N-doped bismuth molybdate decorated with Pt nanoparticles removal azo dyes efficiently via the synergistic effect of adsorption and photocatalysis

Article abstract of DOI:10.1016/j.jallcom.2020.158336

A promising and highly efficient 0.5%Pt/N-Bi₂MoO₆ photocatalyst was synthesized by two-step hydrothermal route and NaBH₄ reduction method to obtain outstanding adsorption performance and photocatalytic efficiency. The degradation yield obtained by 0.5%Pt/N-Bi₂MoO₆ composite catalyst for orange II was 90.13%, which was 4 times and 1.8 times higher than that of pure Bi₂MoO₆ and N-Bi₂MoO₆, respectively. Such prominent photodegradation efficiency was primarily ascribed to the introduction of Pt and N narrowed the band gap of the bulk Bi₂MoO₆ material and surface plasmon resonance (SPR) effect triggered by Pt nanoparticles (NPs), which significantly broadened the optical absorption range and improved separation efficiency of electron-hole pairs. Furthermore, the effects of the initial pH of the solution, the initial concentration of the catalyst, the optical properties and the loading of Pt were discussed. Notably, the adsorption process could be described by the Freundlich model, where 1/n = 0.536, indicating that the concentration had a weak influence on the adsorption capacity. Finally, it had been determined that superoxide anion radicals (?O₂^-) and photogenerated holes (h⁺) were the main active species for the degradation of orange II.

Full text of DOI:10.1016/j.jallcom.2020.158336

Journal of Alloys and Compounds 863 (2021) 158336
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
N-doped bismuth molybdate decorated with Pt nanoparticles removal
azo dyes efficiently via the synergistic effect of adsorption and
photocatalysis
a
a
a
b
a
a
a
Qi Jia , Phuc Khanh Nguyen , Zehao Gu , Xiujun Zhang , Min Liu , Xuefeng Tian , Lei Ma ,
a
a
a,⁎
Li Gong , Xiao Mu , Yanlong Chang
a
College of Chemistry and Chemical Engineering, Laboratory of Special Function Materials and Structure Design of the Ministry of Education, Gansu Provincial
Engineering Laboratory for Chemical Catalysis, Lanzhou University, Lanzhou 730000, PR China
b
College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2020
Received in revised form 11 December 2020
Accepted 13 December 2020
Available online 12 January 2021
A promising and highly efficient 0.5%Pt/N-Bi MoO6 photocatalyst was synthesized by two-step hydro-
2
thermal route and NaBH
catalytic efficiency. The degradation yield obtained by 0.5%Pt/N-Bi
was 90.13%, which was 4 times and 1.8 times higher than that of pure Bi
Such prominent photodegradation efficiency was primarily ascribed to the introduction of Pt and N nar-
rowed the band gap of the bulk Bi MoO material and surface plasmon resonance (SPR) effect triggered by
4
reduction method to obtain outstanding adsorption performance and photo-
MoO composite catalyst for orange II
MoO and N-Bi MoO , respectively.
2
6
2
6
2
6
2
6
Keywords:
Pt nanoparticles (NPs), which significantly broadened the optical absorption range and improved separation
efficiency of electron-hole pairs. Furthermore, the effects of the initial pH of the solution, the initial con-
centration of the catalyst, the optical properties and the loading of Pt were discussed. Notably, the ad-
sorption process could be described by the Freundlich model, where 1/n = 0.536, indicating that the
concentration had a weak influence on the adsorption capacity. Finally, it had been determined that su-
Visible light photocatalysis
Pt-supported and N-doped
Adsorption
Orange II
Photodegradation
-
+
peroxide anion radicals (•O
gradation of orange II.
2
) and photogenerated holes (h ) were the main active species for the de-
©
2021 Elsevier B.V. All rights reserved.
1. Introduction
Bi
2 6
MoO as a typical bismuth-based photocatalyst is composed of
2+
alternating perovskite and [Bi
O
2 2
]
layers. This special layered
Nowadays, wastewater containing organic compounds has at-
structure triggers the existence of an embedded electric field, which
is beneficial to the separation of photogenerated carriers.
Significantly, the sp hybridization of O 2p and Bi 6 s orbitals resulted
in a relatively narrow band gap, enhancing the ability to capture
tracted widespread attention due to its large proportion, difficulty in
treatment and high toxicity to organisms [1,2]. Traditional de-
gradation methods such as microbial degradation, physical sedi-
mentation, and chemical reaction degradation have the
disadvantages of cumbersome processing, incomplete degradation,
and secondary pollution by-products [3,4]. Therefore, it is particu-
larly imperative to develop an efficient and environmentally friendly
degradation technology. In recent years, semiconductor-based ma-
terials have been widely used in photocatalysts, which can make full
use of renewable solar energy to degrade organic compounds into
green and non-toxic inorganic small molecules [5–9].
2 6
visible light [10–14]. Therefore, Bi MoO stands out in the fields of
H
2
production, organic synthesis and pollutant degradation as a
novel material due to its advantages of small band gap, low price,
easy availability, stable performance, and excellent visible light
catalytic activity [15–21]. However, fast carrier recombination and
2 6
low quantum yield attribute to the inherent structure of Bi MoO ,
which makes it difficult for photogenerated electrons and holes
migrate to the surface of catalyst to participate in redox reaction,
resulting in lower photocatalytic applications. To solve these bot-
tlenecks, a variety of methods were proposed, including doping of
nonmetallic elements [22–27], deposition of the (noble)metallic
nanoparticles [28–34], compounding between semiconductor ma-
terials [35–40] and optimization of material morphology [41–44].
⁎
Correspondence to: State Key Laboratory of Applied Organic Chemistry College of
Chemistry and Chemical Engineering Lanzhou University, Lanzhou 730000, P.R. China.
E-mail address: changyanlong@lzu.edu.cn (Y. Chang).
https://doi.org/10.1016/j.jallcom.2020.158336
0925-8388/© 2021 Elsevier B.V. All rights reserved.

