Hollow micro-spherical bismuth oxy-chloride for superior visible light induced dye-sensitized photocatalytic activity and its theoretical insight

Article abstract of DOI:10.1016/j.materresbull.2020.110778

Highly-crystalline BiOCl samples exhibiting hollow microsphere like morphology with exposed (110) crystal facets have been synthesized in the present work. The as-synthesized BiOCl samples were characterized in detail by several sophisticated techniques for proper understanding and subsequent analysis of the phase formation, nature of the chemical bonds existing within the samples, composition, morphology and different properties. Variations in synthesis conditions by varying the synthesis temperature and time duration were performed to compare the changes in phase formation, morphology and catalytic activity of the material. The samples showed effective degradation of toxic Rhodamine B dye under both visible light and UV light irradiation. A detailed mechanism of the photocatalytic activity has been proposed by extensive first principles calculations. The hollow microsphere like morphology along with exposed (110) crystal facets proved beneficial for the superior catalytic activity mediated by dye sensitization under visible light.

Full text of DOI:10.1016/j.materresbull.2020.110778

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Hollow Micro-spherical Bismuth Oxy-chloride for Superior Visible Light
Induced Dye-sensitized Photocatalytic Activity and its Theoretical Insight
Ratna Sarkar, Dimitra Das, Bikram Kumar Das, Anuradha Mitra,
Nirmalya S. Das, Subrata Sarkar, Kalyan K. Chattopadhyay
PII:
S0025-5408(19)32438-9
DOI:
https://doi.org/10.1016/j.materresbull.2020.110778
MRB 110778
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To appear in:
Materials Research Bulletin
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Revised Date:
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19 September 2019
10 January 2020
10 January 2020
Please cite this article as: Sarkar R, Das D, Kumar Das B, Mitra A, Das NS, Sarkar S,
Chattopadhyay KK, Hollow Micro-spherical Bismuth Oxy-chloride for Superior Visible Light
Induced Dye-sensitized Photocatalytic Activity and its Theoretical Insight, Materials Research
Bulletin (2020), doi: https://doi.org/10.1016/j.materresbull.2020.110778
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Hollow Micro-spherical Bismuth Oxy-chloride for Superior Visible Light Induced Dye-
sensitized Photocatalytic Activity and its Theoretical Insight
Ratna Sarkara, Dimitra Dasb, Bikram Kumar Dasa, Anuradha Mitraa, Nirmalya S. Dasb, Subrata
Sarkara and Kalyan K. Chattopadhyaya,b*
*Corresponding Author E-mail: kkc.juphy@gmail.com
aDepartment of Physics, Jadavpur University, Kolkata 700032, India
bSchool of Materials Science and Nanotechnology, Jadavpur University, Kolkata 700032, India
Graphical Abstract
Highlights
 BiOCl hollow micro-spherical morphology was synthesized with exposed (110) facets
 Complete degradation of RhB dye in presence of visible light within 6 minutes
 The photocatalytic mechanism was supported by first principles calculations
 The superior catalytic activity was mediated by dye sensitization under visible light
irradiation.
ABSTRACT
Highly-crystalline BiOCl samples exhibiting hollow microsphere like morphology with
exposed (110) crystal facets have been synthesized in the present work. The as-synthesized
BiOCl samples were characterized in detail by several sophisticated techniques for proper
understanding and subsequent analysis of the phase formation, nature of the chemical bonds
1

existing within the samples, composition, morphology and different properties. Variations in
synthesis conditions by varying the synthesis temperature and time duration were performed
to compare the changes in phase formation, morphology and catalytic activity of the material.
The samples showed effective degradation of toxic Rhodamine B dye under both visible light
and UV light irradiation. A detailed mechanism of the photocatalytic activity has been
proposed by extensive first principles calculations. The hollow microsphere like morphology
along with exposed (110) crystal facets proved beneficial for the superior catalytic activity
mediated by dye sensitization under visible light.
Keywords
Bismuth Oxychloride, (110) crystal facet, hollow microsphere, dye sensitization, DFT
calculation
1. Introduction
Water purification is of utmost importance in modern times owing to the increased discharge
of effluents from textile and chemical industries into the water bodies thereby leading to
severe detrimental effects on aquatic ecosystem. Around one thousand tons of toxic non-
degradable chemical dyes are ejected into the water bodies each year primarily from textile
industries. According to the reports by Konstantinou and Albanis [1], dyes and effluents
emitted from textile industries forms the primary groups of toxic organic compounds. Water
soluble reactive coloured dyes like Rhodamine B and Methyl Orange are the most
troublesome dyes to be removed. Rhodamine B dye comprises of nitrogen groups which
renders its photolysis-resistant nature and subsequent anaerobic degradation results in
carcinogenic and mutagenic products [2]. A study by Leena and Raj [3] revealed that textile
dyes possess high stability as compared to other effluents and has the capability to remain
unaltered under varying environmental conditions. Moreover those dyes which are properly
dispersed in aquatic medium do not possess ionization tendency and can bio-accumulate in
aquatic organisms [4]. Hence it is of immense importance to design economically viable
photocatalytic materials to effectively degrade toxic dyes in order to achieve a step towards
conserving environmental resources.
Significant progress has been achieved in recent time to effectively design semiconductor-
based photocatalysts for efficient degradation of toxic dyes to non-toxic degradable products
by employing UV or visible irradiation sources. Anatase titanium oxide (TiO2) has been the
most sought after material in this regard because of its impressive photochemical reactivity
and relatively insignificant environmental toxicity [5]. However several limitations of TiO2
2

