|Yayuk Astuti||Chemistry Department, Faculty of Sciences and Mathematics, Diponegoro University, Jl. Prof. Soedharto, S. H., Tembalang, Semarang, Central Java 50275, Indonesia|
|Darul Amri||Chemistry Department, Faculty of Sciences and Mathematics, Diponegoro University, Jl. Prof. Soedharto, S. H., Tembalang, Semarang, Central Java 50275, Indonesia|
|Didik S. Widodo||Chemistry Department, Faculty of Sciences and Mathematics, Diponegoro University, Jl. Prof. Soedharto, S. H., Tembalang, Semarang, Central Java 50275, Indonesia|
|Hendri Widiyandari||Department of Physics, Faculty of Mathematics and Natural Sciences, University of Sebelas Maret, Jl. Ir Sutami No.36A, Jebres, Surakarta, Central Java, 57126, Indonesia|
|Ratna Balgis||Department of Chemical Engineering, Faculty of Engineering, Hiroshima University, Japan, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527, Japan|
|Takashi Ogi||Department of Chemical Engineering, Faculty of Engineering, Hiroshima University, Japan, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527, Japan|
The potential of bismuth oxide (Bi2O3) as a photocatalyst, due to its a wide band gap (2.3-3.3 eV), was successfully synthesized using the solution combustion method with several fuels: urea, glycine, and citric acid. The synthesis was started by dissolving bismuth nitrate pentahydrate in nitric acid and then adding the fuel. The solution formed was heated for 8 h at 300°C. After heating, calcination was carried out for 4 h at 700°C. The resulting three products were in a yellow powder form. Fourier Transform InfraRed (FTIR) spectra of the samples confirmed that Bi2O3 had formed, as indicated by the functional groups of Bi-O-Bi observed at approximately 830–850 cm-1 and Bi-O at 1380 cm-1. X-ray diffractograms indicated that Bi2O3 synthesized using urea and glycine fuels was present in the mixed phases of ?-Bi2O3 at 2? of 27.7, 33.3, 27.2 and ?-Bi2O3 at 2? of 30.5, 41.8, 45.5, based on the Joint Committee on Powder Diffraction Standards (JCPDS) database 41-1449 and 27-0050, respectively. However, Bi2O3 produced by citric acid fuel comprised only ?-Bi2O3. Furthermore, different fuels produced different crystallite product sizes; urea generated the smallest crystallite, followed by glycine and citric acid. Additionally, the photocatalytic activity on the degradation of methyl orange of Bi2O3 synthesized using urea fuel exhibited better photocatalytic activity than the other products, with degradation rate constants of 4.38×10-5 s-1, 3.38×10-5 s-1, 2.33×10-5 s-1 for bismuth oxide synthesized by urea, glycine, and citric acid, respectively.
Bismuth oxide (Bi2O3); Photocatalytic activity; Photocatalyst; Solution combustion
Bismuth oxide (Bi2O3) is a semiconductor that has attracted considerable attention because it exhibits good optical and electrical properties, such as a wide band gap of 2.3–3.3 eV (Hashimoto et al., 2016), high refractive index (n?Bi2O3 = 2.9), high dielectric permittivity (?r = 190), and good photoconductivity (Bedoya Hincapie et al., 2012). These properties have led to the use of Bi2O3 for the development of gas sensors, anti-reflection coatings, photo-voltaic cells, fuel cells, and optoelectronic devices (Jalalah et al., 2015). In addition, among the active photocatalysts such as titanium dioxide (TiO2) (Rahman et al., 2018) and ZnO (Winatapura et al., 2016), Bi2O3 has been demonstrated to be a valuable alternative photocatalyst due to its direct band gap energy.
It has been observed that the chemical and electrical properties of Bi2O3 depend on the synthesis procedure (Goti? et al., 2007). Therefore, careful selection of a synthesis method is necessary. Various techniques have been introduced to synthesize Bi2O3, including sol-gel (Mallahi et al., 2014), precipitation (Astuti et al., 2017), hydrothermal treatment (Liu et al., 2011), chemical deposition (Cheng and Kang, 2015), and solution combustion (La et al., 2013, Astuti et al., 2019). Most of these methods require high temperatures, long reaction times, or a particular instrument, which are inefficient from the point of view of energy consumption, production cost, and time.
