Preparation of a novel flower-like MnO2/BiOI composite with highly enhanced adsorption and photocatalytic activity

Article abstract of DOI:10.1039/c5ra02402a

A novel flower-like MnO₂/BiOI composite has been fabricated by a simple and cost-effective approach. Multiple experiments are carried out to optimize the molar ratio of MnO₂/BiOI for the decomposition of methyl orange (MO) and Rhodamine B (RhB). The adsorption property of the as-prepared samples in dark conditions and the photocatalytic activity under visible light and simulated solar light irradiation are investigated in detail. When the MnO₂/BiOI molar ratio is 4 : 5, the highest photocatalytic activity for degradation of MO (97.8%) and RhB (91.7%) is achieved under visible light irradiation for 40 min as well as simulated solar light irradiation (93.4% for MO and 88.2% for RhB within 40 min). The excellent photocatalytic performances are attributed to the synergistic effect of BiOI and MnO₂ and the unique flower-like morphology of the MnO₂/BiOI composite as verified by relative experiments.

Full text of DOI:10.1039/c5ra02402a

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Preparation of novel flower-like MnO2/BiOI composite with highly
enhanced adsorption and photocatalytic activity
Suqin Liu*, Huizhen Liu, Guanhua Jin, Hao Yuan
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A novel flowerꢀlike MnO2/BiOI composite has been fabricated by a simple and costꢀeffective approach.
Multiple experiments are carried out to optimize the molar ratio of MnO2/BiOI for the decomposition of
methyl orange (MO) and Rhodamine B (RhB). The adsorption property of asꢀprepared samples in dark
condition and photocatalytic activity under visible light and simulated solar light irradiation are
10 investigated in detail. When the MnO2/BiOI molar ratio is 4 : 5, the highest photocatalytic activity for
degradation of MO (97.8%) and RhB (91.7%) is achieved under visible light irradiation for 40 min as
well as simulated solar light irradiation (93.4% for MO and 88.2% for RhB within 40 min). The excellent
photocatalytic performances are attributed to the synergistic effect of BiOI and MnO2 and the unique
flowerꢀlike morphology of MnO2/BiOI composite as verified by relative experiments.
systems.
MnO2 is of considerable importance in technological
15 1. Introduction
The toxicity and carcinogenicity of colored dyes to humans in
environment are the major concerns all the time.1 As the wellꢀ
known typical dyes, methyl orange (MO) and Rhodamine B (RhB)
are widely used in textiles, printings, papers, foods,
20 pharmaceutical industries and research laboratories. 2, 3 Therefore,
more and more efforts have been made to explore efficient
treatment strategies for their degradation.4ꢀ10
applications including electrochemical supercapacitor, molecular
50 adsorption, catalysis and ionꢀexchange due to its masses of merits
like inexpensive, nonꢀtoxic and ease of fabrication. As a typical
catalyst, MnO2 is extensively investigated for oxidation of
organic pollutants in wastewater because of its strong adsorption
and oxidation ability.9, 27ꢀ30 Dong et al. found that MnO2 possesses
55 high adsorption capacity to MO with a removal efficiency of
78.6%.28 It is reported that MnO2 has a narrow band gap of 0.25
eV, which enable it to absorb the light at infrared wavelengths in
theory and increase the utilization rate of solar energy when
combination with other type photocatalyst such as TiO2.31ꢀ33
60 Recently, to improve the photocatalystic activity of BiOI, MnOx
is considered as a derivingꢀholeꢀtype cocatalyst to enhance the
transfer of the photoꢀinduced holes.26 Ye et al. reported that
MnOxꢀBiOI can deliver higher photocatalytic activity than BiOI
for the degradation of RhB dye under visible light irradiation.
