photocatalytic applications accepted...
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A review on BiVO4 photocatalyst: Activity enhancement methods for solar
photocatalytic applications
A. Malathia, J. Madhavan,a* Muthupandian Ashokkumarb, Prabhakarn Arunachalamc
aSolar Energy Lab, Department of Chemistry, Thiruvalluvar University, Vellore-632 115, India.
bSchool of Chemistry, University of Melbourne, Parkville, Victoria-3010, Australia.
cElectrochemistry Research Group, Chemistry Department, College of Science, King Saud
University, Riyadh 11451, Saudi Arabia.
*Corresponding author
E-mail address: [email protected] (J. Madhavan)
Graphical abstract
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Highlights
Review of recent trends in BiVO4:solar photocatalytic applications
Morphology control and growth mechanism
Modification of BiVO4 with metal and non-metal elements
Construction of BiVO4 composite materials
Highlights
Review of recent trends in BiVO4:solar photocatalytic applications
Photocatalytic degradation
Morphology control and growth mechanism
Modification of BiVO4 with metal and non metal elements
Construction of BiVO4 composite materials
Abstract
Bismuth vanadate (BiVO4) is a promising visible-light driven semiconductor
photocatalyst with various benefits such as low production cost, low toxicity, high photostability,
resistance to photo-corrosion and narrow band gap with a good response to visible-light excite.
However, the fast recombination of photoinduced charge carriers restricts their photocatalytic
activity. In the past decades, many attempts were adopted to enhance the photocatalytic activity
of BiVO4. Significant advances in understanding the fundamental issues and the development of
an efficient photocatalyst have been made in current years. In this review, we have provided a
comprehensive overview of the latest progress on the morphology control and growth
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mechanism of BiVO4 micro/nano-structures, doping with metal and non-metal elements and
semiconductor coupling along with some highlights in the photodegradation of organic
pollutants under visible-light illumination. This review may benefit the researchers and engineers
in the arena of material chemistry for designing new BiVO4 based photocatalysts with low
production cost and high efficiency.
Keywords: BiVO4; morphology control; growth mechanism; photocatalysis; environmental
application.
1. Introduction
Clean water is an essential constituent for the survival of any form of life. The earth is
surrounded by 70% water, however, only 2.5% is available for drinking, agriculture, domestic
and industrial consumption [1]. In the 21st century, environmental protection and remediation are
the greatest challenges for the human beings due to the rapid industrial developments, exhausted
water resources, uncontrolled ground water development, environmental pollution and global
warming causing abnormal climate changes [2-3]. Various industrial sectors such as textiles,
dyeing and printing industries discharge large amounts of synthetic organic dyes as effluents
[4,5]. Among them, textile industries discharge annually 15% (one thousand tons) of non-
biodegradable dye stuff as an effluent. Therefore, it is very important to eliminate these
pollutants from wastewater in order to protect the environment. To address this issue,
semiconductor photocatalysis has been considered as a low cost, sustainable and environmentally
friendly approach by making use of solar energy [6,7].
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To date, research has been focused on titanium dioxide (TiO2) photocatalyst for energy
and environmental applications owing its outstanding photocatalytic activity, non-toxicity and
high photostability. But, TiO2 is only sensitive under UV-light and utilizes only 4% of available
solar energy, which highly restricts its potential application. Hence, there is a need to develop
new strategies to improve the use of solar energy [8]. Researchers have devoted much attention
to fabricate visible-light driven (VLD) photocatalysts for the wastewater treatment process.
Currently, more than 150 semiconductor materials are available for the environmental
purification applications, which include metal oxides, sulfides, carbides, halides, chalcogenides,
oxyhalides and hydroxides [9-11]. Among them, semiconductor metal oxides, especially,
bismuth-based metal oxides BiVO4, Bi2WO6, Bi2MoO6, Bi4Ti3O12, BiFeO3, Bi2Fe4O9,
Bi5FeTi3O15, BiOX (X=Cl, Br, I), Bi5O7I, etc., are reported as emerging materials due to their
excellent photocatalytic efficiency by virtue of the enhanced charge transfer [12-16]. Among
them, bismuth vanadate (BiVO4) has attracted significant interest due to its outstanding features,
such as low band gap, good dispersibility, non-toxicity, resistance to corrosion and outstanding
photocatalytic result in organic pollutant degradation under visible-light illumination [17-19].
