Ultrasonics Sonochemistry xxx (2014) xxx–xxx
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Short Communication
Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO 4 in water under different pH conditions Abulikemu Abulizi a,b,c, Guo Hai Yang b, Kenji Okitsu a,⇑, Jun-Jie Zhu b,⇑ a
Graduate School of Engineering, Osaka Prefecture University, Osaka 599-8531, Japan State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China c College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, PR China b
a r t i c l e
i n f o
Article history: Received 27 December 2013 Received in revised form 26 March 2014 Accepted 27 March 2014 Available online xxxx Keywords: Sonochemical reduction Permanganate Manganese dioxide Effect of pH
a b s t r a c t MnO2 was synthesized by sonochemical reduction of MnO 4 in water under Ar atmosphere at 20 °C, where the effects of solution pH on the reduction of MnO 4 were investigated. The obtained XRD results showed that poor crystallinity d-MnO2 was formed at pH 2.2, 6.0 and 9.3. When solution pH was increased from 2.2 to 9.3, the morphologies of d-MnO2 changed from aggregated sheet-like or needle-like structures to spherical nanoparticles and finally to cubic or polyhedron nanoparticles. After further irradiation, MnO2 was readily reduced to Mn2+. It was confirmed that H2O2 and H atoms formed in the 2+ sonolysis of water acted as reductants for both reduction for MnO 4 to MnO2 and MnO2 to Mn . The optimum irradiation time for the effective synthesis of MnO2 was 13 min at pH 2.2, 9 min at pH 6.0, 8 min at pH 9.3, respectively. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Manganese oxides can be applied in catalysts, molecular-sieves, ion-sieves, batteries and magnetic materials due to their excellent physicochemical properties [1,2]. MnO2 is also one of the most important materials and a number of researchers pay attention to the influence of MnO2 addition on the electromagnetic properties of ferrite materials [3]. Various approaches have been developed to prepare nanoscale MnO2 such as self-reacting microemulsion [4], precipitation [5], room temperature solid reactions [6], sonochemical [7] and hydrothermal methods [8]. A sonochemical method is known as a unique synthesis method based on cavitation phenomena which are induced by ultrasound irradiation in a liquid. They provide interesting chemical and physical effects due to cavitation bubbles with high temperatures and high pressures as well as the formation of shear forces, microjets and shock waves in a liquid. Such interesting effects have been actively applied in the field of the synthesis of inorganic nanoparticles and nanostructured materials [9–11]. MnO2 nanoparticles have also been synthesized by several sonochemical methods based on the following reactions: (1) sonochemical reduction of MnO 4 [12–15], (2) ultrasound-assisted reaction between MnO4 and Mn2+ [16], and (3) ultrasound-assisted oxidation of Mn2+ ⇑ Corresponding authors. Tel.: +81 722521161 (K. Okitsu). E-mail addresses: [email protected] (K. Okitsu), [email protected] (J.-J. Zhu).
[7,17]. For example, Zhu et al. [14] reported the sonochemical reduction of MnO 4 to MnO2 in the presence of ordered mesoporous carbon template to suppress the growth of MnO2 nanoparticles. Raj et al. [15] reported that spherical MnO2 particles in the range from 10 to 20 nm were formed by the sonochemical reduction of MnO 4 in the presence of polyethylene glycol. It was also reported that the sonochemically prepared MnO2 nanoparticles act as excellent electrochemical capacitors [14,15]. From environmental and energetic points of view, sonochemical reduction methods with MnO 4 can be considered as green approaches for synthesizing MnO2, because a specific chemical reductant is not needed and synthesis can be performed under mild experimental conditions such as relatively low reaction temperature and ambient pressure. Although there are several reports using sonochemical reduction of MnO 4 in an aqueous solution, most of the reports are focusing on the characterization and application of the formed MnO2 nanoparticles and the effects of irradiation time and pH on the change in absorption spectra of the sample solutions are not investigated clearly. Recently, Okitsu et al. [18] investigated the changes in absorption spectra during sonochemical reduction of MnO 4 , however, the effects of pH on the reduction of MnO4 and the characteristics of the formed MnO2 are not investigated at all. In this study, we have investigated two reduction processes for 2+ MnO during ultrasound irradiation of 4 to MnO2 and MnO2 to Mn argon saturated aqueous solutions under different pH conditions. The reduction processes were discussed on the basis of the
http://dx.doi.org/10.1016/j.ultsonch.2014.03.030 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Abulizi et al., Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO4 in water under different pH conditions, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.030
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reactivity of the reductants formed during the sonolysis of water. The effects of pH on the characteristics of MnO2 formed under different pH conditions were also investigated. 2. Experimental 2.1. Materials KMnO4, H2SO4, NaOH, and H2O2 were obtained from Wako Pure Chemical Industries, Ltd. All the chemicals used here were of analytical grade and were used without further purification. Milli-Q water was used throughout the experiments. 2.2. Synthesis and characterization Ultrasound irradiation was performed using a 65 mm / oscillator (Kaijo; Lot. No. MFG34C3) and a 200 kHz ultrasound generator (Kaijo 4021 type, 200 W; Lot. No. 1033). The calorimetric power measured under a typical experimental condition was 13 W. Details of the irradiation system are described in a previous paper [19]. As a typical synthesis procedure, a 60 mL argon saturated aqueous solution containing 0.1 mM KMnO4 in a glass vessel was irradiated in the water bath, which was maintained at 20 °C by a cold water circulation system (TAITEC CL-150R). The vessel was mounted at a constant position (4 mm from the oscillator) and was closed from air during irradiation. The initial pH of the solution was changed by using H2SO4 or NaOH to control solution pH as pH 2.2 or pH 9.0–9.4, respectively. No pH controlled solution had pH 5.7–6.0. The pH value of the solution was measured by a pH meter (Horiba B-112). The concentration of MnO 4 in the irradiated solution was determined by a spectrophotometric method. The absorption spectra of the irradiated solutions were measured by a UV–vis spectrophotometer (Shimadzu UV-2550) with a 1 cm path length quartz cell. In this case, the measured concentration corresponds to the apparent concentration at the time of the measurement, because the reduction
