Characterization and molecular dynamic studies of chitosan–ironcomplexes

T FAHMY1,2,* and A SARHAN2

1 Physics Department, College of Science and Humanities, Prince Sattam Bin Abdulaziz University, 11942 Al-Kharj,

Kingdom of Saudi Arabia2 Polymer Research Group, Physics Department, Faculty of Science, Mansoura University, Mansoura 35516, Egypt

*Author for correspondence (tfahmy_5@yahoo.com)

MS received 6 November 2020; accepted 14 December 2020

Abstract. Chitosan–iron (Cs–Fe) complexes are prepared electrochemically in an aqueous acidic medium in one-

compartment cell at different times. XRD pattern of Cs–Fe complex samples has been investigated in the range from 5� to

50� and revealed that chitosan is characterized by certain crystalline peaks at 8.73�, 11.92� and 18.96�. In addition, the

crystallinity of Cs–Fe complex samples is increased with increasing the content of Fe3?. Ultraviolet–visible (UV–Vis) and

Fourier transform-infrared (FTIR) spectroscopies have been used to investigate the optical properties of Cs–Fe complex

samples. UV analysis showed that pure chitosan is characterized by absorption band at 214 nm resulted from the amide

linkages and at 311 nm, as a shoulder which is attributed to intraligand n ? p and p ? p* transitions of the chro-

mophoric C=O group. On the other hand, two new bands are observed in Cs–Fe complex samples at nearly 350 and 389

nm with increasing Fe3? content. The optical parameters of all the samples, such as optical band gap energy (Eg), Urbach

energy (EU), dispersion energy (Ed) and oscillator energy (Eo) have been estimated. It is found that these parameters are

significantly affected due to the Fe3? content. FTIR spectra revealed that many of the characteristic bands of pure chitosan

have been affected either in its position or its intensity due to the presence of Fe3?, confirming that the formation of

complex between chitosan and Fe3? is occurred. Dielectric relaxation spectroscopy technique has been used to investigate

the dielectric properties of pure chitosan and Cs–Fe complex samples in a wide range frequency and a temperature range

extended from RT to 433 K. The investigation showed that the existence of Fe3? resulted in a modification in the

dielectric constant (e0) and dielectric loss (e0 0) behaviour. Dielectric loss tangent (tan d) showed that pure chitosan is

characterized by two different types of relaxations, whereas Cs–Fe complex samples are characterized by only one

relaxation process.

Keywords. Electrochemical-oxidation; chitosan; complexation; dispersion energy; oscillator energy; interfacial polar-

ization; dipolar relaxation.

1. Introduction

Chitosan is a linear polysaccharide consisting of

b(1 ? 4)-linked 2-amino-2-deoxy-d-glucose and 2-ac-

etamido-2-deoxy-d-glucose. It offers biocompatible and

biodegradable properties and can be formed into many

derivatives through further modifications of its hydroxyl

and amino groups. Chitosan is obtained by the alkaline

deacetylation of chitin, one of the most abundant

biopolymers in nature that can be extracted from fungal

biomass, shrimp shells, crabs, squid pen and insect form

[1]. Chitosan has hydrophilicity, biodegradability, bio-

compatibility and antibacterial properties. So, it can be

used in a wide range of applications, such as food

packaging, separation membrane, drug delivery system,

wound healing and treatment of waste water [2–7]. Chi-

tosan can be easily manufactured into capsules, tablets,

nanoparticles, microspheres, gels, films and beads for

various types of applications [8].

Chitosan, a non-toxic biodegradable material, is known

to be an effective cleaning agent for heavy metals due to its

flexible structure, presence of hydroxyl, amino groups and

nitrogen on its chemical structure [9]. Thus, there are sev-

eral published works about the ability of chitosan and some

of its derivatives complexation with a polyvalent metal ion

to treat various hazardous waste waters [10]. In the process

of complexing chitosan and metal ions, various coordina-

tion mechanisms have been suggested, i.e., the metallic ion

is bonded to four nitrogen atoms either of the same chain or

in different chains or the metallic ion is bound to the amino

group as a pendulum [11]. Most studies for the Fe metal

complexes have indicated that both –OH and –NH2 groups

are linked to metal ions and more than one polymer chain is

participated in the complex formation [12]. It is suggested

Bull. Mater. Sci. (2021) 44:142 � Indian Academy of Scienceshttps://doi.org/10.1007/s12034-021-02434-1Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

that the chitosan–iron complex formed by iron adsorption

onto chitosan is either penta- or hexa-coordinated Fe3?. It is

found that for every Fe3? ion, there are 4 moles of oxygen

atoms and 2 moles of amino groups from two different

chitosan chains. At least one water molecule would be

expected to participate in the coordination complex [13].

Chitosan–metal complexes can be used in the removal or

sequestration of metal ions, catalysis, dyeing, antimicrobial

activity and many other industrial processes [14].