There are three crystal structures of BiVO4, tetragonal zircon phase (t-z), monoclinic scheelite
phase (m-s) and tetragonal scheelite phase (t-s). Among these crystalline structures, monoclinic
scheelite phase (m-s) BiVO4 exhibits a greater photocatalytic performance in visible-light
illumination owing to the lone pair distortion of Bi 6s orbital in BiVO4 semiconductor. The
distinct overlap of O 2p and Bi 6s orbitals in the valence band (VB) is an advantage for the
mobility of photogenerated charge carriers resulting in an improved photocatalytic activity
[17,20].
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Since the first photocatalytic report by Kudo et al. in 1999, [21] the photocatalytic
activity of BiVO4 has been extensively studied. The total number of publications dealing with
BiVO4 during 2006-2016 is shown in Fig. 1. A total of 450 research papers on the photocatalysis
of BiVO4 was published in the past ten years. It is evident that publications related to BiVO4
photocatalyst steadily increased from 2006 to 2016 except in the year of 2012. Undoubtedly it
shows an increasing trend, indicating that BiVO4 photocatalyst is attractive to be an emerging
topic in the field of photocatalysis.
BiVO4 is a n-type semiconductor with a good chemical and photostability. Moreover, its
flexible optical and electronic properties with a band gap ~2.4 eV make it an important candidate
for harvesting solar energy [17,18,22]. On the other hand, the photocatalytic behavior of pure
BiVO4 photocatalyst needs to be further enhanced because of the fast recombination of
photoinduced carriers as a result of narrow band gap energy. To overcome this barrier, enormous
effort has been made to develop the separation of photogenerated charge carriers. Different
methods have been adopted, including controlling morphology, doping with metal and non-metal
elements, semiconductor coupling and exposed reactive facets.
A few outstanding reviews on BiVO4 and bismuth based materials are available focusing
on the photo-anodes for water oxidation and photocatalytic application [20,23-25]. However, a
complete review focusing on the BiVO4 photocatalyst is still essential to deliver readers with a
good understanding of the current research development in this arena. In this review, we
concentrated on the morphological control and approaches for the improvement of BiVO4
photocatalyst efficiency. This comprehensive review deals with recent development in the
preparation, modification and photocatalytic activity of BiVO4 photocatalyst.
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2. Semiconductor photocatalysis: General principles and mechanism
The role of a photocatalyst is to accelerate specific oxidation and reduction process in the
attendance of light energy. Generally, a photocatalytic reaction follows three basic steps: (i)
photo-generation of electron and hole pairs; (ii) separation of electron and hole pair and their
diffusion to the surface of the semiconductor; (iii) photo-reduction and photo-oxidation reactions
on the surface catalytic active sites, as shown in Fig. 2. Generally, semiconductor photocatalyst
absorbs photons (light energy) of energy higher than the bang gap (EBG) resulting in the creation
of holes (h+) in the VB and electrons (e-) in the conduction band (CB). The photogenerated
charge carriers can migrate to the surface of the photocatalyst to facilitate redox reactions. The
photogenerated holes in the VB and electrons in the CB react with O2 and H2O on the catalysts
surface to produce superoxide radical anions (O2•-) and hydroxyl radical (•OH). These radicals
have strong oxidation and reduction potentials for the decomposition of aqueous toxins. The
main reactions that take place during photocatalysis are exposed below reactions (1-4).
Photocatalysts hνe-
CB + h+VB (1)
Photocatalyst (e-CB) + O2 O2
•- (2)
Photocatalyst (h+VB) + H2O H+ + •OH (3)
Pollutants + •OH + O2•- Degradation products (4)
3. Controlled synthesis of BiVO4 micro/nano-structures for photocatalytic applications
It is well evidenced that the photocatalytic reactions occur on the surface of the
photocatalyst. Hence, the photocatalytic activity of a material is intimately connected to the
particle size, crystallinity and morphology of the photocatalysts [6,26]. The larger is the crystal
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size greater is the surface recombination of photogenerated charge carriers. For randomly
produced electrons and holes, the normal diffusion time from the bulk to the surface is
represented by the following Eq. (1),
τ = r2/π2D (1)
where r is the grain radius and D is the diffusion co-efficient of the charge carrier. Therefore,
when the grain radius decreases, the recombination rate decreases and a large number of
photoinduced charge carriers will move to the surface for photocatalytic reaction. The
preparation of particles with small and uniform size is considered to be the vital issues to
increase the photocatalytic performances of materials [27-30].
