Solvent-Free Synthesis of Bismuth Oxychloride Microflower/Nanosheet Homojunctions for Photoactivity Enhancement

Article abstract of DOI:10.1002/cctc.201800357

Solvent-free synthesis of bismuth-based photocatalysts with hierarchical architectures has been rarely reported yet. Herein, BiOCl hierarchical microflowers (MFs) were prepared via a solvent-free route for the first time, using chlorobutanol as the chlorine source. The reaction temperature (T) plays a key role for formation of MFs. The MFs were formed at T >120 °C and some MFs merged into large nanosheets (LNs). MF/LN homojunctions were constructed because considerable differences in sizes and exposed facets between the MFs and the LNs cause differences in their energy band levels. The sample prepared at 120 °C possesses the highest contents of MFs and homojunctions, specific surface area (S_BET, 13 m² g^?1), and photoactivity, indicating the high photoactivity of the sample arises from fabrication of homojunctions as well as the high S_BET. This work provides a solvent-free way to simultaneously synthesize BiOCl MFs and homojunctions, which paves an avenue for solvent-free synthesis of other photocatalysts.

Full text of DOI:10.1002/cctc.201800357

Accepted Article
Title: Solvent-free synthesis of bismuth oxychloride microflower/
nanosheet homojunctions for photoactivity enhancement
Authors: Hanyu Yao, Haiping Li, Tingxia Hu, and Wanguo Hou
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10.1002/cctc.201800357
ChemCatChem
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Solvent-free synthesis of bismuth oxychloride
microflower/nanosheet homojunctions for photoactivity
enhancement
Hanyu Yao,[a] Haiping Li,*[a] Tingxia Hu,[b] and Wanguo Hou*[b]
[2]
Abstract: Solvent-free synthesis of bismuth-based photocatalysts
with hierarchical architectures has been rarely reported yet. Herein,
BiOCl hierarchical microflowers (MFs) were prepared via a solvent-
free route for the first time, using chlorobutanol as the chlorine source.
The reaction temperature (T) plays a key role for formation of MFs.
The MFs were formed at T >120 °C and some MFs merged into large
nanosheets (LNs). MF/LN homojunctions were constructed because
considerable differences in sizes and exposed facets between the
MFs and the LNs cause differences in their energy band levels. The
sample prepared at 120 °C possesses the highest contents of MFs
and homojunctions, specific surface area (SBET, 13 m2 g-1), and
photoactivity, indicating the high photoactivity of the sample arises
from fabrication of homojunctions as well as the high SBET. This work
provides a solvent-free way to simultaneously synthesize BiOCl MFs
and homojunctions, which paves an avenue for solvent-free synthesis
of other photocatalysts.
availability and cost
. However, the hydro-/solvo-thermal
reactions are accompanied with production of massive waste
solvents and high-temperature solid state reactions need injection
of high energy. In contrast, solvent-free low-temperature
synthesis processes are able to avoid these problems. For
example, mechano-chemistry [3] and low-temperature solid state
reactions [4] have been extensively researched in past years, even
though they still apply to only a few reactions. To reduce more
and more industrial wastewater discharge and energy
consumption, more solvent-free low-temperature synthesis
technology should be developed.
Bismuth oxychloride (BiOCl), regarded as
a promising
photocatalyst, has attracted much interest of scientists [5]. The
BiOCl crystal with a layered structure comprises a repetitive unit
composed of one [Bi2O2]2+ layer sandwiched between two Cl–
layers [5a, 6]. The layered structure leads to formation of an internal
electric field which is beneficial for enhancing the separation of
photoinduced electron-hole pairs and thus improving photoactivity
[7]. Because the activity of BiOCl photocatalysts significantly
depends on their morphologies (such as size, shape, and
exposed facets) [7-8], it is reasonable and promising to improve
their photoactivity by manipulating the morphologies via designing
rational synthetic routes. The hydro-/solvo-thermal reactions are
the most extensively used method to prepare BiOCl
photocatalysts with different morphologies. In these reactions,
Introduction
When more and more fossil energy is consumed to meet high-
speed development of our society, the eventual exhaustion of
energy in future and use of fossil energy induced environmental
deterioration become unneglectable problems. Artificial
photosynthesis based on semiconductor photocatalysis brings us
a bright future, by transforming inexhaustible solar energy to
chemical energy through photocatalytic hydrogen production,
carbon dioxide reduction, selective organic transformation and
toxic contaminant degradation [1]. The photocatalytic performance
of semiconductors fundamentally depends on their structure and
morphology gained by different methods, such as hydro/solvo-
thermal reactions, high-temperature solid state reactions,
electrodeposition, chemical vapor deposition, microwave
reactions, coordination chemistry and photochemistry. The first
two methods are most popularly used in consideration of
solvents played significant roles. For example, BiOCl nanosheets
tend to be formed in water
formed in organic solvents like ethylene glycol [5c] and glycerol [9]
Nonetheless, to our best knowledge, solvent-free methods for
preparing BiOCl photocatalysts have been little reported yet.
As is known, hierarchically structured photocatalysts usually
possess higher specific surface areas, expose more active sites,
and thus exhibit higher photoactivity than bulky ones. Preparation
[7]
and hierarchical microstructures
.
of BiOCl photocatalysts with hierarchitectures like microspheres,
microflowers and nanobelts
[10]
is an important research area in
recent years. Because of the layered structure of BiOCl,
nanosheets are easy to form in reactions. To construct
hierarchical morphologies which are actually composed of small
nanosheets with different shapes, structural directing agents,
such as organic acid (e.g. acetic acid), surfactants (e.g. CTAB and
OP-10) and organic solvents (e.g. glycerol and ethylene glycol)
must be used [10a, 11]. Unfortunately, nearly all of the structural
directing agents are environmentally harmful and expensive,
which limits the industrial production of BiOCl hierarchitectures.
