Effect of the molecular structure and surface charge of a bismuth catalyst on the adsorption and photocatalytic degradation of dye mixtures

Article abstract of DOI:10.1039/c5ra14931b

[Bi₆O₆(OH)₃](NO₃)₃·1.5H₂O plates were prepared by a facile hydrothermal method. The effect of the pH value, hydrothermal temperature, reaction time, and concentration of Bi(iii) ion and additives on the morphology and properties of the samples were investigated. The as-synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Brunauer-Emmett-Teller (BET) surface area and UV-vis diffuse reflectance spectroscopy (UV-DRS). It was found that under ultraviolet light irradiation (≤420 nm), the sample prepared at pH = 0.72 displays a higher photocatalytic activity than those prepared at higher pH values, which has been mainly ascribed to the promoted oxygen adsorption at low pH values. Moreover, we investigated the effects of the molecular structure and surface charge on the adsorption and photocatalytic degradation of dye mixtures (methyl orange (MO)-methylene blue (MB) and MO-rhodamine B (RhB)). The results show that in the MB-MO mixture, the adsorption and degradation of both MO and MB are promoted greatly by electrostatic forces; while in the RhB-MO mixture, the degradation of RhB is greatly reduced by MO due to competitive adsorption. These findings help us to understand degradation processes for real wastewater samples containing more than one pollutant.

Full text of DOI:10.1039/c5ra14931b

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Effects of molecule structure and charging property on adsorption and
photocatalytic degradation of dye mixture wastewater
∗
Shurong Wang, Fei Teng , Yunxuan Zhao
Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials
(ECM), Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution
Control (AEMPC), Jiangsu Joint Laboratory of Atmospheric Pollution Control (APC),
Collaborative Innovation Center of Atmospheric Environment and Equipment Technology
(AEET), School of Environmental Science and Engineering, Nanjing University of
Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China.
Abstract
6 6 3 3 3 2
The [Bi O (OH) ](NO ) ·1.5H O plates are prepared by a facile hydrothermal method.
The effects of pH value, hydrothermal temperature, time, the concentration of Bi(III) ion and
additive on the samples are investigated. The asꢀsynthesized samples are characterized by
Xꢀray diffraction (XRD), scanning electron microscopy (SEM), highꢀresolution transmission
electron microscopy (TEM), BrunauerꢀEmmettꢀTeller (BET) and UVꢀvis diffuse reflectance
spectrum (UVꢀDRS). It is found that under ultraviolet light irradiation (≤ 420 nm), the sample
prepared at pH=0.72 shows a higher photocatalytic activity than those at higher pH values,
which has been mainly ascribed to the promoted oxygen adsorption by the low pH value.
Moreover, we mainly investigate the effects of molecule structure and charge on adsorption
and photocatalytic degradation of dye mixtures (methyl orange (MO)/methylene blue (MB)
∗
Corresponding author. Email: 001880@nuist.edu.cn (F. Teng); Phone/Fax: +86ꢀ25ꢀ58731090
1

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and MO/rhodamine B (RhB)). The results show that in the MB/MO mixture, the adsorption
and degradation of both MO and MB are promoted greatly via an electrostatic force; while in
the RhB/MO mixture, the degradation of RhB are greatly reduced by MO, due to a
competitive adsorption. The finding benefits us to understand the degradation process for
practical wastewater containing more than one pollutant.
Keywords: Electrostatic force; Competitive adsorption; Degradation; Dye mixture wastewater;
6 6 3 3 3 2
Bi O (OH) ](NO ) ·1.5H O
Main finding
In a dye mixture, their degradation efficiencies of are much different due to their
different interactions, which favors to understand the degradation process for practical
wastewater.
2

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1
. Introduction
Today, water pollution becomes more serious with the developments of chemical
1
industries. Since the discovery of photocatalysis, a large number of photocatalysts
have been investigated. However, it remains a big challenge to develop an efficient
photocatalyst for the practical application. Among various photocatalysts,
bismuthꢀcontaining compounds as efficient photocatalysts have been researched
2
ꢀ9
intensively.
Besides, bismuth compounds are easier to be recovered from aqueous
. To the best of our knowledge,
reported the plateꢀlike
O photocatalyst. In the pioneering works, Christensen et al.
(OH) ](NO ·1.5H O using thermogravimetric
synthesized [Bi (OH) ](NO ·1.5H by
synthesized the
Bi O (OH) ](NO ) ·1.5H O photocatalysts via a hydrothermal method, and
system due to its higher molecular weights than TiO
2
1
0ꢀ13
however,
only
fewer
·1.5H
studies
have
[
Bi
6
O
6
(OH)
3
](NO
3
)
3
2
1
0
obtained the composition of [Bi
6
O
6
3
3
)
3
2
11
analysis. Xie et al.
6
O
6
3
3
)
3
2
O
a
1
2
microwaveꢀassisted hydrothermal method. Afterward, Xie et al.
[
6
6
3
3 3
2
13
investigated the degradation activity of malachite green (MG). Recently, Yang et al.
investigated the photocatalytic activity of [Bi (OH) ](NO ·1.5H O for the
degradation of phenol. In these studies above, however, the influences of preparation
conditions on [Bi (OH) ](NO ·1.5H O have not been investigated intensively. It
6
O
6
3
3
)
3
2
6
O
6
3
3
)
3
2
is well known that preparations conditions have great influences on the performances
of materials. It is desirable to investigate in detail the influences of various
preparation
Bi (OH)
Moreover, practical wastewaters generally contain more than one pollutant. As a
parameters
so
as
to
understand
the
formation
of
[
6
O
6
3
](NO ·1.5H
3
)
3
2
O.
result, the photocatalytic degradation process of practical wastewater becomes
3

