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A novel sonochemical synthesis of antlerite nanorods
Ultrasonics Sonochemistry 22 (2015) 30–34
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Short Communication
A novel sonochemical synthesis of antlerite nanorods Elad Segal, Ilana Perelshtein, Aharon Gedanken ⇑ Department of Chemistry, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel
a r t i c l e
i n f o
Article history: Received 10 February 2014 Received in revised form 7 July 2014 Accepted 23 July 2014 Available online 31 July 2014 Keywords: Antlerite Nanorods Minerals Sonochemistry
a b s t r a c t Antlerite – Cu3(OH)4SO4 was prepared, for the first time, by the sonochemical method from an aqueous solution of CuSO4, without any additives. The source of OH is a result of protonation of SO24 forming HSO4 and OH . The extreme local conditions inside the cavity that are developed during the bubble collapse (pressure is above 1000 atm and the temperature is higher than 5000 K) lead to the formation of the crystalline mineral. A suggested mechanism for the mineral formation is proposed. Due to the collapse of the bubbles, the distances between the opposite charge Cu2+ and SO24 ions is shortened and a crystallization process is initiated. In addition, the reaction is a one-step process with short irradiation time of less than 30 min. The chemo-physical analysis of the sonochemically obtained product has revealed the presence of single phase antlerite nanorods. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Previous reports on the sonochemical reduction of metallic ions by ultrasound have been presented and discussed. These reports include the sonochemical reductions of metallic ions that were carried out without a reducing agent. For example, the reduction of Ag+ in an aqueous solution of AgNO3 and the deposition of metallic silver upon silica sub-micron spheres by ultrasonic method under argon atmosphere was reported [1]. Fabrication of Pt NPs (nanoparticles) with no stabilizer or capping agent, by reducing PtIV to Pt0 in pure water under Ar atmosphere, in a 2-step process [2] was reported. In the case of copper the sonochemical reduction of Cu2+ from CuSO4 and CuCl2 as starting compounds, has led to the formation of Copper hydride (CuH) [3]. Needless to point out that many examples of the sonochemical reduction of metallic ions in the presence of a reducing agent have been published [4–9]. The sonochemical reduction of metallic ions is of great interest because it is considered a cheap technique which yields small NPs of the corresponding metal. In the absence of a reducing agent the reaction occurs as a result of the collapse of the cavitation bubbles, leading to the homolytical cleavage of water into H and OH radicals [10–12]. It is clear that the number of metals that can be reduced just by radicals formed from the dissociation of water is limited to those having a highly positive standard reduction potential.
⇑ Corresponding author. E-mail address: [email protected] (A. Gedanken). URL: http://www.ch.biu.ac.il/gedanken (A. Gedanken). http://dx.doi.org/10.1016/j.ultsonch.2014.07.020 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Hasin [3] has reported on the sonochemical synthesis of CuH when the reaction was conducted under argon. In the current paper, we report on the same reaction employing the same reactant, CuSO4 as described in Ref. [3] and obtained Cu3(OH)4SO4. The main difference between the work of Hasin and ours, is that we have performed the reaction under air and as a result a different product, the mineral antlerite (Cu3(OH)4SO4) was synthesized. This mineral is well known – and was synthesized previously in a few different ways. Koga and co-workers [13] have shown a largescale microwave-assisted hydrothermal method for the synthesis of micron-size needle-like (Cu3(OH)4SO4). Another research [14] has demonstrated the use of urea and carbonate for the precipitation of antlerite. Vilminot et al. [15] has prepared antlerite in a long process of six days, under 170 °C with NaOH as the hydroxide source. The main disadvantages of the available techniques for the synthesis of antlerite are either long processes or the involvement of special chemicals added as a source of hydroxide. The current article introduces a new preparation technique, i.e. the sonochemical method. The current manuscript reports on a onestep process conducted under ultrasonic waves to produce the mineral antlerite from CuSO4 aqueous solution in a non-inert atmosphere within less than 30 min. Nanorods of up to 150 nm width and 1.5 lm in length were prepared and characterized by common materials science techniques. Herein we report on applying ultrasonic waves on a solution of inorganic salt under air. When ultrasonic irradiation is applied to an aqueous solution of an inorganic salt, the acoustic bubbles that are being formed during the process adsorb the ions of the salt on their surface [16,17]. When the bubbles collapse, extreme local (inside the cavity) conditions of pressure (above 1000 atm) and