like poor utilization of the visible spectrum and inferior adsorption capability of hydrophobic
pollutants hinder its application feasibility [6]. Till then there has been a steady and constant
urge to develop some new photocatalytic materials which could overcome the conventional
drawbacks and prove to be highly efficient in degradation of toxic pollutants. Photocatalyst
system can be broadly classified into oxides-based [7], sulphides-based [8], oxysulfides-
based [9], nitrides-based [10] and oxynitrides-based materials [11,12].
In recent times Bismuth Oxyhalides (BiOX, X= Cl, Br, I and F) has been extensively
investigated and employed in photocatalytic activities owing to their layered crystal structure
[13] and appropriate band gap. This class of wide band gap (3.2-3.5 eV), V–VI–VII ternary
semiconductor materials possessing distinctive opto-electronic features, impressive chemical
stability and non-toxicity contributes its applicability in numerous fields. There are several
reports on different synthetic strategies of BiOX samples like reverse micro-emulsions, wet
chemical methods, low temperature chemical vapour deposition, sonochemical process etc.
[14]. Zhang et al. developed a one-pot solvothermal method utilizing ethylene glycol solvent
to synthesize BiOX nanoplate microspheres [15] whereas Michel et al. carried out a non-
aqueous surfactant-assisted synthetic technique to develop BiOCl material [14]. Sopha et al.
prepared BiOCl having nano-platelet like morphology by breakdown anodization technique
[16]. Applications of BiOX materials in multiple areas have been demonstrated in the
following reports: Li et al. designed dye-sensitized BiOCl catalyst for efficient air and water
purification [17]; Jiang et al. prepared BiOBr flakes for visible light degradation of methyl
orange dye [18]; Wang et al. synthesized plate-like BiOI for visible light photocatalysis [19].
Among all the reported BiOX photocatalysts, BiOCl has proved to be the most efficient in
degrading pollutants mainly because of its unique layered structure which provides sufficient
space to achieve polarization of the corresponding atoms and orbitals which results in the
effective separation of photogenerated electron and hole pairs thereby enhancing the
photocatalytic property [12]. The theoretical band gap of BiOCl makes it suitable for dye
degradation under UV light irradiation. Recently many modifications of pure BiOCl have
been performed to employ the material for visible-light assisted dye degradation in order to
establish its practical viability. Li et al. developed (001) dominant crystal facets of BiOCl for
Biphenol A degradation [20], Pan et al. reported CdS quantum dots coupled BiOCl
nanosheets [21], Pare et al. developed Mn-doped BiOCl [22], Shamaila et al. designed
WO3/BiOCl heterojunction [23], Chai et al. synthesized BiOCl/Bi2O3 based heterojunction
[24], Liu et al. developed BiOCl1-xBrx (x=0–1) [25], Xu et al. synthesized La-doped BiOCl
[26] and Yang et al. reported the synthesis of a composite based on Fe3O4/Pr-BiOCl/Luffa
3

[27] to improve the photocatalytic activity of BiOCl under visible, UV and solar light
irradiation. Hu et al. fabricated TiO2/BiOCl composite for photocatalytic degradation of
Cr(VI) and tetracycline [28]. Moreover, there are a number of reports regarding the structural
modifications of BiOCl for performance enhancement where various groups have synthesized
different morphologies like single-crystal nanodisks [29], ultrathin BiOCl nanosheets [30],
micro-spheres [31] and flower like structures [32].
The crystal structure of BiOCl is tetragonal bipyramidal. This tetragonal matlockite [33]
structure (PbFCl) forms the basic backbone of BiOCl which subsequently undergoes
crystallization to give rise to a layered structure comprising of [Bi2O2]2+ slabs. Non-bonding
weak van der Waals interactions aid in the combined stacking of these slabs mediated by
halogen (Cl, Br, I and F) atoms [34]. The central Bismuth atom in each of the [X–Bi–O–Bi–
X] pattern is encompassed by adjacent four oxygen atoms and four halogen atoms which
essentially results in asymmetric decahedral symmetry [35]. The significant intralayer
covalent bonding along with the associated interlayer van der Waals bonding renders the
compelling anisotropic physical, electrical and optical properties of BiOX compounds [36].
Moreover, BiOCl has unsaturated dangling bonds only at the edges rather than at the surfaces
as implied from the bonding anisotropy. The significantly impressive photo-stability of
BiOCl arises from the fact that the photo-excited antibonding states cannot weaken the O-Bi-
Cl bond [35]. The electronic state configuration of BiOX compounds reveal that the valence
band (VB) is mainly composed of oxygen 2p and Bi 6s hybrid orbitals which are in contrast
to metal oxides where only the oxygen 2p orbitals form the VB. The conduction band (CB) is
comprised of Bi 6p orbitals [37]. The significantly wide VB as resulted from the largely
diffused 6s orbitals of Bismuth attributes to the superior mobility of photogenerated charge
carriers [38].
In the present work, pure BiOCl with hollow microsphere like morphology and (110)
dominant exposed crystal facet has been synthesized following a facile chemical synthesis
route. Variation in synthesis time and temperature duration has been performed to study the
effect of synthetic conditions on phase formation and morphology of the samples. The
samples exhibited superior photocatalytic activity under both visible and UV light irradiation.
The best sample could completely degrade Rhodamine B (RhB) dye in a very short time.
Different variations in photocatalytic parameters (like pH, dosage variations and source of
irradiations) have been performed to study their respective effects on catalytic activity. A
detailed mechanism supported by the plausible occurrence of dye sensitization under visible
light has been proposed to understand the impressive photocatalytic activity of pure BiOCl.
4

Extensive first principles calculations were further employed to support the reaction
mechanism. Till date, very few of the existing literatures on BiOCl photocatalysts provide an
extensive photocatalytic study with variation of different parameters along with support from
DFT calculations. The present report is an extensive work in this regard and the BiOCl
sample developed in this work proved to be a promising visible-light assisted photocatalytic
material in degrading toxic textile dyes of different types.
2. EXPERIMENTAL DETAILS
Scheme 1. Synthesis procedure of BiOCl samples.
2.1. Materials
Bi(NO3)3•5H2O (Bismuth Nitrate Pentahydrate), 37 % HCl (Hydrochloric Acid),
C6H8O7•H2O (Citric Acid Monohydrate) were purchased from Merck. PVP (K30) (Poly
Vinyl Pyrrolidone) was obtained from Loba Chemie. Deionized Water (18.3 MΩ) was used
for all synthesis purposes.
2.2. Preparation of BiOCl powders
All the reagents were of analytical grade and were used without further purification. The
samples were synthesized following previous report by Zhang et al. [39] with some
modifications. The precursor, the molar concentration of the reagents and the synthesis time
period was modified to certain extent in the present work. Initially 3.75 mmol of
Bi(NO3)3•5H2O was dispersed in 22.5 mL of DI water followed by subsequent addition of 2.5
mL of HCl (37 %) into the above solution. The mixed solution (termed as solution A) was
5

