Tungsten Oxide Nanostructures Peculiarity and Photocatalytic Activity for the Ecient Elimination of the Organic Pollutant
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Tungsten Oxide Nanostructures Peculiarity and Photocatalytic
Activity for the Efficient Elimination of the Organic Pollutant
Deepika Jamwal
Panjab University
Vishal Mutreja
Chandigarh University
Rahul .
Panjab University
Surinder Kumar Mehta
Panjab University
Akash Katoch ( katochakash@gmail.com )
Panjab University https://orcid.org/0000-0003-2770-6866
Sang Sub Kim
Inha University College of Engineering
Research Article
Keywords: Tungsten oxide, Nanostructures, Shape, Twin tail surfactants, C14TAB, Organic pollutant, Photocatalysis
Posted Date: January 16th, 2023
DOI: https://doi.org/10.21203/rs.3.rs-2218955/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License
Version of Record: A version of this preprint was published at Environmental Science and Pollution Research on May 30th, 2023. See
the published version at https://doi.org/10.1007/s11356-023-27891-5.
Page 1/18
Abstract
For the first time, the effect of gemini based twin-tail and conventional surfactant on tungsten oxide nanostructures and their efficacy
for the elimination of the organic pollutant is studied. The tungsten oxide nanostructures were synthesized by a simple hydrothermal
route in the presence of C14TAB and gemini based twin-tail surfactant. The impact of using these special shape and size directing
agents for the synthesis of nanostructures was observed in the form of different shapes and sizes. The tungsten oxide web of chains
type nanostructure was obtained using C14TAB in comparison to the cube shaped nanoparticles through twin-tail surfactant. On
contrary, the twin-tail surfactant provides sustainable and controlled growth of cube shape nanoparticles of size ~ 15 nm nearly half
of the size ~ 35 nm obtained using conventional surfactant C14TAB, respectively. For the detailed structural features, the Williamson-
Hall analysis method was implemented to find out the crystalline size and lattice strain of the prepared nanostructures. Owing to the
strong quantum confinement effect, the WO3 cube shaped nanoparticles with an optical band gap of 2.69 eV of the prepared
nanoparticles showed excellent photocatalytic efficacy toward organic pollutant (Fast green FCF) compared to the web of chain
nanostructures with an optical band gap of 2.66 eV. The mechanism has been discussed in detail in the respective section. The ability
of the prepared systems to decompose the organic pollutant (Fast green FCF) in water was tested under visible light irradiations. The
percentage degradation was found to be 94% and 86% for WO3 cube shaped nanoparticles and WO3 web of chains, respectively. The
simplicity of the fabrication method and the high photocatalytic performance of the systems can be promising in environmental
applications to treat water pollution.
1. Introduction
Nowadays, the engineered nanostructures and their exceptional surface features such as high surface-to-volume ratio, multi-
functional possibility, and illustrious surface reactivity have achieved great promises for a variety of consumers and industrial
applications, by making the materials lighter, more robust, and more proficient in comparison to their bulk equivalents (Hou et al.
2018). In contrast to the traditional semiconductors (with a band gap ‹ 2 eV), metal oxides are recognized as wide bandgap (›2 eV)
semiconductors with electron transition energy in the range of ultraviolet and visible light which results in a solution for the
inadequacies with the traditional semiconductors (Park et al. 2017; Guo et al. 2019). Furthermore, wide band-gap semiconductors
materials are also stable at high temperatures, can perform efficiently under high currents and voltage, and show higher carrier
density and mobility (Chaves et al. 2020). Because of the wide band gap of most of the metal oxides, they are also comparable to the
electrode potential of various significant reactions, such as the oxidation of organic molecules, splitting of water to produce both
hydrogen and oxygen reduction of CO2, and degradation of the various pollutants and dyes (Basith et al. 2018; Jamwal et al. 2022).
