Bismuth oxyiodide heterojunctions in photocatalytic degradation of phenolic molecules

Article abstract of DOI:10.1007/s11164-015-1976-7

Bismuth oxyiodide composites were synthesized and tested as photocatalysts under visible light irradiation. Calcination of bismuth oxyiodide, BiOI, at 420 °C for various lengths of time led to composites with varying compositions of Bi₇O₉I₃ and α-Bi₅O₇I. The ease of forming iodide-poor bismuth oxyhalides can be attributed to ready loss of the strongly reducing iodide ion. Thermogravimetric measurements show that BiOI is not thermally stable above 350 °C. The Bi₇O₉I₃/a- Bi₅O₇I composites formed active visible-light photocatalysts for the degradation of phenol, p-chlorophenol, p-cresol, and 4-tert-butylphenol. Compared to BiOI, the rate constant for the photodegradation of phenol was 5-6 times higher over the Bi₇O₉I₃/α-Bi₅O₇I composites. The excellent activity can be attributed to the efficient separation of photogenerated charge carriers at the intimately contacted heterojunctions of the in situ-generated composites.

Full text of DOI:10.1007/s11164-015-1976-7

Res Chem Intermed
DOI 10.1007/s11164-015-1976-7
Bismuth oxyiodide heterojunctions in photocatalytic
degradation of phenolic molecules
Aijuan Han1 Siew Fung Chian1 Xiu Yi Toy2
•
•
•
Jiulong Sun2 Stephan Jaenicke1 Gaik-
Khuan Chuah1
•
•
Received: 27 June 2014 / Accepted: 21 October 2014
ꢀ Springer Science+Business Media Dordrecht 2015
Abstract Bismuth oxyiodide composites were synthesized and tested as photo-
catalysts under visible light irradiation. Calcination of bismuth oxyiodide, BiOI, at
420 ꢁC for various lengths of time led to composites with varying compositions of
Bi7O9I3 and a-Bi5O7I. The ease of forming iodide-poor bismuth oxyhalides can be
attributed to ready loss of the strongly reducing iodide ion. Thermogravimetric
measurements show that BiOI is not thermally stable above 350 ꢁC. The Bi7O9I3/a-
Bi5O7I composites formed active visible-light photocatalysts for the degradation of
phenol, p-chlorophenol, p-cresol, and 4-tert-butylphenol. Compared to BiOI, the
rate constant for the photodegradation of phenol was 5–6 times higher over the
Bi7O9I3/a-Bi5O7I composites. The excellent activity can be attributed to the effi-
cient separation of photogenerated charge carriers at the intimately contacted
heterojunctions of the in situ-generated composites.
Keywords Bismuth oxyiodides ꢀ Composites ꢀ Heterojunction ꢀ Photocatalysts ꢀ
Phenolic
Introduction
Wastewaters containing organic pollutants are of major environmental concern.
These pollutants include industrial chemicals, dyes, pesticides, pharmaceuticals and
personal care products that are continuously released and enter into natural water
[1]. Photocatalysts attract increasing interest as a means to treat the water and to
destroy these pollutants [2]. Most of the proposed materials are semiconductors,
&
Gaik-Khuan Chuah
chmcgk@nus.edu.sg
1
2
Department of Chemistry, National University of Singapore, Singapore 117543, Singapore
NUS Environmental Research Institute, National University of Singapore, 5A Engineering
Drive 1, Singapore 117411, Singapore
123

