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Synthesis and characterization of MnO2 colloids
ARTICLE IN PRESS Radiation Physics and Chemistry 78 (2009) 939–944
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Synthesis and characterization of MnO2 colloids Pooja Yadav a,, Richard T. Olsson b,1, Mats Jonsson a,2 a b
School of Chemical Science and Engineering, Nuclear Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Department of Fiber and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden
a r t i c l e in fo
abstract
Article history: Received 22 August 2008 Accepted 18 February 2009
This work addresses the issue of radiation chemical synthesis of MnO2 nanoparticles and also illustrates the ease of formation of nanorods and sheets by adroit manipulation of experimental conditions. The radiation chemical yield (G-value) for reduction of Mn (VII) by the hydrated electron was found to be 0.27 mmol J1 and 0.17 mmol J1 respectively, when tert. butanol and isopropanol were used as scavengers in nitrogen-saturated solutions. The colloids formed upon irradiation of air-saturated solution and N2-purged solution with tert. butanol as scavenger were found to be most stable. Irradiation of air-saturated solution containing 4 104 M KMnO4 at a dose of 1692 Gy resulted in the formation of nanorods of the dimension 100–150 nm and nanospheres in the range 10–20 nm. Irradiation of N2-purged solution containing tert. butanol as scavenger for dOH-produced reticulated structure of nanorods with length varying from 50 to 100 nm at a dose of 1692 Gy. Elemental analysis was performed using scanning electron microscope on MnO2 formed by reduction and oxidation and the purity was found to be 98% of elemental Mn content. & 2009 Published by Elsevier Ltd.
Keywords: Nanostructures Electron microscopy (TEM and SEM) Oxidation Gamma radiolysis Metal oxide
1. Introduction Nanomaterials have captured the imagination of researchers lately due to the significant difference in their properties compared to their coarse-grained counterpart. The greater surface to volume ratio and specific binding sites of nanoparticles enhance catalytic properties (Hiroki and La Verne, 2005). Manganese dioxide is a fascinating inorganic metal oxide owing to its wide range of applications in catalysis, ion exchange, molecular adsorption and particularly in energy storage and also because of its low cost and environmentally benign nature (Nalwa, 2000; Burda et al., 2005). Amongst other things activated MnO2 is widely used in lithium batteries as lithium intercalation host and also as cathode material in primary alkaline batteries (Yuan et al., 2003). One of the challenges facing the chemists today is to synthesize well-defined mono disperse nanoparticles. Various polymorphs of MnO2 exist in nature as the basic octahedral unit (MnO6) and can be linked in different ways. Their properties depend on the crystallographic forms. For example, a-MnO2 phase is very favorable to intercalation and b-MnO2 is passive to it. Therefore, the controlled synthesis of MnO2 has always been the objective of synthetic chemists. It has been Corresponding author. Tel.: +4687908789; fax: +4687908772.
E-mail addresses: [email protected] (P. Yadav), [email protected] (R.T. Olsson), [email protected] (M. Jonsson). 1 Tel.: +4687907640. 2 Tel.: +4687909123; fax: +4687908772. 0969-806X/$ - see front matter & 2009 Published by Elsevier Ltd. doi:10.1016/j.radphyschem.2009.02.006
shown by the density functional theory (DFT) calculations that g-MnO2 is the energetically favored structure (Balachandran et al., 2003; Sayle et al., 2005). Several methods have been developed for MnO2 synthesis ranging from simple reduction (Kim and Popov, 2003; Jeong and Manthiram, 2002), oxidation (Wang and Li, 2002), co-precipitation (Burda et al., 2005; Toupin et al., 2002), sol-gel (AL-Sagheer and Zaki, 2000), thermal decomposition, etc. (Lee and Goodenough, 1999). Radiation chemistry is an effective tool for the synthesis of particles of nanometer dimension owing to facile manipulation of dose and experimental conditions to obtain the required size distribution. It was effectively shown by Henglein that the colloids formed by radiation-induced reduction were smaller than those formed by co-precipitation (LumePereira et al., 1985, Baral et al., 1985, 1986). Henglein et al. reported the synthesis of MnO2 colloids by radiolytic reduction of KMnO4 in air-saturated solution at pH 10 (Lume-Pereira et al., 1985). They further studied the reaction of colloids with various radicals. A size range 3–5 nm was reported for a dose of 700 Gy and the radiolytically produced colloids were smaller in size compared to those prepared by co-precipitation. However, there was no information about the nature and shape of the particles. Henglein also described the formation of colloids by oxidation of Mn2+ by dOH, formation of colloids in the pH range 3.5–9 was observed (Baral et al., 1985). Size of the particles were not reported, however, based on the UV-absorption spectra a larger size was predicted compared to that formed from Mn (VII) reduction. Further, they studied the reaction of sols with 1-hydroxy-1-methylethyl radical and suggested the formation of
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Mn3+ centers. More recently the MnO2 colloids were synthesized by radiolytic reduction of KMnO4 with isopropanol as scavenger and using polyvinyl alcohol (PVA) polymer and sodium dodecyl sulphate (SDS) as surfactant (Liu et al., 1997). In this study, particles of 6 nm for a dose of 2.01 kGy were obtained. XPS studies by the same group showed a valency of +4 for the MnO2 colloids. The aim of this work is to identify the optimal conditions for generation of MnO2 nanoparticles by radiolysis. An effort has been made to synthesize colloids by reduction of KMnO4 by various reducing radicals and also to characterize the MnO2 particles. Alternatively, the synthesis of MnO2 by radiation-induced oxidation of Mn (II) has also been attempted.
