Polyethylene glycol-doped BiZn2VO6 as a high-efficiency solar-light-activated photocatalyst with substantial durability toward photodegradation of organic contaminations

Article abstract of DOI:10.1039/c8ra06896h

In this study, we focus on a simple, low-priced, and mild condition hydrothermal route to construct BiZn₂VO₆ nanocompounds (NCs) as a novel photocatalyst with strong solar light absorption ability for environmental purification using solar energy. NCs were further doped with polyethylene glycol (PEG) to improve their photocatalytic efficiency for photodegradation processes through inhibition of fast charge carrier recombination rates and higher charge separation efficiency. Surface morphology, phase structure, optical characteristics, and band structure of the as-prepared samples were analyzed using XRD, EDX, XPS, SEM, UV-vis spectroscopy, CL, and BET techniques. PEG-doped BiZn₂VO₆ NCs were applied as effective materials to degrade various kinds of organic pollutants including cationic and anionic types, and these NCs exhibited excellent photocatalytic efficiency as compared to traditional photocatalysts. In particular, the PEG-doped BiZn₂VO₆ (0.10% w/v) photocatalyst exhibited highly enhanced photocatalytic performance with improvements of about 46.4, 28.3, and 7.23 folds compared with PEG-doped ZnO nanorods (NRs), pristine BiVO₄, and BiZn₂VO₆ samples, respectively, for the decomposition of congo red (CR) dye. After 40 minutes of sunlight irradiation, 97.4% of CR was decomposed. In this study, scavenging experiments indicated that both hydroxyl radicals and holes play dominant roles in CR photodegradation under simulated solar light irradiation. Meanwhile, the optimal photocatalyst demonstrated good reproducibility and stability for successive cycles of photocatalysis.

Full text of DOI:10.1039/c8ra06896h

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PAPER
Polyethylene glycol-doped BiZn2VO6 as a high-
efficiency solar-light-activated photocatalyst with
substantial durability toward photodegradation of
organic contaminations
Cite this: RSC Adv., 2018, 8, 37480
ab
a
Aziz Habibi-Yangjeh,b Galia Pozina,c
*
Mahsa Pirhashemi,
Sami Elhag,
Magnus Willandera and Omer Nur
a
In this study, we focus on a simple, low-priced, and mild condition hydrothermal route to construct
BiZn2VO6 nanocompounds (NCs) as a novel photocatalyst with strong solar light absorption ability for
environmental purification using solar energy. NCs were further doped with polyethylene glycol (PEG) to
improve their photocatalytic efficiency for photodegradation processes through inhibition of fast charge
carrier recombination rates and higher charge separation efficiency. Surface morphology, phase
structure, optical characteristics, and band structure of the as-prepared samples were analyzed using
XRD, EDX, XPS, SEM, UV-vis spectroscopy, CL, and BET techniques. PEG-doped BiZn2VO6 NCs were
applied as effective materials to degrade various kinds of organic pollutants including cationic and
anionic types, and these NCs exhibited excellent photocatalytic efficiency as compared to traditional
photocatalysts. In particular, the PEG-doped BiZn2VO6 (0.10% w/v) photocatalyst exhibited highly
enhanced photocatalytic performance with improvements of about 46.4, 28.3, and 7.23 folds compared
with PEG-doped ZnO nanorods (NRs), pristine BiVO4, and BiZn2VO6 samples, respectively, for the
decomposition of congo red (CR) dye. After 40 minutes of sunlight irradiation, 97.4% of CR was
decomposed. In this study, scavenging experiments indicated that both hydroxyl radicals and holes play
dominant roles in CR photodegradation under simulated solar light irradiation. Meanwhile, the optimal
photocatalyst demonstrated good reproducibility and stability for successive cycles of photocatalysis.
Received 17th August 2018
Accepted 6th October 2018
DOI: 10.1039/c8ra06896h
rsc.li/rsc-advances
photocatalysts such as TiO2 and ZnO is severely restricted
because of their wide band gap energy ($3.2 eV), which indi-
1. Introduction
cates that they perform poorly as photocatalysts under visible
light. Regrettably, fast recombination of photo-induced charges
in these semiconductors also limits their photocatalytic appli-
cations.5,6 In this eld, studies are focused on the chemical
modication of photocatalysts with wide band gap, aiming to
alter the band gap or enhance the separation of electron–hole
pairs; also, single-phase photocatalysts with satisfactory pho-
tocatalytic properties for the degradation of organic pollutants
are developed, and they include g-C3N4,7 Bi2O2CO3,8 Bi2Ga4O9,9
and BiFeO3.10 Hence, robust and solar-responsive photo-
catalysts are urgently needed as the next-generation photo-
catalysts, which can provide favorable condition to use all
wavelengths of sunlight.
With rapid industrialization and increasing population, energy
deciency and environmental pollution have become increas-
ingly drastic obstacles in recent years.1,2 In particular, industrial
dyes are accountable for water-pollution, and these organic
pollutants can produce hazardous compounds through hydro-
lysis, oxidation, or other chemical reactions in the wastewater.3
Therefore, the elimination of these poisonous organic
compounds from wastewater with efficient methods remains
a challenging task. One of the promising methods to overcome
this obstacle is entire degradation of these aqueous contami-
nants using semiconductor-based photocatalysis utilizing solar
energy under relatively mild operating conditions.4 As is well-
known, utilization of the most widely studied and used
A novel type of mixed metal oxides represented by the main
formula BiM2AO6 with M ¼ Mg, Ca, Cd, Cu, Pb, Mn, and Zn and
A ¼ V, P, As has become a research focus as potential alternative
semiconductors for sunlight-driven photocatalysts in recent
years.11–16 These compounds have been examined broadly based
on their luminescence, magnetic, and photocatalytic properties
as well as nonlinear optical characteristics. Among these
aDepartment of Science and Technology (ITN), Linkoping University, Campus
¨
¨
¨
Norrkoping, SE-601 74 Norrkoping, Sweden. E-mail: mahsa.pirhashemi@liu.se;
mahsapirhashemi@uma.ac.ir
bDepartment of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P. O.
