Synthesis, structural and optical properties of Eskolaite nanoparticles derived from Cr doped polyanthranilic acid (CrPANA)

Journal of Molecular Structure 1122 (2016) 117e122

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Synthesis, structural and optical properties of Eskolaite nanoparticles derived from Cr doped polyanthranilic acid (CrPANA) Nasser Mohammed Hosny a, *, Mohamed Shafick Zoromba a, b, Ghada Samir a, Samir Alghool a, c, ** a b c

Chemistry Department, Faculty of Science, Port-Said University, 23 December Street, Port-Said, Egypt Department of Chemical and Materials Engineering, King Abdulaziz University, Rabigh, Saudi Arabia Department of Chemistry, Faculty of Science, Taif University, 5700, Saudi Arabia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 January 2016 Received in revised form 20 May 2016 Accepted 20 May 2016 Available online 24 May 2016

Cr doped polyanthranilic acid (Cr PANA) has been used as a molecular precursor of Cr2O3 nanoparticles. Potassium dichromate acted as an oxidant and a dopant in Cr PANA synthesis. The spectral, optical and thermal properties of the precursor have been described. Thermogravimetric Analysis (TGA) and differential scanning calorimetric (DSC) were used to clarify the thermal stability of Cr PANA. The optical band gap (Eg) measurements indicated that Cr PANA has wider optical band than the pure PANA. Calcination of Cr PANA at 600
C produced Eskolaite (Cr2O3) nanoparticles. The obtained nanoparticles have been characterized by XRD and TEM. The average size of the nanoparticles was found to be 70 nm. The measured optical band gap of Eskolaite nanoparticles is 0.35 eV wider than the bulk. © 2016 Elsevier B.V. All rights reserved.

Keywords: Spectral characterization Nanoparticles Optical properties

1. Introduction The use of coordination compounds as molecular precursors in metal oxide nanoparticles synthesis is an important synthetic route [1e3]. Among the categories of coordination compounds are the doped polymers. There are several advantages in use of doped polymers as precursors in the synthesis of metal oxide nanoparticles as, the high purity, homogeneity of the morphology, the relatively low temperature used in processing, the environmental friendliness of the method and the applicability on commercial scale [4e7]. The common trend in nano-composite synthesis is the addition of inorganic compounds to the polymer [4]. Recently, synthesis of metal oxides [8e14], selenides [15] and sulfides [16,17] nanoparticles from the doped polymers as molecular precursors has attracted much more attention. Many of these previous reports indicated the relation between both the particle size and the shape of the synthesized nanoparticles with that of the molecular precursor used [18].

* Corresponding author. ** Corresponding author. Chemistry Department, Faculty of Science, Port-Said University, 23 December Street, Port-Said, Egypt. E-mail addresses: [email protected] (N.M. Hosny), [email protected] (S. Alghool). http://dx.doi.org/10.1016/j.molstruc.2016.05.071 0022-2860/© 2016 Elsevier B.V. All rights reserved.

Chromium oxide (Cr2O3) is a hard oxide, with high corrosion resistance, low friction and good optical properties. These characteristic properties enable Cr2O3 to be a good protective coating in microelectronic and tribological applications. It has also, important applications in space fields [19e22]. In the present work, CrPANA has been used as a precursor of Cr2O3 nanoparticles. The precursor and the synthesized Cr2O3 nanoparticles have been characterized by different techniques. The optical band gap of Cr2O3 nanoparticles was measured to clarify its optical properties. The present work has several advantages as it provides the synthesis of Cr2O3 nanoparticles by simple green chemistry method from cheap available chemicals. The synthesized Cr2O3 nanoparticles have wider optical band gap than the bulk. This wide band gap makes Cr2O3 nanoparticles applicable in optoelectronic devices. 2. Experimental The chemicals used were of analytical grade and were used without further purification. Anthranilic acid (98%) and potassium dichromate (99%) were purchased from (Sigma-Aldrich). IR spectra were carried out on a Mattson 5000 FTIR Spectrometer equipped with fast recovery deuterated triglycine sulfate (DTGS) detector, the resolution is 4 cm1 and the scan no is 32 scan. The

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samples were carried out in the form of KBr discs. Thermal analyses (TGA) and differential scanning calorimetric (DSC) were carried out on a Schimadzu model 50 instrument, with nitrogen flow rate 20 cm3/min and heating rate 10
C/min. The sample weight is 20 mg sample. The electronic spectra were measured on a UV2 Unicam UV/Vis. Spectrometer with 1 cm silica cell by using DMF as solvent. Philips XPERT-PRO device with nickel filtered Cu Ka (l ¼ 1.5405 Å) radiation was used to record XRD pattern. Electron microscopes of the type CM 20 PHILIPS was used to take TEM images.