A. Han et al.
where photo-generated holes act as strong oxidizing agents. Among these, TiO2 has
been most extensively used due to its photoactivity, insolubility, stability, low cost
and non-toxic nature [3]. Unfortunately, TiO2 is not ideal as it has a wide band gap
of 3–3.2 eV (387–413 nm) and therefore can only utilize the ultraviolet portion of
the solar spectrum. However, only a small fraction (ca. 4 %) of the incoming solar
energy lies in the ultraviolet region, whereas the visible light (400–700 nm) in the
solar spectrum is far more abundant (ca. 46 %). Therefore, the development of
photocatalysts that are activated by visible light is important as they will utilize a
larger portion of the solar spectrum.
Recently, bismuth-based compounds such as Bi2O3 [4, 5], Bi2S3 [6], Bi oxyhalides
[7–11], Bi2WO6 [12–14] and BiVO4 [15–18] have attracted much attention because
they are able to degrade organic pollutants using visible radiation. In particular,
composites such as BiOCl/BiOI [19, 20], BiOBr/BiOI [21, 22], BiOCl/BiOBr [23–
25], BiOCl/Bi2O3 [26], Bi2O3/NaBiO3 [27], NaBiO3/BiOCl [28], Bi2O3/Bi2WO6 [29–
31] and Bi2O3/BiVO4 [32] have shown high photocatalytic efficiency. Generally, the
composite comprises a sensitizer semiconductor with a narrow band gap in contact
with a main semiconductor that has a wider band gap. The vectorial transfer of
electrons or holes from one semiconductor to the other reduces the intrinsic
recombination rate, thereby increasing the probability of interfacial charge-transfer
processes over that of a single phase material. The absorption of radiation is extended
to the visible range due to the existence of low band gap semiconductor. In this study,
we prepared tightly adhering composites of different bismuth oxyiodides by in situ
thermal treatment of BiOI. Five stoichiometric bismuth oxyiodides with different
oxygen content are known, namely, BiOI, Bi4O5I2, Bi7O9I3, and a- and b-Bi5O7I [33–
38]. The atomic arrangement in BiOI consists of tetragonal 2-dimensional [Bi2O2]2?
layers which are ‘‘sandwiched’’ by two halogen layers (Fig. 1a). The crystal structures
of the bismuth oxyhalides, BixOyIz, are closely related but with different halogen
Fig. 1 Crystal structures of a BiOI and b a-Bi5O7I
123

Bismuth oxyiodide heterojunctions in photocatalytic degradation…
content in the sandwich layer, depending on the stoichiometry. In a-Bi5O7I, the
[Bi2O2]2? sheets aligned parallel to (001) are mutually connected by folded
1-dimensional [Bi4O8]4- ribbons with axes parallel to [010] and oriented parallel to
(100) (Fig. 1b). The [Bi2O2]2? induces an internal electric field perpendicular to the
layer sheets, which is instrumental in the separation of photo-generated electron–hole
pairs. We envision that the similarity in crystal structures of the bismuth oxyiodides
will enable the formation of heterojunctions between domains of BixOyIz with
different composition, thus facilitating the electron–hole separation. The facile
transformation of BiOI to Bi4O5I2, Bi7O9I3, and a-Bi5O7I upon thermal treatment can
be advantageously used to form these ‘‘tight’’ mixtures of composites required for the
formation of a heterojunction.
The photoactivity of some of these compounds has been reported, especially for
the degradation of dyes by visible light [39–44]. However, the use of dyes as model
compounds may not prove that the material indeed exhibits an intrinsic photocat-
alytic activity as the dye itself absorbs visible light [45]. Hence, we investigated the
photodegradation of non-absorbing compounds such as phenol and phenolic
compounds. These molecules are common pollutants of aquatic systems. The usual
treatment of phenol containing wastewater is by chemical adsorption using activated
carbon; however, regeneration of the used carbon is not possible, and the spent
carbon has usually to be disposed of by controlled incineration [46]. Photocatalysis
offers a promising method for the removal of these phenolic pollutants.
Experimental
Sample preparation
The preparation of BiOI was adapted from Ref. [47]. In a typical synthesis, 0.372 g
of potassium iodide and 0.53 g of sodium acetate were dissolved into approximately
21 ml of water to form solution A. Solution B was prepared by adding 1.57 ml of
glacial acetic acid to 1.088 g of bismuth nitrate pentahydrate. Solution B was added
quickly to solution A with intense stirring. The resultant orange precipitate was
filtered, washed with ethanol and deionized water, and dried at 80 ꢁC in air. The
dried sample was calcined at 420 ꢁC for different lengths of time using a Ney 2-525
Series II oven.
Characterization
Powder X-ray diffractometry was performed using a Siemens D5005 diffractometer
˚
equipped with a Cu anode (k = 1.5418 A) and variable slits. Typically, measure-
ments were made for 2h from 5 to 70ꢁ at a scan rate of 0.02ꢁ s-1 and a dwell time of
1 s step-1. The relative composition of the composites was determined from the
peak areas. Nitrogen adsorption and desorption isotherms were obtained at 77 K
using a TriStar 3000 v.5.02 (Micromeritics) porosimeter. The specific surface area
was calculated using the BET (Brunauer–Emmett–Teller) equation. The pore size
distribution was obtained from the desorption branch of the isotherm using the
123

