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ChemistryOPEN^ if 01 ^ Open Access
DOI: 10.1002/open.201300013
Metastable p-Bi 2 0 3 Nanoparticles with Potential for
Photocatalytic Water Purification Using Visible Light
Irradiation
Maik Schlesinger, [a] Marcus Weber, [a] Steffen Schulze, [b] Michael Hietschold, [b] and
Michael Mehring* [a]
Photocatalytic studies under visible light irradiation using
nanosized (3-Bi 2 0 3 are reported. (3-Bi 2 0 3 nanoparticles are pre-
pared starting from the well-defined bismuth oxido cluster
[Bi 38 0 45 (OMc) 24 (DMSO) 9 ]-2 DMSO-7 H 2 0 (OMc = 0 2 CC 3 H 5 ) using
a straightforward hydrolysis and annealing protocol. Powder
X-ray diffraction studies, transmission electron microscopy, dif-
fuse reflectance UV/Vis spectroscopy and nitrogen adsorption
measurements (using the Brunauer-Emmett-Teller (BET)
theory) are used for the characterization of the as-prepared
|3-Bi 2 0 3 . By time-dependent annealing, the crystallite size can
be controlled between (17±2)nm and (45±5)nm with BET
surface areas of 7 to 29 m 2 g _1 . The indirect band gap of the
as-prepared |3-Bi 2 0 3 amounts to (2.1 5 ±0.05) eV. The decompo-
sition rates for rhodamine B (RhB) solutions are in the range of
2.46 x10" 5 to 4.01 x10" 4 s _1 and depend on the crystallite size,
amount of catalyst and concentration of RhB. Photocorrosion
experiments have shown the formation of Bi 2 0 2 C0 3 after sever-
al catalytic cycles. However, the catalyst can be recycled to
phase-pure p-Bi 2 0 3 nanoparticles by annealing for one hour
under argon atmosphere at 380 °C. Furthermore, the photoca-
talytic activity of as-prepared |3-Bi 2 0 3 nanoparticles for the
decomposition of phenol, 4-chlorophenol, 2,4-dichlorphenol,
4-nitrophenol, triclosan and ethinyl estradiol is demonstrated.
Introduction
Since the discovery of the Honda-Fujishima effect in 1972, [1]
the research in the field of semiconductor photocatalysis has
evolved into two disciplines, the photolysis of water to obtain
hydrogen and oxygen from water [2] and the photocatalytic oxi-
dation of pollutants. [3] Notably, photocatalytic degradation of
organic dyes and pollutants might become one of the main as-
pects in modern decentralized purification systems for air and
water. Until now, Ti0 2 seems to be the most promising material
for such purification systems based on its environmentally
benign nature, commercial availability and photochemical sta-
bility. However, as a result of the band gap of 3.0-3.2 eV only
approximately 7% of the sunlight (A<380 nm) can be effec-
tively used. Several approaches have been reported to improve
[a] Dr. M. Schlesinger, M. Weber, Prof. Dr. M. Mehring
Fakultat fur Naturwissenschaften
Institut fur Chemie, Professur Koordinationschemie
Technische Universitat Chemnitz
StraBe der Nationen 62, 09111 Chemnitz (Germany)
E-mail: [email protected]
[b] Dr. S. Schulze, Prof. Dr. M. Hietschold
Fakultat fur Naturwissenschaften
Institut fur Physik, Professur Analytik an Festkorperoberflachen
Technische Universitat Chemnitz
Reichenhainer StraRe 70, 09126 Chemnitz (Germany)
Dl Supporting information for this article is available on the WWW under
http://dx.doi.Org/1 0.1 002/open.201 30001 3.
© 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution Non-Commercial License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited and is not used for commercial purposes.
the photocatalytic activity and include the addition of precious
metals such as platinum, gold or silver/ 41 which makes an in-
dustrial process quite expensive (see ref. [5]). Thus, in terms of
a sustainable "green chemistry" approach, it is worth to study
other nontoxic semiconductor materials which show better re-
sponse in the visible light region without addition of noble
metals. Bismuth-based materials, such as BiV0 4 , [6] BiOCI, [7]
Bi 2 0 2 C0 3 , [8] Bi 2 Mo0 6 , [9] Bi 2 WO 6 [6c ' 10] and Bi 2 Sn 2 0 7 , [11] were previ-
ously reported to show promising photocatalytic activities
under visible light. Notably, pure bismuth oxide, namely
a-Bi 2 0 3 [6c ' 12] and the metastable polymorphs |3-Bi 2 0 3 [12e ' 13] and
6-Bi 2 0 3 , [14] exhibit photocatalytic behavior as well, and their
large scale production seems to be interesting with regard to
the commercial availability of bismuth (see ref. [5]). Among the
bismuth oxide polymorphs, |3-Bi 2 0 3 is the most active hetero-
geneous photocatalyst. However, the controlled synthesis of
monodisperse |3-Bi 2 0 3 nanoparticles is still a challenge. We
have recently reported a strategy that is based on a straightfor-
ward hydrolysis route starting from well-defined, nanoscaled
bismuth oxido clusters. [13e] The structural relationship between
the bismuth oxido clusters and |3-Bi 2 0 3 is the key point for the
rather mild synthesis method (see Figure S1), which is based
on fast hydrolysis at room temperature followed by short time
annealing at elevated temperature. [15] Note, that starting from
easily accessible [Bi 38 0 45 (OMc) 24 (DMSO) 9 ]-2 DMSO-7 H 2 0 [16]
(OMc = 0 2 CC 3 H 5 ), the synthesis of |3-Bi 2 0 3 nanoparticles with