E. Segal et al. / Ultrasonics Sonochemistry 22 (2015) 30–34
high temperature (higher than 5000 K) occur next to the ions [11]. So unlike sonochemical reaction of volatile materials that happen inside the collapsing bubble, in our case the reaction takes place on the surface of the collapsing bubble. 2. Experimental section 2.1. Materials, equipment, and experimental methods Analytically pure grade of chemicals were used as received without any further purification. Typically, CuSO45H2O was dissolved in deionized water [DDW] to obtain a 0.5 M aqueous solution. The dissolution of the CuSO45H2O was done by stirring the solution magnetically for 15 min at room temperature until a transparent bluish solution was obtained. Then, 50 ml of the solution was poured into a 100 ml beaker. The ultrasonic treatment was done by immersing the high-intensity Ti horn (20 kHz, 750 W, 1.3 cm horn diameter) with 2X booster into the reaction vessel, the reaction time varied from 10 to 25 min. The reaction was carried out without external cooling, and the temperature inside the beaker was 60 °C. The efficiency of the sonicator remained constant during the sonication at 25%, meaning amplitude of 25% out of the maximum sonotrode vibration. After 25 min a bluish-green precipitate was noticed at the bottom of the beaker, the solution was allowed to cool down to room temperature under ambient conditions (the reaction temperature was 60 °C at the end of the sonication) and the desired product was collected by centrifugation, washed twice repeatedly with DDW and dried under vacuum at room temperature. Good reproducibility was obtained for all of the 5 repeated experiments, under the same conditions. In the experiment that probed the possibility of obtaining a higher yield, the reaction conditions were kept as described above and urea was added until a pH of 4 was measured. 3. Characterization The structural characterization of the product as the mineral antlerite was done by XRD with Cu Ka = 1.541 Å radiation, using a Bruker D8 diffractometer. The imaging and morphology of the mineral was studied by a high resolution transmission electron microscope (HR-TEM, JEOL JEM 2100 instrument), and in addition by an environmental scanning electron microscope (ESEM)
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employing the FEI QUANTA 200F device. Elemental analysis characterization was made by Energy-dispersive X-ray analysis (EDAX) attached to the ESEM instrument, and selected area electron diffraction (SAED) crystallographic analysis was also collected by the HR-TEM device. The Cu2+ ions concentration was characterized by elemental analysis using an inductively coupled plasma (ICP) spectrometer (Ultima 2, Jobin Yvon Horiba). Classic calibration method with standard solutions was used in order to analyze the elementals mention above.
4. Results and discussion The product obtained from the sonochemical reaction was characterized by XRD (Fig. 1). The diffraction peaks observed from the pattern of the product are assigned to the orthorhombic structure of antlerite (PDF 01-84-2037), and its main peaks are depicted in Fig. 1. The product can be assigned to highly pure antlerite due to the absence of other impurity peaks. It is important to emphasize that the diffraction peaks of metallic Cu or CuSO4 compound were not detected in the XRD diffraction pattern of the product. The morphology and elemental characterization of the obtained product was made by ESEM (Fig. 2). The image depicts that the product was formed having a needle-like morphology. The length of the needles varies between 0.5 and 1.5 lm and the width ranges between 80 and 150 nm. In order to confirm the formation of antlerite, elemental analysis using Energy-dispersive X-ray analysis (EDAX) were performed. The results indicate the following composition of atoms: Cu (31.3% ± 1.5), S (11.9% ± 1.9), O (56.8% ± 1.8), these values are given in At(%). The measurements determine a 2.64 as the Cu:S molar ratio which is close to the expected Cu:S = 3:1 molar ratio according to the antlerite formula. In any case it is very different from the 1:1 Cu:S molar ratio of the starting material or from other well-known compound Cu2S, and the less known Cu3S2 compounds. The structural nature of the synthesized particles was also confirmed by HR-TEM measurements. The HR-TEM images were taken at low (Fig. 3A) and at high (Fig. 3B) magnifications, and similarly to the SEM results depict a nanorods structure. The needle-like structure size ranges up to 150 nm width and up to 1.5 lm length. The crystalline nature of the particles was studied by measuring the electron diffraction of the selected area (Fig. 3C). The highly crystalline nature of the nanorods is evident from the SAED
Fig. 1. XRD pattern of Cu3(OH)4SO4 particles.
