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Facile synthesis and optical band gap calculation of Mn3O4 nanoparticles
Materials Chemistry and Physics 137 (2012) 637e643
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Facile synthesis and optical band gap calculation of Mn3O4 nanoparticles Nasser Mohammed Hosny a, *, A. Dahshan b, c a
Chemistry Department, Faculty of Science, Port-Said University, 23 December Street, Port-Said, Egypt Physics Department, Faculty of Science, Port-Said University, 23 December Street, Port-Said, Egypt c Physics Department, Faculty of Girls, King Khalid University, Abha, Saudia Arabia b
h i g h l i g h t s < Synthesis of manganese oxides nanoparticles by using metal complexes as precursors. < Characterization of the isolated nanoparticles using XRD, SEM, STM, and HRTEM. < The optical band gap has been calculated and related to the particle size.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 May 2012 Received in revised form 10 August 2012 Accepted 30 September 2012
Mn3O4 nanoparticles were prepared by a simple solid state decomposition method. Four manganese benzoic acid complexes were synthesized through semi-solid phase reaction method as precursors for the preparation of Mn3O4 nanoparticles. The calcination temperature of the precursors was determined from thermal gravimetrical analyses (TGA). The resulting nanoparticles were characterized by XRD, SEM, STM and HRTEM. The obtained particle size is in the range 39e90 nm. HRTEM indicated the formation of spherical nanoparticles. The optical absorption measurements for the obtained nanoparticles showed that the fundamental absorption edge obeys Tauc’s relation for the allowed direct transition. It was found that, the optical band gap (Eg) increases with the decrease of the particle size of the Mn3O4 nanoparticles. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Optical properties Optical materials
1. Introduction Nanometer-size semiconductors have attracted considerable interest in the recent years due to their size-dependent properties and their wide applications in nonlinear optics, photoelectrochemical cells, heterogeneous photocatalysis, optical switching, and single electron transistors [1e7]. Manganese oxides form several phases such as MnO, MnO2, Mn2O3 and Mn3O4. Because of the potential applications of these materials in catalysis and battery technologies [8], many efforts have been put on their synthesis. One of the most important oxides is Mn3O4 (hausmanite) with the spinel structure, which can be used as an active catalyst for the decomposition of the methylene blue dye [9], oxidation of methane, carbon monoxide [10] and the selective reduction of nitrobenzene [11]. So far, various nanostructures of Mn3O4, such as nanoparticles [10], nanorods, nanowires [11,12], mesoporous hollow structures [13,14], urchins corallike and other structures [15] have been synthesized by different * Corresponding author. Tel.: þ20 1021644684. E-mail addresses: [email protected] (N.M. Hosny), [email protected] (A. Dahshan). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.09.068
methods. Ganguli and co-workers [16] prepared 100 nm nanorods of Mn3O4 by using manganese oxalate as precursor. Yang et al. synthesized Mn3O4 nanocrystals from MnCl2 and Mn(CH3COO)2, respectively [17]. Nanoparticles of Mn3O4 have been prepared from manganese acetate and manganese acetylacetonate [18]. Lawes et al. used a co-precipitation method to prepare Mn3O4 nanoparticles [19]. Mn3O4 nanoparticles were synthesized from [bis(salicylidiminato) manganese(II)] precursor by a thermal decomposition method [20]. Mn3O4 nanocrystals have been prepared from manganese acetylacetonate [21]. MnO and Mn3O4 have been prepared via thermolysis of manganese formate in coordinating solvent [22]. Qian et al. [23] synthesized Mn3O4 nanorods by vacuum calcining of MnOOH precursor at 500 C Mn3O4 was also prepared through calcining nanostructure of cMnOOH precursor at 500e800 C for 4e16 h [9]. Baykal and coworkers have reported several papers in synthesis of Mn3O4 nanostructure by using different methods as the oxidation of manganese sulfate [24]. Water-soluble Mn3O4 nanocrystals were prepared through thermal decomposition in a high temperature boiling solvent, 2-pyrrolidone [25]. Mn3O4 was prepared by heating hydrous manganese hydroxide gel at 85 C for 12 h [26]. Also, Mn3O4 nanocrystals were prepared in ionic liquid media at room
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temperature [27]. Nanoparticles of Mn2O3 has been prepared from a manganese supramolecule precursor by thermolyses in oleic acid at 200 C under nitrogen [28]. Solid state decomposition route has been used to prepare Mn3O4 (hausmanite) nanoparticles, with a tetragonal structure by using [bis(2-hydroxy-1-naphthaldehydato)manganese(II)] complex as a new precursor [29]. Here, we prepared hausmanite nanoparticles with average particle size 39e90 nm by relatively mild solid state decomposition method. This work has many advantages as, it provides green chemistry solvent, cheap available chemicals and a simple method could be applied to produce nanoparticles of Mn3O4 on commercial scales. The optical absorption measurements have been made on the formed nanoparticles to obtain the band structure and the energy gap.