In recent decades, nanomaterials have attracted significant attention in a variety of areas as a result of Tungsten oxide is an exclusive
semiconductor material with a characteristic wide and direct band gap of 2.6 to 3.7eV for various structures, which can be affected by
particle size and shape of the material. Tungsten oxide nanostructures have remarkable applications in many fields such as solar
cells, nanolasers, gas sensors, electron field emitters, light-emitting diodes (LEDs), and as a photocatalyst due to their amazing
properties (Aliasghari et al. 2020). Subsequently, to improve the various chemical features, researchers have been following different
synthesis methods such as sol-gel, microwave-assisted synthesis, pulsed laser deposition, solvothermal, hydrothermal method, etc. to
prepare Tungsten oxide nanostructures (Mardare and Hassel 2019). In addition, the nanostructured systems can exhibit new features
and improved performance in comparison to the bulk form due to the increased surface-to-volume ratio, quantum confinement effect,
and alteration in the surface energies of the systems. In arrears to these improved performances, the nanostructured tungsten oxide
hydrate shows greater efficiency and unique features in contrast to the bulk form (Shinde and Jun 2020). In recent times, different
structural forms of tungsten oxide materials such as nanoparticles, nanorods, nanotubes, nanofibers, etc. have been synthesized by
using various physical and chemical methods like sol-gel, microwave-assisted synthesis, chemical vapor deposition, spray pyrolysis,
template mediated synthesis, and hydrothermal method (Lavanya et al. 2017). Out of all these methods we have used the
hydrothermal method, which is known as one of the most prominent techniques for the preparation of nanostructures. At higher
temperatures and pressure, the hydrothermal method is known for the solubility of nearly all the inorganic substances in water and
results in the crystallization of the dissolved precursor materials from the fluid. Hydrothermal synthesis of the nanostructures offers
many opportunities to control the nanoparticle size as well as shape. Reducing the size to the nanoscale and maintaining the shape
of the synthesized material are significant structural amendment strategies to improve the performance of tungsten oxide (Medhi et
al. 2020).
Page 2/18
The controlled size and shapes of the synthesized materials have been approved as important steps for the exploration of the
material for use in photocatalytic, electrocatalytic, thermochromic, photochromic devices, etc. (Yang et al. 2019). In this respect, the
use of surfactants in the synthesis method can lead to nanomaterials with controlled size and morphology with desired properties.
The unique properties of the special type of surfactants i.e. Gemini surfactants encouraged us to use these molecules for the
synthesis of tungsten oxide nanoparticles. In comparison with conventional surfactants (Jamwal et al. 2019), Gemini surfactants
have distinctive features such as their very low critical micelle concentration (cmc), good water solubility, greater efficacy for reducing
the surface tension of water, and low Kraft point (Rana et al. 2019).
The extensive use of synthetic dyes in many fields and their inadvertent release into the environment results in a promising risk to
human health and the ecological system. The different commercial dyes are categorized by sturdy structural and color stability owing
to their high degree of aromaticity. Therefore, adequate removal of synthetic dyes from waste water is extremely important. All
through the past decades, photocatalysis utilizing semiconductors nanomaterials facilitates pollutant decomposition and has
acknowledged significant attention (Cheng et al. 2013).
To date, hydrated tungsten oxide nanostructures with Gemini surfactants are not reported. In this work, for the first time, we have
synthesized the tungsten oxide nanostructures in the presence of Gemini surfactants by using the simple hydrothermal method and
also compared their photocatalytic activity with the conventional surfactant i.e. C14TAB derived tungsten oxide nanostructures. The
effect of using the Gemini surfactants and C14TAB on size and morphology as well as the impact of change of size and morphology
in photocatalytic degradation of the fast green (FCF) dye were discussed in detail.
2. Experimental Section
2.1 Chemicals
Sodium tungstate (NaWO4.2H2O), 1-bromododecane, N,N,Nʹ,Nʹ-tetramethylethylenediamine (TEMED), Ethyle acetate, acetone,
Tetradecyltrimethylammonium bromide (C14TAB) (cmc = 4.6 mM) (Sanchez-Fernandez et al. 2017), Hydrochloric acid (HCl), and FCF
dye were purchased from Sigma-Aldrich and were used without further purification. The cationic twin tail surfactant (TTS) i.e.
dimethylenebis(dodecyldimethylammonium bromide) (cmc = 0.84 mM) was synthesized by the method as reported in the literature
(Zana et al. 1991; Wettig and Verrall 2001) and was used after repeated recrystallization with ethyl acetate and acetone.