2. Experimental KMnO4 and MnSO4 H2O were obtained from Kebo chemicals with a purity of 99%. The rest of the chemicals were purchased from SDS, Fluka, Sigma–Aldrich and Merck. The N2O, N2 and O2 gases were procured from Air Liquide and Strandmollen. All solutions were freshly prepared using deionised water purified by a Millipore-Milli-Q system having a resistivity of 18 MO cm1 and the experiments were carried out at room temperature (22 1C). The pH of the solution was adjusted by using the NaOH (1 104 M) or HClO4. The mean particle size and size distribution was measured using photon correlation spectroscopy with laser of 488 nm wavelength and a fixed scattering angle of 901 (BI–90 particle size, Brookhaven instruments co., USA). The detection range of PCS is between 10 nm and 3 mm and while calculating the mean size the software assumes globular particles. However, formation of nonsperical particles can account for the loss of intensity of the signal for PCS measurements as the PCS is valid for globular systems. The geometry affects the translation motion and also the scattering angle will be different for rods and spheres. The count rate (photon counts per second) is proportional to the concentration of a specific size, when the size distribution is nearly constant. The refractive index and geometry of the particle can influence the intensity of the signal. Wide size distribution presents greater difficulty as the scattered intensity is a function of size. Hence, larger particles contribute more to the measured signal. The zeta potential was measured by means of a ZetaPALS zeta potential analyzer (Brookhaven instruments co., USA), where the z potential was deduced from the particle velocity using Smoluchowski’s equation (Ledin et al., 1993). Irradiations were carried out using a gamma cell (Elite 1000) 137 Cs source. The dose rate was 9.4 Gy/min as determined using the Fricke dosimeter. A brief description of generation of radical follows. d The major products of water radiolysis are free radicals e aq, H , d OH, and molecular products like H3O+, H2 and H2O2. The G-value is defined as moles of species formed or consumed per joule of absorbed energy. The G-values are given in parentheses (mmol J1) (Spinks and Woods, 1990). H2 O
d OHð0:28Þ; e
aq
ð0:28Þ; Hd ð0:047Þ; H2 O2 ð0:073Þ; H2 ð0:047Þ (1)
The reactions of the hydrated electron were studied in N2saturated aqueous solutions containing 0.2 M tert. butanol to effectively scavenge dOH.
OHðH Þ þ ðCH3 Þ3 COH ! ðCH3 Þ2 CH2 COH þ H2 OðH2 Þ
in
(2)
The reaction of the 2-hydroxy-2-propyl radical was studied aqueous solutions containing 0.2 M N2O-saturated
iso-propyl alcohol.
OHðH Þ þ ðCH3 Þ2 CHOH ! ðCH3 Þ2 COH þ H2 OðH2 Þ
(3)
The reactions of OH radical were studied in N2O-saturated solutions. The solubility of N2O in water is 2.5 102 M at 25 1C d and at this concentration, e aq is quantitatively converted into OH. d
H2 O
N2 O þ e aq ! OH þ OH þ N2
(4)
3. Results and discussion 3.1. Reduction of permanganate The aqueous solution of 4 104 M KMnO4 at pH 10 was g irradiated under various conditions to generate the radical of interest. The MnO 4 concentration was measured by UV–Vis spectroscopy at 540 nm, which is the absorption maximum for permanganate. Table 1 lists the G-values obtained for reduction of KMnO4 by various reducing radicals. In case of air-saturated solution the G-value for reduction of KMnO4 is 0.08 mmol J1 and in N2-purged solutions containing tert. butanol the G-value is 0.27 mmol J1, i.e. almost identical to the G-value for the solvated electron. However, the G-value is lowered when isopropanol (0.17 mmol J1) was used as a scavenger despite the increase in yield of reducing radicals. Permanganate is a strong oxidant capable of oxidizing alcohols and other organic reagents. In cases where 2-propanol and HCO 2 were used a significant amount of MnO 4 was consumed in background reactions. The background reaction was also measured and corrected for. However, the radiolytical reduction of MnO 4 is probably not completely independent of the background reaction and the G-values obtained in the systems where background reactions occur are not completely reliable. Furthermore, the 2-hydroxy-2-propyl radical has been shown to react with MnO2 colloids (dose ¼ 700 Gy, size ¼ 3–5 nm) at a rate of 8 106 M1 s1 giving rise to Mn3+, this competing reaction can account for reduced G-value for 3+ are reduction of MnO 4 (Lume-Pereira et al., 1985). Further Mn reduced from an organic radical, the resulting Mn2+ ions undergoes a rapid conproportionation with Mn4+ (Lume-Pereira et al., 1985). Also Mulvaney et al., have shown using both thermal- and radiation-induced dissolution of metal oxides that the Mn3+ Table 1 The G-values for reduction of 4 104 M KMnO4 by various radicals, since KMnO4 oxidizes most alcohols and organic reagents the absorbance was normalized to correct the background reaction. Reaction conditions
Reactive species
Ga value at
l540 nm (mmol J1)
Air saturated N2-purged solution with (CH3)3COH as d OH scavenger N2 purged solution with (CH3)2CHOH as dOH scavenger N2O-purged solution with tert. butanol N2O-purged solution with isopropanol N2O-purged solution with 4 mM sodium formate O2-purged solution with 4 mM sodium formate a
Percent background reaction
(dOH, e aq and Hd) eaq
0.08
–
0.27
–
e aq, (CH3)d2 COH
0.17
20
Tert. butyl radical (CH3)d2 COH
No reduction 0.14
29
COd 2
0.17
38
Od 2
0.15
37
The G value was corrected for background reaction.
