Fabrication of La-Doped Bi2O3 Nanoparticles with Oxygen Vacancies for Improving Photocatalytic Activity

Article abstract of DOI:10.1007/s10562-019-02970-w

Abstract: La-doped Bi₂O₃ were prepared via a simple sol–gel method following a facile low temperature calcination process, and citric acid as an organic complexing agent. La³⁺ doping could not only create oxygen vacanices (OVs) on the surface of Bi₂O₃, but also as the efficient scavenger to capture photogenerated electrons. The novel x% La–Bi₂O₃ photocatalysts exhibited oxidation capacities for degrading azo dye methyl orange (MO) and colorless pollutant phenol, and 2% La–Bi₂O₃ sample showed the highest photocatalytic activity. The enhanced photocatalytic activity in MO and phenol photo-degradation were attributed to the strong synergistic effects of La³⁺-doping and oxygen vacancies (OVs), which La³⁺ ions capture, transfer, and release the photo-generated electrons for conversion from O₂ to ·O2- to prevent the hot e^? of CB to jump into the level of OVs, thereby increase separation efficiencies of photo-generated electrons and holes. The present study may present a new perspective for the utilization of rare earth ion doping to improve the performance of other photocatalytic materials. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02970-w

Catalysis Letters
https://doi.org/10.1007/s10562-019-02970-w
Fabrication of La‑Doped Bi2O3 Nanoparticles with Oxygen Vacancies
for Improving Photocatalytic Activity
Ting Li1 · Shanyu Quan1 · Xuefeng Shi1 · Linmei Yang1 · Cong Liu1
Received: 1 August 2019 / Accepted: 22 September 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
La-doped Bi2O3 were prepared via a simple sol–gel method following a facile low temperature calcination process, and citric
acid as an organic complexing agent. La3+ doping could not only create oxygen vacanices (OVs) on the surface of Bi2O3,
but also as the efcient scavenger to capture photogenerated electrons. The novel x% La–Bi2O3 photocatalysts exhibited
oxidation capacities for degrading azo dye methyl orange (MO) and colorless pollutant phenol, and 2% La–Bi2O3 sample
showed the highest photocatalytic activity. The enhanced photocatalytic activity in MO and phenol photo-degradation were
attributed to the strong synergistic efects of La3+-doping and oxygen vacancies (OVs), which La3+ ions capture, transfer,
−
⋅
−
and release the photo-generated electrons for conversion from O2 to O to prevent the hot e of CB to jump into the level
2
of OVs, thereby increase separation efciencies of photo-generated electrons and holes. The present study may present a
new perspective for the utilization of rare earth ion doping to improve the performance of other photocatalytic materials.
Graphic Abstract
Keywords Bi2O3 · La3+ doped · Oxygen vacancies · O2-TPD · Photocatalytic activity
1 Introduction
* Shanyu Quan
Bismuth oxide (Bi2O3), as the most industrially important
compound of bismuth with non-toxicity, chemical stability
to light illumination, and strong oxidizing ability of holes
in the valance band (VB) [1, 2], has been emerged as a
shyquan@sut.edu.cn
1
Department of Physics, School of Science, Shenyang
University of Technology, Shenyang 110870,
People’s Republic of China
Vol.:(0123456789)
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T. Li et al.
promising photocatalytic material in the decomposition of
various organic contaminants in waste water [3, 4]. How-
ever, Bi2O3 is now confronted with greater recombination
rate of the e−/h+ pairs, which decrease the degradation
efciency of organic pollutants [5]. To improve the pho-
tocatalytic performance of Bi2O3, the coupling with other
semiconductors to construct heterojunctions have been
extensively studied such as Bi2O3/ZnO [6], Bi2O3/TiO2
[7], Bi2O3/Bi2WO6 [8], Bi2O3/BiPO4 [9], Bi2O3/g-C3N4
[10], Bi2O3/NaBiO3 [11], Bi2O3/BiVO4 [12], and Bi2O3/
Bi3O4Br [13]. All of the results show that phototcatalytic
activity has been improved to the extent due to efcient
separation of photoinduced carriers. In addition, doping
is another strategy adopted to improve the photocatalytic
activity. Doing metal [14, 15] or non-metal elements [16,
17] have been proven to be efective method to extent
the absorption regime to visible. Recently, doping with
lanthanide ions are of technological interest due to par-
ticular electronic structures [18, 19]. Lanthanide ions
could capture photoelectrons as electron scavengers to
inhibit the recombination, and improve the separation of
the photogenerated carrier [20, 21]. Doping TiO2 with
La (La–TiO2) can enhance the visible light absorption
by adjusting the electronic structure, and exhibited an
excellent photocatalytic performance for acetone and NO
removal [22, 23]. An optimal amount of La3+ doping in
MgO can remarkably increase the photocatalytic degrada-
tion of fumequine antibiotic (FLQ), which is attributed
to the crystalline defect [24]. M. Bilal Tahir et al. fabri-
cate RM-doped WO3 (RM = La, Gd, Er) nanostructures,
which show excellent photocatalytic performance with 2%
Gd-doping WO3, which could be accredited to suppressed
recombination of charge-carriers [25]. In fact, doping is
also considered to be an efective approach to create oxy-
gen vacancies [26–28]. The introduction of oxygen vacan-
cies (OVs) has been extensively investigated to enhance
the photo-absorption and photocatalytic activity [29–33].