high yield on a multigram scale is possible, and preliminary in-
vestigations have shown that (3-Bi 2 0 3 nanoparticles prepared
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Et 2 N
A/.A/.AT.AT-tetraethyl-
rhodamine
(rhodamine B)
Wastewater-containing
pollutants,
e.g., rhodamine B as
model compound
P-Bi 2 0 3
nanoparticles
Clean water
Scheme 1. Illustration of a potential tube reactor for water purification using sun light irradiation and (3-Bi 2 0 3
nanoparticles as photocatalysts. Similar systems were already tested in pilot-plant scale, e.g., HIDROCEN (Madrid,
Spain). [3f < 19]
by this approach show promising activity in the photocatalytic
degradation of organic dyes in aqueous solution. [13e]
Herein, we report detailed studies on the photocatalytic be-
havior of the as-prepared |3-Bi 2 0 3 nanoparticles under visible
light irradiation (Scheme 1). The influence of the crystallite size,
the catalyst and dye concentration is investigated by using
rhodamine B (RhB) as model pollutant. [17] Additionally, the ac-
tivity of p-Bi 2 0 3 is tested using typical organic pollutants, such
as phenol, 4-chlorophenol, 2,4-dichlorophenol, 4-nitrophenol,
triclosan and ethinyl estradiol, in water. [18]
glomerated particles with parti-
cle sizes of (20±3)nm for p-
Bi 2 0 3 -5, (21±3)nm for P-Bi 2 0 3 -
10, (23±4)nm for p-Bi 2 O 3 -30,
(33±4)nm for p-Bi 2 O 3 -120,
(38±6)nm for p-Bi 2 O 3 -180 and
(40±7)nm for p-Bi 2 O 3 -300
(Figure 2), which is in line with
the results obtained from
powder X-ray diffraction (PXRD).
The decrease of the crystallite
size from (45±5)nm for
P-Bi 2 O 3 -300 to (17±2)nm for
p-Bi 2 0 3 -5 results in an increase
of the Brunauer-Emmett-Teller
(BET) surface areas from 7 m 2 g _1
(P-Bi 2 O 3 -300) to 29m 2 g 1 (p-
Bi 2 0 3 -5). The latter is compara-
ble to previously reported meso-
porous p-Bi 2 0 3 thin films (20-
30 m 2 g _1 ) and thus is among the
highest values reported for p-
Bi 2 O 3 . [20] However, nitrogen ad-
sorption measurements reveal
type II isotherms, indicating the
A)
Results and Discussion
Influence of particle size
The hydrolysis of [Bi 38 0 45 (OMc) 24 (DMSO) 9 ]-2 DMSO-7 H 2 0 with
an aqueous sodium hydroxide solution results in the formation
of an amorphous powder, which is annealed at 370 °C in an
argon atmosphere to give pure |3-Bi 2 0 3 (Figure 1, see Figur-
es S2-S8 in the Supporting Information). The formation of
pure p-Bi 2 0 3 was additionally checked by electron diffraction
measurements of samples p-Bi 2 0 3 -5 (see Figure S9) and
P-Bi 2 O 3 -300. The composition of Bi 2 0 3 was confirmed by elec-
tron dispersive X-ray spectroscopy (Bi 89.5 wt%; 0 10.5 wt%).
By increasing the annealing time at the same temperature
from five to 300 minutes, the crystallite size of the as-prepared
P-Bi 2 0 3 nanoparticles increased moderately. Values between
17±2nm (p-Bi 2 0 3 -5) and 45±5nm (p-Bi 2 O 3 -300) were ob-
tained. A plot of crystallite size versus annealing time results in
a curve which can be described by a function of the type
"a-(1 -b-e~ Kt )" (Figure 1). Transmission electron microscopy
(TEM) images of the as-prepared p-Bi 2 0 3 show partially ag-
1 1 1 1 1
■ , 1 , 1 | 1 , T , • | 1
P-BLO -300
K 2 3
I.I.
V P-Bi 2 0 3 -5 "
1. 1. - ■ ."
25 26 27 28 29
B)
30
20 1°
31 32 33 34 35
55
50
45
: 40
Ui 35
I 30
GO
^25
O
20
15
10
■ , i i i
i 1 i 1
1 i 1
i 1
■„ - - -
_ _ - - ~4
y
0 30 60
90 120 150 180 210
Annealing time / min
240 270 300
Figure 1. A) Cutout of PXRD patterns of p-Bi 2 0 3 -5 and p-Bi 2 O 3 -300 to show
the broadening of the reflexes in dependence of the annealing time at
370 °C (reference: (3-Bi 2 0 3 , ICDD 00-027-0050). B) A plot of the particle size
determined by the Scherrer equation versus annealing time at 370 °C.
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Figure 2. TEM images of the samples A) p-Bi 2 0 3
Bi 2 O 3 -120, E) |J-Bi 2 O 3 -180 and F) p-Bi 2 O 3 -300.
5, B) p-Bi 2 o 3 -io, C) p-Bi 2 o 3 -30, D) In-
formation of nonporous p-Bi 2 0 3 nanoparticles (see Figure SI 0,
S1 1 ). The absorption edge [(542±2)nm; see Figure S1 2] and
the optical band gaps are not significantly influenced by the
crystallite size. The indirect band gap amounts to (2.1 5 ±
0.05) eV and the direct band gap to (2.44 ± 0.03) eV (see
Table 1, see Figure SI 3, S14). A complete summary of the mea-
sured properties is given in Table 1. Notably, the calculation of
the band gaps was carried out by Tauc plots, [21] in order to esti-
mate the allowed indirect and direct band gap by plots of
[ahv ) 1/2 versus hv and {ahv) 2 versus hv, respectively. A plot of
the absorption coefficient versus wavelength offers the possi-
bility to investigate the behavior of the band gap of a semicon-
ductor material. [22] A sharp onset of the absorption at the band
gap energy (E g ) combined with a large absorption coefficient
for hv > E g is typical for a direct band gap semiconductor. An
indirect band gap semiconductor shows a broad, weak onset
Table 1. Crystallite size, BET surface area and photocata lytic performance
of the as-prepared f3-Bi 2 0 3 samples.