of MnO 4 could occur even after stopping the irradiation if the sonochemically formed reductants were existed in the irradiated solution. For comparison, the absorption spectra of MnO 4 during stirring experiments with H2O2 were also measured in situ by using an optical fiber spectrophotometer system with a UV–vis probe, CCD detector and light source unit (Ocean Optics TP300-UV/VIS probe, DH-2000-BAL and USB2000+UV–VIS-ES). The morphologies and crystal structures of the particles were characterized by a scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEOL JEM-1011 and JEOL JEM-2010) and X-ray diffractometer (XRD, Shimadzu XRD-6000) with Cu Ka radiation in the range of 10–80°. Fourier transformed infrared (FT-IR) spectra were recorded between 400 and 4000 cm1 by a KBr disk method with a FT-IR spectrophotometer (JASCO, FT/IR-6100). Since the amount of particles formed from the sonolysis of 0.1 mM MnO 4 was too small to characterize particles, the sonolysis of 1 mM MnO 4 was performed to prepare the samples for XRD, SEM, TEM and FT-IR analyses. The sample solutions including particles were centrifuged for 6 min at 13,000 rpm to collect particles and then washed with water and then vacuum dried at 50 °C. The obtained particles were dispersed in water by an ultrasound cleaning bath and then the obtained dispersions were dropped on an ITO glass for SEM sample preparation and on a carbon coated Cu grid for TEM sample preparation. To understand the mechanism of the sonochemical reduction of MnO 4 , the reduction of MnO4 with H2 gas was investigated in the absence of ultrasound, where 0.1 mM KMnO4 aqueous solutions at pH 2.2, 6.0 and 9.4 were bubbled with pure H2 gas at flow rate of more than 200 mL/min for 20 min at ca. 20 °C. 3. Results and discussion 3.1. Reduction of MnO 4 and formation of MnO2 The reduction of MnO 4 in water did not occur in the absence of ultrasound, but the reduction readily started when ultrasound
Fig. 1. Changes in the absorption spectra of 0.1 mM MnO 4 aqueous solutions during ultrasound irradiation at different irradiation times under Ar atmosphere. ((a) changes in absorptions spectra during irradiation of 0–13 min, (a1) 13–25 min) at pH 2.2, ((b) 0–9 min, (b1) 9–14 min) at pH 6.0 and ((c) 0–8 min, (c1) 8–12 min) at pH 9.3.
Please cite this article in press as: A. Abulizi et al., Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO4 in water under different pH conditions, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.030
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irradiation was performed under Ar atmosphere. Fig. 1 shows the changes in the UV–vis absorption spectra during ultrasound irradiation of MnO 4 solutions, where the effects of pH were investigated. As seen in Fig. 1a–c, before ultrasound irradiation, MnO 4 has several characteristic absorption peaks in the range of 450–600 nm. It can be seen that these absorption peaks due to MnO 4 gradually disappeared with increasing irradiation time. On the other hand, a new broad peak at around 360 nm at pH 6.0 and 9.3 or around 380 nm at pH 2.2 emerged. The new peak at around 360 nm can be attributed to colloidal MnO2 particles based on the literatures [12,20]. The formation of d-MnO2 nanoparticles was confirmed by XRD, SEM TEM and FT-IR analyses as seen in the next section. Interestingly, after the absorption attributed to colloidal MnO2 particles achieved a certain intensity, the absorption started to decrease with further irradiation (Fig. 1a1–c1). This behavior indicates that the reduction of MnO2 to Mn(II) ions readily occurs by ultrasound irradiation [18]. According to the obtained result, it is found that the irradiation time must be controlled carefully to synthesize high yield MnO2 particles from the sonochemical reduction of MnO 4 in water. Fig. 2 shows the changes in the absorbance of the sample solutions at 400 nm during ultrasound irradiation, where the absorbance at 400 nm is monitored as an index of the yield of MnO2. The reason why the absorbance at 400 nm is chosen is to reduce the overlap the absorbance between MnO2 and MnO 4 during the conversion from MnO 4 to MnO2. Khan et al. [21] and Kai et al. [22] reported that the absorption intensity corresponding to MnO2 colloidal solution is linearly related to the concentration of MnO2 and obeys the Beer–Lambert law. Taking into account these literatures, we regarded the absorbance at 400 nm as the relative amount of MnO2 in each solution. It can be seen that the formation of MnO2 occurs quickly by irradiation and the formed MnO2 is reduced consecutively by further irradiation. The yield of MnO2 became the maximum at 13 min at pH 2.2, 9 min at pH 6.0, 8 min at pH 9.3, respectively. These times can be considered to be the optimum irradiation time for the effective synthesis of MnO2. In addition, at pH 2.2, the peak attributed to MnO2 formed at the optimum irradiation time red-shifted and became broader compared with those at pH 6.0 and 9.3 as seen in Supplementary data (Fig. S1). During the formation of MnO2 at pH 2.2, the particles aggregate to form secondary particles, however, they were still as colloidal dispersion during the UV–vis analysis. According to the literature [23], the point of zero charge for d-MnO2 is reported to be pH 2.2, where the supporting electrolyte of 10 mM NaClO4, and HClO4 or NaOH were used to adjust the pH of the solution. Therefore, the broad and peak shifted spectrum observed in the