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complicate, even invalid. It is important to clearly understand the mutual interaction
among different pollutants components. Nevertheless, this work has been scarcely
studied. In this work, we systematically investigated the influences of various
preparation parameters on the formation of [Bi O (OH) ](NO ) ·1.5H O, so as to
6
6
3
3 3
2
determine
Bi (OH)
what
](NO
parameter
·1.5H
plays
the
crucial
role
in
forming
[
6
O
6
3
3
)
3
2
O. Moreover, we have mainly researched the mutual
interactions of different dyes in the methyl orange (MO)/methylene blue (MB) and
MO/rhodamine B (RhB) mixtures. The study could be useful to promote the
development of photocatalysis in practices.
2
2
. Experimental
.1 Sample preparation
All reagents were of analytical grade, purchased from Beijing Chemical
Reagents Industrial Company in China, and were used without further purification.
Typically, 1 mmol of Bi(NO ·5H O was dissolved in 20 mL of distilled water,
and then 3 mL 1M diluted HNO solution was added into the above solution under
3
)
3
2
3
magnetic stirring, followed by adding distilled water to form 40 mL of solution. After
being magnetically stirred for 30 min, the resulting white suspension was transferred
into a Teflonꢀsealed autoclave and maintained at 180 °C for 24 h. After being cooled
to room temperature naturally, the product was separated by centrifugation, washed
with distilled water and ethanol for several times, and dried at 60°C for 5 h. The
experimental parameters, including pH value, hydrothermal temperature, time, the
concentration of Bi(III) ions and ethylene glycol/water ratio, were varied to
understand the formation of [Bi
6
O
6
3 3 3 2
(OH) ](NO ) ·1.5H O, which was listed in Table
4

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1
ꢀ3.
2
.2 Characterization
The crystal structures of the samples were determined by Xꢀray powder
polycrystalline diffractometer (Rigaku D/maxꢀ2550VB), using graphite
monochromatized Cu K radiation (λ= 0.154 nm), operating at 40 kV and 50mA. The
α
◦
◦
ꢀ1
XRD patterns were obtained in the range of 20ꢀ80 (2θ) at a scanning rate of 5 min .
The samples were characterized on a scanning electron microscope (SEM, Hitachi
SUꢀ1510) with an acceleration voltage of 15 keV. The samples were coated with
5
ꢀnmꢀthick gold layer before observations. The fine surface structures of the samples
were determined by highꢀresolution transmission electron microscopy (HRTEM,
JEOL JEMꢀ2100F) equipped with an electron diffraction (ED) attachment with an
acceleration voltage of 200 kV. UVꢀvis diffused reflectance spectra of the samples
were obtained using a UVꢀvis spectrophotometer (UVꢀ2550, Shimadzu, Japan).
BaSO was used as a reflectance standard in a UVꢀvis diffuse reflectance experiment.
4
ꢀ4
Nitrogen sorption isotherms were performed at 77 K and < 10 bar on a
Micromeritics ASAP2010 gas adsorption analyzer. Each sample was degassed at 150
ºC for 5 h before measurements. Surface area was calculated by the
BrunauerꢀEmmettꢀTeller (BET) method.
2
.3 Photocatalytic activity evaluation
The photocatalytic activities of the samples were evaluated by degradation of
MO, MB, RhB, MO/RhB and MB/MO aqueous solutions under ultraviolet
light irradiation (λ≤420nm), using a 500 W Xe arc lamp (CELꢀHXF 300) equipped
with an ultraviolet cutoff filter as a light source. The reaction system was placed in a
5