The thermal, optical and dielectric properties of chitosan/

PVA biopolymer blend have been investigated intensively

in our previous work [15]. The structure of chitosan con-

tains one amino and two hydroxyl groups on the main-chain

of hexosaminide residue [16], thus, understanding the

relation between polymer segmental relaxation and ion

transport in polymer complexes is considered the key for

developing new energy devices [17]. Hence, the purpose of

this study is to investigate the relaxation behaviour of chi-

tosan–Fe3? complex materials using dielectric relaxation

spectroscopy (DRS) method. The main advantage of this

technique compared to other techniques used to investigate

the molecular dynamics is the very wide range of the

covered frequency [18–25]. Additionally, different char-

acterization tools, such as ultraviolet–visible (UV–Vis)

spectroscopy and Fourier transform infrared spectroscopy

(FTIR) are used for our study to investigate the effect of

Fe3? content on the optical properties of chitosan.

2. Experimental

2.1 Materials

Chitosan with high molecular weight (600,000 g per mole)

with a degree of deacetylation [75% is supplied from

Aldrich Chemical Co. and iron plate of (20 9 40 9 2) mm

with purity of 99.995% is supplied from Sigma-Aldrich.

The iron plate is well polished before use via a very fine

emery paper and then is cleaned by de-ionized water, ace-

tone and ethanol (90%) for use as working electrode (an-

ode). Platinum sheet with dimensions of (20 9 40 9 2)

mm is obtained from Sigma-Aldrich and is used as counter

electrode (cathode). De-ionized water with resistivity

*[2 9 108 X cm is in samples preparation process. Acetic

acid of analytical grade is obtained from Fine Chemical EL-

Naser Co., Egypt.

2.2 Preparation of Cs–Fe complexes

Cs–Fe complexes are prepared electrochemically in an

aqueous acidic medium. The electrochemical-oxidation is

occurred at fixed potential of 2 volt, in a one-compart-

ment electrochemical cell. Electrolytic solutions were

produced by dissolving 1 wt% Cs into 2% acetic acid,

and stirred for 48 h to obtain a transparent solution. This

viscous solution was kept overnight until completely

dissolved and any bubbles removed. Fe (anode) and pt

(cathode) plates were separated by 5 cm in the elec-

trolytic solution and contacted to a proper changing

resistor. The temperature of the reaction mixture was

maintained at 25�C with constant stirring and nitrogen

gas was present in the electrolyte throughout the entire

production process. Preliminary experiments were carried

out to determine the time needed to reach the equilibrium

concentration of the metal ion in the electrolytic solution,

this is accomplished when some iron atoms started to

deposit on the cathode, and cathode becomes yellowish.

It is found that the required time to attain the maximum

complexation was about two days. Then, the centrifuge

was operated at a speed of 4000 rpm for 10 min and the

product was filtered before used to eliminate any impu-

rities (scheme 1).

2.3 Characterization methods of Cs–Fe complexes

X-ray diffraction (XRD) pattern of chitosan–Fe (Cs–Fe)

films are obtained by using Philips PW 1390 X-ray

diffractometer. XRD is carried out with a beam

monochromator and CuKa radiation with applied voltage

of 40 kV and current intensity of 40 mA at k = 1.5406 A.

The data is recorded in the range from 5� to 50� with a

scanning speed of 2� min-1. UV–Vis spectroscopy of the

Cs–Fe complexes is obtained in the range of 200–800 nm

by ATI-Unicom UV–Vis spectrophotometer. Mattson

5000 FT-IR spectrometer is used to investigate FT-IR

spectroscopy of Cs–Fe complexes in the range of

400–4000 cm-1.

2.4 Dielectric measurements

The measurements of dielectric properties are carried out in

the frequency range of 1 Hz–105 Hz using a standard

resistance decade box (Model RD-5, Tech Instruments Co.

Ltd., Japan). Stanford model (SR830) lock-in amplifier

(USA) was utilized for measurements in the frequency

range of 1 Hz–105 Hz with the accuracy of 25 ppm ? 30

lHz and resolution of 4.5 digits. Data are collected, while

samples are heated.

3. Results and discussion

3.1 XRD

Figure 1 displays XRD pattern of pure Cs and Cs–Fe

complex samples. XRD pattern of pure Cs showed distinct

crystalline peaks at around 2h values at 8.73�, 11.92� and

18.69�. The high degree of crystallization of pure chitosan

is possibly due to the presence of intermolecular

142 Page 2 of 12 Bull. Mater. Sci. (2021) 44:142

and intramolecular hydrogen between many of the –OH

and –NH2 groups in the structure of chitosan. The crys-

talline peak at 11.92� is attributed to anhydrous crystalline

structure, whereas the peak at 18.69� is characteristic of

chitosan chains which aligned through intermolecular

interaction [26,27]. On the other hand, Cs–Fe complex

samples’ XRD pattern displayed many crystalline peaks

located at 29.56�, 36.04�, 39.46�, 43.19�, 47.70� and 48.63�,respectively. It is found that the intensity of these crystalline

peaks is increased with increasing the content of Fe3?,

indicating the penetration and distribution of Fe3? into the

matrix of chitosan film. The values of d-spacing of these

main peaks are 3.01, 2.48, 2.28, 2.09, 1.91 and 1.87 A,

respectively. The particle size is estimated using Debye–

Scherrer equation as follows:

