Fabrication of a Novel p–n Heterojunction BiOCl/Ag6Si2O7 Nanocomposite as a Highly Efficient and Stable Visible Light Driven Photocatalyst

Article abstract of DOI:10.1007/s10562-018-2631-x

Abstract: Herein, a visible-light-active BiOCl/Ag₆Si₂O₇ nanocomposite with a strong interfacial interaction p–n heterojunction structure was fabricated via a simple deposition–precipitation method and subsequently investigated as a novel photocatalyst for the first time. The structure, morphology, and optical properties of the prepared samples were thoroughly characterized by field-emission scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, X-ray diffraction, diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The photocatalytic performance was evaluated by monitoring the degradation of methyl orange (MO) and phenol and the photocurrent generated under visible-light irradiation. The BiOCl/Ag₆Si₂O₇ photocatalyst increased significantly its photocatalytic performance compared to the pristine BiOCl and Ag₆Si₂O₇ materials. This enhancement could be ascribed to the strong visible light absorption and the effective separation of the photogenerated electrons (e^?) and holes (h⁺) by the internal electrostatic field generated at the junction region. In addition, BiOCl/Ag₆Si₂O₇ showed stable photocurrent over long times and cyclic degradation of MO, thereby demonstrating potential applications in the field of environmental remediation. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2631-x

Catalysis Letters
https://doi.org/10.1007/s10562-018-2631-x
Fabrication of a Novel p–n Heterojunction BiOCl/Ag6Si2O7
Nanocomposite as a Highly Efficient and Stable Visible Light Driven
Photocatalyst
Jibo Qin1 · Nan Chen1 · Chuanping Feng1 · Huiping Chen1 · Zhengyuan Feng1 · Yu Gao2 · Zhenya Zhang3
Received: 17 September 2018 / Accepted: 4 December 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Herein, a visible-light-active BiOCl/Ag6Si2O7 nanocomposite with a strong interfacial interaction p–n heterojunction struc-
ture was fabricated via a simple deposition–precipitation method and subsequently investigated as a novel photocatalyst for
the first time. The structure, morphology, and optical properties of the prepared samples were thoroughly characterized by
field-emission scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, X-ray dif-
fraction, diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy.
The photocatalytic performance was evaluated by monitoring the degradation of methyl orange (MO) and phenol and the
photocurrent generated under visible-light irradiation. The BiOCl/Ag6Si2O7 photocatalyst increased significantly its photo-
catalytic performance compared to the pristine BiOCl and Ag6Si2O7 materials. This enhancement could be ascribed to the
strong visible light absorption and the effective separation of the photogenerated electrons (e−) and holes (h+) by the internal
electrostatic field generated at the junction region. In addition, BiOCl/Ag6Si2O7 showed stable photocurrent over long times
and cyclic degradation of MO, thereby demonstrating potential applications in the field of environmental remediation.
Graphical abstract
Keywords BiOCl/Ag6Si2O7 nanocomposite · Heterojunction structure · Visible light · Phenol and MO degradation ·
Stability
Extended author information available on the last page of the article
Vol.:(0123456789)
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J. Qin et al.
C3N4 [28] being the only systems described. Despite hav-
ing broadened light absorption ranges and improved pho-
tocatalytic activities, these catalysts are expensive, require
complex preparation procedures, and present low photo-
catalytic efficiency towards the degradation of methyl
orange (MO) or phenol. In order to overcome these draw-
backs, it is necessary to combine BiOCl with other suit-
able semiconductors.
1 Introduction
Photocatalysis using semiconductors has been recognized
as an effective method for mitigating environmental pol-
lution and energy shortage problems owing to the supe-
rior solar energy capture and utilization abilities of these
materials [1–5]. Numerous semiconductors (e.g., g-C3N4,
Bi2WO6, CeO2, and AgVO3) have been recently used in a
number of processes, including organic pollutant degrada-
tion to H2O and CO2 [6, 7] and water splitting to H2 and O2
[8]. However, the photocatalytic efficiency of the existing
photocatalysts is currently limited by many factors, includ-
ing the high recombination rate of the photogenerated
e−–h+ pairs, the narrow light absorption range, and the
low photocatalytic efficiency [9]. These issues have been
widely tackled by researchers by using new methods such
as metal doping [10–12], noble metal loading [13] and
preparation of composite photocatalysts with fine hetero-
structure [14]. Among these methods, the preparation of
photocatalysts with heterojunctions has been considered to
be the most effective and feasible approach [15, 16] since
electron–hole (e−–h+) pair recombination is effectively
suppressed in the presence of heterojunctions. However,
highly efficient heterojunction systems are very difficult
to prepare since the fine heterojunction are required com-
bine wide light absorption ranges with high photogen-
erated charge–carrier separation rates. Coupling of two
semiconductor photocatalysts require them to present a
well-matched band structure and a suitable molar ratio.