To our knowledge, there was rare report on formation of BiOCl
[a]
[b]
Mr. H. Yao, Dr. H. Li
National Engineering Research Center for Colloidal Materials
Shandong University
Jinan 250100, P.R. China
Fax: (+86) 53188364750
E-mail: hpli@sdu.edu.cn
Ms. T. Hu, Prof. Dr. W. Hou
Key Laboratory for Colloid and Interface Chemistry (Ministry of
Education)
Shandong University
hierarchitecture without use of structural directing agents [12]
.
Besides the morphological engineering, construction of
heterojunctions is another extensively researched way to
enhance the photoactivity of BiOCl, because the heterojunctions
Jinan 250100, P.R. China
Fax: (+86) 53188364750
E-mail: wghou@sdu.edu.cn
Supporting information for this article is available on the www under
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10.1002/cctc.201800357
ChemCatChem
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can considerably increase the separation of photogenerated
accelerates the merging process to generate more LNs. If the T
is high enough and the tr is long enough, like 160 °C and 20 h, all
the MFs will merge into the LNs (see Figures 2j and S1l).
charge carriers [13]. Hitherto, many compounds have been used to
fabricate heterojunctions with BiOCl, such as BiOI [13], g-C3N4
,
[14]
Bi2O2CO3 [15], BiF3 [16], Bi3O4Cl [6], and Bi24O31Cl10 [17]. Compared
with heterojunctions, homojunctions in which the components
possess the same composition, can also realize the high efficient
separation of charge carriers, but not need the introduction of any
new element. Thus, the homojunctions draw scientists’ great
interest. However, constructing homojunction is quite difficult
because there are few ways to adjust the energy band levels of
semiconductors without changing their chemical composition.
Most homojunctions were constructed by combination of
semiconductors with different crystal phases, such as
anatase/rutile TiO2 [18], α-β phase Ga2O3 [19], and hexagonal-cubic
phase CdS [20]. Several papers reported homojunction fabrication
via adjusting morphologies including crystal facets and particle
sizes [21]. For BiOCl, there is only one paper referring to the
homojunction formation via selective etching with triethanolamine
[21c]. More cost-effective and eco-friendly methods need be
explored to fabricate BiOCl homojunctions.
In this work, BiOCl hierarchical microflowers were synthesized
firstly via a solvent-free low-temperature reaction without use of
any structural directing agent. The microflowers (MFs) composed
of small nanosheets with highly exposed (001) facets can partially
merge into large nanosheets (LNs) with dominantly exposed
(010) facets. The close contacted MFs and LNs with different
energy band levels can form homojunctions which significantly
enhance the separation of photogenerated charge carriers and
thus photocatalytic performance. This work reported a facile
solvent-free route to synthesize BiOCl MFs and homojunctions.
c
B160-20-1
B160-20-2
B160-20-3
B120-12-3
B120-12-2
B120-12-1
JCPDS No. 85-0861
20
40
60
2 / degree
Figure 1. XRD patterns of BiOCl samples prepared by heating (a) at 60–160 °C
for 12 h, (b) at 120 °C for 0–16 h, and (c) at 120 °C for 12 h and at 160 °C for
20 h with Cl/Bi molar ratios of 1–3. I001/I110 in (b) denote intensity ratios of (001)
to (110) peaks. In the name of “Bx-y-z”, x, y, and z symbolize the temperature,
the time, and the Cl/Bi molar ratio, respectively.
Results and Discussion
Figure 1b shows the evolution of XRD patterns of the BiOCl
samples with tr. The reaction at 120 °C is a quick process,
because after heating for only 0.5 h, all the impurity peaks
disappeared. To distinctly show the structure variation of the
sample with tr through the XRD patterns, the peak intensity ratios
of (001) to (110) are shown in the figure. They increase gradually
with increasing tr from 0.5 to 8 h and almost stay constant from 8
to 16 h, indicating 8 h is the minimum time needed at 120 °C to
form thermodynamically stable structure for the sample. SEM
images of the samples are shown in Figures 2e–g and S1g–i.
Pure Bi(NO3)3ꞏ5H2O shows the aggregating structure of large
blocks with an average size of ~500 nm (Figure 2e). B120-0-1
displays small nanosheet-assembled aggregates which possess
a similar shape as the Bi(NO3)3ꞏ5H2O blocks (see the inset in
Figure 2e). The MFs start forming from B120-0.5-1 and exhibit
almost no change from B120-1-1 to B120-16-1. The similar sizes
of Bi(NO3)3ꞏ5H2O blocks, aggregates of B120-0-1, and MFs
indicate that every MF comes from one Bi(NO3)3ꞏ5H2O block.
XRD patterns of the samples with different R are shown in
Figure 1c. With the R increasing from 1 to 3 at T of 120 and 160 °C,
the diffraction peaks of the samples do not show obvious change.