–
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Table 2 A comparison of the mean particle size and zeta potential values. 4 104 M KMnO4 at pH 10 was irradiated. Radicals and reaction conditions
Dose/Gy
Mean diameter/nm
Zeta potential/mV
d Air saturated (dOH, e aq and H )
846 1692 2256
9071 8971 9771
38.471.8 46.770.5 36.270.9
d e aq With (CH3)3COH as OH scavenger and N2 saturation
846 1692 2256
2874 2474 2979
8.671.4 3.471.0 17.471.9
centre present in colloids are continually depleted to produce Mn2+ (Mulvaney et al., 1990). The G-values for solution containing d formate (COd 2 ) and formate with O2 (O2 ) were comparable. 3.2. PCS and zeta potential measurements The mean particle sizes and size distributions were measured for all the colloids and are listed in Table 2 along with zeta potential values. Colloidal MnO2 is formed when permanganate is reduced by a multitude of organic reagents (Perez-Benito and Arias, 1992). Therefore, a more controlled reaction and smaller size was obtained when there was no background reaction. This was observed in case of air-saturated solutions and N2-purged solutions containing tert. butanol. In rest of the cases permanganate was reduced giving rise to MnO2 nanoparticles which acted as seeds for further growth rendering it difficult to control the size and also this background reaction was difficult to monitor. As can be expected, aggregation of MnO2 was much faster in cases where salts were added leading to precipitation at lower doses. 4 104 M KMnO4 air-saturated solution was reduced radiolytically and the mean colloid size was found to be 90 nm for a dose of 846 Gy and only a marginal increase in size was observed even after increasing the dose by nearly a factor of three. The particle size upon reduction of Mn (VII) by the hydrated electron was much smaller for the same dose and the corresponding zeta potential absolute values were lower than those obtained for airsaturated reduction. As a consequence, the colloids were less stable compared to colloids produced in air-saturated solution. It should be noted that PCS assumes globular particles which need not necessarily mean that other shapes may not be present. There would be certain discrepancy in readings if the shape is other than globular for example nanorods with greater aspect ratios. 3.3. Oxidation of Mn (II) Colloids were also synthesized by oxidation of Mn (II). PickKaplan, Rabani (1976) and Baral et al. (1986) reported the formation of colloids by oxidation of Mn(ClO4)2 6H2O. Rabani measured the half life of MnO2 nucleation at various [Mn (II)] and concluded that on increasing [Mn (II)] concentration at constant dose slows down the nucleation (Pick-Kaplan and Rabani, 1976). Hengelein carried out a more detailed study on formation of colloids and observed formation of colloidal solutions at pH 3.5–9. Since manganese (II) sulphate monohydrate is colourless the formation of colloid can be followed by change in colour, where the colloidal solution is yellowish brown. Of about 2 104 M of Mn(SO4)2 H2O was irradiated at pH 10, for a dose of 564 Gy a particle size of 51779 nm was recorded on increasing the dose to 846 Gy the size increased to 7317123 nm. The zeta potential value for these particles at a dose of 564 and 846 Gy was 29.673 mV
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and 17.371, respectively. However, these colloids were not stable and precipitated after half an hour. Therefore, the concentration was reduced to 1 104 M accordingly the particle size reduced to 266740 and 571714 nm for a dose of 564 and 846 Gy, respectively, and subsequently there was an increase in absolute zeta potential value. These colloids were some what more stable and precipitated after few hours. When the pH was varied from 2.7 to 11 for 1 104 M of Mn(SO4)2 H2O pale brown-coloured colloids were formed only at pH 11. Henglein reported formation of colloids at pH 5, 7 and 9 (Baral et al., 1986). PCS measurements showed a particle size of 354 nm for a dose of 846 Gy with a corresponding zeta potential of 58.973 mV. 3.4. Characterization 3.4.1. X-ray diffraction The colloid formed by reduction in air-saturated solution was precipitated by addition of 0.5 M NaCl and was then filtered and repeatedly washed with deionised water and later dried in oven at 60 1C. The black powder was then analyzed by X-ray powder diffraction. The diffraction pattern is shown in Fig. 1. As shown from figure the diffraction pattern of the solid conforms to the lines for MnO2 and was amorphous. Amorphous phases studied by solution calorimetry (zirconia and silica) have significantly lower surface enthalpies than their dense crystalline counterparts, indicating that amorphous phases may be thermodynamically as well as kinetically preferred under constraint of small particle size (Pitcher et al., 2004; Piccione et al., 2000). However, due to poor signal to noise ratio the exact polymorph could not be identified. Manganese (IV) oxides and manganese are divided into two structural families: ramsedellite a-MnO2 and pyrolusite b-MnO2. Other forms of manganese dioxies are a random intergrowth of ramsedellite and pyrolusite (Kohler et al., 1997). The imperfections are characterized as microtwinning and the de Wolff disorder and are generally believed to be responsible for poor X-ray diffraction pattern of manganese oxides and oxyhydroxides (Maclean and Tye, 1996). The d-spacings of 0.775, 0.374, 0.242 and 0.141 nm were calculated using the Bragg equation (nl ¼ 2d sin y) and matched well with the data published for birnessite (Matocha et al., 2001). The corresponding indices are (1 0 0), (2 0 0), (3 1 0) and (5 2 1). XRD pattern for colloid formed by oxidation of Mn (II) was also studied and powder was identified as MnO2 and the polymorph could not be determined. 