synthesis of MnO2 at pH 2.2 would be considered to be the occurrence of the aggregation of MnO2 particles to form secondary particles. (In our experimental condition, the initial concentrations of + H+, SO2 4 and K at pH 2.2 are calculated to be 6.3, 3.2 and 0.1 mM, respectively.) When the sonochemical reduction of MnO4 is performed in water under Ar atmosphere, the thermolysis of water occurs due to the formation of cavitation bubbles with high temperatures [24],
H2 O!H þ OH
ð1Þ
where H atoms and OH radicals are formed. Since the formed H atoms and OH radicals have high reactivity, some of them recombine to form H2O, H2 and H2O2 as seen in reactions (2)–(4) [25–27],
H þ OH!H2 O
ð2Þ
2H !H2
ð3Þ
2 OH!H2 O2
ð4Þ
H atoms and H2 molecules are well-known as reducing species for the reduction of metal ions. Taking into account the standard electrode potential of MnO 4 , it is expected that the reduction of MnO 4 with H2 would occur in aqueous solutions. To confirm the reactivity of H2, the reduction of MnO 4 with H2 gas was investigated in the absence of ultrasound. In our experimental result seen in Supplementary data (Fig. S2), the rates of reduction of MnO 4 with H2 were extremely slow, indicating that the reduction of MnO 4 with H2 would be negligible under our sonication condition. Since MnO 4 can act as a scavenger for H atoms [28], the following reaction would occur,
MnO4 þ 3H !MnO2 þ OH þ H2 O MnO 4
In addition, since be reduced by H2O2 [29],
ð5Þ
is a strong oxidizing agent,
2MnO4 þ 3H2 O2 !2MnO2 þ 3O2 þ 2OH þ 2H2 O
MnO 4
can
ð6Þ
There are still a lot of arguments for the mechanism of the reduction of MnO2 with H2O2 even in the absence of ultrasound [30]. For example, MnO2 is well known as a catalyst for the degradation of H2O2 [31]. In our study, the catalytic degradation of H2O2 with MnO2 would be negligible, taking into account the reduction behavior of MnO2 seen in Fig. 2: it seems that the sonochemical reduction of MnO2 proceeds very quickly in our synthesis system. The representative reactions of MnO2 with the sonochemically formed reducing speices could be shown as follows,
MnO2 þ 2H !Mn2þ þ 2OH
ð7Þ
MnO2 þ H2 O2 !Mn2þ þ 2OH þ O2
ð8Þ
From Fig. 2, it was confirmed that the rates of MnO2 formation and of MnO2 reduction were in the order of pH 2.2 < pH 6.0 < pH 9.3. In addition, the irradiation time needed for MnO 4 to MnO2 was about 8, 9, 13 min and the irradiation time needed for MnO2 to Mn2+ was about 2, 5, 7 min at pH 9.3, 6.0, 2.2, respectively. From this result, it was confirmed that the rate of the reduction from MnO2 to Mn2+ was about 2–4 times higher than that from MnO 4 to MnO2. To consider the reactions between MnO 4 to MnO2 and MnO2 to Mn2+, we focused on the material balance of electron in the following reactions,
MnðVIIÞ þ 3e !MnðIVÞ
ð9Þ
MnðIVÞ þ 2e !MnðIIÞ
ð10Þ MnO 4,
Fig. 2. Changes in absorbance at 400 nm during ultrasound irradiation at different pH solutions under Ar atmosphere. (j) pH 2.2, ( ) pH 6.0 and ( ) pH 9.3.
where Mn(VII), Mn(IV) and Mn(II) correspond to MnO2 and Mn2+, respectively. Based on reactions (9) and (10), the consumption of electron in reaction (9) should be 1.5 times larger than that
Please cite this article in press as: A. Abulizi et al., Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO4 in water under different pH conditions, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.030
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H2O2 under stirring conditions is shown in Supplementary data (Fig. S3). Since the reduction of MnO 4 with H2O2 cannot stop during the existence of H2O2 under stirring conditions, the reaction was monitored with an in situ optical fiber spectrophotometer system with a CCD detector. Fig. S3 shows that the rate of MnO 4 reduction with H2O2 is faster at pH 9.0 than those at pH 2.2 and 5.7 and it takes relatively long time (about 2–33 min) to complete the reduction of about 50% MnO 4 . From this result, it is suggested that the observed rate under ultrasound irradiation consists of the apparent rate and thus is dependent on the amount of reductants, because the spectrum measurement for the irradiated solution takes several 10 min after stopping irradiation. To discuss the mechanism, the effect of the redox potential of MnO 4 was considered on the basis of Pourbaix diagram [32]. However, the rate of reduction did not obey the redox potential of MnO 4 . Although the effect of pH on the formation of manganese oxides was investigated in the radiolysis of MnO 4 solutions [33], the rate of reduction was not discussed clearly. To discuss the mechanism why pH value affected to the rate of reduction, further experimental investigation is required.
Fig. 3. XRD patterns of the samples prepared by ultrasound irradiation under different pH conditions: (a) pH 2.2, (b) pH 6.0 and (c) pH 9.3.
in reaction (10), suggesting that the reaction (10) could be faster than reaction (9). This tendency is roughly in agreement with the results seen in Fig. 2. As seen in Figs. 1 and 2, the apparent rates of MnO 4 reduction is in the order of pH 2.2 < pH 6.0 < pH 9.3. To consider this behavior, the effect of pH on the rate of MnO 4 reduction with H2O2 was investigated under stirring conditions without ultrasound irradiation. The change in the concentration of MnO 4 in the presence of
3.2. Material characteristics To understand the characteristics of the sonochemically formed particles, the analysis by XRD, SEM, TEM and FT-IR was performed.
a
b
c
Fig. 4. SEM (left) and TEM (right) images of sonochemically synthesized MnO2 at (a) pH 2.2, (b) pH 6.0 and (c) pH 9.3.
Please cite this article in press as: A. Abulizi et al., Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO4 in water under different pH conditions, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.030
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4. Conclusion
Fig. 5. FT-IR spectra of MnO2 samples formed under different pH conditions: (a) pH 2.2, (b) pH 6.0 and (c) pH 9.3.