2.2 Synthesis of WO3 nanostructures
The synthesis of WO3 nanoparticles was carried out by taking the 1 mM of NaWO3.H2O solution prepared in 20 mL of deionized water
with constant stirring, 1 mM of TTS in 15 mL of deionized water followed by 4 mL of 2M HCl aqueous solution in a dropwise manner
with constant stirring. The solution was left for constant stirring for the next 30 minutes. At this point, the texture of the solution was
milkish white. Finally, the resultant solution was transferred to the sealed teflon-lined stainless steel autoclave. The sealed autoclave
was heated to 110 ºC for 5 hours. After completion of the reaction, the yellow-colored precipitates were collected by centrifuging and
washed several times with deionized water, followed by vacuum oven drying. On the other hand, for the second sample, the TTS was
replaced with the conventional C14TAB, and the rest of the procedure was similar for the preparation of tungsten oxide
nanostructures.
2.3 Characterization
The formation of WO3 nanoparticles through the hydrothermal method is confirmed by X-ray diffraction (XRD) analysis. The X-ray
diffraction patterns were recorded using Panalytical’s X’Pert Pro diffractometer with graphite monochromatized CuKα irradiation (λ =
1.5418 Å). The infrared spectrum was measured by Perkin Elmer-Spectrum RX- IFTIR, Fourier transform infrared spectrometer (FTIR)
at room temperature in the range of 4000 − 400 cm− 1. High-Resolution Transmission Electron Microscopy (HR-TEM) images, EDS, and
elemental mapping were recorded on FEI Tecnai G2 20S-TWIN. UV-Visible spectra were analyzed by using a UV-Visible
spectrophotometer (Systronics 2202) in the range of 200–900 nm.
2.4 Photocatalytic analysis
The photocatalytic aptitudes of synthesized WO3 nanostructures were assessed via the degradation of FCF pollutant under sunlight
irradiation. All photocatalytic investigations were carried out under parallel conditions on sunny days between 10 am and 2 pm. Open
Page 3/18
borosilicate glassware was used for the reaction (Balakrishnan et al. 2017). In the degradation studies, 20 mg of the synthesized
photocatalyst was suspended in 50 mL of an aqueous solution comprising FCF dye at a concentration of 1 X 10− 5 M. At the start of
the experiment, before visible light irradiation, to attain adsorption-desorption equilibrium between the pollutants and tungsten oxide
nanoparticles, the solution was magnetically stirred in the dark for 30 minutes. At the given time interval, 3 mL of the suspension was
sampled and centrifuged at 4000 rpm for 15 minutes to separate the tungsten oxide nanoparticles from the solution. The supernatant
liquid was examined for the residual concentration of the FCF dye under study. The FCF dye was studied by recording the deviations
in absorbance at the representative absorption peak of λ = 620 nm in addition to scanning over the wavelength from 300 to 800 nm
using a UV-Vis spectrophotometer.
To evaluate the photocatalytic activity of tungsten oxide nanoparticles for pollutant degradation, assorted reaction parameters such
as reaction time, amount of photocatalyst, the concentration of the organic pollutant, and the effect of different pH were analyzed in
detail. The aliquots of the mixture were taken at a definite interval of time during the irradiation, and after centrifugation of the
photocatalyst, absorbance was measured. The degradation percentage of the FCF after photoreaction for the time (t) was analyzed
as (Ramanathan et al. 2020).
(C 0 − C t )
Degradationrate% = × 100%
C0
1
C0 = initial dye concentration, Ct = residual dye concentration at t minutes.
