Single step hydrothermal synthesis of beyerite, CaBi2O2(CO3)2 for the fabrication of UV-visible light photocatalyst BiOI/CaBi2O2(CO3)2

Article abstract of DOI:10.1039/c6ra06723a

The mineral beyerite (CaBi₂O₂(CO₃)₂), a member of the "sillen" family possessing a layer sequence, (Bi₂O₂)²⁺-CO₃^2--Ca²⁺-CO₃^2--(Bi₂O₂)²⁺ has successfully been synthesized under hydrothermal conditions in a single step for the first time. Uniform rectangular plates of beyerite crystallites with a surface area of 33 m² g^-1 were obtained by heating a solution containing bismuth nitrate and calcium carbonate in ethylene glycol along with the addition of a saturated solution of sodium carbonate at 140 °C. The sample was characterized by powder X-ray diffraction combined with Rietveld refinement, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared spectra and diffuse reflectance measurements. The band gap estimated for beyerite was found to be 3.97 eV and was utilized subsequently to form heterostructure with BiOI (band gap of 1.76 eV), another member belonging to the sillen family. The formation of the composite BiOI/CaBi₂O₂(CO₃)₂ has been confirmed by a variety of techniques and its photocatalytic properties as a UV-visible photocatalyst has been demonstrated for the degradation of aqueous solutions of rhodamine B and phenol. Photocatalytic experiments in presence of scavengers suggested the possible mechanism occurring through holes and peroxide radicals rather than the hydroxyl radicals.

Full text of DOI:10.1039/c6ra06723a

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Single step hydrothermal synthesis of beyerite,
CaBi O (CO ) for the fabrication of UV-visible light
2
2
3 2
Cite this: RSC Adv., 2016, 6, 38252
photocatalyst BiOI/CaBi O (CO ) †
2
2
3 2
Vidhu Malik, Meenakshi Pokhriyal and S. Uma*
The mineral beyerite (CaBi
2
O
32ꢀ
2
(CO
3
)
2
), a member of the “sillen” family possessing a layer sequence,
2+
32ꢀ
2+
2+
(Bi O
2
2
)
–CO –Ca –CO –(Bi
2
O
2
)
has successfully been synthesized under hydrothermal
conditions in a single step for the first time. Uniform rectangular plates of beyerite crystallites with
2
ꢀ1
a surface area of 33 m
g
were obtained by heating a solution containing bismuth nitrate and calcium
carbonate in ethylene glycol along with the addition of a saturated solution of sodium carbonate at 140
ꢁ
C. The sample was characterized by powder X-ray diffraction combined with Rietveld refinement,
scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, Fourier
transform infrared spectra and diffuse reflectance measurements. The band gap estimated for beyerite
was found to be 3.97 eV and was utilized subsequently to form heterostructure with BiOI (band gap of
1
.76 eV), another member belonging to the sillen family. The formation of the composite BiOI/
Received 14th March 2016
Accepted 7th April 2016
CaBi (CO has been confirmed by a variety of techniques and its photocatalytic properties as a UV-
2
O
2
3 2
)
visible photocatalyst has been demonstrated for the degradation of aqueous solutions of rhodamine B
and phenol. Photocatalytic experiments in presence of scavengers suggested the possible mechanism
occurring through holes and peroxide radicals rather than the hydroxyl radicals.
DOI: 10.1039/c6ra06723a
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been investigated extensively. Most of these materials have also
been made into different heterostructures in order to improve
1
. Introduction
Semiconductor photocatalysis has emerged as one of the most the photocatalytic efficiency specially under visible light irra-
investigated research topics over the past decade specially from diation. In this regard (Bi CO ), bismuth subcarbonate,
the point of view of environmental remediation by decompos- naturally occurring as mineral bismutite, has been found to be
2
O
2
3
ing harmful organics and to generate hydrogen as an alternate an important compound. Bi
2
O
2
CO
3
belongs to the family of
2
+
1
fuel from water by efficiently making use of the solar energy.
Although the focus of research has been centered on TiO
its modications, a rapid surge in the identication of materials (CO
sillen phases and is an intergrowth of [Bi
2
O
2
]
layers and
2
ꢀ
ꢀ
and (CO
)
)
layers formed in such a way that the plane of the
group is orthogonal to the plane of Bi–O layers.
2
3
3
2
16
for the decomposition of toxic organic pollutants and/or for Bismuth subcarbonate has long been the material of investi-
1
7
water splitting has resulted in several semiconducting photo- gation for medical purpose because of its antibacterial activity.