The SEM images of B120-12-(1–3) show similar MF structures
except that many MFs in the B120-12-(2 and 3) are disassembled
with generation of some stacked nanosheet structures (Figure 2a,
h, and i), and the LN contents in the samples remarkably
Structure and morphology
Influences of temperature (T, °C), reaction time (t, h) and Cl/Bi
molar ratios (R) on structures and morphologies of BiOCl were
well investigated. The prepared BiOCl samples were symbolized
as BT-t-R. Figure 1a shows XRD patterns of the BiOCl samples
prepared at different temperature. At T of 60 °C, BiOCl was not
formed because there are no diffraction peaks of BiOCl observed,
but at T > 80 °C, pure BiOCl samples were obtained because only
diffraction peaks well indexed to a pure tetragonal structure of
[22]
BiOCl (JCPDS No. 85-0861)
were observed. This suggests
that 80 °C is the minimum temperature for formation of BiOCl.
Figures 2 and S1 show SEM images of the BiOCl samples. B(80
and 100)-12-1 are completely composed of nanosheets with
lateral sizes of ~100–800 nm and thickness of ~10–100 nm
(Figure S1a and b), but for B(120–160)-12-1, the MFs, composed
of small thin nanosheets with average thickness of ~15 nm, were
formed with size of ~0.5–1.2 μm (Figure 2a–c), besides some LNs
formed with lateral sizes of several to several tens of micrometers
and thickness of several hundred of nanometers (Figures 2a–c
and S1c–f). The contents of LNs in the samples prominently
increases with increasing T from 120 to 160 °C. Some MF-loaded
LNs are also clearly observed (Figures 2d and S1c–e), indicating
the LNs were formed through merging of the MFs. The high T
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10.1002/cctc.201800357
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decrease with increasing R (Figure S1c, j, and k). SEM images of
B160-20-(1–3) show more clearly gradual reduction of LN
contents with increasing R (Figures 2j–l and S1l–n). The B160-
20-2 and B160-20-3 comprise MFs, but some of the MFs are also
disassembled with stacking of thin nanosheets (Figure 2k and l).
Both the B120-12-3 and the B160-20-3 contain almost no LNs.
Besides, there are almost no LNs observed in the whole reaction
process of the B160-20-3 (Figure S2). These indicate that the high
R is unfavorable for the formation of LNs.
Figure 2. SEM images of BiOCl samples synthesized at various temperatures, reaction time and Cl/Bi molar ratios: (a) B120-12-1; (b) B140-12-1; (c and d) B160-
12-1; (Inset in e) B120-0-1; (f) B120-0.5-1; (g) B120-1-1; (h) B120-12-2; (i) B120-12-3; (j) B160-20-1; (k) B160-20-2; and (l) B160-20-3. (e) The SEM image of
Bi(NO3)3ꞏ5H2O. In the name of “Bx-y-z”, x, y, and z symbolize the temperature, the time, and the Cl/Bi molar ratio, respectively. Scale bars in insets: 100 nm.
TEM and HRTEM were used to further characterize structures
of MFs and LNs. As shown in Figure 3, the MFs (Figure 3a) and
LNs (Figure 3d) in B120-12-1 can be clearly observed. Some
small nanosheets belonging to the MFs adhere to the surface of
the LN (Figures 3d and S3a). Figure 3b shows the HRTEM image
of one small nanosheet in the MF. The lattice fringe spacing of
0.275 nm is ascribed to the (110) facet of BiOCl. Figure 3c shows
the corresponding SAED pattern. The orderly diffraction spots
indicate the single crystal structure of the small nanosheet. The
confirmed (200) and (110) facets in Figure 3c and the (110) facet
in Figure 3b are all vertical to the (001) facet, demonstrating that
the small nanosheet exposes (001) facets at top and bottom
surfaces (Figure 3g) [23]. Figure 3d shows some small nanosheets
adhere to the surface of the LN, and the lattice fringe at the
interface between an adhesive small nanosheet and the LN is
shown in Figure 3e. A clear boundary is observed, as shown by
the dotted line. The fringe spacing of 0.274 nm for the small
nanosheet corresponds to the (110) facet of BiOCl, similar to that
in Figure 3b, which means the small nanosheets come from the
MFs and further suggests the LNs were formed through merging
of the MFs (Figure 3g). The fringe spacing of 0.343 nm for the LN
is ascribed to the (101) facet of BiOCl (Figures 3e and S3b). The
orderly diffraction spots in the SAED pattern (Figure 3f) also
reveal the single crystal structure of the LN. The determined (102)
and (200) facets in Figure 3f and the (101) facet in Figure 3e
manifest that the LN exposes the (010) facet at top and bottom
[13,
surfaces (Figure 3g)
24]. To more clearly show the SAED
difference between the LNs and the MFs, Figure 3c was
superimposed on Figure 3f, and distinctly different diffraction spot
arrangements are observed (Figure S3c). The close contact of the
MFs and the LNs with different sizes and exposed crystal facets
is quite favorable for construction of MF/LN homojunctions
(Figure 3g).
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10.1002/cctc.201800357
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Figure 4. Schematic illustration for formation and growth process of BiOCl
microflowers and nanosheets with increasing reaction time and temperature.
XPS was used to further characterize the structure of BiOCl
samples. Figure 5a and b show the Bi 4f and Cl 2p core-level
spectra of B160-20-1, B120-12-3, and B120-12-1, respectively.