La-doped CeO2 has shown to improved the catalytic ef-
ciency for combustion of methane, and the substitution of
La enhance the oxygen vacancy concentration and promote
the migration of the surface oxygen, leading to improve
the catalytic activity of CeO2 [34].
2 Experimental
2.1 Chemicals and Catalysts Synthesis
Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), nitric
acid, and citric acid were purchased from Guangdong
xilong Chemical Co., Ltd., P.R. China. Lanthanum nitrate
hexahydrate (La(NO3)3·6H2O) was purchased from Tianjin
Guangfu Fine Chemical Research Institute. The organic
pollutants were obtained from Beijing Chemical Reagents
Company. All reagents used are of analytically grade
without further purifcation. Deionized water was used
throughout the whole research process.
La-doped Bi2O3 catalysts were synthesized via a sim-
ple sol–gel-calcination method (a facile low temperature
calcination method) with citric acid as organic com-
plexing agent. In the typical procedure, frstly 5 mmol
of Bi(NO3)3·5H2O was dissolved in 10 mL of 1.5 M
dilute nitric acid, while the expected stoichiometric
La(NO3)3·6H2O was dissolved 20 mL deionized water;
Secondly La(NO3)3 solution was added into the Bi(NO3)3
solution under stirring conditions. Next, 10 mL of citric
acid (6 M) was slowly dropped into the above solution
with vigorous magnetic stirring for 40 min and continu-
ously heated to 70 °C until it became gel. The resulting
gel was dried at 80 °C for 12 h, and then ground into
powders. Finally, the powder was calcined at 400 °C in a
fow of 5% H2/N2 atmosphere for 1 h with a heating rate
of 2 °C/min. In this way, we obtained a series of x% La-
doped Bi2O3 nanoparticles with diferent molar ratios of
La(NO3)3/Bi(NO3)3(x = 1,2 and 3), which were marked as
1% La–Bi2O3, 2% La–Bi2O3 and 3% La–Bi2O3, respec-
tively. For comparison, the pure Bi2O3 was prepared in the
same method without adding La(NO3)3.
2.2 Catalyst Characterization
X-ray difraction (XRD) pattern of pure Bi2O3 and La-
doped Bi2O3 catalyst were carried out on SuperNova X-ray
difraction system with Cu Kα radiation(λ = 0.15406 nm).
The surface morphology of the sample was characterized
with a FEI QUANTA 250 FEG scanning electron micro-
scope (SEM) attached with energy-dispersive X-ray spec-
troscopy (EDS). Transmission electron microscopy (TEM)
and high-resolution transmission electron microscopy
(HRTEM) analysis were conducted on Tecnai G2 F20 to
observe the microstructure and the inter planar spacings
of the catalysts. The chemical states of La-doped Bi2O3
nanoparticles were investigated using X-ray photoelectron
spectroscopy (XPS) studies (Thermo Scientifc X-ray pho-
toelectron spectrometer: K-ALPHA surface analysis) with
In this work, La doped Bi2O3 samples were prepared
by a simple sol–gel method, and oxygen vacancies are
introduced via calcining in inert gas. The content of oxy-
gen vacancies can be efectively changed by the amount
of La3+ used in the synthesis process. The photocatalytic
activities of as-prepared samples were evaluated by deg-
radation of MO/Phenol. La-doped Bi2O3 sample exhibited
excellent photocatalytic performance than pure Bi2O3, and
the possible photodegradation mechanisms for MO/phenol
removal by La-doped Bi2O3 with oxygen vacancies were
proposed in detail.
