Effects of rare earth substitution on the optical properties of Bi2MoO6 for coloring applications

Article abstract of DOI:10.1016/j.materresbull.2015.04.025

A new class of colored inorganic compounds, BiREMoO₆ (RE = Pr, Nd, Sm, Tb and Yb) has been synthesized by a solid state route. The substitution of different rare earths for Bi³⁺ in Bi₂MoO₆ produces visible light responsive compounds by gently red shifting the absorption edge to low energy side. The visible light absorption is based on the charge transfer transitions from O_2p valence band to conduction band made of primary Mo_4d and secondary Bi_6p. The substitution of RE³⁺ introduces partially occupied 4f electronic levels in between the band gap and the position of 4f level depend on the number of f electrons which allows tuning the band gap. Thus the rare earth substitution provides an opportunity to tailor the band gap of Bi₂MoO₆ from 2.99 eV to 2.19 eV. The developed compounds exhibited different shades of yellow hue and demonstrated good coloration to plastics.

Full text of DOI:10.1016/j.materresbull.2015.04.025

Materials Research Bulletin 70 (2015) 93–98
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Effects of rare earth substitution on the optical properties of Bi MoO
2
6
for coloring applications
a
b,
b
b
b
L. Sandhya Kumari , P. Prabhakar Rao *, S. Sameera , Vineetha James , Peter Koshy
a
Department of Optoelectronics, University of Kerala, Thiruvananthapuram 695 581, India
Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695 019, India
b
A R T I C L E I N F O
A B S T R A C T
A new class of colored inorganic compounds, BiREMoO (RE = Pr, Nd, Sm, Tb and Yb) has been synthesized
Article history:
Received 15 July 2013
Received in revised form 11 March 2015
Accepted 12 April 2015
Available online 14 April 2015
6
3+
2 6
by a solid state route. The substitution of different rare earths for Bi in Bi MoO produces visible light
responsive compounds by gently red shifting the absorption edge to low energy side. The visible light
absorption is based on the charge transfer transitions from O2p valence band to conduction band made of
primary Mo4d and secondary Bi6p. The substitution of RE3 introduces partially occupied 4f electronic
levels in between the band gap and the position of 4f level depend on the number of f electrons which
allows tuning the band gap. Thus the rare earth substitution provides an opportunity to tailor the band
+
Keywords:
A. Oxides
A. Optical materials
B. Optical properties
gap of Bi
2 6
MoO from 2.99 eV to 2.19 eV. The developed compounds exhibited different shades of yellow
hue and demonstrated good coloration to plastics.
ã 2015 Elsevier Ltd. All rights reserved.
1
. Introduction
gap (2.63 eV) has the ability to harness more sunlight [8]. Visible
light absorption of Bi MoO was revealed to be due to the
transition from the valence band consisting of the O2p orbitals to
2
6
Bismuth molybdates, having the general chemical formula
nMoO , where n = 3, 2, 1, and their metal ion doped analogs
Bi
2
O
3
Á
3
the conduction band primarily derived from Mo4d orbitals in MoO
octahedra and the secondary Bi6p orbitals [6].
6
have attracted much attention for several years because of their
catalytic application in the area of selective oxidation/oxidative
dehydrogenation or ammoxidation of lower olefins [1–4]. Recently,
bismuth molybdates have also been demonstrated to exhibit
visible-light driven photocatalytic activity [5,6]. Many studies
about the effects of the crystallinity, size, and morphology on the
Oxo salts of bismuth (III) exhibit interesting properties as ionic
conductors, yellow pigments and selective oxidation catalyst
[9–11]. Pigments on the base of Bi O seem to be interesting,
2 3
because they provide interesting color hues from yellow to orange
[12]. Intense colors of these pigments are based on the incorpo-
photocatalytic property of Bi
2
MoO
6
synthesized by the hydrother-
ration of doped Ln (lanthanide) ions into the host lattice of Bi
2 3
O
mal process have been carried out [7]. Bismuth mixed oxides with
[12–16]. The mixed pigment 4BiVO MoO exhibit brilliant
4
 3Bi
2
6
2+
the Aurivillius structure are represented by (Bi
2
O
2
)
(AnÀ1
6
B
n
O
3
n
greenish yellow to reddish yellow shades [17]. To exploit materials
for optoelectronic applications, it is essential to tune the optical
band gap which can be done by way of doping with various metal
ions. Band gap tailoring at the ultraviolet end and visible region of
the solar spectrum is of considerable interest for large area optical
coatings. Generally, the band gap transition is mainly related to
M-d (metal d orbitals) and O-p (oxygen p orbitals) states, but the
localized character of RE 4f levels would make contribution to tune
the band gap. Zou et al. have reported the modification of band
2À
+
1
)
(A = Ba, Bi, Pb; B = Ti, Nb, W, Mo). The
g-Bi
2
MoO with the
Aurivillius structure has been found to possess unique layered
structure in which perovskite slabs of corner-sharing, distorted
2+
MoO
Shimodaira et al. have reported the photocatalytic activity of
Bi MoO and found that among the various bismuth molybdates,
AS-Bi MoO (AS-Aurivillius) with a corner sharing structure of
MoO octahedra possessed low band gap influenced by the
octahedral coordination of oxygen ions to a molybdenum ion [6].
