Co2+ substituted for Bi3+ in BiVO4 and its enhanced photocatalytic activity under visible LED light irradiation

Article abstract of DOI:10.1039/c9ra04188e

We investigated the fabrication of Co-doped BiVO₄ (Bi_1-xCo_xVO_4+δ, 0.05 < x < 0.5) by the substitution of Co ions for Bi sites in BiVO₄. The X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) results indicated that the substitution of Co²⁺ ions for Bi³⁺ sites in BiVO₄ was successful, although a change in the crystal phase of BiVO₄ did not occur. UV-vis DRS and PL results suggested that the Co-incorporation could slightly improve the visible light absorption of BiVO₄ and induce the separation of photoinduced electron-hole pairs; therefore, a significant enhancement of photocatalytic performance was achieved. The Bi_0.8Co_0.2VO_4+δ sample showed superior photocatalytic activity in comparison with other samples, achieving 96.78% methylene blue (MB) removal within 180 min. In addition, the proposed mechanism of improved photocatalytic activities and the stability of the catalyst were also investigated.

Full text of DOI:10.1039/c9ra04188e

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Co2+ substituted for Bi3+ in BiVO4 and its enhanced
photocatalytic activity under visible LED light
irradiation†
Cite this: RSC Adv., 2019, 9, 23526
a
*
Trinh Duy Nguyen,
Quynh Thi Phuong Bui,b Tien Bao Le,b T. M. Altahtamouni,c
Khanh Bao Vu,d Dai-Viet N. Vo,
Nhan Thi Hong Le,e Tuan Duy Luu,e
a
Seong Soo Hongf and Kwon Taek Lim
g
We investigated the fabrication of Co-doped BiVO4 (Bi1ꢀxCoxVO4+d, 0.05 < x < 0.5) by the substitution of Co
ions for Bi sites in BiVO4. The X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS)
results indicated that the substitution of Co2+ ions for Bi3+ sites in BiVO4 was successful, although a change
in the crystal phase of BiVO4 did not occur. UV-vis DRS and PL results suggested that the Co-incorporation
could slightly improve the visible light absorption of BiVO4 and induce the separation of photoinduced
electron–hole pairs; therefore, a significant enhancement of photocatalytic performance was achieved.
The Bi0.8Co0.2VO4+d sample showed superior photocatalytic activity in comparison with other samples,
achieving 96.78% methylene blue (MB) removal within 180 min. In addition, the proposed mechanism of
improved photocatalytic activities and the stability of the catalyst were also investigated.
Received 3rd June 2019
Accepted 12th July 2019
DOI: 10.1039/c9ra04188e
rsc.li/rsc-advances
aiming to enhance the photocatalytic performance of m-s BiVO4
have been explored. Such approaches could include doping
BiVO4 with metals and/or non-metals,16–21 controlling the
morphology and facets of BiVO4,5,13,22,23 constructing a novel p–n
heterojunction photocatalyst,24–27 and controlling the
monoclinic-tetragonal heterostructure of BiVO4.28
1 Introduction
Monoclinic scheelite bismuth vanadate (m-s BiVO4), a p-type
semiconductor with a direct band gap (Eg) of 2.4 eV, has
generated considerable interest as a visible light-responsive
photocatalyst.1–4 Reportedly, different morphologies of mono-
clinically structured BiVO4 crystallites, such as nanospheres,5
nanorods,6,7 nanoowers,8 nanosheets,9 nanotubes,10–12
olive,13,14 and hyperbranched,15 have been clearly created by the
hydrothermal or solvothermal technique. Nevertheless, the
poor charge transport and weak surface adsorption properties
associated with pristine BiVO4 could cause prompt recombi-
nation of photogenerated electron–hole pairs, reducing its
overall photocatalytic efficiency. Hence, various strategies
Among the many methods for improving the photocatalytic
activity, metal and/or non-metal doping is regarded as a simple
and effective strategy. For metal-doped BiVO4, metal ions can
substitute the V or Bi sites in the crystal lattice of BiVO4,
exhibiting a quite different photocatalytic performance. For