Box 179, Ardabil, Iran
c
¨
Department of Physics, Chemistry and Biology (IFM), Linkoping University, S-581 83,
¨
Linkoping, Sweden
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semiconductors, bismuth zinc vanadate (BiZn2VO6) is a prom- several experimental studies have conrmed that hydrogen
ising material due to its outstanding properties: suitable band improves electrical conductivity mainly by increasing the free
gap of 2.4 eV, low price, high chemical durability, and accessi- carrier concentration and/or the carrier mobility.37–40 Also, it is
bility.17–19 For example, Nunes et al.17 reported a computational benecial for decreasing the recombination rate of the light-
study based on density functional theory (DFT), which demon- induced eꢁ/h+ pairs and stabilizing the charge carriers on the
strated that BiZn2VO6 has a unique crystal structure and elec- surface of photocatalyst, eventually elevating the photocatalytic
tronic properties as a narrow band gap semiconductor. Luo reaction efficiency. Adhering to this idea, several researchers
et al.20 synthesized BiZn2VO6 ceramic by the solid state reaction have also obtained high photoconductivity and infrared
method, which showed desirable microwave dielectric proper- absorption with hydrogen-related impurity in ZnO struc-
ties and also exhibited good sintering behavior such as great tures.41–43 Recently, our group reported the introduction of PEG
ꢀ
chemical compatibility with Ag electrodes at 780 C grown for as a hydrogen source in a ZnO NR structure, promoting elecro-
4 h. In addition, BiZn2VO6 as a thin lm electrode can also be catalytic properties and boosting the performance of ZnO NRs
applied for oxygen evolution applications due to its electrical as a sensing electrode. In that study, Elhag et al. completely
and optical properties. For instance, Liu et al.18 presented that characterized and demonstrated enhanced optical and elec-
the photocurrents due to water oxidation at BiZn2VO6 particu- tronical properties of PEG-doped ZnO NRs with retention of
late thin lm electrodes can be efficiently enhanced by pre- morphology and structure.44 Most recently, Elhag et al. have also
treatment with an aqueous TiCl4 solution due to the facilita- intensively investigated photoelectrochemical properties of
tion of the transport of photogenerated electrons within PEG-doped BiZn2VO6 as a photoelectrode and demonstrated
BiZn2VO6 by the necking effect of the ock similar to a TiO2 that PEG doping can enhance the charge collection efficiency
overlayer. In another example, Liu et al.19 reported that BiZn2- and photocurrent density of BiZn2VO6 semiconductor.32
VO6 has a higher specic surface area compared to BiVO4 and
Accordingly, the main aim of the present research is to study
exhibits excellent visible-light-driven photocatalytic O2 evolu- the photocatalytic degradation of various dyes that are exten-
tion efficiency; also, photoactivity can be remarkably increased sively used by the textile industry in the presence of PEG-doped
by chemical etching with warm H2SO4.
BiZn2VO6 as a suitable solar-light-activated photocatalyst. The
According to the interesting properties and high potential of synthesis of BiZn2VO6 semiconductor is mostly carried out
BiZn2VO6 as a photocatalyst and to the best of our knowledge, under extreme conditions including long reaction duration and
no study has been conducted to investigate the photocatalytic high temperature.18–20 Therefore, it is imperative to develop
activity of this unique semiconductor in the photodegradation methods that are simple and fast for the fabrication of BiZn2-
of various organic dyes. However, a considerable impediment in VO6 photocatalysts that yield high photocatalytic activity and
employing this semiconductor as a solar-light-driven photo- stability.
catalyst is the rapid recombination of its photo-generated eꢁ/h+
In the synthetic procedure applied here, we demonstrate an
pairs, resulting in poor performance. Among the various strat- attempt to synthesize PEG-doped BiZn2VO6 NCs using PEG-
egies employed to boost the overall photocatalytic efficiency,21,22 doped ZnO nanorods (NRs) along with BiVO4 through a facile
an interesting strategy to achieve a more efficient photocatalyst hydrothermal method. The novelty of this route lies in its
is the addition of organic agents as a dopant source.23,24 It has simplicity, utilization of low temperature, and the capability of
been reported that PEG is a polymeric material commonly used producing solar light-responsive photocatalysts by a quick and
as a growth template for the synthesis of nanomaterials, and it template-free approach. The photocatalytic performance of the
can be employed to boost the morphology, specic surface area, as-fabricated photocatalysts was evaluated with respect to these
and crystallographic properties of nanoparticles.25–27 Unlike the processing conditions by kinetic analysis of various dye pho-
observations for many reports, in this study, PEG was used as todegradation experiments. The results were assessed to
a doping source, which could introduce a rich hydrogenated- determine the photocatalytic performance of these remarkable
environment around the synthesized photocatalysts. In this solar-driven photocatalysts. Interestingly, our photo-
line, Zhang et al.28 demonstrated that water-soluble PEG has degradation efficiency towards cationic and anionic dyes is
strong hydrogen bonding interactions between hydrogen in comparable to those of many recently reported metal oxide-
water and the hydroxyl oxygen in PEG, resulting in the disrup- based photocatalysts.45–47 The results of these experiments
tion of hydrogen bonding when PEG dissolved in water provides demonstrated enhanced photocatalytic ability. Due to relatively
a rich hydrogenated-environment for the growth of nano- low cost and convenience, PEG-doped BiZn2VO6 NCs can be
particles. In addition, theoretical as well as experimental used as an available photocatalyst for the disposal of industrial
studies have revealed a novel type of behavior for hydrogen as effluents.
an impurity in semiconducting materials.29–31 It is generally
acknowledged that hydrogen exhibits amphoteric effect, which
indicates that it can play the role of a donor (H+) or an acceptor
2. Experimental details
2.1. Materials
(Hꢁ) depending on the Fermi level of semiconductors.32–35
Consequently, this amphoteric behavior precludes hydrogen
from acting as a dopant, i.e., from being a source of conduc- All chemicals of analytical grade were obtained from Sigma-
tivity. Based on rst-principle calculation, van de Valle theo- Aldrich, Germany, and were used without further purication.
retically reported this feature of hydrogen in ZnO.36 Moreover, Also, deionized water was employed for all experiments.