Sample
Time
Size
BET surface area
c/c 0 of RhB
[min] [al
[nm] [b]
[m 2 g - 1 ]
[%] [c]
[s- 1 ]
|5-Bi 2 0 3 -5
5
17±2
(20±3)
29
0
4.01 x 10 4
|5-Bi 2 O 3 -10
10
18±2
(21 ±3)
29
0
4.01 x 10 4
P-Bi 2 O 3 -30
30
23±2
(23±4)
26
1
4.00x10 4
|5-Bi 2 O 3 -120
120
34±3
(33±4)
20
2
3.71 x 10 4
P-Bi 2 O 3 -180
180
40±4
(38±6)
10
14
2.07x10 4
P-Bi 2 O 3 -240
240
43±5
(n.m.)*
7
16
1.97x10 4
p-Bi 2 O 3 -300
300
45±5
(40±7)
7
19
1.78x10 4
[a] Annealing time at 370 °C. [b] Crystallite size determined by Scherrer's
equation (PXRD) and TEM (values in brackets), [c] After 150 min. [*] n.m.
not measured.
of absorption which starts at hv<E g . A typical plot of
the absorption coefficient versus wavelength and
{ahv) y2 versus hv is given for p-Bi 2 O 3 -10 in Figure 3.
This represents a typical indirect band gap semicon-
ductor behavior similar to that recently proposed for
|3-Bi 2 0 3 materials. [23] George et al. have reported on
an indirect band gap of (1.74 ±0.05) eV for |3-Bi 2 0 3
thin films, which is significantly lower compared with
that of the as-prepared |3-Bi 2 0 3 nanoparticles [(2.1 5 ±
0.05) eV] and might be attributed to the different
morphologies. [23a] The direct band gap values for the
as-prepared samples [(2.44 ± 0.03) eV] are compara-
ble to the values for (3-Bi 2 0 3 nanowires (diameter
^7nm) reported by Qiu et al. (2.47 eV), but signifi-
cantly lower than the values for (3-Bi 2 0 3 films de-
scribed by Brezesinski et al. (3.4 e V). [12e ' 20]
The photocatalytic activities of the as-prepared
samples were investigated by UV/Vis studies on the
degradation of a 10" 5 m aqueous solution of rhoda-
mine B (RhB) under visible light irradiation (420 nm<
A<700 nm). RhB is degraded by photooxidation processes
that can follow two principle pathways. [24] A decrease of the in-
tensity of the characteristic absorption band at 553 nm, which
is accompanied by a sequential blueshift, implies a de-ethyla-
tion process of the /V^/V^/V'-tetraethylrhodamine. The degra-
A)
1.0x10 5 -
8.0x1 0 4 -
6.0x1 0 4
E
o
4.0x1 0 4
2.0x10 4 -
B)
o
2.4 2.6
hvleV
Figure 3. A) Plot of the absorption coefficient versus wavelength and
B) {ahv) y2 versus hv for p-Bi 2 O 3 -10 indicating an indirect band gap semicon-
ductor behavior.
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dation products, A/,A/,AMriethylrhodamine (A max = 539 nm), A/,A/'-
diethylrhodamine (A max = 522 nm), A/-ethylrhodamine (A max =
510 nm), and rhodamine (A max = 498 nm), will induce a shift to
lower wavelengths. [25] Another pathway is given by a full de-
composition to give C0 2 and water, which results in a decrease
of the absorption band without a shift in wavelength.
A decrease of the RhB absorption band at 553 nm without
a blueshift is observed for all samples, exemplarily shown for
P-Bi 2 O 3 -10 in Figure 4 A. This indicates a fast decomposition of
the conjugated chromophore system. However, a detailed un-
derstanding of the degradation processes of RhB at the surface
of |3-Bi 2 0 3 is still lacking. Thus, we have performed UV/Vis
measurements in diffuse reflectance mode with p-Bi 2 O 3 -10,
which was covered by adsorbed RhB on the surface prior to
the investigation. As shown in Figure 5, a shift from 556 nm to
522 nm is observed within 15 min by continuous irradiation
with the instrument light source (^100 W). The observed blue-
shift of the absorption band, which represents the formation
of A/^A/'-triethyl rhodamine and A/,A/'-diethylrhodamine, indi-
cates an ongoing de-ethylation process of RhB at the (3-Bi 2 0 3
surface. In solution, the blueshift is not observed, which might
A)
1.0
0.8
0)
o 0.6
CO
_Q
s_
O n A
to 0.4
_Q
<
0.2
0.0 ■
1 I 1 I 1 I 1 I
553 nm
1 i
1 1 1 1 1
f=0min
f=10min
t = 20 min
t= 30 min
t = 40 min
f=50min
t = 60 min
t = 80 min
t= 100 min
t = 120 min
4= 150 min
i i i i i i i i
1 1 1 1 1 1 1
450 475 500 525 550 575 600 625 650
XI nm
B)
3
03
c
o
'-4— '
o
X
CD
"D
CD
N
03
E
300 350 400 450 500 550 600 650 700 750 800
A/ nm
Figure 4. A) UV/Vis spectra of the photodegradation of RhB using sample
p-Bi 2 O 3 -10 as photocatalyst (f=-30 min: start of stirring in the dark;
f = 0 min: start of irradiation with visible light). B) UV/Vis absorption spectra
of dicyanobis(1,10-phenanthroline)iron(ll) complex adsorbed onto p-Bi 2 0 3 -
120.
d)
o
c
03
_Q
-Q
<
t = 0 min
t = 15 min
> nm
-539 nm
- 52% nm
400
450
500
550
A I nm
600
650
700
Figure 5. Diffuse reflectance UV/Vis spectra of solid p-Bi 2 O 3 -10 with ad-
sorbed RhB.
be a result of adsorption/desorption kinetics. We assume that
the degradation process of RhB at the surface of the |3-Bi 2 0 3
nanoparticles is faster than the desorption processes of de-
ethylated RhB intermediates.
