Ag2O-decorated electrospun BiVO4 nanofibers with enhanced photocatalytic performance

Article abstract of DOI:10.1039/c9ra10952h

Semiconductor photocatalysts are emerging as tools for pollutant degradation in industrial wastewater, air purification, antibacterial applications, etc. due to their use of visible light, which is abundant in sunlight. Here, we report a new type of p-n junction Ag₂O/BiVO₄ heterogeneous nanostructured photocatalyst with enhanced photocatalytic performance. P-type Ag₂O nanoparticles were in situ reduced and assembled on the surface of electrospun BiVO₄ nanofibers using ultraviolet (UV) irradiation; this process hindered the recombination of localized photogenerated electron-hole pairs, and hence resulted in the enhanced photocatalytic activity of the BiVO₄/Ag₂O nanocomposites. The photocatalytic activities of the obtained BiVO₄ and BiVO₄/Ag₂O nanocomposites were assessed by measuring the degradation of rhodamine B (RhB) under visible light. The 10 wt% Ag₂O/BiVO₄ sample yielded the optimum degradation of RhB (98.47%), much higher than that yielded by pure BiVO₄ nanofibers (64.67%). No obvious change in the XRD pattern of an Ag₂O/BiVO₄ sample occurred as a result of its use in the photocatalytic reaction, indicating its excellent stability. The high photocatalytic performance observed was attributed to the large surface-to-volume ratio of the essentially one-dimensional electrospun BiVO₄ nanofibers and to the in situ growth of p-type Ag₂O on the surface of the n-type BiVO₄ nanofibers.

Full text of DOI:10.1039/c9ra10952h

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Ag2O-decorated electrospun BiVO4 nanofibers
with enhanced photocatalytic performance†
Cite this: RSC Adv., 2020, 10, 6114
*
*
and Yongyong Zhu
Junpeng Ren
Semiconductor photocatalysts are emerging as tools for pollutant degradation in industrial wastewater, air
purification, antibacterial applications, etc. due to their use of visible light, which is abundant in sunlight.
Here, we report a new type of p–n junction Ag2O/BiVO4 heterogeneous nanostructured photocatalyst
with enhanced photocatalytic performance. P-type Ag2O nanoparticles were in situ reduced and
assembled on the surface of electrospun BiVO4 nanofibers using ultraviolet (UV) irradiation; this process
hindered the recombination of localized photogenerated electron–hole pairs, and hence resulted in the
enhanced photocatalytic activity of the BiVO4/Ag2O nanocomposites. The photocatalytic activities of the
obtained BiVO4 and BiVO4/Ag2O nanocomposites were assessed by measuring the degradation of
rhodamine B (RhB) under visible light. The 10 wt% Ag2O/BiVO4 sample yielded the optimum degradation
of RhB (98.47%), much higher than that yielded by pure BiVO4 nanofibers (64.67%). No obvious change
in the XRD pattern of an Ag2O/BiVO4 sample occurred as a result of its use in the photocatalytic
reaction, indicating its excellent stability. The high photocatalytic performance observed was attributed
to the large surface-to-volume ratio of the essentially one-dimensional electrospun BiVO4 nanofibers
and to the in situ growth of p-type Ag2O on the surface of the n-type BiVO4 nanofibers.
Received 27th December 2019
Accepted 25th January 2020
DOI: 10.1039/c9ra10952h
rsc.li/rsc-advances
development.15,16 Nevertheless, by controlling the morphologies
and structures of BiVO4 and BiVO4 materials modied with noble
1. Introduction
metals and its oxides, further improvements of the photocatalytic
performances of BiVO4 materials can be realized.17 With regards
to the control of morphology, in comparison with decahedral
BiVO4 structures and BiVO4 nanorods, nanobelts, nanotubes and
nanospheres, one-dimensional (1D) BiVO4 nanobers (NFs)
display many advantages: (1) large surface-to-volume ratios,
which could prolong the photocarrier lifetime; (2) very large
aspect ratios, forming a unique charge transfer path along the 1D
direction; and (3) abundant surface active sites, which could
provide many active sites for subsequent surface modications of
the BiVO4.18–24 As for modications with noble metals, doping n-
type noble metals oxides onto the surfaces of p-type bismuth
vanadate semiconductors is a feasible way to construct p–n het-
erojunctions with enhanced photocatalytic performances.25–28
Based on these considerations, we prepared p–n junction
Ag2O/BiVO4 heterogeneous structures by in situ reducing Ag2O
nanoparticles on the surfaces of 1D electrospun BiVO4 NFs. As-
obtained 10 wt% Ag2O/BiVO4 NFs showed enhanced photo-
catalytic activity towards degradation of rhodamine B (RhB)
pollutants under visible light (photocatalytic efficiency: 98.47%
within 100 minutes). Moreover, aer three photodegradation
cycles, the degradation efficiency was still about 90%, indi-
cating its outstanding cycle stability. Theoretical studies indi-
cated this enhanced photocatalysis to be due to the 1D nature of
the electrospun BiVO4 NFs and due to the construction of p–n
junction Ag2O/BiVO4 heterogeneous structures.