Fig. 3 shows the XRD patterns of the prepared particles under different pH conditions. A broad diffraction peak located at around 23° would be due to a glass substrate used here, because of the low amounts of particles. All of the patterns show that the prepared particles have low crystallinity. However, weak two peaks at 37° and 67° were observed. These peaks are in good agreement with those of d-MnO2 [JCPDS No. 80-1098] [34,35]. The XRD pattern at pH 6.0 is almost the same as that at pH 9.3. However, the particles prepared at pH 2.2 have slightly broader two peaks compared with those at pH 6.0 and 9.3, indicating that d-MnO2 with poorer crystallinity is formed at pH 2.2 and the crystallinity of MnO2 is affected by the solution pH. Nam et al. reported that the ultrasound assisted mixing reaction of Mn(VII) and Mn(II) produced MnO2 particles in an aqueous solution at less than pH 8.8 [16]. The XRD patterns were almost the same as the results obtained here, although they characterized the crystal structure of the formed MnO2 as a-MnO2. Although a simple mixing method of Mn(VII) and Mn(II) has been often used to prepare MnO2, in our synthesis method, only one starting chemical, Mn(VII), is enough to synthesize MnO2 at 20 °C. Fig. 4 shows the SEM images of the sonochemically prepared MnO2 under different pH conditions. It was clear that the pH of the solution affected on the size, shape and morphology of the formed MnO2. At pH 2.2, the aggregation of sheet like nanostructures was observed. However, at pH 6.0, MnO2 nanoparticles with 50–150 nm were observed. At pH 9.3, MnO2 cubic or polyhedron particles with 0.2–1 lm were observed. Based on the results of Fig. S1 and the SEM images, the colloidal stability of MnO2 was not good at pH 2.2 and the size of MnO2 was relatively large at pH 2.2 and 9.3. Fig. 4 also shows the TEM images. As seen in the TEM images, the aggregated sheet-like structures or needle-like structures, spherical nanoparticles and large irregular shape materials were observed at pH 2.2, 6.0 and 9.3, respectively. As an example, a high magnification TEM image of the particles prepared at pH 6.0 is also shown in Supplementary data (Fig. S4), where an unevenness surface can be observed. To investigate the chemical structure of the particles, the FT-IR analysis was performed. Fig. 5 shows FT-IR spectra of the particles prepared at pH 2.2, 6.0 and 9.3, respectively. The absorption band located at 500–800 cm1 is the characteristic absorption band of Mn–O bending vibrations of [MnO6] octahedral in d-MnO2 [34]. Two absorption bands located at around 3400 and 1620 cm1 correspond to O–H and H–O–H [34]. On the other hand, it can be seen that the FT-IR spectrum observed in the particles prepared at pH 2.2 was unclear compared with those prepared at pH 6.0 and 9.3. We were not able to explain this reason at present.
MnO2 nanoparticles were synthesized by the sonochemical reduction of MnO 4 in water under Ar atmosphere at 20 °C and the influences of the initial solution pH on the reduction of MnO 4 and morphology of the formed MnO2 were investigated. By analyzing XRD, SEM, TEM and FT-IR, the formation of d-MnO2 with different size and morphology was confirmed: the morphologies of d-MnO2 changed from sheet-like or needle-like nanostructures (pH 2.2) to spherical nanoparticles (pH 6.0) and finally to cubic or polyhedron nanoparticles (pH 9.3). It was also suggested that the 2+ reduction of MnO would be due to 4 to MnO2 and MnO2 to Mn the reactions with the sonochemically formed H2O2 and H atoms. Since the reduction of MnO2 to Mn2+ proceeded quickly under ultrasound irradiation, it was confirmed that the irradiation time was one of the most important factors to synthesize MnO2 particles effectively: the irradiation time must be controlled carefully to get high yield of MnO2 in the sonochemical synthesis method. The sonochemical method for MnO2 synthesis could be considered to be a green method compared with a conventional synthesis method, because the synthesis can be done at room temperature without a chemical reductant and Mn(II). Acknowledgments We acknowledge Masako Konagaya (Osaka Prefecture University) for the help of optical fiber spectrophotometer and FT-IR measurements. Kenji Okitsu acknowledges the support of the JSPS KAKENHI Grant No. 22560760(Japan). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ultsonch.2014.03. 030. References [1] R.N. Rddy, R.G. Reddy, J. Power Sources 132 (2004) 315. [2] Y. Chen, C. Liu, F. Li, H. Cheng, J. Alloys Compd. 397 (2005) 282. [3] X. Zhang, Y. Duan, H. Guan, S. Liu, B. Wen, J. Magn. Magn. Mater. 311 (2007) 507. [4] C. Xu, B. Li, H. Du, F. Kang, Y. Zeng, J. Power Sources 180 (2008) 664. [5] V. Subramanian, H. Zhu, B. Wei, Chem. Phys. Lett. 453 (2008) 242. [6] A. Yuan, X. Wang, Y. Wang, J. Hu, Electrochim. Acta 54 (2009) 1021. [7] A. Zolfaghari, F. Ataherian, M. Ghaemi, A. Gholami, Electrochim. Acta 52 (2007) 2806. [8] D. Yan, P. Yan, S. Cheng, J.T. Chen, R.F. Zhuo, J.J. Feng, G.A. Zhang, Cryst. Growth Des. 9 (2009) 218. [9] K.S. Suslick, G.J. Price, Annu. Rev. Mater. 29 (1999) 295. [10] A. Abulizi, K. Okitsu, J.-J. Zhu, Ultrason. Sonochem. 21 (2014) 1174. [11] A. Gedanken, Ultrason. Sonochem. 14 (2007) 418. [12] T. Fujimoto, Y. Mizukoshi, Y. Nagata, Y. Maeda, R. Oshima, Scripta Mater. 44 (2001) 2183. [13] S.M. Zhu, Z.Y. Zhou, D. Zhang, H.H. Wang, Microporous Mesoporous Mater. 95 (2006) 257. [14] S.M. Zhu, H.S. Zhou, M. Hibino, I. Honma, M. Ichihara, Adv. Funct. Mater. 15 (2005) 381. [15] B.G.S. Raj, A.M. Asiri, A.H. Qusti, J.J. Wu, S. Anandan, Ultrason. Sonochem. (2013), http://dx.doi.org/10.1016/j.ultsonch.2013.11.018. [16] H.S. Nam, J.K. Yoon, J.M. Ko, J.D. Kim, Mater. Chem. Phys. 123 (2010) 331. [17] J.G. Deng, L. Zhang, H.X. Dai, Y.S. Xia, H.Y. Jiang, H. Zhang, H. He, J. Phys. Chem. C 114 (2010) 2694. [18] K. Okitsu, M. Iwatani, B. Nanzai, R. Nishimura, Y. Maeda, Ultrason. Sonochem. 16 (2009) 387. [19] K. Okitsu, A. Yue, S. Tanabe, H. Matsumoto, Y. Yobiko, Y. Yoo, Bull. Chem. Soc. Jpn. 75 (2002) 2289. [20] J.Z. Sostaric, P. Mulvaney, F. Grieser, J. Cem, Soc. Faraday Trans. 91 (1995) 2843. [21] Z. Khan, P. Kumar, K. Din, J. Colloid Interface Sci. 290 (2005) 184–189. [22] K. Kai, Y. Yoshida, H. Kageyama, G. Saito, T. Ishigaki, Y. Furukawa, J. Kawamata, J. Am. Chem. Soc. 130 (2008) 15938–15943. [23] P. Mulvaney, L. Denison, F. Grieser, R. Cooper, J.V. Sanders, D. Meisel, J. Colloid Interface Sci. 121 (1988) 70.
