Enhanced Visible Light-Driven Photocatalytic Activities and Photoluminescence Characteristics of BiOF Nanoparticles Determined via Doping Engineering

Article abstract of DOI:10.1021/acs.inorgchem.0c01811

We report a new type of highly efficient visible light-driven photocatalyst, Sm3+-activated BiOF nanoparticles, developed by a facile solid-state reaction technology. The corresponding phase compositions, morphological nature, and chemical states along with complementary theoretical calculation insights are investigated systematically. Upon 404 nm laser excitation, the photoluminescence performance of the synthesized nanoparticles is explored and the optimal properties are achieved in BiOF:xSm3+ (x = 0.07). The dipole-quadrupole interaction is attributed to the concentration quenching mechanism. Under visible light irradiation, the degradation of the RhB dye by utilizing the Sm3+-activated BiOF nanoparticles is studied. In comparison with the BiOF nanoparticles, the resultant compounds doped with Sm3+ ions demonstrate improved photocatalytic performance. Moreover, on the basis of density functional theory, the electronic structure of the BiOF impacted by Sm3+ ion doping is studied in detail by first-principles calculations, revealing the generation of an impurity energy level that is beneficial for enhancing the photocatalytic properties. Importantly, the h+ and ·O2- active species play a deterministic role in promoting the degradation of the RhB dye. Compared to commercial ZnO nanoparticles, the developed nanoparticles exhibit superior photocatalytic activities, further elaborating that the Sm3+-activated BiOF nanoparticles are poised to be one of most promising visible light-driven photocatalyst candidates.

Full text of DOI:10.1021/acs.inorgchem.0c01811

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Article
Enhanced Visible Light-Driven Photocatalytic Activities and
Photoluminescence Characteristics of BiOF Nanoparticles
Determined via Doping Engineering
Can Wang,∥ Weiguang Ran,∥ Peng Du,* Weiping Li, Laihui Luo, and Dandan Wang*
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ABSTRACT: We report a new type of highly efficient visible light-driven photocatalyst, Sm3+-activated BiOF nanoparticles,
developed by a facile solid-state reaction technology. The corresponding phase compositions, morphological nature, and chemical
states along with complementary theoretical calculation insights are investigated systematically. Upon 404 nm laser excitation, the
photoluminescence performance of the synthesized nanoparticles is explored and the optimal properties are achieved in BiOF:xSm3+
(x = 0.07). The dipole−quadrupole interaction is attributed to the concentration quenching mechanism. Under visible light
irradiation, the degradation of the RhB dye by utilizing the Sm3+-activated BiOF nanoparticles is studied. In comparison with the
BiOF nanoparticles, the resultant compounds doped with Sm3+ ions demonstrate improved photocatalytic performance. Moreover,
on the basis of density functional theory, the electronic structure of the BiOF impacted by Sm3+ ion doping is studied in detail by
first-principles calculations, revealing the generation of an impurity energy level that is beneficial for enhancing the photocatalytic
properties. Importantly, the h+ and •O2 active species play a deterministic role in promoting the degradation of the RhB dye.
−
Compared to commercial ZnO nanoparticles, the developed nanoparticles exhibit superior photocatalytic activities, further
elaborating that the Sm3+-activated BiOF nanoparticles are poised to be one of most promising visible light-driven photocatalyst
candidates.
photocatalysts can capture only the ultraviolet light, leading
to photodegradation under ultraviolet light irradiation.
Evidently, these semiconductors (i.e., TiO2 and ZnO) cannot
utilize the solar energy efficiently. To the best of our
knowledge, ultraviolet light is only 4% of the total solar energy
spectrum, while 43% of solar light energy is represented by
visible light.7,8 Therefore, finding photocatalysts that can take
advantage of visible light would be very helpful for solving the
world’s environmental problems.
1. INTRODUCTION
With the spread of technology and industrialization, humans
are suffering from some serious issues, such as energy shortage,
environmental pollution (i.e., water pollution and atmospheric
pollution), etc. To solve these issues, photocatalysis based on
semiconductors have been intensively studied because they not
only can degrade the pollutants but also can split the water.1,2
Semiconductor-mediated photocatalysts have demonstrated
high efficiency in treating wastewater and further purification.
Importantly, the development of visible light-responsive
photocatalysts has stimulated a new surge of research.3,4 To
date, some commercial semiconductors, including ZnO and
TiO2, have been developed and used as photocatalysts to
promote the degradation of pollutants because of their
advantages of high stability, nontoxicity, low cost, high
photocatalytic activity, etc.5,6 Notably, these commercial
Received: June 18, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c01811
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Recently, bismuth-based semiconductors have been widely
studied in the field of photocatalysis due to their unique spatial
structure, proper energy band gap, and nontoxicity.9,10 Huang
et al. proposed that several types of bismuth-based semi-
conductors, such as BiOI@Bi12O17Cl2, Bi2O2(OH)(NO3), and
Bi4NbO8X (X = Cl or Br), had admirable photocatalytic
activity upon visible light illumination.11−13 Today, the level of
interest in BiOX (X = F, Cl, Br, or I) is increasing because of
its distinctive layered structure that consists of stacked [-X-Bi-
O-O-Bi-X-] layers. In particular, the positive [Bi2O2]2+ slabs
are interleaved by two negative X− (X = F, Cl, Br, or I) ions
with the aid of weak nonbonding van der Waals interaction
along the c-axis, which can trigger an internal electric field
along the c-axis.14 Because of the existence of an internal
electric field, the migration and separation of the hole−
electron pairs will be boosted, resulting in the good intrinsic
photocatalystic activity of BiOX (X = F, Cl, Br, or I). It has
been reported that the organic pollutants (i.e., RhB and methyl
orange) can be degraded by BiOX (X = F, Cl, Br, or I)
nanoparticles.15−17 In spite of these results, the practical
applicability of BiOX (X = F, Cl, Br, or I) compounds is still
restricted because of the relatively high recombination ratio of
the photogenerated hole−electron pairs. Consequently, some
strategies for modifying the photocatalytic performance of
BiOX (X = F, Cl, Br, or I) semiconductors should be put
forward.