3.4.2. Transmission electron microscopy Since PCS gives ambiguous picture about the size of the particles and none whatsoever what about the shape of the particles, colloids that were further characterized by transmission electron microscopy. 4 104 M KMnO4 was reduced in airsaturated solution and for a dose of 1692 Gy nanorods or needles of dimension 100–150 nm were formed and also visible were a few 20 nm rods that were thicker than the others and nanospheres or dots in the range 10–20 nm were also seen. This is pictorially shown in Fig. 2A. Similar results in terms of appearance of nanospheres and nanodots were obtained on increasing the dose to 2256 Gy (Fig. 2B). On moving to a cleaner reducing system of hydrated electron with tert. butanol as scavenger a reticulated structure of nanorods was formed with length varying from 50 to 100 nm for a dose of 1692 Gy. The corresponding TEM image is shown in Fig. 3A. In the same system increasing the dose to 2256 Gy gave clearly defined nanorods of 25–100 nm with 2–3 nm thickness as shown from Fig. 3B. The formation of reticulated rods can be due to the fact that at lower doses reduction of adsorbed ions at the surface of
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Fig. 1. X-ray powder diffraction pattern for the product formed by air-saturated reduction of 4 104 M KMnO4 compared to the expected lines for MnO2. Dose ¼ 1692 Gy.
Fig. 3. TEM image of nanoparticles formed from reduction of 4 10–4 M KMnO4 by hydrated electron with tert. butanol as scavenger at pH 10. (A) Dose ¼ 1692 Gy and (B) dose ¼ 2256 Gy.
Fig. 2. TEM image of nanoparticles formed from reduction of air-saturated 4 10–4 M KMnO4 at pH 10. (A) Dose ¼ 1692 Gy and (B) dose ¼ 2256 Gy.
clusters is predominant that results in less growth centers and larger clusters. In radiolytic reduction the samples were radiolysed for 3–4 h. Formation of nanorods and nanospheres in air-saturated reduction of Mn (VII) suggest that oriented aggregation is taking place. As on moving to a cleaner reducing system nanorods were formed exclusively. The occurrence of nanospheres in the former case could be explained by oxidative termination of growing nanorods. For reduction of Mn (VII) by hydrated electron for a lower dose (Fig. 3A) reticulated structure was obtained and on increasing the dose to 2256 Gy more well-defined rods were obtained. Oxidation of 1 104 M MnSO4 H2O at pH 10 gave a mixture of reticulated structure of 20-nm thick rods with the length ranging from 60 to 100 nm, also seen were sheets of the 200 to 300 nm length for a dose of 282 Gy. The TEM pictures are shown in Fig. 4A and B, respectively. With further increase in dose (846 Gy) only sheets were seen and the TEM image is reproduced in Fig. 4C. The sheets had folded thickness of 10 nm and length was greater than 400 nm. When the concentration was increased to 2 104 M sheets were obtained. This could be due to sorption of Mn2+ on the colloids. 2 104 M sodium hexametaphosphate was added to 1 104 M Mn(ClO4)2 solution before irradiation by Henglein et al
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Fig. 4. TEM image of nanoparticles formed in the reaction of dOH radical with 1 10–4 M Mn(SO4)2. (A), (B) Dose ¼ 282 Gy and (C) dose ¼ 846 Gy.
and they reported adsorption of 50% of Mn2+ ions on polyphosphate anions (Baral et al., 1986). Also d and l MnO2 polymorphs have been reported to assume a layer structure, with sheets made from MnO6 octahedra, separated by alkali or other ions, and water molecules (Burns and Burns, 1975a,b).
3.4.3. SEM measurements Elemental analysis was performed with scanning electron microscope and the purity was found to be 98% of elemental Mn content in MnO2 formed from both by reduction and oxidation. Trace amount of Fe and silica was also found and the latter can be from the glass used.
4. Conclusions This work helped in identifying the conditions for MnO2 nanoparticle generation by radiolysis. Colloids formed in N2-purged solutions of KMnO4 with tert. butanol as scavenger produced homogenous nanorods, whereas in case of air-saturated reduction additional nanospheres were produced also these colloids were stable and devoid of any background reactions. Nanosheets were produced upon oxidation of Mn (II) and the dose required was much less.
Acknowledgements The financial support from the Carl Tryggers Stiftelse fo¨r Vetenskaplig Forskning is gratefully acknowledged. The authors would like to thank Dr. Susanna Wold for the useful discussions.