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1729 with Cu Ka radiation source. Scanning electron micros-
copy (SEM) observations were performed by means of a LEO
1550 Gemini eld emission gun at 5 kV. Purity and elemental
analysis of the photocatalyst were obtained by EDX at 20 kV on
the same SEM instrument. Defect analysis was performed using
cathodoluminescence (CL) on a Gatan MonoCL4 system
combined with the SEM apparatus. Also, X-ray photoelectron
spectroscopy (XPS) data were captured on a Scienta ESCA200
spectrometer with Al Ka radiation. The binding energies were
tted by employing C 1s of adventitious carbon (284.8 eV) as
a reference. N2 gas sorption analysis of the photocatalysts was
performed on a Belsorp mini II apparatus at 77 K. Fourier
transform-infrared (FTIR) spectra were recorded using a Perkin
Elmer Spectrum RX I. Optical properties of the samples were
conducted using a UV-visible spectrophotometer on a Perki-
nElmer Lambda 900 apparatus.
2.2. Preparation of pristine and PEG-doped ZnO nanorods
Briey, pristine and PEG-doped ZnO NRs were synthesized
using a simple hydrothermal method, as reported previously.44
For the preparation of pristine ZnO NRs, 0.05 M aqueous
solutions of Zn(NO3)2$6H2O and C6H12N4 were mixed. Aer
magnetically stirring at room temperature, the homogenous
solution was placed in an oven for 5 hours at 90 ꢀC. Aerwards,
the separated precipitates were washed with deionized water
and acetone and then dried overnight at 60 ꢀC. Upon adding
0.10% (w/v) PEG 2000, PEG-doped ZnO NRs were prepared by
the same method as stated above.
2.3. Synthesis of PEG-doped BiZn2VO6 photocatalysts
In a typical synthetic route for the preparation of PEG-doped
BiZn2VO6 (0.10% w/v), 0.10% (w/v) represents 0.10 g of PEG-
doped ZnO. First, 0.02 M of Bi(NO3)3$5H2O and NH4VO3 were
dissolved in 10 ml of nitric acid (70%) solution. Then, 20 ml
deionized water was added to this solution with constant stir-
ring until the salts were mixed properly. Thereaer, a certain
amount of NaHCO3 was used to adjust the pH to ꢂ6.5 and an
apparently homogeneous yellow solution was formed. Next,
0.10 g of the as-prepared PEG-doped ZnO NRs was dispersed
into this yellow solution and then, the suspension was placed in
a preheated oven for 4 hours at 80 ꢀC. The resulting nano-
particles were rinsed with deionized water and acetone and
then dried at 60 ꢀC for 10 h in an oven. The same procedure was
repeated for non-doped BiZn2VO6 with the exception of the
addition of PEG. A schematic diagram for the fabrication of the
photocatalyst is presented in Fig. 1.
2.5. Photocatalytic tests
The photocatalytic performances of the prepared samples were
measured by photodegradation of 20 mg lꢁ1 congo red (CR),
methylene blue (MB), and rhodamine b (RhB) solutions as
pollutants. The light source was a 300 W xenon lamp used
without any lter to simulate sunlight irradiation. First, 50 mg
of the photocatalyst was dispersed in 100 ml of the dye solution;
the suspension was stirred in dark for 30 minutes to obtain
adsorption–desorption equilibrium between the photocatalyst
and the dye molecules prior to exposure to light. Aer the lamp
was turned on, the samples were taken in specic desired time
intervals within 90 minutes of the reaction time. The change in
the concentration of the dye solution was evaluated using UV-
vis absorption spectroscopy at characteristic absorption wave-
lengths of 497, 664, and 553 nm for CR, MB, and RhB,
respectively.
2.4. Characterization instruments
X-ray power diffraction (XRD) patterns ranging from 10ꢀ to 70ꢀ
of the as-synthesized samples were provided using a Philips PW
3. Results and discussion
3.1. Morphological and structural characterization of the
photocatalysts
XRD evaluation was performed to investigate the crystal struc-
tures and phase purities of the samples of ZnO NRs, PEG-doped
ZnO NRs, BiVO4, BiZn2VO6, and PEG-doped BiZn2VO6; the
patterns are illustrated in Fig. 2. Compared with the typical
diffraction peak of standard ZnO (JCPDS no. 36-1451), ZnO NRs
with and without PEG were suggested to be hexagonal wurtzite
phase ZnO.48 In addition, the XRD pattern of BiVO4 showed
good agreement with the reference card (JCPDS no. 74-4894)
corresponding to the monoclinic phase of BiVO4.49 With respect
to the BiZn2VO6 sample, the diffraction peaks at 23.5ꢀ, 28.9ꢀ,
31.6ꢀ, 34.2ꢀ, 36.9ꢀ, 39.1ꢀ, 46.6ꢀ, and 48.5ꢀ were indexed to (101),
(103), (110), (006), (105), (114), (200), and (116) planes, respec-
tively, of orthorhombic BiZn2VO6 (JCPDS no. 01-075-9214).50
The above results demonstrate that PEG-doped BiZn2VO6
samples are successfully obtained, and the crystalline phase of
BiZn2VO6 is not affected seriously aer loading of PEG. Besides,
no additional crystal phases or impurities were observed,
implying that the as-synthesized samples are unadulterated.
Fig. 1 Schematic illustration for synthesis procedure of the PEG-
doped BiZn2VO6 photocatalysts.