instance, Luo and co-workers observed that the photocurrent of
the BiVO4 semiconductor could be substantially improved by
Mo6+ or W6+ doping, and further demonstrated the substitution
of Mo6+ or W6+ ions in V5+ positions.29 Contrarily, the incorpo-
ration of lanthanide ions CeIII,IV occurring at Bi3+ sites in the
BiVO4 lattice caused a pronounced change in the crystal phase,
optical properties, and the separation of photoinduced elec-
tron–hole pairs, resulting in a prominent improvement of
photocatalytic activity.30 Substitution of Fe3+ ions for Bi3+ sites
in BiVO4 also signicantly enhanced photocatalytic perfor-
mance for the decomposition of ibuprofen and greatly inacti-
vated Escherichia coli bacteria under visible light irradiation.31
However, modication of BiVO4 by doping of cobalt ions
seemed to yield contradictory results. Specically, Regmi and
co-workers found that Co substituted for V in the crystal lattice,
indicated by the shi in the symmetric V–O stretching mode to
lower wavenumber, was conrmed by the Raman result.32 To
the contrary, another study showed that Co2+ substituted for
Bi3+ in BiVO4, which was conrmed by Bo Zhang et al.33 On the
aCenter of Excellence for Green Energy and Environmental Nanomaterials (CE@
GrEEN), Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam. E-mail:
ndtrinh@ntt.edu.vn; Fax: +84-028-39-404-759; Tel: +84-028-3941-1211
bFaculty of Chemical Technology, Ho Chi Minh City University of Food Industry, 140
LeTrong Tan, Tan Phu District, Ho Chi Minh City, Vietnam
cMaterials Science and Technology Program, College of Arts and Sciences, Qatar
University, Doha 2713, Qatar
dNTT Hi-Tech Institute, Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam
eDepartment of Chemical Engineering, HCMC University of Technology, VNU-HCM, Ho
Chi Minh City, Vietnam
fDepartment of Chemical Engineering, Pukyong National University, Busan, Korea
gDepartment of Display Engineering, Pukyong National University, Busan, Korea
† Electronic supplementary information (ESI) available: XRD analysis, Raman
spectra, SEM images, photo-stability tests, trapping experiments, and
a
comparison of photocatalytic activity of photocatalysts. See DOI:
10.1039/c9ra04188e
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other hand, it was suggested by Zhou et al. that the incorpora-
tion of cobalt as an oxide on the surface of larger BiVO catalysts
caused no change in the crystal structure of the composites.34
Co-doped BiVO was also investigated by Geng et al.,35 but they
did not identify the substitution of Co ions for Bi3+ or V3+ sites
in BiVO4.
Therefore, the present study aimed to identify the substitu-
tion of Co ions for Bi3+ sites in BiVO4. We proposed the
synthesis of Co-doped BiVO4 (Bi1ꢀxCoxVO4+d, 0.05 < x < 0.5) by
the hydrothermal method. The effect of Co2+ ions substituted
for Bi3+ sites in BiVO4 on the crystal structure, morphology,
optical behavior, photoinduced charges separation, and pho-
tocatalytic performance for decomposition of methylene blue
(MB) under visible LED light irradiation was carefully explored.
The proposed mechanism for the improved photocatalytic
activities in Co2+ doped BiVO4 is discussed. In addition, the
stability and reusability of catalyst were also investigated.
2.3 Photocatalytic test
The assessment of the photocatalytic performance of the bare
BiVO4 and Co-doped BiVO4 was executed by the photodecom-
position of MB dye in aqueous media under white LED light
irradiation. Six LED lamps (XLamp XT-E White, Cree X6) with
a max power of 10 W and max light output of 1040 lm acted as
the white light source for this study. To perform this experi-
ment, a mixture containing catalyst powder (0.5 g Lꢀ1) and MB
aqueous solution (100 mL, 15 mg Lꢀ1) was mixed in a 250 mL
double layer interbed interlayer glass beaker and vigorously
stirred for 100 min to ensure the adsorption–desorption equi-
librium between the surface of BiVO4 and the MB molecules.