1 3

Fabrication of La-Doped Bi2O3 Nanoparticles with Oxygen Vacancies for Improving…
Al kα radiation. The BET specifc surface areas of the as-
prepared samples were recorded on nitrogen-adsorption
system (ASAP-2020) through the nitrogen adsorption–des-
orption isotherms at 77 K. The electron paramagnetic reso-
nance (EPR) spectra were carried out on a JES-FA200
ESR spectrometer at 100 K in liquid N2. Photolumines-
cence spectra were tested by fuorescence spectrometer
(HITACHI F-4600) at 25 °C. Photocurrent studies were
obtained by CHI660D electrochemical workstation. A
PCA-1200 chemisorption analyzer (Beijing BUILDER,
China) was used to perform the O2 temperature program-
ming desorption (O2-TPD) experiment.
CEL-HXF300D2) with a 420 nm cut-of flter. During the
degradation process, an aliquot of 2 mL was withdrawn at
fxed time intervals and centrifuged to monitor the pollut-
ant concentrations by a UV–Vis spectrophotometer (PER-
SEE, TU-1950) at their maximum absorption wavelengths.
To identify the active species in the photocatalytic process,
isopropanol (IPA, 10 mmol/L), disodium ethylenediamine-
tetraacetate (EDTA-2Na, 1 mmol/L), benzoquinone (BQ,
6 mmol/L), was added into the MO/phenol degradation sys-
tem as a quencher of hydroxyl radicals (·OH) [35], holes (h+)
⋅
−
[36], and superoxide radicals ( O2 ) [37] respectively.
2.3 Photocatalytic Activity Tests
3 Results and Discussion
The photocatalytic activities of the as-prepared samples
were evaluated by degrading methyl orange (MO, 10 mg/L)
and phenol (50 mg/L). To investigate methyl orang/phenol
degradation performance of the as-prepared samples, batch
experiments were carried out. Typically, 100 mg of photo-
catalysts were added into 100 mL of the methyl orange/phe-
nol solution and well dispersed under ultrasonic treatment
for 30 s. Then the mixture was stirred in darkness for 1 h
to reach the adsorption/desorption equilibrium. Finally the
mixture was irradiated by a 300 W Xe lamp (CEAULIGHT,
3.1 Morphology, Structure, and Physical Properties
The morphology of as-prepared pure Bi2O3 and La-doped
Bi2O3 samples were investigated with SEM, TEM, and
HRTEM analysis. Figure 1a displayed SEM image of
undoped Bi2O3 particles, which were irregular, and aggre-
gated to form a larger particle. The surface of undoped Bi2O3
particles were relatively smooth and fat. After La doping
(Fig. 1b–d), the particles were also irregular, and the particle
size was found to be gradually decreased with the increment
Fig. 1 SEM images of Bi2O3 (a), 1% La–Bi2O3 (b), 2% La–Bi2O3 (c) and 3% La–Bi2O3 (d)
1 3

T. Li et al.
of La content from 1 to 3%. The surface of La-doped Bi2O3
particles were relatively rough. The morphology change of
the La-doped samples could be ascribed to the substitution
of the smaller sized of La3+ ions for Bi3+ ions in Bi2O3. The
decrease with increasing La content could be attributed to.
The smaller particle size in most cases would give rise to a
larger surface area and higher adsorption capacity, which is
usually benefcial for photocatalytic activity [38].
(202) and (121) crystal planes of Bi2O3, respectively. The
lattice fringe of crystal phase corresponds to La species
is not observed, confrming that most of the La species in
La–Bi2O3 enter into Bi2O3 lattice to form Bi–La oxides solid
solution [34].
The chemical composition is tested by EDS attached with
SEM. Figure 2e exhibited the EDS spectrum of as-prepared
2% La–Bi2O3 sample, and the peaks of Bi, O and La ele-
ments are detected. The atomic percentage was about 34.53
and 64.50% of Bi and O in the EDS spectrum, while the La
concentration was ca. 1.57 wt%.