of precursors (3) and (4), respectively. The mixture was ground for 20 min in a mortar. Another 5 ml of water have been added to have a semi-solid paste. The mixture was dried at 100 C for 2 h, pink powder was obtained for all the four metal-organic composite. Anal. Found for [MnHL.5H2O]Cl2.4H2O (1) (where HL ¼ the ligand (benzoic acid)): MnC7H24O11Cl2: C, 20.2; H, 5.3; Mn, 12.5; Cl, 17.0. Calc.: C, 20.6; H, 5.9; Mn, 12.8; Cl, 17.4% and Anal. Found for [Mn(HL)2.4H2O]Cl2.2H2O (2): Mn(C14H24O10)2: C, 39.6; H, 5.2; Mn, 16.2, Cl, 12.6 Calc.: C, 39.1; H, 5.0; Mn, 16.5; Cl, 12.1% and Anal. Found for [MnHL.5H2O] SO4 (3): Mn(C7H16O11S): C, 23.0; H, 3.9; Mn, 14.9. Calc.: C, 23.3; H, 4.4; Mn, 14.4% and Anal. Found for [Mn(HL)2.4H2O] SO4 (4): Mn(C14H20O10): C, 35.9; H, 3.8; Mn, 11.6. Calc.: C, 36.2; H, 4.3; Mn, 11.2%. Scheme 1 represents the formation of precursors. 2.3. Synthesis of manganese oxides nanoparticles
2. Experimental All the chemicals used were of analytical grade and used without further purifications.
The as-prepared Mn-benzoic acid precursors were ignited in a muffle furnace at a rate of 50 C min1. in air. Manganese oxides nanoparticles were synthesized at 500 C for 2 h.
2.1. Technique
2.4. Absorption measurements
The metal analyses were carried out by the standard methods [30]. Molar conductance measurements of the complexes (103 M) in DMSO were carried out with a conductivity bridge YSI model 32. Infrared spectra were measured using KBr discs on a Mattson 5000 FTIR Spectrometer. Calibration with the frequency reading was made with polystyrene film. Thermal analysis measurements (TGA) were recorded on a Schimadzu model 50 instrument using 20 mg samples. The nitrogen flow rate and heating rate were 20 cm3 min1. and 10 C min1, respectively. XRD patterns were recorded using Philips XPERT-PRO with nickel filtered Cu Ka (l ¼ 1.5405 A) radiation. Scanning electron microscopy (SEM) images were taken by ZEISS MC 63A microscope. HRTEM images of the products were obtained by CM 20 PHILIPS electron microscopes.
A double beam (Jasco V-630) spectrophotometer was used to measure the absorption spectra for the manganese oxides nanoparticles in the spectral range of wavelength from 400 to 2500 nm. 3. Results and discussion All the precursors are pink in color, stable in air and soluble in water. The presence of ionizable chloride and sulfate ions are ensured by AgNO3 and BaCl2 respectively, without digesting the complexes with HNO3. The molar conductivities values in H2O (103 M) at 25 C are in the range 180e200 U1 cm2 mol1. These values suggest that all precursors are electrolytic in nature [31]. Table 1 collects the most important infrared bands of benzoic acid and its manganese complexes. FT-IR spectrum of benzoic acid shows several bands at 1688, 1328 and 1381 cm1 assigned to nas (COO), ns (COO) and d (OH), respectively [32]. The spectra of all precursors (Fig. 1) show bands in the region 1631e1686 cm1 assigned to nas (COO) of coordinated carboxyl group [33]. This band is shifted to lower wave-number in comparison with its position in the benzoic acid. Also, the spectra of all precursors show, weak band in the regions 1413e1421 assigned to d (OH). This band is shifted to higher wave-number compared with its position in the
2.2. Synthesis of precursors The investigated precursors have been prepared by mixing (1:1) and (1:2) molar ratios of the hydrated manganese chloride and benzoic acid in 5 ml distilled water for preparation of precursors (1) and (2), respectively. The same molar ratios of manganese sulfate and benzoic acid in 5 ml distilled water were used for preparation
O OH
H O H2O O H
MnSO4 or
O Mn
H OH2
X. nH2O
OH2
OH2
500 o C
X = Cl2 or SO4 n = 0 or 4
+
MnCl2.4H2O
̉
H O H2O
O
2
OH
O M
H
O OH2
O O H OH2H
X . nH2O
X = Cl2 or SO4 n = 0 or 2
Scheme 1. Formation of precursors and manganese oxide nanoparticles.
N.M. Hosny, A. Dahshan / Materials Chemistry and Physics 137 (2012) 637e643 Table 1 Some FT-IR spectral bands of benzic acid and its manganese complexes.
Table 2 Thermo analytical results (TG) of precursors (1), (2), (3) and (4).
Precursors
d(OH)
nas(COO) ns(COO)
n(CeO)
n(MeO)
(HL) Benzoic acid [MnHL.5H2O]Cl2.4H2O (1) [Mn(HL)2.4H2O]Cl2.2H2O (2) [MnHL.5H2O] SO4(3) [Mn(HL)2. 4H2O] SO4 (4)
1381 1423 1413 1421 1421
1688 1631 1635 1685 1686
1300 1292 1288 1290 1290
e 669 665 624 661
m w w s s
s s s s s
1328 1391 1388 1360 1363
m w sh w m
m m w w w
639
w w w w
Precursor
T range ( C)
Mass loss estim. (Calcd%)
Assignment of fragment lost
[MnHL.5H2O]Cl2.4H2O (1)
25e102 102e248
18.1 (17.7) 47.3 (47.9)
248e536 25e140 140e364
17.2 (17.4) 9.0 (8.3) 64.1 (65.6)
364e630 29e180 180e318 318e500 28e187
16.0 24.7 30.5 23.0 55.5
187e535
22.0 (23.2)
4H2O Ph, CO, and 5H2O Cl2 2H2O 4H2O, 2Ph, and 2CO Cl2 5H2O Ph and CO SO3 4H2O, 2Ph and CO CO and SO3
[Mn(HL)2.4H2O]Cl2.2H2O (2)
HL ¼ The ligand (benzoic acid).