Compared with Bi WO (2.77 eV), Bi MoO with a narrower band
6
octahedra are sandwiched between (Bi
2
O
2
)
layers [6].
2
6
2
6
6
3
+
structure by controlling the ionic radius (r
R 2 7
) in Bi RNbO
2
6
2
6
photocatalysts and the dependence of band gap on the rare earth
f bands have been well explained [18]. Optical and electronic
properties of a series of novel rare earth bismuth tungstate
compounds have been recently reported [19]. Liu et al. have
reported formation of solid solution as the feasible method to
*
Corresponding author. Tel.: + 91 471 2515311; fax: + 91 471 2491712.
E-mail address: padala_rao@yahoo.com (P. Prabhakar Rao).
http://dx.doi.org/10.1016/j.materresbull.2015.04.025
025-5408/ã 2015 Elsevier Ltd. All rights reserved.
0

9
4
L.S. Kumari et al. / Materials Research Bulletin 70 (2015) 93–98
adjust the conduction band and valence band to obtain a visible
light driven photocatalyst by synthesizing BiYWO [20]. The rare
obtaining a polished surface. The color coordinates of the
pigmented compacts were examined to assess the coloration.
6
earths, Yb and Er have been used in the fiber for realizing optical
switching to reduce the switching power [21,22]. A low band gap
3. Results and discussion
2 6
material like Bi MoO offers wide scope of tuning the band gap by
substituting different metal ions to produce various shades. The
strongly localized f shell in rare earth metals which determines the
similar chemical and physical properties of the lanthanides can
influence the band structure of bismuth molybdate. The band gap
change depends on the number of f electrons of the substituted
rare earth ion. In the present study, new series of rare earth based
3.1. Structural studies
The powder XRD patterns of Bi
Nd, Sm, Tb and Yb) are presented in Fig. 1. Bi
with a monoclinic phase with the space group P21/c and all the
reflections are indexed as per the JCPDS data base number 00-033-
2
MoO
6
and BiREMoO
6
(RE = Pr,
2
MoO crystallizes
6
compounds, BiREMoO
prepared. The influence of RE f bands on the optical property of
Bi MoO has been investigated through optical absorption studies
and band structure analysis. Their color properties, as possible
potential yellow inorganic pigments have been investigated for
coloration of plastics.
6
(RE = Pr, Nd, Sm, Tb and Yb) have been
0208. For BiREMoO
a monoclinic BiREMoO
depending on the rare earth element. Crystallographic analysis
reveals that the BiTbMoO and BiYbMoO compounds belong to
the monoclinic system with space group P2/c. Both BiPrMoO and
BiNdMoO crystallize with monoclinic crystal lattice (space group
6
, all the diffraction peaks could be attributed to
6
type structure with different space groups
2
6
6
6
6
6
C2/c). The formation of monoclinic BiSmMoO
can be judged from the diffraction pattern.