Therefore, it is urgent and crucial to prepare a coupled
photocatalyst with a well-matched conduction and valence
bands (CB and VB, respectively).
Ag6Si2O7 is a typical n-type semiconductor in which
the photogenerated e−–h+ are effectively separated by
the unique internal electric field structure of this material
[29]. Ag6Si2O7 has a very narrow band gap (1.58 eV) [30]
such that it absorbs light over the entire UV and Visible
regions (<740 nm) [31]. Therefore, Ag6Si2O7 can be used
as a highly efficient photosensitive modifier for improving
the visible-light photocatalytic activity of wide band-gap
semiconductors. To our delight, by investigating the energy
levels of Ag6Si2O7, we found that the band structures of
n-Ag6Si2O7 and p-BiOCl match well. Thus, both materials
can be used to construct p–n heterojunction photocatalysts
with high visible-light catalytic performance. However, to
the best of our knowledge, the preparation and application
of p-BiOCl/n-Ag6Si2O7 heterojunction photocatalyst has not
been reported yet.
In this study, BiOCl/Ag6Si2O7 nanocomposites with
p–n junction structure were developed by a simple depo-
sition–precipitation method at room temperature. Various
characterization techniques were used to study the structure,
morphology, and optical properties of these heterojunctions.
MO (as a dye model) and phenol (as representative of color-
less contaminants) were used to evaluate the photocatalytic
activity of the BiOCl/Ag6Si2O7 p–n heterojunction nano-
composite under visible-light illumination. The effect of the
Bi/Ag molar ratio on the photocatalytic performance and
the stability of the hybrids were investigated. Besides, the
intermediate products, the photodegradation mechanism,
and the degradation pathways were also demonstrated and
studied in detail.
Bi-based compounds are currently used as photocata-
lysts for pollution control and environmental restoration
[17]. These compounds include Bi2O3 [18], Bi2WO6 [19],
BiVO4 [20], Bi2MNbO7 (M = Al, Sm, and Fe) [21], and
BiOX (X = Cl, Br, and I) [22]. Among these photocata-
lysts, BiOCl has been widely used for photooxidation and
disinfection of polluted wastewaters owing to its unique
physical structure and high photocatalytic stability [23].
However, BiOCl has a large bandgap and therefore the
excitation of the photogenerated charge carriers, which
can degrade organic pollutants are carried out only under
ultraviolet (UV) irradiation, thus severely limiting its
practical application [24]. Therefore, constructing hetero-
junction structures by coupling BiOCl with other suitable
semiconductors is a reasonable method to enhance its light
absorption capability and photocatalytic activity. However,
examples of p–n type composite heterogeneous systems
containing BiOCl-based junction photocatalytic materi-
als are scarce in the literature, with BiPO4/BiOCl [25],
BiOCl/Bi2MoO6 [26], BiOCl–SrFe12O19 [27], and BiOCl/
2 Experimental
2.1 Materials and Methods
All reagents used were of analytical grade and were used
without further purification. For the synthesis of BiOCl,
0.458 g Bi(NO3)3·5H2O powder was dissolved in 25 mL of
ethylene glycol to prepare a Bi(NO3)3 glycol solution, which
was placed in an ultrasonic bath for 15 min to obtain a uni-
formly mixed solution. At the same time, a certain amount
of KCl was added to 15 mL of deionized water. Then, the
as-prepared KCl solution was added to the Bi(NO3)3 glycol
solution. After continuous magnetic stirring for 30 min, a
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Fabrication of a Novel p–n Heterojunction BiOCl/Ag6Si2O7 Nanocomposite as a Highly…
white solid product was collected by filtration, washed, and
dried for 12 h.