free ligand, indicating the participation of the carboxyl group in bonding in the protonated form. The difference between nas (COO) and ns (COO) of all precursors (Table 1) is greater than 200 cm1, confirming the coordination of the carboxyl in a mono-dentate manner [33,34]. Two new weak bands are observed in the region 962e971 and 624e630 cm1 in the spectra of precursors (3) and (4). These bands are assigned to vibrations of ionizable sulfate group [33,35]. The spectra of all precursors show broad band centered at w3400 cm1 resulted from the presence of water molecules. This band overlaps with the stretching vibrations of the benzoic acid hydroxyl group. FT-IR spectrum of Mn3O4 nanoparticles shows bands at 625 and 513 cm1 characteristic to Mn3O4 [28,36]. From the above findings it could be concluded that, benzoic acid binds to manganese ion in a neutral mono-dentate manner through carboxylate oxygen. This behavior is supported by the results which reported that, when a coordinating ligand is water molecule, the most stable coordination mode of carboxylate group is the mono-dentate. Where, the structures are stabilized by a very strong hydrogen bond between the non-coordinating carboxylate oxygen and the in-plane water hydrogen [37].
[MnHL.5H2O]SO4 (3)
[Mn(HL)2.4H2O]SO4 (4)
(16.5) (25.0) (29.1) (22.2) (54.7)
HL ¼ The ligand (benzoic acid).
that, water of hydration is lost firstly followed by coordinated water then, the organic moiety and finally chlorides or sulfates. 3.2. Electronic spectra and magnetic moment The electronic spectra of the four precursors in DMF show three bands around 19,000, 23,500 and 28,500 cm1 which may be assigned to 6A1g / 4T1g(G), 6A1g / 4T2g(G) and 6A1g / 4Eg, respectively. These bands are clearly observed indicating octahedrally coordinated manganese(II) compounds [38].
a 10 3.1. Thermal analyses
9 8 7
(mg)
The thermal analyses (TGA) curves of the complexes were carried out within a temperature range from room temperature up to 800 C. The estimated mass losses were computed based on the TGA results and the calculated mass losses are computed using the results of microanalyses (Table 2). The results of thermogravimetric analyses of the precursors (1) and (2) are given in Fig. 2a and b. The TGA curves help in deciding the calcination temperature for the precursors. The thermal analyses curves of all precursors indicate
6 5 4 3 2 1
(4)
0
200
400
600
800
Temperature (C)
b 15
(2)
(mg)
% Transmittance
(3)
(1)
4000
3500
3000
2500
2000
1500 -1
wavenumber (cm ) Fig. 1. FT-IR spectra of precursors (1)e(4).
1000
500
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0
200
400
600
800
Temperature (C) Fig. 2. (a) TGA of precursor (1). (b) TGA of precursor (2).
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N.M. Hosny, A. Dahshan / Materials Chemistry and Physics 137 (2012) 637e643
The values of magnetic moments (5.7e5.9 B.M.) for the manganese precursors fall in the range reported for high-spin Mn(II) compounds. 3.3. XRD, SEM and HRTEM Fig. 3 shows XRD pattern of manganese oxides derived from precursors (1e4) after calcination at 500 C. The crystallite size of manganese oxides were calculated from the major diffraction peak of manganese oxide, using DebyeeScherrer formula D ¼ 0.89 l/ b cosq [39]. Where l is the wavelength of x-ray (1.5406 Å) for Cu Ka radiation, b is full width at half maximum and q is the peak position. The estimated particle size was found to be 90, 78 and 39 nm for samples of Mn3O4 which resulted from the ignition of precursors (1), (2), and (4), respectively. The peaks are indexed to hausmanite lattice (Mn3O4) (JCPDS Card No. 16-0154). Body-centered cubic lattice is indexed for Mn2O3 obtained from precursor (3) (JCPDS Card No. 71-0636). The particle size obtained for Mn2O3 is 70 nm. Some peaks correspond to unreacted manganese chloride and sulfate are observed in spectra of the samples derived from precursors 1 and 4, respectively. So, Precursor (2) is considered the most suitable one for preparation of Mn3O4 nanoparticles. Formation of Mn2O3 from the ignition of precursor (3) could be explained on the basis that, all MnO is directly oxidized to Mn2O3, it is expected that the presence of SO4 2 in precursor (3) plays a role in the formation of this oxide through redox reaction with the other decomposition products [24]. It could be concluded that, changing the anion does not affect the morphology of the particles, since all the samples have the same shape. Also, Mn3O4 obtained from precursors with higher ratios (1:2) of manganese to benzoic acid, are smaller than that obtained from lower ratio precursors. This behavior could be explained on the basis that, the presence of two molecules of the organic moiety save good coating of the metal and decreases aggregation. The following mechanism could be suggested for the formation of Mn3O4 nanoparticles. 500 C
