long-term stability of SnS-GR nanocomposite was investigated by recycling experiment under visible light irradiation. The re-used sample was collected and vacuumdried at 100 °C overnight. After five cycles experiments, the photo-decolorization rate decreases from 87 % to 78 % as shown in Fig. 8. To the certain degree, 9 % decrease rate indicates that the SnS-GR nanocomposite has good repeatability. The reduction in repeatability of photodegradation property may ascribe to inevitably generation of some by-products. And the progressive accumulation of by-productss give rise to block some cavities and the active surface sites of photocatalysts, which lead to reduce its photocatalytic activity.

Fig. 9 shows us the scheme of excitation and charge transfer process between SnS particles and graphene nanosheet. Under visible irradiation, the graphene nanosheet acting as good electron acceptors can accept the electrons. Then some photo electrons were generated in valence band (VB) of SnS and transfered into conduction band (CB) of SnS. Meanwhile the electrons accepted by graphene nanosheet from light can transfer into the CB of SnS, thus enhancing the number of electrons as well as the rate of electron-induced redox reactions. Oxygen peroxide radical $O 2 · −$ was produced while generated electrons (e-) reacts with dissolved oxygen molecules in solution. Meanwhile, hydroxyl radical OH was generated while the positive charged hole (h+) reacts with the OH- derived from H2O. The MB molecules then can be photocatalytically degraded by oxygen peroxide radical $O 2 · −$ and hydroxyl radical OH• to CO2, H2O and other mineralization.

## 4. Conclusion

In this study, a facile microwave assisted solvothermal method was designed to synthesize SnS-GR nanocomposite in a domestic microwave oven within 5 min. The spherical SnS nanoparticles triggered by precipitation reaction uniformly distributed on the graphene nanosheets were observed. The results reveal that SnS-GR composite exhibits the highest photocatalytic activity, excellent structure, great repeatability. The present research demonstrates that the as-prepared graphene based semiconductor nanocomposites can be applied as a novel photocatalyst and provides new insights into the photocatalytic degradation of pollutants under visible light irradiation.

## Author Information

Jun-Hui Wang Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province,Yancheng Institute of Technology, Student

Yi-Kai Zeng Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province,Yancheng Institute of Technology, Student

Hao Gu Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province,Yancheng Institute of Technology, Student

Lei Zhu Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province,Yancheng Institute of Technology, Assistant Professor

Won-Chun Oh Hanseo University, Professor

## Figure

Synthesis process of spherical SnS decorated graphene nanocomposite.

(a) FT-IR spectra of graphene oxide and SnS-GR composite, and (b) XRD pattern of graphene, SnS and SnS-GR composite.

SEM images of as-prepared composites: (a,b) graphene, (c,d) SnS-GR composite.

TEM images of samples (a) graphene, (b) SnS-GR composite.

EDX microanalysis (a) and element weight % (b) of SnS-GR composite.

Nitrogen adsorption isotherms obtained from the (a) graphene, (b) SnS-GR composite.

UV/vis spectra of MB concentration against the composite under various time conditions?(a) control sample TiO2, (b) SnS, (c) SnS-GR composite?Apparent first-order linear transforms –ln(c/cads) vs. t of MB degradation on control sample TiO2, SnS and SnS-GR composites under visible light irradiation (d). The concentration of MB solution is 3×10-5 M; the amount of catalyst is 0.03 g.

UV-vis spectra of DPCO extract liquors (a), and of the liquors in the presence of SnS-GR composite under visible light irradiation 60 mins (b), 90 mins (c) and 120 mins (d).

Reuse effect of SnS-GR nanocomposite for photodegradation methylene blue.

Schematic diagram of the charge transfer between graphene and SnS nanoparticles under visible light irradiation.

