Visible light photocatalytic degradation of crystal violet dye and electrochemical detection of ascorbic acid & glucose using BaWO4 nanorods

Article abstract of DOI:10.1016/j.materresbull.2018.03.049

Barium tungstate (BaWO?) rod like nanostructures were synthesized by slightly modified hydrothermal technique using sodium tungstate dihydrate (Na₂WO?. 2H₂O) as tungstate ion source and barium nitrate [Ba(NO?)?] as barium ion source.

Full text of DOI:10.1016/j.materresbull.2018.03.049

Accepted Manuscript
Title: Visible light photocatalytic degradation of crystal violet
dye and electrochemical detection of ascorbic acid and
glucose using BaWO4 nanorods
Authors: M. Younis Ali Khan, Mehvish Zahoor, Ayesha
Shaheen, Nuzhat Jamil, M. Imran Arshad, Sadia Zafar Bajwa,
Naveed Akhtar Shad, Rehman Butt, Israat Ali, M. Zubair
Iqbal, Aiguo Wu, Ghulam Nabi, Sajad Hussain, Tariq
Mahmood, Imran Aslam, Waheed S. Khan
PII:
S0025-5408(17)34758-X
DOI:
Reference:
https://doi.org/10.1016/j.materresbull.2018.03.049
MRB 9930
To appear in:
MRB
Received date:
Revised date:
Accepted date:
28-12-2017
26-3-2018
26-3-2018
Please cite this article as: Khan MYA, Zahoor M, Shaheen A, Jamil N,
Arshad MI, Bajwa SZ, Shad NA, Butt R, Ali I, Iqbal MZ, Wu A, Nabi
G, Hussain S, Mahmood T, Aslam I, Khan WS, Visible light photocatalytic
degradation of crystal violet dye and electrochemical detection of ascorbic
acid and glucose using BaWO4 nanorods, Materials Research Bulletin (2010),
https://doi.org/10.1016/j.materresbull.2018.03.049
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Visible light photocatalytic degradation of crystal violet dye and
electrochemical detection of ascorbic acid & glucose using BaWO4 nanorods
M. Younis Ali Khana,b, Mehvish Zahoora, Ayesha Shaheena, Nuzhat Jamila, M. Imran Arshadb,
Sadia Zafar Bajwaa**, Naveed Akhtar Shada , Rehman Buttc , Israat Alic , M. Zubair Iqbalc, Aiguo
Wuc, Ghulam Nabid, Sajad Hussaine, Tariq Mahmoodf, Imran Aslamg, Waheed S. Khana,c*
aNanobiotechnology Research Group, National Institute for Biotechnology and Genetic
Engineering (NIBGE), Jhang Road, Faisalabad-38000, PAKISTAN.
bDepartment of Physics, Government College University Faisalabad, Jail Road, Faisalabad,
PAKISTAN.
cNingbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of
Sciences (CAS), Ningbo City, Zhejiang 315201, P. R. CHINA.
dDepartment of Physics, University of Gujrat, Hafiz Hayat Campus Jalalpur Road Gujrat,
PAKISTAN
eUniversity of Education, Faisalabad Campus, Samnabad, Faisalabad-38000- PAKISTAN.
fGovernment College Women University Sialkot , Kutchehry Road Sialkot, PAKISTAN.
gDepartment of Basic Sciences & Humanities, University of Engineering and Technology (UET)
Lahore, Narowal Campus, Narowal, PAKISTAN.
*Corresponding Author: waheedskhan@yahoo.com , waheedskhan@nibge.org, (Waheed S.
Khan)
Phone: +92 41 9201404, Fax: +92 41 9201322
** sadya2002pk@yahoo.co.uk (Sadia Zafar Bajwa)

Scheme: Graphical Abstract
Research Highlights
Barium Tungstate (BaWO4) nanorods were prepared by modified
hydrothermal technique using Na2WO₄. 2H2O as tungstate ion source and
Ba(NO₃)₂ as barium ion source.


BaWO4 nanorods exhibited excellent visible light driven photocatalytic activity
against the complete degradation of crystal violet dye within one hour at pH
of 12.


