Page 1 of 15
European Journal of Applied Sciences – Vol.10, No.1
Publication Date:February 25, 2022
DOI:10.14738/aivp.101.11506.
Adem, J. A., Agumba, J. O., Barasa, G. O., &Ochung, A. A. (2022). Hydrolysis Temperature Dependent Structural, Optical Band Gap
and the Associated Urbach Tail Energy of Cellulose Nanocrystals Fabricated from Water Hyacinth.European Journal of Applied
Sciences, 10(1). 88-102.
Services for Science and Education – United Kingdom
Hydrolysis Temperature Dependent Structural, Optical Band Gap
and the Associated Urbach Tail Energy of Cellulose Nanocrystals
Fabricated from Water Hyacinth
Jack A. Adem
Department of Physical Sciences, Jaramogi Oginga OdingaUniversity
of Science and Technology, (P.O. Box 210-40601), Bondo, Kenya
John O. Agumba
Department of Physical Sciences, Jaramogi Oginga OdingaUniversity
of Science and Technology, (P.O. Box 210-40601), Bondo, Kenya
Godfrey O. Barasa
Department of Physical Sciences, Jaramogi Oginga OdingaUniversity
of Science and Technology, (P.O. Box 210-40601), Bondo, Kenya
Angeline A. Ochung’
Department of Physical Sciences, Jaramogi Oginga OdingaUniversity
of Science and Technology, (P.O. Box 210-40601), Bondo, Kenya
ABSTRACT
This study has thus systematically investigated the fingerprints of hydrolysis
temperature on the optical and structural properties of the resultant CNCs. From
the study, it was observed that increase in hydrolysis temperature decreases the
grain size from ~21.7 nm to ~19.3 nm. However, crystallinity increased from
57.9% to 60.3% as calculated using Scherrer equation. By performing UV-Vis
spectroscopy, the value of the peak wavelength for maximum absorption, Xc
increased from ~247.52 nm for 50oC to ~247.76 for 90oC. The value of FWHM
equally increased from ~12.17 nm for 50oC to ~13.48 nm for 90oC. Additionally,
increasing hydrolysis temperature from 50oC to 90oC decreased the bandgap
energy, Eg from ~ 5.31 eV to ~5.14 eV. However, the Urbach energy increased from
~116 meV to ~217 meV respectively. From a plot of Eg versus Eu, the optical band
gap energy of the CNCs when there is no disorder in their microstructure was
found to be ~5.43 eV. The CNCs were found to have zero optical band gap energy
when hydrolysis is done at ~1297.6oC. Further, we have shown that the Urbach
energy is absent when hydrolysis is done at 14.23oC. This is the energy when the
localized defect states in an optical band gap region are completely screened. The
hydrolysis temperature dependent increase in the electronic disorder in the
crystal (Urbarch energy) with increase in associated band gap energy has thus
been estimated to account for the optical disorder in the CNCs.
Keywords: Cellulose Nanocrystals, Urbach Energy, Hydrolysis, Absorbance, Band gap
Energy
Page 2 of 15
89
Adem, J. A., Agumba, J. O., Barasa, G. O., &Ochung, A. A. (2022). Hydrolysis Temperature Dependent Structural, Optical Band Gap and the
Associated Urbach Tail Energy of Cellulose Nanocrystals Fabricated from Water Hyacinth. European Journal of Applied Sciences, 10(1). 88-102.
URL: http://dx.doi.org/10.14738/aivp.101.11506
INTRODUCTION
Cellulose nanofibrils and nanocrystals have good mechanical properties, high transparency,
and low coefficient of thermal expansion, among other properties that facilitate both active
and inactive roles in electronics and related devices [1]. For instance, these nanomaterials
have been demonstrated to operate as substrates for flexible electronics and displays [2, 3], to
improve the efficiency of photovoltaics [4], to work as a component of magnetostrictive
composites [5] and to act as a suitable lithium ion battery separator membrane [6]. More so,
the nanomaterials have gained great utility in the manufacture of sensory devices [7], energy
harvest as well as storage tools [8]. The common sources of cellulose are plants, both aquatic
and non-aquatic with wood and plant cell walls being the major sources. However, it can also
be obtainedfrom other biomass sources, such as banana rachis [9] and potato tubers [10].
Interestingly, cellulose can also be synthesized from marine animals (tunicates) [11], bacteria
(Gluconacetobacterxylinus) [12], and even algae (Valonia) [13]. Water hyacinth (Eichhornia
crassipes) has the potential to be used as raw material for cellulose base polymers [14]. It
basically consists of 25% cellulose, 33% hemicellulose, and 10% lignin. Since it is heavily rich
in cellulose abundant in Lake Victoria, it is quite of interest to this study.
The extraction of cellulose particles from cellulose source materials such as water hyacinth is
a two-stage procedure, the pretreatment stage and the hydrolysis stage. In the pretreatment
stage, the source material is subjected to high degree of purification and a further
homogenization pretreatment to enable it react more consistently in subsequent treatments.