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E. Segal et al. / Ultrasonics Sonochemistry 22 (2015) 30–34
Fig. 2. ESEM image of the antlerite particles obtained from the sonochemical process.
pattern exhibiting a single crystal structure. The measured distances between the fringes are 0.68, 0.21 and 0.22 nm, which fit the distances between the (1 0 1), (3 1 3) and (2 1 4) lattice planes, respectively. These values match very well the distances reported in the literature for the orthorhombic lattice of the Cu3(OH)4SO4 (PDF No. 01-84-2037).
5. Mechanism of sonochemical formation of antlerite In the current study, the antlerite mineral was synthesized from aqueous solution of copper sulfate under ultrasound irradiation.
This process does not involve an addition of external hydroxide source. The pH of the copper sulfate pentahydrate solution before sonication was 3.23, and after sonication the value changed to 2.54. The yield obtained in the reaction under the current conditions was low, 0.07%. Increasing the reaction time to 60 min, had led to complete dissolution of the product. The explanation for the low yield might be due to the fact that the antlerite is known to be soluble in acidic environment, and the by product in the present reaction is H2SO4. Therefore, the low yield is a result of the solubility of the antlerite in the formed acid. In order to increase the yield, the reaction was conducted in the presence of urea (pH = 4). At the end of the reaction the pH was 3.79. The obtained yield of antlerite was 50 times larger, namely 3.5% and this is due to the fact that antlerite is stable between pH 2.5 and 4 [18]. Above pH 4 antlerite is no longer stable and another structure of mineral is formed: brochantite [Cu4SO4(OH)6], this form is stable in pH of 4–6. The increase in the reaction time has also an influence on the yield and by applying reaction time of 60 min, in the presence of urea, a significant improve in the amount of antlerite was obtained (21.7%). Further increase of the reaction time leads to the following yields: 90 min reaction results in 17.2% product that is not far from the yield of 60 min. However, when a reaction was conducted for 120 min, the yield was higher (30.1%). The crystalline structure of the products was verified by XRD, and confirmed as pure antlerite phase (PDF 01-84-2037). In addition, the morphology of antlerite from different irradiation times was analyzed. TEM images of 4 antlerite samples (A–D) collected from different irradiation times: 25, 60, 90 and 120 min are depicted in Fig. 4. The results reveal that the particle’s size is not influenced by time up to 90 min, and slightly increased when a reaction time of 120 min was applied. The length of the rods was increased from 1 lm to 2.2 lm, and the width up to 370 nm.
Fig. 3. HR-TEM of: (A) antlerite nano-rods (scale bar 100 nm); (B) high magnification of (A) (0.8M, scale bar 5 nm); (C) SAED of (A).
E. Segal et al. / Ultrasonics Sonochemistry 22 (2015) 30–34
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Fig. 4. TEM images (A–D) of product obtained at 25, 60, 90 and 120 min of reaction time, respectively.