3. Results And Discussion
3.1 Morphology analysis
The morphology of WO3 nanostructures synthesized using C14TAB and TTS were studied by employing high-resolution transmission
electron microscopy, shown in Fig. 1. It can be observed from that WO3 nanostructures of different shapes and sizes were obtained by
the use of C14TAB and TTS surfactants. The WO3 web of chains obtained with the C14TAB, is shown in Fig. 1a. The HR-TEM image in
Fig. 1b reveals that the surfactant directs the continuous growth of the WO3 web of chains possessed of fused nanostructure. On the
other hand, cube shaped WO3 nanoparticles were obtained using TTS surfactant, as shown in the low and high magnification TEM
images Fig. 1c and 1d. The inset in Fig. 1d demonstrates the high-resolution image confirming the d-spacing value 0.35 nm
corresponds to the (111) plane of the WO3 cube shaped nanoparticles. The average particle size observed from the HR-TEM analysis
for the WO3 web of chains was ~ 35 nm and for WO3 cube shaped nanoparticles was ~ 15 nm, respectively. The nanoparticles size
reduction by 50% confirms that the lower cmc value of TTS surfactant compared to conventional surfactants is significant to tune the
size of metal oxide-based nanomaterials and acts as shape directing and stabilizing agent for the development of nanostructures. As
it is clear from the molecular structure that CTAB is a single tail/monomeric surfactant and TTS is a double tail/dimeric surfactant,
which is schematically shown in scheme S1. There is a considerable difference in the shape and size of the WO3 nanostructures. The
double tail surfactants demonstrate very high potential for the shape as well as size control effects because of their greater interfacial
adsorption ability, and great hydrophobicity are known as more surface-active in comparison to their monomeric units or conventional
surfactants and therefore they have been successfully used as shape directing agent for the synthesis of desired morphologies
(Jamwal et al. 2016a). The TTS surfactant leads to the formation of cube shaped nanoparticles due to the presence of twin-tails
which results in major growth along the (111) and (020) planes (see XRD spectra in Fig. 2). On the other hand, in the case of WO3 web
of chains, planes responsible for the particles growth were the same i.e. (111) and (020) but there is a change in the morphology as
well as size, the formation of the diffused chain-like structures in WO3 web of chains may be due to the coalescence in the particles
supported by the conventional surfactant consist of single chain. The coalescence behavior in the nanoparticles arises with contact
and initial fusion trailed by the orientational alignment of the coalescing planes at the interface amongst the particles during contact.
Furthermore, the crystal structure analysis of the samples was also performed by selected area diffraction pattern (SAED). The SAED
pattern of WO3 web of chain and WO3 cube shaped nanoparticles are shown in Fig. 1e and 1f indicate the polycrystalline nature of
both nanostructures. The indexed SAED rings confirm the orthorhombic structure of the WO3 nanostructures. Similar lattice spacing
values were obtained for both WO3 nanostructures synthesized using C14TAB and TTS surfactants. However, the ordered SAED
Page 4/18
pattern of TTS-derived WO3 cube shaped nanoparticles indicates the better crystalline character of cube shaped nanoparticles.
Clearly, the TTS surfactant has an edge over conventional surfactant for synthesizing the nanoparticles, which can be due to the
lower cmc and existence of twin-tails in the TTS surfactant executes stable growth of nanoparticles in comparison to the single tail
C14TAB. In our recent investigation, the chain length and spacer between the twin-tail has been also found effective to control the
shape and size of gold nanoparticle (Rana et al. 2016). The composition of the samples was analyzed through EDX and elemental
mapping. Figure 1g and 1h show the EDX of the WO3 nanostructures representing the presence of W and O elements, validate the
formation of the WO3 nanoparticles with no extra impurity peaks. The presence of carbon in both samples is attributable to the
organic stabilizing agent as well as to the carbon-coated microgrid mesh supporting the samples for the analysis. The presence of
copper peaks also belongs to the microgrid. The atomic percentages of W and O in the case of the web of chains nanostructure were
found as 4.99% and 15.56%, respectively. In the same way, for the WO3 cube shaped nanoparticles, the atomic percentage of W and O
were found as 5.55% and 17.67% respectively and the ratio for the respective elements was also found around 1:4. Figure 1i and 1j
shows the elemental mapping for the distribution of the elements i.e. W, O, and C in the prepared WO3 web of chains and WO3 cube
shaped nanoparticles, respectively.
