Adsorption and UV/Visible photocatalytic performance of BiOI for methyl orange, Rhodamine B and methylene blue: Ag and Ti-loading effects

Article abstract of DOI:10.1039/c3ce42654h

We synthesized echinoid-like BiOI microspheres with various doped concentrations of Ag (0.1, 1.0, 5.0, 10.0 mol%) and Ti (1.0, 5.0, 10.0, 30.0, 50.0 mol%) in ethylene glycol and then examined their fundamental properties by scanning electron microscopy, t

Full text of DOI:10.1039/c3ce42654h

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Adsorption and UV/Visible photocatalytic
performance of BiOI for methyl orange,
Rhodamine B and methylene blue:
Ag and Ti-loading effects†
Cite this: CrystEngComm, 2014, 16,
3155
a
a
b
c
a
*
Yohan Park, Yulyi Na, Debabrata Pradhan, Bong-Ki Min and Youngku Sohn
We synthesized echinoid-like BiOI microspheres with various doped concentrations of Ag (0.1, 1.0, 5.0,
10.0 mol%) and Ti (1.0, 5.0, 10.0, 30.0, 50.0 mol%) in ethylene glycol and then examined their fundamental
properties by scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis,
UV-visible absorption, FT-IR, Raman, photoluminescence and Brunauer–Emmett–Teller (BET) surface area
measurements. We also measured the adsorption and photocatalytic dye-degradation performance of
the catalysts using methyl orange (MO), Rhodamine B (RhB) and methylene blue (MB). The adsorption
performance was found to be in the order of MO < RhB < MB, and to depend somewhat upon the Ag
and Ti-loadings. MO was degraded in the order of BiOI < Ag–BiOI < Ti–BiOI under UV and visible light
irradiation, while the degradation of RhB was in the order of Ag–BiOI ≈ Ti–BiOI ≪ BiOI, and Ag–BiOI <
BiOI < Ti–BiOI, respectively. MB showed poor photodegradation under UV and visible light. Finally, we
used an indirect chemical probe method with active species scavengers and photoluminescence spec-
Received 30th December 2013,
Accepted 17th January 2014
DOI: 10.1039/c3ce42654h
troscopy to clarify the dye-sensitized photodegradation mechanism. In the mechanism, ˙O2 and h+ were
−
www.rsc.org/crystengcomm
active species under visible light irradiation. No ˙OH radicals were found by luminescence spectroscopy.
Recently, more extensive research has focused on designing
hybrid materials with BiOI to further enhance the catalytic
1. Introduction
Water treatments and impurity removal aided by adsorption
and photocatalysis using a catalyst are very important. Bismuth
oxyhalides (BiOX, X = Cl, Br and I) have recently been exten-
sively investigated for use as adsorbents and photocatalysts for
the removal of unwanted pollutants.1–47 Since the adsorption
and photocatalytic performance are commonly determined by
morphology (e.g., sizes and crystal facets) and composition,
many studies have been conducted to tailor the morphology
and synthesize hybrid materials. Two morphologies of 3D-
echinoid-like (or 3D-hierarchical flower-like) and 2D-sheets (or
2D-plates) have commonly been obtained and reported for
BiOX. In many tested systems such as those that evaluate dye
removal, the 3D BiOX structure has shown much higher
adsorption and photocatalytic activities than the 2D structure.35
activity under visible light irradiation. The reported hybrid
materials include AgI/BiOI1,2,4,47 Ag/AgI/BiOI,10,11 Ag/BiOI,29,30
TiO2/BiOI,31,35,36 MnOx–BiOI,3 BiOI/ZnSn(OH)6,5 BiOI/Bi2WO6,6,7
BiOI/BiPO4,13 Ag3PO4/BiOI,18 Bi/BiOI,19 BiOI–MWCNT,21 BiOI/
n-ZnTiO320 BiOI/BiOX,22,24,26,37,41,42 Bi2S3/BiOI,27 Bi2O2CO3/BiOI,28
BiOI/B2O3,38,39 ZnO/BiOI,40 and Pt/BiOI.46 The heterojunction
material commonly enhances visible light absorption and
further facilitates good electron–hole transport and separation,
which results in the effective removal of impurities by
photoabsorption. When an n-type (e.g., ZnO and TiO2) material
with a higher band gap (3.0–4.0 eV) is junctioned with p-type
BiOI with a lower band gap of ~1.8 eV, visible light irradiation
creates an electron and a hole in the conduction and valence
bands in BiOI, respectively.35,36,40 The electrons are then effi-
ciently transferred to the conduction band of the p-type mate-
rial, leaving the hole in the valence band of BiOI. This process
facilitates the electron–hole pair separation without recombi-
nation. Metallic nanoparticles such as Ag have been reported to
facilitate the capture of photogenerated electrons by surface oxy-
a Department of Chemistry, Yeungnam University, Gyeongsan 712-749,
Republic of Korea. E-mail: youngkusohn@ynu.ac.kr; Fax: +82 53 810 4613;
Tel: +82 53 810 2354
b Materials Science Centre, Indian Institute of Technology, Kharagpur
721 302, W.B., India
c Instrumental Analysis Center, Yeungnam University, Gyeongsan 712-749,
Republic of Korea
−
gen to form superoxide (˙O2 ) radicals, which act as oxidants30
Cheng et al. demonstrated that AgI/BiOI hierarchical hybrids
show much higher photocatalytic 2,4-dichlorophenol decompo-
sition than unhybridized BiOI, and the activity was shown to
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3ce42654h
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decrease with the decreasing Ag nanoparticle size.1 It has been
reported that photoinduced holes in the AgI/BiOI hybrid mate-
rials are the main factor responsible for the degradation of
organic pollutants.47 Major active intermediate species include
adding KI, after which we followed the same procedures
described above.