stirred vigorously at room temperature until the Bi(NO3)3•5H2O was completely dissolved. A
separate set of solution (solution B) was prepared by mixing 42 mL of DI Water, 50 mL of
absolute ethanol (C2H6O), 1.16 x 10-4 mmol of PVP (K30) and 1.31 mmol of Citric Acid
Monohydrate (C6H8O7•H2O) which was heated to 80 °C. 1.19 mmol of the previously
prepared solution A was drop-wise added into the solution B under continuous stirring at 80
°C for 5 min to form solution C. After this, the solution C was heated at 80 °C for different
time durations (0h, 1h, 3h, 6h and 18h) along with vigorous stirring. For temperature
variations study, solution A was drop-wise added into solution B to form solution C which
was then subsequently heated to different temperatures (40, 60, 80, 100 and 120 °C) and the
system was kept under stirring for 5 min. After the reaction procedure was completed, the
solution was cooled down naturally to ambient temperature; the precipitates were collected
by filtration; and washed with DI water and absolute ethanol until the pH of the solution
turned neutral (pH = 7). The final white coloured powdered products were obtained after
overnight drying of the washed samples in an oven at 80 °C. The detailed synthesis procedure
has been illustrated in Scheme 1. B0h/B1h/B3h/B6h/B18h refers to the BiOCl samples
synthesized with 5 min stirring and subsequent heating for 0, 1, 3, 6 and 18 hr. at 80 °C.
B40/B60/B80/B100/B120 refers to the BiOCl sample synthesized with only 5 min stirring at
40, 60, 80, 100 and 120 °C.
2.3. Theoretical methods
The first principles calculations were carried out using Vienna ab-initio Simulation Package
(VASP) [40-42] with projector-augmented-wave (PAW) [43] approach. The contribution
from the exchange-correlation terms were dealt with Perdew-Burke-Ernzerhof (PBE) [44]
functional within the generalized gradient approximation (GGA). Plane wave basis sets up to
an energy cut-off 500 eV were implemented in all the calculations. As sufficiently large cells
were considered for the study of both water and Rhodamine B (RhB) adsorption process on
various BiOCl surfaces, the Brillouin zone integrations were performed using (1×1×1) k-
point mesh centered at the Γ point. The structures were allowed to relax until the total
energies converged below 10-5 eV/ atom. All the calculations were performed in spin
unrestricted manner and the corrections due to dispersive forces were incorporated via the
PBE + D2 force field method (Grimme’s method) [45] as implemented in VASP. During the
6

relaxation of the BiOCl surfaces and composite structures a vacuum slab of 24 Å was
deployed perpendicular to the surface to ward off any spurious interaction. Water or (RhB)
adsorption ability of various BiOCl surfaces was determined comparing the adsorption
energy values which were determined using the following formula:
EAds = ECompo – EP – EW/Rho-b
(1)
Where,
ECompo = is the energy of the water or RhB adsorbed BiOCl surface,
EP = is the energy of the pristine BiOCl surface and
EW/Rho-b = is the energy of the free water or RhB molecule.
Lower values of adsorption energies, as obtained from the above, implies stronger adsorption.
3. RESULTS AND DISCUSSION
3.1. XRD Analysis
The XRD diffraction pattern of the as-synthesized BiOCl samples is shown in Fig. 1(a and b).
The peaks at 12.10, 24.25, 26.00, 32.70, 33.62, 36.67, 41.04, 46.82, 49.90, 54.30, 55.14,
58.76 and 60.64 ο corresponds to (001), (002), (101), (110), (102), (003), (112), (200), (113),
(211), (104), (212) and (114) lattice planes accordingly [46], which are in conformation to the
tetragonal matlockite structure (space group-P4/nmm) (JCPDS Card No. 85-0861). The
highest intensity peak for the B0h/B80 sample is situated at 32.70 ο which corresponds to the
(110) crystal facets whereas for the other samples, the intensity of this (110) peak is more or
less comparable to those of the (101) and (102) planes. This indicates the dominant exposure
of the particular plane in case of B0h/B80 sample. This result is also evident from the texture
coefficients (TC) analysis of all the time and temperature varied samples given in Fig. S1(a
and b) of the ESI along with the details of the TC analytical techniques.
3.2. FTIR Analysis
The FTIR (Fourier transport infrared spectroscopy) spectra of the as-synthesized time and
temperature varied pure BiOCl hollow microsphere samples are shown in Fig. 1(c and d)
respectively. The peak appearing at 527 cm-1 is due to the A2u-type symmetrical vibrations of
Bi-O bond, the large band at 1664 cm-1 arises due to C=O carbonyl stretching vibration and
the weak band occurring at 665 cm-1 is due to the Bi-O bond, the band for ring stretching
vibration occurs at 837 cm-1, whereas the stretching vibration for C-O epoxy bond arises at
1209 cm-1. The peaks at 1285, 1423 and 1458 cm-1 can be attributed to the Bi-Cl bonds, and
that at 2298 cm-1 occurs for C-H alkali stretching vibration. The small hump at 2953 cm-1
occurs due to N-H bond for PVP and the broad hump at 3456 cm-1 arises due to O-H
7