Under light exposure, aliquots of the reaction solution were
withdrawn at dened time intervals, and the solid was imme-
diately isolated by centrifugation at 8000 rpm. The concentra-
tion of MB was determined by the intensity change of
absorption peak at 664 nm using a UV-visible spectrophotom-
eter (Model Evolution 60S, Thermo Fisher Scientic). In addi-
tion, to investigate the stability of catalyst, repeated
photodecomposition of MB dye over ve consecutive cycles was
conducted using a method similar to the above. Aer each
reaction run, the catalyst was washed with water three times
and isolated by centrifugation at 8000 rpm.
2 Experimental
2.1 Synthesis of photocatalysts
The hydrothermal procedure employing Bi(NO3)3$5H2O,
Co(NO3)2$6H2O, and NH4VO3 as the precursors were adopted to
synthesize the Co-doped BiVO4 samples. Firstly, two clear
solutions were prepared as follows: (i) Bi(NO3)3$5H2O (4 mmol)
and a predetermined amount of Co(NO3)2$6H2O were dissolved
in 20 mL of HNO3 2 M; (ii) 4 mmol of NH4VO3 was dissolved in
40 mL of NaOH 0.3 M. Estimation of the concentration of
Co(NO3)2$6H2O followed the stoichiometric formulation of
Bi1ꢀxCoxVO4+d (where x ¼ 0, 0.05, 0.1, 0.2, 0.3, and 0.5; abbre-
viated as BVO, 0.05Co–BVO, 0.1Co–BVO, 0.2Co–BVO, 0.3Co–
BVO, and 0.5Co–BVO, respectively). Then, a mixture was created
by mixing the two solutions under vigorous stirring for 30 min;
pH was adjusted to 10 using NaOH 4 M solution. Next, the
resulting suspension was adjusted for volume to 80 mL using
distilled water and then transferred to a 100 mL hydrothermal
autoclave reactor with a Teon chamber vessel. The reactor then
underwent thermal treatment at 180 ꢁC in an oven for 24 h. The
obtained yellow paste was rinsed with water until the pH of the
ltrate solution was around 7. Finally, the obtained powder was
dried at 110 ꢁC for 24 h, followed by calcination at 300 ꢁC for 2 h.
2.4 Analysis of hydroxyl radical productions
We also used the PL technique with benzene-1,4-dicarboxylic
acid (H2BDC) as the sensor molecule to detect the $OH radi-
cals produced by 0.2Co–BVO during the photocatalytic
processes. H2BDC molecule reacted with the $OH radicals to
form a uorescent molecule (2-hydroxy-1,4-benzenedicarboxylic
acid, H2BDC-OH). The experimental procedures were conduct-
ed based on the above photocatalytic activity test, but MB
solution was replaced by H2BDC solution ([H2BDC] ¼ 5 ꢂ
10ꢀ4 M and [NaOH] ¼ 2 ꢂ 10ꢀ3 M).14 Aer 60 min of irradiation,
the reaction solution aer eliminating catalyst was analyzed by
a Hitachi F4500 uorescence spectrometer at an excitation
wavelength of 315 nm.
3 Results and discussion
3.1 Crystal structure
Fig. 1A presents XRD patterns of the bare BiVO4 and Co-doped
BiVO4 samples. The bare BiVO4 sample had a pure monoclinic
2.2 Characterization of photocatalysts
To characterize the crystal structure of the undoped and doped scheelite phase structure (m-s BiVO4, JCPDs 14-0688) with sharp
BiVO4 photocatalysts, multiple techniques including X-ray characteristic peaks at 28.9ꢁ, and 30.5ꢁ, and double peaks at
diffraction (XRD, D8 ADVANCE-Bruker), Raman spectra 18.9ꢁ, 35ꢁ, and 47ꢁ. Similarly, all XRD peaks of the 0.05Co–BVO,
(HORIBA Jobin Yvon), and X-ray photoelectron spectroscopy 0.1Co–BVO, and 0.2Co–BVO samples were also correctly
(XPS, Thermo VG Multilab 2000) were employed. Morphological indexed to the m-s BiVO4, and no peaks of tetragonal phase (t-z
characteristics were observed by Field Emission Scanning BiVO4) were detected. This result indicated that the presence of
Electron Microscopy (FE-SEM, JSM-6700F, Jeol) and trans- Co with x ¼ 0.05–0.2 caused no signicant change in the crystal
mission electron microscopy (TEM, JEM 1400, JEOL). Energy phase of BiVO4, which was consistent with previous studies.33–35
band structure was evaluated through the UV-visible diffuse However, when further increasing the Co molar ratio to 0.3,
reectance spectrum (UV-Vis DRS, Shimadzu UV-2450) with diffraction peaks of impurities including Bi2O3 (JCPDs 41-1449)
BaSO4 as the standard. Photoluminescence (PL) spectroscopy and Co2V2O7 (JCPDs 70-1189) were detected. The XRD pattern of
was performed using an F-4500 Spectro-uorometer (Hitachi) the 0.5Co–BVO sample also showed the mixed phases of BiVO4,
with an excitation wavelength of 315 nm.