A. Han et al.
Barrett–Joyner–Halenda equation. Prior to the measurement, the samples were
degassed for 4 h at 100 ꢁC for BiOI and 300 ꢁC for the composites. Scanning
electron microscopy (SEM) images were obtained with a JEOL JSM-6701F SEM
(field-emission) microscope. Thermogravimetry was carried out using a TA
Instruments SDT 2960 simultaneous DTA–TGA. The sample was dried at 100 ꢁC
for 30 min before increasing the temperature to 800 ꢁC at 10 ꢁC min-1 in air.
Diffuse reflectance UV–visible spectra were measured with a Shimadzu UV-2450
spectrophotometer using BaSO4 as a reference.
Photocatalytic activity
A suspension of 0.10 g photocatalyst and 100 ml aqueous solution containing
24 mg/L of the phenolic compound (phenol, 4-chlorophenol, p-cresol and tert-
butylphenol) was placed in a 250-ml cylindrical quartz reactor fitted with a water
cooling jacket. After stirring for an hour to establish the adsorption–desorption
equilibrium in the dark, an aliquot was removed, centrifuged to remove any
particulates, and the concentration of the organic compound (Co) was determined by
UV–visible spectroscopy (Shimadzu UV-1601). The reactor was irradiated with a
22-W fluorescent lamp (KES, E27), and further aliquots were removed at different
time intervals for analysis (C). The degradation efficiency (DE) was calculated from
the following equation:
ðCo ꢁ CÞ
DE ¼
ꢂ 100
Co
Results and discussion
Characterization
The thermal stability of BiOI was investigated using thermogravimetry. The TGA
profile shows a small weight loss of *2 % when the temperature was raised from
100 ꢁC to about 350 ꢁC, indicating the stability of BiOI in this low temperature
regime. However, from 350 to 510 ꢁC, a steep weight loss of *23 % occurred
followed by a smaller weight loss of *8 % from 510 to 750 ꢁC (Fig. 2). The total
weight decrease was in the order of 33 %, which agrees with the decomposition of
BiOI to Bi2O3 as the final compound. Above 350 ꢁC, iodine is lost from BiOI, in
turn forming the various oxyhalides as follows:
4BiOI þ 1/2O2 ! Bi4O5I2 þ I2
7Bi4O5I2 þ 1/2O2 ! 4Bi7O9I3 þ I2
5Bi7O9I3 þ 2O2 ! 7Bi5O7I þ 4I2
2Bi5O7I þ 1/2O2 ! 5Bi2O3 þ I2
123