3.1 BiVO4 one-dimensional (1D) and two-dimensional structures
In general, it is believed that an increase in surface to volume ratio of a photocatalyst is
one of the essential ways to enhance the photocatalytic activity [26,31]. Hence, various 1D
nanostructured BiVO4 such as micro-tube [32], nano-fibers [33,34] and micro-ribbon [35] have
been fabricated for photocatalysis application. Often, electro-spinning method was adopted to
fabricate BiVO4 nanofibers due to its ability of quantity production and tunable properties by
varying the technical parameters. For instance, Cheng et al. [34] prepared BiVO4 porous 1D
nanofibers by electro-spinning method using polyvinyl pyrrolidone (PVP)/acetic
acid/ethanol/N,N-dimethylformamide/bismuth nitrate/vanadium (IV) oxy acetylacetonate as a
precursor. The prepared electrospun precursor was calcined at four different temperatures, 400
oC, 450 oC, 500 oC and 550 oC, and the photocatalytic efficiency was estimated towards the
photodegradation of rhodamine (RhB). They reported that the 500 oC calcined BiVO4 sample
display higher photocatalytic efficiency for RhB than that of other calcined temperatures. This
might be due to the greater surface area and higher degree of crystallinity. From the result, it can
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be found that the calcination temperature and the crystallinity of the final products have strong
effects on the photocatalytic efficiency. Similarly, by replacing acetic acid, N,N-
dimethylformamide and vanadium (IV) oxy acetylacetonate with citric acid, hydrochloric acid
and ammonium vanadate as a precursor, Liu et al. [35] synthesized BiVO4 micro-ribbons with
size of ~2-3 µm width, tens of microns in length and a thickness of about 0.4 µm. They also
described about the impacts of calcination temperature on the photocatalytic behavior of BiVO4
towards the visible-light driven photodegradation of methylene blue (MB). They reported that
the 500 oC BiVO4 material exhibited highly enhanced (~93.3%) photodegradation of MB in 60
min compared with other samples. The advancement of photocatalytic performances was
credited to the high specific surface area (5.38 m2/g), offering more active sites for the reacting
materials by improving adsorption, desorption and diffusion behaviors of reactants and products.
Liu et al. [33] synthesized bamboo-shaped BiVO4 nanofibers by an electro-spinning method and
followed by calcining at two different temperatures, 500 oC and 600 oC for 1 h and evaluated the
photocatalytic activity towards the photodegradation of MB. They reported that 500 oC calcined
BiVO4 nanofibers was found to be the best photocatalyst which decomposed ~87% of MB dye in
4 h of visible-light illumination.
The tubular morphology could result in a larger effective surface area, rapid and effective
diffusion of pollutant molecules in tubular structure and more active sites available for
photocatalytic reactions. For instance, Ying et al. [32] prepared BiVO4 micro-tubes and micro-
rods by a hydrothermal method. The BiVO4 micro-tubes morphology was prepared by
hydrothermal processes using CTAB as a surfactant and the BiVO4 with micro-rod structure has
been prepared without the addition of CTAB. The photocatalytic behavior was investigated
towards the decomposition of methyl orange (MO) under the visible-light. They reported that
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CTAB assist synthesized BiVO4 micro-tube displayed the higher photocatalytic properties, with
nearly 5-fold enhancement in comparison with BiVO4 micro-rods. This enhanced activity was
attributed due to the hollow and tubular structures, large specific surface area and the porous
properties, which facilitated the photocatalytic activity, and also CTAB played an important
effect on the formation of the BiVO4 hierarchical micro-tubes.