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Morphologies of the synthesized samples are further inves-
tigated through SEM images, as depicted in Fig. 4. It is clear that
the ZnO sample presents uniform nanorod morphology. The
ZnO nanorod particles are formed with the hexagonal wurtzite
structure with diameters of around 400 nm and lengths of up to
several micrometers. Furthermore, the PEG-doped ZnO sample
(Fig. 4b) exhibits the same features with slightly smaller diam-
eters than that of ZnO NRs. The widths of these PEG-doped ZnO
NRs are between 250 nm and 300 nm with length of about one
micrometer. In addition, the BiVO4 nanocrystals indicate
a spherical conguration (Fig. 4c) and are composed of
numerous BiVO4 sub-nanoparticles; also, the degree of
agglomeration is high. Interestingly, aer introducing PEG-
doped ZnO NRs in the BiVO4 environment, due to strong
interactions and etching effect on the nanorods in the growth
solution at pH ꢂ6.5, the BiZn2VO6 particles start to grow and
exhibit notable changes in size and features compared with
pristine ZnO NRs and BiVO4 samples (Fig. 4d). As clearly seen
from the SEM image of the PEG-doped BiZn2VO6 (0.10% w/v)
sample, it has aggregated structures of large-sized and denser
particles.
Fig. 2 XRD patterns for ZnO NRs, PEG-doped ZnO NRs, BiVO4,
BiZn2VO6, and PEG-doped BiZn2VO6 with different (w/v) percentages
of PEG.
Surface-sensitive XPS analysis further conrms the compo-
sition and valence states of the constituent elements of the PEG-
doped BiZn2VO6 (0.10% w/v) photocatalyst. The survey XPS
spectrum (Fig. 5a) of the PEG-doped BiZn2VO6 (0.10% w/v)
sample exhibits peaks corresponding to Bi 4f, Zn 2p, V 2p, O
1s, and C 1s without any impurities. Zn 2p high resolution XPS
spectrum in Fig. 5b exhibits binding energies of Zn 2p3/2 and Zn
2p1/2 at 1022.4 and 1045.2 eV, respectively, which are an indi-
cation of the presence of Zn2+ in PEG-doped BiZn2VO6 (0.10% w/
v).51 As shown in Fig. 5c, there are two intense peaks at 159.0 eV
and 164.4 eV for the PEG-doped BiZn2VO6 (0.10% w/v) photo-
catalyst, which are ascribed to Bi 4f7/2 and Bi 4f5/2 and corre-
spond to Bi3+.52,53 Also, the XPS data for pristine BiVO4 are
shown in Fig. 5c for comparison. The binding energies of Bi 3f7/
Elemental analysis has been performed to verify the chem-
ical constituents of PEG-doped BiZn2VO6 (0.10% w/v) sample
using spot EDX analysis (Fig. 3a). Except for some Au peaks of
the substrate, EDX spectrum indicates that the sample contains
only Bi, Zn, V, and O, conrming high degree of purity of the as-
prepared PEG-doped BiZn2VO6 (0.10% w/v) sample. Addition-
ally, EDX mappings are performed to investigate the elemental
distribution of PEG-doped BiZn2VO6 (0.10% w/v), and the
results are presented in Fig. 3b–g. It can be observed that Bi,
Zn, V, and O are uniformly distributed over the whole sample
area.
and Bi 3f5/2, at 159.8 and 165.1 eV, respectively, for pristine
2
Fig. 3 (a) EDX spectrum and (b–g) EDX mapping for PEG-doped Fig. 4 SEM images of (a) ZnO NRs, (b) PEG-doped ZnO NRs, (c) BiVO4,
BiZn2VO6 (0.10% w/v) sample.
and (d) PEG-doped BiZn2VO6 (0.10% w/v) samples.
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ZnO NRs only absorb UV light, whereas according to literatures,
PEG-doped ZnO NRs show small red-shi in the absorption
edge.44,56 However, as seen in Fig. 6, the BiVO4 sample clearly
exhibits enhanced absorption in the UV-visible region. It is also
clear that PEG-doped BiZn2VO6 (0.10% w/v) exhibits clear
absorbance in UV and visible regions. Accordingly, improved
light-harvesting ability of the photocatalyst can boost light
utilization and consequently enhance its photocatalytic
performance.
The CL technique is also a benecial method for deter-
mining the optical properties of nanostructures.57 Besides, CL
uses a high-energy electron beam for excitation and all CL
spectra are measured at room temperature. Due to the poly-
crystalline nature of PEG-doped BiZn2VO6 (0.10% w/v) photo-
catalyst, uniformity of the electronic characteristics of different
points on the surface was investigated by CL and the results are
illustrated in Fig. 7. The measured CL spectra of the as-prepared
PEG-doped BiZn2VO6 (0.10% w/v) sample at three points are
quite similar, suggesting isotropic electronic features of the
sample. Furthermore, the results exhibit an evident lumines-
cence band between 540 and 610 nm, which is ascribed to deep
level emission (450–660 nm),58 demonstrating the presence of
interstitial oxygen (Oi) or hydroxyl groups on the surface of the
photocatalyst. As a result, green emission is caused from the
recombination between holes trapped at the surface defects and
electrons trapped at the oxygen vacancies, which can boost
photocatalytic activity by reducing the recombination rate of eꢁ/
h+ pairs.59,60
Fig. 5 XPS spectra for the PEG-doped BiZn2VO6 (0.10% w/v): (a)
survey scan and high-resolution spectra for: (b) Zn 2p, (c) Bi 4f, (d) V 2p,
and (e) O 1s compared with pristine BiVO4 nanoparticles.
BiVO4 are slightly lower than those for Bi in the PEG-doped
BiZn2VO6 (0.10% w/v) material. Moreover, Fig. 5d provides the
spectrum for V 2p. For PEG-doped BiZn2VO6 (0.10% w/v), the
peak at 517.0 eV is ascribed to V 2p3/2 and the other peak at
525.1 eV belongs to V 2p1/2, which are characteristics of V5+ ions.