The influence of the amplitude on the yield was also studied, while the reaction time was kept constant, namely 25 min. By increasing the amplitude from 25% to 30% and 40%, the yield has been increased from 3.5% to 9.1% and 15.65%, respectively. In addition the effect of temperature was studied and we performed the reaction for 25 min at 30 °C by using an ice-water bath. In such conditions, no precipitation was obtained at the end of the reaction. In order to prove the effect of ultrasound irradiation on the formation of crystalline antlerite, the reaction was carried out without sonication and the reactants were heated up to 70 °C for 1 h. Materials have not precipitated. This indicates that the conditions that are developed during the bubble collapse (pressure is above 1000 atm and the temperature is higher than 5000 K) lead to the formation of the crystalline mineral. The fact that US irradiation can induce strong local heating is probably the reason of the formation of antlerite at 60 °C under US. The stability of the antlerite mineral was also examined by probing its solubility in the solution. We have followed the dissolution by ICP measurement of the Cu2+ ions. The concentration of the ions was measured after 3, 18, 24 and 48 h. A constant level of the ions was found indicating that the crystal is stable and does not dissolve in the aqueous solution of pH = 3.79. The basic question to be addressed in this section is why different conditions of the sonochemical reaction of an aqueous solution of CuSO4 lead to the formation of CuH at one time [3] and to Cu3(OH)4SO4 in another experiment. It is clear that in both cases the copper salts as non-volatile materials are found on the circumference of the collapsing bubble. When the reaction is conducted under argon [3], the H radicals that are produced inside the cavity reach the copper ions (Cu2+) and reduces them to Cu+, forming Cu(I)H. In the current paper, the current reaction is carried out under air and a different product is obtained. When the cavity collapse occurs, the adsorbed ions (Cu2+, SO24 and OH ) are
exposed to extreme localized conditions of temperature and pressure. As a result, these ions impact one another and nanoparticles of the antlerite crystal are produced. The liquid–gas interface attendance, in other words-the surface of the acoustic bubble, immensely reduces the nucleation work needed for crystallization, and as discussed in the literature already [19], the crystal nucleation process is accelerated. To conclude, the novelty of the current paper is the fact that it is possible to obtain the antlerite mineral from an aqueous solution of copper sulfate without addition of hydroxide source. The yield of the product is influenced by changing the reaction parameters such as: pH, time, amplitude, and temperature.
6. Conclusions By probing the question of reducing metallic ions by radicals of OH and H formed in water by the sonochemical irradiation process, a novel way to produce nanorods structure of the antlerite mineral was found. A simple one-step procedure using only an aqueous solution of copper sulfate, under ultrasonic irradiation, and the product precipitates after no longer than 25 min. This manuscript reports on a facile synthesis to achieve the antlerite product in the low nano range, which was analyzed and found to be pure and highly crystalline. The yield of the mineral is influenced by changing various reaction parameters. References [1] V.G. Pol, D.N. Srivastava, O. Palchik, V. Palchik, M.A. Slifkin, A.M. Weiss, A. Gedanken, Sonochemical deposition of silver nanoparticles on silica spheres, Langmuir 18 (2002) 3352–3357. [2] T. Chave, N.M. Navarro, S. Nitsche, S.I. Nikitenko, Mechanism of PtIV sonochemical reduction in formic acid media and pure water, Chem. Eur. J. 18 (2012) 3879–3885.
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[3] P. Hasin, Y. Wu, Sonochemical synthesis of copper hydride (CuH), Chem. Commun. 48 (2012) 1302–1304. [4] K. Okitsu, K. Sharyo, R. Nishimura, One-pot synthesis of gold nanorods by ultrasonic irradiation: the effect of pH on the shape of the gold nanorods and nanoparticles, Langmuir 25 (14) (2009) 7786–7790. [5] K. Okitsu, H. Bandow, Y. Maeda, Sonochemical preparation of ultrafine palladium particles, Chem. Mater. 8 (1996) 315–317. [6] K. Okitsu, A. Yue, S. Tanabe, H. Matsumoto, Y. Yobiko, Y. Yoo, Sonolytic control of rate of gold(III) reduction and size of formed gold nanoparticles: relation between reduction rates and sizes of formed nanoparticles, Bull. Chem. Soc. Jpn. 75 (2002) 2289–2296. [7] I.A. Wani, A. Ganguly, J. Ahmed, T. Ahmad, Silver nanoparticles: ultrasonic wave assisted synthesis, optical characterization and surface area studies, Mater. Lett. 65 (2011) 520–522. [8] S.Y. Jeon, J.S. Yun, D.H. Son, C.H. Hwang, S.S. Hong, S.S. Park, Preparation of nickel powders from nickel salt by ultrasonic chemical reduction method, Stud. Surf. Sci. Catal. 159 (2006) 773–776. [9] Z. Zhanjiang, L. Jinpei, Synthesis and characterization of silver nanoparticles by a sonochemical method rare metal, Mater. Eng. 41 (10) (2012) 1700–1705. [10] D. Sunartio, K. Yasui, T. Tuziuti, T. Kozuka, Y. Iida, M. Ashokkumar, F. Grieser, Correlation between Na⁄ emission and ‘‘chemically active’’ acoustic cavitation bubbles, Chem. Phys. Chem. 8 (2007) 2331–2335. [11] E.B. Flint, K.S. Suslick, The temperature of cavitation, Science 253 (1991) 1397– 1399.