Bi2O3, Co2V2O7, and Co3V2O8 (JCPDs 74-1487). The presence of
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Fig. 1 XRD patterns (A) and Raman spectra (B) of the pristine BiVO4 and Co-doped BiVO4 samples.
Bi2O3 could be due to the hydrolysis of Bi(NO3)3.36 Co2V2O7 and symmetric V–O stretching mode ys(V–O)), which were similar to
Co3V2O8 could be formed when the excess of Co2+ content under those of the m-s BiVO4 phase. It was indicated that the existence
hydrothermal condition.37 In addition, there was no peak of Co ion did not affect the crystal phase of BiVO4, which was
indexed to the cobalt oxide or cobalt hydroxide for all Co-doped conrmed by the XRD results. Also, a downtrend according to
BiVO4 samples, suggesting the possible doping of the cobalt the increase of doped ion content in the intensity the symmetric
ions into the lattice of BiVO4 instead of loading to the BiVO4 V–O stretching mode was observed (Fig. S2†), which was caused
surface as hydroxide or oxide crystal grains. For the 0.2Co–BVO by the weak deformation of the VO4 tetrahedron. A similar effect
sample, we also observed that the peak at 28.9ꢁ had slightly was reported previously in terms of Fe-doped BiVO4 (ref. 31) and
shied to a higher angle (Fig. S1†), implying that Co could be Ce-doped BiVO4,30 in which this weak deformation could be due
doped into the lattice of BiVO4 crystal.31,32
to the metal ion substitution at the Bi3+ site in the lattice of the
The crystallite size (d, nm) of Bi1ꢀxCoxVO4+d samples were BiVO4 crystal.
evaluated from {121} peak of XRD patterns using the Scherrer's
The presence of substitution sites of Co in Co-doped BiVO4
equation.14,30 The d values of the Bi1ꢀxCoxVO4+d samples are was also validated by the XPS spectrum. The survey and high-
presented in Table 1. As shown in Table 1, the crystallite size of resolution XPS spectrum of the pristine BiVO4 and 0.2Co–BVO
Co-doped BiVO4 slightly increased compared to that of bare photocatalysts are shown in Fig. 2A–F. Meanwhile, character-
BiVO4 sample, implying that the presence of Co2+ could inu- istic peaks of Bi, V, and O were present for pristine BiVO4 and
ence the formation of crystalline particles of BiVO4.
0.2Co–BVO photocatalysts exhibiting Bi, V, O, and Co peaks
The as-synthesized samples then had their phase structure (Fig. 2A). In addition, according to the peak area, the molar
conrmed by Raman spectra as plotted in Fig. 1B. It was ratios of Bi/Co in 0.2Co–BVO photocatalysts reached the value
observed that all samples expressed main Raman shis at of 3.59/1, which is close to the estimated value of 4/1, indicating
around 131 cmꢀ1 (external mode), 212 cmꢀ1 (external mode), well-dispersion of Co in BiVO4. The C 1s peak at 284.6 eV
326 cmꢀ1 (the asymmetric bending mode das(VO43ꢀ)), 368 cmꢀ1 (Fig. 2B), assigning to the signal from carbon in the instrument,
(the symmetric bending mode ds(VO43ꢀ)), 639 cmꢀ1 (the asym- was used for calibration. Co 2p could be deconvoluted into two
metric V–O stretching mode y0asðV ꢀ OÞ), 709 cmꢀ1 (the asym- pairs of individual peaks at 780.9 eV (Co 2p3/2) and 797.3 eV (Co
metric V–O stretching mode yas(V–O)), and 832 cmꢀ1 (the 2p1/2) (Fig. 2C), which were identied as the major binding
energies (BE) of Co2+
.