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
Recently, especially noble metal NPs (such as Au Ag, Pt, etc.)
supported semiconductors have very broad prospects in accelerating
the catalytic activity [45–49]. On the one hand, a vibrating electric
field is generated on the surface of the precious metal NPs due to its
SPR effect under the excitation of light, which makes the electron
migration rate on the surface and interface become faster [48,50,51].
On the other hand, photo-excited electrons can be transferred from
semiconductors with higher Fermi levels through the interface to
precious metals with lower Fermi levels loaded on the semi-
conductor surface, which can separate the electron-hole pairs ef-
fectively [52]. For instance, Di [53] and co-workers fabricated a novel
200 mL of ammonia solution (6 M) was added into the round bottom
flask, stirred evenly for 30 min at a certain speed, and the yellow
green suspension was transferred to Teflon-lined autoclave for a
hydrothermal reaction (130 ℃ for 14 h). After cooling down natu-
rally, the prepared bright yellow samples were washed several times
with ethanol and ultrapure water followed by drying for 12 h at
60 °C and ground well for further use.
2.1.2. Synthesis of Pt/N-Bi
A certain amount of H
pared N-Bi MoO
taining 15 mL of deionized water, the flask was sonicated for 30 min
and stirred for 3 h to make the mixing uniform. NaBH (6 mL, 0.1 M)
was slowly added dropwise to the mixed solution to obtain Pt/N-
Bi MoO photocatalyst. Inductively coupled plasma optical emission
spectrometry (ICP-OES) analysis showed that Pt loadings of 0.5%, 1%
and 1.76% were obtained in Pt/N-Bi MoO by using 850ul, 1.7 mL and
3.4 mL of H PtCl solution, respectively. Samples with different Pt
contents were prepared and labeled as x% Pt/N-Bi MoO , where x
represented the mass ratio of Pt/N-Bi MoO in the photocatalyst. The
MoO microspheres was
2
MoO
6
photocatalyst
2
PtCl
6
(850 μL, 3.115 M) and the as-pre-
2
6
(0.25 g) was added to a round-bottom flask con-
2 6 3 4
Pd-mediated Z-scheme Bi MoO /g-C N Photocatalysts that could
promote interfacial charge transfer and had high photocatalytic ac-
4
tivity. Li [51] and his colleagues found that Pt-loaded catalysts was
-
beneficial to generate more •O
2
and •OH radicals, which effectively
2
6
degraded special organic substances(4-chlorophenol) that are diffi-
cult to be mineralized. Li et al. [33]. reported that the prepared
2
6
Pt/γ-Bi
2
MoO
6
composite greatly improved the alkylation yield of
2
6
benzyl alcohol and acetophenone under visible light irradiation.
Generally, Pt-based composite semiconductors are acknowledged as
one of the most efficacious photodegradation catalysts for noble
metal supported semiconductors, this is attributed to the fact that Pt
particles can inhibit the recombination of electron-hole pairs by
forming electron traps.
2
6
2
6
possible formation process of Pt/N-Bi
shown in Scheme 1.
2
6
2.2. Photocatalytic tests
At present, nitrogen doping has been used as another effective
method to improve the visible light activity of catalyst, and excellent
results have been obtained in visible light oxidation degradation of
organic pollutants [54]. Nevertheless, a single dopant would form
relatively onefold defect level between the forbidden bands of the
catalyst, which is not suitable for the capture site of photogenerated
charge to produce the optimal charge transfer [4]. Hence, the re-
searchers make full use of the synergistic effect between metals and
non-metals(C, N, B, etc.) [55–61] to weaken the recombination rate
of photogenerated charges and enhance the absorption efficiency of
visible light, such as Cr-C, Mo-C, Cu-N, V-N, Fe-N, N-Zr, Co-B, B-Bi,
B-Fe, etc. [62–66]. For example, Yang et al. [23]. synthesized the
The visible light degradation experiment of Orange II was carried
out at Phchem III photoreactor (Beijing NBET Technology Co., Ltd.)
equipped with 500 W long-arc xenon lamp (XE500) at room tem-
perature. Firstly, 70 mg photocatalyst was suspended in orange II
solution(50 mg L−1,70 mL) by magnetic stirring, and reacted in dark
for 2 h to make the reaction system reach the equilibrium of ad-
sorption and desorption. Then,
the solution was exposed to simulated sunlight (without ultra-
violet cut-off filter) and visible light (with ultraviolet cut-off filter),
respectively. The reaction concentration was measured with an
Ultraviolet–visible (UV-Vis) spectrophotometer (Cary 5000, Agilent
Technologies Co., Ltd.).
3
Pt/N-rGO/N-NaNbO photocatalysts, which could effectively im-
prove the electron transfer performance and significantly reduce the
electron-hole pair binding rate. Wu et al. [67]. fabricated Pt/C@
Detailed information of characterization techniques and photo-
electrochemical tests were presented in the Supporting information.
2 6
Bi MoO composites has excellent efficiency and stability for appli-
cation in degradation of organic pollutants. However, to the best of
our knowledge, there are few studies on the synthesis of Pt-sup-
3. Results and discussion
ported and N-doped Bi
2
MoO
6
composite catalysts with highly effi-
3.1. Characterization of structure and morphology
cient adsorption and photodegradation activities.
Herein, a unique nanostructure with Pt uniformly dispersed on
X-ray diffraction (XRD) patterns of different Pt contents were
used for phase analysis of samples (Fig. 1). There were obvious dif-
fraction peaks at 2θ = 28.23°, 32.59°, 46.66°,55.53° and 58.39°, which
corresponded to (131), (200), (202), (331) and (262) crystal faces of
the surface of nitrogen doped Bi
by simple two-step solvothermal method and NaBH
method. In this work, the adsorption properties and photocatalysis
mechanism of Pt/N-Bi MoO nanocomposites are further evaluated
through the degradation of azo dye orange Ⅱ. Remarkably, the ex-
cellent degradation ability of 0.5% Pt/N-Bi MoO is obtained under
visible light irradiation compared with the original Bi MoO
2
MoO
6
is designed and synthesized
4
reduction
2
6
molybdenite Bi
no extra impurity peak was found in the spectrum, demonstrating
that Pt/N-Bi MoO was successfully synthesized by this method. The
position of the main characteristic peaks did not shift after mod-
ifying N and Pt, indicating that the crystal structure of Bi MoO was
almost unchanged. Notably, the intensity of characteristic diffraction
peaks of samples contained Pt were weaker than that of Bi MoO
and N-Bi MoO , which could be concluded that the combination of
Pt and N-Bi MoO reduced the diffraction efficiency of Bi MoO [26].
2 6
MoO (JCPDS No. 84–0787), respectively. Moreover,
2
6
2
6
2
6
.
2
6
2
. Experimental
2
6
2
.1. Synthesis of photocatalysts
2
6
2
6
2
6
All chemical reagents used in the experiment were analytical
grade and had not been purified. High purity water was provided by
water purification microporous system.
In addition, the diffraction patterns of the samples with different
amounts of Pt doping were generally consistent with that of bismuth
molybdate. There was no obvious diffraction peak of Pt, which might
be due to the small amount of loading and the failure to reach the
minimum detection limit of XRD.
the morphology and ultrastructure of the catalysts were obtained
by using Field emission scanning electron microscope (FE-SEM) and
transmission electron microscope (TEM). As showed in Fig. 2a, it
2
.1.1. Synthesis of N-Bi
The preparation of Bi
30], and the detailed steps were shown in the Supporting
information. Primarily, 1.5 g of the above prepared yellow green
Bi MoO was took and placed in a 250 mL round bottom flask,
2
MoO
6
composite
2
MoO
6
was described in the prior reports
[
2
6
2 6
could be seen that Bi MoO were flower-like sphere structure with a
2