The normalized absorbance changes as a function of the ir-
radiation time of the samples are given in Figure 6. The RhB
solution was stirred for 30 min in the dark after addition of
|3-Bi 2 0 3 nanoparticles to establish the adsorption/desorption
equilibrium. The p-Bi 2 0 3 samples do not show adsorption of
a significant amount of RhB from the solution. However, stud-
ies in terms of a detailed characterization of |3-Bi 2 0 3 surfaces
are still lacking in the literature. For Bi 2 W0 6 and a-Bi 2 0 3 , a bis-
muth-rich surface with a high concentration of M— OH ad (M =
W, Bi) and H 2 O ad is assumed. [26] With this assumption in mind,
we probed the surface polarity for the as-prepared |3-Bi 2 0 3
nanoparticles. In general, the adsorption behavior of the dicya-
nobis(1,10-phenanthroline)iron(ll) complex on a surface can be
used to determine the hydrogen-bond-donating ability of
a metal oxide surface, which is expressed as a. The value of
a can be determined by using the Equation (1): [27]
-7.49 + 0.46v max [10- 3 cm- 1 ]
(1)
where v max represents the observed UV/Vis absorption maxi-
mum.
The adsorption of the dicyanobis(1,10-phenanthroline)iron(ll)
complex on the |3-Bi 2 0 3 nanoparticles results in a broad ab-
sorption band at 547.92 nm and thus gave a = 0.91 (Fig-
ure 4 B). This value is significantly lower than that for silica
gel 60 (a = 1.14), Al 2 0 3 (a =1.32), ZnO (a = 1.56) and W0 3 (a =
1.62). [28] Thus, we assume that the hydrogen-bonding ability of
the as-prepared |3-Bi 2 0 3 nanoparticles is lower compared to
the other mentioned oxides. Note that, Saison et al. described
the absence of Bronsted acid sites on a-Bi 2 0 3 which might be
taken as confirmation of our assumption that |3-Bi 2 0 3 nanopar-
ticles show a low hydrogen-bonding ability. [6c]
The photocatalytic experiment under irradiation with visible
light (f>0 min) without addition of a catalyst shows negligible
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tl min
80 100
k =1 .78*1 0" 4 s" 1 Ac =5.89*1 0" 6 s" 1
k=2.07*10~ A s" 1 *
^ Jc =5.98*10" 4 s" 1
140 160
/c 2 =1 .83*10 s
ff =1.67*1 0" 3 s" 1
^ pure RhB
p-Bi 2 O 3 -10
p-Bi 2 O 3 -120
p-Bi 2 O 3 -240
20 40 60 80 100 120 140 160
1 1 min
1200 2400 3600 4800 6000 7200 8400 9600
t/S
Figure 6. A) Time-dependent conversion and B) semilogarithmic plots of the photodegradation of an aqueous RhB solution (1CT 5 m) under visible light irradia-
tion (f>0 min) using (3-Bi 2 0 3 nanoparticles with different crystallite sizes as photocatalysts. f<0 min shows the adsorption behavior of the (3-Bi 2 0 3 nanoparti-
cles for RhB (inset in A).
decomposition of RhB after 150 min (5%; see Figure 6). The
best photocatalytic activities are observed for |3-Bi 2 0 3 nanopar-
ticles with a crystallite size of 17-34 nm (p-Bi 2 0 3 -5, p-Bi 2 O 3 -10,
P-Bi 2 O 3 -30, p-Bi 2 0 3 -1 20), which degrade approximately 100%
of the initial RhB within 150 min. The |3-Bi 2 0 3 nanoparticles
with a crystallite size of approximately 40-50 nm (p-Bi 2 0 3 -1 80,
P-Bi 2 O 3 -240, p-Bi 2 O 3 -300) decompose between 80% and 85%
of the initial amount of RhB. Differences in activity between in-
dividual samples are expressed in terms of reaction rate con-
stants. As shown in Figure 6, the degradation process of the
aqueous RhB solutions in the presence of the photocatalysts
follows pseudo-first-order reaction kinetics. In the absence of
any catalyst, the degradation process is quite slow (/c = 5.89x
10" 6 s _1 ). The addition of |3-Bi 2 0 3 accelerates degradation at
the beginning of the reaction by a factor of approximately 70
(/c, =4.01 x10" 4 s _1 ; p-Bi 2 0 3 -5). The lowest rate constant is ob-
served for p-Bi 2 O 3 -300 (k= 1.78x 10" 4 s" 1 ). The samples
P-Bi 2 O 3 -240 and p-Bi 2 O 3 -180 show slightly faster kinetics with
k values of 1.97x10" 4 s _1 and 2.07 x 10" 4 s" 1 , respectively. It is
noteworthy that the samples with a crystallite size up to ap-
proximately 34 nm display two distinct linear regimes with two
different rate constants k. At the beginning of the degradation
the rate constants are determined to 4.01 x10" 4 s _1 (p-Bi 2 0 3 -5,
P-Bi 2 O 3 -10), 4.00x10" 4 s" 1 (p-Bi 2 O 3 -30) and 3.71x10- 4 s~ 1 (p-
Bi 2 O 3 -120). After 110 min the reaction is accelerated by
a factor of 4.5. This phenomenon was described in the litera-
ture previously and most likely results from strong light ab-
sorption of an intensively colored RhB solution at low degrada-
tion rates. [20 ' 29]
As might be expected, our investigations show a strong in-
fluence on the activity in dependence of the crystallite size. A