PANA red Cr PANA black

110 100

Transmittance (%)

90

2.1. Preparation of PANA by ammonium persulphate oxidation

80 70 60 50 40

Anthranilic acid (0.014 mol) was dissolved in 5 mL concentrated HCl (diluted by 30 mL distilled water) by stirring the solution. 3.99 g (0.017 mol) of ammonium persulphate dissolved in 20 mL distilled water was added dropwise to the anthranilic acid solution. The mixture was kept overnight at room temperature, and then 12 mL (33%) NH4OH diluted with 12 mL distilled water was added till pH 10. The resulting polymer was filtered off, washed with distilled water, and then dried at 40
C [23]. FTIR (KBr; cm1): 3438 (OeH), 1682 (COO), 1566 (C]C), 1500 (C]N), 1239 (CeN), 1084(d(OH)).

30 20 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 1. IR spectra of PANA and Cr doped PANA.

vibrations of benzoid and quinoid rings, respectively. Two bands at 1239 and 1170 cm1 belong to n CeN of the quinoid and benzoid rings, respectively [24e28]. The bands at 3431, 3208 and 3170 cm1 were attributed to n OH, n NH and n CH, respectively. The spectrum exhibits a band at 759 cm1 due to n CeC and n CeH stretching of the benzoid moiety. The bands g COOH and r COOH vibrations were observed at 650 and 515 cm1, respectively. The above mentioned bands confirm that the compound formed is PANA (Fig. 2). Doping of chromium ion into the (PANA) matrix led to deformation in the electron density of some active sites of the pure PANA. This deformation appears in the form of shifts in the coordinated group bands. The comparison of Cr PANA spectrum with that of pure PANA shows that the band at 1500 cm1 due to n C]N in the spectrum of pure PANA was reduced in intensity and turned to a shoulder, shifted to 1512 cm1 in the spectrum of Cr PANA. The shift resulted from the coordination of the polymer orbits with that of the chromium ion. This shift supports that the azomethine group takes part in coordination to Cr ion. The bands at 3438 and 1084 cm1 due to n OH and d OH, respectively in the spectrum of PANA was shifted in the spectrum of Cr PANA to 3424 and 1070 cm1, respectively. This shift suggests the participation of COOH group in bonding without the displacement of hydrogen ion. The bands of the COO group was shifted to lower wavenumber (1614 cm1) in case of Cr PANA. This shift confirms that the carboxyl group takes

2.2. Synthesis of the precursor Cr PANA Cr PANA was synthesized by mixing 0.014 mol (2.0 g) of anthranilic acid in 5 mL concentrated HCl (diluted with 30 mL distilled water) with 4.29 g (0.014 mol) of potassium dichromate in 60 mL distilled water. The reaction mixture was allowed to stand for 24 h at room temperature. Ammonia solution (33%) was added until ̉ precipitation is formed. The formed polymer was isolated by filtration, washed with distilled water and dried at 40
C. Anal. Calc. for [Cr(C4H4NO2)Cl3.4H2O according to monomeric unite: C, 23.0; H, 3.3; Cr, 14.2. Found: C, 22.5; H, 3.7; Cr, 14.50%. FTIR (KBr; cm1): 3424 (OeH), 1614 (COO), 1566 (C]C), 1512 (C]N), 1246 (CeN), 1070 (d(OH)). 2.3. Synthesis of Cr2O3 nanoparticles 1 g of Cr doped (PANA) precursor was calcined in a muffle furnace at 600
C for 2 h in air. Cr2O3 nanoparticles were resulted. 3. Results and discussion Potassium dichromate oxidizes the monomer of anthranilic acid forming polyanthranilic acid (PANA) and Cr(III) ion. When polyanthranilic acid is kept in contact with Cr(III) ion, it coordinates to it forming Cr PANA. The doped polymer was calcined at 600
C to produce Cr2O3 nanoparticles. 3.1. IR spectra