65 However, the reported highest photocatalytic degradation
efficiency of the prepared MnOxꢀBiOI via photoꢀdeposition
method for RhB under visible light in 30 min is only 78.8%.26
As is well known, the photocatalytic activity is much depended
on the morphology, fabrication strategy and the component of
70 cocatalyst. In general, the catalyst with a morphology which has
larger surface area, trends to show better photocatalytic activities
due to the masses of exposed active sites. In addition, the
combination structure also remarkably affercts the photocatalytic
performance. Based on these, in this manuscript, a novel flowerꢀ
75 like MnO2/BiOI hybrid composite is fabricated by a simple twoꢀ
step method. And the catalytic activity of MnO2/BiOI composites
with different molar ratio is investigated by decomposing MO
and RhB under dark condition, visible light and simulated solar
Semiconductor photocatalysis is of great potential to
decompose organic water pollutants into CO2 and H2O using
25 solar light.7, 11ꢀ13 As a representative of traditional photocatalytic
materials, TiO2 is widely studied by virtues of its nontoxicity,
high chemical stability, low cost and outstanding oxidative
power.14 However, TiO2 could only respond to ultraviolet
radiation because of its large band gap (3.0ꢀ3.2 eV), which leads
30 to low photocatalytic efficiency. Recently, bismuth oxyhalides
(BiOX, X = F, Cl, Br, and I) with layered structures have been
put forward as promising candidate photocatalysts under both
ultraviolet and visible light irradiation.6, 15, 16 Among them, BiOI
exhibited the best photocatalytic activity because of its smallest
35 band gap (1.77ꢀ1.92 eV).15,
Even so, the photocatalytic
17, 18
activity of BiOI has been still restricted by some disadvantages,
such as low efficiency of light absorption, slow rate of charge
transfer and high recombination probability of the photogenerated
electron-hole pairs.19,
In the past few years, coupling with
20
40 second semiconductors, such as Bi2S3/BiOI,6 AgI/BiOI,18
ZnO/BiOI,21 TiO2/BiOI,22 BiOI/BiOX,23ꢀ25 and MnOxꢀBiOI,26 has
been proposed to notably reduce the recombination possibility
and accelerate the separation rate of photogenerated charge
carries, thus effectively improve the photodegradability of
45 semiconductor oxide photocatalyst. As a result, an increasing
number of studies have focused on the coupled semiconductor
1

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light irradiation at room temperature. As expected, the asꢀ 55 The photocatalytic activities were evaluated by the photoꢀ
prepared MnO2/BiOI composite demonstrates much higher
photocatalytic activity than the bare BiOI and MnO2.
Furthermore, the possible reasons and photocatalytic mechanism
have been put forward based on the investigation of reactive
species.
degradation of MO and RhB under the irradiation of dark, visible
light and simulated sunlight, respectively. The pH value was not
adjusted when the reaction was conducted. Experiments were
carried out at room temperature. A 300 W Xe lamp (PLSꢀ
60 SXE300, Beijing Trusttech Co., Ltd.) equipped with a 400 nm
cutoff filter was used as the visible light source, and the distance
between the liquid surface of the suspension and the light source
was set about 20 cm. A 150 W solar simulator (Beijing Zolix
Instrument CO., Ltd.) was used as the simulated sunlight source,
5
2. Experimental
2.1 Preparation of catalyst
All the reagents were analytically pure and used as received 65 and the distance between the liquid surface of the suspension and
10 without further purification. The NaI was bought from Aladdin
Chemistry Co., Ltd. Other chemicals were purchased from
Sinopharm Chemical Reagent Co., Ltd.
the light source was set about 15 cm. The photodegradation
experiments were carried out with the sample power (100 mg)
suspended in MO or RhB solution (100 mL, 10 mg Lꢀ1) with
constant stirring. Prior to irradiation, the suspensions were
In a typical experiment, 316 mg of KMnO4 was dissolved in
112 mL of deionized water by strong magnetic stirring. Then, 11 70 magnetically stirred in the dark for 30 min to obtain the
15 mL of 6 mol Lꢀ1 HCl solution was slowly dropped into the
previous solution under stirring. The asꢀobtained MnO2
precipitates were collected by centrifugation, after washing
several times with distilled water and ethanol and dried in an
oven overnight at 60 °C finally.
desorptionꢀadsorption equilibrium. A magnetic stirrer was
employed for continuous mixing during the dark and visible light
excluding simulated solar light irradiation. At certain time
intervals, 10 mL suspensions were sampled and centrifuged to
75 remove the particles. For catalytic MO and RhB degradation,
each experiment was repeated three times in order to obtain error
bars on the plots. The filtrates of MO and RhB degradation were
analyzed by a UVꢀvis spectroꢀphotometer (UVꢀ1780, Shimadzu)
at wavelength 464 nm and 574 nm, respectively.