[12] C. Browne, R.F. Tabor, D.Y.C. Chan, R.R. Dagastine, M. Ashokkumar, F. Grieser, Bubble coalescence during acoustic cavitation in aqueous electrolyte solutions, Langmuir 27 (2011) 12025–12032. [13] N. Koga, A. Mako, T. Kimizu, Y. Tanaka, Thermal decomposition of synthetic antlerite prepared by microwave-assisted hydrothermal method, Thermochim. Acta 467 (2008) 11–19. [14] C.H. Yoder, R.W. Schaeffer, P. McWilliams, A. Rownad, X. Liu, J. Shambeda, The synthesis of copper/zinc solid solutions of hydroxyl carbonates, sulphates, nitrates, chlorides and bromides, Mineral. Mag. 75 (5) (2011) 2573–2582. [15] S. Vilminot, M. Richard-Plouet, G. Andre´, D. Swierczynski, F. Boure´e-Vigneron, E. Marino, M. Guillot, Synthesis, structure and magnetic properties of copper hydroxysulfates, Cryst. Eng. 5 (2002) 177–186. [16] S. Kiel, O. Grinberg, N. Perkas, J. Charmet, H. Kepner, A. Gedanken, Forming nanoparticles of water-soluble ionic molecules and embedding them into polymer and glass substrates, Beilstein J. Nanotechnol. 3 (2012) 267–276. [17] I. Perelshtein et al., Tannic acid NPs – synthesis and immobilization onto a solid surface in a one-step process and their antibacterial and antiinflammatory properties, Ultrason. Sonochem. (2013). [18] R.L. Frost, Raman spectroscopy of selected copper minerals of significance in corrosion, Spectrochim. Acta A 59 (6) (2003) 1195–1204. [19] A. Kordylla, T. Krawczyk, F. Tumakaka, G. Schembecker, Modeling ultrasoundinduced nucleation during cooling crystallization, Chem. Eng. Sci. 64 (2009) 1635–1642.
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Short Communication
A novel sonochemical synthesis of antlerite nanorods Elad Segal, Ilana Perelshtein, Aharon Gedanken ⇑ Department of Chemistry, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel
a r t i c l e
i n f o
Article history: Received 10 February 2014 Received in revised form 7 July 2014 Accepted 23 July 2014 Available online 31 July 2014 Keywords: Antlerite Nanorods Minerals Sonochemistry
a b s t r a c t Antlerite – Cu3(OH)4SO4 was prepared, for the first time, by the sonochemical method from an aqueous solution of CuSO4, without any additives. The source of OH is a result of protonation of SO24 forming HSO4 and OH . The extreme local conditions inside the cavity that are developed during the bubble collapse (pressure is above 1000 atm and the temperature is higher than 5000 K) lead to the formation of the crystalline mineral. A suggested mechanism for the mineral formation is proposed. Due to the collapse of the bubbles, the distances between the opposite charge Cu2+ and SO24 ions is shortened and a crystallization process is initiated. In addition, the reaction is a one-step process with short irradiation time of less than 30 min. The chemo-physical analysis of the sonochemically obtained product has revealed the presence of single phase antlerite nanorods. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Previous reports on the sonochemical reduction of metallic ions by ultrasound have been presented and discussed. These reports include the sonochemical reductions of metallic ions that were carried out without a reducing agent. For example, the reduction of Ag+ in an aqueous solution of AgNO3 and the deposition of metallic silver upon silica sub-micron spheres by ultrasonic method under argon atmosphere was reported [1]. Fabrication of Pt NPs (nanoparticles) with no stabilizer or capping agent, by reducing PtIV to Pt0 in pure water under Ar atmosphere, in a 2-step process [2] was reported. In the case of copper the sonochemical reduction of Cu2+ from CuSO4 and CuCl2 as starting compounds, has led to the formation of Copper hydride (CuH) [3]. Needless to point out that many examples of the sonochemical reduction of metallic ions in the presence of a reducing agent have been published [4–9]. The sonochemical reduction of metallic ions is of great interest because it is considered a cheap technique which yields small NPs of the corresponding metal. In the absence of a reducing agent the reaction occurs as a result of the collapse of the cavitation bubbles, leading to the homolytical cleavage of water into H and OH radicals [10–12]. It is clear that the number of metals that can be reduced just by radicals formed from the dissociation of water is limited to those having a highly positive standard reduction potential.