For B160-20-1 composed of LNs, peaks around binding energies
(BE) of 164.6 and 159.3 eV are assigned to 4f5/2 and 4f7/2 of Bi3+
and those at BE of 199.6 and 198.0 eV are ascribed to 2p1/2 and
2p3/2 of Cl– in the LNs [7]. For B120-12-3 composed of MFs, the
peaks at BE of 164.8 and 159.5 eV should correspond to Bi3+ and
those at BE of 199.8 and 198.2 eV should be assigned to Cl– in
the MFs. For the B120-12-1 composed of LNs and MFs, the peaks
in Bi 4f and Cl 2p spectra were both deconvoluted into two groups.
One group lies at similar BE positions as the peaks of B160-20-1
and the other group are located at similar BE positions as the
peaks of B120-12-3. The dashed lines indicate that the BE
positions of peaks of B160-20-1 (or LNs) are ~0.2 eV lower than
Figure 3. (a and d) TEM and (b and e) HRTEM images, (c and f) SAED patterns,
and (g) schematic illustration for microflower (MF)/large nanosheet (LN)
homojunction and exposed facets of (a–c) MFs and (d–f) LNs in B120-12-1.
those of B120-12-3 (or MFs), indicating the different surficial
[27]
electron density
resulting from different shapes and exposed
facets of nanosheets [21d] between the LNs and the MFs (Figures
3g and 4). According to the peak area ratio, the MF content in
B120-12-1 is ~90.1% (Figure 5a). The only one group of Bi 4f or
Cl 2p peaks gained for B160-20-1 and B120-12-3 also indicates
that all the MFs merged into the LNs for B160-20-1 and nearly no
LNs formed in B120-12-3. According to the XPS data, Cl/Bi ratios
for B160-20-1, B120-12-3, and B120-12-1 are 1.06, 1.03, and
1.03, respectively, which are close to the theoretical value (1.00).
Besides, there are no Bi 4f and Cl 2p peaks belonging to oxygen
vacancies observed [28], indicating oxygen vacancies were not
generated, which can also be evidenced by white color of all the
samples. The ESR spectrum of B160-20-1 does not show any
Based on above results, the formation process of BiOCl MFs
can be deduced. Since the chlorobutanol can be slowly
hydrolyzed in water when heated, according to the following
chemical
equation,
CH3
CH3
O
C
H O
(1.5-0.5x)
3
+
+
CH3
+ (1-x)
x CO H3C
HCl
x
C
OH
COOH
H3C
C CCl3
+
2
H3C
OH
,
where x is close to 1 [25], when heating the mixture of
chlorobutanol and Bi(NO3)3ꞏ5H2O, the chlorobutanol will react
with the crystal water to form HCl which quickly reacts with
Bi(NO3)3 to form BiOCl. The minimum T for fast proceeding of this
reaction should be 80 °C, according to discussion on the XRD
results (Figure 1a). As shown in Figure 4, under heating and
etching of HCl, the Bi(NO3)3ꞏ5H2O blocks turn into nanosheet
aggregates (e.g. B120-0-1), and then grow into dispersed
nanosheets at T < 100 °C (e.g. B100-12-1) or MFs at T >120 °C
(e.g. B120-12-1). At higher T and longer tr, the MFs with highly
exposed (001) facets may merge into the LNs dominantly
peaks at 300–360 mT (Figure S4), also demonstrating the
[5b, 5c, 29]
absence of oxygen vacancies
in as-prepared BiOCl
samples. FT-IR spectra indicate that no organic compounds
remain in the samples (Figure S5) [30]
.
Valance-band (VB) XPS spectra were tested to determine
*
distance (EVF ) between the VB edge and the Fermi level. As
exposing (010) facets (e.g. B160-20-1) because the (010) facet of
shown in Figure 5c, there is only one linear part in the curves for
B120-12-3 and B160-20-1, corresponding to the EVF* of 2.28 and
2.06 eV, respectively, but for the B120-12-1, there are obviously
two linear parts in the curve, corresponding to the EVF* of 2.06 and
[26]
BiOCl possesses lower surficial energy than the (001) facet
.
The Higher R means that the generated HCl is excessive
compared with the Bi(NO3)3. The surplus HCl molecules can
adsorb on the (001) facets, which significantly decreases the
surficial energy of (001) facets to be lower than that of (010) facets
[26]. This is why the LNs are difficult to form at higher R (e.g. B120-
12-3), even though when heated at 160 °C (e.g. B160-20-3).
*
2.28 eV, respectively. As shown by the dotted lines, the EVF of
*
2.06 eV should belong to the LNs and the EVF of 2.28 eV is
assigned to the MFs. The VB XPS spectra reveal the energy band
level difference between the MFs and the LNs, which is favorable
for the formation of MF/LN homojunctions (Figure 3g).
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10.1002/cctc.201800357
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p3/2
f7/2
(b) Cl 2p
360 nm), the photoabsorption of samples increases gradually with
increasing T from 100 to 160 °C and decreases with increasing R
(a) Bi 4f
f5/2
p1/2
B120-12-3
B120-12-3
B120-12-1
from
1 to 3, with the B160-20-1 exhibiting the highest
photoabsorption (Figure 6b). The energy band gaps (Eg) of the
samples were determined by the Tauc’s plot [32], according to the
B120-12-1
B160-20-1
S1
= 90.1%
S1 +S2
equation, αhν = (hν − Eg )
n/2, where α, h, and ν are the
1
2
photoabsorption coefficient, the Planck constant, and the light
frequency, respectively. n equals 4 for BiOCl, an indirectly
bandgap semiconductor [14]. As shown in Figure 6c, the Eg
increases gradually from 3.14 to 3.24 eV with the increase of T in
100–160 °C and decreases from 3.16 to 3.01 eV with increasing
R in 1–3 (Table 1). The B160-20-1 (or LNs) exhibits the greatest
Eg value of 3.32 eV, while the B120-12-3 (or MFs) shows the
smallest one of ~3.01 eV. The larger Eg of LNs than that of MFs
further demonstrates the influence of sizes and exposed facets
on the energy band levels of BiOCl.