2
3
4
5
catalysts such as BiVO , NaBiO , AgSbO , Ag PO , CaCu - More importantly, because of its band gap of 3.38 eV, it has
Ti O , Ag SbS , and nitrogen incorporated CsCa Ta O , etc. been investigated as a potential photocatalyst under UV light
The materials containing Bi cation offer an attractive choice irradiation for the degradation of many organic dyes. Subse-
by providing nontoxic, photostable semiconductors with quently, several research groups have attempted to enhance the
reduced band gap because of the overlap of its 6s band with the photocatalytic efficiency through different synthetic methods to
4
3
3
3
4
3
6
7
8
4
12
3
3
2
3 10
3
+
12
2
17
12
18
19
2
p band of the anionic oxygen. Apart from BiVO ,
4
other achieve nanotubes, plates, sponge, nanosheets, nanoscale
9
10
2
0
2
1
bismuth containing compounds such as Bi
Bi
2
MoO
6
, Bi
2
WO
6
,
crystallites, and crystallites with ower like morphologies.
have Recently, single crystal nanoplates of Bi CO have also been
11
13–15
O
2
3
2
CuO
4
,
Bi
2
O
2
CO
3
(ref. 12) and BiOX (X ¼ Cl, Br, I)
2
2
2
engineered with enhanced photocatalytic activity. Apart from
synthetic modications, many studies have focused on forming
2
3
Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi 110 photocatalytic heterostructures such as Bi
2
O
Bi
2
CO
3
/Bi
2
WO
6
3
,
,
24
25
26
0
07, India. E-mail: suma@chemistry.du.ac.in
BiVO
/Bi
O
2
CO
3
,
Fe
3
O
4
/Bi
O
2
CO
,
S
3
/Bi
2 2
O CO
4
2
2
3
2
†
Electronic supplementary information (ESI) available: Table of crystallographic
27
28
29
Bi O CO /Bi NbO , BiOI/Bi O CO , Bi O CO /BiOCl, Ag O/
2
2
3
3
7
2
2
3
2
2
3
2
parameters for the Rietveld renements of beyerite, TGA plot, PXRD patterns of
the BiOI/CaBi (CO before and aer the photocatalysis of Rh and
30
31
Bi O CO , g-C N /Bi O CO , graphene–Bi O CO compos-
2
2
3
3
4
2
2
3
2
2
3
2
O
2
3
)
2
B
32
33
ꢀ5
ites, and the ternary composite of Bi O /Bi O CO /Sr Bi O .
2 3 2 2 3 6 2 9
temporal changes in absorbance spectra of Rh B dye (10 M) under visible
irradiation. See DOI: 10.1039/c6ra06723a
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While
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a
vast literature has developed for bismutite from different characterizations are described in the present
(
Bi O CO ), the other bismuth carbonate containing calcium, work. Since the results from the preliminary characterization
2 2 3
known as mineral beyerite, CaBi O (CO ) has been relatively suggested beyerite to be a semiconductor with a wide band gap
2
2
3 2
34
less explored except for a brief study of luminescence. Beyerite of 3.97 eV, we further considered the construction of a hetero-
2
+
2+
has the [Bi
the layer sequence is (Bi
Fig. 1). One probable reason might be the difference in the eV has been chosen to append with beyerite and the hetero-
synthetic conditions of Bi CO and CaBi (CO . The structure BiOI/CaBi (CO has been synthesized, character-
former could easily be precipitated simply by treating Bi ized and studied as a photocatalyst for the degradation of
containing solution (e.g. Bi(NO ) $5H O) with a solution of aqueous solutions of Rh B and phenol under UV-visible light.
2
O
2
]
layers along with the additional Ca ions and structure in order to increase its capability as a photocatalyst.
2
+
2ꢀ
2+
2ꢀ
2+
15
2
O
2
) –CO
3
–Ca –CO
3
–(Bi
2
O
2
)
Towards this purpose, BiOI with a reported band gap of 1.76
(
2
O
2
3
2
O
2
3
)
2
2
O
2
3 2
)
3
+
3
3
2
carbonates preferably followed by a hydrothermal treatment
ꢁ
20
around 120–180 C for 12–24 h. On the other hand, the
beyerite was reported to have synthesized around 300 bar, at 220
2
. Experimental section
ꢁ
C starting from CaCO
3
and Bi
2
O
2
CO
3
. We successfully
2 2 3 2
2.1. Synthesis of CaBi O (CO )
attempted and synthesized CaBi
2
O
2
(CO
3
)
2
for the rst time
All of the reagents employed in this study were of analytical
grade and were used without further purication. For the
synthesis of CaBi
(Thomas baker 99%) was dissolved in 10 ml ethylene glycol
under relatively milder hydrothermal conditions in a single step
by treating directly the bismuth nitrate solution along with
calcium carbonate and a saturated solution of sodium
2 2 3 2 3 3 2
O (CO ) , 0.485 g (1 mmol) of Bi(NO ) $5H O
ꢁ
carbonate around 140 C for 72 h. The as synthesized beyerite
(
Merck) and to this solution 0.05 g (0.5 mmol) of solid CaCO3
was characterized using a variety of techniques such as powder
X-ray diffraction (PXRD), Scanning Electron Microscopy (SEM),
Transmission Electron Microscopy (TEM), thermogravimetric
analysis (TGA), Fourier transform infrared spectroscopy (FTIR)
and UV-visible diffuse reectance spectroscopy measurements.