−
2.2. Catalyst characterization
hydroxyl radicals (˙OH), superoxide radicals (˙O2 ), electrons (e−)
and holes (h+). DMPO (5,5-dimethyl-1-pyrroline N-oxide) EPR
(electron paramagnetic resonance) spin trapping techniques
have been employed to detect these species.48,49 LC-MS (liquid
chromatography-mass spectrometry)19 and LC-SIR (selected ion
recording)-MS were employed to detect the intermediates of the
photocatalytic reactions for tetrabromobisphenol50 and carba-
mazepine.51 As an indirect chemical probe method, scavengers
of the active species were added during the photoreaction,22,27,49
after which the dye degradation rate was measured.
Ideally, a catalyst should be multifuncational under any
environmental conditions. In the present study, we exam-
ined the adsorption and photocatalytic performance of hier-
archical echinoid-like (or flower-like) BiOI with different
doped concentrations of Ag and Ti for the degradation of
methyl orange (MO), Rhodamine B (RhB) and methylene
blue (MB), which have different molecular structures and
UV-visible absorption bands. We chose two different (Ag
and Ti-loaded) catalytic systems for comparison. Ag was
chosen because the Ag+ ion reacts with I− of BiOI to form
AgI with a band gap of ~2.8 eV47 higher than that (~1.8 eV)
of BiOI. On the other hand, Ti does not chemically react
with BiOI, and TiO2 is the most well known photocatalyst
with a band gap of 3.2 eV.16 It was reported that a higher
band gap material can be combined with the lower band
The surface morphology of the powder samples was observed
by scanning electron microscopy (SEM, Hitachi SE-4800). The
microstructures of the catalyst powder were examined by trans-
mission electron microscopy (TEM) and high resolution TEM
(HRTEM) using a Tecnai F20 G2 FEI-TEM operated at 200.0 kV.
The structural property of samples was further obtained from
the electron diffraction patterns. The energy-dispersive X-ray
spectroscopy (EDX) measurement was carried out to check the
elemental composition of the samples. Additionally, the
powder X-ray diffraction (XRD) patterns were evaluated using a
PANalytical X'Pert Pro MPD diffractometer with Cu Kα radia-
tion (40 kV and 30 mA). The UV-visible absorption spectra of
the powder samples were obtained using a Varian Cary 5000
UV-visible-near IR (UV-Vis-NIR) spectrophotometer (Agilent
Technologies). The Fourier-transform infrared (FT-IR) spectra
were acquired using a Thermo Scientific Nicolet iS5 spectro-
meter. The Raman spectra of the pelletized samples were
recorded using a Bruker Senterra Raman spectrometer with a
laser excitation energy of 758 nm. The photoluminescence
spectra were collected using a SCINCO FluoroMate FS-2 at exci-
tation wavelengths of 250 and 280 nm with a Xe lamp source.
The Brunauer–Emmett–Teller (BET) surface area was measured
using a Quantachrome ChemBET TPR/TPD analyzer with a
thermal conductivity detector and 30% N2/He.
gap material BiOI to obtain
a synergic photocatalytic
effect.1,2,4,10,11,29–31,35,36,47 The originality of this work is fur-
ther discussed later. The results presented herein provide a
clearer understanding of hybrid BiOI adsorption and degra-
dation mechanisms and therefore a deeper scientific insight
that will be useful for the development of catalysts/adsorbents
for use in water treatment facilities and environmental reme-
diation projects.
2.3. Catalyst performance tests
To test the adsorption performance of the catalysts, we dis-
persed 10 mg of a sample into 50 mL of a dye (MO, RhB and
MB at 5 mg L−1) solution and then measured the change in
the concentration of the dye solution with time under dark
conditions. Specifically, the concentration was measured
based on the UV-visible absorption intensity using a Jasco
V-530 UV-Vis absorption spectrophotometer. For the photo-
catalytic dye degradation experiments, we dispersed 10 mg of
BiOI, 5 mol% Ag–BiOI, or 10 mol% Ti–BiOI catalysts in a dye
(MO, RhB and MB) solution (10 mg L−1, 50 mL). Following the
adsorption of the dye for 2 hours under dark conditions, we
irradiated the samples with visible (using a halogen lamp) or
UV (using an UV lamp irradiating 352 nm) light. After selected
irradiation times, we measured the concentration of the dye
solution using a UV-visible absorption spectrophotometer
(Jasco V-530). An indirect scavenger method was employed to
2. Experimental section
2.1. Catalyst preparation
Echinoid-like BiOI nanostructures were synthesized as fol-
lows. Stoichiometric amounts of Bi(NO3)3·5H2O (98%, Junsei,
Japan) and KI (99.5%, Samchun Pure Chem., Korea) were
completely dissolved in 20 mL ethylene glycol (99.5%,
Samchun Pure Chem., Korea), after which the transparent
yellow solution was transferred to a Teflon bottle that was
tightly capped and placed in an oven (120 °C) for 12 hours.