Basically, the pretreatment of the source material is done to remove the non-cellulosic
constituents. The non-cellulosic molecules in cellulose include lignin, hemicelluloses, protein,
etc. Once the non-cellulosic materials are removed the cellulose molecules become accessible
to reagents in the subsequent stages. The pretreatments for wood and plants involve the
complete or partial removal of matrix materials (hemicellulose, lignin and protein) and the
isolation of individual complete fibers. The second stage, the hydrolysis process, generally
involves the separation of the purified cellulose materials into their microfibrillar and or
crystalline components. The procedure is a well-controlled and a chemically induced
treatment (chemical hydrolysis). During the chemical hydrolysis process a complete
regioselective degradation of the less organized and more accessible fraction of the cellulosic
material takes place, thereby releasing the crystalline domains from the purified material. The
three basic separation approaches are mechanical treatment, acid hydrolysis, and enzymatic
hydrolysis [15, 16]. These approaches can be used separately to obtain the desired particle
morphology. Acid hydrolysis has been widely used to extract the crystalline particles from a
variety of cellulose sources. The process preferentially removes (hydrolyzes) the amorphous
regions within the cellulose microfibrils. In general, the purified source material is mixed into
deionized water with a given concentration of acid. After reacting for a set amount of time at a
desired temperature, the mixture is diluted with deionized water to quench the reaction. This
mixture then undergoes a series of separation through centrifugation or filtration and
washing/ rinsing steps followed by dialysis against deionized water to remove the remaining
acid or neutralized salt. One of the first successful preparation of CNCs was done through acid
hydrolysis processes. In 1947 Nickerson and Habrle were the first to prepare CNC through
hydrolyzing cellulose with hydrochloric acid and sulfuric [17]. Later In 1951, Ranby managed
to prepare the stable CNCs colloidal suspensions though sulfuric acid hydrolysis of wood fiber
[18]. The typically used mineral acids are sulfuric acid [19], hydrochloric acid [20],
phosphoric acid [21], hydrobromic acid [22], and their mixed acids [23]. However, Sulfuric
Page 3 of 15
90
European Journal of Applied Sciences(EJAS) Vol.10, Issue 1, February-2022
Services for Science and Education – United Kingdom
acid and hydrochloric acid are the most widely used due to their inherent unique advantages
over the other mineral acids coupled with their great abundance.
After a careful preparation of cellulose nanocrystals, the optical properties can be mapped.
This involves performing UV-Vis spectroscopy on the nanocrystals and the determination of
band gap energy, Eg. It is thus important to note that the initial stage of electronic transitions
between the highest occupied valence band and the lowest unoccupied conduction is the
absorption edge of the absorption spectrum. The difference in energy between these two
bands is given by the band gap energy. Tauc’s model is one of the techniques used to calculate
the band gap energyusing absorbance spectra measured by a UV-Vis spectrophotometer. The
relation between the absorption coeffiecient, α and the incident photon energy, hν can be
determined by Tauc’s relationship in the high absorption region of a spectra.
In the present study, optical band-gap energy (Eg) of CNCs having different microstructures
are investigated using an X-ray diffractometer (XRD) and UV-Vis spectrophotometer. The
underlying dependence of microstructure on the evolution of Eg are addressed by assessing
the associated Urbach energy tail (Eu). The correlation between the preparation techniques,
the structure, the Optical Band Gap and the associated Urbach Tail Energy of the CNCs were
mapped.
EXPERIMENTAL
Extraction of Cellulose Nanocrystals from Water Hyacinth
The materials used in this study were water hyacinth stalks and stems obtained from Lake
Victoria. The chemicals and reagents used in this work were Sodium Hydroxide (NaOH,
toluene and ethanol for pretreatment process and Hydrochloric acid (HCl), Sulphuric acid
(H2SO4), Nitric acid (HNO3) for the acid hydrolysis procedure. Cellulose Nanocrystals from
water hyacinth were extracted through a rigorous process. The harvested water hyacinth
stalks and stems were first cut off from the plants and cleaned to get rid of mud, sand,
minerals and any other aquatic impurity that could be lying on their surfaces then rinsed
using deionized water and dried in open air. The cleaned stalks and stems were then chopped
into small cubes then fed into an oven where they were dried at 100oC for 30 minutes. This
was followed by ball milling process that crushed the dry cubes into fine powder. The fully
dried water hyacinth chops were reduced to half powder form by ball milling using a ball
milling machine. The milling machine was set to crush at a low rotating frequency of 60 rpm
for a shorter duration of 5 minutes. The low frequency and shorter crushing duration was to
ensure that the crystalline regions are not broken. The coarsely ground fibers were subjected
to sieving using a sieve of hole diameter 1mm to obtain course powder free of chuff. A total of
20g mass of the powder was prepared ready for the cellulose extraction. As was explained
earlier the water hyacinth powder obtained is 25% cellulose, 33% hemicellulose, and 10%
lignin. The lignin and the hemicellulose components must therefore be eliminated in the
pretreatment stage and in the subsequent acid hydrolysis process.