out and filtered (0.22 µm drainage membrane) to remove
the solid photocatalyst particles. Finally, the above solu-
tions containing MO and phenol were analyzed on a UV–Vis
spectrophotometer (DR6000, HACH, USA) and a high-effi-
ciency liquid chromatograph (Agilent 1206, Agilent Tech-
nologies, USA), respectively. The degradation efficiency (%)
was calculated by using the following equation:
The BiOCl/Ag6Si2O7 nanocomposite photocatalyst was
prepared by a simple co-precipitation process at room tem-
perature. Typically, 1 mM AgNO3 and 1 mM BiOCl aque-
ous solutions were prepared by dissolving these solids in
50 mL of deionized water. Then, a Na2SiO3 solution (20 mL,
0.05 M) was added dropwise to the above mixed solution.
After that, the solid product was collected and washed
several times with deionized water. Finally, the samples
were dried in an oven at 65 °C for 12 h to yield the BiOCl/
Ag6Si2O7 photocatalyst. A series of x BiOCl/Ag6Si2O7 (with
x being the molar ratio of Bi to Ag; x = 1/4, 2/1, 1/1, 1/2,
and 4/1) were synthesized by using the same above method,
while pure Ag6Si2O7 was prepared similarly but without
adding BiOCl.
Degradation efficiency (%) = Ct∕C0 × 100%
(1)
where C0 (mg/L) and Ct (mg/L) are the concentrations of
the target organic solution at irradiation times 0 and t (min),
respectively.
3 Results and Discussion
3.1 Characterization
2.2 Characterization
3.1.1 FESEM and TEM
The morphology of the as-obtained samples was charac-
terized by field emission scanning electron microscopy
(FESEM, GeminiSEM500, Zeiss, Germany) and transmis-
sion electron microscopy (TEM, JEM-2100, Jeol, Japan).
The phase structure and chemical composition of the pho-
tocatalysts were evaluated by X-ray diffraction (XRD, D8
Focus, Bruker, Germany) and X-ray photoelectron spec-
troscopy (XPS, ESCALAB 250Xi, Thermo Fisher, USA),
respectively. The UV–Vis diffuse reflection spectra (DRS)
of the samples were recorded on a UV–Visible-near infrared
spectrophotometer (Cary 5000, Varian, USA). Fourier trans-
form infrared spectroscopy (FTIR, Vertex 70V, Bruker, Ger-
many) was used for analyzing the surface functional groups
of the BiOCl, Ag6Si2O7, and BiOCl/Ag6Si2O7 samples. The
photocatalytic degradation intermediates and the end prod-
ucts resulting from the degradation of MO were detected by
a gas chromatography (GC, Agilent 7890A, Agilent Tech-
nologies, USA).
The surface morphology and particle size of the obtained
BiOCl, Ag6Si2O7, and 1/1 BiOCl/Ag6Si2O7 nanocomposites
were characterized by TESEM and TEM (Fig. 1). Figure 1a
shows the FESEM images of pure Ag6Si2O7, which con-
sisted of regular and smooth spherical particles of 100 nm
in diameter. In contrast, the BiOCl powders (Fig. 2b) exhib-
ited an irregular and non-uniform spherical structure with
a diameter of ca. 1 µm. Figure 1c, d show the FESEM and
TEM images of the 1/1 BiOCl/Ag6Si2O7 composite, respec-
tively. As shown in Fig. 1c, d, Ag6Si2O7 was tightly associ-
ated with BiOCl forming particles of 30 nm in diameter.
In order to further determine the existence of a het-
erojunction structure between BiOCl and Ag6Si2O7, the
1/1 BiOCl/Ag6Si2O7 nanocomposite was investigated by
HRTEM. The magnified HRTEM image (Fig. 1e) clearly
revealed fringes with lattice spacings of 0.264 and 0.245 nm,
which could be indexed to the (1 2 4) and (0 0 3) planes of
Ag6Si2O7 and BiOCl, respectively. These results confirmed
the existence of a BiOCl/Ag6Si2O7 heterojunction. There-
fore, the BiOCl/Ag6Si2O7 photocatalyst could effectively
inhibit recombination of the photogenerated e−–h+ pairs
and greatly improve the efficiency of light capture and its
utilization.