Metal oxide semiconductor photocatalysts have been used to
greatly alleviate such problems as water pollution and those
involving the environment and energy issues by using visible
light of the solar spectrum, in particular collecting it to generate
charge carriers, and promoting charge separation and trans-
fer.1–5 TiO2 is an excellent photocatalyst and is also strongly
oxidizing, chemically stable, inexpensive and non-toxic, and has
been widely studied.6,7 However, the band gap of TiO2 is
approximately 3.2 eV, which leads to a rapid recombination of
the photogenerated electron–hole pairs and absorption of only
ultraviolet (UV) light (only about 4% of total sunlight), and
hence TiO2 is not a suitable and effective photocatalyst for water
purication.8,9 Recently, n-type bismuth vanadate (BiVO4) with
a monoclinic scheelite-like structure has become considered as
a promising class of photocatalysts owing to its narrower band
gap of 2.4 eV compared to that of TiO2, resulting in an easy
separation of photogenerated electrons from holes and excel-
lent photocatalytic performance.10–14
The morphology, structure and specic surface area of BiVO4
materials obtained via traditional approaches, however, have not
met the requirements for more efficient photocatalysis
College of Weapons Engineering, Naval University of Engineering, Wuhan, 430033
China. E-mail: 693301875@qq.com; zyy99515@126.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra10952h
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bers with an average diameter of 500 nm. Fig. S1b–d† display the
SEM images acquired of the BiVO4 precursors calcinated at 400 ꢀC,
500 ꢀC and 600 ꢀC. Inspection of these images showed the
roughness and variation in diameter of the BiVO4 NFs aer calci-
nation, features attributed to the volatilization of the DMF and
PVP. When the temperature reached 500 ꢀC, many pores were
observed evenly distributed on the surfaces of the BiVO4 NFs, and
the average NF diameter decreased to about 250 nm. When the
annealing temperature was further increased, the NFs gradually
broke apart, so we chose the BiVO4 NFs annealed at 500 ꢀC as the
template to grow Ag2O particles. The XRD of the BiVO4 NFs
(Fig. S2†) calcinated at the three temperatures also demonstrated
2. Experimental
2.1. Synthesis of BiVO4 NFs
BiVO4 NFs were synthesized by employing the electrospinning
method. In a typical process, 1.21 g of Bi(NO3)3$5H2O and 0.662 g
of vanadyl acetylacetonate (VO(acac)2) were dissolved in 5 mL of
N,N-dimethylformamide (DMF) under magnetic stirring for 2 h.
Aerwards, a mass of 0.7 g of polyvinyl pyrrolidone (PVP) was
added to the above mixture under magnetic stirring for 8 h to
form a bottle green precursor spinning solution. This solution
was drawn up into a 10 mL plastic syringe via a stainless steel
needle with an inner diameter of 0.5 mm, and then a voltage of 18
kV was applied over a distance of 15 cm between the needle tip
and an aluminium foil collector. Aer carrying out the collection
process for 1 h, the generated precursor was dried in a vacuum
ꢀ
that the crystallinity of the NFs annealed at 500 C was the best.