~0.2 eV
197
~0.2 eV
B160-20-1
201
200
199
198
164
160
156
Binding Energy / eV
Binding Energy / eV
(c)
B120-12-3
2.28 eV
B100-12-1
(c)
(b)
B120-12-1
B140-12-1
B160-12-1
B160-20-1
B120-12-2
B120-12-3
B120-12-1
B160-20-1
B160-12-1
B140-12-1
B120-12-1
B100-12-1
B120-12-1
B160-20-1
(a)
3
2
1
0
80
40
0
200
400
600


Wavelength / nm
1
10
Pore size / nm
B160-20-1
2.06 eV
B120-12-2
B120-12-3
2
3
4
5
2
3
4
5
0
1
2
3
4
0.0 0.2 0.4 0.6 0.8 1.0
h / eV
Binding Energy / eV
Relative pressure / P/P0
2
3
4
5
h / eV
Figure 5. (a) Bi 4f and (b) Cl 2p core-level and (c) valance-band (VB) XPS
spectra of BiOCl samples. “S” in (a) denote the peak area.
Figure 6. (a) N2 adsorption-desorption isotherms, (b) UV-vis diffuse reflectance
spectra and (c) corresponding Tauc plots of different BiOCl samples. Insets in
(a) is the pore size distribution curves gained via calculating adsorption
branches of the isotherms using a Barrett-Joyner-Halenda (BJH) method.
Physicochemical properties
Properties of as-prepared samples, such as specific surface
areas (SBET), porous structures and photoabsorbability, were
characterized. The B120-12-1 exhibits a type IV isotherm
featuring a type H3 hysteresis loop (Figure 6a), suggesting the
formation of mesopores, which is similar as reported BiOCl
hierarchical structures [11b, 31]. Its pore size distribution curve (Inset
in Figure 6a) indicates that the average pore sizes of the sample
are ~2.0 and 10.0 nm, respectively. The smaller pores stem from
the stack of nanosheets and the larger ones from the aggregation
of MFs or other stacked structures. B(100–160)-12-1, B120-(1–
16)-1 and B120-12-(2 and 3) show similar results as the B120-12-
1 (Table 1 and Figure S6). The isotherm of B160-20-1 shows no
hysteresis loop and its average pore size is <1.7 nm, suggesting
there are no mesopores in the B160-20-1. The SBET of samples
increases then decreases with increasing T with the B120-12-1
possessing the highest SBET (13.0 m2 g-1, Table 1), and almost
keeps changeless with increasing tr from 1 to 16 h. Increasing the
R from 1 to 3 causes the decrease in the SBET of samples from
13.0 to 8.7 m2 g-1 (Table 1), probably because of the disassembly
of some MFs and stacking of some small nanosheets (Figure 2h
and i). The B160-20-1 possesses the lowest SBET of 3.5 m2 g-1
because of the merging of MFs into LNs. On the whole, the SBET
of samples depends on the MF contents in the samples.
Photocatalytic activity
The SA was used as the photoreaction substrate to evaluate the
photocatalytic performance of the samples. The adsorption
amounts of SA on all of the BiOCl samples are quite low (Figure
S7). As shown in Figure 7a, the photodegradation efficiencies of
the SA on the samples increase then decrease with increasing T
from 80 to 160 °C. The B120-12-1 exhibits the highest efficiency
which is higher than those of BiOCl nanosheets (B001 and B010)
(Figure 7b). To quantitatively compare the photocatalytic
efficiencies, these data were fitted to a pseudo-zero-order kinetic
model, Ct/C0 = 1 – k0/C0ꞏt, where k0, t, C0, and Ct are the rate
constant, the reaction time, the initial SA concentration, and the
SA concentration at t, respectively. As shown in Figure 7a–c and
Table 1, this model can well describe the photodegradation
process. The k0(B120-12-1) is 0.46, 0.30, 0.10 and 0.24 times
higher than the k0(B120-12-3), the k0(B160-20-1), the k0(B001)
and the k0(B010), respectively. We failed to fit the data to a
pseudo-first-order and a pseudo-second-order kinetic models
(Figure S8), as shown by Adj. R2 values of much less than 1 in
Table 1. The pseudo-zero-order kinetic model was also used in a
few BiOCl-containing photodegradation processes before
[33]
,
even though the pseudo-first-order kinetic model was utilized in
most photodegradation reactions of BiOCl [7, 10b, 21c, 28b, 29a, 34]
Photoabsorption performance of samples was shown in the
UV-vis diffuse reflectance spectra. In the ultraviolet region (λ <
.
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10.1002/cctc.201800357
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Table 1. Physicochemical properties of BiOCl samples.
