Solution combustion synthesis of γ(L)-Bi2MoO6 and photocatalytic activity under solar radiation

Article abstract of DOI:10.1016/j.materresbull.2011.03.035

The facile method of solution combustion was used to synthesize γ(L)-Bi₂MoO₆. The material was crystallized in a purely crystalline orthorhombic phase with sizes varying from 300 to 500 nm. Because the band gap was 2.51 eV, the degradation of wide variety of cationic and anionic dyes was investigated under solar radiation. Despite the low surface area (<1 m²/g) of the synthesized material, γ(L)-Bi ₂MoO₆ showed high photocatalytic activity under solar radiation due to its electronic and morphological properties.

Full text of DOI:10.1016/j.materresbull.2011.03.035

Materials Research Bulletin 46 (2011) 1252–1256
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Solution combustion synthesis of
solar radiation
g(L)-Bi2MoO6 and photocatalytic activity under
*
Dipankar Saha, Giridhar Madras, T.N. Guru Row
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
A R T I C L E I N F O
A B S T R A C T
Article history:
The facile method of solution combustion was used to synthesize
g(L)-Bi2MoO6. The material was
Received 15 November 2010
Received in revised form 26 March 2011
Accepted 31 March 2011
Available online 8 April 2011
crystallized in a purely crystalline orthorhombic phase with sizes varying from 300 to 500 nm. Because
the band gap was 2.51 eV, the degradation of wide variety of cationic and anionic dyes was investigated
under solar radiation. Despite the low surface area (<1 m2/g) of the synthesized material,
g(L)-Bi2MoO6
showed high photocatalytic activity under solar radiation due to its electronic and morphological
properties.
Keywords:
ß 2011 Elsevier Ltd. All rights reserved.
A. Oxides
B. Chemical synthesis
C. X-ray diffraction
D. Catalytic properties
1. Introduction
photocatalyst [26]. g(L)-Bi2MoO6 is an Aurivillius phase perovskite
and one of the Bi3+ oxides that show photocatalytic activity under
solar radiation.
The development of new catalysts for the treatment of
hazardous wastes like dyes, halogenated hydrocarbons from waste
effluents, organic solvents, chlorinated volatile organics, is an
active research area. The application of semiconductor photo-
catalysis is being explored as a supplementary and complementary
methodology to the conventional approaches. Several factors like
band gap energies, features associated with cation vacancy, oxygen
deficiency and surface area influence the performance of materials
used in photocatalytic degradation [1–3]. TiO2 (Degussa P-25) is
one of the most widely used semiconductor photocatalysts for
environmental applications. TiO2 has band gap of 3.2 eV and,
therefore, requires UV radiation to show photocatalytic activity.
Because solar radiation encompasses only ꢀ4% UV radiation of its
total spectrum, developing new catalysts that are active under
solar radiation is a foremost area of research in photocatalysis.
Several studies [4–15] have been attempted to synthesize narrow
band gap TiO2.
The low temperature phase,
noncentrosymmetric space group Pca21, which consists of perov-
2+ (for n = 1, M = Mo and
X = O), intercalated between Bi2O22+ layer [27]. Beyond 685 8C, the
low temperature orthorhombic form transforms to a monoclinic
phase, which crystallizes in a centrosymmetric space group P21/c
g(L)-Bi2MoO6, crystallizes in a
skite layers having formula (Binꢁ1MnX3n+1
)
˚
˚
with
cell
parameters
a = 17.2627(1)A,
b = 22.4296(1)A,
c = 5.5848(1) and
b
= 90.4974(6) [28,29]. Various synthetic pro-
cedures such as co-precipitation [30], hydrothermal [31,32], hard
template method, citrate complex method [26], solvothermal [33],
microwave solvothermal route [34,35], low temperature molten
salt method [36], amorphous precursor method [37] and ultrasonic
assisted method [38] have been used to synthesize
g(L)-Bi2MoO6.
Moreover, substituted (L)-Bi2MoO6 (Bi2MoxW1ꢁxO6) has also
g
been synthesized and tested for photocatalytic activity [39] and
the effect of Mo substitution by W on the photocatalytic activity
has been studied.
Apart from titania, there are other photocatalysts like BiVO4,
AgVOx, CaBi2O4, InMO4 (M = V, Nb, Ta), Bi2O3, Bi2S3, Bi12TiO20
,
Recently, solution combustion technique, as summarized in
reviews and books [40–42], has been extensively used for
synthesis of a wide variety of materials. This method can be
used to synthesize materials with different morphologies and
particle sizes. Bi2WO6 [43] and BiVO4 [44] were synthesized by the
solution combustion technique and these materials were shown
to have higher activity than the parent compounds that were
synthesized by other synthetic procedures. Furthermore,
this synthetic procedure is simple and effective in obtaining
crystalline materials.
Bi2Ti2O7, Bi2InNbO7, Bi2WO6, Ln0.95Mo0.15V0.85O4 (Ln = Ce, Pr, Nd)
that exhibit photocatalytic activity under solar radiation [16–25].
Bi3+ containing oxides show special characteristics due the
presence of s2 configuration which in turn combine with oxygen
2p orbital to decrease the band gap of the semiconductor
* Corresponding author. Tel.: +91 80 2292796; fax: +91 80 3601310.
E-mail address: ssctng@sscu.iisc.ernet.in (T.N. Guru Row).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.03.035