smaller crystallite size results in higher surface areas and thus
provides more active catalyst sites, which is expressed in
higher degradation rates. All of the |3-Bi 2 0 3 nanoparticles
tested possess an excellent photocatalytic activity and are
quite significantly more active than the (3-Bi 2 0 3 nanoflakes syn-
thesized by Chen et al. (degradation of 55% after 120 min.),
which have been tested under similar conditions for the pho-
tocatalytic degradation of aqueous RhB solutions. [13d]
Influence of catalyst amount and initial RhB concentration
The effect of varying the amount of the catalyst from
0.025 mgmL" 1 to 4mgml_~ 1 of |3-Bi 2 0 3 nanoparticles (p-Bi 2 0 3 -
10) for the degradation of a 10" 5 m aqueous solution of RhB
was investigated. As shown in Figure 7 A, the efficiency of the
degradation process increases with an increasing amount of
the catalyst. The rate constants vary from 2.46x10" 5 s _1
(0.025 mgmL" 1 ) to 4.48x10" 4 s _1 (3 mgmL" 1 ). A linear region
of c(|3-Bi 2 0 3 ) versus k is observed up to a concentration of
2 mgmL" 1 . At higher catalyst concentrations, the reaction rates
become independent from the photocatalyst concentration as
a result of agglomeration of catalyst particles, which reduces
the number of catalytically active sites. Furthermore, stronger
absorption and light-scattering effects are present, which
reduce the ability of light to fully penetrate the solution. [30] As
a consequence, less OH' radicals are formed. A maximum of
the reaction rate constant at a specific amount of the catalyst
was also reported in the literature. For tungsten-doped Ti0 2
and for |3-Bi 2 0 3 photocatalysts this value was determined to be
8 mgmL" 1 and 2 mgmL" 1 , respectively. [23b,30] In our studies, the
maximum of the reaction rate constant was observed for
P-Bi 2 O 3 -10 with approximately 3 mgmL" 1 . The higher concen-
tration as compared to the results reported for |3-Bi 2 0 3 particles
(41 nm) by Eberl and Kisch is assigned to the smaller crystallite
size of p-Bi 2 O 3 -10. [23b]
The photocatalytic degradation properties of the (3-Bi 2 0 3
nanoparticles (p-Bi 2 O 3 -10) were also investigated in depend-
ence of the initial RhB concentration. As shown in Figure 7 B,
the reaction rate constants decrease by increasing the initial
RhB concentration. This might be a result of the light absorp-
tion of higher-concentrated RhB solutions. [29] Furthermore, the
photocatalytically active sites are blocked, which results in a re-
duction in the interaction of photons. [20,30 " 31] By using the
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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
c(p-Bi 2 0 3 ) / mg ml_" 1
B)
7.0x10" -
I
6.0x1 0" 4 -
5.0x1 0" 4 -
T c/5 4.0x1 0" 4 -
3.0x1 0" 4 -
2.0x1 0" 4 -
1.0x10" 4 -
-i — i — | — i — | — i — | — i — | — i — | — i — | — i — |—
_i i i i i i i_
0 10 20 30 40 50 60 70 80
c(RhB) / umol L" 1
Figure 7. A) Plot of the rate constant versus the catalyst concentration
(10 5 m RhB solution). B) Plot of the rate constant versus the initial RhB
concentration (1.0 mgmL 1 p-Bi 2 O 3 -10).
Langmuir-Hinshelwood kinetic model the reaction parameters
were determined by Equation (2): [30,32]
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Vk app = V(k r *ks) + c 0 /k r
(2)
where /c app [s _1 ] is the apparent rate constant, k x [jwnolxL -1 s" 1 ]
is the reaction rate constant, k s [Ljumol" 1 ] is the adsorption
rate constant and c 0 is the initial RhB concentration. A plot of
c 0 versus 1//c app shows approximately linearity, which confirms
the applicability of the Langmuir-Hinshelwood kinetic model
for the investigated system (see Figure S1 5). By determining
the intercept and the slope, the values of k x and k s were calcu-
lated to be 5.91 x10" 3 jumol x L" 1 s" 1 and 1.89x10 _1 Ljumol" 1 ,
respectively. For tungsten-doped Ti0 2 , Li et al. observed values
of 4.87x1 0" 3 jwnolL'V 1 and 6.23 x 10" 2 Ljumol" 1 by using ap-
proximately eight-times higher catalyst concentrations as re-
ported here. [30] However, the as-prepared |3-Bi 2 0 3 nanoparticles
show a similar value for k r and a significantly larger value for
k s . This might express better adsorption properties along with
higher degradation rates of |3-Bi 2 0 3 nanoparticles compared to
tungsten-doped Ti0 2 .
Stability of p-Bi 2 0 3 nanoparticles
The stability of the |3-Bi 2 0 3 nanoparticles in multiple photocata-
lytic cycles was tested by using 40 mg of the catalyst (P-Bi 2 0 3 -
10) suspended in 40 mL of a 10" 5 m aqueous RhB solution.