MnðC7 H6 O2 Þn ! CO2 þ H2 O þ MnO n ¼ 1 or 2
2MnO þ 1=2O2 /Mn2 O3 Mn2 O3 þ MnO/Mn3 O4
(111)
(311) (400)
(331)
(511) (440)
(4)
Lin(Counts)
(3)
(020) (211) (112) (103)
(220) (105) (312)
(314)
(2)
SEM images (Fig. 4a and b) show aggregation of spherical nanoparticles forming flower-like shape of Mn3O4. HRTEM images (Fig. 4c and d) shows Mn3O4 nanoparticles are spherical in shape. The average diameter of the spherical Mn3O4 nanoparticles calculated from HRTEM is 75 nm. This value is in agreement with that calculated from XRD. 3.4. Optical band gap The absorption spectra for the manganese oxides nanoparticles (samples 1, 2, 3 and 4) which resulted from the ignition of precursors (1), (2), (3) and (4), respectively are shown in Fig. 5. The absorption coefficient (a) can be calculated from the optical absorption spectra using the following relationship
a ¼ A=d
(1)
where d is the thickness of the specimen. According to Tauc’s relation [40,41] for allowed direct transitions, the photon energy dependence of the absorption coefficient can be described by
ðahnÞ2 ¼ B hn Eg
(2)
where B is a parameter that depends on the transition probability and Eg is the optical energy gap. Fig. 6 shows the absorption coefficient in the form of (ahn)2 versus hn for the manganese oxides nanoparticles. The intercepts of the straight lines with the photon energy axis yield values of the optical band gap. The values of Eg and particle size of the Mn3O4 nanoparticles (samples 1, 2 and 4) which resulted from the ignition of precursors (1), (2) and (4), respectively are shown in Fig. 7. As shown in this figure the optical band gap increases while the particle size of the Mn3O4 nanoparticles decreases. The optical direct band gap were found to be 3.28, 3.51, 3.75 eV for Mn3O4 derived from precursors 1, 2 and 4, respectively and 3.69 eV for Mn2O3 derived from precursors 3. These values reveal that the manganese oxide nanoparticles are semi-conductors. Also, the values of Eg are in the range reported for the high efficient photovoltaic materials. So, the present compounds could be considered potential materials for harvesting solar radiation in solar cell applications [42]. It is clear from Fig. 5 that, the samples exhibit absorption edges which are blue-shifted with decreasing particle size. This blue shift of the absorption edges for different sized nanoparticles arises from quantum size effect in the nanoparticles. This phenomenon causes the continuous band of the solid to split into discrete, quantized levels and the “band gap” to increase [43]. The obtained values of the band gap of Mn3O4 nanoparticles are higher than that of the samples prepared by Dubal et al. (2.30 eV) [44], Xu et al. (2.54 eV) [45]. This shift of the band gap takes place because of the quantum confinement effect [46]. The wider optical band gap of the present compounds compared with the previously reported values, makes them more applicable in optoelectronic devices, specially light emitting devices in the short wavelength region of visible light. 4. Conclusion
(1)
10
20
30
40
50
60
70
2-Theta-scale Fig. 3. XRD powder pattern of manganese oxides nanoparticles.
80
Hausmanite nanoparticles with spherical shape have been obtained from manganese benzoic acid precursors. A simple benign semi-solid method has been used. The optical properties of manganese oxides reveals that, direct electronic transition is mainly responsible for the photon absorption inside the investigated manganese oxides nanoparticles. The values of the optical band gap for the nanoparticles increase while the particle size
N.M. Hosny, A. Dahshan / Materials Chemistry and Physics 137 (2012) 637e643
641
Fig. 4. (A),(B) SEM images (C), (D) HRTEM of Mn3O4 of samples(2) and (4), respectively after calcination at 500 C.
0.8
0.8
Absorbance
b 1.0
Absorbance
a 1.0
0.6 0.4 0.2 0.0 200
300
400
500
600
700
800
0.0 200
900
Wavelength (nm)
300
400
500
600
700
800
900
800
900
Wavelength (nm)
d 1.0 0.8
Absorbance
0.8
Absorbance
0.4 0.2
c 1.0
0.6 0.4
0.6 0.4 0.2
0.2 0.0 200
0.6
300
400
500
600
700
Wavelength (nm)
800
900
0.0 200
300
400
500
600
700
Wavelength (nm)
Fig. 5. Absorption spectra for the manganese oxides nanoparticles ((a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4).
642
N.M. Hosny, A. Dahshan / Materials Chemistry and Physics 137 (2012) 637e643
a
b
0.20 0.16
-1
( h ) x 10 (cm eV)
0.12
14
3
2
0.08
2
2
14
-1
( h ) x 10 (cm eV)
2
2
4
1
0 3.0
3.2
3.4
3.6
3.8
0.04 0.00 3.0
4.0
3.2
d ( h ) x 10 (cm eV)
14
2
2
14
2
3.8
4.0
4
3
-1
3
-1
( h ) x 10 (cm eV)
3.6
2
4
2
c
3.4
h (eV)
h (eV)
1
0 3.0
3.5
4.0
4.5
5.0
h (eV)
2
1
0 3.0
3.5
4.0
4.5
5.0
h (eV)
2
4.0
100
3.8
80
3.6
60
3.4
40
3.2
20
3.0
1
2
3
4
Particle size (nm)
Eg (eV)
Fig. 6. Absorption coefficient in the form of (ahn) versus hn for the manganese oxides nanoparticles ((a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4).