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
Scheme 1. The possible formation process of Pt/N-Bi
2
MoO
6
flower like microspheres. a-c: hydrothermal synthesis of Bi (NO
3 3 2 2 4 2 2 6
) ·5H O and Na Mo ·2H O to form Bi MoO ;
d: preparation of Pt/N-Bi MoO photocatalyst by hydrothermal synthesis and NaBH reduction.
2 6 4
0
.5%Pt/N-Bi
tributed, which fully indicated that Pt NPs had been successfully
loaded on N-doped Bi MoO and dispersed uniformly. The elemental
2 6
MoO photocatalyst and all elements were evenly dis-
2
6
line scan of Pt nanoparticles (inset in Fig. 3g) also further confirmed
the above result. Meanwhile, the High-resolution TEM (HRTEM)
analysis (Fig. 3i) showed that the lattice fringes of 0.315 nm and
0
.225 nm were observed, which corresponded to the (131) plane of
Bi MoO and the (111) plane of Pt NPs, respectively. Simultaneously,
in the HRTEM images of Bi MoO (Fig. S1c) and N-Bi MoO (Fig. S1f),
the (131) plane lattice fringes of Bi MoO could also be clearly seen.
Based on the above conclusions, the possible formation process of
Pt/N-Bi MoO microspheres was shown in Scheme 1. In the initial
stage of the reaction, the generated Bi MoO nanoparticles were
2
6
2
6
2
6
2
6
2
6
2
6
aggregated into a spherical shape in order to reduce the surface
energy under high temperature and pressure. In addition, the small
2+
2-
crystallites gradually dissolved and generated Bi
O
2 2
and MoO
4
ions due to the difference in concentration between the bulk phase
and the particle surface [30]. With the increase of solvothermal time,
2+
2-
many Bi
O
2 2
and MoO
4
ions moved to the surface of spherical
Fig. 1. XRD patterns of different photocatalysts.
solid to nucleate and perform the Ostwald ripening, nanocrystals
were grown on the surface to form nano flakes. Finally, the flower-
diameter of about 1–2 µm assembled and stacked from many
random nanosheets. And the general morphology of Bi MoO had
not changed significantly after doping N, which could be seen clearly
in the Fig. 2b. However, the morphology of the composite after the
introduction of Pt was slightly different from before, it seemed to be
agglomerated, which might be caused by the strong reducibility of
like Bi
secondary hydrothermal process with ammonia as the solvent, N
atoms entered the Bi MoO lattice to replace part of the O atoms and
formed N-Bi MoO . Subsequently, Pt NPs was loaded on N-Bi MoO
through NaBH reduction method, and finally Pt/N-Bi MoO was
2 6
MoO microspheres were formed (Scheme 1a–c) [68]. In the
2
6
2
6
2
6
2
6
4
2
6
formed (Scheme 1d).
NaBH
Fig. 2d. The energy dispersive X-ray spectroscopy (EDX) of the 1%
Pt/N-Bi MoO catalyst associated with SEM (Fig. 2e) showed that
N, O, Mo, Pt and Bi elements had presented in the composite and the
mass fraction of nitrogen atom in 1% Pt/N-Bi MoO using NH ·H O as
the nitrogen source was approximately 0.94%.
4
during the preparation of catalyst, As shown in Fig. 2c and
3.2. Surface structure analysis
2
6
According to the isothermal curve of nitrogen adsorption-deso-
rption and the Barrett–Joyner–Halenda (BJH) pore size distribution
plots (Fig. S2), it could be seen that the three photocatalysts had
similar classical IV adsorption desorption isotherms. The hysteresis
2
6
3
2
In addition, the element mapping was shown in Fig. 3. It could be
seen that Mo, Bi, O, N and Pt elements were obviously present in the
loop at the relative pressure (P/P ) of 0.8–1.0 belonged to the H3
0
3