3.2 XRD Analysis
The structural identification of the as-prepared nanostructures was confirmed by using the X-Ray Diffraction (XRD) analysis. The XRD
pattern of WO3 nanostructures prepared with C14TAB and TTS surfactants are summarized in Fig. 2. The diffraction peaks centered at
16.5º, 25.6º, 30.5º, 33.4º, 34.2º, 35.1º, 38.2º, 38.9º, 49.7º, 52.7º, 56.2º, 57.2º, 61.36º and 62.9º were corresponding to the (020), (111),
(031), (040), (200), (131), (220), (022), (202), (222), (311), (113), (331) and (133) planes of the orthorhombic phase for the tungsten
oxide hydrate (ICDD:01-084-0886). Notably, a nearly identical XRD pattern was observed for both cases. The Debye-Scherrer equation
was applied to calculate the average crystallite size of the prepared WO3 nanostructures. The crystallite size calculated from XRD was
found as 50 and 40 nm for the C14TAB and TTS-derived nanoparticles, respectively. The crystallite size acquired from the Debye-
Scherrer formula of size calculation lacks the contribution from the lattice strain parameters which also indicates the broadening in
the XRD peaks (Bakr et al. 2021). For thorough structural properties investigation of the nanostructures, it is important to acquire
information on the other parameters such as the size, strain, stress, and energy density. Thus, Williamson-Hall (W-H) plot method is
used to excerpt an appropriate understanding of the aforesaid parameters by using the XRD analysis (Sheikh et al. 2018; Kibasomba
et al. 2018). The aforementioned parameters were obtained from the various models i.e. uniform deformation model (UDM), uniform
stress deformation model (USDM), and uniform deformation energy density model (UDEDM) under W-H analysis are summarized in
Table 1. The plots for the W-H analysis models are shown in the Fig. S1. Through these models, the parameters for instance lattice
strain, stress, and energy density of the system were determined within a certain approximation. It is clear from the data that with the
increase in the crystal size there is a decrease in the value of strain and a similar trend was also observed for the stress as well as
energy density values of the given systems.
Table 1
Geometric parameters of WO3 web of chain and cube shaped nanoparticles.
WO3 Nano- Scherrer Williamson-Hall Method
structures Method
UDM USDM UDEDM
Size, D Size, Strain, ε Size, Strain, ε Stress, Size Strain, ε Stress, Energy
D D D Density,
(nm) (Compressive) (nm) (Compressive) σ (Tensile) σ u
(nm) (MPa) (nm) (MPa) (KJm−
3
)
Web of 50 22 0.1x10− 3 90 1.6 x10− 3 0.69 77 16.2x10− 6.57 0.05
chains 3
Cube 43 8.9 4.4x10− 3 25 0.4 x10− 3 1.64 77 7.1x10− 3 2.87 10.8
nanoparticles x10− 3
3.3 UV-Vis Analysis
Page 5/18
The optical properties of the samples were analyzed by using UV-VIS absorption spectra. Figure 3a shows the absorption spectra of
the synthesized WO3 web of chains and WO3 cube shaped nanoparticles. Here, the characteristic spectrum with its fundamental
absorption sharp edge rises at around 407 nm and 400 nm, correspondingly. The red shift in the absorption edge in the case of the
WO3 web of chains was observed, where, it was slightly shifted toward a higher wavelength with a difference of ~ 7 nm. The optical
band gap energy of the synthesized nanostructures was calculated by using the Tauc plot (O’Connor and Tauc 1982) equation:
n
ahv = A(hv − Eg )
2
Where α is the absorption coefficient, A is a constant, hv is the photon energy, Eg is the optical band gap and n is the possible
electronic transition. The band gap energies of the WO3 web of chains and WO3 cube shaped nanoparticles were valued to be 2.66
and 2.69 eV, respectively (Fig. 3b).
As is clear, the band gap energy is higher for the smaller-sized nanoparticles, and directly supported by the quantum confinement
theory, which proposes that the electrons in the conduction band and holes in the valence band are confined by the potential barriers
of the surface or potential well of the quantum box. Thus, due to the confinement of the electrons and holes, the band gap energy
increases between the valence band and conduction band with decreasing particle size (Edvinsson 2018; Singh et al. 2019).