The details regarding the successful synthesis and the outcome
(Thomas baker 99.5%) was added under vigorous stirring, fol-
lowed by the addition of 5 ml saturated solution of sodium
carbonate and 15 ml of distilled water. The resulting precursor
suspension of dense milky nature was transferred to a 60 ml
ꢁ
Teon-lined stainless-steel autoclave and heated at 140 C for
72 h. The hydrothermal vessel was cooled down to room
temperature. The solid product was collected by normal ltra-
tion followed by repeated water washings and was nally dried
at room temperature. The formation of calcium bismuth
carbonate has been investigated (Table 1) by varying the
synthetic conditions along with the use of various carbonate
sources such as ammonium carbonate, urea and citric acid and
the ndings are discussed in detail in the Results and discus-
sion section.
2
.2. Synthesis of BiOI/CaBi O (CO ) heterostructure
2 2 3 2
35
First, BiOI synthesis was carried out by the addition of 7.2 ml
of acetic acid to a solution obtained by dissolving bismuth
nitrate (5 g) in 1 : 1 nitric acid. Separately 1.75 g of potassium
iodide and 2.5 g sodium acetate were dissolved in 100 ml of
water. Subsequently, the bismuth nitrate solution was added to
the potassium iodide and sodium acetate solution. The whole
mixture was kept stirring for nearly 3 h and the brick red
precipitate was ltered and dried in air and used to synthesize
the heterostructure with beyerite aer verication by PXRD
measurement.
The preparation of 10/90 (in weight percentage) BiOI/
CaBi (CO
2
O
2
3 2 2
) was carried out by dispersing 0.90 g of CaBi -
O
2
(CO ) in 20 ml of deionized water by sonication, aer its
3 2
formation corroborated by various characterizations. To this
suspension was added 0.1 g of BiOI and sonication was
continued for another thirty minutes. The pale pink colored
solid was collected by normal ltration and dried at room
temperature. The reason for selecting the targeted composite
with a 10/90 weight percentage was based on the initial
screening experiments using ratios 1/99 and 5/95. The PXRD
2 2 3 2
Fig. 1 The layered structure of beyerite, CaBi O (CO ) .
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2 2 3 2
Table 1 Experimental conditions for the synthesis of CaBi O (CO )
a
Sample
Reagent
Temperature/duration
pH
ꢁ
ꢁ
CBCS1
CBCS2
CBCS3
CBCS4
CBCA1
CBCA2
CBCC1
CBCC2
CBCU1
CBCU2
Saturated sodium carbonate solution (5 ml) + ethylene glycol (10 ml)
120 C/2 days
12.10
140 C/2 days
ꢁ
140 C/3 days
ꢁ
170 C/3 days
ꢁ
Saturated ammonium carbonate solution (5 ml) + ethylene glycol (10 ml)
Saturated sodium carbonate solution (5 ml) + citric acid (10 ml)
Saturated urea solution (5 ml) + ethylene glycol (10 ml)
140 C/3 days
9.35
7.25
2.60
ꢁ
170 C/3 days
ꢁ
140 C/3 days
ꢁ
170 C/3 days
ꢁ
140 C/3 days
ꢁ
170 C/3 days
a
1
3 3 2 3
mmol of Bi(NO ) $5H O and 0.5 mmol of CaCO were used.
patterns of the composites registered the presence of BiOI only temperature. A typical experiment of degradation was carried
for the 10/90 composition.
out as follows: the catalyst (0.1 g) was taken along with 50 ml of
an aqueous solution of Rh B and phenol with an initial
ꢀ
5
ꢀ1
ꢀ4
ꢀ1
2.3. Characterization
concentration of 1 ꢂ 10
mol l
and 1 ꢂ 10
mol l
respectively in the pyrex container with constant stirring. Prior
to irradiation, the suspension of the catalyst and dye solution
was stirred in dark for thirty minutes to sixty minutes, so as to
reach the equilibrium adsorption. Aer light irradiation, ve
milliliter aliquots were pipetted out periodically from the reac-
tion mixture. The solutions were centrifuged, and the concen-
tration of the solutions was determined by measuring the
maximum absorbance (lmax) at 552 and 268 nm for Rh B and
phenol solutions respectively. To differentiate between the
photocatalysis and the photolysis of the dye molecules, experi-
ments were carried out in the presence and in the absence of
PXRD patterns were recorded using high resolution PAN-
analytical diffractometer, equipped with pixel detector
˚
employing Cu K radiation (l ¼ 1.5418 A) obtained with a scan
a
ꢁ
rate of 200 s per step and step size of 0.013 at 298 K. LeBail
method is used to obtain the cell dimensions from the PXRD
patterns and the Rietveld renement of the PXRD patterns were
36
carried out using GSAS + EXPGUI program. The structure
37
diagram were generated by DIAMOND, version 3. Raman
spectra were recorded using a Renishaw spectrometer via
+
microscope system equipped with an Ar laser (l ¼ 785 nm).