Figure 2 shows the TEM and high resolution TEM
(HRTEM) images of 2% La–Bi2O3 sample. As shown, 2%
La–Bi2O3 sample (Fig. 3a–c) were irregular particles, which
was in consistent with the SEM results. The HRTEM image
for the 2% La–Bi2O3 sample (Fig. 3d) identifes the crys-
tal lattice fringes with two diferent interplanar distances
of 2.68 nm and 3.24 nm, which could be assigned to the
X-ray powder difraction patterns of the prepared samples
are shown in Fig. 3a. Obviously, the XRD pattern of the pure
Bi2O3 exhibited the monoclinic α-Bi2O3 (JCPDS PDF-76-
◦
◦
◦
1730). The main difraction peaks at 2ꢀ = 27 , 27.5 , 31.1 ,
Fig. 2 TEM images of 2% La–
Bi2O3 (a–c), HRTEM 2% La–
Bi2O3 (d), and EDS spectrum of
2% La–Bi2O3 (e)
1 3

Fabrication of La-Doped Bi2O3 Nanoparticles with Oxygen Vacancies for Improving…
Fig. 3 XRD patterns (a) and FTIR spectra (b) of Bi2O3 powders with diferent La contents
◦
Table 1 Physical properties of the prepared samples
and 33.4 could be attributed to the lattice planes (112),
(121), (122), and (202), respectively. After low content of
La doping, the crystal structure of Bi2O3 did not change, and
no characteristic peaks of La2O3 and other phase of La were
observed, indicating the formation of Bi–La oxides solid
solution. However, the extra difraction peaks emerge after
La-doping increase to 3% molar fraction of La3+. The phe-
nomena would be attributed to the fact that the ionic radius
of La3+ (1.030 Å) was greatly close to the Bi3+ (1.062 Å)
[39], and La3+ was easily to replace Bi3+ and occupied the
regular lattice site in Bi2O3 crystal lattice.
Samples
Crystallite BET surface Total pore
Band gap
(eV)
size (nm)
area (m2/g)
volume
(cm3/g)
Bi2O3
39.5
2.54
8.65
12.6
5.68
0.0308
0.0987
0.1321
0.0538
2.853
2.855
2.849
2.838
1% La–Bi2O3 25.7
2% La–Bi2O3 22.8
3% La–Bi2O3 21.7
To identify the presence of various bonds in the samples,
the FTIR spectra of the as-prepared samples were carried
out shown in Fig. 3b. It could be seen that all as-prepared
samples had similar profles of FTIR spectrum. Two peaks
at 424 and 503 cm−1 were attributed to the vibrations of
the Bi–O bond of BiO6 octahedron, and the Bi–O–Bi bond
stretching peak was present at 845 cm−1 [40]. In addition, the
O–H stretching vibration of adsorbed water corresponded to
3425 cm−1, and the peak at 1380 cm−1 was the characteristic
peak of the nitrate group. However, no La–O bonds were
detected in La–Bi2O3 as a result of the relatively low content
of La3+, which is accordance with the XRD results.
that of Bi3+ (1.062 Å). Catalysts with smaller crystallites
were generally known to exhibit better catalytic properties
[42].
The BET surface areas and pore parameters of the pre-
pared samples were tested and analyzed by N2 adsorption/
desorption method. The isotherms of Bi2O3, 1% La–Bi2O3,
2% La–Bi2O3 and 3% La–Bi2O3 samples were presented in
Fig. 4. It could be concluded all the isotherms were identi-
fed as a combination of types IV. A distinct H3 hysteresis
loop was observed in the range of 0.8–1.0 P/P0, indicating
that these samples had a porous structure.
The detail information of BET surface areas and Pore
Volume of the prepared samples are listed in Table 1. The
1% La–Bi2O3 and 2% La–Bi2O3 composites had BET
surface areas of 12.6 m2/g and 8.65 m2/g, respectively,
which were higher than that of Bi2O3 (2.54 m2/g) and
3% La–Bi2O3 (5.68 m2/g). The total pore volumes of 1%
La–Bi2O3 and 2% La–Bi2O3 composites were 0.0987 cm3/g
and 0.0471 cm3/g, respectively, which were higher than that
of Bi2O3 (0.0308 cm3/g) and 3% La–Bi2O3 (0.0538 m2/g).
The higher BET surface areas and porous structure would
provide more active sites and transport paths for reactant
molecules and products, which was extremely useful in pho-
tocatalytic process.
The average crystallite size (D) was estimated according
to the following classical Scherrer equation [41]:
Kꢀ
D =
(1)
ꢁ cos ꢂ
where D is the average crystallite size, K is a shape fac-
tor (0.89), λ is the wavelength of the X-ray radiations (Cu
Kα = 0.15418 nm), β is the full width at half maximum
(FWHM) intensity, and θ is the Bragg angle. Table 1 shows
the calculated results. It can been seen La3+ doping could
decrease the crystallite size of the Bi2O3, consistent with the
fact that the ionic radius of La3+ (1.030 Å) was smaller than
1 3

T. Li et al.
Fig. 4 Nitrogen adsorption–desorption isotherms of Bi2O3 and La–Bi2O3
Fig. 5 a UV–Vis difuse refectance absorption spectra for Bi2O3 and La doped Bi2O3, b determination of the band gap energies for Bi2O3 and
La doped Bi2O3
2
The UV–Vis DRS spectrum was used to investigate
the optical adsorption properties of the prepared sam-
ples, and recorded in Fig. 5. It is observed that La doping
lead to an increase in absorption intensity in the region
of 250–450 nm and the absorption band edge a little bit
red shift. These changes might be ascribed to the oxygen
vacancy in the La–Bi2O3 introduces shallow-trap levels
in the band gap of the Bi2O3 which could trap the excited
electrons [43].