Cyclic voltammetry method was adapted to assess the potential of as-obtained
nanostructures for the electrochemical detection of important molecules such
as glucose and ascorbic acid.
ABSTRACT:
Barium tungstate (BaWO₄) rod like nanostructures were synthesized by slightly modified
hydrothermal technique using sodium tungstate dihydrate (Na2WO₄. 2H2O) as tungstate ion source
and barium nitrate [Ba(NO₃)₂] as barium ion source. The prepared nanostructures were
characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy
Dispersive X-ray Spectroscopy (EDX) and Fourier Transform Infrared (FTIR) Spectroscopy.
BaWO₄ nanorods were explored for their photocatalytic activity through decomposition of crystal
violet (CV) dye under visible light under different pH conditions (acidic to basic) from 2 to 12. It
was observed that BaWO₄ completely degrades CV within 60 min at highest pH of 12. BaWO₄
nanorods were also investigated for electrochemical detection of important analytes such as
glucose and ascorbic acid using cyclic voltammetric technique. These studies exhibit the potential
of BaWO₄ nanostructures for applications in environmental sciences as well as biosensor areas.
Keywords: BaWO4 Nanorods; Photocatalytic Degradation; Crystal Violet Dye; Electrochemical
Detection; Ascorbic Acid

Introduction
Owing to unique chemical and physical features of nanomaterials strongly dependent on shape and
size, synthesis techniques with immense control on morphology and size have attracted much
focus from researchers community. Considerable efforts have been dedicated on nanoscale
materials with diversified morphologies for achieving the desired properties and targeted
applications. [1]. Metal tungstates are an important class of materials which find immense
applications in various areas. Compared with bulk materials, nanosized structures of these
materials exhibit more prominent features owing to the increased surface area and quantum
confinement effects. [2]. For example zinc tungstate (ZnWO4) has received considerable attention
due to its applications as photoanodes, solid-state laser host, X-ray and γ-scintillator,
photocatalysts, acoustic and optical fibers, magnetic and luminescent materials, gas sensing and
ceramics, etc [3-6]. Bismuth tungstate (Bi2WO6) being an n-type direct band gap semiconductor
with a band gap of 2.75 eV has shown large potential in electrode materials, solar energy
conversion and catalysis applications [7-9]. Cadmium tungstate (CdWO4) is also an important
member of metal tungstate family with wide-band gap value and finds much interest from
researchers because of its unique physical, chemical and structural properties as well as
multifarious applications including X-ray scintillator, advanced medical X-ray detector in CT
Scan, photocatalysis, phosphors, laser crystals [10]. Similarly manganese tungstate (MnWO4) is
considered as the most promising tungstate material which gains much interest for potential
applications such as humidity sensors, scintillator materials, photocatalysts, optical fibers, and
photoluminescence. These exciting applications are expected to stem from its electrical
conductivity features dependent on the shape of the products [11]. Lead tungstate (PbWO4) based
materials with a tetragonal scheelite-type structure are of much technological worth owing to their
stimulated Raman scattering (SRS), thermoluminescence and excitonic luminescence features
[12-13]. Among tetragonal scheelite-type metal tungstate materials, BaWO4 is a significant
material due to its excellent PL emission and stimulated Raman scattering active crystal [14-16].
Other applications of this useful material are nuclear spin optical hole burning hosts [17], radiation
detection [18], scintillating devices [19], and other electro-optic applications [18]. Environmental
pollution is global issue which requires due attention for healthy life. Industrial effluents are main
source of pollution which lead to contaminated water and crops particularly in backward societies.
Conventional methodologies for the treatment of effluents are costly, time taking, less-efficient.