Adj. R2[d]
Sample
SBET[a] (m2 g-1)
Pore size[b] (nm)
Pore volume (cm3 g-1)
Eg[c] (eV)
Zero
First
Second
0.8464
0.3744
0.6526
0.7727
0.7654
0.5954
0.6102
0.5879
0.6496
0.7712
0.6586
B100-12-1
B120-12-1
B140-12-1
B160-12-1
B160-20-1
B120-12-2
B120-12-3
B120-1-1
B120-4-1
B120-8-1
B120-16-1
8.2
13.0
9.5
2.1, 10.2
2.1, 10.2
1.9, 10.4
2.2, 6.0
≤1.7
0.020
0.050
0.027
0.022
0.017
0.013
0.030
0.054
0.050
0.069
0.058
3.14
3.16
3.20
3.24
3.32
3.03
3.01
\
0.9993
0.9975
0.9991
0.9997
0.9997
0.9964
0.9969
0.9955
0.9992
0.9997
0.9993
0.9603
0.7764
0.9269
0.9513
0.9489
0.8689
0.8987
0.8598
0.9275
0.9469
0.9288
5.0
3.5
8.7
2.1, 10.2
1.9, 10.1
2.2, 10.0
2.1, 9.7
2.4, 9.8
2.2, 10.4
8.7
15.3
15.5
11.8
14.2
\
\
\
[a] BET surface areas of samples. [b] Average pore sizes of samples obtained from pore size distribution curves. [c] Energy band gaps of samples. [d] Adjusted
determination coefficients obtained by fitting photodegradation data to a pseudo-zero-, first-, or second-order kinetic model.
1.0
0.5
0.0
1.0
0.5
0.0
1.0
0.5
0.0
a
b
c
morphologies and SBET of samples hardly change (Figures 2g and
S1g–i). With increasing R from 1 to 3, the photoactivity of samples
decreases distinctly with the k0 decreasing from 0.139 to 0.095
mg L-1 s-1 (Figure 7b and d). The B160-20-1 exhibits the lowest
photoactivity (Figure 7d).
B120-1-1
B120-4-1
B120-8-1
B120-12-1
B120-16-1
B160-20-1
None
B120-12-1
B120-12-2
B120-12-3
B001
B80-12-1
B100-12-1
B120-12-1
B140-12-1
B160-12-1
B010
0
40
80
120
0
40
80
120
0
40
80
120
Time / min
Time / min
Time / min
1.0
0.5
0.0
a
b
0.140
0.139
0.14
d
0.135
0.134
0.133
0.126
0.126
0.12
0.10
0.118
0.115
0.112
0.106
BQ
N2
0.103
CCl4
0.0954
t-BuOH
B120-12-1
Ox
0.0844
0
120 240 360 480
Time / min
0
60 120
0.0159
0.02
0.00
Figure 8. (a) cyclic experiments of photodegradation of SA on B120-12-1, and
(b) influences of scavengers (20 mM tert-butyl alcohol (t-BuOH), 0.2 mM sodium
oxalate (Ox), 5 mM CCl4, and 0.1 mM benzoquinone (BQ)) on photocatalytic
efficiencies of B120-12-1.
Figure 7. Photocatalytic degradation of salicylic acid (SA) on BiOCl samples
prepared at different (a) temperatures, (b) Cl/Bi molar ratios, and (c) reaction
time, and (d) corresponding pseudo-zero-order kinetic rate constants (k0). In the
name of “Bx-y-z”, x, y, and z symbolize the temperature, the time, and the Cl/Bi
molar ratio, respectively. B001 and B010 are BiOCl nanosheets with dominantly
exposed (001) and (010) facets, respectively, prepared according to a reported
Five consecutive photoreactions were conducted to test the
stability of B120-12-1. As shown in Figure 8a, no obvious
decrease in photocatalytic efficiency is observed, and the XRD
patterns of the samples before and after the cyclic experiments
are similar (Figure S9a). The SEM image of the sample after the
cyclic experiment shows that the MF structure almost remains
unchanged (Figures S9b and 2a) during the reaction. On the
method [7]
.
The samples show similar photocatalytic efficiencies with tr
increasing from 1 to 16 h (Figure 7c and d) because the
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10.1002/cctc.201800357
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whole, the BiOCl MFs exhibit a satisfying physicochemical
stability. Main reactive oxygen species in the photoreactive
process, such as superoxide radicals, hydroxyl radicals, free
electrons and holes were detected by adding scavengers,
benzoquinone, tert-butyl alcohol, CCl4, and sodium oxalate to the
suspensions, respectively (Figure 8b) [35]. The addition of tert-
butyl alcohol and CCl4 hardly caused the change of photocatalytic
efficiency, demonstrating the hydroxyl radicals and free electrons
are not active species in this reaction, but the addition of
benzoquinone or purging with N2 to remove O2 significantly
decreased the photocatalytic efficiency, manifesting the important
role of superoxide radicals in the photoreaction. Interestingly, the
addition of sodium oxalate led to a prominent increase of the
photocatalytic efficiency. This is because the sodium oxalate
consumed a lot of holes, decreasing the recombination of
electrons and holes [36], and more electrons remained to reduce
oxygen to generate superoxide radicals, further demonstrating
the superoxide radicals are the major active species.
12-1 (~3.3 μm cm-2) is much higher than those of B001 (~1.5 μm
cm-2), B010 (~1.0 μm cm-2), B120-12-3 (~0.8 μm cm-2) and B160-
20-1 (~0.6 μm cm-2) (Figure 10a), indicating higher charge carrier
separation of B120-12-1 [27]. EIS Nyquist plots of B120-12-1
shows a smaller semicircle radius than B001, B010, B120-12-3,
and B160-20-1 (Figure 10b), also manifesting a higher charge
[34a]
carrier separation rate
for the B120-12-1. On the whole, the
B120-12-1 exhibits higher efficient separation of photogenerated
charge carriers than the other prepared BiOCl samples. Besides,
the PL intensity variations with the T and the R are contrary to the
corresponding changes of k0 (Figures
9 and 7d). These
demonstrate that the enhanced charge carrier separation leads to
the improvement of photocatalytic performance. In view of the
lower PL intensity and higher photocurrent density of the B120-
12-1 than those of B001, B010, B120-12-3 and B160-20-1 which
contain no MF/LN homojunctions, the high separation rate of
charge carriers of B120-12-1 should arise from the construction
of MF/LN homojunctions. The B120-12-3 should contain the most
homojunctions in all the BiOCl samples.