COOH *

Table 1 contains some important IR bands of PANA and Cr PANA and the spectra are represented in Fig. 1. From the spectrum of PANA, a band is observed at 1682 cm1 due to nas COO. The bands at 1566 and 1500 cm1 are characteristic to n C]C and n C]N

H N

COOH H N

COOH N

COOH N

*

n Fig. 2. Suggested structure of PANA.

Table 1 Some important IR bands of PANA and its doped Cr PANA polymers. Compound

n OH

n NH

nas COOH

n C]C

n C]N

ns COOH

n CeN

n MeN

PANA CrPANA

3438 3424

3208 3249

1682 1614

1566 1566

1500 1512

1402 1368

1239 1246

e 506

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119

3.2. Thermal analyses of Cr PANA

*

N

*

H2O

Cl Cr

O

O H Cl

Cl

Fig. 3. Suggested structure of CrPANA.

part as an active site of coordination. The difference between the asymmetric and symmetric stretching of COO group is 248 cm1, indicating the monodentate nature of this group. The spectrum of Cr PANA shows broad band centered at 3400 cm1 due to OH group of water. The new weak bands at 560 and 506 cm1 attributed to n M  O and n M  N vibrations, respectively. Fig. 3 represents the suggested structure of Cr PANA.

TGA, and DSc curves of CrPANA are presented in Fig. 4A and B. Thermogravimetric measurements are used to clarify the thermal stability of the Cr PANA to determine the calcination temperature required for production of Cr2O3 nanoparticles. The TG curve of Cr PANA indicates that the precursor loses 8.0% of its weight as water of hydration. This value agrees with the presence of two molecules of water (calcd. 8.2%). This stage is endothermic process and takes place from room temperature to 100
C. After that, 8.4% of the precursor weight is lost in the temperature range 120e225
C. This step is an endothermic one. The weight loss value is close to the calculated 8.2% corresponds to two molecules of water one of them coordinates to the metal ion and the other exists in the crystal structure but un-coordinated to the metal ion [29]. After removal of water molecules, the solvent free Cr PANA begins to decompose. It was observed that the dry Cr PANA loses 10% of its weight at 327
C. The Cr PANA could not be considered as thermally stable, where the thermally stable polymer is that which loses 10% of its weight in a temperature range higher than 400e450
C [30]. The higher stability of Cr PANA confirms the higher strength of the bonds formed between the polymer and chromium ion. 3.3. Absorption spectra and optical band gap (Eg) of the precursor CrPANA Electronic absorption spectroscopy is a helpful tool in characterization of the organic compounds and in determining the stereochemistry of the coordination compound from the DMSO-d6 transitions. The electronic spectrum is the finger print of the coordination. The absorption spectra was used to confirm the formation of PANA and to determine the stereochemistry of CrPANA. Beside that it was used to measure the optical band gap of the two compounds as follows: The UVevisible spectrum of PANA in DMF shows two absorption bands at 291 and 320 cm1 belong to the pep* transition in C]C group and n-p* transition in C]N group, respectively in the benzoid ring. The band at 595 cm1 was attributed to the quinoid rings excitation absorption [31,32]. The electronic spectrum of Cr PANA in DMF exhibits a band at 334 cm1 due to LMCT transition. Three DMSO-d6 transition inside Cr(III) ion. The first band at 600 cm1 assigned to the 4B1g / 4Eg, transition and the second band at 430 cm1 assigned to 4 B1g / 4B2g, while, the third band at 376 cm1 belongs to 4 A2g / 4T1g transition, in an octahedral Cr(III) complexes [33]. The optical band gap energy (Eg) is the gap between the highest band in the valence band and the lowest one in the conduction band. The absorption spectrum of Cr PANA in DMF was used to determine the optical band gap of pure PANA and Cr PANA. The absorption band gap Eg can be described by Tauc’s relation for allowed direct transitions [34e37]. The relation a ¼ A/d was used to calculate the absorption coefficient (a) from the absorption spectra (Where, d is the path length). The optical band gap for allowed direct transitions is determined from equation (1):

ða h nÞ 2 ¼ B hn  Eg

Fig. 4. (A) TGA curve and (B) DSC of CrPANA.