Zhang et al. [36] prepared 2D BiVO4 single crystal nanosheets with thickness of ~10-40
nm via a hydrothermal route using dodecyl benzene sulfonate (SDBS) as a surfactant. It is well
known that SDBS is an anionic surfactant, which can form micelles in an aqueous solution. The
ionic SDBS can be adsorbed on the surface of BiVO4 nanoparticles and control the growth and
nucleation rate of nanoparticles. The photocatalytic activity of BiVO4 nanosheets comparing
with BiVO4 obtained from the solid state reaction (SSR) techniques. The catalytic activity of
BiVO4 nanosheets synthesized in the presence of SDBS is greatly enhanced for the
photodegradation of the RhB. Similarly, by replacing SDBS with polyethylene glycol with a
molecular weight of 1000 (PEG-1000), Liu et al. [37] synthesized BiVO4 sandwich-like sheets
with a width about ~8 µm and a thickness about ~100 nm via a microwave assisted process. The
PEG-1000 is a long chain polymer, the oxygen on the chain is prone to adsorb and react with
positive Bi3+ ions in the reaction medium to form chain-like co-ordination complexes. Owing to
the chain-like co-ordination complexes between two PEG chains, the BiVO4 particles may
aggregate onto long rods resulting in the formation of solid BiVO4 sheets. The photocatalytic
activity of sandwich-like BiVO4 (S-BiVO4) sheets were compared with that of irregular BiVO4
(I-BiVO4) material, which was fabricated without the addition of PEG-1000. For 150 min of
illumination, ~86% of MO dye was degraded over the S-BiVO4 sample while only about 58%
was removed on an I-BiVO4 sample. From the result, it can be found that surfactants play a great
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impact on the photocatalytic behaviors. Sun et al. [38] synthesized nanoplate-stacked star
morphological structured BiVO4 via hydrothermal method using ethylenediamine tetraacetic acid
(EDTA) as a chelating agent. They also reported that the molar ratio of EDTA to Bi3+ was
considered to be a vital factor in defining the morphology of the products. With the molar ratio
of EDTA to Bi3+ was 0.75 to 1, star-like products fashioned (BiVO4-1 Fig. 3(b)). On the other
hand, in the absence of EDTA, irregular shaped small particles dominated (BiVO4-2, Fig. 3(a)).
When the molar ratio of EDTA to Bi3+ was increased to 1, 3D regular structured microcrystals
were attained (BiVO4-3, Fig. 3(c)). It is evidence that the strong ligand, EDTA, is not only
essential to form a stable complex with Bi3+, but also behaves as a capping agent that directly
influences the facet growth of the nanocrystals. The star-like BiVO4 (BiVO4-1) structure
exhibited higher efficiency for photodegradation of MB, degradation within 25 min under
visible-light irradiation.
3.2 BiVO4 dendritic structures
Perez et al. [39] successfully synthesized BiVO4 dendrite morphology via a hydrothermal
method using polyethylene oxide/polypropylene oxide/polyethylene oxide (PEO100PPO65PEO100)
triblock copolymer (Pluronic F-127) as surfactant and evaluated the photocatalytic activity
towards the degradation of RhB under visible-light irradiation. Pluronic F-127 is a non-ionic
surfactant, which has a comparatively high critical micelle concentration credited to the low
hydrophobicity of PPO blocks. The Bi+ ions are bonded to the surfactant as a result of the
electrostatic attractions between ions and terminal groups (-OH) of the hydrophilic part of
surfactant molecules and control the growth and nucleation rate of BiVO4 nanoparticles.
Furthermore, the photocatalytic activity of BiVO4 dendrite sample is comparable to that of
BiVO4 obtained from SSR process. The dendrite BiVO4 showed 67% elimination of the RhB.
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Lei et al. [40] fabricated BiVO4 dendritic structure by additive-free hydrothermal method at
dissimilar hydrothermal temperatures such as, 100 oC, 140 oC and 180 oC, and evaluated the
photocatalytic performances towards the photodegradation of the RhB. Fig. 4 displays typical
FE-SEM images of the BiVO4 samples attained from dissimilar hydrothermal temperatures.
After 210 min of irradiation, ~99% of the RhB dye was removed over BiVO4 (140 oC) while
only about 58% and 79% removals were observed with BiVO4 (100 oC) and BiVO4 (180 oC)
samples, respectively. The dendritic BiVO4 (140 oC) sample exhibited a superior photocatalytic
activity than other samples. The improved activity might be due to the high surface area (2.1
m2/g) to dendritic BiVO4 (140 oC) sample.