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Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Synthesis and characterization of MnO2 colloids Pooja Yadav a,, Richard T. Olsson b,1, Mats Jonsson a,2 a b
School of Chemical Science and Engineering, Nuclear Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Department of Fiber and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden
a r t i c l e in fo
abstract
Article history: Received 22 August 2008 Accepted 18 February 2009
This work addresses the issue of radiation chemical synthesis of MnO2 nanoparticles and also illustrates the ease of formation of nanorods and sheets by adroit manipulation of experimental conditions. The radiation chemical yield (G-value) for reduction of Mn (VII) by the hydrated electron was found to be 0.27 mmol J1 and 0.17 mmol J1 respectively, when tert. butanol and isopropanol were used as scavengers in nitrogen-saturated solutions. The colloids formed upon irradiation of air-saturated solution and N2-purged solution with tert. butanol as scavenger were found to be most stable. Irradiation of air-saturated solution containing 4 104 M KMnO4 at a dose of 1692 Gy resulted in the formation of nanorods of the dimension 100–150 nm and nanospheres in the range 10–20 nm. Irradiation of N2-purged solution containing tert. butanol as scavenger for dOH-produced reticulated structure of nanorods with length varying from 50 to 100 nm at a dose of 1692 Gy. Elemental analysis was performed using scanning electron microscope on MnO2 formed by reduction and oxidation and the purity was found to be 98% of elemental Mn content. & 2009 Published by Elsevier Ltd.
Keywords: Nanostructures Electron microscopy (TEM and SEM) Oxidation Gamma radiolysis Metal oxide
1. Introduction Nanomaterials have captured the imagination of researchers lately due to the significant difference in their properties compared to their coarse-grained counterpart. The greater surface to volume ratio and specific binding sites of nanoparticles enhance catalytic properties (Hiroki and La Verne, 2005). Manganese dioxide is a fascinating inorganic metal oxide owing to its wide range of applications in catalysis, ion exchange, molecular adsorption and particularly in energy storage and also because of its low cost and environmentally benign nature (Nalwa, 2000; Burda et al., 2005). Amongst other things activated MnO2 is widely used in lithium batteries as lithium intercalation host and also as cathode material in primary alkaline batteries (Yuan et al., 2003). One of the challenges facing the chemists today is to synthesize well-defined mono disperse nanoparticles. Various polymorphs of MnO2 exist in nature as the basic octahedral unit (MnO6) and can be linked in different ways. Their properties depend on the crystallographic forms. For example, a-MnO2 phase is very favorable to intercalation and b-MnO2 is passive to it. Therefore, the controlled synthesis of MnO2 has always been the objective of synthetic chemists. It has been Corresponding author. Tel.: +4687908789; fax: +4687908772.
E-mail addresses: [email protected] (P. Yadav), [email protected] (R.T. Olsson), [email protected] (M. Jonsson). 1 Tel.: +4687907640. 2 Tel.: +4687909123; fax: +4687908772. 0969-806X/$ - see front matter & 2009 Published by Elsevier Ltd. doi:10.1016/j.radphyschem.2009.02.006
shown by the density functional theory (DFT) calculations that g-MnO2 is the energetically favored structure (Balachandran et al., 2003; Sayle et al., 2005). Several methods have been developed for MnO2 synthesis ranging from simple reduction (Kim and Popov, 2003; Jeong and Manthiram, 2002), oxidation (Wang and Li, 2002), co-precipitation (Burda et al., 2005; Toupin et al., 2002), sol-gel (AL-Sagheer and Zaki, 2000), thermal decomposition, etc. (Lee and Goodenough, 1999). Radiation chemistry is an effective tool for the synthesis of particles of nanometer dimension owing to facile manipulation of dose and experimental conditions to obtain the required size distribution. It was effectively shown by Henglein that the colloids formed by radiation-induced reduction were smaller than those formed by co-precipitation (LumePereira et al., 1985, Baral et al., 1985, 1986). Henglein et al. reported the synthesis of MnO2 colloids by radiolytic reduction of KMnO4 in air-saturated solution at pH 10 (Lume-Pereira et al., 1985). They further studied the reaction of colloids with various radicals. A size range 3–5 nm was reported for a dose of 700 Gy and the radiolytically produced colloids were smaller in size compared to those prepared by co-precipitation. However, there was no information about the nature and shape of the particles. Henglein also described the formation of colloids by oxidation of Mn2+ by dOH, formation of colloids in the pH range 3.5–9 was observed (Baral et al., 1985). Size of the particles were not reported, however, based on the UV-absorption spectra a larger size was predicted compared to that formed from Mn (VII) reduction. Further, they studied the reaction of sols with 1-hydroxy-1-methylethyl radical and suggested the formation of
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Mn3+ centers. More recently the MnO2 colloids were synthesized by radiolytic reduction of KMnO4 with isopropanol as scavenger and using polyvinyl alcohol (PVA) polymer and sodium dodecyl sulphate (SDS) as surfactant (Liu et al., 1997). In this study, particles of 6 nm for a dose of 2.01 kGy were obtained. XPS studies by the same group showed a valency of +4 for the MnO2 colloids. The aim of this work is to identify the optimal conditions for generation of MnO2 nanoparticles by radiolysis. An effort has been made to synthesize colloids by reduction of KMnO4 by various reducing radicals and also to characterize the MnO2 particles. Alternatively, the synthesis of MnO2 by radiation-induced oxidation of Mn (II) has also been attempted.