## Reference

1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 306, 666 (2004).
2. C. Lee, X. Wei, J. W. Kysar and J. Hone, Science, 321, 385 (2008).
3. B. Tang, G. X. Hu and H. Y. Gao, Appl. Spectros. Rev., 45, 369 (2010).
4. S. M. Peak, E. J. Yoo and I. Honma, Nano Lett., 9, 72 (2009).
5. B. Seger and P. V. Kamat, J. Phys. Chem. C, 113, 7990 (2009).
6. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev., 95, 69 (1995).
7. N. S. Lewis, Nature, 414, 589 (2001).
8. M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber, R. Russo and P. D. Yang, Science, 292, 1897 (2001).
9. M. A. Kolb, W. F. Maier and K. Stowe, Catal. Today, 159, 64 (2011).
10. G. C. Chinchen and M. S. Spencer, J. Catal., 112, 325 (1988).
11. B. L. Abrams and J. P. Wilcoxon, Crit. Rev. Solid State Mater. Sci., 30, 153 (2005).
12. A. N. Cao, Z. Liu, S. S. Chu, M. H. Wu, Z. M. Ye, Z. W. Cai, Y. L. Chang, S. F. Wang, Q. H. Gong and Y. F. Liu, Adv. Mater., 22, 103 (2010).
13. C. Nethravathi, T. Nisha, N. Ravishankar, C. Shivakumara and M. Rajamathi, Carbon, 47, 2054 (2009).
14. H. T. Hu, X. B. Wang, F. M. Liu, J. C. Wang and C. H. Xu, Synth. Met., 161, 404 (2011).
15. N. K. Reddy and K. T. R. Reddy, Phys. B (Amsterdam, Neth.), 368, 25 (2005).
16. X. L. Gou, J. Chen and P. W. Synthesis, Mater. Chem. Phys., 93, 557 (2004).
17. M. Jayalakshmi, M. M. Rao and B. M. Choudary, Electrochem. Commun., 6, 1119 (2004).
18. P. S. Tang, H. F. Chen, F. Cao, G. X. Pan, K. Y. Wang, M. H. Xu and Y. H. Tong, Mater. Lett., 65, 450 (2011).
19. W. C. Oh, M. L. Chen, K. Zhang, F. J. Zhang and W. K. Jang, J. Korean Phys. Soc., 56, 1097 (2010).
20. W. C. Oh and F. J. Zhang, Asian J. Chem., 23, 875 (2011).
21. P. Z. Sun, M. Zhu, K. L. Wang, M. L. Zhong, J. Q. Wei, D. H. Wu, Z. P. Xu and H. W. Zhu, ACS Nano, 7, 4428 (2013).
22. Y. Liu, Y. Zhang, G. H. Ma, Z. Wang, K. Y. Liu and H. T. Liu, Electrochim. Acta, 88, 519 (2013).
23. D. Li, M. B. Muller, S. Gilje and G. G. Wallace, Nat. Nanotechnol., 3, 101 (2008).
24. L. Zhu, Z. D. Meng, T. Ghosh, M. M. Peng, K. Y. Cho and W.C. Oh, Fresenius Environ. Bull., 21, 1675 (2012).
25. S. G. Hickey, C.Waurisch and B. Rellinghaus, J. Am. Chem. Soc., 130, 14978 (2008).
26. H. Zhang, X. J. Lv, Y. M. Li, Y. Wang and J. H. Li, ACS Nano, 4, 380 (2010).
27. D. Cai, M. Song, J. Mater. Chem., 17, 3678 (2007).
28. S. Das, A.K. Mukhopadhyay, S. Datta and D. Basu, Bull. Mater. Sci., 32, 1 (2009).
29. K. Ullah, S. Ye, L. Zhu, Z. D. Meng, S. Sarkar and W. C. Oh, Mater. Sci. Eng., B, 180, 20 (2014).
30. Z. D. Meng, M. M. Peng, L. Zhu and W. C. Oh, Appl. Catal. B. Environ., 141, 113 (2012).
31. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 54, 2201 (1982).
32. J. Wang, Y. Guo, B. Liu, X. Jin, L. Liu, R. Xu, Y. Kong and B. Wang, Ultrason. Sonochem., 18, 177 (2011).
33. Y. W. Guo, C. P. Cheng, J. Wang, Z. Q. Wang, X. D. Jin, K. Li, P. L. Kang and J. Q. Gao, J. Hazard. Mater., 192, 786 (2011).
34. Z. D. Meng, T. Ghosh, L. Zhu, J. G. Choi, C. Y. Park and W. C. Oh, J. Mater. Chem., 22, 16127 (2012).