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
2 6 2 6 2 6 2 6 2 6
Fig. 2. The corresponding FESEM images of Bi MoO (a), N-Bi MoO (b), 2%Pt/Bi MoO (c) and 0.5%Pt/N- Bi MoO (d). The FESEM and EDX images of 1%Pt/N-Bi MoO (e).
category, which were generally manifested as the slit hole of the
sample stacked by the sheet sample. The inflection points between
was that the doping of N element and the loading of Pt increased the
density of the particles, reduced the surface area, and might also
cause pore plugging. The pore size distribution curve of BJH showed
that the sample was mainly composed of mesopores, and the
average pore size distribution was about 30 nm. After the in-
troduction of N element and noble metal Pt, the pore size distribu-
tion range was wider than that of pure Bi MoO samples. The
2 6
possible reason was that the catalyst was micro-flower-like structure
composed of disordered stacking and assembly of nano chips, many
P/P
and precious metal Pt, which indicated that the original pore part of
Bi MoO was filled. In addition, as could be seen from the data in
Table S1, the specific surface area of pure Bi MoO decreased from
3.61 m g to 25.55 m g after the introduction of N element, and
0
= 0.7–0.9 became smaller after the introduction of N element
2
6
2
6
2
-1
2
-1
3
2
-1
it continued to decrease to 24.00 m g after loading Pt nano-
particles. The main reason for the decreased in specific surface area
4

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
Fig. 3. The mapping images (b), (c), (d), (e), (f) correspond to Mo, Bi, O, N, and Pt in the (a) region of 0.5%Pt/N-Bi
2
MoO
6
catalyst, respectively. The elemental line-scan of Pt
2 6
nanoparticles (g), TEM (h) and HRTEM (i) images of 0.5%Pt/N-Bi MoO .
pseudopores of different sizes were formed on the surface of the
catalyst after doping and loading, which contributed to the pore size
distribution data. The enhanced photocatalytic activity of 0.5% Pt/N-
vibration of Bi-O bond. Meanwhile, the infrared spectrum of the 0.5%
Pt/N-Bi MoO composite was consistent with previous reports. The
peak at 1621.1 cm could be attributed to H-O-H bending vibration
of adsorbed water on the surface of 0.5%Pt/N-Bi MoO [54]. The IR
peaks at 1560.7 cm-1 was ascribed to the N-H bending vibration [69].
2
6
-1
Bi
2
MoO
6
might be attributed to the increased number of active sites
2
6
on the catalyst and the synergistic effect between N-Bi
Pt NPs.
2 6
MoO and
-1
Besides, the wide peak near 3418.7 cm could be assigned to N-H
and O-H stretching vibrations [70,71]. The spectra of N-Bi MoO was
approximately the same as 0.5%Pt/N-Bi MoO In order to
further determined the element composition and valence state of
0.5%Pt/N-Bi MoO , the samples were analyzed by X-ray photoelec-
tron spectroscopy (XPS). Fig. 4b–f showed the XPS spectrum of 0.5%
Pt/N-Bi MoO , including the full spectrum and the high-resolution
Fourier Transform Infrared spectroscopy (FT-IR) was an effective
method to characterize the vibrational structure and functional
groups of compounds. Fig. 4a presented that the changed of the
vibration frequency of different functional groups in the
2
6
2
6
.
2
6
-
1
4
00–4000 cm wave number range. It could be concluded that the
characteristic peak of bismuth molybdate was in the range of
2
6
-
1
-1
-1
4
00–900 cm , among which 727.2 cm and 842.5 cm ascribed to
the asymmetric and symmetric stretching vibration of oxygen atom
at the top of MoO octahedron, respectively. And the absorption
spectrum of each element. There were six signals of Mo, Bi, N, O, C
and Pt in the general spectrum (Fig. 4b), among which C 1 s was used
for calibration as the element carbon on the instrument. The high-
resolution Mo 3d spectra was shown in Fig. 4c, it could be seen that
6
-
1
-1
peaks of 443.3 cm and 567.6 cm belonged to the stretching
5