The final reddish-color products were fully washed with
Millipore water and ethanol repeatedly, after which they
were dried in an oven (80 °C) for 24 hours. We added appro-
priate amounts of a 0.1 M silver nitrate solution (Yakuri,
Japan) or titanium(IV) isopropoxide (98%, Junsei, Japan) to
dope the samples with Ag (0.1, 1.0 5.0 and 10.0 mol%) and Ti
(1.0, 5.0, 10.0, 30.0 and 50.0 mol%), respectively, before
−
examine the roles of the active species such as ˙OH, ˙O2 and
h+, with isopropyl alcohol (IPA), benzoquinone (BQ) and EDTA
−
used as scavengers for ˙OH, ˙O2 and h+, respectively.22,27,41,49
Following irradiation with visible (λ > 420 nm) light, we
measured the UV-visible absorption of the dye solutions
containing the scavenger. Photoluminescence was employed
to detect active ˙OH radicals using a terephthalic acid solution
with the catalyst after visible light irradiation.26
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3. Results and discussion
3.1. Morphology, crystal structure and growth mechanism
Fig. 1 and 2 show the SEM images of the undoped, Ag-loaded
(0.1, 1, 5 and 10 mol%) and Ti-loaded (1, 5, 10, 30 and
50 mol%) BiOI prepared in ethylene glycol, respectively. The
SEM image of the undoped BiOI shows an echinoid-like
microsphere structure with a size of about 1–2 μm and a
surface that appears like a connection of vertically staked
walls. When 0.1 mol% Ag was loaded into the BiOI, the mor-
phology showed no critical change. However, when 1.0 mol%
Ag was loaded into the BiOI, the surface changed signifi-
cantly. Specifically, as the Ag-loading increased, the size of
the microspheres became smaller and distorted. In addition,
many small clusters were shown on the surface. This change
in the morphology is very similar to that of the AgI/BiOI hier-
archical hybrid structures reported by Cheng et al.1 The
observation of the SEM images of the Ti-loaded samples
revealed
a little change with increasing the Ti-loading
amount compared to the Ag-loaded samples. As the Ti-loading
increased, the surface showed a wave-like structure and the
sphere size became smaller.
As shown in Fig. 3, the 3D-hierarchical echinoid (or
flower)-like microspheres appear to be formed by
a
Fig. 2 SEM images of BiOI doped with different concentrations (1, 5,
10, 30 and 50 mol%) of Ti.
3D-assembly growth mechanism. In this process, small BiOI
crystal units (with Ag and Ti units) are formed by a reaction
of Bi3+ + I− + H2O → BiOI + 2H+. These small units initially
grow into sheets that subsequently dissolve and re-grow on
larger particles by making connections between vertically
stacked sheets via the Ostwald ripening process.25,52,53
The 3D-self-assembly process minimizes the surface energy
required to consequently form the microspheres. The Ag or
Ti units are embedded inside the 3D structure during
crystal growth.
We further examined the microstructures of the catalysts
using TEM and HRTEM. Fig. 4 displays the TEM, HRTEM
and HAADF images and the selected area electron diffraction
(SAED) patterns of the BiOI, Ag–BiOI (10 mol%) and Ti–BiOI
(50 mol%) samples. The HRTEM image of BiOI showed clear
lattice fringes with a spacing of 0.303 nm, corresponding to
Fig. 1 SEM images of BiOI doped with different concentrations (0.1, 1,
Fig. 3 A plausible growth mechanism of the echinoid-like Ag and
5 and 10 mol%) of Ag.
Ti-loaded BiOI nanostructures.
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Fig. 4 TEM (left and middle columns), HRTEM (right column) and high-angle annular dark field (HAADF) TEM images (inset, left column) of the
BiOI, Ag–BiOI (10 mol%) and Ti–BiOI (50 mol%) samples. The insets (right column) show the corresponding electron diffraction patterns.
the {102} facet of tetragonal BiOI. The SAED pattern also indi-
cates the crystalline nature of the sample. For Ag–BiOI, small
clusters appear to be embedded in the microstructure as
shown in the TEM image. The surface area may be decreased
due to the formation of the clusters, which is discussed later.
The HRTEM image shows the various lattice fringes with
spacings of 0.282 nm, 0.329 nm and 0.223 nm. These corres-
pond to the {110} facet of BiOI, the {003} facet of BiO and the
{220} facet of AgI, respectively. For the HRTEM image of
Ti–BiOI (50 mol%), interestingly the lattice spacing became
wider with 0.345 nm, and the planes show a staked structure.
It appears that during the crystal growth, titanium(IV) iso-
propoxide may incorporate between the BiOI layers to expand
the lattice spacing. Furthermore, the ring SAED suggests the
polycrystalline nature of the sample as confirmed from the
broad XRD feature (discussed later). Since the size of the Ti
atom is smaller than that of the Bi atom, Ti may easily incor-
porate into the BiOI lattice.54 Li et al. synthesized BiOBr–
Bi2WO6 mesoporous nanosheet composites with and without
titanium(IV) isopropoxide.55 They found that titanium(IV) iso-
propoxide induces pores in the nanosheets and impacts on
the facet formation.
patterns of the undoped flower (or echinoid)-like 3D-BiOI
matched those of tetragonal (P4/nmm) BiOI (JCPDS 10-0445)
well.32,34,35,39 The four major peaks at 2θ = 29.3°, 31.8°, 45.6°
and 55.1° were assigned to the (102), (110), (200) and (212)
planes, respectively.34,35,39 Other minor peaks were also
assigned to the corresponding XRD peaks. As the Ag-loading
increased, new XRD peaks appeared, although the XRD peaks
of BiOI were still predominant. The new patterns of the
10 mol% Ag-loaded BiOI matched those of hexagonal
(P63mc) AgI (JCPDS 9-0374). In addition to the XRD peaks of
AgI, a shoulder near the (102) peak was found, which was
tentatively attributed to rhombohedral (JCPDS 1-075-0995)
bismuth oxide (BiO). During the precipitation and crystal
growth process, Ag+ reacts with I− to form AgI1,2,4,10,11 via an
ion exchange process (e.g., BiOI + Ag+ → AgI + BiO+). During
this process, AgI is embedded in the 3D-BiOI structure. For
the Ti-loaded BiOI, the XRD peak positions were very similar;
however, as shown in the inset of Fig. 5, some XRD peaks
became much broader than others. No XRD peak of TiO2 was
observed. The (102) peak of BiOI was significantly broadened
with the increasing Ti amount, and the (110) peak was
shifted to a larger 2θ angle. This is possibly due to the facets
affected by Ti and the incorporation of Ti into the BiOI lattice
during the crystal growth. Although, no XRD peaks were
obtained from TiO2 embedded in the lattice, the EDX
Fig. 5 displays the powder XRD patterns of undoped, Ag-
loaded (0.1, 1, 5 and 10 mol%) and Ti-loaded (1, 5, 10, 30
and 50 mol%) BiOI prepared in ethylene glycol. The XRD
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Fig. 5 The power X-ray diffraction patterns of BiOI with increasing Ag (left) and Ti (right) loadings. The inset (right) shows the difference in the
(102) and (110) peaks between undoped BiOI and 10 mol% Ti–BiOI.