2. Experimental KMnO4 and MnSO4 H2O were obtained from Kebo chemicals with a purity of 99%. The rest of the chemicals were purchased from SDS, Fluka, Sigma–Aldrich and Merck. The N2O, N2 and O2 gases were procured from Air Liquide and Strandmollen. All solutions were freshly prepared using deionised water purified by a Millipore-Milli-Q system having a resistivity of 18 MO cm1 and the experiments were carried out at room temperature (22 1C). The pH of the solution was adjusted by using the NaOH (1 104 M) or HClO4. The mean particle size and size distribution was measured using photon correlation spectroscopy with laser of 488 nm wavelength and a fixed scattering angle of 901 (BI–90 particle size, Brookhaven instruments co., USA). The detection range of PCS is between 10 nm and 3 mm and while calculating the mean size the software assumes globular particles. However, formation of nonsperical particles can account for the loss of intensity of the signal for PCS measurements as the PCS is valid for globular systems. The geometry affects the translation motion and also the scattering angle will be different for rods and spheres. The count rate (photon counts per second) is proportional to the concentration of a specific size, when the size distribution is nearly constant. The refractive index and geometry of the particle can influence the intensity of the signal. Wide size distribution presents greater difficulty as the scattered intensity is a function of size. Hence, larger particles contribute more to the measured signal. The zeta potential was measured by means of a ZetaPALS zeta potential analyzer (Brookhaven instruments co., USA), where the z potential was deduced from the particle velocity using Smoluchowski’s equation (Ledin et al., 1993). Irradiations were carried out using a gamma cell (Elite 1000) 137 Cs source. The dose rate was 9.4 Gy/min as determined using the Fricke dosimeter. A brief description of generation of radical follows. d The major products of water radiolysis are free radicals e aq, H , d OH, and molecular products like H3O+, H2 and H2O2. The G-value is defined as moles of species formed or consumed per joule of absorbed energy. The G-values are given in parentheses (mmol J1) (Spinks and Woods, 1990). H2 O
d OHð0:28Þ; e
aq
ð0:28Þ; Hd ð0:047Þ; H2 O2 ð0:073Þ; H2 ð0:047Þ (1)
The reactions of the hydrated electron were studied in N2saturated aqueous solutions containing 0.2 M tert. butanol to effectively scavenge dOH.
OHðH Þ þ ðCH3 Þ3 COH ! ðCH3 Þ2 CH2 COH þ H2 OðH2 Þ
in
(2)
The reaction of the 2-hydroxy-2-propyl radical was studied aqueous solutions containing 0.2 M N2O-saturated
iso-propyl alcohol.
OHðH Þ þ ðCH3 Þ2 CHOH ! ðCH3 Þ2 COH þ H2 OðH2 Þ
(3)
The reactions of OH radical were studied in N2O-saturated solutions. The solubility of N2O in water is 2.5 102 M at 25 1C d and at this concentration, e aq is quantitatively converted into OH. d
H2 O
N2 O þ e aq ! OH þ OH þ N2
(4)
3. Results and discussion 3.1. Reduction of permanganate The aqueous solution of 4 104 M KMnO4 at pH 10 was g irradiated under various conditions to generate the radical of interest. The MnO 4 concentration was measured by UV–Vis spectroscopy at 540 nm, which is the absorption maximum for permanganate. Table 1 lists the G-values obtained for reduction of KMnO4 by various reducing radicals. In case of air-saturated solution the G-value for reduction of KMnO4 is 0.08 mmol J1 and in N2-purged solutions containing tert. butanol the G-value is 0.27 mmol J1, i.e. almost identical to the G-value for the solvated electron. However, the G-value is lowered when isopropanol (0.17 mmol J1) was used as a scavenger despite the increase in yield of reducing radicals. Permanganate is a strong oxidant capable of oxidizing alcohols and other organic reagents. In cases where 2-propanol and HCO 2 were used a significant amount of MnO 4 was consumed in background reactions. The background reaction was also measured and corrected for. However, the radiolytical reduction of MnO 4 is probably not completely independent of the background reaction and the G-values obtained in the systems where background reactions occur are not completely reliable. Furthermore, the 2-hydroxy-2-propyl radical has been shown to react with MnO2 colloids (dose ¼ 700 Gy, size ¼ 3–5 nm) at a rate of 8 106 M1 s1 giving rise to Mn3+, this competing reaction can account for reduced G-value for 3+ are reduction of MnO 4 (Lume-Pereira et al., 1985). Further Mn reduced from an organic radical, the resulting Mn2+ ions undergoes a rapid conproportionation with Mn4+ (Lume-Pereira et al., 1985). Also Mulvaney et al., have shown using both thermal- and radiation-induced dissolution of metal oxides that the Mn3+ Table 1 The G-values for reduction of 4 104 M KMnO4 by various radicals, since KMnO4 oxidizes most alcohols and organic reagents the absorbance was normalized to correct the background reaction. Reaction conditions
Reactive species
Ga value at
l540 nm (mmol J1)
Air saturated N2-purged solution with (CH3)3COH as d OH scavenger N2 purged solution with (CH3)2CHOH as dOH scavenger N2O-purged solution with tert. butanol N2O-purged solution with isopropanol N2O-purged solution with 4 mM sodium formate O2-purged solution with 4 mM sodium formate a
Percent background reaction
(dOH, e aq and Hd) eaq
0.08
–
0.27
–
e aq, (CH3)d2 COH
0.17
20
Tert. butyl radical (CH3)d2 COH
No reduction 0.14
29
COd 2
0.17
38
Od 2
0.15
37
The G value was corrected for background reaction.