After 60 min, the reaction is stopped and the catalyst isolated
by centrifugation. The |3-Bi 2 0 3 particles are again dispersed in
a 10" 5 m RhB solution (40 mL) and exposed to visible light irra-
diation. As shown in Figure 8, the first two cycles exhibit iden-
tical photocatalytic performances with rate constants of 3.79 x
10" 4 s _1 . However, the following catalytic runs show a steady
loss in photocatalytic activity. After ten catalytic cycles, only
33% of the initial RhB is decomposed, resulting in a reaction
rate constant of 1.10x10" 4 s" 1 . One technical problem, but not
the major one, is the partial loss of photocatalyst during the
work-up procedure. After ten cycles, only 32 mg (80%) of the
catalyst was isolated. In a second experiment, a small amount
of the photocatalyst after each cycle was used for PXRD stud-
ies. The diffraction patterns show the formation of Bi 2 0 2 C0 3
after several catalytic cycles (Figure 9), which results in a step-
by-step loss of the photocatalytic performance. For further
A)
B)
100
90
80
70
60
50
40
30
20
-I — I — I — I — I — I — I — I — I — ' — I — i — I — I — I — 1 — [-
dark
5 * <
I
♦
▼ t
_i i i , i , i , i i i , i i i , i , i_
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-40 -30 -20 -10
0 10 20 30 40 50 60 70
1 1 min
10
20
tl min
30 40
50
60
— i 1 1 1 . 1 1 1 1-
! I I J JW-3TW 4 » 1
: * * 1 *i-
v T * ... | ; *
10*10" s" 1
1 *
▼ 4
\ *=2.09*1<TV A
/c=3.79*10" 4 s" 1 , m
_i , i i i , i_
600 1200
1800 2400
f/S
3000 3600
Figure 8. A) Time-dependent conversion and B) semilogarithmic plots of the photodegradation of an aqueous RhB solution (10 5 m) under visible light irradia-
tion (f>0 min) using p-Bi 2 O 3 -10 as photocatalyst in several runs. f<0 min shows the adsorption behavior of the p-Bi 2 O 3 -10 nanoparticles towards RhB.
© 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemistryOpen 201 3, 2, 1 46 - 1 55 151
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20 25 30 35 40 45 50 55 60 65 70
2el°
Figure 9. PXRD patterns of p-Bi 2 O 3 -10 used in different numbers of catalytic
cycles and recycled (3-Bi 2 0 3 after 10 runs (reference: (3-Bi 2 0 3 , ICDD 00-027-
0050). Asterisks represent reflections of Bi 2 0 2 C0 3 (ICDD 00-041-1488).
consideration, Bi 2 0 2 C0 3 nanoparticles were synthesized accord-
ing to ref. [33] and tested in their photocatalytic degradation
properties using our standard procedure. The Bi 2 0 2 C0 3 nano-
particles only degrade approximately 45% of a 10" 5 m aqueous
RhB solution within 60 min (see Figure S1 6), and thus a lower
photocatalytic activity is obtained compared to p-Bi 2 O 3 -10.
Based on the determination of the carbon content, it is as-
sumed that the product consists of approximately 55% (3-Bi 2 0 3
and 45% Bi 2 0 2 C0 3 after ten runs. It is suggested that the for-
mation of Bi 2 0 2 C0 3 results from the reaction of the (3-Bi 2 0 3
with the in situ formed C0 2 , which is released during the deg-
radation process of RhB. For a-Bi 2 0 3 and |3-Bi 2 0 3 , this behavior
was described recently by photodegradation of phenol after
several catalytic cycles. [23b,34] In the case of a-Bi 2 0 3/ formation
of (BiO) 4 C0 3 (OH) 2 and Bi 2 0 2 C0 3 is described. |3-Bi 2 0 3 was re-
ported to give a-Bi 2 0 3 , (BiO) 4 C0 3 (OH) 2 and Bi 2 0 2 C0 3 . However,
we observed Bi 2 0 2 C0 3 as the only photocorrosion product in
our experiments. Notably, Bi 2 0 2 C0 3 shows a structural relation-
ship to |3-Bi 2 0 3 , and it was assumed that the catalyst might be
easily recycled. [12a ' 34a ' 35] A temperature dependent PXRD study
at a heating rate of 10 Kmin" 1 of the Bi 2 0 2 C0 3 /|3-Bi 2 0 3 mixture
shows the formation of phase-pure |3-Bi 2 0 3 between 370 °C
and 380 °C. Above 420 °C, Bi 12 SiO 20 is formed as a result of the
reaction of the |3-Bi 2 0 3 nanoparticles with the quartz glass ca-
pillary (see Figure S17). [13e] Annealing of the Bi 2 0 2 C0 3 /|3-Bi 2 0 3
mixture at 380 °C for one hour in a furnace under argon atmos-
phere results quantitatively in phase-pure |3-Bi 2 0 3 with a crystal-
lite size of (31±3)nm (Figure 9). The recycled |3-Bi 2 0 3 was
tested in terms of the photocatalytic activity under the same
conditions described above. As shown in Figure S1 8, the recy-
cled |3-Bi 2 0 3 degrades approximately 67% RhB within 60 min
and gave a reaction rate constant of 3.16x10" 4 s _1 .The slightly
slower degradation process compared to the starting |3-Bi 2 0 3
nanoparticles can be explained by the larger crystallite size of
the recycled |3-Bi 2 0 3 , but its photocatalytic activity is still quite
high.