Comparatively photocatalytic degradation of toxic materials contained in the effluents is a cheaper,
efficient and cost-effective technique. Crystal violet, rhodamine, methylene blue, malachite green
are common examples of dyes usually released from industries in the form of waste. Similarly
detection of different types of harmful materials in samples is also highly desired before taking
action for their decomposition through photocatalysis. In this regard electrochemical
determination of materials, bacterias, viruses, toxic molecules using nanomaterials as sensing
platform is quite effective technique.
To the best of our knowledge, there are no reports in literature regarding the use of BaWO4
nanostructured material for photocatalytic degradation of crystal violet dye as well as
electrochemical detection of molecules. In this research work, barium tungstate (BaWO₄) rod-like
nanostructures were obtained by facile modified hydrothermal technique. Following their
characterized by XRD, SEM, EDX and FTIR Spectroscopy, BaWO₄ nanorods were investigated
for their performance as nanophotocatalyst through photocatalytic degradation of crystal violet
(CV) dye under the simulated solar irradiation. Material was also investigated for electrochemical
detection of important analytes such as glucose and ascorbic acid. These studies exhibit the
potential of BaWO₄ nanostructures for applications in environmental pollutant degradation as well
as biosensor areas.
Experimental
Synthesis of BaWO4 Nanorods
BaWO4 nanomaterials were successfully prepared from hydrothermal method. Barium nitrate (Ba
(NO3)2) and sodium tungstate dihydrate (Na2WO4.2H2O) were purchased from Alfa Aesar of
analytical reagent grade and used without purification. Deionized water (D.I.W) was used as
solvent to prepare all required solutions. D.I.W and ethanol were used to wash the prepared
samples. Barium Nitrate (Ba(NO3)2) and Sodium Tungstate dihydrate (Na2WO4.2H2O) were used
as major precursors to prepare barium tungstate nanomaterials.

Formation of Solution-A: 2g of Ba(NO3)2 was added into 6ml mixed solution of acetic acid
(CH3COOH) into 14ml D.I.W. Mixture was continuously stirred for 5 minutes using a magnetic
stirrer at room temperature.
Formation of Solution-B: 0.44g of Na2WO4.2H2O was mixed in 20 ml D.I.W and mixture was
continuously stirred for 5 minutes in a magnetic stirrer at room temperature.
Mixing of Solutions-A &B: After the formation of solution A & B, the solution-A was mixed drop
wise into solution-B under gentle stirring at room temperature. After sometime, white precipitate
was formed by stirring the mixed solution for 40 minutes. pH of solution was adjusted to 10
through digital pH meter by adding some drops of 2.5 M sodium hydro-oxide (NaOH)
concentrated solution. The resulting solution was then transferred to the Teflon liner autoclave of
200ml capacity. This Teflon lined autoclave was kept into microwave heating oven at 180oC for
24 hours. The obtained yellow precipitates were washed with D.I.W (40%) and ethanol (60%).
The products were washed in a centrifuge at 14000 revolutions for several (10, 7, 5) minutes to
remove any kind of impurities. The process was repeated for six (06) times for each sample.
Samples were dried in a vacuum oven at 60oC for whole night in order to remove water and ethanol
from the samples. Whole step wise synthesis process has been shown in flowchart in Fig.1 whereas
overall reaction for the synthesis can be represented as follows by equation (1).
Ba(NO3)2 + Na2WO4.2H2O
BaWO4 + 2NaNO3 +2H2O
(1)
Crystal structure and phase purity of BaWO4 product was carried out by X-ray Diffraction
(XRD) in a range of 20–60ᵒ (2θ) using PANalytical- X’ Pert PRO, Japan at room temperature
using Cu-Kα radiation (λ= 1.54059 Å). The size, surface features and morphology were observed
by using field emission scanning electron microscope (FESEM (JEOL-7500) while chemical
composition of the product was performed by Energy-dispersive X-ray spectroscopy (EDX)
attached with FESEM. FTIR spectra were recorded by a Bruker Alpha spectrophotometer using
using potassium bromide (KBr) pellets at in the spectral range of 400-4000 cm-1.
Photocatalytic Experiment
Photocatalytic performance of as-synthesized BaWO4 nanomaterial product was explored
by its de-colorization effect on crystal violet dye dissolved in water. For this purpose, all