(1)

B is a parameter depends on the transition probability Plotting (a hn)2 vs. hn gives the direct transitions for PANA and Cr PANA (Fig. 5). The extrapolation of the linear part of the curve to (a hn)2 ¼ 0. The optical band gaps (Eg) of PANA and Cr PANA were found to be 2.7 and 3.4 eV, respectively. The increase in the optical band gap of Cr PANA may be attributed to the strong interaction between the polymer and the d orbits of Cr(III). The observed values

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Fig. 5. Optical band gap of PANA (black) and Cr PANA (blue).(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

of optical band gaps reveal semi-conductivity of the two polymers [38].

3.4. XRD and TEM Fig. 6 shows the XRD pattern of Cr2O3 nanoparticles obtained from calcination of Cr PANA at 600
C. The hexagonal structure characteristic to Eskolaite, (Cr2O3) with lattice constants, a ¼ 4.95876, b ¼ 4.95876, c ¼ 13.59420 Å and a ¼ b ¼ 90
, g ¼ 120
JCPDS Card No. 04-003-5821. The particle size of Cr2O3 was calculated by applying Debye-Scherrer formula D ¼ 0.89 l/b cosq [39]., from the major diffraction peak the particle size was estimated to be 63 nm for the sample of Cr2O3. The peak at 28
does not belong

to Cr2O3 crystal, it may belong to an intermediate product formed during the formation of Cr2O3 crystal. Fig. 7A shows TEM micrograph of Cr2O3 nanoparticles. The particles have irregular spherical hexagonal shapes, some particles have deformed shape due to preparation conditions and random aggregation of the nano-rods. Fig. 7(B) represents the particle size distribution, which indicates that the major particles have size around 70 nm. This value is in good agreement with the value calculated from XRD pattern. Cr2O3 crystals tend to grow in one dimensional, this behavior may be due to the structure of the precursor, where the polymer propagates in one dimensional and/ or due to the calcination conditions [33]. Fig. 8 shows the electronic absorption spectrum of Cr2O3 nanocrystals. Three bands appear in the spectrum at 367, 519 and 425 cm1 assigned to 4A2g / 4T1g, 4B1g / 4Eg and 4B1g / 4B2g, respectively [40]. The optical band gap Eg of Cr2O3 was measured from Tauc’s relation [34e37] for allowed direct transitions. Fig. 9 represents the optical band gap of Cr2O3 as a direct transition in the form (ahn)2 versus hn. The optical band gap (Eg) of Cr2O3 was found to be 3.75 eV, revealing the semi-conductivity nature of Cr2O3 nanocrystals. This Eg value is in the range of some efficient photovoltaic materials [38]. The optical band gap of the Cr2O3 is wider than the bulk (3.4 eV) due to the presence of the particles in the nanoscale [41]. This wider Eg value makes Cr2O3 more suitable in optoelectronic devices. The presence of Cr2O3 in nanoscale is responsible for increasing Eg where, the relation between the size and Eg is inverse relation [12]. 4. Conclusion Cr PANA has been synthesized and characterized by different techniques. Cr PANA has been used as a precursor of Cr2O3 nanoparticles via solid state decomposition rout. The optical band gap of the obtained Cr2O3 nanoparticles was measured from the absorption spectrum and found to be wider than the bulk. The current

Fig. 6. XRD pattern of Cr2O3 derived from Cr PANA after calcination at 600
C.

N.M. Hosny et al. / Journal of Molecular Structure 1122 (2016) 117e122

121

Fig. 9. Optical band gap of Cr2O3 nanoparticles.

method enables the use of Cr PANA in commercial production of crystalline chromium oxide nanoparticles by a simple benign method. References

Fig. 7. (A) TEM image of Cr2O3 nanoparticles and (B) Particle size distribution calculated from TEM image.

2

Intensity

367

425 519

0 300

400

Wavelength(nm) Fig. 8. Electronic spectrum of Cr2O3.

500

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