stretching mode of vibration. The presence of all the characteristic peaks confirms the
successful production of highly pure BiOCl nano-materials.
Fig. 1. XRD pattern of (a) time varied and (b) temperature varied samples; FTIR spectra of
(c) time varied and (d) temperature varied samples.
3.3. XPS Analysis
X-ray photoelectron spectroscopic (XPS) study was executed to analyse the different
chemical bonds present in the samples with respect to binding energy and the corresponding
spectra are shown in Fig. 2(a-d).
In Fig. 2(a) represents the elemental survey scan revealing the existence of the Bi, O, Cl and
C in the as-synthesized samples. The C1s peak at 284.6 eV occurs due to the presence of
atmospheric carbon. The strong peaks at 163.25 and 168.74 eV shown in Fig. 2(b) can be
ascribed to Bi 4f7/2 and Bi 4f5/2 respectively. As evident from Fig. 2(c), the O1s peak can be
deconvulated into two peaks corresponding to at 534.48 and 536.02 eV. The 1st peak can be
assigned to the O2 or O2- of the Bi-O bond whereas the other peak can be primarily associated
to H2O or OH- attached on the surface of the sample. The Cl 2p spectra shows two
deconvulated peaks at 197.84 and 199.37 eV in Fig. 2(d) belonging to Cl 2p3/2 and Cl 2p1/2
8

respectively which corresponds to Cl- of the Bi-Cl bond. Hence the XRD and XPS evaluation
approve that the as-synthesized samples are pure BiOCl [47].
Fig. 2.(a) XPS elemental survey scan; (b) Bi 4f spectra (c) O 1s spectra; (d) Cl 2p spectra of
B0h/B80.
3.4. Morphological and EDS study
Fig. 3. FESEM images of (a) B0h/B80, (b) higher magnified image of B0h/B80 sample.
9

The morphology of the as-prepared B0h/B80 sample along with the magnified image taken at
100 nm is shown in Fig. 3(a) and Fig. 3(b) respectively. The FESEM (Field emission
scanning electron microscopy) images of all other time and temperature varied samples are
given in Fig. S2(a-h) and Fig. S3(a-h) respectively. It is evident from the images that all the
samples exhibit hollow microspheres like structure. The FESEM image of the B0h/B80
sample reveals that the thickness of the walls of the hollow microsphere is narrower than the
other samples thus making the hollow more pronounced in case of B0h/B80. The average
inner diameter of hollow sphere is about 200 nm and the outer diameter is around 580 nm as
calculated for the B0h/B80 sample. For the B120 sample, the thickness of the walls is
increased thus making the internal diameter of the hollow sphere minimum. The uniform
distribution of the hollow spheres was significantly disturbed for the time varied BiOCl
samples. The hollow microspheres are composed of small units having average length around
25 nm ~ 100 nm and thickness about 10 nm ~ 25 nm. The small units are uniformly arranged
in all the as-synthesized BiOCl samples and significant changes in grain/crystalline sizes and
particle dimensions are not observed.
The TEM and HRTEM image of the as-synthesized B0h/B80 pure BiOCl sample is shown in
Fig. 4(a and b) respectively. The images conform that the as-synthesized samples exhibit
hollow micro-spherical morphology. The distinct lattice fringes are observed from Fig. 4(b)
which suggests the highly crystalline nature of the sample and the corresponding high intense
(110) planes along with the other two intense planes (101) and (102) are assigned. This
finding is in accordance with the XRD results which reveals that the highest intensity peak of
BiOCl corresponds to the (110) plane. The scheme representation of the BiOCl crystal
structure is shown in Fig. 4(c) where the blue slab indicates the (110) facet of the BiOCl
supercell.
(c)
10

Fig. 4.(a) TEM image and (b) HRTEM image of B0h/B80 sample; (c) Schematic
representation of the BiOCl crystal structure, the blue slab indicating the (110) facet of the
BiOCl supercell.
Fig. S4(a) shows the Energy Dispersive X-ray Spectra (EDS) of B0h/B80 which reveals the
existence of Bi, O and Cl elemental peaks. Fig. S4(b-d) shows the corresponding elemental
mapping images. The weight and atom percentages of the respective elements of all the
samples are given in Table S1. The atomic ratios of the elements of the as-synthesized BiOCl
samples are approximately stoichiometric, according to the classical formula unit.
3.5. Raman Spectra Analysis
Fig. S5(a and b) shows the Raman Shift of the as-synthesized time and temperature varied
pure BiOCl samples. Pure BiOCl shows three peaks corresponding to the A1g, B1g and Eg
Raman active modes. The asymmetric vibrations modes show less intensity as compared to
the symmetric vibrations. The most active band at 148 cm-1 (also called A1g band) is
attributed to the Bi-Cl internal stretching mode of vibration whereas the weak band at 104
cm-1 is assigned to the external A1g band arising due to 1st order scattering [48]. The other
strong band at 204 cm-1 (called the Eg band) occurs due to the Bi-Cl internal stretching mode
of vibration. The weak band at 484 cm-1 can be assigned as the Eg and B1g band which arises
due to the motion of the O2- atoms. This peak is difficult to observe. Moreover, no peak
shifting has been observed in both the time and temperature varied samples.
3.6. BET Analysis
The measurement of the surface area, porosity and distributions of pore size of the as-
synthesized B0h/B80 sample is evaluated from the nitrogen adsorption-desorption isotherm
obtained from BET analysis. Fig. S5(c) shows the nitrogen adsorption-desorption isotherms
Fig. S5(d) reveals the pore size distribution of the B0h/B80 sample. The calculated surface
area is 27.108 m2g-1 and the pore volume is 0.055 cm3g-1 whereas the average pore diameter
is 8.56 nm as measured by applying the BJH (Barret Joyner Halenda) technique. The
obtained results indicate that the hollow microsphere like morphology of B0h/B80 has a
significant surface is to volume ratio that is important for the photocatalytic application [47].
3.7. Optical property study
11