Photocatalytic degradation of selected organic pollutants
A photocatalyst with potential applications in the field of
water treatment has to be active in the degradation of differ-
ent organic substances. Our results demonstrate that |3-Bi 2 0 3
nanoparticles show a high photocatalytic activity in the degra-
dation of 4x10" 5 m aqueous solutions of various model dyes
such as orange G, methylene blue, methyl orange and rhoda-
mine B. [13e] Thus, we report on the extension of our investiga-
tions to typical organic water pollutants such as phenol,
4-chlorophenol, 2,4-dichlorphenol, 4-nitrophenol, triclosan, and
ethinyl estradiol. These pollutants were demonstrated to be
present in water and are acting as endocrine disruptors, which
for example results in feminization of male fish. [18,36] The photo-
catalytic degradation experiments were performed using
40 mL of an 4x10" 5 m aqueous solution of the appropriate
pollutant and 40 mg of p-Bi 2 0 3 nanoparticles (p-Bi 2 O 3 -10) as
catalyst. The organic pollutants are completely decomposed
within 30 min (Figure 10). The highest rate constant is ob-
served for the decomposition of triclosan (/c = 6.71 x 10" 3 s" 1 )
followed by that of ethinyl estradiol (/c = 4.74x 10" 3 s" 1 ), 4-ni-
trophenol (/c = 4.22x10~ 3 s" 1 ) and 2,4-dichlorophenol (k=
A)
100 -
90 -
80 -
70 -
60 -
50 -
40 -
30 -
20 -
10 -
0 -
— I — 1 — I — ' — I — ' — I — ' — I-
* *
dark
• ♦
• ■
♦ ■
• * f A I
-40 -30 -20 -10
0 10 20
1 1 min
30 40 50 60
B)
10
tl min
15 20
25
30
1 I 1 I 1 I
i i i i i i i
■ k =2.03*1 0~ 3 s" 1
fr=6.71*10~ 3 s" 1 \ \ \«
k =4.74*1 0" 3 s" 1 \
k =3.33*1 0" 3 s" 1
♦
k=4.22*10~ 3 s~ 1
■
•
I i
■ 4-Chlorophenol
• 2,4-Dichlorophenol
a Phenol
• Triclosan
♦ 4-Nitrophenol
★ Ethinyl estradiol
300
600
900 1200 1500 1800
tfs
Figure 10. A) Time-dependent conversion and B) semilogarithmic plots of the photodegradation of phenol, 4-chlorophenol, 2,4-dichlorophenol, 4-nitrophenol,
triclosan and ethinyl estradiol under visible light irradiation (f>0 min) using (3-Bi 2 0 3 nanoparticles as photocatalyst.
© 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemistryOpen 201 3, 2, 1 46 - 1 55 152
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3.33 x 10" 3 s" 1 ). The high activity results from the effective ad-
sorption of the pollutants at the surface of the |3-Bi 2 0 3 nano-
particles and might be explained by bismuth jt interactions of
the present aromatic system with a bismuth-rich surfaced 371
The lowest rate constants are observed for 4-chlorophenol (k =
2.03x10" 3 s" 1 ) and phenol (k= 1.96x 10" 3 s" 1 ). |3-Bi 2 0 3 particles
prepared by Cheng et al. are reported to decompose approxi-
mately 80% of a 1.56x10~ 4 m aqueous solution of 4-chloro-
phenol within 90 min under similar conditions. [13a] Eberl and
Kisch described a 94% mineralization of a 3.13x10" 4 m aque-
ous solution of 4-chlorophenol within 2 h by irradiation at
a wavelength of A > 455 nm. [23b] Li et al. investigated BiOI/Bi 2 0 3
heterostructures in terms of their photocatalytic behavior in
the decomposition of phenol and 4-chlorophenol under visible
light irradiation using a 500 W xenon lamp. [38] The BiOI/Bi 2 0 3
sample containing 20% BiOl, which exhibits the best photoca-
talytic activity, was reported to give reaction rate constants of
8.4x10" 5 s _1 and 2.6x10" 5 s _1 in the degradation of phenol
and 4-chlorophenol, respectively. However, the meaningful
comparison of obtained rate constants with reported values is
barely possible as a result of the lack of standardized reactor
systems and procedures.
Conclusions
Detailed information about the degradation parameters and
the decomposition of typical organic pollutants using (3-Bi 2 0 3
particles are scarce. Here, we have shown the synthesis of
|3-Bi 2 0 3 nanoparticles starting from the well-defined bismuth
oxido cluster [Bi 38 0 45 (OMc) 24 (DMSO) 9 ]-2 DMSO-7 H 2 0 [16] with
control of the crystallite size between (17nm±2)nm and
(45±5)nm and BET surface areas between 29 m 2 g _1 and
7 m 2 g _1 . The indirect band gap was determined to (2.1 5 ±
0.05) eV. The |3-Bi 2 0 3 nanoparticles were used as photocatalysts
to investigate the influence of the crystallite size, the concen-
tration of the catalyst and the concentration of the dye solu-
tion using rhodamine B (RhB) dye as a model system. A de-
crease of the crystallite size, a higher concentration of the cata-
lyst as well as lower concentrations of RhB solutions result in
significantly higher decomposition rates. Especially at high
concentrations of RhB solutions, absorption of light limits the
photocatalytic activity. |3-Bi 2 0 3 is transformed into Bi 2 0 2 C0 3
after several catalytic cycles, which resulted in lower decompo-
sition rates. However, the photocatalyst can be easily recycled
by annealing under argon atmosphere at 380 °C for one hour.
The recycled |3-Bi 2 0 3 (crystallite size (31±3)nm) shows only
marginally lower performance compared with as-prepared
|3-Bi 2 0 3 . The |3-Bi 2 0 3 nanoparticles photocatalytically decom-
pose phenol, 4-chlorophenol, 2,4-dichlorphenol, 4-nitrophenol,
triclosan and ethinyl estradiol with excellent degradation rates.