FTIR spectra were collected using a Perkin Elmer 2000 spec-
trometer using KBr pellet technique. UV-vis diffuse reectance
data were obtained over the spectral range 200–800 nm using
Perkin-Elmer Lambda 35 scanning double beam spectrometer
2 2 3 2
catalyst BiOI/CaBi O (CO ) under UV-visible light irradiation.
The rate of photocatalytic degradations were considered only
aer considering the extent of photolysis of the dye solutions.
For comparison, photocatalytic degradation using single phase
CaBi O (CO ) and BiOI as catalysts were also investigated
2 2 3 2
under similar experimental conditions.
4
equipped with a 50 mm integrating sphere. BaSO was used as
a reference. The data were transformed into absorbance with
the Kubelka–Munk function for the estimation of the band gap.
The SEM micrographs of the samples were recorded using
a FESEM Hitachi S-3700M and TEM was obtained using FEI
2
3. Results and discussion
Technai G 20 electron microscope operating at 200 kV. Ther-
mogravimetric experiments were performed using Netzsch STA 3.1. Synthesis, crystal structure and physical properties of
ꢁ
ꢀ1
4
49F3 instrument with heating rate of 10 C min over a range beyerite
ꢁ
ꢁ
of 50 C to 1000 C. Surface area was measured by nitrogen
adsorption–desorption technique with Quantachrome,
Autosorb-1C TCD (Model: ASIC-X-TCD6) analyzer.
2 2 3 2
The formation of the beyerite (CaBi O (CO ) ) phase under
different hydrothermal treatments (Table 1) was closely moni-
tored by recording the PXRD patterns. Initial experiments,
ꢁ
ꢁ
carried out at temperatures between 120 C and 170 C for
a duration of 2–3 days, clearly indicated a monophasic forma-
2
.4. Photocatalytic experiments
ꢁ
Photocatalytic studies were carried out using a 450 W Xenon arc tion at 140 C for three days. The products obtained aer the
lamp (Oriel, Newport, USA) with a water lter to cut down IR reactions at 120 C and at 140 C for two days (CBCS1 and
ꢁ
ꢁ
radiation. Irradiation was carried out over an external pyrex CBCS2) consisted of both beyerite and bismutite. When the
ꢁ
container of volume 250 ml and water circulation was carried reaction was extended for another day at 140 C a pure beyerite
4
out to prevent any decomposition by thermal effect. Light product (CBCS3) was obtained as endorsed by its PXRD pattern
3
8

uxes were established using ferrioxalate actinometer and the (Fig. 2). Subsequent increase in the reaction temperature to 170
ꢁ
incident photon rate for UV-visible irradiation (250 < l < 800
C for three days also resulted in a monophasic beyerite
ꢀ7
ꢀ1
nm) experiments was I ¼ 1 ꢂ 10 einsteins s . The dyes and (CBCS4). The PXRD patterns of the single phase products
0
the organic compounds were dissolved in doubly distilled (CBCS3 and CBCS4) could readily be indexed in orthorhombic
water. All the experiments were carried out at room symmetry (S.G Immm) with the rened lattice parameters of a ¼
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out reactions by modifying the experimental conditions such as
the types of reactants, their concentration, reaction tempera-
ture, and the type of solvent might assist in understanding the
detailed mechanism of beyerite formation. In addition such
experiments might result in beyerite with interesting
morphologies with varying surface related properties and
investigation is currently underway along this direction.
The morphologies of the different beyerite samples were
further investigated by SEM and TEM techniques. The SEM
images of CBCS3 and CBCS4 indicated the existence of beyerite
as uniform rectangular plate like crystallites in the range 50–150
nm with the appearance of thinner plates in the latter (Fig. 4).
The respective TEM images conrmed both the formation of
rectangular plates with a decrease in thickness with increase in
ꢁ
ꢁ
synthesis temperature from 140 C (CBCS3) to 170 C (CBCS4)
(
(
Fig. 4). Irregular thicker plates were obtained for CBCA1
Fig. 4c). A similar feature has been noted for Bi CO
3
Fig. 2 PXRD patterns of (a) CBCS1, (b) CBCS2, (c) CBCS3 and (d)
CBCS4 * shows reflections corresponding to Bi O (CO ).
2 2 3
2
O
2
18
formation using ammonium carbonate solution. BET surface
area for the sample synthesized at 140 C (CBCS3) was 33 m
g
conducted till 1000 C has been presented in Fig. S1 (ESI†). The
sample did not show any loss in weight till 300 C. The weight
loss is occurring in two stages starting around 400 C and
extending up to 800 C. It has been ascribed to the decompo-
sition of CaBi O (CO ) corresponding to the loss of two mole-
2 2 3 2
cules of CO . The observed weight loss of 18 wt% from 400 to
8
ꢁ
2
ꢀ1
. TGA measurement of the as synthesized CaBi O (CO )