experiments were conducted in a homemade wooden box used as reactor under the simulated solar
irradiation (500 Watt mercury bulb) with dye solution contained in a glass beaker. The reaction
was carried out by adding 50 mg catalyst powder into 200 mL glass beaker containing 100 mL
of dye solution. The suspension was magnetically stirred in dark for 30 minutes to obtain
desorption/absorption equilibrium before irradiating the solution to the light. The solution was
irradiated under mercury lamps and aliquots were drawn at regular time intervals of 15 minutes,
centrifuged and dye solution was analyzed for absorbance using visible spectrophotometer.
Preparation of working electrode for electrochemical sensing measurements
A glassy carbon electrode (GCEs) used in this study was polished by 1.0 µm of alumina slurries
for 1 min followed by rinsing thoroughly with deionized water and ethanol. After washing, the
electrode was cycled 10 times in supporting electrolyte between−0.2 and 1 V at 0.1 Vs−1. For
electrode modification, 0.5 mg of as-synthesized BaWO4 nanorods were dispersed in 1 mL
deionized water. 10 µL of dispersed nanomaterial was placed on GC electrodes and dried in oven
at 90 °C for 1 min, then 2–3µLof 5% nafion was dropped and dried at room temperature. All
solutions for sensor measurements were prepared in de-ionized water and the required
concentrations were obtained by the appropriate dilutions of the respective stock solutions.
Sensor measurements
The electrochemical measurements were carried out using Autolab Potentiostate/Galvanostate
(PGSTAT) and a three-electrode system; a glassy carbon electrode modified with BaWO4
nanomaterials as the working electrode, a glassy carbon rod as the counter electrode and Ag/AgCl
electrode as the reference electrode. All the experiments were conducted by using a sodium
phosphate monohydrate solution (25 mL; 50 mM) as the background electrolyte. Prior to
electrochemical measurements each time, electrolyte solution was degassed with nitrogen gas for
5 min. All CV experiments were carried out at a scan rate of 50 mVs −1 (except for observing the
effect of scan rate) and potential range−0.2–1.0V.

Figure. 1. Flowchart for the depiction of BaWO4 nanorods synthesis.
Results and discussion
Figure 2a-b shows the representative micrographs of BaWO4 nanostructures obtained from
scanning electron microscope (SEM) at different magnification. SEM micrographs of as

synthesizedsample revealed the homogeneous rod-like structural features. SEM micrograph at low
magnification confirms the formation of nanorods in excessive amount. Small aggregations of
nanorods are observed at higher resolution that might because of inter-particle interaction during
the hydrothermal process. Close look at nanorods reveals that they have rough outer surface
possibly due to extremely small pores generated during growth process.
Phase composition and crystal structure of BaWO4 was investigated using X-ray diffraction
technique. Figure 2c shows powder XRD pattern of the synthesized BaWO4 nanostructures. The
diffraction peaks present in the pattern correspond to (112), (004), (200), (211), (204), (220), (116),
(132), (224) and (008) planes of BaWO4 matched with JCPDS card number 72-0746. XRD pattern
confirm the formation of pure single-phase barium tungstate (BaWO4) with indexing belong to
tetragonal phase have a= 5.6140 Å and b= 12.7190 Å.

Figure 2: SEM images of BaWO4 nanorods (a) at low magnification and (b) high magnification;
(c) XRD spectrum; and (d) EDX Pattern of BaWO4 nanorods.
Figure 3a shows FTIR spectrum of the synthesized BaWO4 nanorods. Spectrum indicates the
presence of two very close strong bands around 792 cm-1 and 814 cm-1 which are attributed to W-
O vibrations of [WO4]2- group [20]. Relatively weaker and broad bands observed near 1552 cm-1,
1411 cm-1 and 1316 cm-1 are owing to adsorbed water molecules on the sample surface. Broadness
is attributed to characteristic nanoscale features of the nanorods [21].

Figure 3. (a) FTIR Spectrum and (b) Tauc Plot for energy band gap calculation of BaWO4.
Energy band gap measurement of materials is an important parameter of semiconductor
nanomaterials for studying their properties and exploring their applications. UV- visible spectra of
synthesized nanostructured BaWO4 was taken and optical band gap of the material was calculated
using Tauc relation (2) given below:
(αhν) =A(hν-Eg)n
(2)
Where‘α’is the absorption coefficient, ‘A’ is a constant and n= 1 / 2 for direct band gap material.
The energy band gap (Eg) of the BaWO4 nanorods was estimated by extrapolating the linear portion
of the plot of (αhν)2 against hν to the energy axis as shown in Fig.3(b). Eg value came out to be 3.7
eV for the BaWO4 which is in close match with the reported values [22, 23].
Effect of CV dye solution pH on Photodegradation
The photocatalytic activity of BaWO4 nanorods was examined by degradation of crystal violet
(CV) as model dye with chemical formula as C25H30ClN3 and chemical structure shown in Fig.S1
(ESI†). In general, the pH of the dye solution is an important factor which greatly affects the
degradation of the organic compounds or pollutants in the photocatalytic process and it is equally
influential parameter while actual wastewater treatment [24]. The variation in solution pH can
change the catalytic reaction owing to the surface charge of nanoparticles used as photocatalyst
and it can easily influence the adsorption/desorption rate of the dye which in turn affect the photo-
catalytic capability of the proposed material [25]. Therefore, degradation of nanostructures is