The diffuse reflectance spectra (DRS) taken in the range of 220 to 700 nm for all the as-
synthesized time and temperature varied white coloured powder samples are displayed in Fig.
5(a and b) respectively. BaSO4 has been used as the standard for the reflectance
measurement. It can be observed that all the samples exhibit very high reflectance around 93
% and a sharp fall occurs near 320 nm wavelength. The indirect band gaps of the as-
synthesized samples have been calculated from the diffuse reflectance spectra by employing
Kubelka-Munk method and the corresponding plots of Kubelka-Munk function vs. photon
energy (hν) are shown inset of Fig. 5(a and b). The indirect band gaps as calculated lie in the
range of 3.46 to 3.59 eV and the corresponding values are given in the inset of Fig. 5(a and
b).
The energy band gap diagram for the time and temperature varied samples are shown in Fig.
5(c and d) respectively. The electronic band structures are necessary to describe the excitation
and transition of photogenerated charge carriers that occurs during the photocatalytic activity.
The band diagrams determine the position of the conduction band (CB) and the valence band
(VB) which is used to describe the series of redox reactions taking place during
photocatalysis.
The conduction band edge and the valence band edge potentials can be calculated by a set of
equations as described by the Mulliken’s electronegativity theory [49],
EVB = χSemiconductor– Ee + 0.5Eg
(2)
Where,
EVB = the valence band (VB) edge potential,
χSemiconductor= the electronegativity of the semiconductor,
Ee = the energy of the free electrons of hydrogen scale (ca. 4.5 eV) and
Eg = the band gap energy of the semiconductor.
The VB edge potentials can be calculated from the above equations. The corresponding CB
edge potentials are evaluated by subtracting the VB edge potentials from the electronic band
gaps as measured from DRS.
12

Fig. 5. Diffuse Reflectance Spectra of (a) time varied and (b) temperature varied BiOCl
samples and inset: Kubelka-Munk plots for calculation of indirect band gaps; energy band
diagrams of (c) time and (d) temperature varied samples. Photoluminescence emission
spectra of (e) time varied and (f) temperature varied BiOCl samples.
The photoluminescence emission (PL) spectra of the as-prepared time and temperature varied
BiOCl samples are represented in Fig. 5(e and f) respectively. All the samples exhibit
emission spectra around 390 nm for an excitation wavelength of 280 nm. The B0h/B80
sample shows the least emission intensity as compared to other samples. This observation
indicates the successful separation of the photogenerated electrons and holes in the B0h/B80
sample. The charge carriers do not recombine and annihilate which proves to be beneficial
for the photocatalytic activity [39].
3.8. Mott-Schottky and Nyquist Analysis
The Mott-Schottky plots of the B0h/B80 BiOCl sample measured in two frequencies is
shown in Fig. 6(a). The 1/C2 Vs. potential plots shows the existence of negative slopes which
indicates the ‘p’ type characteristics of the semiconductor. The Mott-Schottky plots of the
other time and temperature varied BiOCl samples are given in Fig. S6(a-h).
13

The electrochemical impedance spectroscopy (EIS) was recorded in the semi-circular Nyquist
graphs. It is a known fact that the smaller semi-circular arc of the Nyquist plots indicates the
efficient separation of the charge carriers. The Nyquist plots of the as-synthesized time and
temperature varied BiOCl samples are shown in Fig. 6(b and c). The B0h/B80 sample
exhibits the smallest arc radius as compared to all other samples which determines the
impressive separation efficiency of the charge carriers along with inferior charge transfer
resistance. This observation establishes the B0h/B80 sample to be an effective photocatalyst
owing to its superior interfacial charge transfer capability.
Fig. 6.(a) Mott-Schottky plot of B0h/B80 sample at frequencies of 2000 Hz and 2500 Hz;
Nyquist plots of (b) time and (c) temperature varied samples.
3.9. Photocatalytic activity of BiOCl samples
The photocatalytic activity of as-synthesized time and temperature varied BiOCl samples
were evaluated by the degradation of aqueous solution of Rhodamine B (RhB) Dye under
visible light irradiation. The absorbance spectra of the dye in presence of different catalysts
are shown in Fig. S7(a-i) from which it can be observed that all the samples were almost able
to degrade the dye completely within significantly less time. The best sample B0h/B80
showed about 99 % degradation efficiency within only 6 minutes of visible-light irradiation.
14

Fig. 7. First order kinetics plots of (a) time varied and (b) temperature varied samples; rate
constant bar graphs of (c) time varied and (d) temperature varied samples; degradation
efficiency of (e) time varied and (f) temperature varied samples.
The reaction kinetics of the degeneration of RhB dye performed under visible light irradiation
by different time and temperature varied BiOCl samples been calculated following equation
(3). As evident from the plots, the photocatalytic reaction rates follow first order kinetics and
the corresponding rate constant values are calculated and tabulated in Table S2.
ln (C0/CT) = kappT
(3)
Where,
C0 = initial concentration of RhB dye,
CT = concentration of RhB dye at time ‘t’,
kapp = apparent 1st order rate constant and
T = time.
The kapp value was calculated from the slope of ln(C0/CT) vs. time plots as shown in Fig. 7(a
and b) for the time varied and the temperature varied samples respectively.
The bar graphs shown in Fig. 7(c and d) depict the first order rate constant values of the time
and temperature varied BiOCl samples respectively.
15

The degradation efficiency of the as prepared samples has been calculated from equation (4)
and the corresponding bar graphs shown in Fig. 7(e and f) depicts the calculated degradation
efficiency of the time and temperature varied BiOCl samples respectively.
η = (η0 – ηT )/η0 %
(4)
Where,
η = degradation efficiency,
η0 = initial concentration of RhB dye and
ηT = concentration of RhB dye at any time ‘t’.
All the samples show impressive efficiency in degrading toxic RhB dye, however, the
B0h/B80 sample proved to be the most effective catalyst having degradation efficiency of
about 99 %.
The stability of the as prepared BiOCl catalysts in degrading the RhB dye under visible light
irradiation was studied by recycling the catalyst up to three cycles. It can be observed from
Fig. 8(a) that the degradation efficiency of the B0h/B80 BiOCl sample remains more or less
unchanged even after reusing for three cycles which suggests that no photo-corrosion occurs
and this in turn establishes the significant stable nature of the catalyst.
The effect of catalyst dosage on degrading RhB dye was studied by varying the amount of
catalyst and irradiating the sample under visible light. Different amount (10, 20, 30, 40 and
50 mg) of time and temperature varied BiOCl (B0h/B80) catalyst was added into the same
concentration of RhB dye solution and the photocatalysis procedure was carried out
following the same procedures. It can be observed from Fig. 8(b) that with increase in the
catalyst loading the photocatalytic activity improved to a great extent and the corresponding
degradation efficiency improved from 70 % to 99 %. This can be attributed to the fact that
with increase in the amount of catalyst, there is a subsequent increase in the number of active
sites which in turn ameliorates the rate of degradation. The dosage amount of 30 mg has been
considered the optimum catalyst loading and the degradation rate is 98.92 % in only 6 min of
irradiations.
16