2 2 3 2
˚
˚
˚
16
3
.76283(5) A; b ¼ 3.75493(8) A; c ¼ 21.6611(5) A. We explored
ꢁ
the hydrothermal reactions for the formation of beyerite using
ammonium carbonate reactant instead of sodium carbonate in
the similar temperature range. Beyerite sample obtained by
using ammonium carbonate solution at 140 C and 170 C for
three days (CBCA1 and CBCA2) essentially resulted in homo-
geneous phases (Fig. 3). On the other hand, the usage of urea as
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
2
ꢁ
00 C roughly matched to the calculated value.
ꢁ
ꢁ
a carbonate source at 140 C (CBCU1) and at 170 C for three
days (CBCU2) resulted in a mixture of bismutite (Bi O CO ) and
The only structural investigation so far on beyerite in the
2
2
3
literature has been the single crystal X-ray diffraction studies
carried out on a naturally occurring beyerite mineral. So in the
present study, we carried out the structural conrmation of the
2 2 3 2
synthesized bulk polycrystalline material of CaBi O (CO ) by
carrying out the Rietveld renement of the PXRD data obtained
for the sample CBCS3 (Fig. 5). The single crystal structural study
conrmed a centrosymmetric orthorhombic space group
beyerite (CaBi
2
O
2
(CO
3
)
2
). The use of citric acid together with
16
ꢁ
sodium carbonate at 140 C for three days (CBCC1), again
resulted in a mixture consisting more of bismutite and less of
beyerite (Fig. 3e). This particular reaction when repeated at
ꢁ
a higher temperature (170 C) for the same duration did not
result in any solid product formation. Based on these experi-
ments, it is clear that under basic conditions formation of
beyerite is followed aer the formation of bismutite. Carrying
(Immm) in order to get a sensible positioning of the two
carbonate groups in the unit cell (Fig. 1). For the powder
renement, therefore we started with a structural model
comprising of the positional parameters arrived on the basis of
single crystal structural study. The renement converged
successfully and the resulting positional and thermal parame-
ters are listed in Table 2. The crystallographic parameters and
the details of the Rietveld renement results are listed in Table
S1 (ESI†). The structure of beyerite consists of two cationic sites
respectively for bismuth and calcium. The polyhedron around
the bismuth atom is square antiprism with Bi–O1 bond lengths
˚
of 2.134 and 2.435 A and with a longer Bi–O3 bond length of
˚
2
.758 A. The calcium atom also has a regular eight coordination
˚
formed by O2 with a distance of 2.387 A (Table 3). The layering
is formed by Bi–O layers and Ca–O layers together with each one
of the carbonate layer alternately pointing in both up and down
directions along the c axis (Fig. 1). In this aspect the structure is
different from that of the simple bismuth oxycarbonate,
2 2 3
Bi O (CO ) (bismutite), wherein all the carbonate groups have
their apex pointing towards only one direction along the c axis,
thereby resulting in a polar non-centrosymmetric space group
Fig. 3 PXRD patterns of (a) CBCA1, (b) CBCA2, (c) CBCU1, (d) CBCU2
2 2 3
and (e) CBCC1 * shows reflections corresponding to Bi O (CO ).
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Fig. 4 SEM images of samples (a) CBCS3, (c) CBCS4 and (e) CBCA1. The respective TEM images are shown in (b), (d) and (f).
ꢀ
1
(
1
S.G Imm2). Both bismutite and beyerite belong to the sillen vibration (n ) for carbonate groups, while the peak at 687 cm
2ꢀ
39
family of materials.
to the CO
Fig. 7 shows the UV-visible diffuse reectance spectra of
(CO , wherein the absorption
at 1469 cm and at 1072 cm could be attributed respectively edges were found to be in the UV region, well below 380 nm. It
3
4
in plane bending (n ).
The infrared spectrum of the sample was recorded in the
ꢀ1
spectral region between 400 and 4000 cm (Fig. 6a). The peaks various samples of CaBi
2
O
2
3 2
)
ꢀ1
ꢀ1
2
ꢀ
to the antisymmetric and symmetric stretching of CO3 group. is clear that the optical behavior of all samples synthesized
The bending modes of triangular CO group gave rise to the under different conditions seemed to be almost similar with
3
ꢀ
1
band at 865 cm . These observed features are in good accor- small variations in the positions of their absorption edges. The
dance with the reported spectra for (BiO) (OH) CO and band gap energies calculated by plotting the Kubelka–Munk
(CO is presented function with photon energy (eV) were found to be 3.97 eV for
in Fig. 6b. Bands observed at lower wavenumber (120–210 cm
4
2
3
39
Bi
2
O
2
CO
3
.
The Raman spectrum of CaBi
2
O
2
3 2
)
ꢀ
1
)
both CBCS3 and CBCA1 and 3.92 eV for CBCS4 (Fig. 7).
n/2
might arise from the lattice vibrations. Strong band positioned Application of the equation; ahn ¼ A(hn ꢀ E
g
)
(where a, n, A
ꢀ
1
at 1070 cm
was attributed to the symmetric stretching and E
g
are the absorption coefficient, frequency of light,
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2 2 3 2
Fig. 5 Rietveld refinement PXRD pattern of CaBi O (CO ) , experi-
mental data; green line, calculated profile; pink line below, difference
profile; vertical bars, Bragg positions.