measurement confirms the presence of Ti (Fig. S1, ESI†). This
suggests that the Ti is embedded in the lattice of BiOI as an
amorphous form. Wei et al. prepared BiOBr–TiO2 catalysts
using tetra-butyl titanate.54 They also did not observe XRD
peaks of crystalline Ti-species, and attributed the Ti to the
amorphous phase.
white-brown when the Ti amount was increased. The optical
band gaps were estimated using the Tauc equation, αhν =
A(hν − Eg)n/2, where A is an empirical constant, and n = 1 for
the direct band gap and n = 4 for the indirect band
gap.2,4,9,20,56 The indirect band gaps were obtained from the
intersection of the plot of [αhν]1/2 versus hν. For undoped
BiOI, the indirect band gap was estimated to be 1.82 eV, and
this value showed a small change upon the Ag loading to
1.83 eV for the 10 mol% Ag-loaded BiOI. For the Ti-loaded
sample, the band gap became gradually increased when the
Ti amount was increased to 1.92 eV for the 50 mol% Ti-
loaded BiOI. Xia et al. reported a band gap of 1.84 eV for the
flower-like porous BiOI microspheres,57 which is in good
agreement with the results of the present study. A band gap
of 1.76 eV for sheet-shaped single-crystalline BiOI was
reported by Chang et al.56 Band gap energies of 1.77 and
1.72 eV were reported for BiOI nanoflakes and nanoplates,
respectively.44 However, a much lower band gap of 1.46–1.63 eV
was reported for BiOI prepared using a microwave irradiation
in a mannitol solution.17
3.2. Band gaps, Raman spectra and photoluminescence
The UV-Vis reflectance absorption spectra of the Ag-loaded
(0.1, 1, 5 and 10 mol%) and Ti-loaded (1, 5, 10, 30 and
50 mol%) BiOI are displayed in Fig. 6. The absorbance Y-axis
data were converted from diffuse reflectance data by the
Kubelka–Munk method. For the Ag-loaded samples, the
absorption edge showed no critical change as the Ag amount
was changed, which is in good agreement with the results
reported by Lv et al.4 However, the edge of the Ti-loaded sam-
ple shifted to a shorter wavelength when the Ti amount was
increased. The brown color of the Ag-loaded samples
remained about the same while the Ti-loaded sample became
Fig. 6 The UV-Visible diffuse reflectance absorption spectra of BiOI with increasing Ag (left) and Ti (right) loadings. The insets (top) show the photo-
graphs of the corresponding samples. The lower left insets show the plots of [αhν]1/2 versus hν for BiOI, 10 mol% Ag–BiOI and 50 mol% Ti–BiOI.
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Fig. 7 displays the Raman spectra of BiOI doped with dif-
ferent concentrations of Ag and Ti. The corresponding FT-IR
spectra are provided in Fig. S2, ESI.† A peak at 148 cm−1
attributed to the Eg mode (internal Bi–I stretching) was found
in all of the Raman spectra, which is in good agreement with
previous studies.24,27 The B1g and A1g modes were nearly
absent. The vibrational modes of BiOI with a tetragonal struc-
ture (P4/mmm) are represented as Γ = 2A1g + B1g + 3Eg + 2Eu +
2A2u, where A1g, B1g and 3Eg are Raman active, and the others
are IR active.2,12,22,24 The major peak position showed no crit-
ical change as the amounts of Ag and Ti changed, indicating
that the major BiOI structure was preserved regardless of the
guests, which agrees well with the XRD results (Fig. 5).
Fig. 8 The photoluminescence spectra of BiOI with increasing Ag
(left) and Ti (right) loadings at an excitation wavelength of 280 nm.
Fig. 8 displays the photoluminescence (PL) spectra of the
BiOI, Ag and Ti-loaded BiOI catalysts at an excitation wave-
length of 280 nm. A broad peak was commonly observed at
around 600 nm.32,46 Yu et al. also observed a broad peak at
~600 nm for the BiOI nanoplates.46 Photoluminescence (PL)
spectroscopy has commonly been employed to predict the
photocatalytic efficiency of a catalyst. Since the electrons and
holes play major roles in the photocatalytic reaction the cata-
lytic activity decreases when an electron and a hole recom-
bine to emit a photon.2,20,58 For this reason, catalysts with a
high PL will generally have a lower photocatalytic perfor-
mance. The PL intensity of BiOI increased by about the same
amount upon the Ag and Ti-loadings. Based on the PL data
alone, the photocatalytic activity of the Ag and Ti-loaded BiOI
should be lower; however, the observed photocatalytic activity
was not consistent with the PL results, indicating that other
factors are also involved in the photocatalytic mechanism.
just 10 min (Fig. 9). For undoped BiOI, about 10% of MO
and 90% of RhB and MB were adsorbed, which suggests
that BiOI is not a suitable catalyst for MO adsorption. For
0.1 mol% Ag-doped BiOI, the adsorption performance was
slightly enhanced, compared with BiOI. The adsorption per-
formance became poor when the Ag-loading was increased
above 1 mol% for all of the dyes i.e. MO, RhB and MB (left
side, Fig. 9). For the Ti-loaded BiOI with MO (top right,
Fig. 9), the adsorption performance was increased with
increasing Ti-loading up to 30 mol%. The 0.1 mol% Ag
3.3. The adsorption of MO, RhB and MB, and the BET
surface area
We tested the adsorption performance of BiOI, Ag-doped BiOI
and Ti-doped BiOI using methyl orange (MO), Rhodamine B
(RhB) and methylene blue (MB) under dark conditions after
Fig. 9 The UV-visible absorption spectra and the corresponding peak
intensities upon the adsorption of MO, RhB and MB (5 mg L−1, 50 mL)
by 10 mg BiOI, Ag-doped BiOI and Ti-doped BiOI. The insets show the
photo images of the corresponding dye solutions after adsorption. All
the absorption peaks were normalized before adsorption.