6
(space group C2)
2. Experimental
2.1. Materials and methodology
3.2. Morphological analysis
Compositions based on BiREMoO
6
(RE = Y, Pr, Sm, Nd, Tb and Yb)
, Pr
(99.9%) supplied by M/s
The primary particles of the pure Bi
of relatively elongated rod shapes, in which the average length was
25 m and were agglomerated. SEM analysis (Fig. 2) of Bi MoO
and BiREMoO (RE = Pr, Nd, Sm, Tb and Yb) reveals the changes in
the morphology from long rod type to small irregular shaped
particles. Rare earth substitution in Bi MoO is found to decrease
2 6
MoO exhibited a mixture
were prepared from the corresponding oxides: Bi
Sm , Nd , Tb , Yb , and MoO
2
O
3
6 11
O ,
2
O
3
2
O
3
4
O
7
2
O
3
3
m
2
6
Sigma–Aldrich. Stoichiometric proportions of the chemicals were
weighed and were thoroughly wet mixed in an agate mortar with
acetone as the wetting medium for 1 h and dried in an air oven.
6
2
6
This process of mixing and drying was repeated thrice to obtain a
the particle size. As seen from the morphological analysis, it can be
concluded that the substitution of smaller rare earth elements in
Bi MoO significantly influences the crystal growth and prefers
2 6
morphology entirely different from the rod shaped structure of the
host material. Even though all the compounds crystallize in the
2 6 6
monoclinic crystal system, Bi MoO and BiREMoO fall under
different space groups depending on their lattice type. This might
have caused the change in morphology with the substitution of
rare earth ions. The effective solid solution formation was checked
by energy dispersive spectrophotometer (EDS) analysis attached
with TEM. Fig. 3 shows the EDS analysis of selected samples
ꢀ
homogeneous mixture. Bi
2
MoO
6
was calcined at 1100 C for 6 h,
ꢀ
BiPrMoO
6
was calcined at 950 C for 9 h and BiREMoO
6
(RE = Nd, Tb
ꢀ
and Yb) were calcined at 1000 C in air. The calcination process was
repeated thrice for the same sample. To refine and homogenize the
particle size after calcinations, the resulting products were ground
in an agate mortar.
2.2. Characterization techniques
The crystalline nature and phase purity of the samples were
investigated using powder X-ray diffractometer with Ni-filtered
Cu-K (K = 1.54060 Å, K = 1.54443 Å), radiation using a PAN-
alytical Philips X’pert Pro diffractometer. Data was collected over a
6 6 6
BiNdMoO , BiTbMoO and BiYbMoO and identifies the presence of
a
a
1
a
2
all the expected elements. The stoichiometric formula calculated
from the semiquantitative EDS results are in close agreement with
the theoretical formulae. This also further confirms the homoge-
neity of the phase formed.
ꢀ
ꢀ
2
u range from 10 to 90 . The morphology of the synthesized
samples was recorded on a scanning electron microscope JEOL
JSM-5600 model, with an acceleration voltage of 15 kV. The diffuse
reflectance of the powdered pigment samples were measured
(
200–780 nm) with a UV–vis Spectrophotometer (Shimadzu
UV-2450 with an integrating sphere attachment, ISR-2200) using
barium sulphate as the reference. The color coordinates were
determined by coupling analytical software (UVPC Color Analysis
Personal Spectroscopy Software V3, Shimadzu) to the UV-2450
spectrophotometer. The CIE 1976 L*a*b* colorimetric method was
used, as recommended by the Commission Internationale de
l'Eclairage (CIE).
Among the pigment compositions prepared, the typical
pigment sample BiTbMoO
like PMMA to test their coloring performance. The pigment,
BiTbMoO (10 wt%) was ultrasonicated in an alcohol–water (1:4)
6
was dispersed in polymer matrix
6
mixture for 10 min to ensure complete dispersion of the
pigment particles. A viscous solution of PMMA (90 wt%) was
made using a conventional electrical coil heater. The pigment
dispersion was slowly added with stirring and converted to a
thick paste. The paste after 2 h curing was compressed
uniaxially into a form of cylindrical disk using a hydraulic
press at a pressure of 25 MPa. Both sides of the pigmented
polymer were lapped using a fine grade emery sheet for
Fig.1. Powder X-ray diffraction patterns of Bi
Tb and Yb) compounds. S.G refers to space group. For BiREMoO
peaks could be attributed to a monoclinic BiREMoO type structure with different
space groups depending on the rare earth element.
2
MoO
6
and BiREMoO
6
(RE = Pr, Nd, Sm,
6
, all the diffraction
6

L.S. Kumari et al. / Materials Research Bulletin 70 (2015) 93–98
95
Fig. 2. SEM photographs of Bi
2
MoO
6
and BiREMoO
6
(RE = Pr, Nd, Sm, Tb and Yb). Substitution of different rare earth metal ions in Bi
2
MoO
6
changes the morphology of the
particles from long rod type particles to small irregular shaped particles.