Table
CaBi
2
Positional, occupancies and thermal parameters of
)
3 2
2
O
2
(CO
Atoms
Wyck
x/a
y/b
z/c
SOF
U(iso) A˚ 2
Bi1
Ca1
C1
O1
O2
O3
4i
2c
4i
4j
8l
4i
0
0
0.5
0.5
0.5
0.5
0
0
0.5
0
0.191(1)
0.5
ꢀ0.30904(4)
0.5
0.4236(7)
0.2626(5)
0.4408(2)
0.3431(5)
1
1
1
1
1
1
0.0263(5)
0.030(1)
0.009(5)
0.032(2)
0.037(2)
0.049(4)
2 2 3 2
Table 3 Selected bond distances (in A˚ ) for CaBi O (CO )
Fig. 6 (a) FTIR and (b) Raman spectrum of CBCS3.
Atoms
Interatomic distances ( A˚ )
Bi1–O1
Bi1–O1
Bi1–O3
Ca1–O2
C1–O2
C1–O3
2.13390(5) ꢂ 2
2.43512(2) ꢂ 2
2.75838(3) ꢂ 4
2.38652(3) ꢂ 8
1.21880(2) ꢂ 2
1.74400(4)
proportionality constant and band gap respectively) in the
2
present case, the plot of (ahn) vs. hn using the value of 3.97 eV,
resulted in nearly two for n, suggesting beyerite to be an
4
0
indirect semiconductor. The band gap energies are higher as
compared to the value of 3.34 eV for Bi CO3. The possible
2 2
O
reason could be the additional layer of calcium carbonate
present in the beyerite. The valence band formation using O 2p
4
1
and Bi 6s might be similar to that of bismutite, while the
liing up of the conduction band might occur because of the
contribution of Ca 4s states. The optical behavior of beyerite
and its wide band gap suggests the compound to be a potential
photocatalyst under UV light irradiation. So we further
considered the option of fabricating heterostructured photo-
catalyst with BiOI in order to check the improvement in terms
Fig. 7 Diffuse reflectance spectra (DRS) of (a) CBCS3, (b) CBCS4 and
c) CBCA1, inset show the respective plots of Kubelka–Munk function
(
of the extended visible light absorption and enhanced photo- vs. photon energy.
catalytic behavior.
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3
.2. Synthesis, structure and physical properties of BiOI/
Raman spectra of the composite clearly showed the bands of
both beyerite and BiOI (inset in Fig. 8d). Specially the peak at
CaBi (CO
2
O
2
3 2
)
ꢀ1
1
50 cm corresponding to the internal Bi–I stretching of BiOI
is seen, thus conrming that the composite is a mixture of
CaBi (CO and BiOI.
BiOI, a p-type semiconductor belonging to the sillen family is an
established bismuth containing material identied for the
successful formation of heterostructures photocatalyst with
2
O
2
3 2
)
42
43
44
Fig. 9a shows the UV-visible diffuse reectance spectra of the
composite along with that of the pristine beyerite and BiOI for
comparison. The absorption spectra of the BiOI/CaBi O (CO )
2 2 3 2
2 3 6 2 2 3
Bi S , ZnSn(OH) , and Bi O CO . The composite formation
of beyerite with BiOI has been attempted using a weight
percentage of 10/90 as pointed out earlier. The PXRD patterns of
the obtained products (Fig. 8a), showed clearly the diffraction
peaks corresponding to the beyerite and BiOI (PDF# 850863).
SEM image showed the existence of plates similar to those
observed for pure beyerite that were not uniform anymore. The
possibility might be the inclusion of BiOI that has resulted in
bringing the irregularity seen in the SEM image of thickness of
about 50 nm (Fig. 8b). The corresponding TEM image
conrmed the square thinner plates in the nm scale (Fig. 8c). A
surface area of 98 m g has been noted which is higher than
the individual surface areas of beyerite. The infrared spectrum
of the composite was dominant by the features of beyerite such
as the appearance of peaks at 1468 cm and 860 cm corre-
sponding to the carbonate antisymmetrical stretching and
bending modes (Fig. 8d). The stretching vibration of Bi–O
exhibit the mixed absorption properties of both the compo-
nents with its absorption edge clearly shied towards visible
region (near 400 nm) along with the broad enhanced visible
absorption in the region 400–670 nm. The band gap determined
from the plot of the Kubelka–Munk function with photon
energy (eV) (Fig. 9a) was found to be 3.18 eV as compared to the
observed values of 3.97 eV and 1.83 eV for beyerite and BiOI
respectively.