2.3 Photocatalytic Performance
The photocatalytic performance of the BiOCl, Ag6Si2O7,
and x BiOCl/Ag6Si2O7 samples towards the photodegrada-
tion of MO (10 mg/L) and phenol (30 mg/L) were carried
out under visible light illumination with an average light
intensity of 40 mW/cm2. At the initial stage of the photocata-
lytic reaction, 100 mg of photocatalyst were ultrasonically
dispersed in a 300 mL glass beaker containing 100 mL of
MO or phenol solutions at room temperature. Subsequently,
the mixed solution was vigorously stirred in dark for 15 min
to complete the adsorption–desorption balance between MO
(or phenol) and the photocatalyst. Afterwards, the Xe lamp
was turned on to start the photocatalytic reaction. At given
intervals, 2.5 mL of the reacted liquid were quickly taken
3.1.2 XRD
The crystal structures of the BiOCl, Ag6Si2O7, and
1/1BiOCl/Ag6Si2O7 nanocomposite catalysts were inves-
tigated by XRD at room temperature, and the results are
displayed in Fig. 2. Regarding BiOCl (Fig. 2a), diffrac-
tion peaks at 2θ = 12.10, 25.90, 32.80, 33.52, 41.06, and
46.67° were clearly observed, which corresponded to the
(0 0 1), (1 0 1), (1 1 0), (1 0 2), (1 1 2), and (2 0 0) planes
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J. Qin et al.
Fig. 1 FESEM images of Ag6Si2O7 (a); BiOCl (b) and 1/1 BiOCl/Ag6Si2O7 (c), TEM (d) and HRTEM (e) images 1/1 BiOCl/Ag6Si2O7
of a tetragonal BiOCl phase (JCPDS 06-0249) [32]. The
XRD pattern of Ag6Si2O7 (Fig. 2b) contained one promi-
nent peak at ca. 34.01°, characteristic of the (1 2 4) plane of
the Ag6Si2O7 phase (JCPD File No. 85-0281) [30, 33]. The
diffraction pattern of the BiOCl/Ag6Si2O7 nanocomposites
(Fig. 2c) revealed the presence of intense sharp peaks, indi-
cating that the catalysts were well-crystallized. Compared
to the fresh sample, the XRD pattern of BiOCl/Ag6Si2O7
after four reaction cycles (Fig. 2d) remained unchanged with
regards to the position and intensity of the peaks, thereby
revealing good stability. In addition, no impurity peaks were
found for the BiOCl/Ag6Si2O7 heterojunction, which sug-
gested that the photocatalyst was only composed of BiOCl
and Ag6Si2O7.
3.1.3 XPS
To further determine the chemical composition and differ-
ent chemical status of the BiOCl/Ag6Si2O7 photocatalyst,
XPS analysis was conducted. The full-range XPS survey
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Fabrication of a Novel p–n Heterojunction BiOCl/Ag6Si2O7 Nanocomposite as a Highly…
absorption (i.e., 1/2 and 2 for direct and indirect transitions,
respectively).
Using n = 1/2, the direct band gap energy of the 1/1
BiOCl/Ag6Si2O7 heterostructure was estimated to be 1.87 eV
(Fig. 4b), which revealed the narrow-band-gap characteris-
tics of this photocatalyst. This value was lower than that of
BiOCl (2.71 eV), thereby demonstrating that the deposi-
tion of Ag6Si2O7 on BiOCl was effective in increasing the
absorbance of visible light and the number of available pho-
togenerated carriers. The Mott–Schottky curve of Ag6Si2O7
was also measured without light (Fig. 4c). The linear plot
showed a positive slope, revealing the n-type semiconduc-
tor nature of Ag6Si2O7. The flat potential was calculated to
be −0.03 eV (vs. standard calomel electrode (SCE)), which
corresponded to 0.21 V (vs. normal hydrogen electrode
(NHE)). N-type semiconductors are well-known for having
CB 0–0.1 eV above the flat potential [37, 38]. Here, the volt-
age difference between the CB and the flat potential was set
to 0.02 eV. Therefore, the CB of Ag6Si2O7 was calculated
to be 0.23 eV. Combing the above results and the band-gap
values extrapolated in the DRS spectra (Fig. 4b), the VB of
Ag6Si2O7 was estimated to be 1.23 eV.