Dhkl ¼kk

bhkl cos h;

where k, k, b and h are particle shape factor *0.9, X-ray

wavelength, full width at half maximum of (hkl) reflection

and Bragg angle corresponds to (hkl) reflection, respec-

tively. It is found that the average particle size is nearly

131 nm.

3.2 UV–visible spectroscopy

UV–Vis spectra of pure Cs and Cs–Fe complex samples

with different time of electrolysis are investigated from 200

to 800 nm, as shown in figure 2a. The chitosan spectrum

displays two absorption bands at 214 nm, resulting from the

amide linkages [28] as it is the only partially deacetylated

chitin, and at 311 nm as a shoulder, this band was attributed

to intraligand n ? p and p ? p* transitions of the chro-

mophoric C=O group. On the other hand, two new bands

are observed in Cs–Fe complex samples at nearly 350 and

389 nm with increasing Fe? content.

The absorption band which is observed at 389 nm is

shifted to higher wavelength *450 nm. The absorption

band at 303 nm in Cs–Fe complex samples is related to the

entry of a new biopolymeric molecule within the coordi-

nation sphere of Fe3? and the band at 350 nm is attributed

to hydroxyl groups (–OH) or because of the existence of

water molecules in metal coordination sphere [29]. The

band at 450 nm is attributed to Fe3?–O4 systems [30]. The

shift of the band from 389 to 450 nm in the sample of

higher concentration of Fe3? indicating the completely

complexation of Cs with Fe. These two peaks tend to

indicate the presence of two different particle sizes for the

resulting Cs–Fe complex samples.

The absorption coefficient (a) of the samples can be

expressed using the following equation [31]:

a ¼ 2:303A

d;

where A and d are the absorbance and sample thickness,

respectively. Figure 2b represents the variation of absorp-

tion coefficient (a) vs. photon energy (hm). This behaviour

showed that the value of a is strongly influenced by Fe3?

content in chitosan matrix. Because of the interaction

between the chitosan matrix and Fe3? ions through OH

group, the absorption edge is decreased until it reaches

3.06 eV for Cs–Fe complex samples compared to pure

chitosan which has 4.99 eV.

The changes observed in the absorption spectra could

indicate the type of electronic transitions that are possible in

OHOH2C

NH2

HO

O

HOH2C

NH2

HOO

OHOH2C

NH2HO

O

CH2OH

H2N

OHO

Fe OH2

H2O

Fe

O

HOH2C

NH2

HOO

O

CH2OH

OH

**

**

**

Scheme 1. Illustration of the electrochemical complexation of cationic CS, [HCS]? with the Fe3? ions

released from oxidation of the anode.

10 20 30 40 500

2000

4000

6000

8000

10000

12000

48.63047.700

43.19039.46036.040

29.560

8.730 11.920

Inte

nsity

(a.u

)

Pure Chitosan CS-Fe (24 hr) CS-Fe (36 hr)

18.690

Figure 1. X-ray diffraction pattern of pure Cs and Cs–Fe

complex samples.

Bull. Mater. Sci. (2021) 44:142 Page 3 of 12 142

the Cs–Fe complex samples. The photon absorption based

on Tauc equation can be written as follows [32]:

ahm ¼ B hm� Eg

� �n; ð1Þ

where a is the absorption coefficient, hm the incident photon

energy, B a constant, Eg the optical band gap energy and n a

parameter that characterizes the band transition nature,

respectively. The values of n = 1/2 and 2 for direct and

indirect allowed transitions, whereas for direct and indirect

forbidden transitions are 3/2 and 3, respectively. The values

of direct optical band gap energy Egd are calculated from

extrapolation of the straight-line part of the ahmð Þ2 vs. hm

graph to ahmð Þ2¼ 0, as shown in figure 3a and listed in

table 1. It is found that with increasing Fe3? content, the

values of Egd decreased until it reaches the value of 3.19 eV

in comparison to pure chitosan which has a value of

5.31 eV. The most interesting thing is the existence of two

or three optical absorption edge with lower and higher

energy gap, as shown in figure 3a. This behaviour has been

reported for different polymers such as PVC and PVA

doped with CoCl2 and NiCl2 [33]. The disorder and defect

states within the polymer structure induce localized states in

the band structure and hence, a decrease in the values of

Egd. Increasing the content of Fe3? in the chitosan matrix

decreased the band gap for the complex samples. The

observed variation in the band gap indicates the change in

the energy states in conduction and valence bands, i.e., the

electronic structure of the pure chitosan matrix has already

changed. The decrease of optical band gap after doping may

be interpreted, based on the fact that the incorporation of

dopant amounts will form charge transfer complexes

(CTCs) in the host network of chitosan. The additional

absorption peaks obtained in Cs–Fe samples support this

opinion. The CTCs will increase the electrical conductivity

by giving additional charges into the network, thus the

optical band gap (Eg) will be decreased [34].