Fig. 7 The Raman spectra of BiOI with increasing Ag (left) and Ti
(right) loadings.
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Table 2 The BET surface areas (m2 g−1) of reference (undoped) BiOI
and Ti-doped BiOI with different Ti concentrations
doped BiOI showed the highest adsorption of RhB (middle
left, Fig. 9). For RhB, the adsorption performance of
Ti-loaded BiOI was found to be almost the same except for
1.0 mol% Ti–BiOI (middle right, Fig. 9). For the MB dye, the
adsorption performance was lower with 1, 5, and 10 mol%
Ti–BiOI, but higher with 30 and 50 mol% Ti–BiOI. Among
these dyes, the adsorption was commonly found to be in the
order of MO ≪ RhB < MB, which is in good agreement with
the literature.59 Zhang et al. reported that the BiOBr cata-
lysts showed a better absorption performance for RhB and
MB than for MO,59 which is consistent with the results of
the present study. They attributed the higher adsorption
ability of RhB and MB to stronger electrostatic interactions
between the dyes and the BiOBr surface. Similarly, because
RhB and MB are positively charged while MO is negatively
charged in solution, the negatively surface charged BiOI will
interact more strongly with RhB and MB.
Since the surface area of a catalyst is related to the
adsorption, the BET surface areas (Fig. S3, ESI†) were
measured. These values are summarized in Tables 1 and 2
for the Ag and Ti-loaded BiOI catalysts, respectively. Before
loading Ag and Ti, the 3D-flower like BiOI exhibited a BET
surface area of 60.8 m2 g−1, which is larger than the values
reported in previously conducted studies.3–5,13,20,27,41,57
Xia et al. measured BET specific surface areas of 28.3 m2 g−1
and 5.3 m2 g−1 for 3D flower-like BiOI and 2D-plates,
respectively.57 Upon loading 0.1% Ag into BiOI, the surface
area increased slightly, while loading more than 1 mol% led
to a significant decrease in the BET area. For the Ti-loading
below 10 mol%, the surface areas were similar compared
with that of undoped BiOI, while the 30 and 50 mol%-Ti
samples showed a dramatic increase in the BET surface area.
Overall, for the higher loading of Ag (1, 5 and 10 mol%) and
Ti (30 and 50 mol%), the BET surface area was correlated with
the adsorption performance. In other words, the surface area
effect was more dominant than other effects. For the lower Ag
and Ti loadings, the adsorption of a dye onto the catalyst was
more likely dependent on the nature (e.g., surface charge and
molecular dipoles) of the dye and the catalyst.59
Ti mol%
0
1
5
10
30
50
Surface area (m2 g−1
)
60.8
57.8
70.5
58.0
98.8
111.5
we tested the adsorption of MO, RhB and MB with a dye
concentration of 10 mg L−1 under dark conditions (Fig. S4,
ESI†). Upon adsorption, we examined the photodegradation
of MO, RhB and MB by the three (BiOI, 5 mol% Ag–BiOI and
10 mol% Ti BiOI) selected catalysts under UV (λ < 420 nm)
and visible (λ > 420 nm) light irradiation (Fig. 10).60 Without
the catalyst, we observed no significant degradation of the
dye under UV or visible light irradiation, indicating that a
charge transfer process between the dye and the catalyst
may be an important factor for improving dye degradation
(discussed in detail below). After the adsorption of the three
different dyes for 2 hours in the dark, we started irradiation
with UV or visible light.
Fig. 10 displays the photodegradation rates (change in
peak position and UV-visible absorption spectra) of the MO,
RhB and MB solutions with BiOI, 5 mol% Ag-loaded BiOI
and 10 mol% Ti-loaded BiOI under UV and visible light irra-
diation. For MO, the degradation rate (or the change in
peak position) was very similar under UV and visible irradia-
tion. Specifically, the degradation performance showed the
order of BiOI < Ag–BiOI < Ti–BiOI, and the Ti-loaded BiOI
showed the best photocatalytic performance for MO dye. For
RhB, the photocatalytic activity differed greatly under UV
and visible irradiation. Specifically, RhB with BiOI was effi-
ciently degraded with time under UV irradiation, while RhB
with the Ag and Ti-loaded BiOI catalysts showed very little
degradation with time. The doped catalysts showed a very
poor photocatalytic activity for RhB under UV irradiation.