The photooxidation properties, the possibility to recycle the
catalyst as well as the opportunity of a straightforward gram
scale production demonstrate that the here presented (3-Bi 2 0 3
nanoparticles are auspicious materials for water purification
photocatalyst systems. Preliminary investigations by irradiation
with sun light over a period of seven hours showed promising
activities in the degradation of RhB (see Figure S1 9). Further
studies are currently under progress to develop an efficient
photocatalytic system on the basis of immobilized |3-Bi 2 0 3
nanoparticles to reduce the leaching as observed upon multi-
ple catalytic cycles.
Experimental Section
General: Powder X-ray diffraction (PXRD) patterns were measured
with a STOE Stadi P diffractometer (Darmstadt, Germany) using
CuK a radiation (40 kV, 40 mA) and a Ge(111)-monochromator. The
crystallite size was estimated using the formula determined by the
Scherrer equation r = KX/ficosO, where r is the volume-weighted
crystallite size [nm], K is the Scherrer constant, here taken as 1.0,
X is the X-ray wavelength, 6 is the Bragg angle and /3 is the full
width of diffraction line at half of the maximum intensity (FWHM;
background subtracted). The FWHM is corrected for instrumental
broadening using a LaB 6 standard (SRM 660) purchased from the
US National Institute of Standards and Technology (NIST). The
value of was corrected from = /3 2 measured - 0f nstrument $ 2 measured and
(instrument are tne FWHMs of measured and standard profiles). Trans-
mission electron micrograms were obtained by a 200 kV high-reso-
lution transmission electron microscope (HRTEM; CM20FEG, Phi-
lips) with an imaging energy filter from Gatan (GIF, CA, USA). The
energy dispersive X-ray (EDX) spectroscopy experiments and mor-
phology investigations were examined using a scanning electron
microscope (SEM; NanoNovaSEM, FEI, OR, USA). Specific surface
analyses were performed at liquid nitrogen temperature (77 K)
using a Micromeritics Gemini 2370 (GA, USA), which were evaluat-
ed by the Brunauer-Emmett-Teller (BET) method in the p/p 0 range
of 0.001-0.25. The adsorption/desorption isotherms were recorded
at liquid nitrogen temperature (-196°C) after activation under
vacuum at 130°C for 1 h using a Sorptomatic 1990 (Fisons Instru-
ments, Ipswich, UK). Diffuse reflectance UV/Vis spectroscopy was
performed using a single-beam simultaneous spectrometer MCS
400 (Carl Zeiss Jena GmbH). The UV and Vis radiation were gener-
ated using a deuterium lamp CLD 300 and a xenon lamp CLX 11,
respectively. CHN analyses were obtained with a Thermo Flash
EA 1112 CHN analyzer (Thermo Fisher Scientific). The in situ UV/Vis
measurements to examine the photocatalytic activity were carried
out by using an Agilent Cary60 UV/Vis (Agilent Technologies)
equipped with fiber optics.
Synthesis of P-Bi 2 0 3 nanoparticles: The precursor [Bi 38 045(OMc)2 4
(DMSO)g]-2 DMSO-7 H 2 0 was synthesized according to the litera-
ture. 1161 In a typical procedure, the precursor was converted into
(3-Bi 2 0 3 nanoparticles as published previously. [13e] In order to con-
trol the particle size, the time of temperature annealing at 370 °C
was varied between 5 min and 5 h. PXRD analyses proved the for-
mation of phase-pure (3-Bi 2 0 3 in every case. CHN and EDX analyses
revealed that the as-prepared products are free of carbon and
sodium, respectively.
Photocatalytic tests: The photodegradation experiments were car-
ried out by using 40 ml_ of an aqueous solution of 1 x10" 5 m rho-
damine B (RhB) or 4x10" 5 m aqueous solutions of the appropriate
organic pollutant and 40 mg of the as-prepared samples in
a water-cooled glass reactor (15°C). If not further specified, (3-Bi 2 0 3
nanoparticles with a crystallite size of approximately 20 nm were
used. The suspension was stirred in the dark for 30 min to reach
the adsorption/desorption equilibrium. The suspension was illumi-
nated with a 300 W xenon lamp (Cermax® VQTM ME300BF, Perkin-
Elmer) equipped with a hot mirror filter U<700 nm) and a UV
cutoff filter (A>420 nm, GG420, Schott) to provide visible light irra-
diation. The effective irradiation area was 4.52 cm 2 (^25% of the
© 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemistryOpen 201 3, 2, 1 46 - 1 55 153
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reactor area). The UV/Vis measurements were carried out in situ by
stopping to stir for 10 s and darkening the light beam using
a cover. The measurements were carried out up to 150 min. Up to
30 min measurements were done with a 5 min interval and up to
120 min with a 10 min interval. The degrees of conversion were
determined by calculating the mathematical area under the char-
acteristic UV/Vis absorption bands of the appropriate compounds.
Acknowledgements
We thank Prof. Dr. Stefan Spange for access to diffuse reflectance
UVA/is spectroscopy, Prof. Dr. Heinrich Lang, Janine Fritzsch for
performing CHN analyses and Dr. Susan Seifert for discussion
about the hydrogen-bond-donating ability of surfaces. We grate-
fully acknowledge the support of this work by the German Re-
search Foundation (DFG) (SPP1415). The article publication costs
were funded by the German Research Foundation (DFG) (Ge-
schaftszeichen INST 270/219-1) and the Chemnitz University of
Technology (Germany) in the funding program Open Access Pub-
lishing.
Keywords: bismuth oxide nanoparticles • bismuth oxido
clusters • organic pollutants • photocatalysis • visible light
irradiation
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Received: March 12, 2013
Published online on July 2, 2013
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