3
.3. Optical properties
around 450 nm, with characteristic absorption bands at about 523,
87, 682 and 750 nm for BiNdMoO . The spectra of BiPrMoO show
several well-resolved bands attributed to the f–f transitions of the
5
6
6
The colors of Bi MoO and BiREMoO are presented in Fig. 4.
2
6 6
3
+
Bi
Bi
2
MoO
MoO
6
exhibits a pale yellow color. Tb and Yb substitution in
leads to yellow color whereas Pr substitution produces
Pr ions. BiPrMoO
blue and red region of the visible spectrum. More red shift was
observed for BiYbMoO and BiTbMoO with steep absorption edge
6
displays greenish yellow hue as it absorbs in
2
6
greenish yellow color. Light green and light yellow colors were
obtained for Nd and Sm substituted Bi MoO , respectively. Fig. 5
shows the combined absorption spectra of Bi MoO and BiREMoO
RE = Pr, Nd, Sm, Tb and Yb). For oxides containing RE , there is
6
6
2
6
at ꢁ500 nm. Tb and Yb substituted bismuth molybdates absorb
2
6
6
blue light efficiently and the compounds exhibit yellow hue. The
3+
3+
(
band gap energy is decreased with the substitution of RE in
strong optical absorption in the near-UV region based on O2p–Mo4d
charge transfer transition. The absorption edge typically extends
into the visible region to give various color shades with different
Bi
2
MoO
6
from 2.99 to 2.19 eV (Table 1).
The reflectance and absorbance of a material can be affected by
a combination of factors such as mean particle size, particle size
distribution, particle shape, particle porosity, packing density,
surface texture, its chemical composition, and wavelength. Rare
rare earths. The optical absorption edge critically depends on the
rare earth element (RE3 ) present in the samples. Bi
+
MoO
absorbs
2
6
below 400 nm with a sharp edge at ꢁ400 nm. Rare earth
2 6
earth substitution in Bi MoO is found to decrease the particle size
and change the morphology from long rod type to small irregular
shaped particles. Even though the particle morphology changes
substitution significantly shifts the absorption edge to low energy
6 6
side. BiSmMoO and BiNdMoO show a sharp absorption edge at
6
Fig. 3. EDS analysis of BiREMoO (RE = Nd, Tb and Yb) pigments and identifies the presence of all the expected elements.

9
6
L.S. Kumari et al. / Materials Research Bulletin 70 (2015) 93–98
Table 1
Color co-ordinates and band gap of Bi
Yb). The band gap decreases with the incorporation of RE into the Bi
2 6 6
MoO and BiREMoO (RE = Pr, Nd, Sm, Tb and
3
+
2
MoO
6
matrix.
Sample
Color co-ordinates
g
E (eV)
ꢀ
L*
a*
b*
C*
h
Bi
BiPrMoO
BiNdMoO
BiSmMoO
2
MoO
6
93.4
84.3
74.2
91.2
79.1
78.3
À4.3
À12.6
4.7
À7.4
16.0
17.7
11.5
49.1
8.25
39.5
61.6
58.8
12.2
50.7
9.49
40.2
63.3
61.4
69.4
104.4
60.3
100.6
75.4
73.4
2.99
2.47
2.51
2.54
2.19
2.20
6
6
6
BiTbMoO
BiYbMoO
6
6
showed a clear absorption edge at around 400 nm, and the
corresponding band gap energy is 2.99 eV. This can be explained by
the reported band structure of the AS-Bi MoO , where O2p forms
2 6
the valence band and the conduction band is derived from the
Mo4d orbitals and the Bi6p orbitals [6]. The contribution of each
orbital is not equivalent, although the LUMO consisted of hybrid
orbitals of Mo4d and Bi6p orbitals. The Mo4d orbitals mainly
contributed to the LUMO rather than the Bi6p orbital. Visible light
absorption of AS-Bi
from the valence band consisting of the O2p orbitals to the
conduction band derived from the primary Mo4d orbitals in MoO
octahedra and the secondary Bi6p orbitals. The band structure
model of BiREMoO (RE = Pr, Nd, Sm, Tb and Yb) can be illustrated
2 6
MoO was revealed to be due to the transition
Fig. 4. Photographs of Bi
compounds. Tb and Yb substitution in Bi
substitution produces greenish yellow color. Light green and light yellow colors
were obtained for Nd and Sm substituted Bi MoO , respectively. (For interpretation
2
MoO
6
and BiREMoO
6
(RE = Pr, Nd, Sm, Tb and Yb)
2
MoO leads to yellow color whereas Pr
6
6
2
6
6
of the references to color in this figure legend, the reader is referred to the web
version of this article.)