–
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Table 2 A comparison of the mean particle size and zeta potential values. 4 104 M KMnO4 at pH 10 was irradiated. Radicals and reaction conditions
Dose/Gy
Mean diameter/nm
Zeta potential/mV
d Air saturated (dOH, e aq and H )
846 1692 2256
9071 8971 9771
38.471.8 46.770.5 36.270.9
d e aq With (CH3)3COH as OH scavenger and N2 saturation
846 1692 2256
2874 2474 2979
8.671.4 3.471.0 17.471.9
centre present in colloids are continually depleted to produce Mn2+ (Mulvaney et al., 1990). The G-values for solution containing d formate (COd 2 ) and formate with O2 (O2 ) were comparable. 3.2. PCS and zeta potential measurements The mean particle sizes and size distributions were measured for all the colloids and are listed in Table 2 along with zeta potential values. Colloidal MnO2 is formed when permanganate is reduced by a multitude of organic reagents (Perez-Benito and Arias, 1992). Therefore, a more controlled reaction and smaller size was obtained when there was no background reaction. This was observed in case of air-saturated solutions and N2-purged solutions containing tert. butanol. In rest of the cases permanganate was reduced giving rise to MnO2 nanoparticles which acted as seeds for further growth rendering it difficult to control the size and also this background reaction was difficult to monitor. As can be expected, aggregation of MnO2 was much faster in cases where salts were added leading to precipitation at lower doses. 4 104 M KMnO4 air-saturated solution was reduced radiolytically and the mean colloid size was found to be 90 nm for a dose of 846 Gy and only a marginal increase in size was observed even after increasing the dose by nearly a factor of three. The particle size upon reduction of Mn (VII) by the hydrated electron was much smaller for the same dose and the corresponding zeta potential absolute values were lower than those obtained for airsaturated reduction. As a consequence, the colloids were less stable compared to colloids produced in air-saturated solution. It should be noted that PCS assumes globular particles which need not necessarily mean that other shapes may not be present. There would be certain discrepancy in readings if the shape is other than globular for example nanorods with greater aspect ratios. 3.3. Oxidation of Mn (II) Colloids were also synthesized by oxidation of Mn (II). PickKaplan, Rabani (1976) and Baral et al. (1986) reported the formation of colloids by oxidation of Mn(ClO4)2 6H2O. Rabani measured the half life of MnO2 nucleation at various [Mn (II)] and concluded that on increasing [Mn (II)] concentration at constant dose slows down the nucleation (Pick-Kaplan and Rabani, 1976). Hengelein carried out a more detailed study on formation of colloids and observed formation of colloidal solutions at pH 3.5–9. Since manganese (II) sulphate monohydrate is colourless the formation of colloid can be followed by change in colour, where the colloidal solution is yellowish brown. Of about 2 104 M of Mn(SO4)2 H2O was irradiated at pH 10, for a dose of 564 Gy a particle size of 51779 nm was recorded on increasing the dose to 846 Gy the size increased to 7317123 nm. The zeta potential value for these particles at a dose of 564 and 846 Gy was 29.673 mV
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and 17.371, respectively. However, these colloids were not stable and precipitated after half an hour. Therefore, the concentration was reduced to 1 104 M accordingly the particle size reduced to 266740 and 571714 nm for a dose of 564 and 846 Gy, respectively, and subsequently there was an increase in absolute zeta potential value. These colloids were some what more stable and precipitated after few hours. When the pH was varied from 2.7 to 11 for 1 104 M of Mn(SO4)2 H2O pale brown-coloured colloids were formed only at pH 11. Henglein reported formation of colloids at pH 5, 7 and 9 (Baral et al., 1986). PCS measurements showed a particle size of 354 nm for a dose of 846 Gy with a corresponding zeta potential of 58.973 mV. 3.4. Characterization 3.4.1. X-ray diffraction The colloid formed by reduction in air-saturated solution was precipitated by addition of 0.5 M NaCl and was then filtered and repeatedly washed with deionised water and later dried in oven at 60 1C. The black powder was then analyzed by X-ray powder diffraction. The diffraction pattern is shown in Fig. 1. As shown from figure the diffraction pattern of the solid conforms to the lines for MnO2 and was amorphous. Amorphous phases studied by solution calorimetry (zirconia and silica) have significantly lower surface enthalpies than their dense crystalline counterparts, indicating that amorphous phases may be thermodynamically as well as kinetically preferred under constraint of small particle size (Pitcher et al., 2004; Piccione et al., 2000). However, due to poor signal to noise ratio the exact polymorph could not be identified. Manganese (IV) oxides and manganese are divided into two structural families: ramsedellite a-MnO2 and pyrolusite b-MnO2. Other forms of manganese dioxies are a random intergrowth of ramsedellite and pyrolusite (Kohler et al., 1997). The imperfections are characterized as microtwinning and the de Wolff disorder and are generally believed to be responsible for poor X-ray diffraction pattern of manganese oxides and oxyhydroxides (Maclean and Tye, 1996). The d-spacings of 0.775, 0.374, 0.242 and 0.141 nm were calculated using the Bragg equation (nl ¼ 2d sin y) and matched well with the data published for birnessite (Matocha et al., 2001). The corresponding indices are (1 0 0), (2 0 0), (3 1 0) and (5 2 1). XRD pattern for colloid formed by oxidation of Mn (II) was also studied and powder was identified as MnO2 and the polymorph could not be determined. 