Compared with the results for pristine BiVO4, a shi of 0.5 eV is
observed in the peak position corresponding to V 2p (516.5 for V
2p3/2 orbital and 524.6 eV for V 2p1/2).54 In the XPS spectrum for
O 1s (Fig. 5e), one characteristic peak at 532.1 eV and a shoulder
at 530 eV are clearly detected, which are in agreement with the
results for lattice oxygen and oxygen species of hydroxyl groups
adsorbed on the surface, respectively.32 It is noticeable that the
peak of O–H in PEG-doped BiZn2VO6 (0.10% w/v) is stronger
than that of BiVO4, which demonstrates that the hydroxyl
content in PEG-doped BiZn2VO6 is higher than that on BiVO4.
Generally, more hydroxyl contents on the surface of a photo-
catalyst have a positive effect on photocatalytic prociency.35
This can be further conrmed by the results obtained from
photocatalytic performance. Consequently, the peak shis
(increase in binding energy of Bi 4f and V 2p in PEG-doped
BiZn2VO6 (0.10% w/v)) are in fact due to the incorporation of
ZnO components into the BiVO4 structure and chemical inter-
actions between the material components, which lead to the
growth of mixed metal oxide semiconductor, as previously re-
ported.32 Therefore, XPS results demonstrated successful
fabrication of PEG-doped BiZn2VO6 photocatalyst.
3.2. Evaluation of photocatalytic performance
It is important to note that CR was selected as the typical
resistant dye to estimate the photocatalytic performance of
obtained photocatalysts. Fig. 8a shows the degradation effi-
ciencies of CR with different photocatalysts. As observed, there
is no distinct CR degradation in the presence of simulated solar
UV-vis absorption spectroscopy was applied to investigate
the optical properties of the samples. In general, the light-
harvesting efficiencies for photocatalysts are of vital impor-
tance for their applications in the degradation of organic dyes.55
As expected from a wide band gap semiconductor, it is clear that
Fig. 6 UV-vis absorption spectra for BiVO4 and PEG-doped BiZn2VO6
(0.10% w/v) samples.
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approximately 11.9% and 23.1%, respectively, in 90 min of light
irradiation. This is related to the rapid recombination of eꢁ/h+
pairs in these semiconductors although BiVO4 can strongly
absorb visible light due to narrow band-gap energy. However, as
a key result, the addition of PEG to ZnO NRs caused 14.6%
increase in CR degradation in comparison with that for the pure
ZnO NR photocatalyst. Also, the relative photodegradation
efficiency of PEG-doped BiZn2VO6 photocatalyst presented
crucial improvements in contrast to that of pure BiZn2VO6. This
conrmed that the presence of PEG inuenced the photo-
catalytic activity of the photocatalyst. Besides, the highest
removal efficiency of CR reached 97.4% for the prepared PEG-
doped BiZn2VO6 (0.10% w/v) sample within 40 min under
simulated solar light irradiation. Clearly, the introduction of
PEG enhanced the photocatalytic degradation ability of BiZn2-
VO6; however, the content of PEG exhibited serious impact on
the prociency of photocatalysts and 0.10% (w/v) of PEG was
determined as the optimal amount. As a result, the synergistic
effect of PEG and BiZn2VO6 appreciably promoted the separa-
tion of the photo-induced eꢁ/h+ pairs, which has a benecial
effect on the photocatalytic reactions. Such a phenomenon has
been previously proved by photoluminescence and photocur-
rent measurements reported by Elhag et al.32
Fig. 7 CL spectra of PEG-doped BiZn2VO6 (0.10% w/v). The inset
shows an SEM image of the sample, where the numbers correspond to
the locations of CL measurements.
Changes in UV-vis absorbance spectra of CR as a function of
irradiation time in the presence of PEG-doped ZnO NR, BiVO4,
BiZn2VO6, and PEG-doped BiZn2VO6 (0.10% w/v) samples are
shown in Fig. 8b–e. Evidently, the characteristic absorption
peak of CR diminishes slightly over PEG-doped ZnO NR, BiVO4,
and BiZn2VO6 samples with prolonged light illumination time.
However, as shown in Fig. 8e, the intensity of the major peak
rapidly weakens and becomes stable, indicating that the CR
structure is successfully degraded by PEG-doped BiZn2VO6
(0.10% w/v) photocatalyst. Additionally, the degradation effi-
ciency of our present photocatalytic system is compared with
those of other recently reported photocatalysts for the photo-
catalytic degradation of CR, and the results are summarized in
Table 1.61–66 The results demonstrate that PEG-doped BiZn2VO6
(0.10% w/v) is a better photocatalyst with respect to the duration
of reaction and degradation efficiency.
To quantitatively evaluate the photocatalytic performance of
the samples, the observed rst-order rate constants (kobs) of
different photocatalysts were obtained according to a pseu-
do-rst-order kinetic model.67,68 The calculated rate constants are
presented in Fig. 9. All as-prepared PEG-doped BiZn2VO6 samples
showed an abrupt increase in CR degradation under the stated
conditions. The degradation rate constant of CR over PEG-doped
BiZn2VO6 (0.10% w/v) was calculated to be 831 ꢃ 10ꢁ4 minꢁ1
,
which is about 46.4, 28.3, and 7.23 times higher than those of
PEG-doped ZnO NRs (17.3 ꢃ 10ꢁ4 minꢁ1), BiVO4 (29.4 ꢃ
10ꢁ4 minꢁ1), BiZn2VO6 (115 ꢃ 10ꢁ4 minꢁ1), respectively. The
origin of superior photocatalytic performance of the PEG-doped
Fig. 8 (a) Photodegradation of CR over ZnO NRs, BiVO4, PEG-doped
ZnO NRs, BiZn2VO6, and PEG-doped BiZn2VO6 NCs with different (w/v)
percentages of PEG. UV-vis spectra for degradation of CR under solar
simulated light irradiation over (b) PEG-doped ZnO NRs, (c) BiVO4, (d) BiZn2VO6 (0.10% w/v) photocatalyst can be ascribed to efficient
separation and rapid transfer of photo-induced eꢁ/h+ as well as
BiZn2VO6, and (e) PEG-doped BiZn2VO6 (0.10% w/v) photocatalysts.
relatively higher light absorption ability of the photocatalyst.
light without any photocatalyst, indicating high stability of CR
under light illumination. In addition, pure ZnO NRs and BiVO4
illustrated low photocatalytic degradation efficiencies of
As an important inuencing factor of photocatalytic perfor-
mance, Brunauer–Emmett–Teller (BET) specic surface areas of
samples were measured by the N2 adsorption/desorption BET
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Table 1 Comparison of CR photodegradation efficiency over different photocatalysts
Photocatalyst
amount
Photodegradation
efficiency
Photocatalyst
Light source
CR concentration
Ref.