2
ꢀ1
3
.3. Photocatalytic properties of BiOI/CaBi
The photocatalytic activities of the composite BiOI/CaBi
CO sample along with the pure BiOI and CaBi (CO were
evaluated for the degradation of Rh B under UV-visible light (l <
00 nm). Fig. 9b shows the change in absorption spectra of Rh B
2 2 3 2
O (CO )
2 2
O -
ꢀ
1
ꢀ1
(
3
)
2
2
O
2
3 2
)
8
ꢀ
1
45
observed around 489 cm in the pure BiOI that is distinctly
different from beyerite has probably been shied to lower wave
numbers and has not been noticed in the composite. The
solution in the presence of the catalysts. The peak intensity of
Rh B at 560 nm decreased with increasing irradiated time and
disappeared completely aer 2 h. The inset in Fig. 9b shows the
28
2 2 3 2
Fig. 8 (a) PXRD, (b) SEM image, (c) TEM image and (d) FTIR (inset shows Raman spectra) of BiOI/CaBi O (CO ) .
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Fig. 9 (a) UV-visible diffuse reflectance absorption spectra (inset shows plot of Kubelka–Munk function vs. photon energy), (b), (c) temporal
ꢀ5
ꢀ4
changes in absorbance spectra of Rh B dye (10 M) and phenol (10 M), under UV-visible irradiation (inset shows the plot of concentration vs.
time (min)) respectively and (d) recycling experiments of the BiOI/CaBi (CO
2
O
2
3 2
) .
ꢀ
1
variation in C/C
0
, where C is the concentration (mg l ) of Rh B aer the addition of IPA, suggesting that cOH radicals were not
ꢀ
1
at time t and C
0
is the initial concentration (mg l ) with time. playing a signicant role (Fig. 10a). While the rate of photo-
While photolysis in the absence of any catalysts is negligible, degradation showed a marginal decrease for the addition of BQ
+
the activity of pure beyerite has been considerably increased and AO, indicating that the likely species involved were h and
ꢀ
ꢀ
aer the composite formation with BiOI. The degradation cO2 . The generation of the active cOH radicals were also fol-
activity is almost comparable or even slightly better than that of lowed by using terephthalic acid as a probe and estimating the
pure BiOI itself under similar experimental conditions. The amount of 2-hydroxyterephthalic acid formed from its photo-
catalyst was further checked for photocatalytic cyclic efficiency luminescent (PL) emission around 426 nm (for the excitation
and we found that the activity of the catalyst remain unchanged wavelength of 312 nm). The PL intensity again conrmed that
for the decomposition of Rh B up to four cycles under UV-visible cOH radicals are probably not the reactive species and may be
+
light irradiation. The rate of decomposition started showing neglected (Fig. 10b). It is therefore expected that only h and
ꢀ
a decrease during the h cycle (Fig. 9d). Aer ve cycles of cO2 are mainly responsible for the photodegradation process.
decomposition the catalyst was characterized by PXRD and was
similar to the synthesized catalyst before photocatalysis expecting beyerite CaBi
Fig. S2†). The photocatalytic oxidation of dyes occurs through similar to that of Bi CO
, that were formed aer be derived (Fig. 11). Based on the optical absorption behaviour
the light absorption and electron–hole formation by the pho- of the heterostructure, it is clear that BiOI/CaBi (CO could
It is well known that BiOI is a p-type semiconductor and
(CO to be a n-type semiconductor
, a possible reaction mechanism may
2
O
2
3 2
)
(
2
O
2
3
+
ꢀ
2
reactive species such as h , cOH or cO
2
O
2
3 2
)
tocatalyst. In order to understand the nature of the reactive be excited by the irradiation of UV-visible (l < 800 nm) light. The
species, the Rh B dye degradation has been carried out in the electrons formed from BiOI aer light irradiation move towards
presence of different scavengers. For example, benzoquinone the conduction band of CaBi O (CO ) because of the effect of
2
2
3 2
(
BQ), isopropyl alcohol (IPA) and ammonium oxalate (AO) were inner electron eld. These electrons react with the adsorbed O2
ꢀ
+
28
ꢀ
used to study the respective inuence of cO
2
, cOH and h . The to form the cO
2
species. The holes on the other hand move in
concentrations of the added BQ, IPA and AO scavengers were 0.1 to the valence band of BiOI, leading to the oxidation of Rh B and
ꢀ1
ꢀ1
ꢀ1
mmol l , 1.0 mmol l and 1.0 mmol l respectively. The the mentioned steps are shown schematically in Fig. 11. The
photodegradation of Rh B seemed to remain almost unaffected formation of heterostructure with photocatalytic properties is
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Fig. 10 (a) Effects of different scavengers on the degradation of Rh B dye in the presence of BiOI/CaBi
2
O
2
(CO
3
)
2
and (b) rate of cOH radical
ꢀ4
formation based on PL spectral changes over BiOI/CaBi
irradiation time.