For the optical transitions which are obtained by photons

with energy (hm) less than the energy gap (Eg), the photons

absorption is related to the existence of localized tail states

within the forbidden gap region. The tail width is called the

Urbach tail, is considered as an indicator to the existence of

defect levels within the forbidden gap [35]. Urbach energy

(EU) is the energy width of the localized tail states in the

Figure 2. (a) UV–Vis spectra of pure chitosan and Cs–Fe complexes and (b) a against hm for all samples.

Figure 3. (a) (ahm)2 vs. hm for all samples and (b) ln a vs. hm for all samples.

142 Page 4 of 12 Bull. Mater. Sci. (2021) 44:142

band gap and measures the thermal fluctuation in the band

gap, can be estimated, using the following formula:

a ¼ a0 exp hm=EUð Þ; ð2Þ

where a0, hm and EU are defined as a constant, incident

photon energy and Urbach energy, respectively. Figure 3b

represents a linear relation between ln a and hm and by

knowing the inverse of slope of this linear relation, the

values of EU of all samples are calculated and summarized

in table 1. As listed in table 1, the values of EU are

decreased with increasing Fe3? content in the Cs–Fe com-

plex samples. This variation in EU values is related to the

variation in density of defect states as well as degree of

disorder [36–38].

The refractive index values of the Cs–Fe complex sam-

ples are evaluated using the following relation, n ¼ 1þffiffiffiR

p

1�ffiffiffiR

p ,

where R is the reflectance and it can be expressed as

R ¼ 1 �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiT exp Að Þ

p, where T and A are the transmittance

and absorbance, respectively [39]. Figure 4a displays the

variation of the refractive index against the wavelength of

pure chitosan and complex samples. The normal dispersion

of the refractive index has been obtained in different ranges

of wavelengths depending on the type of the sample as

shown in figure 4a. The dispersion of refractive index rep-

resents a main role to find out materials with enhanced

optical properties, because it is a significant factor in

designing devices as well as in the optical communication

where spectral dispersion is considered as a guiding factor.

The dispersion parameters of pure chitosan and complex

samples are calculated according to the following equation

based on the single-oscillator model [40].

n2 � 1� �

¼ Ed E0

E20 � ðhmÞ2

; ð3Þ

n2 � 1� ��1¼ E0

Ed

� 1

EdE0

hmð Þ2; ð4Þ

where the parameters n, Ed and E0 are the refractive index,

dispersion energy and oscillator energy, respectively. The

dispersion energy (Ed) expresses the normal electronic

excitation spectrum and measures the average strength of

the inter-band optical transitions, whereas the oscillator

energy (E0) is defined as the energy separation between

centres of conduction and valence bands. The values of Ed

and E0 can be determined by plotting (n2 - 1)-1 against

(hm)2, as depicted in figure 4b. By knowing the values of the

intercept and slope, the values of both Ed and E0 are

determined and listed in table 2. It is observed that the

values of Ed and E0 are decreased with increasing the

content of Fe3?.

Table 1. The values of absorption edge, direct optical band gap energy and Urbach energy.

Sample Absorption edge (eV)

Direct energy (eV)

Urbach energy EU (eV)Egd1 Egd2

Pure chitosan 4.99 5.31 — 0.93

Cs–Fe (24 h) 3.14 3.97 3.28 0.32

Cs–Fe (36 h) 3.24 4.00 3.25 0.18

Cs–Fe (48 h) 3.06 3.92 3.19 0.40

200 400 600 8001.0

1.5

2.0

2.5

2.5 3.0 3.5 4.0 4.5 5.0 5.50.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

5.9 6.0 6.1 6.2

0.24

0.26

0.28

0.30

0.32

n

Wavelength (nm)

Pure Chitosan Cs-Fe (24 hr) Cs-Fe (36 hr) Cs-Fe (48 hr)

(a)

Pure Chitosan Cs-Fe (24 hr) Cs-Fe (48 hr)

(n2 -1

)-1

(h )2 (eV)2

(b)

Cs-Fe (36 hr)35

(n2 -1

)-1

(hu)2 (eV)2

Figure 4. (a) Refractive index against the wavelength for all samples and (b) (n2 - 1)-1 against (hm)2

for all samples.