Fig. 2 XRD patterns of the BiOCl (a); Ag6Si2O7 (b); 1/1 BiOCl/
Ag6Si2O7 (c) and used 1/1 BiOCl/Ag6Si2O7 sample (d)
spectrum of BiOCl/Ag6Si2O7 (Fig. 3a) revealed the pres-
ence of Bi, Cl, O, Ag and Si (the C 1s peak was ascribed to
adventitious hydrocarbon from the XPS instrument itself).
The high-resolution Bi 4f XPS spectrum is shown in Fig. 3b.
The two peaks at 159.09 and 164.50 eV were assigned to
the Bi 4f7/2 and Bi 4f5/2 transitions respectively, and were
characteristics of Bi3+ in BiOCl [34]. As shown in Fig. 3c,
two peaks at 199.80 and 198.20 eV were observed in the Cl
2p spectrum, which were ascribed to Cl 2p1/2 and Cl 2p3/2
transitions, respectively [35]. The two peaks at 374.38 and
368.37 eV (Fig. 3d) were indexed to the Ag 3d3/2 and Ag
3d5/2 transitions of Ag6Si2O7, respectively [30, 33]. The XPS
pattern of pure BiOCl (Fig. 3e) revealed that this sample was
high purity, since it only contained Bi, O, and Cl signals.
Thus, the corresponding band edge positions of BiOCl
was calculated by using the following equation [39]:
EVB = χ − Ee + 0.5Eg
(3)
where χ is the geometric mean of the electronegativity
of the constituent atoms forming the semiconductor, Ee is
the energy of the free electrons on the hydrogen scale (ca.
4.5 eV), and Eg is the band gap energy of the semiconductor.
The χ value for BiOCl was 6.64 eV. From this equation, EVB
and ECB of BiOCl were calculated to be 3.49 and 0.78 eV,
respectively.
3.1.5 FTIR
3.1.4 DRS
The chemical structure and bonding of the samples were
further studied by FTIR. All samples showed significant
absorption peaks at 1664, 2965, and 3400 cm−1, which
may be attributed to the O–H stretch of H2O adsorbed on
the surface of the photocatalyst [40, 41], while the peak
at 1374 cm−1 was assigned to the stretching vibrations of
the CH2 groups in ethanol [7]. In the case of Ag6Si2O7
(Fig. 5a), the absorption peaks at 470 and 570 cm−1 may
be associated to the bending mode of Si–O–Si, while
the vibration band of 987 cm−1 was attributed to the
Si–O bending mode [42]. As shown in Fig. 5b, the peak
observed at 519 cm−1 was produced by Bi–O stretching
vibration [43]. The absorption peaks at 430 and 1050 cm−1
were ascribed to the O–Cl stretching and symmetric
stretching vibrations of the Bi–Cl bond, respectively
[44, 45], which fully confirmed the composition of the
BiOCl photocatalyst. The sharp peaks at 870 cm−1 may
UV–Vis DRS was used to explore the optical properties of
the samples. As shown in Fig. 4a, Ag6Si2O7 possessed a
strong photo-absorption ability in the UV and visible–light
regions, and this photo-absorption extended to 640 nm. In
contrast, BiOCl exhibited a strong absorption only in the UV
region. Coupling of BiOCl and Ag6Si2O7 resulted in BiOCl/
Ag6Si2O7 composites having an enhanced optical absorption
properties than pure BiOCl. Meanwhile, the band gap of the
samples were calculated by the Kubelka–Munk expression
[36]:
(hvꢀ)1∕n = A(hv − Eg)
(2)
where α is the optical absorption coefficient; hv is the inci-
dent photonic energy (eV); A is the proportionality con-
stant; Eg is the band gap energy (eV); and n is a factor that
depends on the kind of optical transition induced by photon
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J. Qin et al.