2
O (CO )
2 3 2
using a solution of 3 ꢂ 10 M terephthalic acid (excitation at 312 nm) with
2 2 3 2
Fig. 11 Schematic diagram of depicting the possible reaction mechanism over BiOI/CaBi O (CO ) under UV-visible light irradiation.
therefore made possible as compared to the pristine CaBi O - (Fig. S3†). Here again, the decrease in the intensity of Rh B was
2
2
3 2
(CO ) with limited photocatalytic efficiency. The efficiency of found to occur nearly in 3.5 h duration which is better than that
the photocatalyst was probed further by carrying out the of pure BiOI, and beyerite. Additional experiments are required
decomposition of phenol which has its absorption maximum in order to evaluate the efficiency of the photocatalyst under
only in the UV region (268 nm). The absence of visible absorp- visible light irradiation.
tion in the phenol unlike that seen in the case of Rh B helps to
evaluate the photocatalytic nature by excluding the decompo-
4
. Conclusions
sition via photosensitization process. Photolysis of phenolic
solution has been found to be negligible and the composite
degraded phenol in almost 5 h under UV-visible irradiation
The calcium bismuth oxy carbonate known as the mineral
beyerite (CaBi (CO ) has been successfully synthesized by
2
O
2
3 2
)
(Fig. 9c). The independent decomposition of phenol by BiOI/
a simple hydrothermal process. The structure of this material
has been conrmed by carrying out the Rietveld renement of
the PXRD measurements. The morphology of the crystallites
under varying synthesis conditions as monitored from their
SEM and TEM pictures together with the observed band gap of
CaBi (CO despite being moderate, conrms the catalyst's
2
O
2
3 2
)
photocatalytic ability. Efficient degradation activity has also
been noted when Rh B solution was irradiated with visible light
(
400 < l < 800 nm) in the presence of BiOI/CaBi O (CO )
2 2 3 2
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3.97 eV from its optical characterization showed beyerite to be 10 (a) T. Saison, P. Gras, N. Chemin, C. Chaneac, O. Durupthy,
a promising wide gap semiconducting material to develop
photocatalysts and an ideal candidate for constructing new
heterostructures. The formation of heterostructures has been
V. Brezova, C. Colbeau-Justin and J.-P. Jolivet, J. Phys. Chem.
C, 2013, 117, 22656; (b) L. Zhou, M. Yu, J. Yang, Y. Wang and
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shown by using a p-type semiconductor BiOI (E
the composite corresponding to 10/90 of weight ratio of BiOI/
CaBi (CO has been characterized by several techniques. 12 Y. Zheng, F. Duan, M. Chen and Y. Xie, J. Mol. Catal. A:
The efficiency of the photocatalyst has been shown by carrying Chem., 2010, 317, 34.
out successful photodegradation of aqueous solutions of Rh B 13 Y. Wu, B. Yuan, M. Li, W.-H. Zhang, Y. Liu and C. Li, Chem.
g
¼ 1.83 eV) and 11 Y. Zhang, G. Li, H. Zhao, F. Tian, F. Xiao and R. Chen,
CrystEngComm, 2013, 15, 8159.
2
O
2
3 2
)
and phenol under UV-visible irradiation. Photodegradation
Sci., 2015, 6, 1873.
ꢀ
experiments carried out to study the inuence of cO , cOH and 14 Y. Chen, M. Wen and Q. Wu, CrystEngComm, 2011, 13, 3035.
2
+
h using respective quenching reagents such as benzoquinone 15 J. Di, J. Xia, Y. Ge, L. Xu, H. Xu, M. He, Q. Zhanga and H. Li, J.
(BQ), isopropyl alcohol (IPA) and ammonium oxalate (AO)
Mater. Chem. A, 2014, 2, 15864.
suggested the likelihood of peroxide and hole being responsible 16 J. D. Grice, Can. Mineral., 2002, 40, 693.
species for the process as compared to hydroxyl radicals.
17 (a) R. Chen, M. H. So, J. Yang, F. Deng, C. M. Che and
H. Z. Sun, Chem. Commun., 2006, 2265; (b) G. G. Briand
and N. Burford, Chem. Rev., 1999, 99(9), 2601.
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Acknowledgements
1
This work was supported by the DST (SR/S1/PC-07/2011), DST
(SB/S1/PC-08/2012), DU-DST Purse Grant phase-II, DST – 19 Y. Liu, Z. Wang, B. Huang, K. Yang, X. Zhang, X. Qin and
Nanomission Government of India, and University of Delhi
Y. Dai, Appl. Surf. Sci., 2010, 257, 172.
under the “Scheme to Strengthen R&D Doctoral Research 20 G. Cheng, H. Yang, K. Rong, Z. Lu, X. Yu and R. Chen, J. Solid
Program”. We thank the USIC for the use of facilities. We also State Chem., 2010, 183, 1878.
gratefully acknowledge Prof. R. Nagarajan, Department of 21 P. Madhusudan, J. Zhang, B. Cheng and G. Liu,
Chemistry, University of Delhi for constant support. VM and MP
thank CSIR for SPMF and SRF fellowship respectively.
CrystEngComm, 2013, 15, 231.
22 H. Huang, J. Wang, F. Dong, Y. Guo, N. Tian, Y. Zhang and
T. Zhang, Cryst. Growth Des., 2015, 15, 534.
2
3 Y.-S. Xu, Z.-J. Zhang and W.-D. Zhang, Dalton Trans., 2014,
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