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
2 6 2 6 2 6 2 6
Fig. 4. (a) FTIR spectra of Bi MoO , N-Bi MoO and 0.5%Pt/N-Bi MoO composite. (b) Overall XPS spectrum of 0.5%Pt/N-Bi MoO , and high–resolution XPS spectra of 0.5%Pt/
N-Bi MoO composite with Mo 3d (c), Bi 4 f (d), N 1 s (e) and Pt 4 f, (f).
2
6
there were four peaks, the binding energies of 232.07 eV and
35.12 eV corresponded to Mo 3d5/2 and Mo 3d3/2, respectively, in-
the O–Bi–N–Mo–O bond [73,74]. And the binding energy of
402.81 eV could be assigned to the adsorption of nitrogen species on
2
6
+
dicating that Mo existed in a single chemical state of Mo , two
small peaks centered on 233.35 eV and 236.16 eV could be attributed
to Mo-O bond. The high-resolution Bi 4 f spectrum (Fig. 4d) showed
two main peaks with binding energy of 158.77 eV and 164.08 eV,
which corresponded to Bi 4 f7/2 and Bi 4 f5/2, respectively. Two small
peaks centered on 158.98 eV and 164.55 eV could be assigned to the
Bi-O metal bond [70]. The high-resolution spectrum of N 1 s (Fig. 4e)
was divided into two peaks. The binding energy 398.59 eV was
higher than Mo-N (the binding energy is 394.2 eV [72]) and Bi-N (the
binding energy is 397.4 eV [73]),which was contributed to the sub-
2 6
0.5%Pt/N-Bi MoO photocatalyst [28,75]. As shown in Fig. 4f, the
0
peaks centered at 74.22 eV and 70.72 eV were assigned to Pt 4 f5/2
0
and Pt 4 f7/2, respectively. While the peaks at 76.80 eV and 71.82 eV
2
+
2+
corresponded to the binding energies of Pt 4 f5/2 and Pt 4 f7/2
respectively. Simultaneously, XPS analysis showed that Pt existed in
two valence states on the surface of 0.5%Pt/ N-Bi MoO , and the
proportions of Pt0 and Pt were 80.48% and 19.52%, respectively,
,
2
6
2+
0
2+
which indicates that a part of Pt was oxidized to Pt . The conclu-
sions of high-resolution spectrum were consistent with FTIR and
XRD, which could further confirm the accuracy of the preparation of
the target catalyst [26].
2 6
stitution of ordinary O atom in Bi MoO lattice by N atom to form
6

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
Fig. 5. (a) Photocatalytic degradation curves of different catalysts under visible light irradiation. (b) Kinetic curves of Orange II degradation by different catalysts under visible
light irradiation.
3.3. Photocatalytic performance test
presence of ultraviolet light could promote the degradation of Or-
ange II by 0.5% Pt/N-Bi MoO photocatalyst.
2
6
In order to investigate the photocatalytic efficiency of the semi-
Fig. 6b showed the photocatalytic degradation kinetics curves of
Orange II with different catalyst concentrations. It could be seen
from the figure that when the concentration of the catalyst was
increased from 0.6 g L-1 to 1 g L , the degradation efficiency of Or-
ange II was greatly improved. With the catalyst concentration in-
creased again, although the degradation efficiency still maintained
an upward trend, the rate of increase slowed down distinctly. The
effect of catalyst concentration on the degradation efficiency of or-
ange II was mainly ascribe to the change of the number of active
sites in the system. Within a certain range, the quantity of active
sites in the system increased, and the efficiency of the degradation
reaction was also improved. Nevertheless, when the concentration of
the catalyst was too high, its density was large, and it was difficult
for light to fully penetrate the suspension, which made the utiliza-
tion rate of the catalyst decline, so the ascent rate became slower.
conductors, the photodegradation of Orange II was performed. It
could be obtained from the absorption wavelength scanning curve of
Orange II (Fig. S3a) that the maximum absorption wavelength of
Orange II was 484 nm. Thus, the absorbance of different concentra-
tions was measured at 484 nm. Fig. 5a showed the degradation curve
of Orange II by photocatalysis with different catalysts. The photolysis
did not occur under visible light irradiation in the absence of a
catalyst. The degradation rate was relatively flat in the absence of
light, showing that the catalyst could be activated by visible-light.
-1
And the original Bi
photocatalytic degradation activity. The adsorption capacity of
Bi MoO had been significantly improved when N was doped, but
there was little difference in photocatalytic effect. However, all Pt/N-
Bi MoO samples showed excellent adsorption and photocatalytic
activity. According to the change trend of Pt/N-Bi MoO degradation
2 6
MoO had very low adsorption capacity and
2
6
2
6
2
6
2 6
The degradation capacity of 0.5% Pt/ N-Bi MoO photocatalyst
curve, it could be easily found that the difference of photocatalysis
was not obvious with the increased of Pt loading amount.
The Langmuir-Hinshelwood (L-H) model (Formula S1.2) was used
to explore the reaction kinetics of Orange II degradation. Fig. 5b
showed the fitting curve of the reaction kinetics under different
was investigated from the side by exploring the effect of initial
concentration on the degradation of Orange II (Fig. 6c). With the
increase of the initial concentration of Orange II, the time for pho-
todegradation was prolonged when the amount of catalyst added
was constant. Obviously, the catalyst could completely degrade or-
-1
conditions. The linear relationship between ln(C
(
0
/C
t) was obtained, which was in line with the first-order kinetics
Formula S1.3). It could be seen that the kinetic rate constant of 0.5%
t
) and light time
ange II at concentrations below 30 mg L . When the concentration
-1
of solution was 50 mg L , the reaction time was extended to 5 h,
and the decolorization rate was up to 90%. Therefore, for maximum
degradation, orange II solution with an initial concentration of
(
-1
2 6
Pt/N-Bi MoO was relatively large (0.00778 min ), and the increase
of the rate constant was small (0.00802 min ) with the increase of
the loading amount of Pt NPs. Simultaneously, the 0.5%Pt/N-Bi MoO
-1
-1
50 mg L was used in the later experiments.
2
6
The initial pH values of orange II solution were adjusted to 3, 5,
6.3 and 8 with 0.5 mol L-1 HCl and NaOH solutions, where pH = 6.3
was the reaction solution without adjustment for photodegradation
experiments as shown in Fig. 6d. It could be seen that the initial pH
of Orange II had a great influence on the degradation reaction. When
pH = 3, the photodegradation rate was slow and the photodegrada-
tion rate enhanced with the increased of pH value. When pH = 6.3,
the degradation rate reached the maximum and the photocatalytic
effect was the best. Continue to increase the pH value, the photo-
catalytic rate decreased instead. It was generally believed that
changing the initial pH of the solution would change the interface
charge properties of the solution accordingly, thus affecting the
dispersion process of the nanoparticles in the solution and the ad-
sorption behavior of the Orange II molecules on the catalyst surface.
Moreover, when the initial pH value of the solution changed, the
redox reaction process on the surface of photocatalyst was different.
Therefore, the effect of changing the initial pH value of the solution
photocatalyst showed outstanding degradation ability (up to 90%) in
the experimental process, which was about 4, 1.8 and 1.3 times
higher than that of pure Bi
respectively. Therefore, considering the experimental factors and
reaction cost, 0.5% Pt/N-Bi MoO was selected as the optimal cata-
lyst for later correlation research.
2 6 2 6 2 6
MoO , N-Bi MoO and 2%Pt/Bi MoO ,
2
6
3.3.1. Effects of different factors on the photodegradation of Orange II
The effects of light source, catalyst concentration, initial con-
centration of Orange II and initial pH on photocatalytic degradation
of Orange II were investigated.
As was shown in Fig. 6a, the decolorization rate of 0.5% Pt/N-
Bi
2 6
MoO under visible light and simulated sunlight was 90.1% and
9
2.4%, respectively (Table S2). Apparently, the degradation ability of
Orange II was better under simulated sunlight, and the degradation
time was 60 min shorter than that under visible light. Clearly, the
7