Fig. 8.(a) Reusability of the B0h/B80 sample up to three cycles; (b) Effect of catalyst dosage
on photocatalytic activity; (c) Effect of pH on the photocatalytic activity of B0h/B80; (d)
Photocatalytic activity of B0h/B80 under visible and UV light irradiation.
The effect of pH on the catalytic dye degradation was studied by varying the pH of the
catalyst-dye solution from acidic to basic. It can be observed from Fig. 8(c) that the
photocatalytic activity of the B0h/B80 sample increased to a great extent at acidic pH,
whereas, the reaction rate got drastically hindered at a basic pH. The ‘k’ values calculated for
the three pH variations are given in Table S3. The B0h/B80 sample showed only 45 %
degradation efficiency at a basic pH whereas the degradation efficiency at an acidic pH is
nearly 100 % for the same irradiation time.
The effect of the light source on the photocatalytic activity was studied by irradiating the
catalyst-dye solution with both UV light source and visible light source. It can be observed
from Fig. 8(d) that under UV light irradiation the photocatalytic rate decreased significantly
and the corresponding rate constant values are given in Table S4. The wavelength in the UV
range thus proved to be quite insufficient in carrying out the photocatalytic reaction as
compared to the visible light wavelength.
17

Fig. 9.(a) First order kinetics plot of degradation of different textile dyes by B0h/B80 sample;
(b) First order kinetics plot of degradation of various mixed dyes by B0h/B80 sample.
Fig. S8(A) shows the zeta potential graph of B0h/B80 for surface charge calculation. The
negative surface charge obtained for the sample makes it potential for degrading positive
cationic dyes like RhB. The cationic dye being positive in nature adsorbs significantly on the
negatively charged surface of BiOCl which in turn facilitates its easy degradation. In order to
establish the practical feasibility of the catalysts, BiOCl was further utilized for the
degradation of anionic dyes like Methyl Orange (MO) and Eosin B. Fig. S9(a and b) shows
the absorbance spectra of MO and Eosin B dye respectively in presence of the B0h/B80
catalyst. Fig. 9(a) shows the corresponding first order kinetics plot of different textile dyes
separately (RhB, MO and Eosin B dyes). The sample was able to degrade the anionic dyes to
considerable extent under visible light irradiation. Moreover, the B0h/B80 sample was
utilized for the degradation of mixed dye solution comprising of equal concentration of RhB
and MO dye, RhB and Eosin B dye, MO and Eosin B dye; and the mixture of these three
dyes. The catalyst was able to completely degrade the mixture of dyes within 210 min of
visible light irradiation. The absorbance spectra are shown in Fig. S9(c-f) and the
corresponding first order kinetics plots are shown in Fig. 9(b). The rate constant values of
RhB, MO, Eosin B and mixture different dyes are given in Table S5.
It is a known fact that the photo-induced electrons, holes, hydroxyl radicals and superoxide
anions are the primary classes of active radicals that participate in the photocatalytic reactions
to effectively degrade the toxic dyes into non-toxic end products. The existence of these
radicals can be confirmed from the free radicals trapping procedure [50]. The activity of the
photocatalytic degradation of RhB dye under visible light irradiation by B0h/B80 BiOCl
sample in the presence of different scavengers like benzoquinone (BQ), isopropyl alcohol
(IPA), t-butanol and ethylene diamine tetra acetate disodium (EDTA-2Na) has been studied
by adding the requisite amounts of these scavengers during the photocatalysis procedure
followed by studying the effect of their addition in the reaction kinetics. The corresponding
CT/C0 plots are shown in Fig. 10(a) from which it can be clearly observed that the
degradation efficiency got significantly hindered upon addition of BQ and EDTA-2Na
whereas no significant changes were observed upon addition of IPA and t-butanol. It is to be
noted that BQ acts as a scavenger of superoxide anion radical; IPA and t-butanol are
scavengers of hydroxyl radicals, whereas EDTA-2Na acts as photogenerated holes scavenger.
Since in dye-sensitization induced photocatalytic activity there are no chances of formation of
18

photogenerated holes, hence the observations prove that the superoxide anions are the main
active radicals which participate in the photocatalytic reactions.
Fig. 10.(a) Scavenger test, (b) XRD pattern of before and after photo-catalysis and (c) after
photo-catalysis FESEM image of B0h/B80.
The XRD plots of B0h/B80 catalyst before photocatalysis and after photocatalysis are shown
in Fig. 10(b). The catalyst sample does not undergo any phase changes after the catalytic
reactions take place. All the characteristics peaks are present in the reused sample with no
additional peaks or absence of any peaks. The FESEM image of the used catalyst B0h/B80
sample is shown in Fig. 10(c) which reveals the exact same morphology that was observed
before the photocatalytic reactions. These observations prove the stability of the catalyst in
degrading toxic organic dyes and thereby establish its practical feasibility.
3.10. Plausible photocatalytic mechanism
Scheme 2 represents the plausible photocatalytic mechanism in degrading RhB dye by
B0h/B80 catalyst under visible light irradiation. The photocatalytic activity of BiOCl samples
were examined by decomposition of toxic organic dyes under visible light irradiation at
ambient temperature and pH. The gradual decoloration of the RhB dye from deep pink to
light pink and finally to completely colorless within 6 min of visible light irradiation in the
presence of BiOCl catalyst occurs due to de-ethylation of RhB and the associated absorbance
spectra has already been illustrated in Fig. S7(a-i). Fig. S7(j) shows the intensity ratio
(CT/C0) plot with respect to irradiation time for the B0h/B80 sample showing significant
adsorption of RhB dye (about 85 %) on the catalyst surface during one hour of dark stirring.
Further dark stirring led to the saturation of the adsorption of dye which can be seen from
inset of Fig. S7(j). The rest 15 % of the dye could not be degraded without light irradiation.
In order to establish the achievement of proper adsorption-desorption of dye on the BiOCl
surface, the dye solution was subjected to uniform heating at 60 °C after sufficient adsorption
has taken place during the one hour dark stirring. The absorption spectra were taken at
19