D. Saha et al. / Materials Research Bulletin 46 (2011) 1252–1256
1253
In this article, we report the synthesis of
g
(L)-Bi2MoO6 via a
35). The calibrations for MB, RB, MG, MV, AG, CR, RBBR and OG
were based on the Beer–Lambert law at its maximum absorption
facile solution combustion method and its solar photocatalytic
activity for the degradation of different classes of both anionic and
cationic dyes.
wavelength,
respectively.
lmax, 664, 554, 617, 584, 648, 497, 590 and 584 nm,
2. Experimental
3. Result and discussions
2.1. Synthesis of g(L)-Bi2MoO6
3.1. Synthesis and characterizations
g
(L)-Bi2MoO6 was synthesized by the solution combustion
method using tartaric acid (S.D. Fine-Chem Ltd., India, 99%) as a
fuel. (Sigma–Aldrich, 99.9%) and
Bi(NO3)3ꢂ6H2O
g
(L)-Bi2MoO6 was synthesized by solution combustion taking
tartaric acid as a fuel. It is of interest to note that pure phase of the
compound was obtained by using solely tartaric acid as precursor
fuel whereas pure phase could not be obtained taking other fuel
like urea, glycine, hydrazinehydrate, oxalyldihydrate as precursor
fuel. Therefore, it may be inferred that the strength of tartaric acid
(NH4)6Mo7O24ꢂ4H2O (S.D. Fine-Chem Ltd., India, 99%) were taken
as the precursors. Bi(NO3)3ꢂ6H2O was dissolved in 5(M) nitric acid
to make clear solution and (NH4)6Mo7O24ꢂ4H2O was dissolved in
double distilled water. Both were mixed in a combustion petri dish
and stirred with magnetic stirrer. Stoichiometric amount of fuel
was added to the clear solution and the resulting mixture was kept
in a preheated furnace maintained at 500 8C. When tartaric acid
undergoes combustion with precursors, it produces heat that is
absorbed by the precursors accelerating the chemical reaction of
the precursors. After the combustion reaction, the final product
was kept in the furnace maintained at 500 8C for 12 h to obtain
is suitable for the formation of the pure phase of g(L)-Bi2MoO6 as
the strength of fuel plays a potential role in forming the product as
well as on the activity of the compound [41]. Pale-yellow colored
product was obtained from the synthesis that was used for further
analysis and degradation experiments.
Powder diffraction data show the formation of single phase
orthorhombic
g(L)-Bi2MoO6. Fig. 1 shows the profile refinement
details of (L)-Bi2MoO6. The profile fit was achieved using
g
pure
g(L)-Bi2MoO6. This resulting yellow product was used for
JANA2000 [45] taking Pseudo-Voigt as peak shape function and
further analysis and degradation experiments. The reaction is
given by
Legendre polynomial as background function (number of
terms = 15).
g(L)-Bi2MoO6 crystallizes in space group Pca21. The
refined cell parameters are a = 5.484(1), b = 16.206(1), c = 5.505(1)
and the fitting parameters are Rp = 4.73, Rwp = 7.49. The surface
area of the materials was less than 1 m2/g, as determined from BET
analysis and the particle sizes (Fig. 2) were found in range of 300–
2BiðNOÞ3ꢂ6H2O þ ð1=7Þ ðNH4Þ6Mo7O24 þ ð2:74Þ ðC4H4O6Þ
! Bi2MoO6 þ 10:96CO2 þ 6:07N2 þ 22:5H2O
500 nm. Fig. 3 displays the diffuse reflectance spectra of
g(L)-
2.2. Sample characterization
Bi2MoO6. The steep shape of the spectra showed that the visible-
light absorption was due to the band-gap transition. The accurate
band gap was calculated from the Tauc plot [46,47] (Fig. 3 inset).
The energy of the band gap was estimated to be 2.51 eV, similar to
that observed by other investigators [26,34].
Morphology analysis of
g(L)-Bi2MoO6 sample was carried out
with the help of FEI Quanta scanning electron microscope. UV–vis
diffuse reflectance spectra were recorded on a Perkin Elmer
Lambda 35 UV–vis spectrophotometer. BET analysis was carried
out using a porosimeter (Belsorp, Japan). After the phase purity of
the obtained samples was confirmed by conventional X-ray
powder diffraction, Rietveld quality data were collected using
3.2. Photocatalysis
The photocatalytic behavior of g(L)-Bi2MoO6 was studied under
the Philips X-pert diffractometer with Cu K
a radiation over the
solar radiation by degrading anionic and cationic dyes. Dye
solutions were stirred in presence of the catalyst for one hour prior
to the solar radiation exposure to monitor the changes in dye
concentration due to adsorption. No significant adsorption (<5%)
was observed. Experiments were also carried out in absence of
angular range of 5 ꢃ 2 ꢄ 100, with a step width of 0.017.
u
2.3. Photodegradation experiments
The degradation of all dyes in presence the compounds
synthesized in this study was carried out simultaneously to
ensure similar exposure to solar radiation. All the experiments
were conducted between 10 a.m. and 2.30 p.m. when the solar
intensity fluctuations were minimal. The average solar intensity in
a day between 10 a.m. and 3.30 p.m. was 0.753 kW mꢁ2. Both
anionic and cationic dyes were chosen for the study. Methylene
blue (MB), Rhodamine B (RB), Malachite green (MG) and Methyl
violet (MV) (all from S.D. Fine-Chem Ltd., India) were chosen as
cationic dyes whereas Alizarine green (AG), Congo red (CR)
Ramazoline brilliant blue (RBBR) and Orange G (OG) (all from S.D.
Fine-Chem Ltd., India) were chosen as anionic dyes. The initial
concentration of all the dyes were 10 ppm except for OG (50 ppm)
and RBBR (25 ppm) and catalyst loading was 1 g/L. The dyes were
dissolved in double distilled Millipore filtered water and taken in
250 ml cylindrical borosilicate glass beaker (diameter 6 cm, height
9 cm) covered with watch glass. The solutions were stirred
continuously and exposed to solar radiation. The supernatant
solutions were collected at regular intervals and centrifuged for
30 min prior to the analysis. The concentration of solutions was
determined by UV–vis spectrophotometer (Perkin Elmer Lambda
Fig. 1. Full pattern matching of the powder pattern of g(L)-Bi2MoO6.