Bull. Mater. Sci. (2021) 44:142 Page 5 of 12 142

The static refractive index (n0) and static dielectric con-

stant (es) are evaluated by knowing the values of both Ed

and E0 according to the following equation [41]:

n20 ¼ 1 þ Ed

E0

;

es ¼ n20: ð5Þ

The relation between refractive index (n) and wavelength

(k) can be expressed as follows [42]:

n2 ¼ eL � e2

pc2

N

m�

� �k2; ð6Þ

where eL, e, c and (N/m*) are the lattice dielectric constant,

electronic charge, speed of light and carrier concentration to

effective mass ratio, respectively.

The variation of n2 against k2 for all samples is shown in

figure 5. The values of (eL) and (N/m*) are estimated by

knowing the intercept and slope of figure 5 and listed in

table 2. The difference between static and lattice dielectric

constants values can be related to the contribution of free

carriers in the different samples. The values of plasma

frequency of pure chitosan and Cs–Fe complex samples are

calculated using the following equation and listed in table 2

[40].

x2p ¼ e2

e0

N

m�

� �: ð7Þ

3.3 FTIR spectroscopy

FTIR measurements are more significant tool used to identify

and prove the interactions between Cs molecules and Fe ions

during the electrochemical process and it is considered as an

evidence for the chelation between O and N atoms. FTIR

spectra of Cs–Fe samples with different electrolysis time in

comparison with pure Cs are shown in figure 6. The strong

broad band of pure Cs in the region of 3500–3100 cm-1 is

characteristic of the stretching vibration of O–H and N–H

groups [25]. The characteristic weaker bands at 2932 and

2882 cm-1 are assigned to stretching vibration of aliphatic

C–H bonds of chitosan. The characteristic transmittance

bands at 1640 and 1557 cm-1 are attributed to stretching

vibrations of C=O bond (amide I) and N–H deformation

(amide II) overlapped with NH2, respectively. Transmittance

bands at 1410 and 1383 cm-1 are attributed to bending

vibrations of –OH and –CH3 of the acetyl group, respec-

tively. The bands at 1153 and 1061 cm-1 are assigned to

glycosidic linkages and C–O–C glucose ring vibrations [43].

The band at 1077 cm-1 is attributed to C–O group indicating

that the –COOH group is existed. On the other hand, FTIR

spectra of Cs–Fe samples as shown in figure 6b showed that

the broad band in the region of 3500–3100 is characterized

by a significant decrease in transmittance and more broad-

ness indicating that the N–H vibration is affected due to Fe3?

[44]. Also, the decreasing of band intensity at 1728 cm-1

(stretching vibration C=O, carbonyl group), indicating that

hydrogen bonding reaction within the acetamido group is

broken down in presence of Fe3? [45]. The change in the

transmittance bands around the band 1150 cm-1 reflects to

changes in the secondary carbon hydroxyl group. The

apparent decrease in intensity, as well as shift in position for

1410 and 1383 cm-1 may be attributed to the formation of

complex between chitosan and Fe3? [46]. The metal–ligand

transmission band is observed at 585 cm-1 and assigned to

stretching Fe–N equatorial and the other short transmittance

bands in the region from 800 to 500 cm-1 are assigned due

to the presence of metal–oxygen (Fe–O) [12,47].

3.4 Dielectric spectroscopy analysis

3.4a Dielectric constant: The study of the dielectric con-

stant and dielectric loss as a function of frequency as well as

Table 2. The values of dispersion energy, single oscillator energy, es, eL, N/m* and plasma frequency.

Sample

Dispersion energy,

Ed (eV)

Single oscillator energy,

E0 (eV) es eL

N/m*

(m-3 kg-1)

Plasma frequency

xp (Hz)

Pure chitosan 8.07 4.76 2.69 12.22 5.66 9 1053 4.04 9 1013

Cs–Fe (24 h) 2.71 2.49 2.09 13.15 2.99 9 1053 2.94 9 1013

Cs–Fe (36 h) 1.31 2.63 1.50 26.28 1.03 9 1054 5.46 9 1013

Cs–Fe (48 h) 2.99 2.18 2.37 14.80 2.44 9 1053 2.65 9 1013

1 2 3 4 54

5

6

7

Pure Chitosan Cs-Fe (24 hr) Cs-Fe (36 hr) Cs-Fe (48 hr)

n2

2 (x10-13) m2

Figure 5. n2 against k2 for pure chitosan and complex samples.

142 Page 6 of 12 Bull. Mater. Sci. (2021) 44:142

temperature is one of the most appropriate and sensitive

methods for examining the polymer structure [19,21,48,49].