0
Samples Fig. 7. Optical band gap (Eg) and particle size of the Mn3O4 nanoparticles.
decreases. The investigated nanoparticles are semi-conductors and could be used as harvesting materials in solar cell devices. References [1] [2] [3] [4] [5] [6] [7] [8]
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Facile synthesis and optical band gap calculation of Mn3O4 nanoparticles Nasser Mohammed Hosny a, *, A. Dahshan b, c a
Chemistry Department, Faculty of Science, Port-Said University, 23 December Street, Port-Said, Egypt Physics Department, Faculty of Science, Port-Said University, 23 December Street, Port-Said, Egypt c Physics Department, Faculty of Girls, King Khalid University, Abha, Saudia Arabia b
h i g h l i g h t s < Synthesis of manganese oxides nanoparticles by using metal complexes as precursors. < Characterization of the isolated nanoparticles using XRD, SEM, STM, and HRTEM. < The optical band gap has been calculated and related to the particle size.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 May 2012 Received in revised form 10 August 2012 Accepted 30 September 2012
Mn3O4 nanoparticles were prepared by a simple solid state decomposition method. Four manganese benzoic acid complexes were synthesized through semi-solid phase reaction method as precursors for the preparation of Mn3O4 nanoparticles. The calcination temperature of the precursors was determined from thermal gravimetrical analyses (TGA). The resulting nanoparticles were characterized by XRD, SEM, STM and HRTEM. The obtained particle size is in the range 39e90 nm. HRTEM indicated the formation of spherical nanoparticles. The optical absorption measurements for the obtained nanoparticles showed that the fundamental absorption edge obeys Tauc’s relation for the allowed direct transition. It was found that, the optical band gap (Eg) increases with the decrease of the particle size of the Mn3O4 nanoparticles. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Optical properties Optical materials
1. Introduction Nanometer-size semiconductors have attracted considerable interest in the recent years due to their size-dependent properties and their wide applications in nonlinear optics, photoelectrochemical cells, heterogeneous photocatalysis, optical switching, and single electron transistors [1e7]. Manganese oxides form several phases such as MnO, MnO2, Mn2O3 and Mn3O4. Because of the potential applications of these materials in catalysis and battery technologies [8], many efforts have been put on their synthesis. One of the most important oxides is Mn3O4 (hausmanite) with the spinel structure, which can be used as an active catalyst for the decomposition of the methylene blue dye [9], oxidation of methane, carbon monoxide [10] and the selective reduction of nitrobenzene [11]. So far, various nanostructures of Mn3O4, such as nanoparticles [10], nanorods, nanowires [11,12], mesoporous hollow structures [13,14], urchins corallike and other structures [15] have been synthesized by different * Corresponding author. Tel.: þ20 1021644684. E-mail addresses: [email protected] (N.M. Hosny), [email protected] (A. Dahshan). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.09.068
methods. Ganguli and co-workers [16] prepared 100 nm nanorods of Mn3O4 by using manganese oxalate as precursor. Yang et al. synthesized Mn3O4 nanocrystals from MnCl2 and Mn(CH3COO)2, respectively [17]. Nanoparticles of Mn3O4 have been prepared from manganese acetate and manganese acetylacetonate [18]. Lawes et al. used a co-precipitation method to prepare Mn3O4 nanoparticles [19]. Mn3O4 nanoparticles were synthesized from [bis(salicylidiminato) manganese(II)] precursor by a thermal decomposition method [20]. Mn3O4 nanocrystals have been prepared from manganese acetylacetonate [21]. MnO and Mn3O4 have been prepared via thermolysis of manganese formate in coordinating solvent [22]. Qian et al. [23] synthesized Mn3O4 nanorods by vacuum calcining of MnOOH precursor at 500 C Mn3O4 was also prepared through calcining nanostructure of cMnOOH precursor at 500e800 C for 4e16 h [9]. Baykal and coworkers have reported several papers in synthesis of Mn3O4 nanostructure by using different methods as the oxidation of manganese sulfate [24]. Water-soluble Mn3O4 nanocrystals were prepared through thermal decomposition in a high temperature boiling solvent, 2-pyrrolidone [25]. Mn3O4 was prepared by heating hydrous manganese hydroxide gel at 85 C for 12 h [26]. Also, Mn3O4 nanocrystals were prepared in ionic liquid media at room
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temperature [27]. Nanoparticles of Mn2O3 has been prepared from a manganese supramolecule precursor by thermolyses in oleic acid at 200 C under nitrogen [28]. Solid state decomposition route has been used to prepare Mn3O4 (hausmanite) nanoparticles, with a tetragonal structure by using [bis(2-hydroxy-1-naphthaldehydato)manganese(II)] complex as a new precursor [29]. Here, we prepared hausmanite nanoparticles with average particle size 39e90 nm by relatively mild solid state decomposition method. This work has many advantages as, it provides green chemistry solvent, cheap available chemicals and a simple method could be applied to produce nanoparticles of Mn3O4 on commercial scales. The optical absorption measurements have been made on the formed nanoparticles to obtain the band structure and the energy gap.