3.4 FTIR Analysis
The perceived bands for the pure C14TAB and TTS as well as both with WO3 web of chains and WO3 cube shaped nanoparticles were
studied to enumerate the surface adsorption of the surfactants on the surface of nanostructures, is summarized in Fig. 4. Focusing
on the pure C14TAB and WO3 web of chains (Fig. 4a), the characteristics bands at 2914 cm− 1 (CH2asym ) shifts to 2919 cm− 1, 2846
cm− 1(CH2sym ) shifts to 2849 cm− 1, 1480 cm− 1 (δ(CH3)asym ) shifts to 1489 cm− 1, 1408 cm− 1 (CH2)nsciss shift to 1376 cm− 1, 909 cm−
1
v(C-N+) shifts to 864 cm− 1 and 715 cm− 1 (CH2)rock shifts to 648 cm− 1 in case of pure C14TAB to WO3 web of chains. The other
bands at 3016 cm− 1 (CH3asym (-N+(CH3)3), 2946 cm− 1 (CH3sym (-N+(CH3)3), 1462 cm− 1 (CH2)nsciss, and 949 cm− 1 v(C-N+) for pure
C14TAB, were not observed in WO3 web of chains, indicating the strong interaction of the surfactant with the web of chains (Borodko
et al. 2009). Furthermore, in the case of TTS (Fig. 4b), the bands at 2954 cm− 1, 2922, and 2851 correspond to symmetric (vsym (C-H))
and asymmetric (vasym (C-H)) stretching vibrations of methylene groups, respectively which shifts to 2950, 2915, and 2850 in the case
of WO3 cube shaped nanoparticles. The band related to NR4+ asymmetric bending (ρasym (NR4+)) at 1636 was shifted to 1625 for
WO3 cube shaped nanoparticles. Furthermore, the scissoring mode of vibration for the methylene chains (C-H) was observed at 1463
and 1378 moved to 1465, an indicator of the gauche defects. The band at 971 can be assigned to the v(C-N+) stretching modes that
shifted to 953. The band at 853 for pure surfactants was observed and showed a shift to 887 which may be a pointer to higher energy
end-gauche defects than in pure TTS and proposes that the head group of the surfactant is intensely attached to the nanoparticles.
The absence of the bands in the case of WO3 cube shaped nanoparticles was an indication of the strong interaction of the
surfactants with the nanoparticles (Jamwal et al. 2016b). The peak assignment for the pure surfactants and nanoparticles
synthesized in the presence of surfactants was mentioned in Table 2.
Page 6/18Table 2
Peak assignment of C14TAB and TTS in the presence and absence of WO3 nanostructures.
Peak Assignment C14TAB WO3 web of chains TTS WO3 cube shaped nanoparticles
CH3asym (N+(CH3)3) 3016 ----- ----- -----
vsym(C–H) 2846 2849 2851 2850
vasym(C–H) 2946 ----- 2954 2950
2914 2919 2922 2915
δs(C–H) 1480 1489 1463 1465
1462 ----- 1378 -----
1408 1376 ----- -----
ρasym (NR4+) ----- ----- 1636 1625
ν(C–N+) 949 ----- 1158 -----
909 864 971 953
----- ----- 904 887
----- ----- 853 -----
ρw(CH2)n ----- ----- 1301 -----
ρt(CH2)n ----- ----- 1275 -----
ρr(CH2)n 715 648 ----- -----
ν = stretching, sym = symmetric, asym = antisymmetric, δs = methylene scissoring, ρr = rocking, ρt = twisting, ρw = wagging.
Based on the above results, we identified a possible formation reaction mechanism of WO3 nanostructures synthesized with C14TAB
and TTS surfactants. In the hydrothermal method, the synthesis of WO3 starts with the formation of tungstic acid. The reaction
between Na2WO4.2H2O and HCl in the presence of C14TAB and TTS has been shown in equations (3) and (4). The formation of
tungsten oxide nanostructures in this hydrothermal process has been considered a direct combination pathway (Yin et al., 2012):
Na2WO4 + 2HCl → H2WO4 + 2NaCl (3)
H2WO4 → WO3.H2O (4)
3.5 Degradation of FCF dye with WO3 nanostructures under sunlight
The photocatalytic activity of the synthesized nanostructures was appraised for the fast green dye degradation as an exemplary
reaction under visible light irradiation. By monitoring the maximum absorbance of the FG at 620 nm, the photocatalytic degradation
of the FCF dye was represented in Fig. 5, showing the plot between normalized concentration and the analogous time. The reaction
for the decomposition of the FG dye obeys the first-order reaction, which could be shown by the following equation (Alzahrani 2018):
C0
ln ( = kt)
C
5
Where C0 represents the concentration of the FG at a time equal to zero (equilibrium), C is the concentration at any time and k
represents the rate constant of the reaction. The first-order equation speculates that the bindings originated from physical adsorption.
