|K. Kusdianto||Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember (ITS), Kampus ITS, Sukolilo, Surabaya 60111, Indonesia|
|W. Widiyastuti||Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember (ITS), Kampus ITS, Sukolilo, Surabaya 60111, Indonesia|
|Manabu Shimada||Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 4-1, Kagamiyama 1-chome, Higashi-Hiroshima, Hiroshima 739-8527, Japan|
|Tantular Nurtono||Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember (ITS), Kampus ITS, Sukolilo, Surabaya 60111, Indonesia|
The degradation of organic pollutants using photocatalysis is more effective than conventional methods, with ZnO being the most widely used of the various semiconductor materials for application in photocatalysis. Unfortunately, degradation efficiency is inhibited by the electron-hole recombination. The photocatalytic activity of ZnO can be enhanced by adding noble metals, such as Ag nanoparticles. However, the fabrication of ZnO?Ag using liquid-phase processes is complicated and often requires multiple steps. In this study, the effects of Ag content, ranging from 0 to 20 wt%, in the photocatalytic activity of ZnO?Ag nanocomposites are investigated. The nanocomposites were fabricated by a one-step process using flame pyrolysis and with zinc acetate and AgNO3 as the precursors. The nanocomposites were collected using an electrostatic precipitator and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and nitrogen adsorption-desorption of the Brunauer-Emmett-Teller (BET) specific surface area. The XRD results confirm the existence of Ag nanoparticles in the prepared nanocomposites, whose crystallite size was not significantly influenced by the presence of the nanoparticles. The SEM indicated that the morphology of the nanocomposites produced was spherical, with some aggregates. Particle size distribution of the nanocomposites increased with higher Ag content. The photocatalytic activity of the produced nanocomposites, estimated by evaluating the degradation of the methylene blue aqueous solution under UV irradiation, showed that the highest photocatalytic performance was attained when the concentration of Ag was equal to 5 wt%.
Ag content; Gas-phase; Methylene blue degradation; Nanocomposite
Organic pollutants from waste water can be degraded in several ways, such as adsorption, oxidation, reduction and photocatalysis (Balu et al., 2018; Kusrini et al., 2018; Mamat et al., 2018). The application of photocatalysis using semiconductor materials is reported to be more effective than the conventional chemical oxidation methods for degradation of these pollutants (Chatterjee & Dasgupta, 2005).
ZnO is the most widely used of the various semiconductor materials for application in photocatalysis due to its suitable band gap, non-toxicity, high chemical stability, cost-effectiveness, strong oxidation ability, and easy availability (Duan et al., 2010). However, electron-hole recombinations inhibit the photocatalytic activity of pristine ZnO. Therefore, many attempts have been made to enhance the photocatalytic performance, such as by modification of the ZnO structure in order to increase the surface area and light absorption (Kadam et al., 2018). Furthermore, doping the ZnO nanoparticles with noble metals (Dermenci et al., 2014), lanthanide groups (Vaiano et al., 2017), natural zeolite (Rahman et al., 2018), and Fe3O4 (Winatapura et al., 2016) have also been reported as promising candidates for enhancing the photocatalytic activity. In general, Ag nanoparticles are more attractive as dopants compared to other noble metals because of their high electrical and thermal conductivity, non-toxicity, cost-effectiveness, and high work function (Kadam et al., 2018). Kusdianto et al. (2017) reported that the addition of Ag nanoparticles enhanced the photocatalytic activity of the nanocomposite produced by up to 35% compared to pristine TiO2 because Ag can be used as an electron acceptor to inhibit the electron-hole recombination.
ZnO-Ag nanocomposite can be fabricated by liquid-phase methods using sol-gel, precipitation, electrodeposition, hydrothermal, and solvothermal approaches (Jianguo et al., 2017; Kadam et al., 2018; Vaiano et al., 2018; Liu et al., 2019). These methods are able to fabricate nanocomposite at ambient temperature and atmospheric pressure. However, the disadvantages of these processes are that they involve a large number of processing steps and then require further steps to remove any residue or impurities, as well as completely removing the solvent. Furthermore, gas-phase methods employing spray drying pyrolysis have been utilized to fabricate ZnO-Ag nanocomposite (Dermenci et al., 2014). These methods are suitable for obtaining particles in a single step. However, decomposition of the precursors occurs inside the tubular furnace, which requires a high electrical power source. Another one-step process using the gas-phase method and flame pyrolysis has also been reported as a good candidate for producing the particles due to high crystallinity of the nanoparticles produced. Moreover, it is not necessary to deal with the solvent after fabrication as it evaporates during decomposition inside the flame reactor (Tani et al., 2002). The high purity of the product, with a relatively narrow size distribution, is another advantage of this method (Kammler et al., 2001). In addition, many types of low-cost precursors can be used as sources (Solero, 2017).
In this study, preference has been given to the flame pyrolysis method, as the energy source of the flame can be generated by simple combustion between the fuel and oxidizer, meaning it is cost-effective. In previous studies, our group has successfully fabricated ZnO by the flame pyrolysis method, in which the morphology of the particles produced was significantly affected by the temperature of the flame (Widiyastuti et al., 2013; Widiyastuti et al.,2014). However, to the best of our knowledge, no studies have reported the effect of Ag content on ZnO-Ag nanocomposite prepared by flame pyrolysis and the characterization of the products for the photocatalytic activity. Inspired by this gap, the objective of this study is to investigate the effect of Ag loading on ZnO-Ag nanocomposite synthesized by the flame pyrolysis method and their photocatalytic performance under UV light irradiation. Methylene blue (MB) was used as the model organic pollutant because it is commonly used as a synthetic dye in the textile industry and is not easily biodegraded by nature (Kusrini et al., 2018). Furthermore, MB is a heterocyclic aromatic compound, which is toxic and highly dangerous to humans (Balu et al., 2018). Using MB as a model organic pollutant, we believe that the results obtained will provide valuable information on MB degradation efficiency to help solve the environmental issue, especially due to the liquid waste of organic pollutants.
ZnO-Ag nanocomposites have been successfully fabricated by a one-step process using flame pyrolysis. The effect of Ag content on the nanocomposites was also investigated. The XRD results indicate that the ZnO produced by flame pyrolysis has a typical hexagonal Wurtzite structure with high crystallinity. The existence of Ag in the nanocomposite could be detected by XRD when the Ag content was greater or equal to 5%wt. The crystallite size of the nanocomposites was not significantly changed by varying the Ag content. The spherical shape of the ZnO, with some agglomeration of particles, was observed by SEM analysis, while particle size increased slightly with increasing Ag content. However, pore diameter did not show any clear tendency with the various Ag loadings, whereas the surface area increased to 0.1%wt, then decreased with increasing Ag content. Finally, photocatalytic activity evaluated by measuring MB degradation under UV light irradiation showed that maximum degradation efficiency of 63% could be achieved when Ag content of 5 %wt was used. The best photocatalytic activity was attained at 5%wt of Ag. We believe that this finding provides valuable information for the fabrication method of ZnO-Ag nanocomposite, as well as the effect of Ag content, with wide future applicability in various fields such as dye-sensitized solar cells, gas sensors, antibacterial applications, and photocatalysis.
The authors are grateful to Herlinda S., Ika Silvia A., and Nurul Ika for their assistance in the experiments. They would also like to thank Mr. Jiang D. and M. Ishihara for their assistance in the SEM observation. This work was financially supported by the Direktorat Riset dan Pengabdian Masyarakat (DRPM) DIKTI, with contract grant No. 955/PKS/ITS/2018.
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