schematically as shown in Fig. 6. As shown in the schematic
diagram, the band gap is therefore the difference in energy
between the top of the O2p band and the bottom of the Mo4d band
with substitution of rare earth metal ion, the expected increase in
reflectance is not observed and instead absorbance is found to
increase in the visible region. This may be attributed to the band
gap engineering in the charge transfer transitions of bismuth
molybdate by the intermediate f orbitals of different rare earth
metal substituent which overrules the effect of particle size effect.
for Bi
substituted bismuth molybdate depends on the partly filled RE
f shell.
Shift of optical absorption edge towards lower energy side is
2 6
MoO . Red shifted absorption edge observed for RE
4
continued from Sm to Pr compounds, suggesting that the filled 4f
levels are positioned between the O2p valence band and Mo4d + Bi6s
g
conduction band. The variation of E can be explained by the
3
6
.4. Band gap engineering in BiREMoO (RE = Pr, Nd, Sm, Tb and Yb)
energy of the RE 4f levels lowering in energy with an increasing
occupation of 4f electrons. Prokofiev et al. have studied periodicity
in the optical band gap variation of Ln sesquioxides [23]. According
The band gap energy of BiREMoO
absorption spectra (Fig. 5) indicates a sudden decrease in band gap
with red shift in the absorption edge of Bi MoO . Pure Bi MoO
6
6
calculated from UV–vis
to their report, the occupied 4f band in Ce
2 3 2 3 2 3
O , Pr O , and Tb O lies
2
6
2
above the O2p band, and thus, 4f–5d transition may determine the
Fig. 6. Band structure models of Bi
Yb). The band gap is therefore the difference between the top of the O2p band and
the bottom of the Mo4d band for Bi MoO . Red shifted absorption edge observed for
2 6 6
MoO and BiREMoO (RE = Pr, Nd, Sm, Tb and
Fig. 5. Absorbance spectra of Bi
The substitution of different rare earths for Bi in Bi
2
MoO
6
and BiREMoO
6
(RE = Pr, Nd, Sm, Tb and Yb).
MoO produces visible light
3
+
2
6
2
6
RE substituted bismuth molybdate depends on the partly filled RE 4f shell. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
responsive compounds by gently red shifting the absorption edge to low energy
side. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)

L.S. Kumari et al. / Materials Research Bulletin 70 (2015) 93–98
97
Table 2
band gap energy. The RE 4f energy level gradually becomes
lowered with an increase of their atomic number, finally lying in
the valence band, which results in a monotonic increase of band
ꢀ
L*a*b*C h values of pigmented polymer compact BiTbMoO
PMMA after exposure to sunlight.
6
(10%) dispersed in
ꢀ
Exposure time (h)
L*
a*
b*
C
h
gap energy from Ce
the band structure of BiSmMoO
consisting of the O2p orbitals and the conduction band primarily
derived from the Mo4d orbitals in MoO octahedra and the
2
O
3
to Sm
2
O
3
. Considering this, we can assume
6
to be composed of valence band
0
1
2
5
80.98
77.50
78.28
83.50
81.98
80.65
80.19
13.2
63.12
58.55
59.53
62.14
60.38
61.72
62.74
64.48
60.49
61.56
64.53
62.49
64.02
64.76
78.18
75.44
75.28
74.34
75.04
74.58
75.62
15.20
15.66
17.40
16.13
17.02
16.08
6
5
secondary Bi6p orbitals. Occupied Sm 4f orbitals are overlapped
1
2
with the valence band. Correspondingly, the band gap is reduced to
24
48
3
+
3
2
.54 eV from 2.99 eV with the substitution of Sm . But the Nd 4f
2
and Pr 4f filled levels are still raised in energy due to the lower
number of 4f electrons. For BiNdMoO the band gap is still lowered
6
3
to 2.51 eV as Nd 4f level lies just above the O2p valence band.