3.4.2. Transmission electron microscopy Since PCS gives ambiguous picture about the size of the particles and none whatsoever what about the shape of the particles, colloids that were further characterized by transmission electron microscopy. 4 104 M KMnO4 was reduced in airsaturated solution and for a dose of 1692 Gy nanorods or needles of dimension 100–150 nm were formed and also visible were a few 20 nm rods that were thicker than the others and nanospheres or dots in the range 10–20 nm were also seen. This is pictorially shown in Fig. 2A. Similar results in terms of appearance of nanospheres and nanodots were obtained on increasing the dose to 2256 Gy (Fig. 2B). On moving to a cleaner reducing system of hydrated electron with tert. butanol as scavenger a reticulated structure of nanorods was formed with length varying from 50 to 100 nm for a dose of 1692 Gy. The corresponding TEM image is shown in Fig. 3A. In the same system increasing the dose to 2256 Gy gave clearly defined nanorods of 25–100 nm with 2–3 nm thickness as shown from Fig. 3B. The formation of reticulated rods can be due to the fact that at lower doses reduction of adsorbed ions at the surface of
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Fig. 1. X-ray powder diffraction pattern for the product formed by air-saturated reduction of 4 104 M KMnO4 compared to the expected lines for MnO2. Dose ¼ 1692 Gy.
Fig. 3. TEM image of nanoparticles formed from reduction of 4 10–4 M KMnO4 by hydrated electron with tert. butanol as scavenger at pH 10. (A) Dose ¼ 1692 Gy and (B) dose ¼ 2256 Gy.
Fig. 2. TEM image of nanoparticles formed from reduction of air-saturated 4 10–4 M KMnO4 at pH 10. (A) Dose ¼ 1692 Gy and (B) dose ¼ 2256 Gy.
clusters is predominant that results in less growth centers and larger clusters. In radiolytic reduction the samples were radiolysed for 3–4 h. Formation of nanorods and nanospheres in air-saturated reduction of Mn (VII) suggest that oriented aggregation is taking place. As on moving to a cleaner reducing system nanorods were formed exclusively. The occurrence of nanospheres in the former case could be explained by oxidative termination of growing nanorods. For reduction of Mn (VII) by hydrated electron for a lower dose (Fig. 3A) reticulated structure was obtained and on increasing the dose to 2256 Gy more well-defined rods were obtained. Oxidation of 1 104 M MnSO4 H2O at pH 10 gave a mixture of reticulated structure of 20-nm thick rods with the length ranging from 60 to 100 nm, also seen were sheets of the 200 to 300 nm length for a dose of 282 Gy. The TEM pictures are shown in Fig. 4A and B, respectively. With further increase in dose (846 Gy) only sheets were seen and the TEM image is reproduced in Fig. 4C. The sheets had folded thickness of 10 nm and length was greater than 400 nm. When the concentration was increased to 2 104 M sheets were obtained. This could be due to sorption of Mn2+ on the colloids. 2 104 M sodium hexametaphosphate was added to 1 104 M Mn(ClO4)2 solution before irradiation by Henglein et al
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Fig. 4. TEM image of nanoparticles formed in the reaction of dOH radical with 1 10–4 M Mn(SO4)2. (A), (B) Dose ¼ 282 Gy and (C) dose ¼ 846 Gy.
and they reported adsorption of 50% of Mn2+ ions on polyphosphate anions (Baral et al., 1986). Also d and l MnO2 polymorphs have been reported to assume a layer structure, with sheets made from MnO6 octahedra, separated by alkali or other ions, and water molecules (Burns and Burns, 1975a,b).
3.4.3. SEM measurements Elemental analysis was performed with scanning electron microscope and the purity was found to be 98% of elemental Mn content in MnO2 formed from both by reduction and oxidation. Trace amount of Fe and silica was also found and the latter can be from the glass used.
4. Conclusions This work helped in identifying the conditions for MnO2 nanoparticle generation by radiolysis. Colloids formed in N2-purged solutions of KMnO4 with tert. butanol as scavenger produced homogenous nanorods, whereas in case of air-saturated reduction additional nanospheres were produced also these colloids were stable and devoid of any background reactions. Nanosheets were produced upon oxidation of Mn (II) and the dose required was much less.
Acknowledgements The financial support from the Carl Tryggers Stiftelse fo¨r Vetenskaplig Forskning is gratefully acknowledged. The authors would like to thank Dr. Susanna Wold for the useful discussions.
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