The acquired SEM images of the Ag2O/BiVO4 NFs are shown in
Fig. 1b and c. Ag2O particles were observed to be uniformly
distributed on the BiVO4 NFs. Fig. 1d shows the acquired SEM
image of a single Ag2O/BiVO4 NF and images of its corresponding
elemental mapping. The dot mapping images of Bi, V, O, and Ag
followed closely the backbone of the NF, suggesting the successful
synthesis of the Ag2O/BiVO4 NFs. A closer TEM observation of an
Ag2O/BiVO4 NF (Fig. 1e) indicated that the equally distributed Ag2O
particles did not change the ber structure of BiVO4. Fig. 1f shows
a high-resolution TEM (HRTEM) image of a marginal area of the
Ag2O/BiVO4 NF shown in Fig. 1e, and indicated the monoclinic
structure of the as-synthesized BiVO4 NFs and Ag2O particles. The
lattice constant measured from the scheelite-like structure of the
BiVO4 NF was 0.308 nm, in agreement with the (112) planes of the
BiVO4 phase.13 A lattice spacing value of 0.236 nm was assigned to
the (200) planes of the Ag2O phase. The selected area electron
diffraction pattern (Fig. 1g) acquired from an Ag2O/BiVO4 NF
showed the same good crystallinity and nanocomposite structure.
The energy dispersive X-ray (EDX) spectrum acquired of an Ag2O/
BiVO4 NF is shown in Fig. 1h, and revealed that the NF was
composed of Bi, V, O and Ag elements. Fig. 1i shows thermogra-
vimetric analysis and differential scanning calorimetry (TGA/DSC)
curves indicative of the synthesis of porous BiVO4 NFs.30 Analysis
of the acquired X-ray diffraction (XRD) pattern (Fig. 1j) further
conrmed that the as-synthesized NFs were monoclinic scheelite-
like-structured BiVO4 NFs (JCPDS card no. 75-2480).17,31,32 None of
the XRD patterns of the samples showed any characteristic
diffraction peak corresponding to Ag2O particles, attributed to the
small quantities of Ag2O in the as-synthesized samples.
ꢀ
oven at 60 C for 12 h. Finally, the dried BiVO4 precursor was
calcinated in a muffle furnace set at various temperatures
(400 ꢀC, 500 ꢀC, 600 ꢀC) each for 1 h and with a constant heating
rate of 1 ꢀC minꢁ1 between these levels.29
2.2. In situ growth of Ag2O nanoparticles on the BiVO4 NFs
Ag2O nanoparticles were in situ reduced and assembled on the
surfaces of BiVO4 NFs by applying ultraviolet (UV) irradiation. First,
in each of series of samples, a mass of 0.1 g of as-prepared BiVO4
calcinated at 500 ꢀC was dispersed in 40 mL of deionized water. To
these samples were added AgNO3 at different mass fractions, of
3%, 5%, 10% and 15%, respectively, to form suspensions under
magnetic stirring. The suspensions were stirred under UV light
illumination for 5 h. The nal powders were washed with deion-
ized water several times, and then dried at 60 ꢀC in the air for 12 h.
2.3. Characterizations
The morphologies, structures and crystalline forms of BiVO4
NFs and Ag2O-doped BiVO4 NFs were investigated by using
a
scanning electron microscope (SEM, MAGELLIAN-400),
transmission electron microscope (TEM, JEM-2200FS), and X-
ray powder diffractometer system (XRD, Rigaku D/max-2550),
respectively. The change in the mass of a BiVO4 precursor as
a function of temperature and time was assessed by acquiring
thermogravimetric analysis and differential scanning calorim-
etry (TGA/DSC) curves in the isothermal mode using a TA SDT
Q600 thermogravimetry analyzer. Raman spectra were acquired
by using a Raman spectroscopy apparatus (Spex 1403) with
a 514 nm-wavelength laser to identify the molecular structures
of the samples. X-ray photoelectron spectroscopy (XPS) data
were measured with an ESCALAB 250 electron spectrometer to
characterize the surface chemical states of the samples. An
absorption spectrum was obtained with a UV-Vis spectropho-
tometer (UV-2450), using BaSO4 as a reference.