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D. Saha et al. / Materials Research Bulletin 46 (2011) 1252–1256
Fig. 2. SEM images of combustion synthesized g(L)-Bi2MoO6.
catalyst (photolysis). Less than 5% degradation was observed
during photolysis, as shown in Figs. 4 and 5.
is primarily a bulk phenomena [30]. The photocatalytic degrada-
tion of RB by Bi2MoO6 nanoparticles [36,37] can occur due to
photocatalysis wherein the irradiation promotes an electron from
the valence band to the conduction band to form the pair electron–
hole or through photosensitization where the irradiation excites an
electron from dye and then it is injected to the conduction band of
the semiconductor oxide. The photocatalytic degradation of RB by
Bi2MoO6 has been established [37] to occur due to the
photosensitization of dye.
Fig. 4 shows the degradation profile of RB in the presence of
g
(L)-Bi2MoO6. Complete degradation of RB was observed after
300 min of solar irradiation, while 80% degradation was observed
at the end of 2 h of solar exposure. Total organic carbon (TOC) for
the degradation of RB at the end of 5 h was measured using the
Shimadzu V-CSN TOC analyser. While >99% decolorization was
observed, more than 65% of TOC was removed. This is consistent
with the observation of Tian et al. [33]. The degradation of RB,
observed in this study, with combustion synthesized Bi2MoO6 can
be compared with the degradation of RB in the presence of
Bi2MoO6 synthesized by other techniques. For example, the
degradation of RB after solar exposure to 500 W Xe lamp for 2 h
was 55% in the presence of solid state synthesized Bi2MoO6 and
78% and 95% in the presence of uncalcined and calcined Bi2MoO6
hollow spheres [33]. A detailed discussion on the photocatalytic
activity for the degradation of RB in the presence of Bi2MoO6,
synthesized by various techniques can be found elsewhere [30].
As the concentration of RB decreases upon irradiation, the
absorption maxima of RB showed a shift from 554 nm to ꢀ500 nm
with increase in irradiation time. The hypsochromic shift can be
attributed to N-de-ethylation of RB [48]. The blue shift is due to the
competition between the de-alkylation step, which is a surface
mediated process and the degradation of the chromophore, which
Fig. 4 shows the degradation profiles for the degradation of
cationic dyes like heteropolyaromatic (MB and MV), triaryl-
methane (MG) in the presence of
g(L)-Bi2MoO6. Upon exposure
to 5 h of solar radiation, complete degradation was observed in
case of MG while degradation was almost 80% and 50% in case of
MB and MV, respectively. Therefore, from the degradation profile
we can conclude that the degradation rate follows the trend
RB > MG > MB > MV. Similarly, the blue shift has also been
observed in case of other cationic dyes. The shift was from
664 nm to ꢀ634 nm for MB which can be attributed to N-
dealkylation of methylene blue, as reported earlier [49]. In case of
MG and MV, the shift was from 617 nm to ꢀ608 nm and from
584 nm to ꢀ566 nm, respectively. The shifting of absorption
maxima in case of MG and MV may be due to N-de-methylation
Fig. 4. Degradation profile for cationic dyes in the presence of g(L)-Bi2MoO6 under
Fig. 3. DR–UV–vis spectrum and Tauc plot of
g(L)-Bi2MoO6.
solar radiation.