Rare-earth ions, which exhibit configurations partially filled
by 4f electrons, are recognized as a promising approach for
enhancing the photocatalytic behaviors of the semiconductors
through adjusting the energy band gap and hindering the
combination of photogenerated hole−electron pairs.18−20
Zhang et al. stated that the photocatalytic properties of the
BiOBr 2D nanosheets were greatly improved through the
addition of Eu3+ ions.21 Thomas et al. reported that the Sm3+-
doped g-C3N4 nanosheets exhibited superior photocatalytic
behaviors compared to those of the undoped g-C3N4
nanosheets.22 In comparison, the Sm3+ ion, as a member of
the rare-earth ions, has been considerably studied as a
photoluminescent activator because it can emit red/orange
emissions triggered by the 4G5/2 → 6HJ transitions (J = 5/2, 7/2,
9/2, and 11/2) within 4fn configurations.23,24 Furthermore, the
BiOX (X = F, Cl, Br, or I) compounds would be the proper
luminescent hosts for the Sm3+ ions because the Bi3+ and Sm3+
ions possess similar ionic radii and the same charge. To date,
the photoluminescence properties of the Sm3+-activated BiOCl
and BiOBr compounds have been reported.25,26 Nevertheless,
to the best of our knowledge, there is no research on the
photocatalytic activity and photoluminescence characteristics
of the Sm3+-activated BiOF compounds. In this work, the Sm3+
ions and BiOF were chosen as activators and the luminescent
host, respectively, to synthesize the Sm3+-activated BiOF
nanoparticles by utilizing a simple solid-state reaction
technology. The crystalline phase, morphology behaviors,
concentration-dependent quenching mechanism, and photo-
luminescent properties of the prepared nanoparticles were
explored. Furthermore, the electron structure of the studied
samples was investigated by employing first-principles density
functional theory (DFT). In addition, via analysis of the
degradation rate of the RhB dye, the photocatalytic activities of
the developed nanoparticles were explored under visible light
excitation.
2. EXPERIMENTAL SECTION
2.1. Preparation of Sm3+-Activated BiOF Compounds. The
facile solid-state reaction technique was used to synthesize the
Bi1−xOF:xSm3+ (BiOF:xSm3+; x = 0.00, 0.03, 0.07, 0.09, or 0.11)
nanoparticles. The raw materials Bi2O3, NH4F, and Sm2O3, which
exhibited high purities of 99%, 98%, and 99.99%, respectively, were
used as the starting materials. These raw materials were first weighted
on the basis of the designed stoichiometric ratio by the electronic
balance. After that, these powders were mixed for 30 min and then
transferred to the crucible. Ultimately, they were placed in a furnace
and sintered at 500 °C for 3 h to generate the final products. In
particular, to obtain the required sintering temperature of 500 °C, the
heating rate was set as 5 °C/min.
2.2. Sample Characterization. The crystallinity and phase
composition characteristics of the nanosized samples were detected
by a Bruker D8 Advance diffractometer with a Cu Kα radiation
source. In terms of the Cu Kα radiation source, its wavelength is
1.5406 Å. The Raman spectrum of resultant nanoparticles was tested
by means of a microfluorescence spectrum measurement system
(Ocean Optics QE pro). The X-ray photoelectron spectroscopy
(XPS) spectrum was recorded using a multifunctional imaging
electron spectrometer (Thermo ESCALAB 250XI). The morpho-
logical information about the studied samples was checked by taking
advantage of the JEM-2100F transmission electron microscope
(TEM) (JEOL) and using Hitachi SU-70 field emission scanning
electron microscopy (FE-SEM) attached with an energy-dispersive X-
ray spectroscopy (EDX). Utilizing the Edinburgh FS5 fluorescence
spectrometer, the photoluminescent properties of the synthesized
nanoparticles were investigated. To measure the temperature-
dependent emission spectra, we attached the Linkam HFS600E-PB2
temperature controlling equipment to the Edinburgh FS5 fluores-
cence spectrometer.
2.3. First-Principles Calculation. With the aid of density
functional theory, the electronic structure of the BiOF influenced
by the Sm3+ ions was examined. Furthermore, the Cambridge
Sequential Total Energy Package code (i.e., CASTEP) was employed
to calculate the partial density of states, band structure, and orbital
population of the BiOF and Sm3+ activation BiOF compounds.27−29
To guarantee the high accuracy of the calculated results, the cutoff
energy of the Vanderbilt ultrasoft pseudopotential was selected as 420
eV. Additionally, the K-point step was carried out by a 7 × 7 × 4
Monkhorst−Pack scheme for the sake of determining the Brillouin
zone integration.30 Geometry optimization, which was performed by
utilizing the Broyden, Fletcher, Goldfarb, Shannon (i.e., BFGS) route,
was performed before the properties were evaluated. The places of the
entire atoms were contemporaneously optimized throughout the
geometry optimization process. With regard to the geometry
optimization, its convergence tolerance was adopted with the
variations in the maximal ionic Hellmann−Feynman force (i.e., 1.0
× 10−2 eV/Å), stress tensor (i.e., 2.0 × 10−2 GPa), total energy (i.e.,
5.0 × 10−6 eV/atom), and maximal displacement (i.e., 5.0 × 10−4 Å).
Furthermore, the calculations were executed on the basis of the
generalized gradient approximation (GGA) scheme with the Perdew−
Burke−Ernzerhof (PBE) function. Throughout the entire calculation
procedures, we set the convergence criterion for the self-consistent
field (SCF) to 5.0 × 10−7 eV/atom.