Fig. 3 The XPS spectra of 1/1
BiOCl/Ag6Si2O7 (a); Bi 4f (b);
Cl 2p (c); Ag 3d (d) and pure
BiOCl (e)
be produced by C–H bond associated with small amounts
of ethylene glycol present on the surface of BiOCl. Since
BiOCl was tightly encapsulated by Ag6Si2O7, the FTIR
spectrum of the BiOCl/Ag6Si2O7 photocatalyst was similar
to that of Ag6Si2O7 (Fig. 5c). All the peaks of the BiOCl/
Ag6Si2O7 composite photocatalysts revealed the presence
of BiOCl and Ag6Si2O7 exclusively. Thus, new phases
other than BiOCl and Ag6Si2O7 were not observed, which
further demonstrated that the BiOCl/Ag6Si2O7 nanocom-
posite was successfully synthesized, in line with the XRD
and XPS results.
3.2 Photocatalytic Activity
The MO degradation curves on BiOCl, x BiOCl/Ag6Si2O7,
and Ag6Si2O7 are shown in Fig. 6a. The degradation effi-
ciencies of pure BiOCl and Ag6Si2O7 after 40 min of
reaction were 19.0 and 16.0%, respectively. The photo-
catalytic activity of x BiOCl/Ag6Si2O7 towards the deg-
radation of MO showed a maximum for Bi to Ag molar
ratio of 1:1 (98.1%) and a decrease thereafter. To the best
of our knowledge, the photocatalytic efficiency of this 1/1
BiOCl/Ag6Si2O7 material was much higher than those of
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Fabrication of a Novel p–n Heterojunction BiOCl/Ag6Si2O7 Nanocomposite as a Highly…
Fig. 4 UV–Vis diffuse reflectance spectra (a); band energy (b) and Mott–Schottky curve (c) of samples
BiOBr/BiPO4 [46], BiOBr0.75 0.25/BiOIO3 [47], BiOBr/
I
RGO [48]. In addition, in order to better understand the
reaction kinetics of the photocatalytic degradation process
of MO, a pseudo-first-order model was applied according
to the following equation:
Ln(C0∕C) = Kapp
t
(4)
where C0 is the initial concentration of MO (mg/L); C is
the concentration of MO at a time t (mg/L); t is the irradia-
tion time (min), and Kapp is the reaction rate constant (1/
min). The Kapp values of the different samples are shown
in Fig. 6b. The Kapp values were calculated to be 0.0710,
0.0211, 0.0170, 0.0041, 0.0032, 0.0047, 0.0024 1/min for
the 1/1 BiOCl/Ag6Si2O7, 2/1 BiOCl/Ag6Si2O7, 4/1 BiOCl/
Ag6Si2O7, 1/2 BiOCl/Ag6Si2O7, 1/4 BiOCl/Ag6Si2O7, BiOCl
and Ag6Si2O7, respectively. Thus, 1/1 BiOCl/Ag6Si2O7
showed the highest Kapp among the samples tested herein.
This value was ca. 15.2 and 29.6 times higher than that of
BiOCl and Ag6Si2O7, respectively.
Fig. 5 FT-IR spectra for the Ag6Si2O7 (a); BiOCl (b); and 1/1 BiOCl/
Ag6Si2O7 (c) samples
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J. Qin et al.
Fig. 6 Photocatalytic activities of the samples towards the MO degra-
dation (a); apparent rate constants for the photocatalytic degradation
of MO solution (b); photodegradation curves of phenol over BiOCl,
Ag6Si2O7 and 1/1 BiOCl/Ag6Si2O7 (c); TOC removal on the MO
solution in the presence of the 1/1 BiOCl/Ag6Si2O7 catalyst (d)
To eliminate the influence of dye sensitization on the
degradation process, we used a colorless and refractory
substance such as phenol as the degradation target to fur-
ther study the photocatalytic activity of BiOCl/Ag6Si2O7.
As revealed in Fig. 6c, only 0.3 and 20% of phenol were
degraded by BiOCl and Ag6Si2O7 within 7 h, respectively.
In contrast, 1/1 BiOCl/Ag6Si2O7 decomposed 83% of phe-
nol within the same time period. The superior photocata-
lytic activity of 1/1 BiOCl/Ag6Si2O7 originated from a
synergistic effect between BiOCl and Ag6Si2O7.