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sealed black box with the top opened, and a distance of 15 cm was maintained from
the solution surface to light source. 100 mg of powders was dispersed in 200 mL of
MO(12.5 mg/L), MB(4.5 mg/L), RhB(7.5 mg/L), MO(12.5 mg/L)/RhB(7.5 mg/L) and
MO(12.5 mg/L)/MB(4.5 mg/L) aqueous solutions in a Pyrex beaker at room
temperature, respectively. Before lamp was turned on, the suspension was
continuously stirred for 30 min in the dark to ensure the establishment of an
adsorption–desorption equilibrium between the powders and dye molecules. During
degradation, 3 mL of solution was collected at intervals of irradiation by pipette, and
subsequently centrifuged to remove the catalysts. UVꢀvis spectra were recorded on a
Spectrumlab 722sp spectrophotometer to determine the concentration of dye. The
photocatalytic reaction was normalized by an apparent firstꢀorder kinetic equation as
follows:
ln(C
0
/C) = k
a
×t, or C = C
0
×exp(ꢀk
a
×t)
(1)
Where C is the initial concentration of dye solution, and C is the concentration of dye
0
a
molecule at different irradiation time, k represents apparent firstꢀorder rate constant.
3
3
. Results and discussion
.1 Effect of pH value on the samples
Table 1 shows the effect of pH value on the samples. The crystal phases of the
products are determined by Xꢀray powder diffraction (XRD) (Fig. 1a). At pH≤5.38, all
the diffraction peaks of the samples can be well indexed to tetragonal
[
Bi O (OH) ](NO ) ·1.5H O (a=b=3.8175Å, c=17.149Å, JCPDS: 53ꢀ1038). At
6 6 3 3 3 2
pH=0.72 and 1.15, the intensities of the diffraction peaks of the samples do not
change significantly. At pH=3.51, the intensities of the diffraction peaks of the sample
6

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increase obviously. Generally, a high pH value favors for the hydrolysis of Bi(III).
Nevertheless, at too high pH values (7, 9.05), the Bi and NH NO have formed.
2
O
3
4
3
Herein, the hydrolysis of Bi(III) can be described as Equation (2) as follows:
+
Bi(NO ) ·+ H O → Bi O (OH) ](NO ) ·1.5H O + H
(2)
3
3
2
6
6
3
3 3
2
herein, the pH value of the precursor solution was adjusted by adding HNO or
3
1
4
NH
HNO
particles, consisting of plates subunits, formed (Fig. 1b). Henry et al. has reported
3
·H
2
O. The pH value would affect the crystal nucleation and growth.
When
3
was added, the hydrolysis of Bi(III) was restrained. As a result, the large
15
−
−
3
that the OH and NO would bond to Bi(III) downward (BiꢀOH bonds) and upward
−
−
2+
(
BiꢀONO bonds), respectively; and the inserted OH and NO between [Bi O ]
2
3
2
2
16
layers are connected through the hydrogen bond. Nevertheless, Li et al. hold that
−
−
3
both OH and NO
layers stack together along the cꢀaxis via Van der Waals force
(nonbonding interactions). Herein we hold that the formation of plateꢀlike subunits
could
result from
the
intrinsic
anisotropic
layered
structure
of
[
Bi O (OH) ](NO ) ·1.5H O. At low pH values, the number of nuclei is fewer, and
6 6 3 3 3 2
then the nuclei grow slowly to form [Bi
6
O
6
(OH)
3
](NO
3
)
3
·1.5H
2
O plates (Fig. 1(b,c)).
·1.5H O plates
1
7
At the moderate pH values (3.51, 5.38), [Bi
6
O
6
(OH) ](NO
3
3
)
3
2
agglomerate or assemble to form thick biscuits (Fig. 1d). At high pH values (7.36), the
thin plates form (Fig. 1e,f), but the impurities phase also form. The high pH value can
not only promote crystal nucleation and growth, but also favor for the dehydration of
BiꢀOH to form Bi
formation of [Bi
2
O
3
. It is obvious that the pH value plays a crucial role in the
6
O
6
(OH) ](NO ·1.5H O.
3
3
)
3
2
Typically, the BS1 sample prepared at pH=0.72 was further characterized by
highꢀresolution electron emission microscopy (HRTEM) and selected area electron
diffraction (SAED) (Fig. 2). As shown in Fig. 2c, the lattice spacings of 0.317nm,
7