2.4. Photocatalytic Activity Test. The photocatalytic activity of
the BiOF:xSm3+ nanoparticles was examined by studying the
decomposition of the RhB dye under visible light irradiation. A 500
W Xe lamp attached with a filter (λ ≥ 400 nm) was employed as the
lighting source. Before the experiment was carried out, 50 mg of
prepared nanoparticles was added to the RhB aqueous solution (20
mg/L) with a volume of 40 mL and stirred in the dark for 30 min to
achieve the absorption−desorption balance. After that, the RhB
aqueous solution containing the designed nanoparticles was exposed
to visible light. During irradiation, 3 mL of the solution was taken at a
fixed time interval and the photocatalysts were removed by
centrifugation. The absorption ability of the aqueous solution
described above was measured by applying the Cary 5000
B
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Figure 1. (a) XRD patterns of BiOF:xSm3+ nanoparticles with diverse Sm3+ ion contents. Rietveld XRD refinement profiles of (b) BiOF and (c)
BiOF:0.07Sm3+ nanoparticles. (d) Crystallographic structure of the BiOF crystal cell.
ultraviolet−visible (UV−vis) spectrophotometer. Furthermore, for
the sake of exploring the stability of the designed nanoparticles, they
were centrifuged and washed three times with high-purity water for
the next cycling experiment. Additionally, a comparison of the
photocatalytic properties of the designed compounds and commercial
ZnO nanoparticles was also performed under full spectrum
irradiation, and its experimental processes were the same as described
above.
was far less than 30%, which indicated that the designed solid
solution of BiOF:xSm3+ compounds could be generated. To
explore the phase compositions of the developed nanoparticles
in depth, Rietveld XRD refinements of the representative
BiOF:xSm3+ nanoparticles with x values of 0.00 and 0.07 were
performed. According to the Rietveld refinement results, one
knows that the calculated XRD profiles were the same as these
measured data, as shown in panels b and c of Figure 1,
illustrating that the resultant compounds exhibited a single
tetragonal phase. Moreover, the calculated cell parameters as
well as the corresponding residual factors of the BiOF:xSm3+
(x = 0.00 and 0.07) nanoparticles are listed in Table S1. As
disclosed, these obtained residual factors were small, which
confirmed that the refinement results were reliable. Addition-
ally, via the introduction of the Sm3+ ions, the cell parameters
of the BiOF:0.07Sm3+ nanoparticles were decreased in
comparison with those of the BiOF nanoparticles (see Table
S1), which was triggered by the unequal ionic radii between
the dopants (i.e., Sm3+) and substituted ions (i.e., Bi3+). These
decreased cell parameters further implied that the Bi3+ ions
could be substituted by the Sm3+ ions, resulting in the
formation of a BiOF:xSm3+ solid-state solution. The crystallo-
graphic structure of the BiOF crystal cell is shown in Figure 1d.
It is shown that the Bi3+ ions were coordinated with four
oxygen ions and five fluoride ions.
2.5. Active Species Trapping and ESR Experiment. Isopropyl
alcohol (IPA; 1 mM), ethylenediaminetetraacetic acid (EDTA; 1
mM), and benzoquinone (BQ; 1 mM) were used as scavengers for
trapping the active species of the hole (h+), superoxide radical
−
(•O2 ), and hydroxyl radical (•OH), respectively. The experimental
test was similar to the photocatalytic process described above. The
5,5-dimethyl-1-pyrroline N-oxide (DMPO) served as the radical
−
scavenger to monitor the intensity of the signals of •O2 and •OH
active species, and the ERS spectrum was measured by using an
electron paramagnetic resonance spectrometer (JES FA200).
3. RESULTS AND DISCUSSION
Utilizing X-ray diffraction (XRD), the crystalline phase
performance of the final compounds was clarified. Figure 1a
depicts the XRD profiles of the resultant compounds as a
function of Sm3+ ion content. As demonstrated, the diffraction
peaks were identical with those of the standard card of BiOF
(JCPDS 861648), implying that the synthesized samples had a
tetragonal phase. It is widely accepted that a new solid solution
cannot be formed if the radius percentage difference (Dr)
between the dopants and substituted ions is >30%. Herein, the
Dr value of Bi3+/Sm3+, which was calculated by utilizing the
function Dr = 100% × (R1 − R2)/R1 [here R1 and R2 refer to
the ionic radii of Bi3+ (1.24 Å) and Sm3+ (1.13 Å),
respectively], was 8.87%.31 Evidently, the estimated Dr value
The Raman spectra of the representative BiOF:xSm3+ (x =
0.00, 0.03, 0.07, and 0.11) nanoparticles were recorded and are
shown in Figure S1. As one can see, the Raman profiles were
the same and were composed of three bands at approximately
165.7, 238.8, and 422.4 cm−1. In particular, the intense band at
165.7 cm−1 was associated with the A1g mode (internal Bi−F
stretching) and the peak at 238.8 cm−1 was attributed to the Eg
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Figure 2. (a and b) TEM graphs, (c) high-resolution TEM graph, (d) SAED pattern, (e) EDX spectrum, and (f−i) elemental mapping of the
BiOF:0.07Sm3+ nanoparticles.
Figure 3. XPS spectra of (a) Bi3+ 4f, (b) O2− 1s, (c) F− 1s, and (d) Sm3+ 3d of the BiOF:0.07Sm3+ nanoparticles.
mode (internal Bi−F stretching), while the third band at 422.4
cm−1 was assigned to the Eg/Bg mode of oxygen atoms.32,33
These achievements further proved that the tetragonal
structure of the BiOF was formed.
FE-SEM and TEM were utilized to identify the morpho-
logical information about the synthesized products. Figure S2
shows the FE-SEM images of the BiOF:xSm3+ compounds. It
is obvious that the final products were all composed of
nanosized particles with anomalous shapes. It is worth noting
that the shape and size of the particles hardly varied with an
increase in the Sm3+ ion concentration, revealing that the
microstructure performance of the synthesized products was
independent of the dopant content. In particular, the average
particle sizes, which were evaluated from the FE-SEM images,
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Figure 4. Calculated energy band structures of the (a) BiOF host and (b) Sm3+-activated BiOF compounds, where the Fermi (EF) level was defined
as the zero energy. Orbital populations of (c) the BiOF host and (d) Sm3+-activated BiOF compounds near the EF level.
of the resultant samples were approximately 390, 360, 410,
420, 380, and 370 nm when the x value was 0.00, 0.03, 0.05,
0.07, 0.09, and 0.11, respectively (see Figure S3). To deeply
explain the morphological features of the prepared products,
the typical TEM graphs of the BiOF:0.07Sm3+ nanoparticles
were examined, as demonstrated in panels a and b of Figure 2.