3.3 Photocurrent Properties and Stability of BiOCl/
Ag6Si2O7
High photocurrent intensities are indicative of high e−–h+
separation efficiencies, which is related to the photocatalytic
performance [49]. Herein, the mechanism governing the
generation, separation, and transformation of photogenerated
charges was investigated by monitoring the variation of pho-
tocurrent time at room temperature. As shown in Fig. 7a, the
instantaneous photocurrent of the 1/1 BiOCl/Ag6Si2O7 nano-
composite was ca. 2.5 and 2.6 times higher than those of
BiOCl and Ag6Si2O7, respectively. This higher photocurrent
of the 1/1 BiOCl/Ag6Si2O7 nanocomposite was attributed to
its higher separation efficiency for the photoinduced e− and
h+ pairs, which resulted in higher photocatalytic activities.
The stability of a photocatalyst is the most important indi-
cator to evaluate its practical applications. We performed
The concentration of total organic carbon (TOC) was
selected as the mineralization index to characterize the
MO degradation degree. As shown in Fig. 6d, 60.1% of
the TOC was mineralized after 90 min under irradiation,
revealing that the BiOCl/Ag6Si2O7 nanocomposite has a
great potential in environmental remediation and polluted
wastewater treatment applications.
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Fabrication of a Novel p–n Heterojunction BiOCl/Ag6Si2O7 Nanocomposite as a Highly…
Fig. 7 Transient photocurrent densities of samples (a) and repeated photocatalytic experiments of the 1/1 BiOCl/Ag6Si2O7 nanocomposite under
simulated visible light (Catalyst dosage=1 g/L, C0 =10 mg/L) (b)
MO degradation cycling tests on 1/1 BiOCl/Ag6Si2O7
(Fig. 7b). The photocatalytic efficiency of 1/1 BiOCl/
Ag6Si2O7 remained unchanged after four successive cycles.
This result was in line with the XRD results (Fig. 2d), indi-
cating that the heterojunction photocatalysts was highly sta-
ble and possessed high catalytic activity.
3.4 Photocatalytic Mechanism
In order to determine the role of these active substances
within the Photodegradation process of MO, three different
radical scavengers were added to the reaction system namely,
benzoquinone (BQ), ammonium oxalate (AO), and isopropyl
−
alcohol (IPA) as superoxide radical (·O2 ), hole (h+), and
hydroxyl radical (·OH) scavengers, respectively. As shown
in Fig. 8, when IPA was added to the reaction system, the
photocatalytic activity of BiOCl/Ag6Si2O7 remained nearly
unchanged, indicating that ·OH was not the main active free
radical species. However, addition of BQ and AO resulted
in significant decreases of the MO degradation efficiency,
indicating that h+ and ·O2− were the mainly active species.
Based on the above results, a possible Z-scheme photo-
catalytic mechanism on the BiOCl/Ag6Si2O7 heterojunction
photocatalyst under visible light was proposed (Fig. 9). As is
well-known, the Fermi level (EF) of a p-type semiconductor
such as BiOCl is close to its VB, while Ag6Si2O7 is an n-type
semiconductor and therefore EF is close to its CB. When
Ag6Si2O7 photocatalyst grown on the surface of the BiOCl
particles and formed a p–n heterojunction, the energy level
of BiOCl would shift to higher values. While, the energy
level of Ag6Si2O7 would shift continuously downward until
the EFs of BiOCl and Ag6Si2O7 reached an equilibrium. At
the same time, a strong internal electric field would be also
formed between BiOCl and Ag6Si2O7.
Fig. 8 Reactive species trapping experiments (catalyst dosage=1 g/L,
C0 =10 mg/L)
Under visible light irradiation, both BiOCl and Ag6Si2O7
were simultaneously excited to produce h+ and e−. Owing
to the strong internal electric field in the closely contacted
BiOCl and Ag6Si2O7, the photoinduced e− on the CB of
Ag6Si2O7 was transferred to the VB of BiOCl. We speculated
that, at this moment, the ECB of BiOCl was more negative
than the standard oxygen reduction potential (−0.33 eV vs.
NHE) [50]. Therefore, it was possible for the photogenerated
−
electrons in the CB of BiOCl to reduce O2 to ·O2 . Although,
the h+ produced in the VB of Ag6Si2O7 may become more
positive, the ECB of Ag6Si2O7 was below the standard redox
potential of OH−/·OH (2.38 eV) [51]. Thus, ·OH species
were not detected in the active substance capture experi-
ment. However, owing to their superior oxidizing power, the
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J. Qin et al.