The complex dielectric constant (e*) as a function of

frequency can be expressed as follows:

e�ðxÞ ¼ e0ðxÞ � je00ðxÞ; ð8Þ

where e0(x) and e00(x) are defined as the real and imaginary

parts of e*(x) and indicate to charge storage capacity and

energy loss, respectively. Both e0(x) and e00(x) can be

estimated from the following equations:

e0ðxÞ ¼ Cd

e0A; ð9Þ

where C, d, e0 and A are defined as the measured capaci-

tance, sample thickness, free space permittivity and sample

surface area, respectively.

e00ðxÞ ¼ e0ðxÞ tan d ; ð10Þ

where tan d is defined as the dielectric loss tangent.

Dielectric measurements are used to investigate the differ-

ent relaxation processes in polysaccharides [15,50].

Figure 7 displays the variation of the dielectric constant of

pure chitosan and Cs–Fe (48 h) complex sample against the

frequency at various temperatures in a wide range of fre-

quencies from 1 to 105 Hz. The polymer dielectric beha-

viour is largely dependent on the polar or non-polar

features. Thus, the presence of polar groups such as C=O or

O–H groups in chitosan molecules that carry a high polarity

group is the cause of the increase in dielectric constant.

Generally, it is observed that the dielectric constant (e0) of

pure chitosan and Cs–Fe samples is frequency- and

temperature-dependent. At lower frequencies and high

temperatures, the dielectric constant behaviour showed a

sharp increase and then diminished with increasing fre-

quency and finally, at higher frequency region displayed

frequency-independent behaviour. High values of dielectric

constant at low frequencies can be explained based on the

electrode-polarization mechanism, i.e., high contribution of

charge carrier accumulation at the electrode–sample inter-

faces [51,52]. The peaking in log r00 against frequency at

low frequencies, as shown in figure 7c, is manifesting the

electrode polarization mechanism [50]. The high values of

dielectric constant at high temperatures are attributed to the

increase in the charge carrier density due to the increase in

the ion aggregates dissociation. At lower frequencies, the

dipoles have enough time to align with the applied electric

field, which in turn leads to a rise in the dielectric constant

values. On the other hand, the periodic reversal of the

electric field at higher frequencies occurs so quickly that

there is no excess ion diffusion in the field direction. Thus,

the depolarization due to the accumulation of charges

decreases resulting in a decrease in the dielectric constant.

Also, it is clear that the dielectric constant increases with

increasing Fe3? concentration and then, decreases at high

concentrations as shown in figure 7d. Typically, the

increase in the value of dielectric constant (e0) of the

dielectric material after filling with a conducting phase is

due to the presence of interfacial polarization between the

insulator phases and the conductor [53]. The high concen-

tration of Fe3? may lead to agglomeration or inhomoge-

neous dispersion causing changes in the microstructure of

chitosan samples, which contribute to a decrease in the

4000 3500 3000 2500 2000 1500 1000 500

Pure Chitosan (a)

(b)

Cs-Fe 48 h

Cs-Fe 36 h

Cs-Fe 24 h

Tra

nsm

itta

nce

(a.u

)

Wavenumber (cm-1)

Figure 6. FTIR spectra of (a) pure chitosan and (b) chitosan–Fe complex samples with different

electrolysis time.

Bull. Mater. Sci. (2021) 44:142 Page 7 of 12 142

dielectric constant [54]. These agglomerations or inhomo-

geneous dispersions of Fe3? will restrict the chain motion of

chitosan.

3.4b Dielectric loss: Dielectric loss gives valuable infor-

mation about the dielectric relaxation process and energy

loss of polysaccharides. Frequency dependence of dielectric

loss e00 for chitosan and Cs–Fe complex samples is depicted

in figure 8. Similar to the behaviour of e0, it is found that e00

is decreased with increasing the frequency for all samples

and showed a frequency-independent behaviour at higher

frequencies. Cs–Fe complex samples can be considered as a

non-lossy dielectric material due to the small values of e00 at

higher frequencies, which makes it suitable for a wide range

of uses in many practical applications. The high values of

dielectric loss at lower frequencies and high temperatures

can be attributed to space charge polarization or the DC

electric conduction. The dielectric loss behaviour of pure

chitosan showed a relaxation in the range of *20 Hz–

103 Hz and is indicated by arrows as shown in figure 8a.

This relaxation has been shifted to higher frequency region

*4 9 102–1 9 104 Hz for Cs–Fe complex sample which is

prepared in 48 h, as shown in the inset of figure 8b. This

shift in the relaxation region may be indicative to a change

in the microstructure of the complex samples.

Figure 9 displays the variation of dielectric loss against

temperature for pure chitosan and Cs–Fe complex samples

at selected frequency *1 kHz. The dielectric loss (e00) was

observed to increase with increasing temperature and also

showed a broad relaxation peak at higher temperatures. This

increase in e00 may be due to an increase in both the con-

centration of charge carrier inside the samples as well as the

movement of the polar groups, which is characteristic of

polar dielectrics, as the dipoles movement is gradually

facilitated with an increase in the temperature and thus, an

increase in the dielectric loss. The isochronal plot of e00

showed a broad relaxation peak for pure chitosan in the

temperature range 383–410 K. This peak could be desig-

nated as glass transition temperature due to the molecular

motion in the amorphous region of chitosan [55]. This

relaxation peak becomes more broadness and covered a

large temperature range with increasing Fe3? content in the

complex samples.