The plot of (ꢀhv) versus (hv) shown in Fig. 5b, the band
gaps were estimated to be 2.853, 2.855, 2.849 and 2.838 eV
for Bi2O3, 1% La–Bi2O3, 2% La–Bi2O3, and 3% La–Bi2O3,
respectively. The results indicated that the band gaps of 2%
La–Bi2O3 sample were slightly narrowed.
3.2 Chemical States
The band gap of the prepared photocatalysts were esti-
mated from refectance data using the following formula:
To elucidate the surface chemical environments of Bi, La
and O elements the prepared catalysts were characterized
by X-ray photoelectron spectroscopy (XPS). The O 1s
XPS spectra of prepared samples (shown in Fig. 6a) are
deconvoluted into three peaks located at 529.48 0.2 eV,
530.88 0.2 eV, 531.58 0.2 eV, which corresponds to lat-
tice oxygen (OL) [45], oxygen vacancies (OV) [46] and sur-
face chemisorbed oxygen (Oa) [47, 48], respectively. With
(
)
n∕2
ꢀhv = A hv − Eg
, where ꢀ, h, v, A and Eg represents
absorption coefcient, Planck constant, light frequency,
a constant, and band gap energy, respectively. And the n
depend on the transition in a semiconductor (n=1 for direct
transition and n=4 for indirect transition). The value of n
for Bi2O3 were reported to be 1 [44].
1 3

Fabrication of La-Doped Bi2O3 Nanoparticles with Oxygen Vacancies for Improving…
Fig. 6 High-resolution XPS spectra of as-prepared samples: a O 1s; b La 3d with peak ftting
the addition of La, the OV/O 1s molar ratio increases from
17.8 to 39.7%, and then decreases to 25.4%, indicating the
OV concentration on the catalysts 2% La–Bi2O3 surface is
higher than that of other catalysts. The oxygen vacancies on
the catalyst surface can serve as photoinduced electron traps
as well as adsorption sites where the electrons in oxygen
vacancy states can reduce the absorption of oxygen on the
For 1% La–Bi2O3 sample, two peaks located at 834.4 eV
and at 851.2 eV are ascribed to 3d5/2 and 3d3/2, respectively.
The rest of the two binding energies at the high binding
energy side of the 3d5/2 and 3d3/2 belong to the satellite peaks
of La3+. With the increase of La content, the intensity of
3d5/2 and 3d3/2 peaks increases and the peak gradually shifts
to higher binding energy, which may attribute to the changes
of the crystal structure or electronic properties due to La-
doing. These results demonstrate La is in the +3 oxidation
state in all La-doping samples, and La3+ ions were success-
fully doped into the surface lattice of Bi2O3 nanoparticles
[53]. These results were in good agreement with the standard
binding energy of La [54].
⋅
−
surface of the samples and produce O , which is beneft
to the enhancement of photocatalytic ac2tivity [49, 50]. The
results demonstrates the oxygen vacancies can be generated
via annealing treatment under inert gases atmosphere. Gen-
erally, oxygen vacancies (Vo) in metal oxides can be formed
by remove oxygen atoms in the lattice sites (Oo) to the gase-
ous state (O2) at a non-oxidizing condition [51, 52].
3.3 Photocatalytic Activity
−1
Oo ↔ Vo + 1∕2O2(gas) + 2e
(2)
In addition, the La 3d XPS spectrum of La-doping sam-
The photocatalytic activity of as-prepared samples was
evaluated by degradation of MO/phenol under visible light
ples presented in Fig. 6b.
Fig. 7 Photodegradation of a 10 mg/L of MO, b 50 mg/L of phenol over prepared samples
1 3

T. Li et al.
(λ≥420 nm) irradiation. Figure 7 presents the time-depend-
ent degradation profle of MO (Fig. 7a) and phenol (Fig. 7b)
over the as-prepared samples. For comparison, dark adsorp-
tion and the direct photolysis were also examined. The
adsorption–desorption equilibriums between photocatalysts
and MO/phenol molecules could establish in 60 min under
the dark with continuous stirring. La-doping enhances the
adsorption properties of the catalysts, which is ascribed to
the result of the slightly increased surface areas than of pure
Bi2O3, arising from the rough surface of La-doping Bi2O3.