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Please cite this article in press as: A. Abulizi et al., Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO4 in water under different pH conditions, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.030
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Short Communication
Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO 4 in water under different pH conditions Abulikemu Abulizi a,b,c, Guo Hai Yang b, Kenji Okitsu a,⇑, Jun-Jie Zhu b,⇑ a
Graduate School of Engineering, Osaka Prefecture University, Osaka 599-8531, Japan State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China c College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, PR China b
a r t i c l e
i n f o
Article history: Received 27 December 2013 Received in revised form 26 March 2014 Accepted 27 March 2014 Available online xxxx Keywords: Sonochemical reduction Permanganate Manganese dioxide Effect of pH
a b s t r a c t MnO2 was synthesized by sonochemical reduction of MnO 4 in water under Ar atmosphere at 20 °C, where the effects of solution pH on the reduction of MnO 4 were investigated. The obtained XRD results showed that poor crystallinity d-MnO2 was formed at pH 2.2, 6.0 and 9.3. When solution pH was increased from 2.2 to 9.3, the morphologies of d-MnO2 changed from aggregated sheet-like or needle-like structures to spherical nanoparticles and finally to cubic or polyhedron nanoparticles. After further irradiation, MnO2 was readily reduced to Mn2+. It was confirmed that H2O2 and H atoms formed in the 2+ sonolysis of water acted as reductants for both reduction for MnO 4 to MnO2 and MnO2 to Mn . The optimum irradiation time for the effective synthesis of MnO2 was 13 min at pH 2.2, 9 min at pH 6.0, 8 min at pH 9.3, respectively. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Manganese oxides can be applied in catalysts, molecular-sieves, ion-sieves, batteries and magnetic materials due to their excellent physicochemical properties [1,2]. MnO2 is also one of the most important materials and a number of researchers pay attention to the influence of MnO2 addition on the electromagnetic properties of ferrite materials [3]. Various approaches have been developed to prepare nanoscale MnO2 such as self-reacting microemulsion [4], precipitation [5], room temperature solid reactions [6], sonochemical [7] and hydrothermal methods [8]. A sonochemical method is known as a unique synthesis method based on cavitation phenomena which are induced by ultrasound irradiation in a liquid. They provide interesting chemical and physical effects due to cavitation bubbles with high temperatures and high pressures as well as the formation of shear forces, microjets and shock waves in a liquid. Such interesting effects have been actively applied in the field of the synthesis of inorganic nanoparticles and nanostructured materials [9–11]. MnO2 nanoparticles have also been synthesized by several sonochemical methods based on the following reactions: (1) sonochemical reduction of MnO 4 [12–15], (2) ultrasound-assisted reaction between MnO4 and Mn2+ [16], and (3) ultrasound-assisted oxidation of Mn2+ ⇑ Corresponding authors. Tel.: +81 722521161 (K. Okitsu). E-mail addresses: [email protected] (K. Okitsu), [email protected] (J.-J. Zhu).
[7,17]. For example, Zhu et al. [14] reported the sonochemical reduction of MnO 4 to MnO2 in the presence of ordered mesoporous carbon template to suppress the growth of MnO2 nanoparticles. Raj et al. [15] reported that spherical MnO2 particles in the range from 10 to 20 nm were formed by the sonochemical reduction of MnO 4 in the presence of polyethylene glycol. It was also reported that the sonochemically prepared MnO2 nanoparticles act as excellent electrochemical capacitors [14,15]. From environmental and energetic points of view, sonochemical reduction methods with MnO 4 can be considered as green approaches for synthesizing MnO2, because a specific chemical reductant is not needed and synthesis can be performed under mild experimental conditions such as relatively low reaction temperature and ambient pressure. Although there are several reports using sonochemical reduction of MnO 4 in an aqueous solution, most of the reports are focusing on the characterization and application of the formed MnO2 nanoparticles and the effects of irradiation time and pH on the change in absorption spectra of the sample solutions are not investigated clearly. Recently, Okitsu et al. [18] investigated the changes in absorption spectra during sonochemical reduction of MnO 4 , however, the effects of pH on the reduction of MnO4 and the characteristics of the formed MnO2 are not investigated at all. In this study, we have investigated two reduction processes for 2+ MnO during ultrasound irradiation of 4 to MnO2 and MnO2 to Mn argon saturated aqueous solutions under different pH conditions. The reduction processes were discussed on the basis of the
http://dx.doi.org/10.1016/j.ultsonch.2014.03.030 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Abulizi et al., Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO4 in water under different pH conditions, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.030
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reactivity of the reductants formed during the sonolysis of water. The effects of pH on the characteristics of MnO2 formed under different pH conditions were also investigated. 2. Experimental 2.1. Materials KMnO4, H2SO4, NaOH, and H2O2 were obtained from Wako Pure Chemical Industries, Ltd. All the chemicals used here were of analytical grade and were used without further purification. Milli-Q water was used throughout the experiments. 2.2. Synthesis and characterization Ultrasound irradiation was performed using a 65 mm / oscillator (Kaijo; Lot. No. MFG34C3) and a 200 kHz ultrasound generator (Kaijo 4021 type, 200 W; Lot. No. 1033). The calorimetric power measured under a typical experimental condition was 13 W. Details of the irradiation system are described in a previous paper [19]. As a typical synthesis procedure, a 60 mL argon saturated aqueous solution containing 0.1 mM KMnO4 in a glass vessel was irradiated in the water bath, which was maintained at 20 °C by a cold water circulation system (TAITEC CL-150R). The vessel was mounted at a constant position (4 mm from the oscillator) and was closed from air during irradiation. The initial pH of the solution was changed by using H2SO4 or NaOH to control solution pH as pH 2.2 or pH 9.0–9.4, respectively. No pH controlled solution had pH 5.7–6.0. The pH value of the solution was measured by a pH meter (Horiba B-112). The concentration of MnO 4 in the irradiated solution was determined by a spectrophotometric method. The absorption spectra of the irradiated solutions were measured by a UV–vis spectrophotometer (Shimadzu UV-2550) with a 1 cm path length quartz cell. In this case, the measured concentration corresponds to the apparent concentration at the time of the measurement, because the reduction