of precursors (3) and (4), respectively. The mixture was ground for 20 min in a mortar. Another 5 ml of water have been added to have a semi-solid paste. The mixture was dried at 100 C for 2 h, pink powder was obtained for all the four metal-organic composite. Anal. Found for [MnHL.5H2O]Cl2.4H2O (1) (where HL ¼ the ligand (benzoic acid)): MnC7H24O11Cl2: C, 20.2; H, 5.3; Mn, 12.5; Cl, 17.0. Calc.: C, 20.6; H, 5.9; Mn, 12.8; Cl, 17.4% and Anal. Found for [Mn(HL)2.4H2O]Cl2.2H2O (2): Mn(C14H24O10)2: C, 39.6; H, 5.2; Mn, 16.2, Cl, 12.6 Calc.: C, 39.1; H, 5.0; Mn, 16.5; Cl, 12.1% and Anal. Found for [MnHL.5H2O] SO4 (3): Mn(C7H16O11S): C, 23.0; H, 3.9; Mn, 14.9. Calc.: C, 23.3; H, 4.4; Mn, 14.4% and Anal. Found for [Mn(HL)2.4H2O] SO4 (4): Mn(C14H20O10): C, 35.9; H, 3.8; Mn, 11.6. Calc.: C, 36.2; H, 4.3; Mn, 11.2%. Scheme 1 represents the formation of precursors. 2.3. Synthesis of manganese oxides nanoparticles
2. Experimental All the chemicals used were of analytical grade and used without further purifications.
The as-prepared Mn-benzoic acid precursors were ignited in a muffle furnace at a rate of 50 C min1. in air. Manganese oxides nanoparticles were synthesized at 500 C for 2 h.
2.1. Technique
2.4. Absorption measurements
The metal analyses were carried out by the standard methods [30]. Molar conductance measurements of the complexes (103 M) in DMSO were carried out with a conductivity bridge YSI model 32. Infrared spectra were measured using KBr discs on a Mattson 5000 FTIR Spectrometer. Calibration with the frequency reading was made with polystyrene film. Thermal analysis measurements (TGA) were recorded on a Schimadzu model 50 instrument using 20 mg samples. The nitrogen flow rate and heating rate were 20 cm3 min1. and 10 C min1, respectively. XRD patterns were recorded using Philips XPERT-PRO with nickel filtered Cu Ka (l ¼ 1.5405 A) radiation. Scanning electron microscopy (SEM) images were taken by ZEISS MC 63A microscope. HRTEM images of the products were obtained by CM 20 PHILIPS electron microscopes.
A double beam (Jasco V-630) spectrophotometer was used to measure the absorption spectra for the manganese oxides nanoparticles in the spectral range of wavelength from 400 to 2500 nm. 3. Results and discussion All the precursors are pink in color, stable in air and soluble in water. The presence of ionizable chloride and sulfate ions are ensured by AgNO3 and BaCl2 respectively, without digesting the complexes with HNO3. The molar conductivities values in H2O (103 M) at 25 C are in the range 180e200 U1 cm2 mol1. These values suggest that all precursors are electrolytic in nature [31]. Table 1 collects the most important infrared bands of benzoic acid and its manganese complexes. FT-IR spectrum of benzoic acid shows several bands at 1688, 1328 and 1381 cm1 assigned to nas (COO), ns (COO) and d (OH), respectively [32]. The spectra of all precursors (Fig. 1) show bands in the region 1631e1686 cm1 assigned to nas (COO) of coordinated carboxyl group [33]. This band is shifted to lower wave-number in comparison with its position in the benzoic acid. Also, the spectra of all precursors show, weak band in the regions 1413e1421 assigned to d (OH). This band is shifted to higher wave-number compared with its position in the
2.2. Synthesis of precursors The investigated precursors have been prepared by mixing (1:1) and (1:2) molar ratios of the hydrated manganese chloride and benzoic acid in 5 ml distilled water for preparation of precursors (1) and (2), respectively. The same molar ratios of manganese sulfate and benzoic acid in 5 ml distilled water were used for preparation
O OH
H O H2O O H
MnSO4 or
O Mn
H OH2
X. nH2O
OH2
OH2
500 o C
X = Cl2 or SO4 n = 0 or 4
+
MnCl2.4H2O
̉
H O H2O
O
2
OH
O M
H
O OH2
O O H OH2H
X . nH2O
X = Cl2 or SO4 n = 0 or 2
Scheme 1. Formation of precursors and manganese oxide nanoparticles.
N.M. Hosny, A. Dahshan / Materials Chemistry and Physics 137 (2012) 637e643 Table 1 Some FT-IR spectral bands of benzic acid and its manganese complexes.
Table 2 Thermo analytical results (TG) of precursors (1), (2), (3) and (4).
Precursors
d(OH)
nas(COO) ns(COO)
n(CeO)
n(MeO)
(HL) Benzoic acid [MnHL.5H2O]Cl2.4H2O (1) [Mn(HL)2.4H2O]Cl2.2H2O (2) [MnHL.5H2O] SO4(3) [Mn(HL)2. 4H2O] SO4 (4)
1381 1423 1413 1421 1421
1688 1631 1635 1685 1686
1300 1292 1288 1290 1290
e 669 665 624 661
m w w s s
s s s s s
1328 1391 1388 1360 1363
m w sh w m
m m w w w
639
w w w w
Precursor
T range ( C)
Mass loss estim. (Calcd%)
Assignment of fragment lost
[MnHL.5H2O]Cl2.4H2O (1)
25e102 102e248
18.1 (17.7) 47.3 (47.9)
248e536 25e140 140e364
17.2 (17.4) 9.0 (8.3) 64.1 (65.6)
364e630 29e180 180e318 318e500 28e187
16.0 24.7 30.5 23.0 55.5
187e535
22.0 (23.2)
4H2O Ph, CO, and 5H2O Cl2 2H2O 4H2O, 2Ph, and 2CO Cl2 5H2O Ph and CO SO3 4H2O, 2Ph and CO CO and SO3
[Mn(HL)2.4H2O]Cl2.2H2O (2)
HL ¼ The ligand (benzoic acid).