Apparently, the recorded TEM images also consisted of
nanosized particles. Figure 2c shows that the monitored high-
resolution TEM image contained legible lattice fringes, and the
interval of the adjacent lattice fringes was proven to be 3.34 Å,
which was in good agreement with the d spacing of the (101)
plane of the tetragonal BiOF. Furthermore, the studied
nanoparticles possessed a single crystalline structure that was
confirmed by the selective area electron diffraction (SAED)
image presented in Figure 2d. The EDX spectrum revealed
that the synthesized nanoparticles consisted of elements Bi, O,
F, and Sm (see Figure 2e). Notably, a small peak of Pt was also
found in the EDX spectrum and was associated with the
platinum electrode for examining the FE-SEM graphs. In
addition, the elements (i.e., Bi, O, F, and Sm) in the resultant
nanoparticles were uniformly distributed, which was proven by
the elemental mapping results, as shown in Figure 2f−i.
The chemical states and elemental components of the
resultant nanoparticles were identified by utilizing XPS, and
the results are depicted in Figure 3. The high-resolution Bi 4f
XPS spectrum presented in Figure 3a is dominated by two
peaks at around 159.1 and 164.4 eV originating from Bi3+ 4f7/2
and Bi3+ 4f5/2, respectively.19,21 Furthermore, the high-
resolution O 1s XPS spectrum can be deconvoluted into two
peaks at 529.6 and 530.7 eV, which were attributed to the O2−
1s and oxygen vacancy, respectively.34 As we know, the visible
light capture capacity of the photocatalyst can be increased
with the help of the oxygen vacancy, resulting in enhanced
photocatalytic activity.35 In the F 1s XPS spectrum, only one
intense peak at 682.8 eV corresponding to F− 1s was observed,
as demonstrated in Figure 3c.36 Additionally, the XPS
spectrum shown in Figure 3d consisted of two bands, and
their binding energies were approximately 1083.3 and 1109.3
eV, respectively, which were ascribed to Sm3+ 3d5/2 and Sm3+
37
3d3/2
.
The XPS results further indirectly confirmed the
successful synthesis of Sm3+-activated BiOF nanoparticles.
For the sake of additionally analyzing the photolumines-
cence and photocatalytic mechanism of the designed nano-
particles, the crystal structure of the BiOF and Sm3+-activated
BiOF compounds was established and its corresponding DFT
calculation was also carried out. The density of states and
energy band structure of the BiOF host and Sm3+-activated
BiOF compounds are illustrated in Figure 4. Figure 4a shows
that the BiOF host, which exhibited an energy band gap (i.e.,
Eg) of 3.197 eV, pertained to the direct semiconductor.
However, upon the introduction of Sm3+ ions, the studied
samples changed to indirect semiconductors, where positions
of the top of the valence band and the bottom of the
conduction band were different from each other, and its Eg
value was found to be ∼3.045 eV. As we know, when the
positions of the top of the valence band and the bottom of the
conduction band were not the same, this improves the
separation of the hole−electron pairs, leading to the enhanced
photocatalytic activity. Note that, although the band gap of the
BiOF host was not changed so much with the addition of Sm3+
ions, an impurity energy level (e.g., 4f level of Sm3+ ions),
which was located between the conduction band and valence
band of the host, was generated, and the corresponding energy
level as well as the electron distribution is shown in Figure S4.
After a gradual increase in the energy of the electron from the
ground state to the high energy level, the distribution of
E
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Figure 5. (a) Excitation and emission profiles of the BiOF:0.07Sm3+ nanoparticles. (b) Emission profiles of the BiOF:xSm3+ nanoparticles excited
at 404 nm. (c) Dependence of the fluorescence intensity on the dopant content. (d) Photoluminescence mechanism of Sm3+ ions in the
BiOF:xSm3+ nanoparticles as well as the possible concentration quenching mechanism. (e) Plot of log(x) vs log(I/x) for the developed
BiOF:xSm3+ nanoparticles.
electron orbitals is displayed in the right side of Figure S4,
which coincided well with the results presented in Figure 4b.
The partial densities of states of the BiOF host and Sm3+-
activated BiOF system are described in panels c and d of
Figure 4, respectively. According to the computational results,
it is clear that the electronic transition process from the ground
state to the high energy level was mainly associated with the
transition from the O 2p orbital to the Bi 6p orbital. However,
when the Sm3+ ions were added to the BiOF host lattices, the
impurity energy level (e.g., 4f level of Sm3+ ions) would be
formed and located in the range of the ground state of the O
2p orbital and the excited level of the Bi 6p orbital (see panels
b and d of Figure 4). Because of this impurity energy level, the
energy band gap between the ground state and excited level
would be decreased, resulting in the enlarged absorption range.
As a consequence, the designed compounds can efficiently
absorb the photon and enhance the photocatalytic activity.
The UV−vis absorption spectra of the BiOF:xSm3+
nanoparticles were examined to check the photoabsorption
capacity of the studied samples, as illustrated in Figure S5. It is
apparent that the BiOF nanoparticles possessed a strong
absorption in the wavelength range of 200−430 nm and were
caused by O2−/F− → Bi3+ charge transfer (Figure S5a).21
Notably, upon the introduction of Sm3+ ions, an extra sharp
demonstrated in Figure S5, which was conducive to obtaining
high photocatalytic activity under visible light irradiation. As
verified in previous reports, the Eg value of the semiconductor
can be obtained by employing the function o αhv = A(hv −
Eg)n (where α, hv, and A refer to the absorption constant,
phonon energy, and coefficient, respectively, while the n value
is dependent on the type of semiconductor).38 As confirmed
1
above, the n value of the BiOF should be /2 because it
pertains to the direct semiconductor. Consequently, the Eg
value of the synthesized BiOF nanoparticles was 3.96 eV (see
Figure S6a). However, the BiOF:xSm3+ nanoparticles (0.03 ≤
x ≤ 0.11) were indirect semiconductors, and the n value is 2.