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
Fig. 6. The effects of different experimental conditions on the degradation efficiency of Orange II: the properties of light source (a) (70 mL 50 mg L-1 Orange II solution, catalyst
dosage = 1 g L-1, reaction time = 240 min, pH = 6.3), catalyst concentration (b) (70 mL 50 mg L-1 Orange II solution, reaction time = 240 min, pH = 6.3), initial concentration of
Orange II (c) (70 mL Orange II solution, catalyst dosage = 70 mg, reaction time = 240 min, pH = 6.3), initial pH of solution (d) (70 mL 50 mg L-1 Orange II solution, catalyst dosage
=
1 g L-1, reaction time = 240 min).
on the photodegradation of orange II dye was the result of the
comprehensive action of the above factors.
adsorption behavior of orange II dye followed the pseudo-second-
order kinetics model, and there was no new characteristic peak in
the infrared spectrum before and after the reaction (Fig. 10b). It
could be concluded that the adsorption rate was controlled by
physical adsorption.
3.4. Adsorption experiment
3.4.1. Adsorption kinetic model
The sorbent adsorption rate and adsorption behavior were re-
vealed by exploring the kinetics of the adsorption process. Fig. 7a
showed that the curve of orange II adsorption amount with time, the
changed curve could be divided into three stages. In the first stage,
the adsorption capacity increased sharply, which was due to the
concentration difference effect between the bulk phase and the
catalyst surface, which made the orange II molecules diffuse rapidly
from the solution to the catalyst surface; the growth rate of ad-
sorption amount in the second stage was relatively slow, the orange
II molecules were transferred from the surface of the catalyst to the
interior through the pore size, and captured by the active sites of the
catalyst through the physical and chemical properties between
them; And the adsorption amount in the third stage tended to be
stable, which was the dynamic equilibrium of adsorption-deso-
rption. The experimental data were fitted by pseudo-first-order ki-
netic model, pseudo-second-order kinetic model and Weber and
Morris(W-M) kinetic model in order to further explore the adsorp-
tion behavior of catalysts [76]. It could be seen from Fig. 7 that the
pseudo-first-order model and W-M model fitted by experimental
data were a nonlinear curve, indicating that adsorption was a pie-
cewise process. The correlation coefficient of pseudo-second-order
kinetics was 0.98, which was higher than that of pseudo-first-order
3.4.2. Adsorption isotherms
The adsorption isotherm curve of 0.5% Pt/N-Bi
at room temperature was shown in Fig. 8. When the initial con-
2 6
MoO for orange II
-1
centration range was within 0–40 mg L , the contact probability
between the catalyst and orange II enhanced with the improvement
of the initial concentration and the adsorption capacity of orange II
increased. Different isotherm models were used to investigate fur-
ther the adsorption capacity of the best performing composite
(Langmuir - Fig. 9a, Freundlich - b, Dubinin-Radushkevich (D-R)
- Fig. 9c and Temkin - d) [77]. Freundlich model was an empirical
model, which could be used for surface adsorption and multi-layer
adsorption. As shown in Table S3, the correlation coefficient fitted by
Freundlich model was higher than Langmuir model, D-R model and
Temkin model, which showed that Freundlich model was more
2 6
suitable to describe the adsorption process of 0.5% Pt/N-Bi MoO
catalyst for orange II and the adsorption of orange II on composite
was consistent with the multi molecular layer adsorption model
[78]. It was generally believed that the value of 1/n in Freundlich
model was usually between 0 and 1, and its value indicated the
strength of the influence of concentration on the adsorption capa-
city. The fitting results showed that 1/n = 0.536, which illustrated
that the catalyst was easy to adsorb the orange II molecules, and the
(
0.95) and W-M model (0.94) (Table S2), indicating that the
8