It can be observed clearly from Fig. 5a and 5c, that in the presence of solar irradiation the WO3 web of chains as a catalyst took 190
Page 7/18minutes for the 86% degradation of the FG dye with the rate constant value as k = 0.0116 min− 1. Similarly, in the case of WO3 cube
shaped nanoparticles, the maximum absorbance of FG at 620 nm was shown and 94% degradation with k as 0.0068 min− 1 was
observed from the plot (Fig. 5b and 5d). Based on the aforementioned data it has been observed that WO3 cube shaped nanoparticles
were showing a worthy performance in comparison to the WO3 web of chains. The higher photocatalytic degradation efficiency can
be attributed to the smaller size nanoparticles synthesized in the presence of TTS. The photoreactions mainly proceed on the surface
of the photocatalyst and a decrease in the size of nanoparticles will lead to the increase in surface area to volume ratio, available
surface-active sites, and interfacial charge carrier transfer rates which directly point towards the higher catalytic activities. By
following Eq. (5), the first-order reaction rate constants were calculated for the WO3 web of chains and WO3 cube shaped
nanoparticles and are shown in Fig. 6a and 6d. The plot shows a straight line fit with a correlation coefficient very close to unity (r2 =
0.988 for the WO3 web of chains and 0.998 for WO3 cube shaped nanoparticles), clearly indicating that the kinetics of the degradation
reaction followed the first-order rate law. Adsorption of FCF onto the catalyst was also evaluated for the pseudo-second-order (Fig. 6b
and 6e) and intraparticle diffusion model Fig. 6c and 6f. (Ahmed et al. 2017). The pseudo-second-order model gives the chemical
adsorption and the equation for the second-order is
t 1 t
= +
Q k2 Q 2 Q
t e e
6
Where, Qe and Qt are the adsorption capacity of tungsten oxide nanoparticles at equilibrium and at time t (min). The initial adsorption
rate was found using k2Qe2.
On the other hand, the intraparticle diffusion model was applied to identify the diffusion mechanism. The equation of the intraparticle
diffusion model is shown as
1
Q = ki t 2 + C
t
7
where ki represents the intraparticle diffusion rate and C is the intercept. The linearized results from the aforementioned kinetic
models were compiled in Table 3.
The correlation coefficient (R2) and the kinetic results obtained from different models signify that the adsorption of FCF is controlled
by the pseudo-first-order model in comparison to the other i.e pseudo-second-order and intraparticle diffusion model.
Page 8/18Table 3
Adsorption kinetic parameters for the adsorption FCF on tungsten oxide nanostructures.
Nanostructures Kinetic Models Kinetic model parameter
WO3 web of chains Pseudo-first-order R2 = 0.988
k1 = 0.0116
Pseudo-second-order R2 = 0.900
k2 = 21.716
Intraparticle diffusion R2 = 0.975
ki = 00297
C = 0.975
WO3 cube shaped nanoparticles Pseudo-first-order R2 = 0.998
k1 = 0.0068
Pseudo-second-order R2 = 0.538
k2 = 35.091
Intraparticle diffusion R2 = 0.964
ki = 0.032
C = 0.964
It has been noted that, in semiconductor materials, the band edge positions have a considerable connection with the oxidation
process of organic compounds. Therefore, it is compulsory to determine the valence band (VB) and conduction band (CB) edge
positions of the as-synthesized WO3 nanostructures. The valance band and the conduction band edge positions for these
nanostructures were calculated by using the Mulliken electronegativity method (Murali et al. 2019) as per the given steps
1
EC B = χ − EC − Eg
2
8
EV B = EC B + Eg
9
Where, χ is the absolute electronegativity of the semiconductor atom and the value of χ for the WO3 was determined as 6.88 eV
(Aslam et al. 2019), EC is the energy of free electrons of the hydrogen scale i.e. kinetic energy of the electrons and Eg for the WO3 web
of chains and WO3 cube shaped nanoparticles were calculated as 2.66 and 2.69 eV, respectively by using the Eq. 1. Based on
equations 8 and 9 the values for the VB and CB for WO3 web of chains were extracted as 1.05 and 3.71 eV. Similarly, for the WO3 cube
shaped nanoparticles VB and CB values were found as 1.03 and 3.72 eV, respectively. A pictorial representation of the photocatalytic
dye degradation showing the band gaps of different catalysts (WO3 web of chains and WO3 cube shaped nanoparticles) for the
degradation of FG is shown schematically in scheme 1. At the time of solar light irradiation, the valance band electrons which are
present at the lower potential (1.05 and 1.03 eV) for both systems got excited by absorbing the photons with energy hv ˃Eg, move to
the conduction band having the higher potential (3.71 and 3.72 eV). The charge carriers generated due to this process participate in
the oxidation, and reduction process during the photocatalytic reaction. In detail, due to the presence of visible light irradiation, the
photocatalyst was activated to generate the pairs of holes (h+) and electrons (e−). The photogenerated holes reacted with H2O to form
hydroxyl radical ·OH, which is the major oxidant species to degrade the different organic pollutants and hydrogen ions (H+), which
Page 9/18acts as a byproduct and resulted in the decrease of pH in the system. The generation of the electrons in the reaction was responsible
to create other oxidants by consuming the oxygen that can be oxidized directly by the active radicals to final products as CO2 and
H2O. Due to the above-mentioned reaction steps, the different nanomaterial photocatalysts perform with admirable photocatalytic
efficiency.