⇑ Corresponding author. E-mail address: [email protected] (A. Gedanken). URL: http://www.ch.biu.ac.il/gedanken (A. Gedanken). http://dx.doi.org/10.1016/j.ultsonch.2014.07.020 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Hasin [3] has reported on the sonochemical synthesis of CuH when the reaction was conducted under argon. In the current paper, we report on the same reaction employing the same reactant, CuSO4 as described in Ref. [3] and obtained Cu3(OH)4SO4. The main difference between the work of Hasin and ours, is that we have performed the reaction under air and as a result a different product, the mineral antlerite (Cu3(OH)4SO4) was synthesized. This mineral is well known – and was synthesized previously in a few different ways. Koga and co-workers [13] have shown a largescale microwave-assisted hydrothermal method for the synthesis of micron-size needle-like (Cu3(OH)4SO4). Another research [14] has demonstrated the use of urea and carbonate for the precipitation of antlerite. Vilminot et al. [15] has prepared antlerite in a long process of six days, under 170 °C with NaOH as the hydroxide source. The main disadvantages of the available techniques for the synthesis of antlerite are either long processes or the involvement of special chemicals added as a source of hydroxide. The current article introduces a new preparation technique, i.e. the sonochemical method. The current manuscript reports on a onestep process conducted under ultrasonic waves to produce the mineral antlerite from CuSO4 aqueous solution in a non-inert atmosphere within less than 30 min. Nanorods of up to 150 nm width and 1.5 lm in length were prepared and characterized by common materials science techniques. Herein we report on applying ultrasonic waves on a solution of inorganic salt under air. When ultrasonic irradiation is applied to an aqueous solution of an inorganic salt, the acoustic bubbles that are being formed during the process adsorb the ions of the salt on their surface [16,17]. When the bubbles collapse, extreme local (inside the cavity) conditions of pressure (above 1000 atm) and
E. Segal et al. / Ultrasonics Sonochemistry 22 (2015) 30–34
high temperature (higher than 5000 K) occur next to the ions [11]. So unlike sonochemical reaction of volatile materials that happen inside the collapsing bubble, in our case the reaction takes place on the surface of the collapsing bubble. 2. Experimental section 2.1. Materials, equipment, and experimental methods Analytically pure grade of chemicals were used as received without any further purification. Typically, CuSO45H2O was dissolved in deionized water [DDW] to obtain a 0.5 M aqueous solution. The dissolution of the CuSO45H2O was done by stirring the solution magnetically for 15 min at room temperature until a transparent bluish solution was obtained. Then, 50 ml of the solution was poured into a 100 ml beaker. The ultrasonic treatment was done by immersing the high-intensity Ti horn (20 kHz, 750 W, 1.3 cm horn diameter) with 2X booster into the reaction vessel, the reaction time varied from 10 to 25 min. The reaction was carried out without external cooling, and the temperature inside the beaker was 60 °C. The efficiency of the sonicator remained constant during the sonication at 25%, meaning amplitude of 25% out of the maximum sonotrode vibration. After 25 min a bluish-green precipitate was noticed at the bottom of the beaker, the solution was allowed to cool down to room temperature under ambient conditions (the reaction temperature was 60 °C at the end of the sonication) and the desired product was collected by centrifugation, washed twice repeatedly with DDW and dried under vacuum at room temperature. Good reproducibility was obtained for all of the 5 repeated experiments, under the same conditions. In the experiment that probed the possibility of obtaining a higher yield, the reaction conditions were kept as described above and urea was added until a pH of 4 was measured. 3. Characterization The structural characterization of the product as the mineral antlerite was done by XRD with Cu Ka = 1.541 Å radiation, using a Bruker D8 diffractometer. The imaging and morphology of the mineral was studied by a high resolution transmission electron microscope (HR-TEM, JEOL JEM 2100 instrument), and in addition by an environmental scanning electron microscope (ESEM)
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employing the FEI QUANTA 200F device. Elemental analysis characterization was made by Energy-dispersive X-ray analysis (EDAX) attached to the ESEM instrument, and selected area electron diffraction (SAED) crystallographic analysis was also collected by the HR-TEM device. The Cu2+ ions concentration was characterized by elemental analysis using an inductively coupled plasma (ICP) spectrometer (Ultima 2, Jobin Yvon Horiba). Classic calibration method with standard solutions was used in order to analyze the elementals mention above.