In the direct photolysis process, the MO/phenol solution was
only undergone the photo irradiation without any photocata-
lysts. It was found that, the direct photolysis of MO and phe-
nol were almost negligible. Figure 7a depicts the variation of
MO concentration (Ct/C0) over as-prepared samples against
reaction time. It was obvious that the photocatalytic activ-
ity was signifcantly enhanced after the appropriate content
of La3+ doping. After 100 min irradiation, 89%, 98%, and
79% of MO dye solution was degraded by 3% La–Bi2O3, 2%
La–Bi2O3 and 1% La–Bi2O3 respectively, while only 27%
degradation rate by Bi2O3. Furthermore, the photocatalytic
performance for degradation of phenol over as-prepared
samples was also studied (Fig. 7b). The photodegradation
rates of phenol over Bi2O3, 1% La–Bi2O3, 2% La–Bi2O3 and
3% La–Bi2O3 were 31%, 63%, 73%, and 59%, respectively.
Additionally, the apparent reaction rate constant k is cal-
culated by the pseudo-frst-order rate equation: ln(C0/Ct)=k,
and is summarized in Table 2. Obviously, the apparent reac-
tion rate constant k of 2% La–Bi2O3 sample was larger than
that of the other samples. The results demonstrated that
appropriate content of La doping could greatly enhance the
photocatalytic activity for both MO and phenol degradation.
occurred in photocatalytic process [55]. Photoluminescence
spectra of as-prepared samples are carried out and the result
was provided in Fig. 8a. As can be seen, the PL spectra
intensity of 2% La–Bi2O3 is much lower than that of the
other prepared samples, indicating that 2% La–Bi2O3 has
a relatively low rate of recombination of photogenerated
electrons and holes. The recombination of e− and h+ on 2%
La–Bi2O3 should be suppressed by the oxygen vacancies,
which is helpful for the transfer and separation of photogen-
erated electron–hole pairs.
Additionally, transient photocurrent responses (Fig. 8b)
and electrochemical impedance spectroscopy (EIS) (Fig. 8c)
were performed to reveal the separation and transfer of pho-
togenerated electron–hole pairs during the photocatalytic
reaction. The higher photocurrent density means a longer
lifetime of the photogenerated charge carriers and a higher
photocatalytic activity [56].
As shown in Fig. 8b, all the prepared samples show a sta-
ble photocurrent response in intermittent 30 s switch-on and
-of cycles under irradiation of visible light, and La-doping
produce more photocurrent response. 2% La–Bi2O3 sam-
ple exhibited the strongest photocurrent intensity compared
to other samples, which was 2.8, 1.9, and 1.2 folders than
those of Bi2O3, 1% La–Bi2O3, and 3% La–Bi2O3, indicat-
ing that more photogenerated electrons and holes are ef-
ciently separated in 2% La–Bi2O3 sample. Simultaneously,
2% La–Bi2O3 has the lowest arc radius in the electrochemi-
cal impedance spectroscopy (EIS) Nyquist plot in Fig. 8c,
which means a lower resistance of electron transfer and an
efective separation of photo-generated electron–hole pairs
in 2% La–Bi2O3 sample [57–59]. These results declared
more oxygen vacancies could bring about a more efcient
separation efciency and lower recombination rate for pho-
togenerated electron–hole pairs, which were in accordance
with the change of photocatalytic activity.
3.4 Separation Efciency of Charge Carriers
In order to clarify the separation efciency and transfer rate
of photogenerated electron and hole, a series of experiments
for charge carrier separation are performed. Photolumines-
cence emission intensity was related to the recombination
rate of photogenerated electrons and holes after excitation
process. The lowest the emission intensity, the lower recom-
bination rate of photogenerated electrons and holes will be
3.5 Mechanism for Photo‑Degradation of MO/
Phenol
To clarify the mechanism of the photo-degradation of MO
and phenol over the as-prepared samples, trapping experi-
ments were carried out to capture the major active species
generated in photocatalytic process, such as superoxide radi-
·
+
⋅
−
cals ( O2 ), hydroxyl radicals( OH) and h [60]. Herein, iso-
propanol (IPA), benzoquinone (BQ) and ethylenediaminetet-
raacetic acid disodium salt (EDTA-2Na) were chosen as the
Table 2 Reaction rate constants (k) of MO and phenol
Sample
MO k/min−1 (×10−3
)
Phenol k/h−1
·
+
⋅
⋅
−
−
scavenger for OH, O2 and h ∕ O2 , respectively. As shown
in Fig. 9a, the addition of isopropanol (IPA) didn’t show
signifcant change on the degradation of MO, suggesting that
hydroxyl radicals (·OH) played a negligible role during the
photo-degradation process. However, an obvious decrease in
the photoactivity upon the addition of BQ indicated that the
Bi2O3
3.2
14.3
37.1
19.8
0.0872
0.203
0.288
0.223
1% La–Bi2O3
2% La–Bi2O3
3% La–Bi2O3
Reaction conditions: [catalyst]=100 mg; [MO]=10 mg/L; [phe-
nol]=50 mg/L
⋅
−
O2 had some contribution to the photo-degradation of MO.