20
The pure BiOI was synthesized according to ref. 34. 0.728 g of
Bi(NO3)3 5H2O was added in 20 mL of absolute ethanol under
.
stirring at room temperature for 30 min. Then, 40 mL of NaI
solution (0.225 g of NaI dissolved in distilled water) was added
dropwise into the previous solution. The pH of the mixture was
.
25 adjusted to 7 by adding 1.5 mol · Lꢀ1 NH3 H2O. Then, the
mixture was transferred to a 250 mL flask and maintained at 80
°C for 3 h by oil bath heating. After completion of the reaction,
the precipitates were collected by centrifugation, washed several
times with distilled water and ethanol and dried in an oven
30 overnight at 60 °C finally.
80 3. Results and discussion
3.1 Characterization of MnO2/BiOI
MnO2
In order to synthesize MnO2/BiOI composite, the asꢀprepared
MnO2 was well dispersed by ultrasound for 1 hour in ethanol
solution containing the same molar of Bi(NO3)3 5H2O. The
.
102
MnO2/BiOI (4/5)
subsequent operations are the same as the preparation of BiOI.
35 For comparison, MnO2/BiOI composites with molar ratio of
.
110
MnO2 to Bi(NO3) 3 5H2O (2/7, 3/5, 4/5, 4/4, 6/3) are designed.
001
10
212
114
200
2.2 Characterization
BiOI
004
40
002
20
The crystal phases of the samples were analyzed by a MXPAHF
Xꢀray diffractometer from 10° to 80° with a Cu Kα of 1.54056
40 Å. Scanning electron microscopy (SEM, JEOL JSMꢀ6360) and
highꢀresolution transmission electron microscopy (HRTEM,
JEOL JEMꢀ2010) were employed to observe the morphologies
and microstructures. The BrunauerꢀEmmettꢀTeller (BET) surface
area was measured using a Quantachrome autosorbꢀ1 automated
45 gas sorption systems at 77K. Xꢀray photoelectron spectroscopy
(XPS) was measured with a Thermo VG Scientific Sigma Probe
tetragonal BiOI
60 70 80
30
50
Two-Theta (deg)
Fig. 1 XRD patterns of the asꢀprepared BiOI, MnO2/BiOI (4/5), and
MnO2
85 Fig.1 presents the XRD patterns of the pure BiOI, MnO2 and
MnO2/BiOI (4/5). All the diffraction peaks of the bare BiOI could
be well assigned to the tetragonal phase (JCPDS card no. 10ꢀ
0445).35 The three diffraction peaks at 2θ = 29.3°, 31.8° and 45.6°
are well assigned to the (102), (110) and (200) planes of BiOI,
90 respectively. The sharp and narrow diffraction peaks indicate that
the sample is well crystallized. The XRD peaks of MnO2 match
well with the layered birnessiteꢀtype MnO2 (JCPDS card no. 80ꢀ
1098).36 The low intensity reveals the poor crystallinity of the
MnO2. The MnO2/BiOI composite (4/5) shows similar diffraction
spectrometer.
A PerkinꢀElmer Optimaꢀ8000 was used to
determine the real amount of MnO2 and BiOI in the samples by
inductively coupled plasma analysis (ICP). The UVꢀVIS diffuse
50 reflection spectra were obtained for the dryꢀpressed disk samples
using a Scan UVꢀVIS spectrophotometer (UVꢀVIS DRS : UVꢀ
2550, Shimadzu) equipped with an integrating sphere assembly,
using BaSO4 as reflectance sample.