As also presented in Fig. 2C, the BE
38
indexed to the cobalt metal or cobalt oxides was not detected,
implying that Co could be doped into the lattice of BiVO4
crystal. Bi 4f spectrum of BVO (Fig. 2D) showed characteristic
peak of Bi 4f5/2 and Bi 4f7/2 at 164.1 eV and 158.7 eV respec-
tively, with DE ¼ Bi 4f5/2 ꢀ Bi 4f7/2 of 5.4 eV. Similarly, Bi 4f5/2
(164.2 eV) and Bi 4f7/2 (158.9 eV) with DE of 5.3 eV were also
observed in the Bi 4f spectrum of 0.2Co–BiVO4 samples
(Fig. 2D). Aer the Gaussian–Lorentzian tting, the
asymmetric V 2p3/2 spectra can be deconvoluted into two pairs
Table 1 The physical properties and photocatalytic activity of the bare
BiVO4 and the Co-doped BiVO4 samples
Samples
d (nm)
Eg (eV)
kapp (10ꢀ3 minꢀ1
)
R2
BVO
25.45
29.09
28.65
31.88
30.31
2.31
2.30
2.29
2.26
2.14
0.775
1.185
1.882
0.633
0.333
0.99742
0.95483
0.98230
0.98225
0.98070
0.1Co–BVO
0.2Co–BVO
0.3Co–BVO
0.5Co–BVO
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Fig. 2 The survey and high-resolution XPS spectrum of the pristine BiVO4 and 0.2Co-BVO.
of individual components referring to the surface V4+ and V5+ doping Co2+ replacing some Bi3+ positions in the internal lattice
states; they were 515.6 and 516.8 eV for BVO, and 515.0 and structure of BiVO4, implying the formation of Co–O–Bi bonds.
516.7 eV for 0.2Co–BiVO4 (Fig. 2E). The V4+/V5+ molar ratios of Also, the varied Oads/Olatt molar ratio was 0.49 for 0.2Co–BVO
BVO and 0.2Co–BVO samples were 0.165 and 0.214, respec- and 0.24 for the bare BiVO4 sample. These results indicated that
tively. These results showed the slight shi of Bi 4f and V 2p for Co-doped BiVO4 further increases the content of Oads and
the 0.2Co–BVO sample as compared with the BVO sample, oxygen vacancy, which are possibly responsible for improving
implying the interaction of Co2+ with Bi3+ and V5+ ions. More- the photocatalytic performance of BiVO4, as discussed below.
over, considering to the ion radius of Co2+, V5+, and Bi3+, the ion
3+
radius of Co2+ (0.79 A)32 was smaller than that of the Bi (1.03
˚
3.2 Morphological characteristics
A) but larger than that of V5+ (0.36 A);
thus, doping with
30
29,39
˚
˚
Co2+ tended to replace some Bi3+ in the internal lattice structure
of BiVO4. This outcome was also supported by Bo Zhang et al.