Contrary to other methods, the solution combustion method offers a time-, energy-, and cost-efficient process and a simple experimental setup (Li et al., 2015). This method is based on an exothermic redox reaction between the fuel and oxidant, which generally provides the energy for the metal oxides’ formation (Lackner, 2010). Another benefit of this method is the exothermicity of the self-sustaining chemical reaction that drives the reaction because of the presence of the oxidant and fuel (Li et al., 2015).
The effect of various fuels on the solution combustion method has been studied in the synthesis of metal oxides, such as aluminum oxide (Al2O3), nickel (II) oxide NiO (Raveendra et al., 2016), and TiO2 (Rasouli et al., 2011). These studies reported that the fuels affected the products’ physicochemical properties; including morphology, crystallite size, crystalline phase, and crystal system. Urea, glycine, and citric acid are the most commonly reported fuels because of their high exothermicity and ability to coordinate with nitrates (Li et al., 2015). Synthesis of Al2O3 using glycine resulted in amorphous phase particles, while the use of urea generated crystalline Al2O3. However, in the case of TiO2 and NiO synthesis, the use of either urea or glycine produced crystalline phase particles, and only TiO2 synthesis using citric acid required further calcination. Regarding morphology, the use of glycine produced particles with higher porosity compared to urea and citric acid, which occurs because of the fuels’ molecular structures. Urea, glycine, and citric acid contain amino (–NH2) groups, amino and carboxyl (–COOH) groups, and hydroxyl (–OH) and carboxyl (–COOH) groups, respectively. The order of reactivities of the functional groups from highest to lowest is amino, hydroxyl, and carboxyl, respectively (Li et al., 2015). Even though the importance of fuel type on metal oxide synthesis has been demonstrated, the effect of fuel reactivity on the synthesis of Bi2O3 using the solution combustion method has never been reported. Therefore, this research aims to investigate the effect of fuels on the physicochemical properties and photocatalytic activity of Bi2O3 synthesized using the solution combustion method.
In this study, the effects of the reactivities of urea, glycine, and citric acid, as fuels, on the physiochemical properties of Bi2O3 were investigated. The fuels’ influence on the structural characteristics of Bi2O3 was also evaluated, and the photocatalytic activity of the synthesized Bi2O3 was measured using dye degradation.
Bi2O3 particles were successfully synthesized using the solution combustion method with various fuels: urea, glycine, and citric acid. The successful synthesis was confirmed by the particles’ yellow color and the presence of a Bi-O-Bi vibration mode at 837–848 cm-1 by FTIR analysis. The different fuels affected the morphology and physical properties of the synthesized particles. ?-Bi2O3 (monoclinic), identified at 2? 27.2, 27.7 and 33.3, was observed to be the major phase in all the prepared particles; however, samples synthesized using urea and glycine exhibited a minor presence of ?-Bi2O3 (tetragonal), observed at 2? 30.5, 41.8, 45.5. Different morphological structures of Bi2O3 particles were found, including thin-flake, porous, and bulky flake-like structures, which were observed in the particles prepared using urea, glycine, and citric acid, respectively. The effect of the fuels was also indicated by the particles’ band gap energies, namely 2.55 eV, 2.3 eV, and 2.75 eV for those prepared with urea, glycine, and citric acid, respectively. Furthermore, the highest photocatalytic activity for the degradation of methyl orange was exhibited by Bi2O3 particles synthesized using urea, followed by glycine and citric acid, with degradation rate constants of 4.38×10-5 s-1, 3.38×10-5 s-1, and 2.33×10-5 s-1, respectively.
The authors wish to acknowledge the Ministry of Research, Technology and Higher Education, Republic of Indonesia, for its financial support through a Penelitian Hibah Kompetensi (HiKom) grant, 2018, with the grant no. 101-71/UN7.P4.3/PP/2018. Moreover, Yayuk Astuti would like to thank Diponegoro University for financial support during the Postdoctoral/Sabbatical Program, 2017, with the grant no. 990/UN7.P/HK/2017, and the Thermal Fluid Lab, Chemical Engineering, Hiroshima University for the use of its SEM instrument facility.
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