regular intervals up to 3 hours of heating and it was found that the absorption intensity
increased with increasing in heating time suggesting the desorption of the dye molecules
from the catalyst surface. The corresponding absorption spectrum is given in Fig. S7(k).
Additionally no other new absorption peaks originate in the total spectral range of the RhB
dye. It is to be noted that though the band gaps of the as-synthesized BiOCl samples lie in the
UV range (3.46-3.59 eV), however, B0h/B80 sample could degrade only 68 % of RhB dye
under UV light irradiation for 10 min whereas 99 % of RhB dye was degraded in lesser
amount of time under visible-light irradiation. The high level of adsorption of the dye along
with better photocatalytic activity under visible light irradiation than under UV light suggest
the probability of dye-sensitized photocatalytic activity of BiOCl. Moreover, the hollow
spherical morphology of B0h/B80 sample with an inner and outer diameter of about 200 nm
and 580 nm respectively and the BET surface area of about 27.108 m2g-1 is also one of the
probable reasons for the superior photocatalytic activity of BiOCl. Additionally from the
XRD data of the B0h/B80 sample, it can be observed that the (110) plane is the most intense
among all other planes whereas for the other temperature varied samples, the peak
corresponding to the (110) plane is more or less comparable with the other XRD peaks; and
for the B120 sample, its intensity decreases than that of the (102) plane. The texture
coefficient values as calculated from XRD data also reveals the highest value of TC (1.75)
along the (110) plane for the B0h/B80 sample which suggests the preferential growth of the
sample along this direction. Moreover, the FESEM image of the B0h/B80 sample reveals that
the thickness of the walls of the hollow microsphere is narrower than the other samples thus
making the hollow more conspicuous in case of B0h/B80. For the B120 sample, the thickness
of the walls is increased thus making the internal diameter of the hollow sphere minimum.
This may be the reason behind the decreased photocatalytic activity of the B120 sample as
compared to B0h/B80.
Based on the above facts and with special weightage on the fact that hollow micro-spherical
BiOCl samples as-synthesized in the present work exhibited visible-light assisted degradation
of RhB dye in-spite of having band gaps in the UV range, suggest the probable conclusion of
dye self-sensitized degradation phenomenon occurring in presence of visible-light source.
Moreover the significant adsorption of RhB on the BiOCl surface leads to the transfer of
photogenerated electrons between activated RhB* and BiOCl catalyst along with
recombination of charge carriers between the RhB dye and the catalyst [51].
Generally photogenerated charge carriers are produced under visible light irradiation with a
wavelength equivalent to the band gap of the catalyst. The electrons travel to the conduction
20

band leaving behind holes in the valence band. However, in the present study, due to the
large band gap of BiOCl, visible-light irradiation is not sufficient to excite the material and
no photogenerated charge carriers are produced. But the visible light energy proves to be
adequate in exciting the RhB dye molecules adsorbed on the catalyst surface thereby forming
an active species RhB*. It is also to be noted that the LUMO and HOMO of RhB lies at -1.10
V and 1.14 V (Vs. NHE) [52] whereas the CB minima and the VB maxima of BiOCl lies at
0.15 V and 3.74 V (Vs. NHE) respectively. Due to the more negative LUMO potential of
RhB than the CB edge of BiOCl, electrons will be transferred from the LUMO of RhB to the
CB of BiOCl. After donating an electron RhB* gets converted to the active species RhB+•.
The hydroxyl active radical is produced by some intermediate reactions taking place in the
CB of BiOCl where O2 reacts with electrons (injected from RhB*) and H+ ions to form H2O2.
Since the redox potential of this reaction is more positive (0.69 V wrt NHE) than the CB of
BiOCl, hence this reduction reaction occurs more feasibly where H2O2 gets reduced to •OH
in presence of electrons. The hydroxyl anion thus produced helps in the degradation of the
organic pollutants to CO2 and H2O. The RhB+• can also effectively degrade the toxic dye to
non-toxic products in presence of the active radicals.
Fig. 11(a) shows the bar graph plot of adsorption energies of water on different planes of
pristine BiOCl as obtained from first principles calculation. For each BiOCl planes, two
different water adsorption sites are considered which are denoted as (h k l) (a/b). The lowest
adsorption energy value (-2.59 eV) for water adsorption is observed for the (110) (a) plane of
BiOCl as revealed from the bar graph. The optimized geometry of water adsorbed in its most
favourable condition is shown in Fig. 11(b). This proves that the (110) plane of BiOCl is
most beneficial for the adsorption of water. Due to the efficiency of the (110) surface to
adsorb water, we further studied the adsorption mechanism of RhB dye on this surface. The
optimized structure of RhB adsorbed on the (110) plane is shown in Fig. 11(c). This
calculation corroborates the dependency of the superior photocatalytic activity of BiOCl on
the most intense (110) crystal plane.
21

Fig. 11.(a) Adsorption energies of water adsorbed on different planes of BiOCl. The plane
indices are shown along the x axis. The top view of each surfaces are shown in the inset. (b)
Optimized structure of water adsorbed on BiOCl (110) plane. (c) Optimized structure of RhB
on BiOCl (110) plane.
The (110) exposed facet of BiOCl shows the presence of large density of edge terminated
oxygen atoms as observed from Fig. 11(c) rendering an anionic nature in the sample. This
proves to be immensely beneficial for the adsorption of cationic RhB dye along the particular
plane assisted with accumulation of photogenerated charge carriers transferred from the
LUMO of RhB dye.
The series of reactions that occur during the photocatalytic activity under visible light
irradiation are given as follows:
RhB(ads) + hν
RhB(ads)* + BiOCl
O2 + e-
RhB(ads)
*
(5)
(6)
(7)
(8)
BiOCl(eCB-) + RhB(ads)
+ •
-
•O2
O2 + 2H+ + 2eCB
H2O2
-
22