of MnO 4 could occur even after stopping the irradiation if the sonochemically formed reductants were existed in the irradiated solution. For comparison, the absorption spectra of MnO 4 during stirring experiments with H2O2 were also measured in situ by using an optical fiber spectrophotometer system with a UV–vis probe, CCD detector and light source unit (Ocean Optics TP300-UV/VIS probe, DH-2000-BAL and USB2000+UV–VIS-ES). The morphologies and crystal structures of the particles were characterized by a scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEOL JEM-1011 and JEOL JEM-2010) and X-ray diffractometer (XRD, Shimadzu XRD-6000) with Cu Ka radiation in the range of 10–80°. Fourier transformed infrared (FT-IR) spectra were recorded between 400 and 4000 cm1 by a KBr disk method with a FT-IR spectrophotometer (JASCO, FT/IR-6100). Since the amount of particles formed from the sonolysis of 0.1 mM MnO 4 was too small to characterize particles, the sonolysis of 1 mM MnO 4 was performed to prepare the samples for XRD, SEM, TEM and FT-IR analyses. The sample solutions including particles were centrifuged for 6 min at 13,000 rpm to collect particles and then washed with water and then vacuum dried at 50 °C. The obtained particles were dispersed in water by an ultrasound cleaning bath and then the obtained dispersions were dropped on an ITO glass for SEM sample preparation and on a carbon coated Cu grid for TEM sample preparation. To understand the mechanism of the sonochemical reduction of MnO 4 , the reduction of MnO4 with H2 gas was investigated in the absence of ultrasound, where 0.1 mM KMnO4 aqueous solutions at pH 2.2, 6.0 and 9.4 were bubbled with pure H2 gas at flow rate of more than 200 mL/min for 20 min at ca. 20 °C. 3. Results and discussion 3.1. Reduction of MnO 4 and formation of MnO2 The reduction of MnO 4 in water did not occur in the absence of ultrasound, but the reduction readily started when ultrasound
Fig. 1. Changes in the absorption spectra of 0.1 mM MnO 4 aqueous solutions during ultrasound irradiation at different irradiation times under Ar atmosphere. ((a) changes in absorptions spectra during irradiation of 0–13 min, (a1) 13–25 min) at pH 2.2, ((b) 0–9 min, (b1) 9–14 min) at pH 6.0 and ((c) 0–8 min, (c1) 8–12 min) at pH 9.3.
Please cite this article in press as: A. Abulizi et al., Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO4 in water under different pH conditions, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.030
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irradiation was performed under Ar atmosphere. Fig. 1 shows the changes in the UV–vis absorption spectra during ultrasound irradiation of MnO 4 solutions, where the effects of pH were investigated. As seen in Fig. 1a–c, before ultrasound irradiation, MnO 4 has several characteristic absorption peaks in the range of 450–600 nm. It can be seen that these absorption peaks due to MnO 4 gradually disappeared with increasing irradiation time. On the other hand, a new broad peak at around 360 nm at pH 6.0 and 9.3 or around 380 nm at pH 2.2 emerged. The new peak at around 360 nm can be attributed to colloidal MnO2 particles based on the literatures [12,20]. The formation of d-MnO2 nanoparticles was confirmed by XRD, SEM TEM and FT-IR analyses as seen in the next section. Interestingly, after the absorption attributed to colloidal MnO2 particles achieved a certain intensity, the absorption started to decrease with further irradiation (Fig. 1a1–c1). This behavior indicates that the reduction of MnO2 to Mn(II) ions readily occurs by ultrasound irradiation [18]. According to the obtained result, it is found that the irradiation time must be controlled carefully to synthesize high yield MnO2 particles from the sonochemical reduction of MnO 4 in water. Fig. 2 shows the changes in the absorbance of the sample solutions at 400 nm during ultrasound irradiation, where the absorbance at 400 nm is monitored as an index of the yield of MnO2. The reason why the absorbance at 400 nm is chosen is to reduce the overlap the absorbance between MnO2 and MnO 4 during the conversion from MnO 4 to MnO2. Khan et al. [21] and Kai et al. [22] reported that the absorption intensity corresponding to MnO2 colloidal solution is linearly related to the concentration of MnO2 and obeys the Beer–Lambert law. Taking into account these literatures, we regarded the absorbance at 400 nm as the relative amount of MnO2 in each solution. It can be seen that the formation of MnO2 occurs quickly by irradiation and the formed MnO2 is reduced consecutively by further irradiation. The yield of MnO2 became the maximum at 13 min at pH 2.2, 9 min at pH 6.0, 8 min at pH 9.3, respectively. These times can be considered to be the optimum irradiation time for the effective synthesis of MnO2. In addition, at pH 2.2, the peak attributed to MnO2 formed at the optimum irradiation time red-shifted and became broader compared with those at pH 6.0 and 9.3 as seen in Supplementary data (Fig. S1). During the formation of MnO2 at pH 2.2, the particles aggregate to form secondary particles, however, they were still as colloidal dispersion during the UV–vis analysis. According to the literature [23], the point of zero charge for d-MnO2 is reported to be pH 2.2, where the supporting electrolyte of 10 mM NaClO4, and HClO4 or NaOH were used to adjust the pH of the solution. Therefore, the broad and peak shifted spectrum observed in the
synthesis of MnO2 at pH 2.2 would be considered to be the occurrence of the aggregation of MnO2 particles to form secondary particles. (In our experimental condition, the initial concentrations of + H+, SO2 4 and K at pH 2.2 are calculated to be 6.3, 3.2 and 0.1 mM, respectively.) When the sonochemical reduction of MnO4 is performed in water under Ar atmosphere, the thermolysis of water occurs due to the formation of cavitation bubbles with high temperatures [24],
H2 O!H þ OH
ð1Þ
where H atoms and OH radicals are formed. Since the formed H atoms and OH radicals have high reactivity, some of them recombine to form H2O, H2 and H2O2 as seen in reactions (2)–(4) [25–27],
H þ OH!H2 O
ð2Þ
2H !H2
ð3Þ
2 OH!H2 O2
ð4Þ
H atoms and H2 molecules are well-known as reducing species for the reduction of metal ions. Taking into account the standard electrode potential of MnO 4 , it is expected that the reduction of MnO 4 with H2 would occur in aqueous solutions. To confirm the reactivity of H2, the reduction of MnO 4 with H2 gas was investigated in the absence of ultrasound. In our experimental result seen in Supplementary data (Fig. S2), the rates of reduction of MnO 4 with H2 were extremely slow, indicating that the reduction of MnO 4 with H2 would be negligible under our sonication condition. Since MnO 4 can act as a scavenger for H atoms [28], the following reaction would occur,
MnO4 þ 3H !MnO2 þ OH þ H2 O MnO 4
In addition, since be reduced by H2O2 [29],