evaluated at different pH conditions. The pH of the dye solutions was adjusted prior to the
illumination using 1 M HCL and NaOH solutions respectively. Figure 4(a-g) represents the series
of spectra recorded at different pH values of 2, 3, 5, 7, 9, 10, 12 during the photocatalytic
degradation of CV dye over barium tungstate nanostructure materials.
Figure 4: UV-vis absorption spectra of crystal violet dye for different irradiation times showing
photocatalytic degradation of crystal violet dye using BaWO4 nanorods at pH (a) 2, (b) 3 (c) 5, (d)
7, (e) 9, (f) 10 (g) 12; (h) Concentration changes (C/Co) of CVdye as a function of irradiation time
(t) in the presence of photocatalyst at different pH; (i) CV degradation profiles over BaWO4
nanorods as a function of irradiation time at different pH values.
All the experiments on photodegradation were carried out at various initial pH values by taking
homogeneous CV dye concentration (10mg/500ml) and catalyst loading (50mg). At pH 2 as
indicated in Fig.4(a), the spectrum of blue colored CV dye solution exhibits absorption peak at
578 nm at t=0 which decreases gradually after irradiation under visible light subject to the effect

of BaWO4 photocatalyst. It shows full degradation after almost 140 min. The degradation
efficiency was calculated using relation (3) and found to be 57.5 %. CV dye solution successfully
degraded at pH 3 as shown in Fig 4(b). Its absorbance decreases with the passage of time and
degradation efficiency is found to be 65.9 %. Fig 4(c) depicts that at pH 5, degradation is
successfully done after 100 minutes. Similarly degradation efficiencies at higher values of pH were
determined and are shown in Table.1
Table 1. Effect of pH of CV dye solution on degradation using BaWO4 nanorods.
pH
2
Degradation Efficiency (%)
57.5
65.9
68.93
76.3
98.7
99.2
99.7
3
5
7
9
10
12
The estimated degradation efficiency is determined according to the following equation;
Degradation Efficiency (%) =(Co-Ct)/Co
(3)
Where;
Co = Initial concentration of dye at time t=0
Ct = Concentration of dye at any time t
Figure 4(h,i) indicate the decolorization efficiency of dye at different pH, with same catalyst
concentration. The reason behind the decrease in concentration of CV is the adsorption of CV
molecules that takes place at the surface of a photocatalyst. On the other hand, with the increasing
pH values, decolorization efficiency of CV molecules also increases with the simulated solar

irradiation. It can be observed from the above figure that material shows maximum decolorization
efficiency at pH=12 which is approximately equal to 99.7% and was found to be lowest at pH= 2
(57.5%). Alkaline pH range promotes more hydroxyl radicals (OH), due to the formation of
hydroxyl ions and the formation of photogenerated holes, which significantly increases the
photocatalytic efficiency of photocatalyst. Degradation of crystal violet dye solution carried out
using BaWO4 nanorods has been compared with other reported materials as shown in Table 2 and
it shows that better performance of our prepared material in nanorods forms.
Table 2. Comparison of Crystal Violet (CV) dye degradation using BaWO4 nanorods with other
materials
Dye
Catalyst
Efficiency Irradiation
source
Time
Refrence
Crystal
Violet
Crystal
Violet
Crystal
Violet
Crystal
Violet
Crystal
Violet
Anatase Nanosphere 99%
UV Light
6 hours
[26]
TiO2
Mn Doped and PVP < 100%
Capped ZnO NPs
UV irradiation 3 hours
[27]
[28]
TG capped ZnS NPs ∼96%
UV–Visible
irradiation.
UV Light
3 hours
90min
60 min
Ag Doped TiO2
>99%
[29]
BaWO4
∼98%
Visible light
present
Nanorods
Possible Mechanism for Photocatalytic Degradation
The photocatalytic mechanism for heterogeneous degradation of crystal violet dye dissolved in
aqueous form subjected to BaWO4 nanorods used as catalyst under visible light shown in Fig.5
can be represented by the following reactions:
BaWO4 + hν
e- + h+ + BaWO4
(4)