2.3 Photocatalytic experiments
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lines to that of the pure BiOI, and no obvious diffraction lines of
MnO2 is observed. This may be due to the low diffraction
intensity of MnO2. Meanwhile, the main peak of MnO2 is very
close to that of BiOI. Similar phenomenon was observed in
previous literatures. 26, 33
5
To further confirm the existence of MnO2 in the composite, Xꢀ
ray photoemission spectroscopy (XPS) (Fig. 2) is applied to
investigate the surface composition and chemical state of the
MnO2/BiOI (4/5) composite and the physical mixed MnO2 and
10 BiOI (marked as MnO2 + BiOI (4 + 5) ). Elements of Bi, O, I and
Mn are clearly observed in the survey spectra (Fig. 2a). The high
resolution XPS of Mn 2p3/2 is displayed in Fig. 2b, from which
two peaks positioned at 642.2 eV and 654.2 eV, which are
ascribed to the Mn 2p3/2 state and Mn 2p1/2, are observed.26 The
15 two strong peaks at 159.0 eV and 164.4 eV in the high resolution
XPS of Bi 4f (Fig. 2c) are assigned to Bi 4f7/2 and Bi 4f5/2 peaks
of Bi3+, in good agreement with the valence state of Bi in BiOI.26
As shown in Fig. 2d, the two peaks of I region at 619.0 eV and
630.5 eV relate to the I 3d5/2 and I 3d3/2, respectively,
20 corresponding to the characteristics of Iꢀ in the BiOI. The O1s
core level spectrum (Fig. 2e) of MnO2/BiOI (4/5) could be fitted
to three peaks at 529.7 eV, 530.4 eV and 531.8 eV,26 which could
be assigned to the MnꢀOꢀMn37 bonds in MnO2, BiꢀO bonds22 in
[Bi2O2] slabs of BiOI layered structure, and hydroxyl groups,
25 respectively. Note that the peaks of MnꢀOꢀMn bonds and BiꢀO
bonds in MnO2/BiOI (4/5) has a slight shift compared with those
of physical sample MnO2+BiOI (4+5) (Fig. 2f), implying the
interaction between MnO2 and BiOI.
35
Fig.3 SEM images of BiOI (a) and MnO2 (b); SEM (c), TEM (d) and
HRTEM (d, f) images of MnO2/BiOI (4/5) composite; Inset is EDS image
of MnO2/BiOI (4/5) composite.
The morphology features of the asꢀprepared products were
40 characterized by SEM, TEM, and HRTEM. Fig. 3a shows that the
pure BiOI is composed of smooth flakes with uneven size less
than 500 nm. The pure MnO2 is of microsphere with a diameter
of 500 nm, but an aggregation takes place (Fig. 3b). The
MnO2/BiOI (4/5) composite demonstrates
a
flowerꢀlike
45 morphology structure, which is constructed by many straight
flakes (Fig. 3c and d). The diameter of the flower is about 1 ꢁm,
reveal a possible combination of MnO2 and BiOI. Note that, in
the preparation process, the MnO2 was dispersed in the Bi(NO3)3
solution by ultrasound firstly. Then, Iꢀ was added. After adjusting
50 the pH to 7, the primary BiOI crystals were formed firstly and
prefer to grow in the surface of MnO2 when precipitating from
solution owing to the tendency to reduce the total surface energy.
As a result, a different flowerꢀlike morphology was formed. In
such a unique flowerꢀlike structure, the BiOI could contact with
55 MnO2 intimately, which will facilitate the fast transfer of photoꢀ
induced carriers. Additionally, as seen from the TEM picture
(Fig. 3d), the flakes stack and intercross with one another,
forming a lot of pores in the surface. The abundant voids could
not only enrich the active sites of BiOI, but also increase the
60 contact between catalyst and MO solution, which will benefit to
the photocatalytic activity.