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
Fig. 7. Adsorption kinetic model. (a) Adsorption capacity of dark reaction varies with time; (b) pseudo-first-order kinetic model; (c) pseudo-second-order kinetic model;
(
d) W-M kinetic model.
activity curve of the sample in five cycles. Although the decolor-
ization rate decreased slightly with the increase of the number of
cycles, the catalyst had an excellent decolorization rate (81.7%) after
5
times of repeated use. The slight reduction in the decolorization
rate of the catalysts might be attributed to the fact that some of the
incompletely degraded Orange II molecules and intermediate pro-
ducts were not effectively separated during repeated use and re-
mained on the surface of the composite photocatalyst, occupying
some active sites of the photocatalyst. It could be seen from FT-IR
spectrum before and after catalyst recycling in Fig. 10b that the
positions of the main characteristic absorption peaks did not move
and no new characteristic peaks were found. Combined with SEM
micrograph in Fig. S4, the morphological characteristics of the cat-
alyst did not change before and after the recovery reaction.
Consequently, the conclusion could be concluded that the 0.5%
2 6
Pt/N-Bi MoO photocatalyst had excellent catalytic ability and good
stability after 5 cycles.
Fig. 8. The change curve of adsorption capacity under different concentrations.
3
.6. Energy band structure analysis
initial concentration of the orange II molecules had a weak influence
on the adsorption capacity.
The optical properties of the catalysts played a crucial role in the
photodegradation activity, UV-Vis analysis of the photocatalyst was
carried out. As shown in Fig. 11a, there were obvious absorption
peaks in the UV-Vis region of 400–550 nm, but the absorption de-
3.5. The stability of photocatalyst
The stability of the catalyst was one of the most critical links to
gree was different. The absorption of 0.5% Pt/ N-Bi
sample was much stronger than that of single-component Bi
2
MoO
6
composite
MoO
measure the performance of the catalyst. The photocatalytic stability
of 0.5% Pt/N-Bi MoO sample was explored under the irradiation of
2
6
,
2
6
which was caused by the doping of nonmetallic element N and the
surface plasmon resonance effect of Pt NPs, thus enhancing the ab-
sorption of visible light and improving the utilization efficiency.
Fig. 11b showed the curve of (F(R)*E)1 changing with E value,
visible light. The suspension after each reaction was separated by
centrifugation to recover the photocatalyst, then dried at 80 ℃ and
reserved for the next cycle of experiment. Fig. 10a showed the
/2
9

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
Fig. 9. (a) Langmuir isotherm model; (b) Freundlich isotherm model; (c) D-R isotherm model; (d) Temkin model.
2 6 2 6
Fig. 10. (a) The cycling performance of 0.5%Pt/N-Bi MoO composite in photocatalytic degradation of Orange II. (b) Infrared spectrum of 0.5%Pt/N-Bi MoO composite material
before and after cycling.
calculated and drawn by Kubelka-Munk formula, from which the
band gap of the semiconductor could be inferred. The calculated
belong to n-type semiconductor. The intersection point of the tan-
gent line and the abscissa was the flat band potential of the sample.
band gaps of Bi
.45 eV and 2.25 eV, respectively. It could be seen from the figure
that when doped with non-metallic N, the band gap of Bi MoO
narrowed from 2.47 eV to 2.45 eV, and then decreased to 2.25 eV
after the surface was uniformly loaded with Pt NPs. The flat band
potential was studied by Mott Schottky diagram of the sample in
order to further estimate the edge potential of semiconductor con-
duction band. The tangent slopes of the linear part of the three
samples in Fig. 11c was positive, indicating that all the samples
2
MoO
6
, N-Bi
2
MoO
6
,0.5%Pt/N-Bi
2
MoO
6
were 2.47 eV,
2 6 2 6 2 6
The flat band potentials of Bi MoO , N-Bi MoO ,0.5%Pt/N-Bi MoO
2
were −0.56 eV, −0.52 eV and −0.45 eV respectively. Generally, com-
pared with the flat band potential, the conduction band edges of n-
type semiconductor were negative 0.1 eV, so the conduction band
edges were −0.66 eV, −0.62 eV and −0.55 eV respectively. According
to the band gap value measured by UV Vis diffuse reflectance and
based on the formula ECB = EVB - E , the price band edges could be
g
estimated to be 1.81 eV, 1.83 eV and 1.7 eV, respectively. The possible
band structure was shown in Fig. 11d.
2
6
10