4. Results and discussion The product obtained from the sonochemical reaction was characterized by XRD (Fig. 1). The diffraction peaks observed from the pattern of the product are assigned to the orthorhombic structure of antlerite (PDF 01-84-2037), and its main peaks are depicted in Fig. 1. The product can be assigned to highly pure antlerite due to the absence of other impurity peaks. It is important to emphasize that the diffraction peaks of metallic Cu or CuSO4 compound were not detected in the XRD diffraction pattern of the product. The morphology and elemental characterization of the obtained product was made by ESEM (Fig. 2). The image depicts that the product was formed having a needle-like morphology. The length of the needles varies between 0.5 and 1.5 lm and the width ranges between 80 and 150 nm. In order to confirm the formation of antlerite, elemental analysis using Energy-dispersive X-ray analysis (EDAX) were performed. The results indicate the following composition of atoms: Cu (31.3% ± 1.5), S (11.9% ± 1.9), O (56.8% ± 1.8), these values are given in At(%). The measurements determine a 2.64 as the Cu:S molar ratio which is close to the expected Cu:S = 3:1 molar ratio according to the antlerite formula. In any case it is very different from the 1:1 Cu:S molar ratio of the starting material or from other well-known compound Cu2S, and the less known Cu3S2 compounds. The structural nature of the synthesized particles was also confirmed by HR-TEM measurements. The HR-TEM images were taken at low (Fig. 3A) and at high (Fig. 3B) magnifications, and similarly to the SEM results depict a nanorods structure. The needle-like structure size ranges up to 150 nm width and up to 1.5 lm length. The crystalline nature of the particles was studied by measuring the electron diffraction of the selected area (Fig. 3C). The highly crystalline nature of the nanorods is evident from the SAED
Fig. 1. XRD pattern of Cu3(OH)4SO4 particles.
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E. Segal et al. / Ultrasonics Sonochemistry 22 (2015) 30–34
Fig. 2. ESEM image of the antlerite particles obtained from the sonochemical process.
pattern exhibiting a single crystal structure. The measured distances between the fringes are 0.68, 0.21 and 0.22 nm, which fit the distances between the (1 0 1), (3 1 3) and (2 1 4) lattice planes, respectively. These values match very well the distances reported in the literature for the orthorhombic lattice of the Cu3(OH)4SO4 (PDF No. 01-84-2037).
5. Mechanism of sonochemical formation of antlerite In the current study, the antlerite mineral was synthesized from aqueous solution of copper sulfate under ultrasound irradiation.
This process does not involve an addition of external hydroxide source. The pH of the copper sulfate pentahydrate solution before sonication was 3.23, and after sonication the value changed to 2.54. The yield obtained in the reaction under the current conditions was low, 0.07%. Increasing the reaction time to 60 min, had led to complete dissolution of the product. The explanation for the low yield might be due to the fact that the antlerite is known to be soluble in acidic environment, and the by product in the present reaction is H2SO4. Therefore, the low yield is a result of the solubility of the antlerite in the formed acid. In order to increase the yield, the reaction was conducted in the presence of urea (pH = 4). At the end of the reaction the pH was 3.79. The obtained yield of antlerite was 50 times larger, namely 3.5% and this is due to the fact that antlerite is stable between pH 2.5 and 4 [18]. Above pH 4 antlerite is no longer stable and another structure of mineral is formed: brochantite [Cu4SO4(OH)6], this form is stable in pH of 4–6. The increase in the reaction time has also an influence on the yield and by applying reaction time of 60 min, in the presence of urea, a significant improve in the amount of antlerite was obtained (21.7%). Further increase of the reaction time leads to the following yields: 90 min reaction results in 17.2% product that is not far from the yield of 60 min. However, when a reaction was conducted for 120 min, the yield was higher (30.1%). The crystalline structure of the products was verified by XRD, and confirmed as pure antlerite phase (PDF 01-84-2037). In addition, the morphology of antlerite from different irradiation times was analyzed. TEM images of 4 antlerite samples (A–D) collected from different irradiation times: 25, 60, 90 and 120 min are depicted in Fig. 4. The results reveal that the particle’s size is not influenced by time up to 90 min, and slightly increased when a reaction time of 120 min was applied. The length of the rods was increased from 1 lm to 2.2 lm, and the width up to 370 nm.