30
Fig. 2 The survey XPS of MnO2/BiOI (4/5) (a), high resolution XPS of
Mn 2p (b), Bi 4f (c), I 3d (d), O 1s (e) in MnO2/BiOI (4/5), and O 1s (f) in
MnO2+BiOI (4+5)
The HRTEM images of the MnO2/BiOI composite are given in
Fig. 3d and Fig. 3e. Three sets of different fringes are clearly
found. The fringe spacing of 0.305 nm, 0.224 nm and 0.199 nm
65 agree well with the spacing of the (102), (004) and (200) lattice
planes of BiOI, respectively. In Fig. 3e, lattice fringes with
3

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spacing of 0.240 nm and 0.718 nm are found, corresponding to
the indirect band gap (Eg) is estimated to be 1.77 eV, in good
the (112) fact of BiOI and the (001) facet of MnO2, 30 accordance with the band gap of BiOI previously reported.5, 17 40
respectively.38 The wellꢀdefined fringes and the high crystallinity
of the MnO2/BiOI composite will facilitate the separation of the
photoꢀinduced carrier and thus improve the corresponding
catalytic activities.18 In the EDS image (Fig. 3f inset), elements of
Although the band edge positions of the BiOI are not easily
determined experimentally, a theoretical prediction is possible
using concepts of electronegativity. The conduction band (CB)
and valence band (VB) potentials can be calculated by the
5
Mn, I, Bi and O are clearly displayed, further confirming the coꢀ 35 following equations:41
existence of MnO2 and BiOI.
EVB = X − E c + 0.5E g
(2)
(3)
3.2 Optical absorption properties
Where EVB is the VB edge potential, ECB is the CB edge potential,
X is the electronegativity of the semiconductor, which can be
40 calculated as the geometric mean of the electronegativity of the
constituent atoms. The X value for BiOI is ca. 5.99 eV according
to relative reports.5 Ec is the energy of free electrons on the
hydrogen scale (about 4.5 eV), Eg is the band gap energy (1.77
eV) of the semiconductor. Accordingly, the VB and CB of BiOI
45 were estimated to be 2.38 and 0.61 eV, respectively. Because the
band edges could move with the change of pH value, the new
potentials of VB and CB for BiOI at pH = 7.0 is calculated
according to Eq. (4): 42ꢀ44
Efp = E0fp −0.05915pH
(4)
50
Where
is the flat band potential of catalyst when the pH is 0.
When the pH is 7.0, the corresponding VB and CB of BiOI are
1.97 and 0.20 eV, respectively.
3.3 Catalytic activity of MnO2/BiOI
MO and RhB were chosen as different types of model
55 pollutants to evaluate the adsorption ability and photocatalytic
activity of the asꢀprepared samples under dark condition and
visible light irradiation, respectively. Fig. 5 shows that the
concentration of dyes declines gradually without visible light
irradiation, which is attributed to the adsorption process.5, 6, 9, 35
60 The pure BiOI shows negligible activity for MO or RhB
adsorption. By contrast, about 57.2% of MO and 40.3% of RhB
are adsorbed by MnO2 after 30 min, illustrating relative strong
sorption ability. When the molar ratio of MnO2 to BiOI is 4/5, the
highest adsorption performance of both MO and RhB are
65 achieved. This phenomenon may have a relationship with their
special BET surface areas (SBET). The SBET of the pure BiOI,
MnO2 and MnO2/BiOI (4/5) composite are 13.19, 32.72 and
47.38 m2 gꢀ1, respectively. It is well accepted that the large
specific surface area will be beneficial to the adsorption ability.23
70 Obviously, the largest SBET of MnO2/BiOI (4/5) composite
origins from its special flowerꢀlike structure, in which amount of
voids exist.
10
Fig. 4 (a) UVꢀVis diffuse reflectance spectra (DRS) of asꢀprepared
MnO2/BiOI samples; (b) the plotting of (αhν)1/2 νѕ. photon energy
The optical absorption and energy band feature of
semiconductor are crucial factors determining its photocatalytic
a
15 performance. Fig. 4 is the UVꢀvis DRS of the asꢀprepared
samples. As seen, BiOI has a strong absorption edge for visible
light at about 650 nm. Increasing the MnO2 content, the
absorption edge of MnO2/BiOI shows an obvious red shift.
Meanwhile, the absorption intensity of MnO2/BiOI is enhanced
20 gradually. The band gap energy of a semiconductor could be
calculated by the following formula: 5, 18, 39
(1)
wherea α, ν, Eg, and A are the absorption coefficient, light
frequency, band gap, and a constant, respectively. Among them, n
25 depends on the characteristics of the transition in
a
semiconductor, including direct transition (n = 1) or indirect
transition (n = 4). The indirect band gaps are obtained from the
intersection of the plot of [αhν]1/2 versus hν. For the pure BiOI,
4

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irradiation. At the same time, it can be seen from Fig. 6d that
most the fuchsia color of the starting RhB solution fades just after
30 visible light irradiation for 10 min. And an evident decrease from
2.109 to 0.083 in RhB absorption at λ = 554 nm is observed.