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0
d
.286nm and 0.271 nm are well consistent to the literature data of d103= 3.173 Å,
006= 2.855 Å and d110=2.697 Å of [Bi (OH) ](NO ·1.5H O, respectively.
6
O
6
3
3
)
3
2
Furthermore, the diffraction rings result from the (1010) and (006) Bragg reflections
(
Fig. 2d), revealing the polycrystalline nature of [Bi O (OH) ](NO ) ·1.5H O.
6 6 3 3 3 2
3
.2 Effects of temperature, time, Bi(III) concentration and VEG/V
samples
W
on the
6 6 3 3 3 2
In order to understand the evolution process of [Bi O (OH) ](NO ) ·1.5H O, the
reaction temperatureꢀ, timeꢀ, Bi(III) concentrationꢀdependent experiments were
carried out (Table 2).
Fig. 3a shows the XRD patterns of the samples prepared at different temperatures.
All the asꢀprepared samples are phaseꢀpure [Bi
JCPDS: 53ꢀ1038) (Fig. 3a), and the large aggregates of plates form (Fig. 3b,c). It is
obvious that with the increase of temperature, the peaks intensities increase. A high
6 6 3 3 3 2
O (OH) ](NO ) ·1.5H O forms
(
18
crystallinity indicates less bulk defects. Usually, the bulk defects of photocatalyst
are considered to be the recombination centers for photogenerated holes and electrons,
1
8
resulting in a low photocatalytic activity.
Thus, the sample obtained at high
temperatures could be beneficial to improve photocatalytic activity. Fig. 3d shows the
XRD patterns of the samples at different reaction times (12ꢀ30 h). All the samples are
well indexed to the phaseꢀpure [Bi O (OH) ](NO ) ·1.5H O (JCPDS: 53ꢀ1038) (Fig.
6
6
3
3 3
2
3
d). At 12 and 18h, the peaks intensities of the samples do not vary obviously. At 24
and 30 h, the peaks intensities of the samples increase, indicating an increased
crystallinity. No obvious difference can be observed from their SEM images (Fig.
3
e,f). Fig. 3(gꢀi) present the XRD patterns and SEM images of the samples at different
Bi(III) concentrations. All the samples can also be well indexed to phaseꢀpure
8

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[
6 6 3 3 3 2
Bi O (OH) ](NO ) ·1.5H O, but the product yield increases with increasing Bi(III)
concentration. It is obvious that the temperature, time and Bi(III) concentration have
little influence on the phase purity and morphology of the sample.
Table 3 and Fig. 4aꢀf present the effect of volumetric ratio of ethylene
glycol/water (V /V ) on the samples. In pure ethylene glycol, only Bi metal form, in
EG
W
1
9,20
which Bi(III) has been reduced by ethylene glycol.
/1 to 1/1, the Bi CO , Bi and an unknown impurities phases form. When
is smaller than 1, the diffraction peaks of metal bismuth disappeared. The
phaseꢀpure [Bi O (OH) ](NO ) ·1.5H O can be harvested only in pure water. Fig.
W
When VEG/V decreases from
3
2
O
2
3
2 3
O
V
EG/V
W
6
6
3
3 3
2
4
(b,c) shows that in the presence of glycol, the formed microspheres are assembled by
thin plates. With increasing VEG/V , the sample is tendency to form microsphere. At
=1, most of microspheres have cracked and a few of hollow microspheres
have formed (Fig. 4d). At VEG/V < 1, the plates with different thicknesses have
formed. It is obvious that in the presence of ethylene glycol, the phaseꢀpure
W
V
EG/V
W
W
[
Bi O (OH) ](NO ) ·1.5H O cannot be obtained.
6 6 3 3 3 2
3
.3 Photodegradation activity of single dye
Fig. 5a presents the photodegradation curves of RhB by the samples prepared at
different pH values. After 30 min, 93.13% of RhB can be removed by the sample
BS1, pH=0.72), while 83.31%, 73.25%, 65.54% and 36.76% by the samples prepared
at pH=0.95, 1.15, 5.38 and rutile TiO , respectively. Meanwhile, the selfꢀdegradation
of RhB is 23.44%. We have measured the activity of Degussa P25 TiO . It is found
that RhB can be degraded completely by Degussa P25 TiO after 10ꢀmin UV
irradiation. Its activity is higher than that of [Bi (OH) ](NO ·1.5H O, since it is a
(
2
2
2
6
O
6
3
3
)
3
2
mixture of anatase and rutile, high BET area and a high light absorbance, and so on.
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Fig. 5c further gives their reaction kinetic curves. The apparent degradation kinetic
−1
rate (kBS1=0.08481 min ) of RhB over BS1 is 1.53, 2.11, 2.60 and 6.90 times as high
as those over the samples at pH=0.95, 1.15, 5.38 and rutile TiO , respectively. The
2
photocatalytic degradation efficiencies of RhB over these samples follow the order:
BS1> BS2 > BS3> BS5. It seems that the activities of the samples prepared at low pH
values are higher than those at higher pH values. It should be pointed out that the BET
2
ꢀ1
areas of all the samples are lower than 2.0 m g . The similar results are also reported
11ꢀ13
in the other reports.
Thus, the BET area has little influence on the activity in our
study. As discussed above, the phase purity of the product is sensitive to the pH value.
+
We hold that under an acidic preparation condition, H ions could endow
[
6 6 3 3 3 2
Bi O (OH) ](NO ) ·1.5H O with the positively charged surfaces. The surface charge
properties would affect the surface adsorption nature of catalyst.
Generally, the surface oxygen vacancies are the active sites for molecular oxygen
adsorption, thus the formation of oxygen vacancies on material surface is a
prerequisite to molecular oxygen activation. Li et al. have reported that under
1
6
common condition, less oxygen vacancies are present on the surface of TiO
2
.
On
the other hand, oxygen vacancy as one kind of point defect can promote the separation
22
of photoꢀinduced electrons and holes, leading to a high activity. Ye et al.
have
reported that because of the low bond energy and long bond length of Bi–O, the
unique atomic structure of bismuth facilitates the formation of oxygen vacancies
2+
under UV light irradiation. [Bi
6
O
6
(OH)
3
](NO
3 3 2 2 2
) ·1.5H O consists of both [Bi O ]
−
−
3
15
and anions (OH , NO
) layers,
thus oxygen vacancies could form through the
breakage of Bi–O bond under UV irradiation. Herein, whether oxygen vacancies
formed or not is still elusive. Limited by our research conditions, however, it is
23
difficult for us to quantitatively investigate defects at present. Moreover, Cui et al.
10