Mesoporous sphere-like Cu2O
BiFeO3/N-doped graphene
ZnO/ZrO2
a-MnO2
ZnAl2O4 nanoparticles
BiOCl–Ag8SnS6
Xe 300 W
Xe 300 W (UV cut-off lter)
UV 15 W
500 W halogen lamp
500 W mercury lamp
Sunlight radiation
Xe 300 W
1.4 ꢃ 10ꢁ5
10 mg lꢁ1
10 ppmꢁ1
40 mg l
M
50 mg
0.02 g
0.05 g
0.10 g
50 mg
100 mg
50 mg
97% in 90 min
55% in 120 min
90% in 120 min
>90% in 70 min
98.3% in 80 min
>99% in 90 min
>99% in 40 min
61
62
63
64
65
66
15 mg lꢁ1
20 mg lꢁ1
20 mg lꢁ1
PEG-doped BiZn2VO6 (0.10% w/v)
This work
method, and the results are presented in Fig. 10. Furthermore, BiZn2VO6 (0.10% w/v) photocatalyst in comparison to that in
the BET surface areas of samples, calculated using the N2 the PEG-doped BiZn2VO6 (0.15% w/v) photocatalyst. Therefore,
adsorption/desorption isotherm data, are presented in Table 2 appropriate loading of PEG is important for achieving the best
together with the mean pore diameters and total pore volumes. It photocatalytic performance.
is evident that all curves exhibit small H1-type hysteresis loops,
The well-known oxidative species in photocatalysis are
which are ascribed to type IV isotherms and are representative of photo-generated holes (h+) and free radicals such as superoxide
mesoporous materials.69 The BET specic surface area of PEG- radicals (cO2ꢁ) and hydroxyl radicals (cOH). To further identify
doped ZnO NRs is 10.5 m2 gꢁ1, which is about 4.8 times higher the predominant active species for CR degradation over the
than that of pristine ZnO NRs (2.22 m2 gꢁ1). This indicates that PEG-doped BiZn2VO6 (0.10% w/v) photocatalyst in the reaction
doping PEG is a feasible method to increase the BET surface area system, three typical scavengers, namely, 2-propanol (2-PrOH, 1
of the host. It is clear that the surface area, mean pore diameter, mM), ethylenediamintetraacetic acid (EDTA, 1 mM), and p-
and total pore volume of PEG-doped BiZn2VO6 NCs are greater benzoquinone (BQ, 1 mM) were used for the estimation of the
than those of the PEG-doped ZnO NR and BiVO4 samples. Thus, roles of cOH, h+, and cO2ꢁ, respectively. Reaction kinetic
because of their large textural properties, PEG-doped BiZn2VO6 constants for processes with and without the relevant scaven-
NCs provide more photocatalytic reaction sites for the adsorption gers were examined, and the results are shown in Fig. 11.
of reactant molecules and capture of the irradiation light. This Clearly, all scavengers could partially suppress the photo-
also increases the efficiency of eꢁ/h+ separation;46 thus, the catalytic efficiency of the optimal photocatalyst. However, the
photocatalytic activity of PEG-doped BiZn2VO6 NCs is enhanced. degradation rate constant of CR was slightly reduced aer the
ꢁ
Among all samples, the PEG-doped BiZn2VO6 (0.15% w/v) pho- addition of BQ, implying that only a few cO2 species are
tocatalyst has the largest specic surface area. However, its produced in the degradation process. Conversely, in the pres-
photocatalytic activity is lower than that of the PEG-doped ence of 2-PrOH and EDTA, the degradation rate constant of RhB
BiZn2VO6 (0.10% w/v) photocatalyst, which can be related to
the efficient separation of charge carriers in the PEG-doped
Fig. 9 The degradation rate constants of CR over ZnO NRs, PEG- Fig. 10 Nitrogen gas adsorption–desorption data over ZnO NRs,
doped ZnO NRs, BiVO4, BiZn2VO6, and PEG-doped BiZn2VO6 NCs PEG-doped ZnO NRs, BiVO4, and PEG-doped BiZn2VO6 NCs with
with different (w/v) percentages of PEG.
different (w/v) percentages of PEG.
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Table 2 Texture properties of ZnO NRs, PEG-doped ZnO NRs, BiVO4, and PEG-doped BiZn2VO6 NCs with different (w/v) percentages of PEG
Photocatalyst
Surface area (m2 gꢁ1
)
Mean pore diameter (nm)
Total pore volume (cm3 gꢁ1
)
ZnO NRs
PEG doped ZnO NRs
BiVO4
PEG-doped BiZn2VO6 (0.01% w/v)
PEG-doped BiZn2VO6 (0.05% w/v)
PEG-doped BiZn2VO6 (0.10% w/v)
PEG-doped BiZn2VO6 (0.15% w/v)
2.22
10.5
3.59
9.29
10.4
12.8
20.8
5.31
8.77
5.46
5.68
11.9
13.1
13.7
0.0049
0.0139
0.0067
0.0142
0.0311
0.0439
0.0679
in Fig. 12. The band edge positions of the conduction band (CB)
and valence band (VB) of semiconductors can be calculated
according to the Mulliken electronegativity theory:70
EVB ¼ X ꢁ Ee + 0.5Eg
ECB ¼ EVB ꢁ Eg
(1)
(2)
Here, X is the absolute electronegativity of the semiconductor,
and the X value for BiZn2VO6 is 5.81; Ee is the energy of free
electrons on the hydrogen scale (about 4.5 eV), and Eg is the
band gap of the semiconductor. The estimated VB and CB
potentials are +0.3 and +2.7 eV, respectively, for BiZn2VO6. In
the presence of solar simulated irradiation, the PEG-doped
BiZn2VO6 NC photocatalyst absorbs photons and creates
excited photo-electrons in CB and holes in VB of the semi-
Fig. 11 The degradation rate constants of CR over PEG-doped conductor. Subsequently, PEG impurities can be regarded as
BiZn2VO6 (0.10% w/v) photocatalyst in the presence of different
scavengers.