Obviously, the MnO2/BiOI (4/5) composite demonstrates much
enhanced catalytic activity than the bare BiOI and MnO2. The
highest photocatalytic activity of MnO2/BiOI (4/5) is mainly
35 attributed to the synergistic effect between MnO2 and BiOI. It
was reported that MnO2 could act as a derivingꢀhole type
cocatalyst to enhance the separation efficiency of the
photoinduced carriers of BiOI.26 The detailed photocatalytic
mechanisms are further discussed in the photocatalytic
40 mechanism part. Moreover, the unique flowerꢀlike morphology
will also contribute to the outstanding photocatalytic
performance. In this structure, the intimate conduct contact
between MnO2 and BiOI is conducive to the charge transfer from
BiOI to MnO2. This could be confirmed by the low photocatalytic
45 activities of the physical mixed MnO2 and BiOI (marked as MnO2
+ BiOI (4 + 5) ) under both dark condition and visible light
irradiation. Note that a simple physical mixing could not provide
a close combination of MnO2 and BiOI, thus the charge transfer
is not fluent.5 20 In addition, the flowerꢀlike morphology has a
50 relative high SBET which increase the surface active sites as well
as the contact area between the dye solution and catalyst. 1, 45, 46
Fig. 5 Adsorption of MO (a) and RhB (b) as a function of time under
dark condition
As seen from the adsorption data, the concentration of the MO
and RhB solution shows negligible decrease after 30 min in the
darkness. Therefore, we irradiated the samples with visible or
simulated solar light after treating the solution under dark
conditions for 30 min. As shown in Fig. 6a and Fig. 6b, the
concentration of dyes decrease slightly in the absence of catalyst,
5
10 which confirm the good chemical stability of MO and RhB. For
MnO2, about 60.5% of MO and 43.4% of RhB are removed in 40
min under visible light irradiation, closing to its adsorption
capacity (59.5% for MO and 42.2% for RhB). The poor
photoctalysis activity may have relationship with its narrow band
15 gap of 0.25 eV, which enable it to absorb the light at infrared
wavelengths.31ꢀ33 It is worth noting that, the photocatalytic
conversion of MO degradation over the MnO2/BiOI (4/5) sample
reach the max value of 94% after just 10 min irradiation. As is
shown in Fig, 6c, the main peak of MO (λ = 464 nm) completely
20 disappears only after 10 min irradiation. Moreover, the color of
the supernatant changes from yellow to colorless observed by the
naked eye, verifying the excellent photocatalytic activity of the
MnO2/BiOI (4/5) under visible light irradiation. Such result is
much higher than that of the bare BiOI (44.4% in 10 min).
25 Significantly, the asꢀobtained MnO2/BiOI (4/5) also shows a
relatively high photocatalytic performance for RhB with a
degradation ratio of 91.7% after 40 min of visible light
Fig. 6 Photodegradation of MO (a) and RhB (b) as a function of
irradiation time; UVꢀVis spectral changes of MO (c) and RhB (d) as a
55
function of irradiation time over MnO2/BiOI (4/5)
To further reveal the excellent photocatalytic performance of
the MnO2/BiOI (4/5) composite, we also evaluated the
photocatalytic activity using MO and RhB under simulated solar
light irradiation. As Fig. 7 displayed, the highest photocatalytic
60 activity is still achieved over the MnO2/BiOI (4/5) sample,
inducing 93.4% for MO and 88.2% for RhB degradation within
40 min. The cause of the slight decrease in activity for the
MnO2/BiOI(4/5) sample compared to the degradation
performance (97.8% for MO and 91.7% for RhB) under visible
65 light is that the photodegradation experiments under simulated
solar light irradiation were performed without constant stirring.