Pretreatment stage that involved alcohol treatment coupled with alkaline hydrolysis and
bleaching processes followed. The main aim was to get rid of the non-cellulosic materials from
the powder. Pretreatment process also exposes the extracted cellulose for more reactions in
the acid hydrolysis stage.10g of the course hyacinth powder was measured using an electronic
beam balance and transferred into a round bottomed flask containing 150 ml homogeneous
Page 4 of 15
91
Adem, J. A., Agumba, J. O., Barasa, G. O., &Ochung, A. A. (2022). Hydrolysis Temperature Dependent Structural, Optical Band Gap and the
Associated Urbach Tail Energy of Cellulose Nanocrystals Fabricated from Water Hyacinth. European Journal of Applied Sciences, 10(1). 88-102.
URL: http://dx.doi.org/10.14738/aivp.101.11506
mixture of toluene and ethanol in the ratio 2:1. The toluene/ethanol mixture was made by
mixing 100ml of toluene to 50 ml of ethanol and then stirred to homogeneity. Toluene and
ethanol mixture are one of the best cellulose extracts from a source material. Once dipped into
the mixture, the water hyacinth sludge was heated at 70oC for 2 hours at a constant stirring
using the rotor vapor instrument. This process was done to remove extractive components
contained in the stalks of the water hyacinth leaving behind lignin, cellulose and
hemicellulose. The sediment was then subjected to a bleaching process using NaClO3 for two
hours at 80oC. This first bleaching process not only increased the whiteness of the cellulose
sludge but also led to the elimination of lignin by breaking the ether bonds. The bleached
sludge was then subjected to alkaline hydrolysis using NaOH 1% at 60oC for 2 hours to
remove the hemicellulose from the main chain cellulose. NaOH reaction with the sludge led to
heavy degradation of lignin molecules due to the termination of major bonds. The resultant
product was further subjected to a second bleaching process using NaClO3 2% at 75oC for
2hours to remove any remaining lignin and hemicellulose that could still be clinging in the
product. A highly systematic and structured acid hydrolysis process was finally done in order
to get rid of the amorphous region of the pure cellulose material so as to remain with the
crystalline region only thereby obtaining cellulose nanocrystals in syrup form. The study
resorted to the use of mineral acids as they yield CNCs with high crystalline index. The
mineral acid used was HCl for different samples. The degree of concentration was kept at 5%,
and the hydrolysis heating temperature was varied from 50oC, 70oC and 90oC. The heating
was done in a water bath for 2 hours.
Each of the prepared samples was then subjected to centrifugation process at a frequency of
3000rpm for 5 minutes. This was done to accomplish the homogenization process for
accurate optical measurements. The essence of parameterizing the process was to enable to
investigate how the three hydrolysis parameters affect the degree of crystallinity of the
extracted CNCs. The three parameters are the concentration of the acid, hydrolysis
temperature and the hydrolysis reaction time. The samples were given unique acid hydrolysis
treatments to help us investigate how change in these parameters (acid concentration,
reaction time and temperature) impact in the optical characteristics of the obtained CNCs and
the subsequent impact on the band gap energy.
Characterization of CNCs
The crystalline structure and phase identification of the cellulose composites were
investigated by a PANalyticalX'Pert PRO diffractometer using a Cu Kα radiation (λ = 1.54 Å)
with a current of 40 mA and an anode voltage of 45 kV. The absorbance spectra were collected
using Chary60 Shimadzu UV-Vis spectrophotometer transmission in the 200-500 nm range at
the wavelength interval of 0.2nm. The Maximum absorbance wavelength measurements were
read and recorded. The band gap energies were further calculated from Tauc’s graphs that
were generated from the absorbance spectral graphs. These optical measurement values were
done for all the parameters used in the extractions and the results compared and evaluated.
RESULTS
Structural Analysis
Cellulose I polymorph has been shown to have both monoclinic and triclinic structure [24]. To
investigate the crystalline structure of the fabricated cellulose nanocrystals, X-ray diffraction
(XRD) analyses were performed. Figure 1 show the X-ray diffraction patterns of samples
Page 5 of 15
92
European Journal of Applied Sciences(EJAS) Vol.10, Issue 1, February-2022
Services for Science and Education – United Kingdom
prepared at 5% concentration and hydrolysis done at temperatures of 50oC, 70oC and 90oC for
2 hours.
10 20 30 40 50
(004)
(200) 50oC
70oC
90oC Intensity [au]
2 Theta Degrees
(110)
Figure 1: X-ray diffraction patterns of samples prepared at 5% concentration and hydrolysis
done at temperatures of 50oC, 70oC and 90oC for 2 hours
These samples indicate sharp diffraction peaks at 2θ around 16.5o, 22.5o and 34.5o
corresponding to the (110) (200) and (004) lattice planes which are representing the typical
cellulose I structure [ 25].
The crystallinity index for CNCs prepared at the different hydrolysis temperatures were
calculated by equation (1).
Crystallinity Index
( 2 0 0 )
( 2 0 0 )
( )
1 0 0 % a m
I I
X
I
........(1)
Where I (200) is the intensity value for the crystalline cellulose (when 2θ = 22.5° for cellulose I
and Iam is the intensity value for the amorphous cellulose (when 2θ = 18° for cellulose I) [26].
The results of crystallinity indices are shown in Table 1.