XPS data were collected to monitor the elemental composition
of the nanocomposites and valence states of the elements, as
shown in Fig. 2. Fig. 2a shows the survey XPS spectrum of Ag2O/
BiVO4 NFs. Analysis of this spectrum indicated that the samples
consisted of Bi, V, O and Ag elements, in agreement with the
results of mapping images and the EDX spectrum. The acquired
high-resolution XPS spectrum of the Ag2O/BiVO4 NFs in the Bi
4f, V 2p, O 1s and Ag 3d regions are shown in Fig. 2b–e, respec-
tively. In the Bi 4f region (Fig. 2b), the two peaks located at
159.35 eV and 164.67 eV can be attributed to the binding energies
of Bi 4f5/2 and Bi 4f7/2 with a spin–orbit splitting energy of 5.32 eV,
3. Results and discussion
Fig. 1a and S1a† show the SEM images acquired of the BiVO4 demonstrating that element Bi was present in the form Bi3+.
precursor. Inspection of the images showed a very smooth surface Fig. 2c displays the V 2p region of the spectrum, which can be
of the BiVO4 precursor, attributed to it containing the high- deconvoluted into two sets of peaks centered at 517.3 eV and
molecular-weight PVP template. The BiVO4 precursor formed 524.9 eV, corresponding to V 2p3/2 and V 2p5/2 with a spin–orbit
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Fig. 1 (a) An SEM image of an obtained BiVO4 precursor. (b and c) Typical SEM images of Ag2O-doped BiVO4 NFs. (d) An SEM image of a single
Ag2O/BiVO4 NF with the corresponding Bi, V, O and Ag elemental mapping images. (e) A TEM image and (f) high-resolution TEM (HRTEM) image
of an Ag2O/BiVO4 NF. (g) Selected area electron diffraction (SEAD) pattern and (h) EDAX data of the Ag2O/BiVO4 NF taken from the area marked
in (b). (i) TGA/DSC curves of Ag2O/BiVO4 NFs (TGA curve: red; DSC curve: black; heating range: 30–600 ꢀC with a rate of 5 ꢀC minꢁ1). (j) XRD
patterns of pure BiVO4 and Ag2O/BiVO4 composites.
Fig. 2 High-resolution XPS spectra of 10 wt% Ag2O/BiVO4 samples: (a) the survey spectrum and (b) Bi 4f, (c) V 2p, (d) O 1s, and (e) Ag 3d spectra.
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splitting energy of 7.6 eV.33,34 In comparison with XPS spectra of photocurrent–time curves of Ag2O/BiVO4 NFs with various mass
pure BiVO4 NFs (Fig. S3†), no obvious change was found in the ratios are displayed in Fig. 3b. These curves showed that the
spin–orbit splitting energy values of the Bi and V elements, but photocurrent density of the 10% Ag2O/BiVO4 NFs was greater
for the O element, a new peak at 534.7 eV and attributed to Ag2O than that of the samples with other proportions of the compo-
appeared in the O 1s region of the XPS spectrum of the hybrid nents and with the same illumination time. This result can be
composites. To further determine the valence state of the attributed to n-type Ag2O in moderate amounts being able to
elements Ag, the high-resolution XPS spectrum in its Ag 3d hinder photonic electron–hole pairs from recombining and
region (Fig. 2e) was further analyzed. Two observed peaks at excess Ag2O nanoparticles in the nanocomposites forming
374.9 eV and 368.9 eV can be ascribed to the binding energies of a load center of carriers and hence impeding the separation of
Ag 3d3/2 and Ag 3d5/2 with a spin–orbit splitting energy of 6 eV, photogenerated electrons and holes.
consistent with the chemical state of Ag2O. All above analysis
Fig. 4a shows the degradation of an RhB pollutant under
results taken together showed that the Ag2O nanoparticles visible light irradiation using pure BiVO4 NFs as well as Ag2O/
became tightly bound to the surfaces of the BiVO4 NFs, benecial BiVO4 samples with various mass ratios. No obvious degrada-
for the photocatalytic performance.