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have reported that after phosphate modification, the increase of surface acidity favors
for the O adsorption on the TiO surfaces, which enhances photogenerated charge
2
2
separation. What is more, they have also demonstrated that after being modified by
different sodium phosphates (Na PO , Na HPO , NaH PO , and H PO ), the crystal
3
4
2
4
2
4
3
4
phase and crystallinity of TiO have not changed, although the phosphate groups have
2
been anchored on the TiO
modification of TiO by H PO
Therefore, we could infer that the surface acidity of [Bi
prepared at low pH values (with HNO ) favors for the adsorption of O , leading to
2
surfaces. They further reported that the surface
or H SO can chemically adsorb a large amount of O2.
2
(OH) ](NO ·1.5H O
2
3
4
2
4
2
3
6
O
6
3
3
)
3
3
2
photogenerated charge separation. Summarily, the surface acidity of
Bi (OH) ](NO ·1.5H O is an important factor affecting the photocatalytic
activity.
[
6
O
6
3
3
)
3
2
Typically, the light absorption properties of the samples prepared at different pH
values were characterized by UVꢀvis diffuse reflection spectroscopy (UVꢀDRS) (Fig.
5
e,f). The BS1 sample prepared at pH=0.72 has a higher absorbance than the others at
pH=1.15, 0.95 and 0.83 in the UV region. The band gap of
2
4
[
Bi
6
O
6
3
(OH) ](NO
3
)
3
·1.5H
2
O can be calculated using Equation (3) as follows :
n/2
αhν = A (hν − E
g
)
(3)
where α, hν, A, and E are optical absorption coefficient, photonic energy,
g
proportionality constant and band gap, respectively. By extrapolating the straight line
to the xꢀaxis in Tauc plot (Fig. 5f), the band gaps are estimated to be 3.33 eV, 3.38 eV,
3
.35 eV and 3.32 eV for the samples prepared at pH=0.83, 0.72, 0.95 and 1.15,
respectively. For layered BiOCl, Zhang et al. have reported that the typical layered
structure can provide a space large enough to polarize the related atoms and orbitals,
25
resulting in the formation of dipoles. The induced dipoles favor for the efficient
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separation of hole–electron pair. From Tauc plot in Fig.5f we can know that
Bi (OH) ](NO ·1.5H O has an indirectꢀtransition bandꢀgap. For an indirect
transition band gap, the excited electrons have to travel a certain space distance to
[
6
O
6
3
3
)
3
2
25,26
reach conduction band.
Although an extra energy is needed, the recombination
probability of photogenerated electrons and holes can be reduced greatly, leading to
2
5,26
an improved photocatalytic activity.
for their UVꢀDRS spectra.
To conclude, there is no obvious difference
Also, we have investigated the photoactivities of the samples prepared at
different temperatures for the degradation of RhB (Fig. 5b,d). After reaction for 30
min, the removal efficiencies of RhB by the BS8, BS9, BS11 BS13 and BS14 samples
are 37.18%, 87.95%, 93.13%, 90.01% and 67.79%, respectively. The apparent
ꢀ1
reaction rate constant of BS11 is 0.08481 min , which is 6.3, 1.29, 1.17 and 2.47
times as high as that of BS8, BS9, BS13 and BS14, respectively. It is obvious that the
o
BS11 sample obtained at 180 C for 24 h shows the highest activity among them. Due
2
ꢀ1
to their similar BET areas (<2 m g ), its high activity can be attributed to the high
18
crystallinity. A high crystallinity means that the samples possess less bulk defects,
which could reduce the recombining rate, leading to an improved activity.
3
.4 Photocatalytic reaction mechanism and cycle stability
2
7,28
In previous study,
Honda et al. have demonstrated that the primary step of
RhB degradation is Nꢀdeethylation, in which the electrons from the excited singlet
*
RhB transport to CdS in the RhB/CdS system under visible light irradiation. Further,
they have confirmed that RhB can undergo oneꢀelectron oxidation by investigating the
29
cyclic voltammogram of RhB. Besides, Hu et al. have reported that for a single
electron transfer (SET) mechanism, the Nꢀdeethylation process is initiated by the
12