when using periodic density functional calculations within the
GGA + U approach for calculating the formation energy.33
Fig. 2F displays O 1s spectrum of the pure BiVO4 sample with
the BE positions at 529.8 eV, 531.4 eV, and 532.5 eV, which
indexed to the lattice oxide (V–O or Bi–O bonds) in BiVO4 (Olatt),
the hydroxide hydrated or defective oxide (Oads) species, and the
surface adsorbed water, respectively.30 As also shown in Fig. 2F,
no other BE positions were observed due to the BE of V–O and
Bi–O bonds being approximately the same. In terms of 0.2Co–
BVO sample, aer the Gaussian–Lorentzian tting, the O 1s
spectrum was also deconvoluted into three pairs of individual
components: (1) the lattice oxide at BE of 530.1 eV; (2) the
hydroxide hydrated or defective oxide at BE of 531.5 eV; (3) and
the surface adsorbed water at BE of 533.2 eV. Notably, these
peaks were slightly shied to the high BE region in comparison
to that of the bare BiVO4. This outcome could be due to the
The presence of Co also considerably altered the morphology of
the as-synthesized samples, as exhibited in Fig. 3. The bare
BiVO4 sample consisted of large quantities of corner-cut trun-
cated bipyramid and granular like structures with smooth
surfaces (Fig. 3a). The 0.05Co–BVO (Fig. S3A†), 0.1Co–BVO
(Fig. S3B†), and 0.2Co–BVO (Fig. 3b) were morphologically
similar to the Co-doped BiVO4 sample. However, the surface of
these Co-doped BiVO4 sample (Fig. 3b) was much rougher than
those of the bare BiVO4 sample and covered amorphous parti-
cles. Consistent with the results of the XRD patterns, the 0.3Co–
BVO (Fig. S3C†) and 0.5Co–BVO (Fig. S3D†) samples mostly had
amorphous nanoparticles because of the presence of the
mixture containing BiVO4, Bi2O3, Co2V2O7, and Co3V2O8. Also,
as presented in Fig. 4, the EDS-Mapping analysis of Bi, V, O, and
Co in the 0.2Co–BVO was both in line with the aforementioned
XPS results and indicative of uniform distribution over the
BiVO4 surface. The TEM images of 0.2Co–BVO sample (Fig. S4†)
also conrms that the crystal of 0.2Co–BVO sample was not
homogeneous with different shapes and sizes.
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Fig. 4 Element mapping images of 0.2Co–BVO sample: SEM image
(a), Bi element (b), V element (c), O element (d) and Co element (e).
Fig. 3 SEM images of the (a) BVO and (b) 0.2Co–BVO samples.
samples was improved due to the efficient separation and
3.3 Optical absorption properties
transfer of photoinduced charges.
UV-vis DRS spectra of the Bi1ꢀxCoxVO4+d samples are displayed
in Fig. 5A. All samples demonstrated intensive absorption
bands in the visible light region with the absorption edges of
550–600 nm, which ascribed to the electron transfer from the
valence band (VB) to the conduction band (CB). The direct band
gap energy (Eg, eV) of the as-synthesized photocatalysts were
derived from the tangent in the Tauc plots when plotting the
(ahn)2 function with respect to hn (a, h, and n are the absorption
coefficient, Plank's constant, and incident light frequency,
respectively) as shown in Fig. 5B.13,30,40,41 The Eg values of the
Bi1ꢀxCoxVO4+d samples are presented in Table 1. Accordingly,
the Eg appeared to be slightly declining with increasing Co
molar ratio. This behavior indicated the slightly narrowed
BiVO4 band gap, which could be due to Co incorporation and
the introduction of an energy level (Ef) near the top of the VB in
the forbidden band gap (Fig. 11), caused by substitution of
cobalt ions to Bi3+ in BiVO4.33
The separation and transfer of photoinduced charges in
Bi1ꢀxCoxVO4+d semiconductors could be claried by PL spectra
as presented in Fig. 6. For the undoped BiVO4 catalyst, the
emission peak at an excitation wavelength of 340 nm was
observed at around 478 nm. For 0.2Co–BVO, the absence of an
emission peak indicated that Co incorporation, due to the
increased electron mobility, could prevent photogenerated
electron–hole pairs from recombining. As a result, it was
apparent that the photocatalytic activity of the Bi1ꢀxCoxVO4+d
3.4 Photocatalytic performance
To demonstrate the improvement in photocatalytic perfor-
mance of Co-doped BiVO4 catalysts in comparison with the bare
BiVO4, we conducted the photodegradation of MB under white
LED light irradiation using the pristine BiVO4 and Co-doped
BiVO4 samples. Fig. 7 shows UV-vis absorption spectra of MB
solution isolated from a reaction mixture containing 0.2Co–
BVO catalyst, water, and MB. As shown in Fig. 7, the intensity of
the absorption spectra decreased aer the rst 20 min in the
dark and then remained unchanged aer 60 min in the dark,
indicating the adsorption–desorption equilibrium between the
surface of the 0.2Co–BVO sample and the MB molecules. When
visible light was provided, the intensity of the absorption peak
(664 nm) dramatically declined, and the position of this peak
tended to shi toward a shorter wavelength. These signs were
attributed to the photodegradation of MB molecules, which was
caused by 0.2Co–BVO catalyst.