Therefore, the Eg values of the BiOF:xSm3+ nanoparticles were
estimated to be around 3.44, 3.41, 3.31, 3.27, and 3.23 eV
when the Sm3+ ion concentration was 3, 5, 7, 9, and 11 mol %,
respectively, as displayed in Figure S6b−f.
Figure 5a illustrates the excitation and emission spectra of
the BiOF:0.07Sm3+ nanoparticles. When the monitored
wavelength was set as 603 nm, the excitation spectrum
comprised several sharp peaks and their central wavelengths
were 363, 376, 404, 418, 442, and 491 nm corresponding to
6
4
4
4
the transitions of Sm3+ ions from H5/2 to D3/2, P7/2, L13/2
,
4G9/2
,
4I15/2, and I9/2 levels, respectively.39,40 To receive
4
6
4
intense photoluminescent properties, we chose the 404 nm
light as the excitation lighting source because it had its
maximum intensity. As presented in Figure 5a, the emission
spectrum excited by 404 nm was composed of three strong
emission bands centered at approximately 563, 603, and 641
nm that were caused by the 4f−4f transitions of Sm3+ ions
absorption peak at 404 nm arising from the H5/2 → L13/2
transition of Sm3+ ions was seen and its intensity increased
gradually with the increment in dopant content (see Figure
S5b−f).23 Aside from the narrow absorption peak, the
absorption edge also showed a red shift tendency with an
increase in Sm3+ ion content (i.e., 0 ≤ x ≤ 0.11), as
F
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Figure 6. (a) Fluorescence intensity mapping of the BiOF:0.07Sm3+ nanoparticles as a function of temperature. (b) Dependence of the
fluorescence intensity on temperature. (c) Plot of 1/kT vs ln(I0/I − 1) for the BiOF:0.07Sm3+ nanoparticles.
4
6
6
6
from the G5/2 excited level to H5/2, H7/2, and H9/2 ground
In general, the concentration-dependent quenching mecha-
nism can be contributed by both electric multipole interaction
and exchange interaction. Utilizing the following function
given by Blasse, the critical distance (Rc) of the dopants (e.g.,
Sm3+) was evaluated to verify the possible quenching
mechanism:46
states, respectively.39,40
A doping strategy has been widely utilized to engineer the
band structures of both bulk and nanoscale materials,
promoting the implementation of multifunctional materials
and devices.41−45 To expose the impression of the Sm3+ ion
content on the photoluminescence features of designed
nanoparticles and to determine the optimum doping
concentration, the emission spectra of the BiOF:xSm3+ (0 ≤
x ≤ 0.11) nanoparticles excited by 404 nm were recorded and
are demonstrated in Figure 5b. As expected, only the
characteristic emissions of Sm3+ ions were exhibited in the
resultant nanoparticles, and the fluorescence intensity was
sensitive to the doping concentration. In particular, the
fluorescence intensity was monotonously aggrandized when
the Sm3+ ion content was changed from 3 to 7 mol %, whereas
it declined when the x value was over 0.07 because of the
concentration quenching effect (see Figure 5c), implying that
the best doping content for the Sm3+ ions in studied samples
was 7 mol %. The energy level diagram of Sm3+ ions is drawn
and shown in Figure 5d to explain the involved luminescent
process and concentration quenching effect. When the studied
samples are excited by 404 nm radiation, electrons can be
1/3
i
y
3V
4πxcZ
j
z
j
z
Rc = 2j
z
z
z
j
j
(1)
k
{
where xc, V, and Z are defined as the critical content of the
dopants, the unit cell volume, and the number of cation
positions in the unit cell that are available to be occupied by
the dopants, respectively. For the studied samples, xc = 0.07, V
= 87.779 Å3, and Z = 2. Hence, the Rc value of the Sm3+ ions in
the studied compounds was determined to be ∼15.6 Å. The
exchange interaction cannot be responsible for the concen-
tration-dependent quenching mechanism only when Rc < 5 Å;
otherwise, the electric multipole interaction plays a major role
in determining the concentration quenching mechanism.47 On
the basis of the discussion presented above, we can deduce that
the concentration-caused quenching effect in the Sm3+-
activated BiOF nanoparticles was triggered by the electric
multipole interaction. As we know, in terms of the electric
multipole interaction, it includes dipole−dipole, dipole−
quadrupole, and quadrupole−quadrupole interactions. To
gain detailed information about the electric multipole
interaction, we examined the relationship between the doping
concentration (x) and the fluorescence intensity (I) by
utilizing the function proposed by Dexter:48
6
4
excited from the H5/2 level to the F7/2 level. Thereafter, the
4G5/2 level is populated from the F7/2 level by means of the
4
4
4F7/2 → G5/2 + hv process. As a consequence, the intense
4
6
4
6
emissions arising from the G5/2 → H5/2, G5/2 → H7/2, and
6
4G5/2 → H9/2 transitions are noted. Note that, when the
doping content is over a certain value (e.g., 7 mol %), the
cross-relaxation (CR) process arising from the G5/2 excited
4
I
x
1
level to other intermediate levels would occur, leading to the
depopulation of the G5/2 level as well as the quenched
=
4
k(1 + βxθ/3
)
(2)
fluorescence intensity. Herein, there are four probable CR
processes (i.e., CR1−CR4) for decreasing the population of
In the function presented above, k and β refer to constants and
the value of θ is dependent on the type of interaction (i.e., θ =
the G5/2 level, as described in Figure 5d.23,38
4
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Figure 7. UV−vis absorption spectra of RhB degradation in the presence of (a) BiOF and (b) BiOF:0.11Sm3+ nanoparticles upon visible light
excitation. The insets of panels a and b show the optical photographs of the RhB aqueous solution as a function irradiation time. (c) Rate of
photocatalytic degradation of RhB by resultant nanoparticles under visible light irradiation. (d) Kinetic rates of the synthesized nanoparticles for
RhB under visible irradiation.