However, under visible light irradiation the Ag and Ti-doped
catalysts showed a good photocatalytic activity comparable
to or better than that of undoped BiOI. Overall, the catalytic
activity was in the order of Ag–BiOI < BiOI < Ti–BiOI. Ti-
loaded BiOI showed a very good photocatalytic activity
toward RhB under visible light irradiation, implying that the
dye degradation occurs via a dye sensitized mechanism,61,62
as discussed in detail below. In addition, the different
exposed facets showed different reactivities between UV and
visible light.45 Although MB was the most efficiently
adsorbed onto the catalyst surface, it was not efficiently
photodegraded under UV and visible light irradiation. The
peak position showed no change with time. The concentra-
tion of the MB solution showed no linear decrease with irra-
diation time (UV and visible) when treated with the BiOI and
Ag and Ti-loaded BiOI catalysts (Fig. S5, ESI†),
suggesting that the MB dye could be removed efficiently by
adsorption, but not by photocatalytic reaction. The photocat-
alytic performance has been shown to be more closely
related to the exposed facets of a catalyst than the surface
area.43,45,63 In the dye-sensitized mechanism, a good charge
3.4. The photocatalytic performance of MO, RhB and MB
under UV and visible light
Prior to examining the detail photocatalytic performance we
selected 5 mol% Ag– and 10 mol% Ti–BiOI as representative
test materials based on the XRD and adsorption data. The
5 mol% Ag– and 10 mol% Ti–BiOI were enough to show dif-
ferences in the XRD and adsorption performance, compared
with those of un-loaded BiOI. For the selected three catalysts,
Table 1 The BET surface areas (m2 g−1) of reference (undoped) BiOI
and Ag-doped BiOI with different Ag concentrations
Ag mol%
0
0.1
1
5
10
Surface area (m2 g−1
)
60.8
66.2
44.3
45.1
44.2
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Fig. 10 Photodegradation (change in peak position and UV-visible absorption spectra) of the MO, RhB and MB solutions (10 mg L−1, 50 mL) by
BiOI (■), 5 mol% Ag-loaded BiOI (▲) and 10 mol% Ti-loaded BiOI (●) under UV (left column) and visible (right column) light irradiation. The insets
show the photos of the corresponding solutions with photodegradation time. The absorption wavelengths of the dyes (Fig. S6, ESI†) were omitted
for clarity.
transfer followed by separation is an important step, and is
mainly determined by the exposed facet. The LUMO of a
dye and the CB of a catalyst is another important factor for
the higher charge transfer. A good interfacial wavefunction
mixing (or a well-aligned energy level) promotes the charge
transfer rate.
(or the shift in peak position), with an order of Ag(5 mol%)–
BiOI < BiOI < Ti(50 mol%)–BiOI. Upon the addition of
scavengers for ˙O2 and h+, the dye degradation (or the
−
shift in peak position) was dramatically suppressed. The
−
dye with the scavenger of ˙O2 showed some degradation,
but no change in the absorption peak position. This indi-
cates that other active species such as h+ dissociate the dye
To clearly examine the photocatalytic mechanism and
the role of the active species, we employed an indirect
chemical probe method using scavengers of isopropyl alcohol
without forming
a secondary product (discussed later).
However, the scavenger for the ˙OH radical showed no sup-
−
(IPA), benzoquinone (BQ) and EDTA for the ˙OH, ˙O2 and
pression and the same degradation rate as the dye solution
−
h+ species, respectively. Fig. 11 displays the photo-
catalytic degradation of RhB for BiOI, Ag(5 mol%)–BiOI
and Ti(50 mol%)–BiOI in the absence and presence of
scavengers (IPA, BQ and EDTA) under visible (λ > 420 nm)
light irradiation for 6 hours. It should be noted that the
photocatalytic activity is dependent on the amount of Ag and
Ti-loading. Jiang et al. prepared Ag and Ti-decorated flower-
like BiOBr microspheres and found that the photocatalytic
activity was highly dependent on the Ag or Ti-content.64
The dye without a scavenger showed the greatest degradation
without a scavenger. These results indicate that the ˙O2 and
h+ species are formed during visible light irradiation,
and that they play important roles in dye degradation,
which agrees well with the available literature.2,3,5,14,19,22
−
Li et al. also found that the ˙O2 and h+ species play major
roles in degradation, but the ˙OH radical showed no effect
on the degradation of phenol with
a BiOI/ZnSn(OH)6
catalyst.5 For the BiOI and Ti–BiOI catalysts, the h+ appears
−
to be more important than ˙O2 . Ye et al. reported a similar
−
result for BiOI.3 Furthermore, they found that ˙O2 became
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To confirm that ˙OH had no effect on the dye degrada-
tion, we employed photoluminescence spectroscopy using
terephthalic acid.20,22,27,51 When ˙OH radicals are formed
in response to visible light irradiation they react with
terephthalic acid to form luminescent (λem = 425 nm)
2-hydroxyterephthalic acid. We observed no significant emis-
sion signal (Fig. S7, ESI†) at 425 nm for the BiOI, Ag(10%)–BiOI
or Ti(5%)–BiOI catalysts after visible light irradiation or UV
irradiation for 6 hours, confirming that no significant ˙OH
radical was produced during the photoirradiation, and that
it was not involved in the photodegradation reaction. For
the Bi2S3/BiOI catalyst under visible light irradiation,
Cao et al. observed a photoluminescence peak at 425 nm,
which was attributed to the formation of ˙OH radicals.27
However, they found that the ˙OH radical effect was not as
strong as that of the ˙O2 and h+ species for the degradation
−
of MO.
We further examined the photodegradation of BiOI with
different amounts of Ag and Ti-loadings without adding any
scavengers. Fig. 12 shows that 0.1 mol% Ag–BiOI is the most
efficient catalyst for RhB dye degradation and there is a shift
in the absorbance position of the dye solution. However, BiOI
with 1, 5 and 10 mol% Ag showed poor degradation and
smaller shift in the peak position. This indicates that RhB is
poorly degraded when the amount of Ag was loaded above
1 mol%. Lu et al. prepared a Ag-modified (0.2–2.0 wt%)
BiOBr catalyst and tested the photodegradation of RhB
under visible light.65 They found a similar catalytic behaviour
for pure BiOBr with a higher catalytic activity than that
of Ag-modified BiOBr. They attributed the poor performance
of the Ag-modified BiOBr to the formation of Ag2O.