tion was observed in the absence of any photocatalyst. When
Aer characterizing the elementary morphologies and any of the as-prepared Ag2O/BiVO4 photocatalysts were added,
structures of the Ag2O/BiVO4 NFs, photocatalytic performance the RhB pollutant almost fully degraded aer irradiation of
studies were systematically conducted. First, UV-Vis diffuse 100 min. In the presence of pure BiVO4, 3%, 5%, 10%, and 15%
absorption spectra (UV-Vis DRS) of as-prepared samples were Ag2O/BiVO4, 64.67%, 90.2%, 96.65%, 98.47%, and 96.65%,
acquired, as shown in Fig. 3a and S4,† to investigate their photo- respectively, of the initial amount of RhB degraded within the
absorption properties. According to the UV-Vis DRS of pure 100 minutes. The 10% Ag2O/BiVO4 NFs exhibited the highest
BiVO4 shown in Fig. S4,† the sample showed relatively strong activity, with the degradation rate increased by 32.8% compared
absorption of light at wavelengths of 300 to 450 nm, but to that of the pure BiVO4 NFs. The kinetic curves of RhB
weakening absorption as the wavelength of the light was degradation are displayed in Fig. 4b, and indicated that the
increased above 450 nm, indicating that BiVO4 by itself can only degradation can be approximated as a pseudo-rst-order
be excited by UV light. Aer BiVO4 was decorated with Ag2O process following the linear transform ln(C0/Ct) ¼ kt, where k
nanoparticles, the UV-Vis DRS of the resulting Ag2O/BiVO4 NFs is the degradation rate constant.35,36 All of the Ag2O/BiVO4
(Fig. 3a) showed a signicant red-shi of the absorption composites exhibited higher photocatalytic activities than did
compared to that of the pure BiVO4 NFs. All the composites with the bare BiVO4. The degradation rate constant (k) of the Ag2O/
different mass ratios exhibited enhanced absorption of visible BiVO4 with
a 10% mass ratio was calculated to be
light, which can be attributed to the heterojunction formed 0.03908 minꢁ1, a value quite remarkably about 1.62, 1.23 and
between Ag2O and BiVO4 NFs having changed the optical 1.96 times those for the Ag2O/BiVO4 samples with 3%, 5% and
properties of the photocatalyst, and suggesting that it would 15% Ag2O loads, respectively. The above two results indicated
display outstanding activity in the visible light region. The p– that decoration of Ag2O nanoparticles on the surface of BiVO4
n heterojunction effectively decreased the coupling of the NFs did indeed improve the photocatalytic properties, and
photogenerated electron–hole pairs, thus resulting in the indicated that using the appropriate mass ratio of Ag2O in the
enhanced visible light absorption. The corresponding as-prepared composites was crucial to the synergistic effects
photocurrent density was measured in
a three-electrode during the photocatalytic reaction. Therefore, we chose 10% of
system in a 0.5 M K2SO4 aqueous electrolyte with a Pt counter Ag2O/BiVO4 for further study. To investigate the effect of pho-
electrode, an Ag/AgCl reference electrode, and as-prepared tocatalyst dosage on the photocatalytic performances, photo-
Ag2O/BiVO4 NFs serving as the working electrode. The catalytic reductions of RhB using various masses of 10% Ag2O/
Fig. 3 (a) UV-Vis diffuse absorption spectra of pure BiVO4 photocatalysts and Ag2O/BiVO4 samples with various mass ratios of the components.
(b) The corresponding photocurrent density curves.
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Fig. 4 (a) Photocatalytic reductions of RhB using pure BiVO4 photocatalysts and Ag2O/BiVO4 samples with various mass ratios. (b) The linear
relationships of ln(Ct/C0) versus time for the degradation of RhB. (c) Photocatalytic reductions of RhB in the presence of various masses of 10 wt%
Ag2O/BiVO4 photocatalyst under visible-light irradiation. (d) Photocatalytic reductions of various concentrations of RhB using 10 wt% Ag2O/
BiVO4.
BiVO4 photocatalysts under visible-light irradiation were per- outstanding cycle performances. Fig. 5b displays the acquired
formed, as shown in Fig. 4c. Notably, a mass of 5 mg of the XRD spectra of the Ag2O/BiVO4 photocatalyst before and aer
Ag2O/BiVO4 photocatalyst degraded nearly all of the RhB the photocatalytic reaction. We found that the phase structure
pollutant. In contrast, when using more of the photocatalyst, of Ag2O/BiVO4 aer the photodegradation was nearly the same
the photocatalytic performance was poorer. This result was as that before the photodegradation, showing the excellent
attributed to excessive photocatalyst in the RhB solution stability and reusability of the photocatalyst.