3.4c Dielectric loss tangent (tan d): In dielectric materials,

many of the polarization mechanisms may have some

Figure 7. Dielectric constant (e0) against frequency of (a) pure chitosan, (b) Cs–Fe (48 h) complex

sample, (c) log r0 0 vs. frequency for Cs–Fe samples at T = 413 K and (d) dielectric constant (e0) vs.frequency for Cs–Fe samples at T = 413 K.

142 Page 8 of 12 Bull. Mater. Sci. (2021) 44:142

relationship with the relaxation behaviour, namely, space

charge, interfacial and dipolar polarizations. Generally, the

dielectric relaxation occurs due to interfacial polarization

(IP), space charge, Maxwell Wagner Sellar polarization

(MWS) and defect relaxation in the low frequency region,

whereas the dipolar relaxation appears in the high frequency

region [56]. In polysaccharides materials, different relax-

ation modes have been found at different temperature ran-

ges, such as c-relaxation, b-relaxation, bwet-relaxation, a-

relaxation and r-relaxation, respectively. c-relaxation is

occurred at low temperature and is attributed to small scale

molecular motions, b-relaxation is occurred in the temper-

ature range from 138 to 293 K and related to motion of the

local chain due to fluctuation in glycosidic bonds, bwet-

relaxation is obtained at room temperature *300 K in

hydrated samples, a-relaxation is occurred at glass transi-

tion temperature (Tg) and r-relaxation is obtained at high

temperatures *453 K and attributed to the migration of

protons [57].

Figure 10 depicts the variation of dielectric loss tangent

(tan d) against frequency of pure chitosan and Cs–Fe

complex sample (48 h) as a representative sample of com-

plexes. In general, the high values of tan d at lower

frequencies of a dielectric material is attributed to interfa-

cial polarization (IP) of charges accumulated at internal

interfaces and/or due to the long-range motion of free

charge carriers [58]. The behaviour of tan d of pure chitosan

against the frequency at different temperatures showed two

different modes of relaxations, as shown in figure 10a. The

first one is obtained in the region of low frequency with

slight change in the peak position and can be attributed to

the interfacial polarization (IP). Interfacial polarization has

been found at low frequency and high temperature in

polysaccharides [25,59]. This relaxation is obtained at

lower frequencies due to the inertia of formed dipoles. The

second one has covered a wide range of frequencies (from

*6 to 1 9 103 Hz) and peak position of each relaxation

peak has been shifted with increasing temperature towards

higher frequency. This behaviour refers to a thermally

active relaxation process, i.e., a-relaxation or dipolar

relaxation [60,61]. Dipolar relaxation has been related to

micro-Brownian motion of the chain segments in the

polymer matrix amorphous phase. Due to the Brownian

motion in the amorphous region in semicrystalline polymer,

such as chitosan, loss tangent behaviour will display a

significant broadness when the sample temperature reaches

glass transition temperature, Tg, as observed in figure 10a

[62]. On the other hand, the behaviour of tan d of the

representative sample of complex samples showed only one

relaxation process in the frequency range from *6 to

1 9 102 Hz. It was also observed that the peak of each

relaxation process shifted to the higher frequency with

increasing temperature. Hence, it is concluded that the

relaxation time (s) is decreased with increasing temperature.

Generally, the dielectric relaxation process depends mainly

on the structure of the molecule and its length. The high

values of e0 and low values of tan d of polymer matrix

composites or polymer complexes are desirable for

promising applications in electronic devices or capacitors

[52]. The characteristic relaxation time value, s of each

relaxation process for pure chitosan and Cs–Fe complex

samples has been calculated using the formula,

Figure 8. Dielectric loss (e00) against frequency for (a) pure chitosan and (b) Cs–Fe (48 h) complex

sample.

300 320 340 360 380 400 420 440 4600

50

100

150

200

T (K)

Pure Cs CS-Fe (24 hr) CS-Fe (36 hr) CS-Fe (48 hr)

at f = 1kHz

Figure 9. e0 0 against temperature for all samples at f = 1 kHz.

Bull. Mater. Sci. (2021) 44:142 Page 9 of 12 142

s ¼ 1=2pfmax, where fmax is the frequency of each relax-

ation peak position of figure 11.

The activation energy (Ea) values of the relaxation pro-

cess in all samples have been estimated by plotting the

variation of ln s against 1/T, as shown in figure 11. These

plots are found to be linear indicating that s obeys Arrhe-

nius equation as follows:

s ¼ s0 exp Ea=kBTð Þ; ð11Þ

where the parameters s0, Ea and kB are defined as pre-ex-

ponential factor, activation energy and Boltzmann’s con-

stant, respectively. Values of Ea have been calculated by

knowing the slope of the relation between ln s and 1/T and

listed in table 3. It is found that the activation energy values

are decreased as the concentration of Fe3? increased in the

complex samples.