The effect of photoabsorption difference on photoactivity of
samples can be excluded because the photoactivity of samples
decreases while their ultraviolet absorption gradually increases
from B120-12-1 to B160-12-1 (Figures 6b and 7d). The influence
of Eg can also be ruled out because the cut-off λ of incident light
a
b
B120-12-3
B160-20-1
B120-12-2
B010
B100-12-1
B120-12-1
B140-12-1
B160-12-1
B001
B120-12-1
for photoreactions is ~365 nm, correspond to a bandgap of ~3.40
[37]
eV (1240/λ)
which is higher than the Eg of all the prepared
BiOCl samples (Table 1). The SBET is a non-ignorable factor
because the k0 of the BiOCl samples basically increase with
increasing SBET (Figure S10). However, the k0 values of B100-12-
1, B120-12-3, and B160-20-1 which do not contain MF/LN
homojunctions, are all smaller than those containing the MF/LN
homojunctions (e.g. B160-12-1 and B120-12-2), even though the
360 400 440 480 520
Wavelength / nm
360 400 440 480 520
Wavelength / nm
Figure 9. Photoluminescence spectra of BiOCl samples at the excitation
wavelength of 280 nm. In the name of “Bx-y-z”, x, y, and z symbolize the
temperature, the time, and the Cl/Bi molar ratio, respectively.
S
BET values of B100-12-1 and B120-12-3 are prominently greater
than that of B160-12-1 and quite close to that of B120-12-2
(Figure S10). This indicates that the MF/LN homojunction is an
important factor leading to high photoactivity of BiOCl samples
containing both MFs and LNs (e.g. B120-12-1). Besides, the
BiOCl nanosheets with dominantly exposed (001) facets exhibited
higher photoactivity than those with highly exposed (010) facets
B120-12-1
B160-20-1
B001
B120-12-3
B010
8
6
4
2
0
B120-12-1
B001
B010
B120-12-3
B160-20-1
b
a
Light on off
4
2
0
(Figure 7d) because of the influence of the internal electric field [7]
.
Given that the MFs are composed of small nanosheets with
dominant exposure of (001) facets, the (001) facet exposure
percentages of as-prepared samples should follow the order,
B160-12-1 < B140-12-1 < B120-12-1 < B120-12-2 < B120-12-3.
However, the B120-12-3 exhibits a smaller k0 value than the
B160-12-1 (Figure S10), suggesting the exposed (001) facet
percentage is not a key factor causing high photoactivity of B120-
12-1.
The construction of MF/LN homojunctions considerably
contributes to the enhancement of photogenerated charge carrier
separation. Generally, Separation rate of photogenerated charge
carriers can be characterized through photoluminescence (PL)
spectra and photoelectrochemical methods. Figure 9 shows the
PL spectra of BiOCl samples. The PL intensity first decreases and
then increases with increasing T from 100 to 160 °C and gradually
increases with increasing R from 1 to 3. The B120-12-1 exhibits
the lowest PL intensity, suggesting slowest recombination
efficiency of charge carriers [27]. The photocurrent density of B120-
0
100
Time / s
200
0
4
8

12
Z /
Figure 10. (a) Photocurrent densities and (b) electrochemical impedance
spectra of different BiOCl samples. The dark current density of all the samples
was set to ~0 for distinct comparison of their photocurrent density. In the name
of “Bx-y-z”, x, y, and z symbolize the temperature, the time, and the Cl/Bi molar
ratio, respectively.