However, Kong et al. found an enhanced photocatalytic
Fig. 11 The photocatalytic degradation (top two lines) of RhB with
BiOI, Ag(5 mol%)–BiOI and Ti(50 mol%)–BiOI in the absence and
presence of scavengers (IPA, BQ and EDTA) under visible light
irradiation for 6 hours. The photocatalytic degradation (bottom two
lines) of RhB over Ag–BiOI and Ti–BiOI with different Ag and Ti-loaded
BiOI in the presence of BQ under visible light irradiation for 6 hours.
more important than the h+ species for Pt–BiOI.3 Similarly,
in the present study, the ˙O2 and h+ species showed
−
comparable activities for Ag–BiOI, which were attributed to
an enhanced electron transfer followed by the generation
− 29,30
of ˙O2 .
Liu et al. used Ag/BiOI composites to degrade
acid orange II, MO and RhB under visible-light irradiation
and found a higher photocatalytic activity than BiOI.29
They explained that the electrons in the CB of BiOI accu-
mulated on the surface of Ag and were more easily trans-
−
ferred to the adsorbed surface oxygen to form active ˙O2 .
−
Since a partial degradation of RhB was found with the ˙O2
scavenger, we further examined the effect of BQ over the
catalyst with different Ag and Ti-loadings (Fig. 11). The dye
degradation was found to be dependent on the Ag or
Ti-content. The BiOI with higher loadings of Ag (1, 5 and
10 mol%) showed
a poor degradation, compared with
undoped and 0.1 mol% Ag-doped BiOI. On the other hand,
the Ti–BiOI catalysts showed a better photodegradation
performance as discussed in Fig. 10 for RhB under visible
light irradiation.
Fig. 12 The photocatalytic degradation (and the change in peak
position) of RhB over BiOI with Ag-loading under visible light irradia-
tion for 6 hours. The UV-visible absorption peaks were normalized
before photoirradiation.
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+
dye + visible light → dye (eCB− + hVB
)
(1)
adsorbed dye (eCB− + hVB+) + BiOI → BiOI (eCB−) + dye (hVB+) (2)
−
+
BiOI + UV and visible light → BiOI (eCB + hVB
)
(3)
BiOI (eCB−) + adsorbed O2(or surface O) → ˙O2 (or ˙O−) + BiOI (4)
−
H2O → H+ + OH−
(5)
(6)
(7)
(8)
˙O2 (or ˙O−) + H+ → ˙O2H (or ˙OH)
OH− + BiOI (hVB+) → ˙OH
−
H2O + BiOI (hVB+) → H+ + ˙OH
˙O2 , h+or ˙OH + adsorbed dye/dye+ → degradation products (9)
−
When a dye absorbs visible light, an electron (e−) and a
hole (h+) are created in the conduction and valence bands
Fig. 13 The photocatalytic degradation (and the change in peak
position) of RhB over BiOI with different Ti-loadings under visible light
irradiation for 6 hours. The UV-Vis. absorption peaks were normalized
before photoirradiation.
(CB and VB), respectively. In
a dye sensitized mecha-
nism,22,61,62 the electron in the CB transfers to the CB of
the catalyst before being recombined with a hole (Fig. 14).
The transfer rate is determined by good interfacial
wavefunction mixing (and good adsorption) between the
dye and the catalyst. Based on the UV-Vis absorption spec-
tra (Fig. 6), the catalyst could directly absorb UV or visible
light to create e− and h+ in the CB and VB, respectively. The
electron in the CB of the catalyst is captured by the
performance with loadings below 0.5 wt% AgBr, and a poor
performance above 1.0 wt% AgBr, compared with pure
BiOBr.66
For the Ti-loaded BiOI (Fig. 13), the UV-visible absorption
intensity showed no linear relationship with the Ti-amount.
However, the blue shift in the peak position was linearly
increased with the increasing the Ti-amount. This indicates
−
adsorbed molecular (or atomic) oxygen to form ˙O2 (or ˙O−)
radicals, which degrade the adsorbed dye.30 As shown in
−
−
that ˙O2 radical became more important as the Ti-amount
Fig. 11, the ˙O2 scavenger suppressed dye degradation.
−
was increased in BiOI. The ˙O2 radical may attack the N-ethyl
Additionally, oxygen vacancies have been shown to play
active roles in dye degradation.68 The electron in the energy
state of the oxygen vacancy was trapped by the surface
group of RhB (as shown in the inset of Fig. 13) to initially
form secondary products, which results in the blue-shift in
the absorption position.67
−
oxygen to create active ˙O2−, and the ˙O2 radical may have
Based on the aforementioned results, the following
photodegradation mechanism was proposed27,43 under visi-
ble (λ > 420 nm) light irradiation for BiOI and doped BiOI
(Fig. 14).
proceeded to form active ˙OH radicals. The adsorbed OH−
species may also capture the hole in the VB of BiOI to form
˙OH radicals.51,59 However, the ˙OH radical was not involved
in the dye degradation because there was no ˙OH scavenger
effect and no PL emission of 2-hydroxyterephthalic acid.
+
The hVB in the catalyst or in the dye acts as an active spe-
cies to finally degrade the dye,47 as confirmed by the hole
scavenger test. Consequently, the O2 and h+ species pri-
−
marily degrade the dye, which is consistent with the available
literature.2,3,5
Finally, to emphasize the originality and new results of
the present study, we summarized the literatures using simi-
lar catalysts in Table 3. The AgI/BiOI (or Ag/BiOI) catalysts
were commonly synthesized as follows: BiOI was first synthe-
sized and then dispersed in an AgNO3 solution.1,2,4,10,11,29,30
AgI was formed at the interface by an ion exchange reaction.