Fig. 8A displays the removal efficiency of MB by the pristine
BiVO4 and Co-doped BiVO4 samples. In comparison with the
bare BiVO4 samples, Co-doped BiVO4 catalysts exhibited
signicantly improved photocatalytic activity aer 180 min of
irradiation in response to the incorporation of Co. The degra-
dation rate of 0.2Co–BVO sample reached its highest value
(about 98%) while the gure was 72% for the bare BiVO4
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Fig. 7 UV-vis absorption spectra of MB solution separated from
catalyst suspensions during illumination using 0.2Co–BVO.
of Bi2O3, Co2V2O7, and Co3V2O8 phase in 0.3Co–BVO and 0.5Co–
BVO samples as described by the above XRD analysis. In addi-
tion, we employed the pseudo-rst-order kinetic equation (ln(C/
Fig. 5 UV-vis DRS spectra (A) and band gap energy (B) of the pristine
BiVO4 (a), 0.05Co–BVO (b), 0.1Co–BVO (c), 0.2Co–BVO (d), 0.3Co–
BVO (e), and 0.5Co–BVO (f).
sample. However, when further increasing Co content (0.3Co–
BVO and 0.5Co–BVO samples), the degradation rate witnessed
a sharp decline. Such an outcome could be due to the formation
Fig. 8 Photocatalytic degradation of MB over BiVO4 samples (A):
BiVO4 (a), 0.05Co–BVO (b), 0.1Co-BV (c), 0.2Co-BV (d), 0.3Co-BV4
Fig. 6 Room temperature PL spectra of the pristine BiVO4 (a) and (e), and 0.5Co-BV (f) and plots of ln(Co/C) versus irradiation time
0.2Co–BVO (b).
representing the fit using a pseudo-first-order reaction rate (B).
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Co) ¼ kt; Co and C are the concentration of MB at 0 min and photodegradation rate is expected in the presence of BQ and
t min of irradiation, respectively) to t the degradation of MB K2Cr2O7. This observation implies that the presence of two
(Fig. 8B) and obtain the reaction rate constant (k, 10ꢀ3 minꢀ1) of dominant active oxidative species, namely $OH and h+, directly
the as-synthesized samples.28 The k values of Bi1ꢀxCoxVO4+d oxidized MB molecules adsorbed on the surface of the 0.2Co–
samples are presented in Table 1, indicating that the 0.2Co– BVO catalyst. Similarly, trapping experiments of photocatalytic
BVO sample exhibited superior photocatalytic performance.
degradation of MB over the pure BiVO4 sample (Fig. S7†) also
A comparison table for photocatalytic MB degradation indicated that $OH and h+ are the main active oxidative species.
performance of the as-synthesized Co-doped BiVO4 with other Therefore, the substitution of cobalt ions to Bi3+ in the BiVO4
materials reported in the literature was also conducted (Table did not inuence the dominant oxidative species.
S1†). As shown in Table S1,† the degradation rate of MB over Co-
The essential role of $OH radicals in the photocatalytic
doped BiVO4 samples was equal to or higher than those re- processes was further proved by the photoluminescence spectra
ported from previous studies, which further indicated that Co- of H2BDC-OH (Fig. 10). As displayed in Fig. 10, the intensity of
doped BiVO4 could be a promising photocatalyst for the H2BDC-OH caused by Co–BVO samples was enhanced as
degradation of organic dyes.
compared to the intensity of H2BDC-OH caused by the bare
To investigate the stability of the 0.2Co–BVO catalyst, ve BiVO4 sample. The highest intensity of H2BDC-OH caused by
cycling photocatalytic tests were conducted by collecting and the 0.2Co–BVO sample corresponded to the peak photocatalytic
reusing the same photocatalyst. The result (Fig. S5†) exhibited activity due to the increased formation of $OH radicals on the
an only negligible decrease in photocatalytic activity aer ve surface of this sample.
runs, which may have been caused by an incomplete collection
The complete mechanism for photodegradation over Co-
of the catalyst aer each runs. Moreover, no obvious differences doped BiVO4 catalyst was suggested based on prior investiga-
were observed in the XRD results of the 0.2Co–BVO catalyst tions3,6,42 and illustrated in Fig. 11. The process included four
before/aer ve runs (Fig. S6†), suggesting the excellent steps: (i) the generation of photo-excited electrons (eꢀ) from the
stability and reusability of the 0.2Co–BVO catalyst.