ð5Þ
is a strong oxidizing agent,
2MnO4 þ 3H2 O2 !2MnO2 þ 3O2 þ 2OH þ 2H2 O
MnO 4
can
ð6Þ
There are still a lot of arguments for the mechanism of the reduction of MnO2 with H2O2 even in the absence of ultrasound [30]. For example, MnO2 is well known as a catalyst for the degradation of H2O2 [31]. In our study, the catalytic degradation of H2O2 with MnO2 would be negligible, taking into account the reduction behavior of MnO2 seen in Fig. 2: it seems that the sonochemical reduction of MnO2 proceeds very quickly in our synthesis system. The representative reactions of MnO2 with the sonochemically formed reducing speices could be shown as follows,
MnO2 þ 2H !Mn2þ þ 2OH
ð7Þ
MnO2 þ H2 O2 !Mn2þ þ 2OH þ O2
ð8Þ
From Fig. 2, it was confirmed that the rates of MnO2 formation and of MnO2 reduction were in the order of pH 2.2 < pH 6.0 < pH 9.3. In addition, the irradiation time needed for MnO 4 to MnO2 was about 8, 9, 13 min and the irradiation time needed for MnO2 to Mn2+ was about 2, 5, 7 min at pH 9.3, 6.0, 2.2, respectively. From this result, it was confirmed that the rate of the reduction from MnO2 to Mn2+ was about 2–4 times higher than that from MnO 4 to MnO2. To consider the reactions between MnO 4 to MnO2 and MnO2 to Mn2+, we focused on the material balance of electron in the following reactions,
MnðVIIÞ þ 3e !MnðIVÞ
ð9Þ
MnðIVÞ þ 2e !MnðIIÞ
ð10Þ MnO 4,
Fig. 2. Changes in absorbance at 400 nm during ultrasound irradiation at different pH solutions under Ar atmosphere. (j) pH 2.2, ( ) pH 6.0 and ( ) pH 9.3.
where Mn(VII), Mn(IV) and Mn(II) correspond to MnO2 and Mn2+, respectively. Based on reactions (9) and (10), the consumption of electron in reaction (9) should be 1.5 times larger than that
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H2O2 under stirring conditions is shown in Supplementary data (Fig. S3). Since the reduction of MnO 4 with H2O2 cannot stop during the existence of H2O2 under stirring conditions, the reaction was monitored with an in situ optical fiber spectrophotometer system with a CCD detector. Fig. S3 shows that the rate of MnO 4 reduction with H2O2 is faster at pH 9.0 than those at pH 2.2 and 5.7 and it takes relatively long time (about 2–33 min) to complete the reduction of about 50% MnO 4 . From this result, it is suggested that the observed rate under ultrasound irradiation consists of the apparent rate and thus is dependent on the amount of reductants, because the spectrum measurement for the irradiated solution takes several 10 min after stopping irradiation. To discuss the mechanism, the effect of the redox potential of MnO 4 was considered on the basis of Pourbaix diagram [32]. However, the rate of reduction did not obey the redox potential of MnO 4 . Although the effect of pH on the formation of manganese oxides was investigated in the radiolysis of MnO 4 solutions [33], the rate of reduction was not discussed clearly. To discuss the mechanism why pH value affected to the rate of reduction, further experimental investigation is required.
Fig. 3. XRD patterns of the samples prepared by ultrasound irradiation under different pH conditions: (a) pH 2.2, (b) pH 6.0 and (c) pH 9.3.
in reaction (10), suggesting that the reaction (10) could be faster than reaction (9). This tendency is roughly in agreement with the results seen in Fig. 2. As seen in Figs. 1 and 2, the apparent rates of MnO 4 reduction is in the order of pH 2.2 < pH 6.0 < pH 9.3. To consider this behavior, the effect of pH on the rate of MnO 4 reduction with H2O2 was investigated under stirring conditions without ultrasound irradiation. The change in the concentration of MnO 4 in the presence of
3.2. Material characteristics To understand the characteristics of the sonochemically formed particles, the analysis by XRD, SEM, TEM and FT-IR was performed.
a
b
c
Fig. 4. SEM (left) and TEM (right) images of sonochemically synthesized MnO2 at (a) pH 2.2, (b) pH 6.0 and (c) pH 9.3.
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4. Conclusion
Fig. 5. FT-IR spectra of MnO2 samples formed under different pH conditions: (a) pH 2.2, (b) pH 6.0 and (c) pH 9.3.
Fig. 3 shows the XRD patterns of the prepared particles under different pH conditions. A broad diffraction peak located at around 23° would be due to a glass substrate used here, because of the low amounts of particles. All of the patterns show that the prepared particles have low crystallinity. However, weak two peaks at 37° and 67° were observed. These peaks are in good agreement with those of d-MnO2 [JCPDS No. 80-1098] [34,35]. The XRD pattern at pH 6.0 is almost the same as that at pH 9.3. However, the particles prepared at pH 2.2 have slightly broader two peaks compared with those at pH 6.0 and 9.3, indicating that d-MnO2 with poorer crystallinity is formed at pH 2.2 and the crystallinity of MnO2 is affected by the solution pH. Nam et al. reported that the ultrasound assisted mixing reaction of Mn(VII) and Mn(II) produced MnO2 particles in an aqueous solution at less than pH 8.8 [16]. The XRD patterns were almost the same as the results obtained here, although they characterized the crystal structure of the formed MnO2 as a-MnO2. Although a simple mixing method of Mn(VII) and Mn(II) has been often used to prepare MnO2, in our synthesis method, only one starting chemical, Mn(VII), is enough to synthesize MnO2 at 20 °C. Fig. 4 shows the SEM images of the sonochemically prepared MnO2 under different pH conditions. It was clear that the pH of the solution affected on the size, shape and morphology of the formed MnO2. At pH 2.2, the aggregation of sheet like nanostructures was observed. However, at pH 6.0, MnO2 nanoparticles with 50–150 nm were observed. At pH 9.3, MnO2 cubic or polyhedron particles with 0.2–1 lm were observed. Based on the results of Fig. S1 and the SEM images, the colloidal stability of MnO2 was not good at pH 2.2 and the size of MnO2 was relatively large at pH 2.2 and 9.3. Fig. 4 also shows the TEM images. As seen in the TEM images, the aggregated sheet-like structures or needle-like structures, spherical nanoparticles and large irregular shape materials were observed at pH 2.2, 6.0 and 9.3, respectively. As an example, a high magnification TEM image of the particles prepared at pH 6.0 is also shown in Supplementary data (Fig. S4), where an unevenness surface can be observed. To investigate the chemical structure of the particles, the FT-IR analysis was performed. Fig. 5 shows FT-IR spectra of the particles prepared at pH 2.2, 6.0 and 9.3, respectively. The absorption band located at 500–800 cm1 is the characteristic absorption band of Mn–O bending vibrations of [MnO6] octahedral in d-MnO2 [34]. Two absorption bands located at around 3400 and 1620 cm1 correspond to O–H and H–O–H [34]. On the other hand, it can be seen that the FT-IR spectrum observed in the particles prepared at pH 2.2 was unclear compared with those prepared at pH 6.0 and 9.3. We were not able to explain this reason at present.