Oxidation reaction occurring at BaWO4 surface
h+ + H2O
.OH + H+
(5)
(6)
2h+ + 2H2O
2H+ + H2O2
and reduction reaction
-
e- + O2
.O2
(7)
When visible light falls upon aqueous solution containing photocatalyst BaWO4 and crystal violet
dye, the electron are excited by light energy (hν) and move from valence band (VB) to conduction
band (CB) leading to electron-hole pairs (h+, e-) generation. Oxidation reaction occurs at VB where
positive hole (h+) react with adsorbed water molecule (H2O) and hydroxyl radical (.OH) are
formed. While reduction reaction occurs at CB where negative electron (e-) causes the generation
of superoxide radical (.O2) by reaction with dissolved oxygen.

Figure 5. Reaction mechanism of crystal violet photodegradation over BaWO4 nanorods under the
simulated solar irradiation.
At the end, these highly reactive species such as superoxide and hydroxyl radicals oxidize the
crystal violet dye molecules (C25H30ClN3) present on the active sites of the barium tungstate
nanorods. Final products may include CO2. H2O, NO3 and other non- or less-toxic simple
molecules [14].
-
C25H30ClN3 + (.OH, .O2 )
CO2 + H2O + NO3 + Other simple fragments
(8)
Increased photocatalytic activity of the as-obtained BaWO4 nanorods can be attributed to large
adsorption of dye molecules on the rough surface of nanorods, less recombination of electron-hole
pairs, high production of reactive radicals, and high separation rate of charge carriers.
Electrochemical Detection of Glucose and Ascorbic Acid
Cyclic voltammetry was used for measurement and detection of different analytes such as glucose
and ascorbic acid for nanorods like nanostructured barium tungstate (BaWO4) material. Cyclic
voltammetry showed response of bare electrode and glassy carbon modified electrode both in
presence and absence of 0.5mM ascorbic acid and glucose at a scan rate of -1 to 1V/s as shown in
Figure 6 (a,b). Bare electrode and modified glassy carbon electrode does not show oxidation peak
both in the absence and presence of glucose molecule as well as ascorbic acid. However, enhanced
current was observed which shows that semiconductor material has potential to change the current
intensity.

Figure 6. Electrochemical analysis of BaWO4 nanorods.
Conclusion
Barium tungstate (BaWO₄) rod like nanostructures were synthesized by adopting slightly modified
hydrothermal technique using sodium tungstate dihydrate (Na2WO₄. 2H2O) as tungstate ion source
and barium nitrate [Ba(NO₃)₂] as barium ion source. The prepared nanostructures were
characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy
Dispersive X-ray Spectroscopy (EDX) and Fourier Transform Infrared (FTIR) Spectroscopy.
BaWO₄ nanorods were investigated for their performance as nanophotocatalyst through
photocatalytic degradation of crystal violet (CV) dye under the simulated solar irradiation.
Material was also investigated for electrochemical detection of important analytes such as glucose
and ascorbic acid. These studies exhibit the potential of BaWO₄ nanostructures for applications in
environmental sciences as well as biosensor areas.
Acknowledgement
This research is funded by Higher Education Commission (HEC) of Pakistan by research Grant
No. NRPU6115. Dr. Waheed S. Khan also acknowledges the support of Chinese Academy of
Sciences under CAS-PIFI Fellowship at Ningbo Institute of Materials Technology and
Engineering (NIMTE), Ningbo City, Zhejiang, P.R. China.

Conflict of Interest
There are no conflicts of interest to declare.
†Electronic Supplementary Information (ESI)
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