VB into the CB of Bi1ꢀxCoxVO4+d when visible LED light was
provided; (ii) the corresponding holes (h+) leaving the VB; (iii)
the photo-excited electrons (eꢀ) being trapped by the acceptor
defects (Co2+); (iv) the holes (h+) being transferred to the surface
to oxidize the H2O and OHꢀ to $OH radicals which played an
essential role in the photocatalytic oxidation process. In
comparison with the pristine BiVO4 samples, the fabrication of
Co-doped BiVO4 catalyst could encourage the efficient separa-
tion and transfer of photoinduced charges, corresponding to
production of a signicant amount of $OH radicals for degra-
dation of MB. As a result of this success, the photocatalytic
performance of BiVO4 was signicantly enhanced.
3.5 Photodegradation mechanism
To explore the mechanism for the reactive species responsible
for MB degradation over 0.2Co–BVO, radical trapping experi-
ments in the photodegradation were performed (Fig. 9). The
experimental steps were similar to the above photocatalytic
activity test; however, radical scavengers were added to the
reaction solution at the beginning of light irradiation.
Employed radical scavengers included tert-butanol (TBA),
ammonium oxalate monohydrate (AO), 1,4-benzoquinone (BQ)
and potassium dichromate (K2Cr2O7) for hydroxyl radicals
In addition, as described above, the photocatalytic perfor-
mance of the Bi1ꢀxCoxVO4+d samples were not linear to the Co
ꢃ
($OH), hole (h+), superoxide ðO2ꢀ Þ, and electron (eꢀ). Following
the addition of TBA and AO, the photodegradation rate showed
a
marked decline. Meanwhile, the reduction in the
Fig. 10 Photoluminescence spectrum of H2BDC-OH that formed
Fig. 9 Trapping experiments of photocatalytic degradation of MB over upon the reaction between the $OH radicals produced by the pristine
0.2Co–BVO with the presence of scavengers: TBA (a), AO (b), BQ (c) BiVO4 (a), 0.05Co–BVO (b), 0.1Co–BVO (c), 0.2Co–BVO (d), 0.3Co–
and K2Cr2O7 (d), and without the presence of scavenger (e).
BVO (e) and 0.5Co–BVO (f) and H2BDC.
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Fig. 11 An entire mechanism for photocatalytic degradation: (a) BiVO4 and (b) Co-doped BiVO4.
ion contents, but tended to increase rst and then steadily
Acknowledgements
decline. This downward trend could be attributed to the
formation of Bi2O3, Co2V2O7, and Co3V2O8 as a result of the
excessive doping Co ion content. Moreover, the excessively
doped Co ion content could lead to the fast recombination of
electron–hole pairs.43 Wang et al. also indicated the excessive
doping metal content could occupy the space charge and hinder
the penetration depth of irradiation into BiVO4 as result of fast
recombination of electron–hole pairs and poor photoactivity.44
Our work rst reported Co-doped BiVO4 materials synthe-
sized according to the chemical formula Bi1ꢀxCoxVO4+d (0.05 < x
< 0.5). Based on the XRD, Raman, and XPS results, as well as the
previous studies, we could conclude that the substitution of
Co2+ ions for Bi3+ sites in BiVO4, resulting in the improvement
of the photocatalytic activity of BiVO4.
This research is funded by Vietnam National Foundation for
Science and Technology Development (NAFOSTED) under grant
number 104.05-2017.315.
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Monoclinic scheelite phase Bi1ꢀxCoxVO4+d have been success-
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There are no conicts to declare.
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23534 | RSC Adv., 2019, 9, 23526–23534
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