1 3

Fabrication of La-Doped Bi2O3 Nanoparticles with Oxygen Vacancies for Improving…
Fig. 8 a PL spectra, b transient photocurrent responses, c electrochemical impedance spectroscopy of as-prepared samples
In addition, the degradation rate of MO further decreased
with the addition of EDTA-2Na. The above results could
demonstrate that the photo-generated h+ was the most criti-
O2 temperature programming desorption (O2-TPD)
experiment were performed with Bi2O3 and 2% La–Bi2O3
to investigate the O2 desorption behavior on the surface of
photocatalysts, which is shown in Fig. 9d. As known, the
low temperature peak below 700 °C is referred to as the
α-oxygen desorption and stemmed from the oxygen spe-
cies adsorbed on the surface of the catalyst during calcina-
tion [62]. It is clear that the desorption peak of O2 from
2% La–Bi2O3 surface is the most intense one, indicating
that 2% La–Bi2O3 can adsorb more O2 than Bi2O3. This
signifcant enhancement of adsorbed α-oxygen is likely
related to the existence of surface oxygen vacancies.
To further identify whether surface oxygen vacancy is
the main factor of O2 adsorption, the electron paramag-
netic resonance (EPR) spectra of 2% La–Bi2O3 and Bi2O3
were conducted on a JES-FA200 ESR spectrometer at
100 K in liquid N2. The comparison of the ESR spectra of
Bi2O3 and 2% La–Bi2O3 were illustrated in Fig. 9e. The
typical ESR signal at g = 2.001 (calculated by g = hν/βH)
emerged in 2% La–Bi2O3 sample, which proves the exist-
ence of oxygen vacancies. The results indicated the La
doping and low temperature calcination could largely
⋅
−
cal species, and O served as a secondary active species
during photocatalyt2ic degradation of MO.
Figure 9b presents the efect of scavengers on photocata-
lytic activity of as-prepared samples with addition of EDTA-
2Na, IPA and BQ, respectively. The addition of IPA and BQ
had negligible efect on the photocatalytic activity, while,
photocatalytic activity was tremendously reduced in case
of EDTA-2Na. The scavenging experiment identifed holes
(h+) as main oxidizing species.
⋅
−
In order to further confrm the generation of O2 radi-
cals in the photocatalytic process, we employed the elec-
tron spin-trap technique (ESR technique) to probe the active
⋅
−
oxygen species based on the fact that O2 can be trapped by
5,5-dimethyl-l-pyrroline-N-oxide (DMPO) [61]. Figure 9c
⋅
−
shows no DMPO- O2 signal was observed when the pho-
tocatalytic reaction was performed with pure Bi2O3. The
⋅
−
characteristic ESR signals of DMPO- O appeared in the
presence of 2% La–Bi2O3 sample, which2provide evidence
⋅
−
of O2 formed.
1 3

T. Li et al.
Fig. 9 Trapping experiments of the photodegradation of MO (a) and
phenol (b) over 2% La–Bi2O3 sample; c DMPO-ESR spectra of Bi2O3
and 2% La–Bi2O3 samples; d O2-temperature programmed desorption
profles of Bi2O3 and 2% La–Bi2O3 samples; e EPR signals of Bi2O3
and 2% La–Bi2O3 samples
−1
La − Bi2O3 + hv → e (CB) + h
+
(3)
(4)
(5)
(6)
(7)
−1
La − Bi2O3 + hv → e (OVs) + h
+
La3+(5d06s0) + e (OVs) → La2+(5d06s1)
−1
O2 + La2+(5d06s1) → ⋅O2 + La3+(5d06s0)
−
⋅
MO∕Phenol + O2 ∕h → CO2 + H2O + …
−
+
The enhanced photocatalytic activity of 2% La–Bi2O3
samples could be ascribed to several factors. Firstly, the
formed OVs states located between the CB and VB as an
impurity level due to the La doping. The OVs states would
take part in a new photoexcitation process, which the pho-
togenerated e− transferred from the VB to the OVs levels
and left h+ in the VB. As a result, more photoinduced h+
with high oxidizing ability would participate in the photo-
degradation process. Secondly, due to particular electronic
structure of La3+ (5s25p65d06s0), La3+ ions have exceptional
electron searching ability [63]. La3+ ions doped into Bi2O3
crystal lattice might act as the efcient scavengers to cap-
ture electrons to be reduced to La2+ ions (5s25p65d06s1),
preventing the hot e− of CB to jump into the level of OVs.