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excitation of dye, followed by an electron transfer to catalyst. Subsequently, the
resultant dye radicals hydrolyze to lose one alkyl group. Further, they have proved
that Nꢀdeethylation of RhB arises from the hydrolysis of the cationic dye radical in
2
9
the absence of active oxygen species. In our study, there is no hypsochromic shift of
the absorption maximum of RhB (Fig. 6a), but an obvious hypsochromic shift can be
+
29
observed in the VO
2
/RhB system.
Thus, in our study, the cleavage of whole
conjugated chromophore structure of RhB, instead of the Nꢀdeethylation, may occur.
The previous reports reported that the mineralization of dyes is mainly attributed to
2
2,26ꢀ29
−·
2
the formation of active oxygen species.
in our experiments.
Thus oxygen species (O ) could form
Herein, the trapping experiments are conducted to determine the active species,
in which Na and dimethyl sulphoxide (DMSO) are employed as the scavengers
2 2 4
C O
of hole and hydroxyl radical, respectively (Fig. 6b). When adding DMSO in reaction
solution, 34.3% of RhB was degraded; while 38.5% of RhB was degraded when
adding Na C O We could deduce that over [Bi O (OH) ](NO ) ·1.5H O, RhB is
2
2
4.
6
6
3
3 3
2
oxidized simultaneously by both holes and hydroxyl radicals.
Thus, a degradation pathway of RhB is proposed, as shown in Fig. 6c. It is
*
wellꢀknown that the organic radical species (RhB ) resulted from the photosensization
of RhB can facilely react with O to produce peroxide free radicals, which can further
2
initiate a series of radical chain reactions and finally mineralize organic compounds.
At the same time, the conduction band (ꢀ0.9
Bi (OH) ](NO ·1.5H O is more negative than the reduction potential (E (O
ꢀ0.33 V vs. N.H.E.) of O
V
vs. N.H.E.) of
◦
−·
[
6
O
6
3
3
)
3
2
2
/O
2
)
−
·
=
2
, indicating that oxygen species (O ) could be generated
2
during the degradation process; whereas its less positive valence band (+2.48 V vs.
◦
N.H.E.) indicates that H O cannot be oxidized to form ·OH (E (·OH/H O) = +2.68 V
2
2
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12,30
31
−·
vs. N.H.E.).
As reported in the literature,
O
2
is unstable in aqueous solution
+
and is easily decomposed into ·OH. In our experiment, both h and ·OH are main
−
2
·
active species, but the generation of O
generation of ·OH.
could be the prerequisite step to the
To investigate the stability of [Bi O (OH) ](NO ) ·1.5H O under UV light
6
6
3
3 3
2
irradiation, the degradation reaction of RhB dye over the BS1 sample was cycled for
three times (Fig. 6d). It is noteworthy that after three cycles, its degradation efficiency
6 6 3 3 3 2
is only reduced by 4.2%, indicating a good stability of [Bi O (OH) ](NO ) ·1.5H O.
3
.5 Photodegradation of MB/MO and RhB/MO mixture dye wastewater
Generally, more than one dye is contained in practical wastewater. In order to
simulate practical wastewater, the degradations of MB/MO and RhB/MO mixture
dyes were investigated over the BS1 sample. As shown in Fig. 6a and 7aꢀd, the
maximum absorption peaks positions of MB, MO and RhB do not change in single
dye or mixture dye solution, suggesting that no intermediates are produced. After 30
min UV irradiation, the removal ratios of MB(Single dye), MO(Single dye),
MB(MB/MO), MO(MB/MO) are 56%, 27.68%, 73.87%, 47.86%, respectively.
Especially, the removal ratio of MB(MB/MO) reaches 71% after 10 min UV
irradiation. It is surprising that the removal efficiencies of MB and MO in the
MB/MO mixture solution are greatly higher than those in single dye, respectively. It is
obvious that a mutual promoting effect may occur between MO and MB. In Fig.s 7e
and 7g, the adsorption value of MB is 55.88% in MB/MO mixture solution, which is
3
.9 times as high as that (14.5%) in single MB solution. Herein, adsorption is mainly
a physical process, in which dye molecules adsorb on the catalyst surface to reach an
adsorptionꢀdesorption balance before light on. Generally, the adsorption of dye
14