The degradation of the organic dye by the active species can be proposed as (Yu et al. 2020):
WO3 + hν → h+ + e− (10)
h+ + OH− → ·OH (11)
h+ + H2O → ·OH + H+ (12)
e- + O2 → ·O2− (13)
·O2− + H+ → H2O → O2 + OH− + ·OH (14)
Organic pollutant + ·OH + O2 → CO2 + H (15)
4. Conclusion
In summary, we have successfully synthesized the WO3 nanostrucutres by a simple one-pot synthesis incorporating the two
completely different shape and size directing agents i.e. TTS and C14TAB, resulting in enormous changes in the shape and size of the
prepared nanoparticles. The detailed structural analysis was supported by XRD, EDS, and elemental mapping. The optical
measurements indicated that the prepared nanoparticles are favorable for the photocatalytic decomposition of the organic pollutant.
The effect of size on the degradation of the organic pollutant was also observed and discussed in detail. It was observed that the
nanoparticles prepared with TTS were smaller in size and shows greater photocatalytic efficiency of 94% for the FCF dye. As a
consequence, the obtained nanoparticles, by controlling the shape and size by varying the parameters could be a potential candidate
for solar light-based wastewater treatment.
Declarations
Author contribution: Deepika Jamwal: investigation, data curation, analysis, original draft writing, visualization, conceptualization;
Vishal Mutreja and Rahul :formal analysis and visualization; Surinder Kumar Mehta: conceptualization, review and editing; Akash
Katoch: review and editing, conceptualization, visualization. Sang Sub Kim: visualization, review and editing.
Funding: Deepika Jamwal acknowledges financial support from a National Post-Doctoral Fellowship (PDF/2017/001869), from the
DST, SERB, India, Chandigarh. Akash Katoch acknowledges financial support from UGC- Startup Grant. Authors also acknowledge the
research and laboratory facility support from Panjab University, Chandigarh.
Data availability: All data generated or analyzed during this study are included in this article.
Ethical approval and consent to participate: Not applicable
Consent for publication: All authors read and approved the final manuscript. All authors are fully aware of this manuscript and have
permission to submit the manuscript for possible publication
Conflicts of interest: The authors declare no conflicts of interest.
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Scheme
Scheme 1 is available in Supplementary Files section.
Figures
Page 12/18Figure 1
(a) HR-TEM image and (b) high magnification HR-TEM image WO3 web of chains, (c) HR-TEM image and (d) high magnification HR-
TEM image of WO3 cube shaped nanoparticles, (e) and (f) SAED pattern, (g) and (h) EDS, (i) and (j) mapping (W, O, and C) of WO3
web of chains and WO3 cube shaped nanoparticles.
Page 13/18Figure 2
XRD patterns of (a) C14TAB (b) TTS-derived WO3 nanostructures.
Page 14/18Figure 3
(a) UV-Visible absorption spectra and (b) Tauc plots, for WO3 web of chains and cube shaped nanoparticles.
Page 15/18Figure 4
FTIR spectra of (a) (i) pure C14TAB and (ii) WO3 web of chains, (b) (i) pure TTS, and (ii) WO3 cube shaped nanoparticles.
Page 16/18Figure 5
(a) and (b) UV-Visible absorption spectra of FCF dye solution under visible irradiation, (c) and (d) dye degradation percentage with
time in the presence of WO3 web of chains and cube shaped nanoparticles, respectively.
Page 17/18Figure 6
(a) and (d) Pseudo first-order kinetic model, (b) and (e) Pseudo second-order kinetic model, and, (c) and (f) for photodegradation of
FCF dye in the presence of WO3 web of chains and WO3 cube shaped nanoparticles.
Supplementary Files
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SupplymentaryInformation.docx
Scheme1.png
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