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
Fig. 11. Optical properties of Bi
of possible electronic energy band structure.
2
MiO
6
, N-Bi
2
MoO
6
2
and 0.5%Pt/N-Bi MoO
6
: a) UV–Vis DRS curves, b) the (F(R)*E)1/2~hν plots, c) the Mott-Schottky plots, d) Schematic diagrams
3.7. Photodegradation mechanism
was consistent with the results of photoluminescence spectra and
electrochemical impedance spectra.
The separation efficiency of photogenerated electrons was closely
In the process of photodegradation of Orange II, different kinds of
free radicals would be produced. Free radical capture experiments
were designed to explore the active species of the reaction system.
related to the recombination of photoexcited carriers. Consequently,
Photoluminescence (PL) test, Electrochemical impedance spectro-
scopy (EIS) test and Transient photocurrent response(I-t) test were
performed to evaluate the separation and transfer efficiency of
photogenerated electrons. Fig. 12a was the emission spectra of
Bi MoO , N-Bi MoO and 0.5%Pt/N-Bi MoO samples measured at
2 6 2 6 2 6
the excitation wavelength of 260 nm. All the sample had obvious
emission peak at 467 nm and 562 nm. The emission peak intensity of
Benzoquinone (BQ, 0.5 mM), isopropanol (IPA, 1 mM) and sodium
-
oxalate (1 mM) were used to capture superoxide anion radicals(•O
2
),
+
hydroxyl radicals(•OH) and photogenerated holes(h ), respectively.
Fig. 12d presented the effect of different free radical scavengers on
the degradation of Orange II. Distinctly, IPA was added to the reac-
tion system, the reaction was nearly unaffected, and its degradation
efficiency was almost the same as that of the system without free
radical scavenger, indicating that the existence of hydroxyl radicals
had no effect on the degradation reaction. The degradation rate of
the photocatalyst was declined to the minimum value of about 15%
when sodium oxalate was added. Synchronously, the degradation
rate of the photocatalyst was about 55.4% when BQ was added,
which was far lower than the photocatalytic efficiency without
scavenger (about 88%). The above data showed that superoxide
0
.5%Pt/N-Bi
2
MoO
6
was significantly reduced compared with pure
MoO , indicating that the electrons and holes in
Bi MoO and N-Bi
2
6
2
6
the composite photocatalyst had a high separation efficiency.
The transfer rate of photogenerated electron holes in the catalyst
could be analyzed by EIS test. The Nyquist diagram of the catalyst
sample was shown in Fig. 12b. Currently, the arc radius was small,
indicating that the carrier migration resistance was low, and the
carrier recombination was difficult. It could be clearly seen from the
-
+
figure that the arc radius of the 0.5%Pt/N-Bi
ple's Nyquist curve was much smaller than that of pure Bi
N-Bi MoO , illustrating that the introduction of Pt NPs and N ele-
2
MoO
6
composite sam-
2
anion radicals (•O ) and photogenerated holes (h ) played a leading
2
MoO and
6
role in the photocatalytic degradation of Orange II.
According to the above characterization analysis and free radical
capture test, the reaction mechanism of enhancing the photo-
2
6
ments could significantly improve the electron migration ability and
reduce photogeneration reorganization of electrons and holes. In
addition, the photocurrent transfer efficiency of the catalyst was
investigated through the transient light response curve of the
sample (Fig. 12c) to further explore the separation of photogenerated
carriers. The results showed that the photocurrent density of 0.5%
degradation activity of 0.5% Pt/N-Bi
posed. For one thing, N doping replaced the original oxygen atoms in
the Bi MoO lattice with N atoms, which made the optical absorp-
2 6
MoO composites were pro-
2
6
tion edge red shifted (Fig. 11b). The shift of the optical absorption
edge indicated that the N 2p orbit might be formed at the top of the
Pt/N-Bi
N-Bi MoO
tively inhibit the recombination of photogenerated carriers, which
2
MoO
6
was much higher than that of pure Bi
2
MoO
6
and
2 6
0.5%Pt/N-Bi MoO valence band, so that more electrons were excited
to the conduction band of the composite photocatalyst. For another,
the electron transfer rate was effectively enhanced and the
2
6
, indicating that Pt-supported and N-doped could effec-
11

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
2 6 2 6 2 6
Fig. 12. (a) PL spectra, (b) transient photocurrent responses, (c) EIS Nyquist plots of Bi MoO N-Bi MoO and 0.5%Pt/N-Bi MoO composite. (d) Effects of different kinds of free
radical scavengers on the photodegradation of Orange II.
2 6
Scheme 2. The mechanism of degradation of Orange II by 0.5%Pt/N-Bi MoO composite under visible light.
recombination of photogenerated electron hole pairs was sig-
nificantly inhibited through the synergistic effect of Pt NPs and
From the Mott Schottky test (Fig. 11c), it was calculated that the top
of the 0.5%Pt/N-Bi MoO valence band and the bottom of the con-
2
6
N-Bi
Pt/N-Bi
Scheme 2. The electrons on the valence band of N-Bi
2
MoO
6
.
Summarily, the degradation mechanism of 0.5%
composite under visible light could be described as
MoO semi-
duction band were 1.7 eV and −0.55 eV, respectively, while the Fermi
energy level of Pt NPs was 0.8 eV, and the work function was 5.36 eV
(relative to the vacuum level). This means that the electrons could
2
MoO
6
2
6
conductor were excited by light to migrate to the conduction band
and generate holes on the valence band at the same time (Eq. 1).
easily flow from the N-Bi
with low Fermi level due to the deposition of noble metals on the
2 6
MoO with high Fermi level to the Pt NPs
12

Q. Jia, P.K. Nguyen, Z. Gu et al.
Journal of Alloys and Compounds 863 (2021) 158336
N-Bi
2
MoO
6
surface (Eq. 2). In addition, under the excitation of light,
Declaration of Competing Interest
the electron transfer rate at the interface was accelerated through
the SPR effect of Pt NPs, which promoted the separation of photo
generated electron-hole pairs, and obtained a higher redox rate (Eq.
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
-
3
). Because the redox potential (−0.33 eV) of O
sitive than that of 0.5%Pt/N-Bi MoO
1.7 eV) of 0.5%Pt/N-Bi MoO was more negative than that of OH /
OH (2.29 eV), accordingly, O
generated charge on 0.5%Pt/N-Bi
not be oxidized to •OH by the photogenerated hole [79]. The results
2
/•O
2
was more po-
2
6
(−0.55 eV), and the VB position
-
(
•
2
6
Acknowledgements
-
2
could be reduced to •O
2
by photo-
-
2
MoO (Eq. 4), and H O or OH could
6
2
Thank you very much for the support of the analysis and test
center of Lanzhou University.
-
2
showed that more superoxide anion radicals (•O ) and photo-
+
-
+
generated holes (h ) could be generated, the •O
Orange II to form CO , H O and some small molecular products (such
as 1, 2-naphthol, formic acid, maleic acid or succinic acid, etc.) [3]
Eq. 5). According to the results of capture experiments, it was also
confirmed that the main active species of the system in the photo-
2
and h reacted with
Appendix A. Supporting information
2
2
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.jallcom.2020.158336.
(
-
and h+ radicals, which had no re-
degradation process were •O
2
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