free ligand, indicating the participation of the carboxyl group in bonding in the protonated form. The difference between nas (COO) and ns (COO) of all precursors (Table 1) is greater than 200 cm1, confirming the coordination of the carboxyl in a mono-dentate manner [33,34]. Two new weak bands are observed in the region 962e971 and 624e630 cm1 in the spectra of precursors (3) and (4). These bands are assigned to vibrations of ionizable sulfate group [33,35]. The spectra of all precursors show broad band centered at w3400 cm1 resulted from the presence of water molecules. This band overlaps with the stretching vibrations of the benzoic acid hydroxyl group. FT-IR spectrum of Mn3O4 nanoparticles shows bands at 625 and 513 cm1 characteristic to Mn3O4 [28,36]. From the above findings it could be concluded that, benzoic acid binds to manganese ion in a neutral mono-dentate manner through carboxylate oxygen. This behavior is supported by the results which reported that, when a coordinating ligand is water molecule, the most stable coordination mode of carboxylate group is the mono-dentate. Where, the structures are stabilized by a very strong hydrogen bond between the non-coordinating carboxylate oxygen and the in-plane water hydrogen [37].
[MnHL.5H2O]SO4 (3)
[Mn(HL)2.4H2O]SO4 (4)
(16.5) (25.0) (29.1) (22.2) (54.7)
HL ¼ The ligand (benzoic acid).
that, water of hydration is lost firstly followed by coordinated water then, the organic moiety and finally chlorides or sulfates. 3.2. Electronic spectra and magnetic moment The electronic spectra of the four precursors in DMF show three bands around 19,000, 23,500 and 28,500 cm1 which may be assigned to 6A1g / 4T1g(G), 6A1g / 4T2g(G) and 6A1g / 4Eg, respectively. These bands are clearly observed indicating octahedrally coordinated manganese(II) compounds [38].
a 10 3.1. Thermal analyses
9 8 7
(mg)
The thermal analyses (TGA) curves of the complexes were carried out within a temperature range from room temperature up to 800 C. The estimated mass losses were computed based on the TGA results and the calculated mass losses are computed using the results of microanalyses (Table 2). The results of thermogravimetric analyses of the precursors (1) and (2) are given in Fig. 2a and b. The TGA curves help in deciding the calcination temperature for the precursors. The thermal analyses curves of all precursors indicate
6 5 4 3 2 1
(4)
0
200
400
600
800
Temperature (C)
b 15
(2)
(mg)
% Transmittance
(3)
(1)
4000
3500
3000
2500
2000
1500 -1
wavenumber (cm ) Fig. 1. FT-IR spectra of precursors (1)e(4).
1000
500
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0
200
400
600
800
Temperature (C) Fig. 2. (a) TGA of precursor (1). (b) TGA of precursor (2).
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N.M. Hosny, A. Dahshan / Materials Chemistry and Physics 137 (2012) 637e643
The values of magnetic moments (5.7e5.9 B.M.) for the manganese precursors fall in the range reported for high-spin Mn(II) compounds. 3.3. XRD, SEM and HRTEM Fig. 3 shows XRD pattern of manganese oxides derived from precursors (1e4) after calcination at 500 C. The crystallite size of manganese oxides were calculated from the major diffraction peak of manganese oxide, using DebyeeScherrer formula D ¼ 0.89 l/ b cosq [39]. Where l is the wavelength of x-ray (1.5406 Å) for Cu Ka radiation, b is full width at half maximum and q is the peak position. The estimated particle size was found to be 90, 78 and 39 nm for samples of Mn3O4 which resulted from the ignition of precursors (1), (2), and (4), respectively. The peaks are indexed to hausmanite lattice (Mn3O4) (JCPDS Card No. 16-0154). Body-centered cubic lattice is indexed for Mn2O3 obtained from precursor (3) (JCPDS Card No. 71-0636). The particle size obtained for Mn2O3 is 70 nm. Some peaks correspond to unreacted manganese chloride and sulfate are observed in spectra of the samples derived from precursors 1 and 4, respectively. So, Precursor (2) is considered the most suitable one for preparation of Mn3O4 nanoparticles. Formation of Mn2O3 from the ignition of precursor (3) could be explained on the basis that, all MnO is directly oxidized to Mn2O3, it is expected that the presence of SO4 2 in precursor (3) plays a role in the formation of this oxide through redox reaction with the other decomposition products [24]. It could be concluded that, changing the anion does not affect the morphology of the particles, since all the samples have the same shape. Also, Mn3O4 obtained from precursors with higher ratios (1:2) of manganese to benzoic acid, are smaller than that obtained from lower ratio precursors. This behavior could be explained on the basis that, the presence of two molecules of the organic moiety save good coating of the metal and decreases aggregation. The following mechanism could be suggested for the formation of Mn3O4 nanoparticles. 500 C
MnðC7 H6 O2 Þn ! CO2 þ H2 O þ MnO n ¼ 1 or 2
2MnO þ 1=2O2 /Mn2 O3 Mn2 O3 þ MnO/Mn3 O4
(111)
(311) (400)
(331)
(511) (440)
(4)
Lin(Counts)