6 for dipole−dipole interaction, θ = 8 for dipole−quadrupole
interaction, and θ = 10 for quadrupole−quadrupole
interaction). When x > xc, the aforementioned eq 2 can be
rewritten as49
following function to comprehend the thermal quenching
effect in depth:50,51
I
I
i
y
ΔE
kT
0
j
j
j
z
z
ln
− 1 = ln A −
z
{
(4)
k
I
θ
i y
j z
j z
j z
log
= C − log(x)
where I0, I, A, and k refer to the initial fluorescence intensity,
the fluorescence intensity when the temperature (i.e., T)
reaches a certain value, a constant, and the Boltzmann
coefficient, respectively. In particular, k presents a fixed value
of 8.629 × 10−5 eV/K. Figure 6c illustrates the plot of 1/kT
versus ln(I0/I − 1) for the BiOF:0.07Sm3+ nanoparticles. Via
linear fitting, one achieves a slope of −0.209. Therefore, the
ΔE value of the Sm3+ ions in the designed nanoparticles was
0.209 eV.
(3)
x
3
k {
where C is a coefficient. For the purpose of achieving the θ
value, the relationship between log(x) and log(I/x) was
determined and is presented in Figure 5e. Figure 5e shows that
these experimental data could be linearly fitted with a slope of
−2.77. Thus, the θ value was 8.31 and close to 8, suggesting
that the content-dependent quenching effect was triggered by
dipole−quadrupole interaction.
Apart from the photoluminescence performance at room
temperature, the temperature-dependent photoluminescence
behaviors of the resultant nanoparticles were also explored to
investigate their thermal stability. The fluorescence intensity
mapping of the representative BiOF:0.07Sm3+ nanoparticles
within the temperature range of 303−463 K is depicted in
Figure 6a. It is significant that the fluorescence intensity
declined gradually with an increase in temperature, which was
attributed to the thermal quenching effect. In spite of the
decreased fluorescence intensity at elevated temperature, it still
retained 58.1% of its starting value (i.e., at 303 K) when the
temperature was increased to 463 K (see Figure 6b),
demonstrating that the prepared downconverting nanoparticles
had satisfactory thermal quenching behaviors and were
potential candidates for practical applications. Furthermore,
the activation energy (i.e., ΔE) was also discussed through the
Under visible light irradiation, the photocatalytic properties
of developed upconverting nanoparticles were investigated by
monitoring the degradation of the RhB dye. The UV−vis
absorption spectra of the RhB aqueous solution with the
addition of BiOF and BiOF:0.11Sm3+ nanoparticles as well as
their corresponding optical photographs at diverse irradiation
times are shown in panels a and b of Figure 7, respectively. As
revealed in panels a and b of Figure 7, upon irradiation by
visible light, the absorption band exhibited a blue shift that was
associated with the de-ethylation of the RhB dye by attacking
the active oxygen species on the N-ethyl group42,53 and the
color of the studied RhB aqueous solution changed gradually
with an increase in exposure time, suggesting that the RhB dye
can be efficiently photodegraded by the synthesized nano-
particles. Prior to visible light irradiation, the RhB dye (i.e.,
N,N,N′,N′-tetraethylated rhodamine) showed an intense
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Figure 8. (a) Recycling degradation performance of RhB by the BiOF:0.11Sm3+ nanoparticles under visible light irradiation. (b) Rate of
−
degradation of RhB by the BiOF:0.11Sm3+ nanoparticles with various scavengers under visible light irradiation. ESR spectra of (c) DMPO-•O2
and (d) DMPO-•OH adducts of BiOF:0.11Sm3+ nanoparticles in the dark and under visible light irradiation.
absorption peak with a wavelength of 554 nm. Upon visible
light irradiation, the absorption band of the aqueous solution
moved to 541 nm (i.e., N,N,N′-triethylated rhodamine), 528
nm (i.e., N,N′-diethylated rhodamine), 506 nm (i.e., N-
ethylated rhodamine), and 497 nm (i.e., rhodamine) with an
increase in irradiation time from 60 to 360 min.52,53 A similar
changing tendency was also observed in the BiOF:0.03Sm3+
and BiOF:0.07Sm3+ nanoparticles upon excitation by visible
light, as presented in Figure S7. Notably, aside from the blue
shift, the intensity of the featured absorption band of the RhB
aqueous solution (i.e., 554 nm) also rapidly declined upon
visible light excitation for 360 min.
conducive to capturing visible light. Furthermore, a blank
experiment was also performed by analyzing the degradation of
the RhB dye without adding any types of catalysts under the
same experimental condition. As one can see, the RhB dye self-
degradation could not be observed upon visible light
irradiation. These results confirmed that the resultant nano-
particles were promising visible light-triggered photocatalysts
for RhB dye degradation.
For the purpose of analyzing the involved reaction dynamics
of the RhB dye photodegradation in the presence of
BiOF:xSm3+ nanoparticles, the reaction kinetics of the
designed photocatalysts were evaluated by the Langimuir−
Hinshelwood model, as defined below:54,55
Figure 7c illustrates the photocatalytic degradation percent-
age of the RhB dye in the presence of different synthesized
nanoparticles. The degradation rate of the RhB dye was
estimated by the following expression:
C
C
i
y
0
j
j
j
z
z
z
ln
= Kt
(6)
k
{
i
y
C
C0
where C0 is the starting concentration of the dye without
irradiation, C is the concentration of the dye after irradiation, t
is the irradiation time, and K is the apparent first-order rate
constant. Figure 7d shows the plot of ln(C0/C) versus t.