Bismuth oxyiodide heterojunctions in photocatalytic degradation…
100
-0.8
-0.6
-0.4
-0.2
0
90
80
70
60
0
200
400
600
800
Temperature (oC)
Fig. 2 TGA and derivative weight loss for BiOI
The continuous weight loss from 350 to *700 ꢁC indicates the ease at which the
bismuth oxyiodides undergo transformation. Being a strong reductant, the iodide is
easily oxidized and lost as iodine upon thermal treatment in air.
The X-ray diffractogram of the as-prepared BiOI shows diffraction lines that can
be indexed to the tetragonal phase with space group P4/nmm (JCPDS 10-0445). No
other phases were observed (Fig. 3). After calcination at 420 ꢁC for 0.5 h, the
diffraction lines of BiOI were shifted to smaller 2h. The identity of this compound
was deduced from the weight loss in TGA to be Bi7O9I3. The measured weight loss
from 100 to 850 ꢁC was 18.5 % which agrees with the transformation of Bi7O9I3 to
Bi2O3 (19.3 %). When the heating of BiOI was extended to 1 h, another bismuth
oxyiodide compound, a-Bi5O7I (PDF 00-40-0548), was formed which coexisted
Bi5O7I (40-0548)
(f)
(e)
(d)
(c)
(b)
(a)
BiOI (10-0445)
0
10
20
30
40
50
60
70
2θ (º)
Fig. 3 XRD patterns of (a) BiOI and samples calcined at 420 ꢁC for (b) 0.5 (c) 1.5 (d) 2 (e) 4 and (f) 8 h
123

A. Han et al.
with Bi7O9I3. From the peak areas in the diffractogram, the proportion of Bi7O9I3/a-
Bi5O7I was 88:12 (Table 1). The proportion of a-Bi5O7I increased with heating
time. However, the X-ray diffractogram of the sample heated for 4 h showed small
peaks at 2h *30ꢁ, 42ꢁ and 52ꢁ which cannot be assigned to Bi7O9I3 or a-Bi5O7I.
These peaks may be due to Bi3O4I although there are no reported X-ray data for this
compound. However, the bromine analog, Bi3O4Br, is known (PDF 04-013-1331).
After 8 h, the sample had completely transformed into the pure a-Bi5O7I phase,
which was stable at this temperature so that, even after heating for 48 h, no further
decomposition to Bi2O3 was observed. Hence, by varying the heating time at
420 ꢁC, different compositions of Bi7O9I3 and a-Bi5O7I, were formed. The closely
similar crystal structures of the oxyiodides should lead to an intimate contact
between the two different components during this in situ transformation.
The electron micrographs show that the as-precipitated BiOI sample forms
nanoplates which agglomerate into 3-D flower-like structures (Fig. 4a). The
diameter of the nanoplates was estimated to be about 180–250 nm. After calcination
at 420 ꢁC, the flower-like agglomerates were partially destroyed, exposing more
nanoplates (Fig. 4b). As a result, the samples calcined for 0.5–1.5 h had higher
surface areas, 11.4–14.0 m2 g-1 (Table 1). In contrast, the as-precipitated BiOI has
a surface area of only 8.90 m2 g-1. The rupture of the flower-like structures appears
to be linked to the transformation of BiOI to Bi7O9I3 and a-Bi5O7I as the iodide ions
were replaced by oxygen. However, the surface area decreased for longer heating
times above 1.5 h.
The UV–visible diffuse reflectance spectrum shows that the as-precipitated BiOI
absorbs from 200 to 600 nm with the absorption edge at about 650 nm (Fig. 5). The
spectra of the other bismuth oxyiodide samples formed after calcination show a blue
shift. In Bi7O9I3 and the Bi7O9I3/a-Bi5O7I composites, the absorption edge is at
*500 and 450 nm, respectively, while in pure a-Bi5O7I (obtained by calcining
BiOI for 8 h), it is at about 400 nm. As the bismuth oxyiodides are indirect
‘
semiconductors, the band gap Eg can be obtained from a plot of [F(R?)ꢀht]
against the photon energy, ht, where F(R?) is the Kubelka–Munk transformation of
the reflectance. For BiOI, Bi7O9I3/a-Bi5O7I, and a-Bi5O7I, Eg were found to be
1.91, 2.54–2.64, and 3.16 eV, respectively (Table 1). These values agree with those
reported in the literature [40, 41, 43, 44]. The electronic structure was calculated by
Density Functional Theory using CASTEP code. The valence band in bismuth
Table 1 Bismuth oxyiodides formed after heating BiOI at 420 ꢁC
Time (h)
BiOI
Bi7O9I3 (%)
a-Bi5O7I (%)
Surface area (m2/g)
Eg (eV)
0
100
0
0
100
88
63
36
0
0
0
8.90
14.1
12.4
11.4
7.90
5.93
4.26
1.91
2.51
2.54
2.56
2.60
2.64
3.16
0.5
1
0
12
1.5
2
0
37
0
64
98a
4
0
8
0
0
100
Contains a small fraction of an unidentified phase, possibly Bi3O4I
123