3
.5. Color characterization
The CIE 1976 color co-ordinates of the powdered compounds
2
Similar band structure with partially filled Pr 4f orbital located
within the band gap above the valence band can be assumed for
BiPrMoO
overlapped with O2p for BiSmMoO
2p bands for BiPrMoO
Tb and Yb substituted compounds show minimum band gap
6
. Thus the highest occupied level is formed by Sm 4f
are summarized in Table 1. The brightness (L*) is slightly lowered
with the introduction of rare earth metals but a* (redness) value is
found to escalate. The b* value corresponding to yellow chroma-
ticity depends on the identity of the RE element. From Table 1, it
follows that the b* coordinate has a wide interval of values ranging
from 11.5 to 61.6. This is also reflected in its chroma value (C*)
which gets enhanced from 12.2 to 63.6. The richest yellow hue was
obtained for BiTbMoO
shade is shifting from off white for Bi
for BiSmMoO , greenish yellow for BiPrMoO
color for Tb and Yb containing bismuth molybdate. The hue angle
values of the samples lie in the yellow region of the cylindrical
6
or by filled Pr 4f lying above the
O
6
.
3
+
values compared to other rare earth compounds in contrast to the
increased number of 4f electrons. For Ce, Pr and Tb the energy of
4
f–5d transition is lower than for the rest of the lanthanides. It is
the transition that determines the absorption edge E of these
oxides (4f band lies above valence band) [23]. This explains the
lower E values of Tb compound. As soon as the f-band enters into
the 2p band, the E values become constant. The 4f ground state
g
6
and BiYbMoO
6
compounds. The color
to light yellow color
and bright yellow
2
MoO
6
g
6
6
g
3
+
3+
3+
3+
energy levels trend for Pr , Nd , Sm and Tb in the current
system is in agreement with the Dorenbose theory [24,25].
ꢀ
color space (h = 70–105 for yellow) [26]. The color co-ordinates of
3
+
However, for Yb the observed band gap points out a higher
ground state energy level than the other lanthanides, which is not
in accordance with the expected trend. It is to be mentioned here
the yellow compounds BiTbMoO
to be higher than that of the powder BiVO
b* = 42.35) reported elsewhere in the literature [27].
6
and BiYbMoO
6
sample are found
(L* = 68.0, a* = 16.5,
4
3
+
3+
that Tb is a ‘hole trap’ and Yb is an ‘electron trap’. Due to this
Yb tend to Yb in the system. For the divalent lanthanides the 5d
levels will be close to the bottom of the conduction band and the
ionization energy EdC is minimal for Yb As a consequence, this
positions the 4f ground level of Yb to much below the conduction
band, which is much above the 4f ground levels of other trivalent
rare earths (Pr , Nd , Sm and Tb ). This difference in RE 4f
levels and the corresponding difference in the band gaps lead to
different color, as can be seen from Table 1.
3
+
2+
3
.6. Coloration of plastics
2
+.
The coloring performance of the typically synthesized samples,
BiTbMoO was tested for its coloring application in a substrate
6
2+
material like PMMA. Typically, 10 wt% pigment samples were
dispersed in PMMA and compressed to a cylindrical disc (Fig. 7).
The color co-ordinates of the test pieces were measured at
different locations. The average L*a*b* values (L* = 80.98, a* = 13.2,
b* = 63.12) obtained were similar indicating the uniform distribu-
tion of pigment particles in the polymer matrix. The light
resistance of the typical pigmented polymers was tested by
exposing it to sunlight at various time intervals and measured the
color coordinates. The L*a*b* values of pigmented compact were
found to be nearly same as that of the unexposed sample which
indicates that the pigmented polymer is resistant to light (Table 2).
Thus, the developed compounds may find potential application in
the coloring of various plastic materials as yellow pigments.
3
+
3+
3+
3+
4
. Conclusions
A series of novel rare earth substituted bismuth molybdate
compounds, BiREMoO
synthesized and elucidated the role of RE 4f levels on the optical
properties of Bi MoO . It is observed that the substitution of rare
earth metals for bismuth in Bi MoO red shifted the absorption
6
(RE = Pr, Nd, Sm, Tb and Yb) have been
2
6
2
6
edge. This leads to yellow color for Sm, Yb and Tb and greenish
yellow color for Pr and light green color for Nd. Rare earth ion
dependence of the band gap is governed by the position of energy
level of RE 4f levels in the band structure, which lowers with
3
+
increasing occupation of f electrons in RE . The partially occupied
f levels close to the valence band edge decreases the band gap of
Bi MoO and produces various color shades. The position of 4f
band depends on the number of f electrons in RE and hence is the
4
2
6
Fig. 7. Photograph of BiTbMoO
uniform distribution of pigment particles in the polymer matrix.