Significantly, these experimental data obeyed the linear
relation, implying that the involved photodegradation reaction
belonged to the pseudo-first-order dynamics. From Figure S8,
one achieves K values for BiOF, BiOF:0.03Sm3+, Bi-
OF:0.07Sm3+, and BiOF:0.11Sm3+ nanoparticles of 0.008,
0.109, 0.0142, and 0.0213, respectively. It is found that the K
value of BiOF:0.11Sm3+ nanoparticles was the highest, which
further proved that the prepared nanoparticles doped with 11
mol % Sm3+ ions had the best photocatalytic activity compared
to those of other components.
j
z
j
z
degradation rate = j1 −
z × 100%
j
j
z
z
(5)
k
{
where C0 is initial concentration of the RhB aqueous solution
and C is the concentration of the RhB aqueous solution after
visible light irradiation. Cleary, after irradiation for 360 min,
the RhB dye was completely degraded (i.e., degradation
efficiency of 100%) with the help of the catalysts (i.e.,
BiOF:xSm3+; 0.03 ≤ x ≤ 0.11). Nevertheless, evident
differences among them were still seen; that is, compared
with the BiOF nanoparticles, these BiOF:xSm3+ (x = 0.03,
0.07, and 0.11) nanoparticles exhibited superior photocatalytic
activity, as shown in Figure 7c. In particular, BiOF:0.11Sm3+
nanoparticles had photocatalytic behavior that was better than
those of other components because it exhibited a broader
absorption edge and a smaller energy band gap that are
To explore the stability and recyclability of the developed
photocatalysts, the cycle experiment for the photodegradation
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Figure 9. Schematic diagram of the possible photocatalytic mechanism of the BiOF:xSm3+ nanoparticles under visible light irradiation.
of RhB by the BiOF:0.11Sm3+ nanoparticles was carried out, as
shown in Figure 8a. As demonstrated, the photocatalytic
efficiency of the BiOF:0.11Sm3+ nanoparticles scarcely varied
after five degradation experiments, implying that the designed
photocatalysts had splendid stability. As is known, several types
ECB = EVB − Eg
(8)
X, Ee, and Eg denote the absolute electronegativity of the
semiconductor, the energy of free electrons on the hydrogen
scale (i.e., Ee ≈ 4.5 eV), and the energy band gap, respectively.
For BiOF, the X value was 7.17 eV32 and the Eg value
calculated from the UV−vis absorption spectrum was 3.96 eV.
Therefore, the ECB and EVB values of the BiOF were found to
be 0.69 and 4.65 eV, respectively. As presented in Figure S9,
the EVB value of the BiOF nanoparticles was more positive than
the potential of •OH/OH− (1.99 eV vs NHE). Thus, the
generated h+ on the valence band can oxidize the H2O/OH−
into •OH species.58 However, the ECB value of the BiOF
nanoparticles was also more positive than the potential of the
−
of active species, such as h+, •OH, and •O2 , can contribute to
the photocatalytic reaction. For the sake of identifying the
primary active categories that participated in the photocatalytic
reaction, the trapping experiment was carried out. Herein, IPA,
EDTA, and BQ were adopted as scavengers for trapping •OH,
−
h+, and •O2 , respectively. As disclosed in Figure 8b, the
photocatalytic activity of the BiOF:0.11Sm3+ nanoparticles was
largely restrained through the addition of BQ (i.e., 38.9%) and
EDTA (i.e., 17.9%) under visible light irradiation for 360 min,
manifesting that both the h+ and •O2 species played
−
−
O2/•O2 group (−0.33 eV vs NHE), which shows the
dominant roles in the photocatalytic process. Furthermore,
when the IPA was added, the photocatalytic activity of the
BiOF:0.11Sm3+ nanoparticles was mildly decreased (i.e.,
95.3%), revealing that the •OH was not the dominant active
species that contributed to RhB decomposition. To further
confirm the generation of the active species by the developed
nanoparticles under visible light irradiation, DMPO was
employed as the trapping agent to perform the ESR spin-
photogenerated electrons on the conduction ban cannot
−
combine with O2 to produce the •O2 species (see Figure
S9). Nevertheless, both the trapping experiment and ERS
results confirmed the existence of •O2− species in the involved
photocatalytic process of the BiOF:xSm3+ nanoparticles that
may be attributed to the fact that the Sm3+ ions can accelerate
the generation of •O2 species.20,59
−
The results presented above demonstrate that prepared
nanoparticles can efficiently decompose the RhB dye upon
visible light excitation, and the photocatalytic properties of the
BiOF nanoparticles can be significantly improved through
Sm3+ ion doping. The probable photocatalyic mechanism of
the resultant nanoparticles is described in Figure 9. Upon
visible light illumination, the BiOF host can capture the energy
and generate the electron (e−) and h+ on its conduction and
valence bands, respectively. Therefore, h+ on the valence band
can either directly react with RhB or react with the H2O/OH−
to form the •OH species. These generated •OH species will
take part in the process of RhB degradation (see Figure 9).
The DFT calculation proves that an impurity energy level was
formed when the Sm3+ ions were induced into the BiOF host
lattices, and it is beneficial for inhibiting the combination of
−
trap test. From panels c and d of Figure 8, no signals of •O2
and •OH species were detected in the dark. However, when
−
visible light was applied, the signals of DMPO-•O2 and
DMPO-•OH appeared, and their intensities were enhanced
with an increase in irradiation time. These results suggested
−
that the •O2 and •OH species were generated during the
photocatalytic process, which was in good agreement with the
results achieved from the trapping experiments.
−
To gain deeper insight into the generation of the •O2 and
•OH species, the conduction band edge (i.e., ECB) and valence
band edge (i.e., EVB) of the studied samples were calculated
with the aid of the following empirical functions:56,57
EVB = X − Ee + 0.5Eg
(7)
J
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electron−hole pairs and achieving the enhanced photocatalytic
activity.59,60 Furthermore, because the 4f orbitals of the Sm3+
ions are partially filled by the electrons, it can capture the
electrons on the conduction band of BiOF to form Sm2+ ions.
Notably, the Sm2+ ions can combine with O2 to generate the
characteristic emission of Sm3+ ions with an optimal doping
concentration of 7 mol %. The complementary theoretical
calculation results reveal that the dipole−quadrupole inter-
action was responsible for the content-dependent quenching
mechanism. Furthermore, the resultant nanoparticles had low
thermal quenching, and the activation energy was 0.209 eV.