active hydrogen-related defects that can act as shallow donors
and enhance n-type conductivity of the photocatalyst, as has
been proven in our previous study.32 In particular, the Mott–
Schottky measurement conrms that hydrogen incorporated
into BiZn2VO6 by PEG doping causes a shi in the Fermi level
was markedly restrained. Therefore, it can be inferred that cOH
and h+ are the critical and responsible active species generated
of PEG-doped BiZn2VO6 towards the conduction band edge in
in the photodegradation process.
comparison with that observed for un-doped BiZn2VO6.32
On the basis of the above experimental results and discus-
These results are also consistent with previously published
sion, a suggested mechanism for the enhanced photoactivity of
PEG-doped BiZn2VO6 photocatalysts is schematically presented
results by Cooper et al.,71 where they have reported an
enhancement in n-type conductivity of BiVO4 due to hydrogen
treatment. In addition, in our pervious study,32 based on the
observed photocurrent and photoluminescence analysis, we
showed that PEG modication can affect the separation effi-
ciency of photo-generated electron/hole pairs. This is under-
stood because the coverage of the nanoparticles by PEG
molecules is due to intermolecular hydrogen bonds. Moreover,
this result is also in agreement with the result of a recent study
by Wang et al.72 Thus, doping of PEG introduces hydrogen
impurities that act as effective shallow donors in BiZn2VO6
semiconductors; also, they play a vital role in the achieved
conductivity and stabilize the photo-generated electrons and
holes on the surface of as-prepared photocatalysts, leading to
increased lifetime of the charge carriers. This consequently
enhances their chance to reach the photocatalyst's surface to
initiate photocatalytic reactions. Ultimately, the electrons
generated in PEG-doped BiZn2VO6 NCs can further react with
resolved O2 and H+ to produce H2O2, which can be further
Fig. 12 A plausible diagram for separation of electron–hole pairs in
reduced to cOH radicals.73 Besides, VB of BiZn2VO6 edge is
the PEG-doped BiZn2VO6 photocatalysts.
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more positive than the standard redox potentials of cOH/H2O showed no degradation without the photocatalyst under light
(+2.4 eV vs. NHE) and cOH/OHꢁ (+1.9 eV vs. NHE); thus, the illumination. As can be observed, the calculated RhB degrada-
photo-induced holes in VB can oxidize adsorbed H2O or OHꢁ tion rate constants were 35.7 ꢃ 10ꢁ4, 32.5 ꢃ 10ꢁ4 and 549 ꢃ
molecules to cOH.74,75 Meanwhile, the VB holes are oxidative 10ꢁ4 minꢁ1 over PEG-doped ZnO NR, BiVO4 and PEG-doped
species and they can also eliminate organic pollutant mole- BiZn2VO6 (0.10% w/v) photocatalysts, respectively. These
cules. Consequently, PEG-doped BiZn2VO6 NCs have high results indicated that PEG-doped BiZn2VO6 exhibited a higher
charge separation and transfer efficiency; when separated degradation rate constant compared to other photocatalysts,
charge carriers react with surrounding water molecules, many and indeed the enhanced activity of this photocatalyst was 15.4
reactive oxygen species are produced, which can effectively and 16.9-folds better than those of PEG-doped ZnO NR and
decompose the surrounding pollutant molecules.
BiVO4 samples, respectively, for the degradation of RhB.
The recyclability and stability are important factors for Furthermore, the PEG-doped BiZn2VO6 photocatalyst showed
practical applications of photocatalysts. For this purpose, maximal reaction rate constant for MB degradation (300 ꢃ
circulating experiments for the photodegradation of CR were 10ꢁ4 minꢁ1), which was about 12.7 and 10.3 times superior than
performed to explore the stability of PEG-doped BiZn2VO6 those of PEG-doped ZnO NRs (23.6 ꢃ 10ꢁ4 minꢁ1) and BiVO4
(0.10% w/v) as a photocatalyst under simulated sunlight irra- (29.1 ꢃ 10ꢁ4 minꢁ1), respectively. Hence, it can be concluded
diation. Fig. 13a reveals that aer four successive recycles, the that the high activity prole of this photocatalyst for both
photocatalyst only exhibited a marginal loss of its photocatalytic cationic and anionic dyes makes it suitable for industrial
performance. A comparison of the XRD patterns and FT-IR wastewater treatment.
spectra of the photocatalyst before and aer four photo-
Finally, the photocatalytic activity of the PEG-doped
catalytic degradation cycles is presented in Fig. 13(b and c). No BiZn2VO6 (0.10% w/v) photocatalyst was compared with
apparent changes are found before and aer the cycling those of other recently prepared photocatalysts, and the
experiments, suggesting that the structure of PEG-doped results are displayed in Table 3. As can be seen, compared to
BiZn2VO6 (0.10% w/v) photocatalyst does not change during some of the reported visible-light-driven photocatalysts used
the photocatalytic process. Hence, it is conrmed that the in the photocatalytic degradation such as Bi2O2CO3 micro-
photocatalyst is not corroded during light irradiation and has spheres,76 Bi2Fe4O9/graphene,77 CeO2/Bi2MoO6,78 Bi2O3/g-C3N4
excellent durability aer multiple uses for photocatalytic composite,79 and BiOBr/TiO2 nanobers,80 PEG-doped BiZn2-
degradation of pollutants in the liquid phase.