Fig. 3. HR-TEM of: (A) antlerite nano-rods (scale bar 100 nm); (B) high magnification of (A) (0.8M, scale bar 5 nm); (C) SAED of (A).
E. Segal et al. / Ultrasonics Sonochemistry 22 (2015) 30–34
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Fig. 4. TEM images (A–D) of product obtained at 25, 60, 90 and 120 min of reaction time, respectively.
The influence of the amplitude on the yield was also studied, while the reaction time was kept constant, namely 25 min. By increasing the amplitude from 25% to 30% and 40%, the yield has been increased from 3.5% to 9.1% and 15.65%, respectively. In addition the effect of temperature was studied and we performed the reaction for 25 min at 30 °C by using an ice-water bath. In such conditions, no precipitation was obtained at the end of the reaction. In order to prove the effect of ultrasound irradiation on the formation of crystalline antlerite, the reaction was carried out without sonication and the reactants were heated up to 70 °C for 1 h. Materials have not precipitated. This indicates that the conditions that are developed during the bubble collapse (pressure is above 1000 atm and the temperature is higher than 5000 K) lead to the formation of the crystalline mineral. The fact that US irradiation can induce strong local heating is probably the reason of the formation of antlerite at 60 °C under US. The stability of the antlerite mineral was also examined by probing its solubility in the solution. We have followed the dissolution by ICP measurement of the Cu2+ ions. The concentration of the ions was measured after 3, 18, 24 and 48 h. A constant level of the ions was found indicating that the crystal is stable and does not dissolve in the aqueous solution of pH = 3.79. The basic question to be addressed in this section is why different conditions of the sonochemical reaction of an aqueous solution of CuSO4 lead to the formation of CuH at one time [3] and to Cu3(OH)4SO4 in another experiment. It is clear that in both cases the copper salts as non-volatile materials are found on the circumference of the collapsing bubble. When the reaction is conducted under argon [3], the H radicals that are produced inside the cavity reach the copper ions (Cu2+) and reduces them to Cu+, forming Cu(I)H. In the current paper, the current reaction is carried out under air and a different product is obtained. When the cavity collapse occurs, the adsorbed ions (Cu2+, SO24 and OH ) are
exposed to extreme localized conditions of temperature and pressure. As a result, these ions impact one another and nanoparticles of the antlerite crystal are produced. The liquid–gas interface attendance, in other words-the surface of the acoustic bubble, immensely reduces the nucleation work needed for crystallization, and as discussed in the literature already [19], the crystal nucleation process is accelerated. To conclude, the novelty of the current paper is the fact that it is possible to obtain the antlerite mineral from an aqueous solution of copper sulfate without addition of hydroxide source. The yield of the product is influenced by changing the reaction parameters such as: pH, time, amplitude, and temperature.
6. Conclusions By probing the question of reducing metallic ions by radicals of OH and H formed in water by the sonochemical irradiation process, a novel way to produce nanorods structure of the antlerite mineral was found. A simple one-step procedure using only an aqueous solution of copper sulfate, under ultrasonic irradiation, and the product precipitates after no longer than 25 min. This manuscript reports on a facile synthesis to achieve the antlerite product in the low nano range, which was analyzed and found to be pure and highly crystalline. The yield of the mineral is influenced by changing various reaction parameters. References [1] V.G. Pol, D.N. Srivastava, O. Palchik, V. Palchik, M.A. Slifkin, A.M. Weiss, A. Gedanken, Sonochemical deposition of silver nanoparticles on silica spheres, Langmuir 18 (2002) 3352–3357. [2] T. Chave, N.M. Navarro, S. Nitsche, S.I. Nikitenko, Mechanism of PtIV sonochemical reduction in formic acid media and pure water, Chem. Eur. J. 18 (2012) 3879–3885.
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