increasing the turbidity of the solution, a feature adverse to the
In order to arrive at a best possible understanding of the
penetration of visible light. The photocatalytic reductions of mechanism of the photocatalysis of a heterostructured photo-
various concentrations of RhB using 5 mg of the Ag2O/BiVO4 catalyst system, we produced a schematic of the energy diagram
(10%) are shown in Fig. 4d. When the initial concentration of and charge separation in the Ag2O/BiVO4 NFs under visible light
the RhB solution was 20 mg Lꢁ1, the degradation efficiency of irradiation (Fig. 6). BiVO4 and Ag2O are n-type and p-type semi-
the Ag2O/BiVO4 sample was about 10% lower than that for conductors, respectively, and the combination of them can form
10 mg Lꢁ1. This result may have been due to the more a p–n heterostructure semiconductor. Under visible-light irradi-
concentrated solutions having been darker and hence showing ation, Ag2O can be excited because of its relatively narrow
worse transmittance of light, with fewer photons arriving at the bandgap of 1.3 eV, and the resulting photo-induced electrons
surface of photocatalyst and leading to the decrease of the would be expected to be able to transfer from the conduction
degradation efficiency. In addition, with the increase of the band (CB) of Ag2O to that of BiVO4 NFs. But considering the
initial concentration of the solution, the surface of the photo- higher CB potential of BiVO4 NFs than those of O2/$O2ꢁ (ꢁ0.33
catalyst would have produced more reactants, decreasing the eV) and O2/H2O2 (0.695 eV), the electrons on the CB of BiVO4 NFs
density of available active sites on the surface of the photo- cannot reduce O2 to generate $O2ꢁ, H2O2, and $OH, which are
catalyst, and hence affecting the utilization of visible light and strong oxidizing groups that, if present, can break down the
causing the decline of the degradation efficiency.37
To assess the practical applicability of the photocatalyst, the the goal of improved photocatalytic performance.34,38–40
cycling stability of the 10% Ag2O/BiVO4 hybrid composite was The possible reactions for the degradation of RhB are as
target RhB into H2O, CO2, and so on, so as to assist in reaching
also assessed, as shown in Fig. 5a. Even aer three photo- follows.
catalytic reduction processes, 90% of initial value was observed
for the Ag2O/BiVO4 photocatalyst, demonstrating its
+
)
Ag2O ꢁ BiVO4 + hv / Ag2O ꢁ BiVO4 (eCBꢁ$$$hVB
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Fig. 5 (a) Three cycles of photocatalytic reductions of RhB using 10 wt% Ag2O/BiVO4 as the photocatalyst. (b) XRD patterns of Ag2O/BiVO4
before and after photocatalysis.
ꢁ
+
ꢁ
Ag2O (eCBꢁ) + BiVO4 / Ag2O + BiVO4 (eCB
)
Here, eCB and hVB denote an electron in the CB and hole in
the VB, respectively.
ꢁ
O2 + eCB / O2cꢁ
h+ + H2O / h+ + OHc
4. Conclusions
In conclusion, a novel p–n junction Ag2O/BiVO4 heterogeneous
photocatalyst has been successfully synthesized by reducing
Ag2O nanoparticles in situ on the surfaces of 1D electrospun
BiVO4 NFs. The obtained Ag2O/BiVO4 NFs with 10% mass ratio
of Ag2O showed 98.47% photocatalytic efficiency within 100
minutes of exposure to visible light, and showed robust cycling
stability, with a degradation efficiency of 90% aer three cycles.
The enhanced photocatalytic performance of the Ag2O/BiVO4
heterogeneous photocatalyst can be attributed to the special 1D
structure of the hybrid composites—and to the synergistic effect
of Ag2O nanoparticles and BiVO4 NFs forming heterogeneous p–
n junction structures and hence impeding the recombination of
photogenerated electron–hole pairs.
ꢂ
O2 ꢁ þ H2O/HOꢂ þ OHꢁ
2
ꢂ
ꢂ
HO2 þ H2O/OH þ H2O2
H2O2 / 2OHc
RhB + 2OHc / CO2 + H2O
RhB + Ag2O (hVB+) / degradation products
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors sincerely acknowledge nancial support from the
China Postdoctoral Science Foundation (2014M550392), and
a Special Financial Grant from the China Postdoctoral Science
Foundation (2017T100799).
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