In addition, we studied the photocatalytic performance for some organic dyes.81 In addition, PEG-doped BiZn2VO6 (0.10%
the degradation of two more organic dyes (MB and RhB as w/v) presented enhanced activity in shorter photo-
VO6 (0.10% w/v) exhibited better photodegradation effect for
a
cationic dyes) as probe reactions and demonstrated the degradation time relative to other solar light-driven photo-
enhancement in activity for PEG-doped BiZn2VO6 in compar- catalysts such as porphyrin-decorated Bi2O2CO3,82 BiOCl/
ison with that for PEG-doped ZnO NRs and BiVO4; the results Bi2Ti2O7,83 PEG-BiOCl,84 and MoS2–FeZnO.85 This veried that
are presented in Fig. 14. According to Fig. 8, similar to CR (as an
anionic dye), RhB and MB dye solutions (as cationic dyes)
Fig. 13 (a) Reusability of the PEG-doped BiZn2VO6 (0.10% w/v) pho- Fig. 14 The degradation rate constants of CR, RhB, and MB over the
tocatalyst; (b and c) XRD patterns and Ft-IR spectra of the photo- PEG-doped ZnO NR, BiVO4, and PEG-doped BiZn2VO6 (0.10% w/v)
catalyst before and after four successive runs.
samples under solar simulated light irradiation.
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Table 3 Comparison of photodegradation performances of PEG-doped BiZn2VO6 (0.10% w/v) with some reported photocatalystsa
Photodegradation
efficiency (%)
Photocatalyst
Experimental conditions
Light source
Bi2O2CO3 microspheres
Bi2Fe4O9/graphene
CeO2/Bi2MoO6
Catalyst ¼ 0.01 g,
300 W Xe* lamp, l $ 420 nm
Visible light irradiation
300 W Xe lamp, l $ 400 nm
35 W Xe lamp
98% in 180 min
95% in 180 min
[MO*] ¼ 10 mg lꢁ1, 100 ml
Catalyst ¼ 30 mg,
[MV*] ¼ 30 mg lꢁ1, 50 ml
Catalyst ¼ 50 mg, [RhB] ¼ 10 mg lꢁ1
[TC*] ¼ 20 mg lꢁ1, 100 ml
Catalyst ¼ 20 mg,
,
About 100% in 75 min,
78.7% in 120 min
88.7% in 120 min
Bi2O3/g-C3N4 composite
BiOBr/TiO2 nanobers
Porphyrin-decorated Bi2O2CO3
BiOCl/Bi2Ti2O7
[AB 10B*] ¼ 10 mg lꢁ1, 50 ml
Catalyst ¼ 0.025 g,
250 W Xe lamp, l $ 400 nm
350 W Xe lamp
About 100% in 180 min
About 100% in 90 min
90% in 120 min
[4-NP*] ¼ 10 mg lꢁ1, 50 ml
Catalyst ¼ 50 mg, [RhB*] ¼ 0.5 mM,
50 ml, pH ¼ 7.0
Catalyst ¼ 100 mg,
300 W Xe lamp
[TC-HCl*] ¼ 50 mg lꢁ1, 100 ml
PEG–BiOCl
Catalyst ¼ 15 mg, [MO] ¼ 10 mg lꢁ1
50 ml, pH ¼ 7.0
,
,
500 W Xe lamp
About 100% in 20 h
95% in 140 min
MoS2–FeZnO
Catalyst ¼ 1 g lꢁ1
,
400 W halogen lamp
300 W Xe lamp
[MB*] ¼ 20 mg lꢁ1, 150 ml
PEG doped BiZn2VO6 (this work)
Catalyst ¼ 50 mg, [CR*] ¼ 20 mg lꢁ1
97.4% in 40 min,
97.7% in 60 min,
96.1% in 100 min
[RhB] ¼ 20 mg lꢁ1
,
[MB] ¼ 20 mg lꢁ1, 100 ml
a
[*]: Xe ¼ xenon, MO ¼ methyl orange, RhB ¼ rhodamine B, MV ¼ methyl violet, TC ¼ tetracycline, TC-HCl ¼ tetracycline hydrochloride, AB 10B ¼
amido black 10B, 4-NP ¼ 4-nitrophenol, MB ¼ methylene blue, CR ¼ congo red.
it can be categorized as one of the most active photocatalysts
for the removal of various dyes.
Acknowledgements
The authors would like to thank University of Mohaghegh
Ardabili – Iran and Linkoping University – Sweden for nancial
support. Sami Elhag acknowledges the partial nancial support
4. Conclusions
˚
from the AForsk through the project no. 17-457.
To sum up, a simple low-temperature hydrothermal method
was applied to synthesize solar-light-activated PEG-doped
BiZn2VO6 photocatalysts using PEG-doped ZnO NRs and
pristine BiVO4 as starting materials. The as-prepared PEG-
doped BiZn2VO6 NCs exhibited remarkably higher photo-
catalytic activities toward degradation of CR compared to PEG-
doped ZnO NRs and pristine BiVO4. Photocatalytic processes
of RhB and MB decomposition were also investigated. The
PEG-doped BiZn2VO6 (0.10% w/v) photocatalyst exhibited the
highest photo-activity, and the degradation efficiencies of CR,
RhB, and MB were 97.4%, 97.6%, and 96.0%, respectively,
which were much higher than those of PEG-doped ZnO NRs
and pristine BiVO4. This distinctively enhanced photocatalytic
activity of the PEG-doped BiZn2VO6 (0.10% w/v) photocatalyst
was ascribed to its increased absorption of visible-light and
higher efficiency in the separation and transfer of photo-
induced electrons and holes. Furthermore, trapping experi-
ments demonstrated that the main reactive species are cOH
and h+ rather than cO2ꢁ. As a result, it can be deduced that this
study can be useful to prepare more progressive and prom-
ising photocatalysts for environmental remediation and other
industrial utilizations.
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