6
(10%) dispersed in PMMA. The test piece exhibits

9
8
L.S. Kumari et al. / Materials Research Bulletin 70 (2015) 93–98
variation of band gap. The developed compounds demonstrated
good coloration to plastics with effective light resistance.
[9] T. Takahashi, H. Iwahara, Mater. Res. Bull. 13 (1978) 1447–1453.
10] A.W. Sleight, Science 208 (1980) 895–900.
[
[11] F. Abraham, M.F. Debreuille-Gresse, G. Mairesse, G. Nowogrocki, Solid State
Ionics 28-30 (1988) 529–532.
[12] P. Prabhakar Rao, M.L.P. Reddy, Dyes Pigm. 63 (2004) 169–174.
13] P. Sulcova, M. Trojan, J. Therm. Anal. Calorim. 83 (2006) 557–559.
14] H. Mizoguchi, H. Hosono, H. Kawazoe, M. Yasukawa, S. Fujitsu, K. Fukumi,
Mater. Res. Bull. 34 (1999) 373–379.
Acknowledgement
[
[
L.S. Kumari would like to acknowledge the Council of Scientific
and Industrial Research for the financial support towards the
Senior Research Fellowship.
[15] P. Sulcova, E. Prokleskova, P. Bystrzycki, M. Trojan, J. Therm. Anal. Calorim. 100
2010) 65–69.
16] P. Sulcova, E. Prokleskova, J. Min. Metall. 44 (2008) 27–33.
17] Erkens, U.S. Patent 5976238, (1999).
[18] Z. Zou, J. Ye, H. Arakawa, J. Phys. Chem. B 106 (2002) 517–520.
(
[
[
References
[
19] Y.Y. Li, W.D. Cheng, H. Zhang, C.S. Lin, W.L. Zhang, L. Geng, G.L. Chai, Z.Z. Luo, Z.
[
[
[
1] A.M. Beale, G. Sankar, Chem. Mater. 15 (2002) 146–153.
2] E. Vila, J.M. Rojo, J.E. Iglesias, A. Castro, Chem. Mater. 16 (2004) 1732–1739.
3] A. Castro, P. Begue, B. Jimenez, J. Ricote, R. Jimenez, J. Galy, Chem. Mater. 15
Z. He, Dalton Trans. 40 (2011) 7357–7364.
[20] H. Liu, J. Yuan, W. Shangguan, Y. Teraoka, J. Phys. Chem. C 112 (2008)
8521–8523.
(
2003) 3395–3401.
[21] Z. Zang, Appl. Opt. 52 (2013) 5701–5708.
[
[
[
4] H. Li, K. Li, H. Wang, Mater. Chem. Phys. 116 (2009) 134–142.
5] J. Yu, A. Kudo, Chem. Lett. 34 (2005) 1528–1529.
6] Y. Shimodaira, H. Kato, H. Kobayashi, A. Kudo, J. Phys. Chem. B 110 (2006)
[22] Z.G. Zanga, Y. Zhang, J. Mod. Opt. 59 (2012) 161–165.
[23] A.V. Prokofiev, A.I. Shelykh, B.T. Melekh, J. Alloys Compd. 242 (1996) 41–44.
[24] P. Dorenbose, J. Lumin. 108 (2004) 301–305.
1
7790–17797.
7] J. Ren, W. Wang, M. Shang, S. Sun, E. Gao, ACS Appl. Mater. Interfaces 3 (2011)
529–2533.
8] L. Zhou, M. Yu, J. Yang, Y. Wang, C. Yu, J. Phys. Chem. C 114 (2010) 18812–18818.
[25] P. Dorenbos, J. Phys.: Condens. Matter 15 (2003) 8417–8434.
[26] P. Sulcova, M. Trojan, J. Therm. Anal. Calorim. 88 (2007) 111–113.
[27] M.C. Neves, M. Lehocky, R. Soares, L. Lapcik, T. Trindade, Dyes Pigm. 59 (2003)
181–184.
[
[
2