D. Saha et al. / Materials Research Bulletin 46 (2011) 1252–1256
1255
Fig. 5. Degradation profile for anionic dyes in the presence of g(L)-Bi2MoO6 under
solar radiation.
Fig. 6. Degradation profile for different initial concentrations of MB. The inset shows
the variation of the initial rate with the initial dye concentration.
[50]. The degradation of these dyes is likely to occur due to the de-
methylation and N-de-ethylation [51].
the conduction band derived from the primary Mo 4d orbitals in
MoO6 octahedra and the secondary Bi 6p orbitals [31].
Fig.
monoazo-sulphonated (OG), anthroquinonic (AG and RBBR) and
diazo-sulphonated (CR) in presence of (L)-Bi2MoO6. Unlike
5
depicts the degradation profile for anionic dyes
The photocatalytic activity of g(L)-Bi2MoO6 can be attributed to
the presence of the Bi–O polyhedra and MoO6 octahedra in the
Bi2MoO6 crystal structure. The Bi–O polyhedra in Bi2MoO6 serve as
the active donor sites, which enhances the electron transfer to
oxygen and eliminates the recombination of electron–hole pairs
[36]. Oxide like Bi2O3Bi12TiO20 similarly plays a role of active
oxygen donors in the photo-oxidation process [36]. Furthermore,
DFT calculation [53] reveals that the MoO6 octahedra contribute to
g
cationic dyes, there was no significant change in absorption
maxima in case of anionic dyes. There was no degradation in case
of OG whereas AG was degraded up to 20% only after 300 min of
solar radiation. The absence of degradation of monoazo dyes is
similar to the observation by others [29], who observed no
degradation of another monoazo dye, methyl orange. But
degradation was almost 80% in case of both RBBR and CR.
Therefore, from the degradation profile it can be concluded that
the rate follows the order, CR > RBBR > AG > OG. The order of
degradation based on the class of the dye is therefore,
diazo > anthraquinonic > monoazo and is consistent with the
order observed with TiO2 [51].
the formation of top part of the valence band. Thus, under visible
ꢅ
light irradiation on the catalyst,
g
(L)-Bi2MoO6 generates OH and
O2ꢁ radical on the surface [31] leading to the formation of oxygen
radicals which enhance the degradation.
4. Conclusions
In order to determine the kinetics of degradation, the
degradation of MB was carried out with four different initial
concentrations. For a first order reaction, the rate of reaction is
given by, r = k C. Similar first order kinetics has been observed for
the degradation of RB in the presence of Bi2MoO6 synthesized by a
polymeric precursor technique [26] and for the degradation of MB
[39] in the presence of Bi2MoO6 nanosheet. The initial rate (r0) for
the degradation for 10, 20, 40 and 60 ppm heteropolyaromatic dye
g
(L)-Bi2MoO6 was prepared via solution combustion method
using tartaric acid as the precursor fuel. The material was
crystalline orthorhombic phase. The particle sizes were in the
range 300–500 nm and had a band gap of 2.51 eV. The degradation
of wide variety of cationic and anionic dyes was investigated under
solar radiation. Despite the low surface area (<1 m2/g),
Bi2MoO6 showed better photocatalytic activity under solar
radiation due to its electronic and morphological properties.
g(L)-
(MB) by
g(L)-Bi2MoO6 was calculated from the change in the
concentration, that is, from (C0 ꢁ Ct)/t, where C0 is initial
concentration of the dye and Ct is the concentration of the dye
at time t where t was taken as 60 min. The rate constant k was
calculated by plotting the variation of the initial rate with initial
concentrations (Fig. 6). This indicates that the order of degradation
is unity and the overall kinetic rate constant, k, for the degradation
Acknowledgements
We acknowledge the funding from DST, India, and financial
support for the XRD machine from the DST-FIST program. Author
thanks Mr. Jarali for BET measurement.
of methylene blue in the presence of
g(L)-Bi2MoO6 was
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