3.3 BiVO4 hollow sphere structures
Yin et al. [41] prepared BiVO4 hollow nanosphere morphology by precipitation method
using colloidal carbon spheres (CCS) as hard template and evaluated the catalytic activity
towards the degradation of the RhB. CCS is a kind of hard template which contains negatively
charged -OH and –C=O groups on their surface. So it is very advantageous to the adsorption of
Bi3+ and VO3- ions on the surfaces. Fig. 5 shows the growth mechanism of the BiVO4 hollow
nanosphere prepared via CCS. When a uniform solution of bismuth nitrate and ammonium
vanadate was annealed at 80 oC, Bi3+ ions were adsorbed on the negatively charged surface of
CCS via electrostatic interaction and functioned as anchors. Moreover, when the calcination
temperature is 100 oC, VO3– ions joined with the anchored Bi3+ ions and amorphous BiVO4
precipitated on the surface of the CCS to form the BiVO4 precipitated CCS. Succeeding
calcination of BiVO4 precipitated CCS lead to the crystallization of amorphous BiVO4 into
monoclinic BiVO4 at nearly 236 oC and the exclusion of the CCS templates at 353 oC. Finally,
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the BiVO4 hollow nanospheres were attained. The photocatalytic performance of BiVO4 hollow
nanospheres was compared with BiVO4 obtained from the SSR and aqueous method (AM). The
BiVO4 hollow nanospheres exhibited almost complete removal of the RhB dye within 70 min
while only about 12.5% of RhB can be decomposed over the SSR-BiVO4 and ~85% over the
AM-BiVO4 after 120 min of illumination. The advancement of photocatalytic performances was
attributed to the high surface area 5.85 m2/g of BiVO4 hollow nanosphere morphology.
Zong et al. [42] synthesized BiVO4 single and multi-shell hollow spheres by a calcination
process using carbonaceous spheres as a hard template and explored the photocatalytic
performances for the photodegradation of MB. Fig. 6 illustrates the growth mechanism of BiVO4
single and multi-shell hollow spheres. When the carbonaceous spheres with the lower quantity of
Bi3+ and VO3- were processed below the annealing rate of R=10 oC/min, Bi3+ and VO3- ions on
the surface of the carbonaceous spheres were distorted into the crystallographic Bi-V-O. At last,
all of the crystallized Bi-V-O nanoparticles were cross-linked to create a single-shell hollow
sphere. Processing the carbonaceous spheres templates in the alkaline media former to the
adsorption is one of the main factors to fabricate the hollow spheres with double- and triple-shell
hollow structures. Under visible-light irradiation for 80 min, about ~100% degradation of MB
was observed over the double shell hollow sphere while only about ~75% and ~65% of MB
degraded over a single shell hollow sphere and BiVO4, respectively.
Lu et al. [43] prepared core-shell structured (CSS) BiVO4 by surfactant and template-free
hydrothermal method using bismuth nirtate/ammonium vanadate/ethanol/acetic acid as
precursors. They also prepared BiVO4 biscuit morphology by hydrothermal method using PVP
as a surfactant, and a plate-like morphology of BiVO4 was prepared by hydrothermal method
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using CTAB as a surfactant. Fig. 7 shows the close up views of CSS BiVO4 hollow spheres.
From the Fig. 7(a-f) it can be monitored that the shell of BiVO4 hollow spheres becomes thinner
with the extending the time duration of hydrothermal reaction. This illustrates that the shell
thickness of CSS BiVO4 hollow spheres can be organized via regulating the duration of time
intervals. Fig. 8(a) represents the absorption spectra of the RhB during dissimilar irradiation
times in the presence of CSS BiVO4 nanoparticles under 4.5 h visible-light irradiation. Fig. 8(b)
demonstrate that the BiVO4 core shell structure exhibited ~99% degradation of RhB in 4.5 h
while the photocatalytic activity of BiVO4 plate morphology (~88%) and BiVO4 biscuit
morphology (~68%) was very low. The exceptional photocatalytic activities can be credited to
the greater surface area of the CSS BiVO4 hollow spheres, which can offer more active surfaces
for the photogenerated charge carriers to interact with the RhB.
Li et al. [44] synthesized BiVO4 hollow microspheres by additive free hydrothermal
method through adjusting the precursor pH and using bismuth nitrate and decavanadate as Bi and
V source. Fig. 9 shows the formation mechanism and the photocatalytic activity of RhB over
BiVO4 at different pH. It can be seen from Fig. 9(a) the morphology of BiVO4 nanoparticles
fabricated at pH=8 are irregular microparticles with the particle sizes varied from ~1 to5 µm.