The formation of MF/LN homojunctions arises from appropriate
energy band levels of MFs and LNs. Mott-Schottky plots of B120-
12-3 and B160-20-1 were tested to confirm conduct band (CB)
edges of MFs and LNs, respectively. As shown in Figure 11a and
b, the electrochemical flat potentials of B120-12-3 (MFs) and
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10.1002/cctc.201800357
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B160-20-1 (LNs), obtained by extrapolation of the Mott-Schottky
plots, are about –1.04 and –1.09 V (vs. Ag/AgCl), respectively,
which are proximately equal to their CB edge potentials [38]. Their
corresponding potentials (ECB vs. RHE) are –0.49 and –0.54 eV,
respectively, according to the equation, E(RHE) = E(Ag/AgCl) +
0.197 + 0.059pH. In association with the VB XPS spectra and the
Eg values, the VB edge potentials (EVB vs. RHE) of MFs and LNs
are 2.52 and 2.78 V, respectively, and their Fermi levels (EF) are
separately 0.24 and 0.72 V, as shown in Figure 11c. Because the
EF of MFs is higher than that of LNs, after their contact, electrons
will migrate from the MFs to the LNs, which causes the EF of MFs
and LNs to simultaneously shift downward and upward,
respectively, to reach a unified EF and generates an interfacial
electric field. At this moment, both the ECB and the EVB of LNs are
higher than those of MFs, and the generated interfacial electric
field promotes the transfer of photogenerated electrons from the
CB of LNs to that of MFs and holes from the VB of MFs to that of
LNs, which enhances the charge carrier separation. Actually, this
mechanism was reported for nearly all of the other homojunctions
constructed via tuning particle shapes, such as hexahedron prism
anchored octahedral CeO2 [21b], teethlike BiOCl [21c], vertically
aligned nanosheets-array-like BiOI [39], and graphite carbon nitride
reaction time of 12 h and a Cl/Bi ratio of 1 (B120-12-1) exhibits
the highest MF content (~90 wt%), SBET and photoactivity. With
increasing Cl/Bi ratios, the contents of LNs in the samples
decrease, but the SBET also decreases because of disassembly of
some MFs, accompanied by the decrease of photoactivity. The
MFs were easy to form because they could be observed after
heating at 120 °C for only 0.5 h, and the photoactivity of the
samples exhibit little change with reaction time from 1 to 16 h. The
B160-20-1 contains only LNs but the higher Cl/Bi ratios restricted
formation of the LNs. The ultraviolet absorption and Eg values of
samples increase gradually with the increase of temperature (or
LN contents in the samples), but decrease with increasing Cl/Bi
ratios (or decreasing LN contents in the samples), indicating the
LNs possess higher ultraviolet absorption and a greater Eg than
the MFs. The MFs are composed of thin nanosheets with
thickness of ~15 nm and dominantly exposed (001) facets while
the LNs highly expose (010) facets with thickness of several
hundreds of nanometers. The close contacted MFs and LNs
formed the homojunctions. The MF/LN homojunctions play a key
role in improving the photoactivity of the B120-12-1. The high
photoactivity arises from the high separation rate of
photogenerated charge carriers which was proved by the low PL
intensity and the high photocurrent density. The energy band
levels of MFs and LNs revealed fundamentally the mechanism for
enhanced charge carrier separation and construction of MF/LN
homojunctions. This work provides a new simple way to
synthesize BiOCl hierarchical architectures and homojunctions.
[40]
.
1000 Hz
500 Hz
100 Hz
1000 Hz
500 Hz
100 Hz
a
b
30
20
10
0
12
8
4
Experimental Section

1.09 V
-0.4

1.04 V
-0.4
Materials
0
-1.2
-0.8
0.0
0.4
-1.2
-0.8
0.0
0.4
Potential / V vs. Ag/AgCl
Potential / V vs. Ag/AgCl
Bi(NO3)3ꞏ5H2O, chlorobutanol (C4H7Cl3Oꞏ0.5H2O), and salicylic acid (SA)
were purchased from Aladdin (Shanghai, P. R. China) and used without
further purification. Ultrapure water with resistivity of 18.2 MΩꞏcm was
produced via a Hitech-Kflow system (Shanghai, P. R. China) and used in
all the experiments.
Preparation of BiOCl samples
Bi(NO3)3ꞏ5H2O (1.5mmol) and chlorobutanol with Cl/Bi molar ratios (R) of
1–3 were mixed together by grinding for 5 min in a mortar. Then the
mixtures were transferred to a 20-mL Teflon-lined stainless steel autoclave
and heated at the temperature (T) of 60–160 °C for 0–24 h. After cooling
naturally to the room temperature, the products were washed carefully with
deionized water and alcohol for five times, respectively, and then dried at
60 °C for 12 h. The samples are symbolized by BT-tr-R where tr is the
reaction time. tr = 0 means the heating was stopped once the temperature
reached the set value. R was equal to 1 if there is no special explanation.
For comparison, BiOCl nanosheets with highly exposed (001) or (010)
facets (marked as B001 or B010) were also prepared according to a
Figure 11. Mott-Schottky plots of (a) B120-12-3 and (b) B160-20-1 and (c)
energy band levels and photogenerated charge carrier transfer pathways of
microflower (MF)/large nanosheet (LN) homojunctions. “+” and “–” denote the
direction of interfacial electric field.
Conclusions
BiOCl microflowers (MFs) were synthesized via a solvent-free
low-temperature route, using the chlorobutanol as the chlorine
source. The temperature played a key part for the formation of
MFs. Some MFs merges into large nanosheets (LNs) in the
reaction process and the MF contents in the samples increase
then decrease with the increase of temperature from 100 to
160 °C, which leads to similar changes of their SBET and
photocatalytic activity. The sample prepared at 120 °C with
reported method [7]
.
Characterization
Powder X-ray diffraction (XRD) patterns were measured using a D8
Advance diffractometer (Bruker, Germany), with Cu Kα radiation (λ =
1.54184 Å). Scanning electron microscopy (SEM) images were collected
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FULL PAPER
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Acknowledgements
This work was supported financially by the National Natural
Science Foundation of China (No. 21603118 and 21573133), the
Natural Science Foundation of Shandong Province of China (No.
ZR2016BQ04) and the Independent Innovation Foundation of
Shandong University in China (No. 2016JC032).
Keywords: BiOCl • homojunction • hierarchical • microflower •
solvent-free
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Table of Contents
FULL PAPER
BiOCl microflower/nanosheet
homojunctions were constructed via a
solvent-free route with enhanced
photogenerated charge carrier
separation and photoactivity.
Hanyu Yao, Haiping Li,* Tingxia Hu, and
Wanguo Hou*
Page No. – Page No.
Solvent-free synthesis of bismuth
oxychloride microflower/nanosheet
homojunctions for photoactivity
enhancement
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