Fig. 9 Z-scheme photocatalytic mechanism of the of the BiOCl/Ag6Si2O7 photocatalyst
h+ produced in the VB of Ag6Si2O7 could directly oxidize
the organic contaminants to form CO2 and H2O. Accord-
ing to recent literature, partial chloride ions were oxidized
to chlorine in the composite photocatalyst containing chlo-
ride ions during the photocatalytic process. Therefore, we
inferred that in this reaction system, the chlorine molecules
adsorbed on BiOCl/Ag6Si2O7 could directly oxidize MO or
phenol and returned to the original ionic state (Cl−) due to
their superior oxidation capacity. In summary, these excel-
lent optical absorption properties and rapid photo-generated
charge transfer rates led to the high photocatalytic activity
for the BiOCl/Ag6Si2O7 heterojunction photocatalyst.
these small molecular organics were oxidized to H2O and
CO2.
4 Conclusions
In summary, a novel BiOCl/Ag6Si2O7 p–n heterojunction
nanocomposite was successfully prepared by a deposi-
tion–precipitation method at room temperature. Various
characterization techniques were used to study the struc-
ture, morphology, and optical properties of these het-
erojunctions. Among the samples tested, the 1/1 BiOCl/
Ag6Si2O7 p–n heterojunction exhibited the highest pho-
tocatalytic activity towards the degradation of different
organic (MO and phenol) degradation under visible light
irradiation. This enhanced photoactivity was attributed to
the intimate p–n heterostructure formed between BiOCl
and Ag6Si2O7, which promoted significantly efficient
separation and transfer of photogenerated charge carri-
ers and thereby inhibited charge–carrier recombination.
Simultaneously, the materials displayed good stability and
recyclability. It was noteworthy that this is the first report
on photocatalytic decomposition of organic compounds
over BiOCl/Ag6Si2O7 nanocomposites under visible light
condition. We believe that these results will encourage
further investigations and the development of p–n BiOCl/
Ag6Si2O7 heterojunction nanocomposites for refractory
polluted wastewater applications.
3.5 Degradation Pathway of MO
In order to better understand the degradation pathway of MO
in the photocatalytic process, we conducted GC-MS analysis
of the intermediate products of reaction, and proposed a rea-
sonable degradation pathway (Fig. 10). The main intermedi-
ate products were benzenesulfonic acid with retention time
(RT) 8.75 min and 2-methyl-N-phenyl with RT of 8.77 min.
These substances were the initial degradation products of
MO and revealed that the catalytic process started with the
cleavage of the azo group linking the two aromatic rings.
Longer irradiation times resulted in the onset of new inter-
mediates (benzene with a RT of 12.21 min and aniline with a
RT of 12.29 min), which were degraded to heptane. Finally,
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Fabrication of a Novel p–n Heterojunction BiOCl/Ag6Si2O7 Nanocomposite as a Highly…
Fig. 10 Degradation pathway proposed on the basis of GC/MS analysis during photodegradation of MO
Acknowledgements The authors acknowledge financial support from
the National Natural Science Foundation of China (NSFC) (Nos.
20407129, 51578519), Major Science and Technology Program for
Water Pollution Control and Treatment (No. 2017ZX07202002) and
the Fundamental Research Funds for the Central Universities (No.
2652017190).
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Affiliations
Jibo Qin1 · Nan Chen1 · Chuanping Feng1 · Huiping Chen1 · Zhengyuan Feng1 · Yu Gao2 · Zhenya Zhang3
2
* Nan Chen
chennan@cugb.edu.cn
College of Chemical and Environmental Engineering,
Shandong University of Science and Technology,
Qingdao 266590, People’s Republic of China
1
School of Water Resources and Environment, MOE Key
Laboratory of Groundwater Circulation and Environmental
Evolution, China University of Geosciences (Beijing),
Beijing 100083, People’s Republic of China
3
Graduate School of Life and Environmental Sciences,
University of Tsukuba, 1-1-1 Tennodai, Tsukuba,
Ibaraki 305-8572, Japan
1 3