Bismuth oxyiodide heterojunctions in photocatalytic degradation…
Fig. 4 SEM images of a as-precipitated BiOI and b after calcination at 420 ꢁC for 1 h
1.4
BiOI
1/2 h
1.2
1 h
2 h
4 h
8 h
1.0
0.8
Bi2O3
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 5 UV–vis diffuse reflectance spectra of BiOI and after heating at 420 ꢁC for 0.5–8 h. (The spectrum
of Bi2O3 is included for reference)
oxyiodide is composed of mainly oxygen 2p and iodine 5p with smaller
contributions from bismuth 6s and 6p orbitals, while the conduction band is made
up of bismuth 6p, iodine 5s and oxygen 2p orbitals. Hence, a loss of iodine in the
bismuth oxyhalides will result in an increase in the band gap.
Photocatalytic activity
The as-prepared BiOI showed low photocatalytic activity for degradation of phenol.
After 4 h, only about *15 % degradation was observed (Fig. 6). Assuming a pseudo-
-1
cat
first-order model where ln(Co/C) = kt, the rate constant is 0.007 min-1
g
(Table 2).
Calcining the BiOI at 420 ꢁC for 0.5 h resulted in Bi7O9I3 which showed higher
123

A. Han et al.
80
60
40
20
0
Bi2O3
0
0.5
1
1.5
2
4
8
Heating time (h)
Fig. 6 Degradation efficiency for phenol over Bi2O3 and BiOI heated at 420 ꢁC for various times
Table 2 Quantum efficiency
for phenol degradation over
bismuth oxyiodide composites
-1
cat
Catalyst
k (min-1
g
)
Quantum efficiency (%)
BiOI
0.5
1
0.007
0.031
0.044
0.046
0.042
0.031
0.018
0.41
2.25
3.04
3.06
2.95
1.83
1.17
1.5
2
4
8
photoactivity. The rate constant, k, was increased to 0.031 min-1
51 % phenol degradation was observed. However, the highest activity, k, of
g
so that, after 4 h,
-1
cat
0.042–0.046 min-1
g , and degradation efficiencies of *66–68 % in 4 h, were
cat
-1
shownbytheBi7O9I3/a-Bi5O7Icomposites. Theformationofpurephasea-Bi5O7Iledto
lower photoactivity. Nevertheless, the degradation efficiency of 33 % over a-Bi5O7I
washigherthanthe15 %observedfortheiodine-freeBi2O3. Thequantumefficiencyfor
visible light (k [ 420 nm) for phenol degradation varied from 0.4 to *3 %, depending
on the photocatalyst (Table 2). Despite its narrow band gap of 1.91 eV, the quantum
efficiencyfor BiOIwas only0.4 %. Incomparison, the Bi7O9I3/Bi5O7I composites have
wider band gaps of 2.54–2.60 eV but the quantum efficiency was *3 %. This
observation supports the hypothesis that close junctions between composites can aid in
higher photocatalytic efficiency through electron–hole separation, although we do not
have any direct measurements of electron–hole recombination in these materials. In
comparison, the pure phase BiOI and Bi2O3 showed significantly lower activity.
In view of the good activity of the Bi7O9I3/a-Bi5O7I composites, the sample with 88 %
Bi7O9I3/12 % a-Bi5O7I was tested for the degradation of other phenolic compounds
(Fig. 7). The substituted phenols with electron-donating alkyl groups such as para-cresol
123