-
H2O2 + eCB
•OH
(9)
(10)
(11)
•OH + RhB(ads)
CO2 + H2O
+•
-
RhB(ads) + •OH/•O2
CO2 + H2O
Scheme 2. Schematic representation of the mechanism of photocatalytic activity of B0h/B80
material under visible light irradiations.
Hence it can be concluded that upon adsorption of RhB dye on BiOCl catalyst surface, an
effective RhB/BiOCl interface is formed due to the existence of the significant energy level
offset between them. This in turn leads to the thermodynamically favoured rapid transfer of
photogenerated electrons from LUMO of dye to CB of catalyst resulting in effective
separation of photogenerated charge carriers, which is an important requisite for
photocatalytic activity [51].
Scheme 2 depicts the plausible photocatalytic mechanism of hollow spherical BiOCl samples
in degradation of toxic organic dyes. Additionally Scheme S2(a) represents the adsorption of
RhB on BiOCl surface and consequent facilitation of the enhanced photocatalytic activity
owing to the unique morphology of the catalyst [53].
The plausible mechanism that took place under UV light irradiation is given in Scheme
S2(b). In this case, due to the favourable band gap of the BiOCl, excitation of the material
occurs under UV light with subsequent generation of electrons in the CB and holes in the VB
23

of BiOCl. The CB of BiOCl (0.15 V vs. NHE) is more positive than the redox potential of
-
O2/•O2 (-0.046 V vs. NHE) hence the reduction reaction to form the superoxide anion is not
feasible in the CB of BiOCl. However, the more positive redox potential of O2/H2O2 (0.69 V
vs. NHE) favours the formation of hydroxyl active radical (•OH) by reduction of H2O2 in
presence of electrons. The VB of BiOCl (3.74 V vs. NHE) is more positive than the redox
potential of H2O/•OH (2.37 V vs. NHE), hence the oxidation reaction of OH- to •OH in
presence of photogenerated holes occurs at the VB of BiOCl. The •OH active radical can
subsequently reduce RhB dye to CO2 and H2O.
A list of all the reported pure BiOCl catalysts in degradation of RhB dye is tabulated in Table
S6 which reveals the superior nature of the catalyst reported in the present work in terms of
degradation time and reaction rate constant value.
4. CONCLUSION
A facile chemical synthetic procedure for the development of pure BiOCl samples by varying
some synthetic parameters like time and temperature duration has been reported in the
present work. The phase formation has been confirmed from the XRD analysis and the most
intense peak as observed from the XRD corresponded to the (110) plane which revealed the
dominant exposure of that crystal facet. Chemical structure analysis have been performed by
XPS and FTIR measurements which have provided detailed insights into the different
chemical bondings and associated modes of vibrations. Raman study corroborated the
findings. The hollow microsphere like morphology was revealed from the FESEM images
and the elemental analysis was studied from EDS. The lattice fringes as observed from the
HRTEM images confirmed the highly crystalline nature of the samples and the (110) plane
was calculated to be the most intense plane. The surface area, porosity and the distribution of
the pore size was measured from BET analysis. The band gaps of the as prepared samples as
measured from DRS were found to lie in the UV region and the corresponding electronic
band edge potentials were measured by following the Mulliken’s electronegativity theory.
The PL emission spectra of B0h/B80 exhibited the lowest intensity which indicated the
spatial separation of photogenerated charge carriers. The Mott-Schottky measurements
revealed the p-type characteristics of the BiOCl semiconductors whereas the smallest arc
radius of B0h/B80 sample as measured from Nyquist analysis establishes its superior charge
transfer efficiency as compared to other samples. The samples were finally utilized for the
effective photocatalytic degradation of toxic organic cationic and anionic dyes. The best
sample B0h/B80 exhibited the best efficiency by completely degrading the dye within very
24

short time of visible light irradiation. Several parameters like catalyst dosage, pH and light
source were varied to observe their effects on the reactions. The B0h/B80 sample was further
utilized for degradation of a mixture of dyes and the catalyst was able to be reused up to 3
cycles which proved its practical feasibility. The effect of the most intense (110) plane of
BiOCl on the superior catalytic activity was further demonstrated from first principles
calculations which confirmed the (110) plane to be the most suitable adsorption site. The high
adsorption of cationic dye RhB on the anionic catalyst BiOCl along with exhibition of
superior catalytic activity under visible-light irradiation in-spite of having band gaps in the
UV range indicated the dye sensitized photocatalytic phenomenon. In conclusion, the as
prepared BiOCl samples emerged to be highly efficient catalysts in degrading organic
pollutants which establish them as potential candidates for water remediation related
applications.
Declaration of Competing Interest
The authors confirm that there are no conflicts to declare.
Data Availability Statement
The data will be provided if requested in future.
Conflicts of interest
The authors declare that there are no conflicts to declare.
Authors statement:
The authors declare that the work reported here is original and the manuscript is sent to
Materials Research Bulletin with the full consent of all the co-authors. This manuscript or
any of its parts are not published elsewhere.
ACKNOWLEDGEMENT
The authors wish to thank the University Grants Commission, the Government of India, for
the ‘University with potential for excellence (UPEII) (Grant No. F-1-10/12(NSPE))’ scheme,
the Department of Science and Technology (DST) and DST Nanomission for financial
support. RS acknowledges the Council of Scientific and Industrial Research (CSIR) (File No.
09/096(0872/2016-EMR-I)), the Government of India for awarding her a Junior Research
25

Fellowship (JRF) and DD and BKD wishes to acknowledge the DST [Reg. No. of DD-
IF170684 and Reg. No. of BKD– IF150499], Govt. of India for awarding them INSPIRE
fellowship. SS is thankful to Science and Engineering Research Board (SERB) for their
financial support (SERB/EEQ/2017/000092).
26

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