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molecules on the catalyst surface is the prerequisite for a degradation reaction. At the
same time, the adsorption value of MO is 14.23% in MB/MO mixture solution, which
is 13.3 times higher than that in single MO solution. We hold that the
negativelyꢀcharged MO (or positivelyꢀcharged MB) may have improved the
adsorption of positivelyꢀcharged MB (or negativelyꢀcharged MO) on the
[
6 6 3 3 3 2
Bi O (OH) ](NO ) ·1.5H O surface. Thus, there is a strong interaction between the
components in the MB/MO mixture solution.
However, such a mutual promoting effect is not obvious in RhB/MO mixture
solution. Compared with single dye solution, however, the adsorption abilities of both
RhB and MO in RhB/MO solution have not changed obviously, but the degradation
activity of RhB decreases obviously. Fig. 7h clearly shows that their photocatalytic
removal ratios decrease in the order as follows: MB(MB/MO) > RhB > MB >
MO(MB/MO) > RhB(RhB/MO) > MO(RhB/MO) ≈ MO. After 60ꢀmin irradiation, the
removal ratios of MB(MB/MO), MO(MB/MO), RhB(RhB/MO) and MO(RhB/MO)
mixture dye solution are 76.30% , 63.78%, 73.02% and 46.75%, respectively. It is
easily understood that in the RhB/MO mixture solution, the reduced activity results
primarily from the competitive adsorption on the catalyst surface.
Our experiment results show that in MB/MO solution, the mutual promotion
effect of both MB and MO on adsorption and degradation could mainly be resultant
from the electrostatic interaction between cationic MB and anionic MO. Our Zeta
6 6 3 3 3 2
potential measurements show that the surface of [Bi O (OH) ](NO ) ·1.5H O is
positively charged. Thus, the anionic dye molecules adsorb on catalyst and are
activated by catalyst preferentially, as shown in Fig. 8. It should point out that the
adsorption of MB in single MB dye solution may be due to the two donor atoms (S, N)
with lone pair electrons. As shown in Fig. 8, we assume that in MB/MO solution, the
15

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adsorption of MB can be improved greatly through the electrostatic interaction with
MO preferentially adsorbed on catalyst. In turn, the adsorbed MB on the catalyst can
also promote the further adsorption of MO on the catalyst. Compared RhBꢀMO with
single dye, on the other hand, the adsorption ability of RhB or MO has not change
nearly (Fig. 7f). However, the degradation ratio of RhB is reduced, while that of MO
does not change obviously (Fig. 7h). We could deduce that the electrostatic interaction
between MO and RhB may be weaker than that between MO and MB. This could be
because MB and RhB have different molecular structures (Fig. 8). It has been reported
30
that the degradation efficiency of MO is high under acidic condition. As an anionic
dye, MO contains azo group (ꢀN=Nꢀ) that links to aromatic sp2ꢀhybridized Cꢀatoms
(Fig. 8). As a cationic dye, RhB contains a carboxyl group that can hydrolyze to
+
+
release H . The adsorption of H favors to increase the positive charge of catalyst,
thus slightly promoting the adsorption of MO. At the same time, the hydrolyzed RhB
ꢀ
is endowed with a negative charge due to the presence of COO , thus a competitive
adsorption could occur between them. As a result, the degradation ratio of RhB
decreased. To conclude, it is the molecule structure and the charging properties of dye
that lead to a different removal efficiency. It should be pointed out that the true
mechanism for mixture dye solution needs further study in future.
4
. Conclusions
The pH value of precursor solution has a significantly influence on the formation
6 6 3 3 3 2
of [Bi O (OH) ](NO ) ·1.5H O. In the MB/MO mixture solution, the adsorption and
the degradation of MB have been greatly improved by MO, which has been mainly
attributed to their electrostatic interaction; while the degradation of RhB has been
refrained obviously by MO due to the competitive adsorption between RhB and MO.
16

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The different results for cationic MB and RhB are mainly ascribed to their different
molecule structures. The finding is useful to promote the application of photocatalysis
in cleaning practical wastewater.
Acknowledgments
This work is financially supported by National Science Foundation of China
(
21377060), the Project Funded by the Science and Technology Infrastructure
Program of Jiangsu (BM201380277), Jiangsu Science Foundation of China
BK2012862), Six Talent Climax Foundation of Jiangsu (20100292), Jiangsu
(
Province of Academic Scientific Research Industrialization Projects (JHB2012ꢀ10,
JH10ꢀ17), the Key Project of Environmental Protection Program of Jiangsu (2013016,
2
012005), A Project Funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD) sponsored by SRF for ROCS, SEM
2013S002) and “333” Outstanding Youth Scientist Foundation of Jiangsu (2011015).
(
17