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IPA affect the photocatalytic activity of MnO2/BiOI (4/5)
20 slightly, suggesting that ·OH does not play a key role for the
degradation of the two dyes. On the contrary, the photocatalytic
degradation of MO or RhB is obviously inhibited after the
−
addition of BQ or EDTA, which implies that h+ and ·O2 played
major role in MnO2/BiOI (4/5) under visible light irradiation.
25 This corresponds well with the previous reported results.
Based on above results, the coupling effects of MnO2 and BiOI
was proposed to explain the enhanced photocatalytic activity of
MnO2/BiOI (4/5) composite. The proposed schematic mechanism
of MnO2/BiOI composite is shown in Fig. 9. It is well known
30 that the photodegradation processes are depended on charge
carrier generation and separation. As shown in Fig. 9, under
visible excitation, the electron of BiOI can be promoted from the
valence band to the conduction band, leaving behind a hole in the
valence band. Then the photoinduced holes transfer to MnO2 and
35 largely reduce the recombination of photogenerated electrons and
holes, thus more effective electrons and holes are taking part in
the photodegradation process.26
Fig. 7 Photocatalytic activity of the bare MnO2, BiOI, and MnO2/BiOI
samples with different molar ratios (MnO2/BiOI) under simulated solar
light irradiation
5
3.4 Possible photocatalytic mechanism
The unique flowerꢀlike morphology of the composite also
attributes to the remarkable photocatalytic performance. Firstly,
40 the relative high SBET and massed of pores increase the surface
active sites as well as the contact area between the dye solution
and catalyst. Moreover, the enhanced sensitization effect between
the dyes and BiOI may also be beneficial to the degradation
efficiency. In the process of dye photosensitization, the electrons
45 on the highest occupied molecular orbital (HOMO) of dye
molecules will be excited to the lowest unoccupied molecular
orbitals (LUMO) under visible light, and subsequently would be
injected to the CB of BiOI. Then the adsorbed molecular oxygen
ꢀ
would capture the electrons in the CB of BiOI to form ·O2
50 radicals, which degrade the adsorbed MO or RhB dye further.5, 6
That is why the ·O2 scavenger suppresses dye degradation as
ꢀ
shown in Fig. 8. At the same time, the photoinduced holes on the
surface of BiOI can also directly oxidize the organic pollutants.
That is to say, the photocatalytic process and photosensitization
55 process jointly involve in the dye degradation.
Fig.8 Effects of different scavengers on the degradation of MO (a) and
RhB (b) over MnO2/BiOI (4/5) composite
As is well known, during the photocatalytic oxidation (PCO)
10 process, a series of reactive oxygen species, such as h+, ·OH, or
·O2 , are supposed to be involved. In order to investigate the roles
Fig. 9 The proposed possible mechanism for the improvement of
photocatalytic activity
−
of these reactive species, we conducted the reactive species
trapping experiments. In our experiments, isopropanol (IPA),47, 48
ethylene diamine tetraacetic acid (EDTA),5, 49 and benzoquinone
4. Conclusion
51
−
15 (BQ),50, acting as the scavengers for ·OH, h+ and ·O2 , are 60 In summary, we report a simple, economical and effective
introduced into the PCO process, respectively.
Fig. 8 shows the variation of MO and RhB degradation with
different quenchers. Both the MO and RhB solution added with
synthesis of a novel flowerꢀlike MnO2/BiOI composite. The
optimum MnO2/BiOI composite (4/5) demonstrate a maximal
photocatalytic degradation efficiency of 97.8% under visible light
6

Page 7 of 7
RSC Advances
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DOI: 10.1039/C5RA02402A
28. W. F. Dong, L. H. Zang and H. Li, Appl. Mech. Mater., 2013, 361,
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and 93.4% under simulated solar light irradiation for MO within
40 min. In addition, the RhB removal efficiency reached 91.7%
and 88.2% in 40 min under visible and simulated solar light
irradiation, respectively. The outstanding photocatalytic activity
is attributed to the cocatalytic effect of MnO2 and BiOI, as well
as the unique flowerꢀlike morphology, which provide a relative
high SBET and intimate contact of MnO2 and BiOI. Our results
suggest that MnO2/BiOI composites may be promising
photocatalyst for the degradation of organic pollutants due to its
70
5
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