MnO2 nanoparticles were synthesized by the sonochemical reduction of MnO 4 in water under Ar atmosphere at 20 °C and the influences of the initial solution pH on the reduction of MnO 4 and morphology of the formed MnO2 were investigated. By analyzing XRD, SEM, TEM and FT-IR, the formation of d-MnO2 with different size and morphology was confirmed: the morphologies of d-MnO2 changed from sheet-like or needle-like nanostructures (pH 2.2) to spherical nanoparticles (pH 6.0) and finally to cubic or polyhedron nanoparticles (pH 9.3). It was also suggested that the 2+ reduction of MnO would be due to 4 to MnO2 and MnO2 to Mn the reactions with the sonochemically formed H2O2 and H atoms. Since the reduction of MnO2 to Mn2+ proceeded quickly under ultrasound irradiation, it was confirmed that the irradiation time was one of the most important factors to synthesize MnO2 particles effectively: the irradiation time must be controlled carefully to get high yield of MnO2 in the sonochemical synthesis method. The sonochemical method for MnO2 synthesis could be considered to be a green method compared with a conventional synthesis method, because the synthesis can be done at room temperature without a chemical reductant and Mn(II). Acknowledgments We acknowledge Masako Konagaya (Osaka Prefecture University) for the help of optical fiber spectrophotometer and FT-IR measurements. Kenji Okitsu acknowledges the support of the JSPS KAKENHI Grant No. 22560760(Japan). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ultsonch.2014.03. 030. References [1] R.N. Rddy, R.G. Reddy, J. Power Sources 132 (2004) 315. [2] Y. Chen, C. Liu, F. Li, H. Cheng, J. Alloys Compd. 397 (2005) 282. [3] X. Zhang, Y. Duan, H. Guan, S. Liu, B. Wen, J. Magn. Magn. Mater. 311 (2007) 507. [4] C. Xu, B. Li, H. Du, F. Kang, Y. Zeng, J. Power Sources 180 (2008) 664. [5] V. Subramanian, H. Zhu, B. Wei, Chem. Phys. Lett. 453 (2008) 242. [6] A. Yuan, X. Wang, Y. Wang, J. Hu, Electrochim. Acta 54 (2009) 1021. [7] A. Zolfaghari, F. Ataherian, M. Ghaemi, A. Gholami, Electrochim. Acta 52 (2007) 2806. [8] D. Yan, P. Yan, S. Cheng, J.T. Chen, R.F. Zhuo, J.J. Feng, G.A. Zhang, Cryst. Growth Des. 9 (2009) 218. [9] K.S. Suslick, G.J. Price, Annu. Rev. Mater. 29 (1999) 295. [10] A. Abulizi, K. Okitsu, J.-J. Zhu, Ultrason. Sonochem. 21 (2014) 1174. [11] A. Gedanken, Ultrason. Sonochem. 14 (2007) 418. [12] T. Fujimoto, Y. Mizukoshi, Y. Nagata, Y. Maeda, R. Oshima, Scripta Mater. 44 (2001) 2183. [13] S.M. Zhu, Z.Y. Zhou, D. Zhang, H.H. Wang, Microporous Mesoporous Mater. 95 (2006) 257. [14] S.M. Zhu, H.S. Zhou, M. Hibino, I. Honma, M. Ichihara, Adv. Funct. Mater. 15 (2005) 381. [15] B.G.S. Raj, A.M. Asiri, A.H. Qusti, J.J. Wu, S. Anandan, Ultrason. Sonochem. (2013), http://dx.doi.org/10.1016/j.ultsonch.2013.11.018. [16] H.S. Nam, J.K. Yoon, J.M. Ko, J.D. Kim, Mater. Chem. Phys. 123 (2010) 331. [17] J.G. Deng, L. Zhang, H.X. Dai, Y.S. Xia, H.Y. Jiang, H. Zhang, H. He, J. Phys. Chem. C 114 (2010) 2694. [18] K. Okitsu, M. Iwatani, B. Nanzai, R. Nishimura, Y. Maeda, Ultrason. Sonochem. 16 (2009) 387. [19] K. Okitsu, A. Yue, S. Tanabe, H. Matsumoto, Y. Yobiko, Y. Yoo, Bull. Chem. Soc. Jpn. 75 (2002) 2289. [20] J.Z. Sostaric, P. Mulvaney, F. Grieser, J. Cem, Soc. Faraday Trans. 91 (1995) 2843. [21] Z. Khan, P. Kumar, K. Din, J. Colloid Interface Sci. 290 (2005) 184–189. [22] K. Kai, Y. Yoshida, H. Kageyama, G. Saito, T. Ishigaki, Y. Furukawa, J. Kawamata, J. Am. Chem. Soc. 130 (2008) 15938–15943. [23] P. Mulvaney, L. Denison, F. Grieser, R. Cooper, J.V. Sanders, D. Meisel, J. Colloid Interface Sci. 121 (1988) 70.
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