(3)
(020) (211) (112) (103)
(220) (105) (312)
(314)
(2)
SEM images (Fig. 4a and b) show aggregation of spherical nanoparticles forming flower-like shape of Mn3O4. HRTEM images (Fig. 4c and d) shows Mn3O4 nanoparticles are spherical in shape. The average diameter of the spherical Mn3O4 nanoparticles calculated from HRTEM is 75 nm. This value is in agreement with that calculated from XRD. 3.4. Optical band gap The absorption spectra for the manganese oxides nanoparticles (samples 1, 2, 3 and 4) which resulted from the ignition of precursors (1), (2), (3) and (4), respectively are shown in Fig. 5. The absorption coefficient (a) can be calculated from the optical absorption spectra using the following relationship
a ¼ A=d
(1)
where d is the thickness of the specimen. According to Tauc’s relation [40,41] for allowed direct transitions, the photon energy dependence of the absorption coefficient can be described by
ðahnÞ2 ¼ B hn Eg
(2)
where B is a parameter that depends on the transition probability and Eg is the optical energy gap. Fig. 6 shows the absorption coefficient in the form of (ahn)2 versus hn for the manganese oxides nanoparticles. The intercepts of the straight lines with the photon energy axis yield values of the optical band gap. The values of Eg and particle size of the Mn3O4 nanoparticles (samples 1, 2 and 4) which resulted from the ignition of precursors (1), (2) and (4), respectively are shown in Fig. 7. As shown in this figure the optical band gap increases while the particle size of the Mn3O4 nanoparticles decreases. The optical direct band gap were found to be 3.28, 3.51, 3.75 eV for Mn3O4 derived from precursors 1, 2 and 4, respectively and 3.69 eV for Mn2O3 derived from precursors 3. These values reveal that the manganese oxide nanoparticles are semi-conductors. Also, the values of Eg are in the range reported for the high efficient photovoltaic materials. So, the present compounds could be considered potential materials for harvesting solar radiation in solar cell applications [42]. It is clear from Fig. 5 that, the samples exhibit absorption edges which are blue-shifted with decreasing particle size. This blue shift of the absorption edges for different sized nanoparticles arises from quantum size effect in the nanoparticles. This phenomenon causes the continuous band of the solid to split into discrete, quantized levels and the “band gap” to increase [43]. The obtained values of the band gap of Mn3O4 nanoparticles are higher than that of the samples prepared by Dubal et al. (2.30 eV) [44], Xu et al. (2.54 eV) [45]. This shift of the band gap takes place because of the quantum confinement effect [46]. The wider optical band gap of the present compounds compared with the previously reported values, makes them more applicable in optoelectronic devices, specially light emitting devices in the short wavelength region of visible light. 4. Conclusion
(1)
10
20
30
40
50
60
70
2-Theta-scale Fig. 3. XRD powder pattern of manganese oxides nanoparticles.
80
Hausmanite nanoparticles with spherical shape have been obtained from manganese benzoic acid precursors. A simple benign semi-solid method has been used. The optical properties of manganese oxides reveals that, direct electronic transition is mainly responsible for the photon absorption inside the investigated manganese oxides nanoparticles. The values of the optical band gap for the nanoparticles increase while the particle size
N.M. Hosny, A. Dahshan / Materials Chemistry and Physics 137 (2012) 637e643
641
Fig. 4. (A),(B) SEM images (C), (D) HRTEM of Mn3O4 of samples(2) and (4), respectively after calcination at 500 C.
0.8
0.8
Absorbance
b 1.0
Absorbance
a 1.0
0.6 0.4 0.2 0.0 200
300
400
500
600
700
800
0.0 200
900
Wavelength (nm)
300
400
500
600
700
800
900
800
900
Wavelength (nm)
d 1.0 0.8
Absorbance
0.8
Absorbance
0.4 0.2
c 1.0
0.6 0.4
0.6 0.4 0.2
0.2 0.0 200
0.6
300
400
500
600
700
Wavelength (nm)
800
900
0.0 200
300
400
500
600
700
Wavelength (nm)
Fig. 5. Absorption spectra for the manganese oxides nanoparticles ((a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4).
642
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a
b
0.20 0.16
-1
( h ) x 10 (cm eV)
0.12
14
3
2
0.08
2
2
14
-1
( h ) x 10 (cm eV)
2
2
4
1
0 3.0
3.2
3.4
3.6
3.8
0.04 0.00 3.0
4.0
3.2
d ( h ) x 10 (cm eV)
14
2
2
14
2
3.8
4.0
4
3
-1
3
-1
( h ) x 10 (cm eV)
3.6
2
4
2
c
3.4
h (eV)
h (eV)
1
0 3.0
3.5
4.0
4.5
5.0
h (eV)
2
1
0 3.0
3.5
4.0
4.5
5.0
h (eV)
2
4.0
100
3.8
80
3.6
60
3.4
40
3.2
20
3.0
1
2
3
4
Particle size (nm)
Eg (eV)
Fig. 6. Absorption coefficient in the form of (ahn) versus hn for the manganese oxides nanoparticles ((a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4).
0
Samples Fig. 7. Optical band gap (Eg) and particle size of the Mn3O4 nanoparticles.
decreases. The investigated nanoparticles are semi-conductors and could be used as harvesting materials in solar cell devices. References [1] [2] [3] [4] [5] [6] [7] [8]
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