However, we mixed bismuth nitrate, AgNO3 (or Ti iso-
propoxide) and KI together in EG, and treated at 120 °C for
12 hours. Cheng et al. synthesized AgI/BiOI using a similar
method, but used a mixed EtOH/H2O solvent, and a thermal
Fig. 14 The photocatalytic degradation mechanism of a dye over
the catalysts.
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Table 3 A summary of the literatures using similar catalyst systems
Catalysts
Experimental conditions
Test systems and performances
AgI(16–400 nm)/BiOI1
BiOI in AgNO3 EG solution, ion exchange
BiOI in AgNO3 aq. solution, ion exchange
2,4-Dichlorophenol, under Vis., enhanced
RhB and phenolunder Vis., enhanced, max at
70.4% AgI
MO, RhB, MB under Vis., enhanced (best at 40%,
and MB > RhB > MO)
MO and phenol enhanced compared with BiOI, best
at 20% AgI
Acid orange II, MO, RhB under Vis., enhanced, best
at 0.6%
AgI(15.7, 28.2, 35.2, 47.0, 70.4%)/BiOI
microspheres2
AgI(10, 40, 60%)/BiOI4
BiOI in AgNO3 aq. solution
AgI(0–100%)/BiOI47
Bi nitrate + AgNO3 + KI in EtOH/H2O
(85 °C, 2 hours)
BiOI in AgNO3 sol. and UV photoirradiation
Ag/BiOI(0.3, 0.6, 0.9%)29
Ag/BiOI(1.1, 2.1, 4.5, 8.0%)30
Ag/AgI/BiOI10
BiOI in AgNO3 sol. and photoirradiation
BiOI in AgNO3 aq. sol., photoirradiation
BiOI in AgNO3 EtOH/H2O sol.,
E. coli 8099, under Vis., enhanced, best at 4.5%
MB and o-nitrophenol, under Vis., enhanced
MO, under Vis., enhanced
Ag/AgI/BiOI11
photoirradiation
Bi nitrate + tetrabutyl titanate aq. sol.: reverse
microemulsions.
Impregnating-hydroxylation method using BiI3
Bi nitrate (EG) + AgNO3 (EG) + TiO2 (H2O)
Bi nitrate + AgNO3 (or Ti isopropoxide ) + KI
in EG (120 °C, 12 hours)
TiO2(0–100%)/BiOI31
MO, enhanced, best at 25%, under Vis.
BiOI-coated on TiO2 nanotube wall35
TiO2(1–2%, 50%)/BiOI36
Echinoid-like BiOI, AgI/BiOI and Ti–BiOI
microsphere (this work)
MO, under Vis., enhanced,
Phenol under Vis., best at 25%
MO, RhB, MB under UV and Vis.
treatment at 85 °C for 2 hours.47 They examined the
photodegradation of MO and phenol under visible light irradi-
ation, and found an enhanced catalytic performance upon
Ag-loading, which is consistent without result for MO.
However, we further examined RhB and MB under UV light as
well as visible light, and the new results were discussed
earlier. There was no report on the photocatalytic study
performed under UV light with the loaded BiOI. The Ti–BiOI
catalyst was synthesized differently in the present work, and
carefully tested for MO, RhB and MB which have different
molecular structures and UV-visible absorption bands. The
Ti-loading enhances the catalytic performance for MO under
UV and visible light. For RhB, the Ti-loading showed an
enhanced effect under visible light, but poor performance
under UV light, compared with un-loaded BiOI.
2) MO was efficiently photodegraded by the catalysts
under UV and visible light in the order of BiOI < Ag–BiOI <
Ti–BiOI. For RhB, BiOI was an efficient photocatalyst under
UV irradiation, while Ag and Ti-loaded BiOI showed a poor
photocatalytic activity. Under visible light irradiation, the
catalysts showed a good catalytic activity with an order of
Ag–BiOI
< BiOI < Ti–BiOI. MB showed poor photo-
degradation with the catalysts under both UV and visible
light irradiation.
3) A dye sensitized mechanism was proposed in which
light absorption by the dye followed by a charge transfer
from the dye to the catalyst is involved. The active species of
−
˙O2 and h+ (more active) play important roles in the
−
photocatalytic process. It was found that the ˙O2 species
initially create secondary dissociation products, while the h+
species showed a total dissociation without forming any
secondary product. The photocatalytic activity was under-
stood by various factors such as surface area, crystallinity,
good interfacial wavefunction mixing (or the well-aligned
energy level for change transfer), and the relative role of
active species.
4. Conclusion
Echinoid flower-like BiOI with various concentrations of Ag
(0.1, 1.0, 5.0, 10.0 mol%) and Ti (1.0, 5.0, 10.0, 30.0,
50.0 mol%) was successfully synthesized in an ethylene
glycol environment and their fundamental properties were
examined by SEM, XRD, UV-visible absorption, Raman,
FT-IR, photoluminescence and BET surface area measure-
ments. The adsorption and photocatalytic performances
were demonstrated for MO, RhB and MB. The adsorption
performance was mainly related to the nature of the dye and
the catalyst surface structure, not the surface area.
The present study provides a new insight that can aid in
the selection of an appropriate adsorbent/catalyst for
pollutant removal by explaining the adsorption and
photodegradation mechanism of MO, RhB and MB with
BiOI and Ag and Ti–BiOI catalysts under UV and visible
light irradiation.
New important findings of the present study are as
follows:
Acknowledgements
1) MB was commonly the most efficiently adsorbed by the
catalysts, while none of the catalysts adsorbed MO well. Over-
all, the adsorption was found to be in the order of MO ≪
RhB < MB, which was attributed to electrostatic interactions
between the dye and the catalyst surface.
This work was financially supported by the India-Korea joint
project, the National Research Foundation of Korea (NRF)
grant funded by the Korean government (MEST) (NRF-2012-
0006296), and the Department of Science and Technology,
New Delhi through the Indo-Korea/P-02 grant.
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