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Title list of Tables and Figures
Table 1. Effect of pH value on the samples
Table 2. Effects of temperature, time and Bi(III) concentration on the samples
Table 3. Effect of volumetric ratio of ethylene glycol (EG) to water (W) on the
samples
Table 4. Adsorption and degradation curves of single methylene blue (MB), single
methylene orange (MO), single rhodamine B (RhB), MB/MO, and RhB/MO solutions
Fig. 1. XRD patterns (a) and SEM images (bꢀf) of the asꢀsynthesized samples at
different pH values: (b) 0.72; (c) 1.15; (d) 5.38; (e) 7.36; (f) 9.05
Fig. 2. XRD patterns (a), TEM images (b), Lattice fringe images (c) and SAED
pattern (d) of the BS1 sample
Fig. 3. XRD patterns of the samples prepared at different reaction temperatures (a),
times (d), Bi(III) concentrations (g); SEM images of the samples prepared at different
o
o
conditions: (b)120 C; (c) 180 C; (e) 12 h; (f) 30 h; (h) 0.5 mmol Bi(III); (i) 2.0 mmol
Bi(III)
Fig. 4. XRD patterns (a) and SEM images of the asꢀsynthesized samples at different
volumetric ratios of ethylene glycol (EG) to water (W): (b) 1:0; (c) 3:1; (d) 1:1; (e)
1
:3; (f) 0:1
Fig. 5. Photodegradation (a, b) and reaction kinetic curves (c, d) of RhB under
ultraviolet light irradiation (λ≤ 420 nm) of the samples at different conditions,
UV–Vis diffuse reflectance spectra (e) and the Tauc plots (f) for the samples prepared
at different pH values
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Fig. 6. UVꢀvis absorption spectra of rhodamine B (RhB) (a), degradation curves of
RhB by the BS1 sample while adding dimethyl sulphoxide (DMSO) and Na
degradation pathways of RhB (c) and the cycle experiment (d)
2 2 4
C O (b),
Fig. 7. UVꢀvis absorption spectra of (a) methylene blue (MB), (b) methyl orange
MO), MBꢀMO(c) and RhBꢀMO(d) solutions at different reaction times;
(
Photodegradation curves (e, f) of different dyes solutions in 30 min; Adsorption (g)
and removal ratio (h) histograms of different dye solutions in 30 min
Fig. 8. Schematic diagram of mutual interaction of MO with MB and RhB
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Table 1
Table 1 Effect of pH value on the samples
Sample
Bismuth(III)
Reagent
pH
Temperature
Reaction time
o
(
mmol)
( C)
(h)
24
24
24
24
24
24
24
BS1
BS2
BS3
BS4
BS5
BS6
BS7
1
1
1
1
1
1
1
HNO
HNO
/
3
3
0.72
0.95
1.15
3.51
5.38
180
180
180
180
180
180
180
HNO
HNO
3
3
+NH
+NH
3
3
·H
·H
2
2
O
O
HNO +NH ·H O 7.36
3
3
2
HNO
3
+NH
3
·H
2
O
9.05
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Table 2
Table 2 Effects of temperature, reaction time and Bi(III) concentration on the samples
Sample
Bismuth(III)
Temperature
Reaction time
o
(
mmol)
( C)
(h)
24
24
24
24
30
18
12
24
24
24
BS8
BS9
1
1
90
120
150
180
180
180
180
180
180
180
BS10
BS11
BS12
BS13
BS14
BS15
BS16
BS17
1
1
1
1
1
0.5
1.5
2.0
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Table 3
Table 3 Effect of volumetric ratio of ethylene glycol (EG) to water (W) on the
samples
Sample
Bismuth(III)
Solvent
Temperature
Reaction time
(h)
o
(
V
EG/V
W
)
( C)
(mmol)
BS21
BS22
BS23
BS24
BS25
BS26
BS27
BS28
BS29
1.0
1/0
3/1
2/1
180
180
180
180
180
180
180
180
180
24
24
24
24
24
24
24
24
24
1.0
1.0
1.0
1.5/1
1/1
1.0
1.0
0.67/1
0.5/1
0.33/1
0/1
1.0
1.0
1.0
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Table 4
Adsorption and degradation ratios for single methylene blue (MB), single methylene
orange (MO), single rhodamine B (RhB), MB/MO, RhB/MO solutions
Dyes
Adsorption
Degradation
(at 30 min)
56.00%
Degradation
(
at 30 min)
14.34%
1.07%
(at 60 min)
MB
/
MO
27.68%
/
RhB
2.62%
57.46%
/
MB(MB/MO)
MO(MB/MO)
RhB(RhB/MO)
MO(RhB/MO)
55.88%
14.23%
2.51%
73.87%
76.30%
63.78%
73.02%
46.75%
47.86%
42.78%
1.25%
28.51%
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Fig. 1
a
JCPDS: 53ꢀ1038
b
#
:
:
NH NO
4
3
Bi O
2
3
#
*
#
*
#
pH=9.05
pH=7.36
*
*
*
*
*
*
pH=5.38
pH=3.51
pH=1.15
pH=0.72
1
0
20
30
40
50 60
70
2θ (degree)
c
d
f
e
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Fig. 2
b
a
BS1
10
20
30
40
50
60
70
2θ(degree)
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Fig. 3
a
b
c
o
180
C
o
150
C
o
120
C
o
9
0 C
10
20
30
40
50
60
70
2
θ
(degree)
d
e
f
30h
24h
1
8h
(
a)
1
2h
(
70
a)
10
20
30
40
50
60
2θ
(degree)
g
h
i
2.0mmol
1.5 mmol
1.0mmol
0.5mmol
10
20
30
40
50
60
70
2
θ
(degree)
29