On the other hand, under visible light irradiation, all of the
prepared samples can efficiently decompose the RhB dye.
Through the doping of Sm3+ ions, the photocatalytic behaviors
of the BiOF nanoparticles were increased and the
BiOF:0.11Sm3+ nanoparticles exhibited the best photocatalytic
activity with a high K value of 0.0213. Moreover, the studied
nanoparticles had excellent stability. Additionally, the h+ and
−
−
•O2 species, as shown in Figure 9. Subsequently, the •O2
species will participate in the process of RhB decomposition.
−
This can be used to explain the existence of •O2 species in
the BiOF:0.11Sm3+ nanoparticles. The extra generation of
−
•O2 species in the Sm3+-activated BiOF nanoparticles can
also contribute to photocatalytic activity that is superior to that
of pure BiOF nanoparticles. These aforementioned photo-
degradation processes can be described as
−
•O2 species played the dominant role in promoting the
BiOF + hv → e− + h+
(9)
degradation of the RhB dye under visible light irradiation.
Ultimately, the prepared BiOF:0.11Sm3+ nanoparticles were
found to exhibit better photocatalytic behaviors than the
commercial ZnO nanoparticles under full spectrum light
illumination. These achievements demonstrate that the
developed Sm3+-activated BiOF nanoparticles with strong
photoluminescence properties show great potential as visible
light-driven photocatalysts in the application of promoting the
degradation of pollutants.
h+ + H2O/OH− → •OH
(10)
Sm3+ + e− → Sm2+
(11)
−
Sm2+ + O2 → •O2
(12)
−
h+ or •O2 or •OH + RhB → CO2 + H2O
(13)
Because of these processes, the RhB dye was efficiently
degraded by the synthesized BiOF:xSm3+ nanoparticles under
visible light illumination.
ASSOCIATED CONTENT
* Supporting Information
■
sı
To further research the photocatalytic activity of the
synthesized nanoparticles, we compared the photodegradation
ability of BiOF:0.11Sm3+ and commercial ZnO nanoparticles
under full spectrum light irradiation. From the UV−vis
absorption spectra, we can see that the characteristic
absorption band of the RhB dye decreased gradually when
the irradiation time was increased (Figure S10a,b). Moreover,
a significant change in the color of the RhB aqueous solution
was also seen. With the help of eq 5 and the recorded UV−vis
absorption spectra, the time-dependent degradation rate of the
BiOF:0.11Sm3+ and commercial ZnO nanoparticles was
calculated and is illustrated in Figure S10c. Evidently, within
a reaction time of 60 min, the degradation efficiencies of the
BiOF:0.11Sm3+ and commercial ZnO nanoparticles were 100%
and 97.1%, respectively, indicating that the resultant
BiOF:0.11Sm3+ nanoparticles had better photocatalytic activity
than the commercial ZnO nanoparticles under full spectrum
light irradiation. This may be because the BiOF:0.11Sm3+
nanoparticles can utilize the visible light to decompose the
RhB dye aside from the ultraviolet light. Furthermore, the plot
of ln(C0/C) versus t is presented in Figure S10d. As disclosed,
these experimental data could be linearly fitted and their slopes
were 0.084 and 0.056 for the BiOF:0.11Sm3+ and commercial
ZnO nanoparticles, respectively, suggesting that the K values of
BiOF:0.11Sm3+ and commercial ZnO nanoparticles were 0.084
and 0.056, respectively. The larger K value of BiOF:0.11Sm3+
nanoparticles further proved that the resultant compounds had
photocatalytic activity superior to those of the commercial
ZnO nanoparticles under full spectrum light irradiation.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01811.
Lattice parameters of BiOF and BiOF:0.07Sm3+ nano-
particles; Raman spectra, FE-SEM images, particle size
distribution, electron distribution and energy level
diagram, UV−vis absorption spectra, and optical band
gap of the BiOF:xSm3+ nanoparticles; UV−vis absorp-
tion spectra of RhB degradation in the presence of
BiOF:0.03Sm3+ and BiOF:0.07Sm3+ nanoparticles; K
value of the BiOF:xSm3+ nanoparticles; and band
positions of the BiOF host and photocatalytic activities
of the BiOF:0.11Sm3+ and ZnO nanoparticles (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Peng Du − Department of Microelectronic Science and
Engineering, School of Physical Science and Technology, Ningbo
University, 315211 Ningbo, Zhejiang, China; orcid.org/
0000-0002-8668-038X; Email: dupeng@nbu.edu.cn,
dp2007good@sina.com
Dandan Wang − GLOBALFOUNDRIES (Singapore) Pte. Ltd.,
Singapore 738406; Email: dandan.wang@
globalfoundries.com
Authors
Can Wang − Department of Microelectronic Science and
Engineering, School of Physical Science and Technology, Ningbo
University, 315211 Ningbo, Zhejiang, China
Weiguang Ran − Department of Chemistry and Chemical
Engineering, Qufu Normal University, Qufu 273165, China
Weiping Li − Department of Microelectronic Science and
Engineering, School of Physical Science and Technology, Ningbo
University, 315211 Ningbo, Zhejiang, China; orcid.org/
0000-0002-5882-8527
4. CONCLUSION
In summary, Sm3+-activated BiOF nanoparticles with splendid
photoluminescence properties and photocatalytic activity were
developed by a facile solid-state reaction method. The FE-SEM
and TEM results confirmed that the final products comprised
nanosized particles in the range of 200−270 nm. Upon 404 nm
light excitation, the resultant nanoparticles emitted the
Laihui Luo − Department of Microelectronic Science and
Engineering, School of Physical Science and Technology, Ningbo
K
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University, 315211 Ningbo, Zhejiang, China; orcid.org/
0000-0002-2370-8277
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01811
Author Contributions
∥C.W. and W.R. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the K. C. Wong Magna Fund of
Ningbo University (xkzw 1507), the Natural Science
Foundation of Ningbo (2018A610076 and 2019A610061),
the Natural Science Foundation of Zhejiang Province
(LQ20F050004), and the key research and development
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