While, La2+ ions easily return to original state by releasing
electrons, the released electron could quickly transfer to the
oxygen molecules adsorbed on the surface of the La–Bi2O3
Fig. 10 A proposed schematic photodegradation mechanisms for MO
over 2% La–Bi2O3 sample under visible light irradiation
induce the formation of oxygen vacancies, which can
adsorb atmospheric oxygen.
Based on a series of experiments mentioned above, a pro-
posed photocatalytic mechanism for the enhanced photocata-
lytic activity of x% La–Bi2O3 was shown in Fig. 10, and the
main degradation processes of MO/phenol are as follows:
1 3

Fabrication of La-Doped Bi2O3 Nanoparticles with Oxygen Vacancies for Improving…
⋅
−
15. Li YT, Zhang ZF, Zhang YY, Sun XG, Zhang JM, Wang CH
(2014) Ceram Int 40:13275
photocatalyst and reduce the oxygen molecules to O2 , i.e.
superoxide radical, which could further degrade MO/phenol.
Obviously, the synergistic efects of La3+-doping and oxygen
vacancies (OVs) play an important role in photodegradation
processes of MO/phenol.
16. Sudrajat Hanggara (2017) Superlattice Microst 109:229
17. Chen Y, Zhang Y, Luo L, Shi Y, Wang S, Li L, Long Y, Jiang F
(2018) Ceram Int 44:2178
18. Suriye K, Praserthdam P, Jongsomjit B (2007) Appl Surf Sci
253:3849
19. Xu A, Gao Y, Liu H (2002) J Catal 207:151
20. He RX, Jiang HQ, Wu F, Zhi KD, Wang N, Zhou CL (2014)
Chem Pap 68:1049
4 Conclusions
21. Wang J, Liu P, Fu X, Li Z, Han W, Wang X (2009) Langmuir
25:1218
La doping Bi2O3 photocatalyst has been successfully syn-
thesized via a facile sol–gel method following a facile low
temperature calcination. 2% La–Bi2O3 exhibited excellent
photocatalytic activity in degradation of MO/phenol. The
content doping of La3+ and the amount of oxygen vacancies
of 2% La–Bi2O3 plays a crucial role in promoting the separa-
tion of photogenerated electrons and holes, and their syner-
gistic efect brings about the improved photocatalytic perfor-
mance. In this research, the amount of oxygen vacancy could
be controlled by the temperature, atmosphere, and time in
calcining process. Optimization of these experimental condi-
tion will be the way to increase and control concentration of
oxygen vacancy. Besides, the present work would be useful
to explore oxygen vacancies induced photocatalytic activity
in rare earth ion doped semiconducting oxides.
22. Ho CC, Kang F, Chang GM, You SJ, Wang YF (2019) Appl Surf
Sci 465:31
23. Huang Y, Cao JJ, Kang F, You SJ, Chang CW, Wang YF (2017)
Aerosol Air Qual Res 17:2555
24. Ali Imran Vaizogullar (2018) J Electron Mater 47:6751
25. Bilal Tahir M, Sagir M (2019) Sep Purif Technol 209:94
26. Zhang J, Zhao Z, Wang X, Yu T, Guan J, Yu Z, Li Z, Zou Z
(2010) J Phys Chem C 114:18396
27. Zhang B, Li D, Wang XY (2010) Catal Today 158:348
28. Yang B, Shen YS, Su Y, Li PW, Zeng YW, Shen SB (2017)
Catal Commun 94:47
29. Chen T, Hao Q, Yang W, Xie C, Chen D, Ma C, Yao W, Zhu Y
(2018) Appl Catal B 237:442
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Mater Sol C 182:113
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Q, Yan WS, Pan BC, Xie Y (2018) J Am Chem Soc 140:1760
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Acknowledgements This work was financially supported by the
National Natural Science Foundation of China (Grant No. 11604222),
and the Liaoning Provincial Natural Science Foundation of China
(Grant No. 201601155).
34. Zhu W, Jin J, Chen X, Li C, Wang T, Tsang CW (2018) Environ
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