3.4d Cole–Cole plot: Cole–Cole plot gives valuable

information about the electrical performance of dielec-

trics in the form of semicircles. It is found that Cole–Cole

plot of pure chitosan at different temperatures, consists of

two semicircles, one in the low frequency range with

large diameter at high temperatures and the other is

compressed in the high frequency range. The observed

semicircles with large diameters in the low frequency

region and high temperature confirmed the existence of

electrode polarization and interfacial polarization (IP).

On the other hand, the complex samples of Cs–Fe showed

only one semicircle, as shown in figure 12b, and the

semicircles’ radius decreased with increasing Fe3? con-

tent, as shown in figure 12c, indicating the presence of an

activated conduction mechanism [63,64]. It is found that

the sample Cs–Fe (48 h) has the smallest semicircle

diameter revealing that it has the least resistance for

flowing the charges.

0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

2.5

0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

Tan

Log f (Hz)

303 K323 K333 K343 K353 K363 K373 K383 K393 K403 K413 K423 K433 K

(b) Cs-Fe (48 hr) 323 K 333 K 343 K 353 K 373 K 383 K 393 K 403 K 413 K 423 K

Tan

Log f (Hz)

(a) Pure Chitosan

Figure 10. tan d vs. frequency for (a) pure chitosan and (b) Cs–Fe (48 h) complex sample.

2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1

-10

-9

-8

-7

-6

-5

-4

-3

Pure Chitosan

Cs-Fe (24 hr)

Cs-Fe (36 hr)

Cs-Fe (48 hr)

1000/T (K-1)

Figure 11. ln s vs. 1/T for all samples.

Table 3. Values of activation energy (Ea).

No. Sample Ea (eV)

1 Pure chitosan 0.96

2 Cs–Fe (24 h) 0.83

3 Cs–Fe (36 h) 0.75

4 Cs–Fe (48 h) 0.64

142 Page 10 of 12 Bull. Mater. Sci. (2021) 44:142

4. Conclusion

XRD pattern of Cs–Fe complex samples has been investi-

gated in the range from 5� to 50�. It is found that pure

chitosan is characterized by certain crystalline peaks at

8.73�, 11.92� and 18.96�. The high crystallinity of chitosan

is attributed to the presence of intermolecular and

intramolecular hydrogen between many of the –OH and

–NH2 groups. In addition, the crystallinity of Cs–Fe com-

plex samples is increased with increasing the content of

Fe3?. The optical properties of the complex samples are

investigated using UV–Vis and FTIR spectroscopy. UV–Vis

spectroscopy analysis revealed that pure chitosan is char-

acterized by two absorption bands located at 214 and

311 nm, respectively. These bands are attributed to amide

linkages and intraligand n ? p and p ? p* transitions of

the chromophoric C=O group. On the other side, Cs–Fe

complex samples are characterized by two new bands are

positioned at 350 and 389 nm. With increasing Fe3? con-

tent, these new bands are shifted to new positions and

observed at 359 and 450 nm. The absorption band at

450 nm is attributed to Fe3?–O4 systems indicating that the

complete complexation between chitosan and iron has been

occurred. In addition, it is found that the optical parameters

characterizing Cs–Fe samples are significantly affected due

to the Fe3? content. The most interesting thing is the

existence of two or three optical absorption edge with lower

and higher energy gaps. The values of Egd are decreased

until to reach the value of 3.19 eV in comparison to pure

chitosan which has a value of 5.31 eV. FTIR spectra

showed that many of the characteristic bands of chitosan

have been changed in its position and intensity due to the

presence of Fe3?, confirming that the formation of complex

between chitosan and Fe3? has occurred. The investigation

of dielectric properties of pure chitosan and Cs–Fe complex

samples revealed that both dielectric constant (e0) and

dielectric loss (e00) behaviour have been modified due to the

existence of Fe3?. The high values of e0 at low frequency

and high temperatures are attributed to the electrode

polarization and interfacial polarization in Cs–Fe samples.

Dielectric loss tangent (tan d) showed that pure chitosan is

characterized by two different types of relaxation, the first

one is obtained in the low frequency region and named as

interfacial polarization, while the second is obtained in the

high frequency region and named as dipolar relaxation. On

the other hand, Cs–Fe complex samples are characterized

by only one relaxation process. The activation energy val-

ues of relaxation process are found to be Fe3? content-

dependent and decreased from 0.96 to 0.94 eV.

Acknowledgements

One of the authors (T Fahmy) would like to thank Scientific

Research Deanship, Prince Sattam Bin Abdulaziz Univer-

sity, KSA.

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