Bismuth oxyiodide heterojunctions in photocatalytic degradation…
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
Time (h)
Fig. 7 Photodegradation efficiency for phenol (filled diamond), 4-chlorophenol (open circle), para-
cresol (filled square), and 4-tert-butylphenol (open triangle) over 88 % Bi7O9I3/12 % a-Bi5O7I
and 4-tert-butylphenol were more easily degraded than phenol or 4-chlorophenol. The
degradation efficiency for the former substrates was[99 % after 2 h, whereas about 8 h
were required to achieve[90 % degradation of phenol and 4-chlorophenol.
The recyclability of the 88 % Bi7O9I3/12 % a-Bi5O7I composite was tested.
Following a batch reaction for phenol degradation, the photocatalyst was recovered
and tested for another cycle without any pretreatment. After 4 h, the used catalyst has a
100
80
60
40
20
0
0
2
4
6
8
Time (h)
Fig. 8 Photodegradation efficiency for phenol over fresh (open circle), untreated used (filled diamond),
and 300 ꢁC-recalcined (open square) 88 % Bi7O9I3/12 % a-Bi5O7I composite
123

A. Han et al.
phenol degradation efficiency of 52 % as compared to 67 % for the fresh catalyst. The
X-ray diffractograms did not reveal any differences between the fresh and used
photocatalyst. However, thermogravimetric analysis showed a sharp drop in weight at
*300 ꢁC, suggesting the presence of adsorbed organics at the surface of the used
photocatalyst. Hence, the used photocatalyst was recalcined at 300 ꢁC for 2 h before
testing. It was found that this pretreatment was sufficient for full recovery of the
photoactivity (Fig. 8). Surprisingly, the compounds proved very stable against iodine
loss, and the UV–visible reflectance spectra and X-ray diffractograms of the fresh and
recycled catalyst showed no significant changes (Fig. 9).
1.4
(a)
1.2
fresh
recycled
1.0
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
Wavelength (nm)
(b)
recycled
fresh
60
10
20
30
40
50
70
2θ (º)
Fig. 9 a UV-vis diffuse reflectance spectra and b XRD of fresh and recycled Bi7O9I3/a-Bi5O7I catalyst
after 4 runs
123

Bismuth oxyiodide heterojunctions in photocatalytic degradation…
Conclusion
The precipitation of bismuth oxyiodide, BiOI, under aqueous conditions in the
absence of any template or surfactant gave rise to hierarchical flower-like
microspheres made up of thin nanoplatelets. A series of composites containing
Bi7O9I3 and a-Bi5O7I was formed by calcining BiOI at 420 ꢁC in air for varying
lengths of times. The identity of Bi7O9I3 and a-Bi5O7I was determined by powder
X-ray diffractometry and weight loss in thermogravimetry. From UV–visible
diffuse reflectance measurements, the band gap energy increased from 1.91 eV in
BiOI to 3.16 eV in a-Bi5O7I. The composites showed very good photoactivity in the
degradation of phenolic compounds under visible light irradiation. Cresol and
4-tert-butyl phenol were degraded at a higher rate than phenol or 4-chlorophenol.
The composites were more active than pure phase BiOI or a-Bi5O7I and maintained
their activity in subsequent batch runs.
Acknowledgments X.Y. Toy and J.L. Sun gratefully acknowledge the support of Singapore-Peking-
Oxford Research Enterprise for a research assistantship and research scholarship, respectively. This work
was supported by Ministry of Education ARC Tier 1 Grants, RP 143-000-476-112 and R-143–000–
550–112.
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