When the pH of the solution was maintained at 6 during the synthesis of BiVO4, the obtained
material still continued as microparticles, but the particle size became comparatively smaller (~1-
3 µm). The microsphere BiVO4 morphology was attained at pH=4 and it displayed uniform size
at~5.0 µm. It can be observed that the shells of the spheres are comprised of numerous small
BiVO4 nanoparticles with the size of ~100-300 nm. The average diameter of the BiVO4
microspheres is nearly ~5.0 µm and hollow morphological structures can be obtained, when
turned the pH to 2. From Fig. 9(b), the sample BiVO4 (pH=4) exhibited 84.1% of RhB was
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decomposed with illumination for 150 min. These studies indicate that the pH of the reaction
solution, morphologies and particle size of the last products have a major impact on the
photocatalytic activity.
3.4. BiVO4 hierarchical and olive structures
Chen et al. [45] synthesized BiVO4 red blood cell, flower-like microsphere and dendrite
morphologies via facile solvothermal method through adjusting the solution pH and using
bismuth nitrate/ammonium vanadate/citric acid/ethylene glycol/ethanol/water as precursors. The
morphologies and microstructure information on the preparations BiVO4 samples are shown by
the FE-SEM micrographs, as illustrated in Fig. 10. Fig. 10(a,b) shows the morphology of BiVO4
red blood cell (S-BiVO4) and it can be achieved by using Na2CO3 as pH controlling agent.
Similarly, by replacing Na2CO3 with NH3.H2O flower-like microsphere BiVO4 nanoparticle (A-
BiVO4) was obtained and the resultant FE-SEM images are depicted in Fig. 10(c,d). Fig. 10(e)
shows the BiVO4 dendrite-like morphology (N-BiVO4), this morphology can be achieved
without the addition of citric acid and Na2CO3 under the similar conditions. The S-BiVO4 sample
with the red blood cell morphology exhibited higher catalytic activity, than the flower and
dendritic morphology of BiVO4. The improved photocatalytic activity was credited to the porous
structure and effective adsorption of the pollutants on their surface.
Jiang et al. [46] synthesized porous olive-like BiVO4 by an alcohol hydrothermal process
with bismuth nitrate and ammonium vanadate as Bi and V metal precursors, NaOH to adjust pH,
ethanol/ethylene glycol as solvent and dodecylamine (DA), oleylamine (OL) or oleic acid (OA)
as a surfactant. Fig. 11 shows the SEM micrographs of the BiVO4 samples derived under
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dissimilar circumstances. An irregular morphology was observed with the particle size of ~0.4-2
µm for the BiVO-1.5 sample prepared at pH=1.5 without the addition of surfactant (Fig. 11(a)).
With the introduction of surfactant DA, though, the attained BiVO-DA-1.5 sample was
comprised of a huge number of olives structured like BiVO4 micro entities with porous
morphology (Fig. 11(b)). With DA as surfactant and at pH=3.0 the attained BiVO-DA-3 sample
also maintained an olive-like structure and a porous structure (Fig. 11(c)). With an increase in
pH from 3.0 to 7.0, though, the morphological structure of the BiVO-DA-7 developed into a
comparatively uniform short-rod-like nanoparticles (Fig. 11(d)). When the pH varied in 11.0,
though, most of the BiVO-DA-11 particles exhibited a spherical morphological structure that
was comprised of many porous Bi4V2O11 micro sheets (Fig. 11(e,f)). It can be observed from
Fig. 11(g,h) when the pH of the metal precursor was 1.5, varying the surfactant (OL or OA) did
not encourage important modifications in particle morphology and pore structure. This
investigation summarized that the major part of pH of the metal precursor on the morphology
and pore structure of the obtained products. This result inferred that a strongly acidic precursor
solution in the attendance of DA preferred the creation of monoclinic BiVO4 with an olive
structured-like and porous morphology. However, when increasing the pH above 11 in presence
of DA some impurity phase of BiVO4 (Bi4V2O11) was observed. Among the BiVO4 samples, the
porous olive-like one with a specific surface area of 12.7 m2/g showed higher photocatalytic
activity for phenol degradation. The higher photocatalytic behavior of the porous olive-like
BiVO4 sample was related with its superior surface area a porous structure and lower band gap.
Wang et al. [47] prepared olive-like BiVO4 hierarchical morphology via a template-free
hydrothermal process and evaluated the photocatalytic activities towards photodegradation of
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MB. Fig. 12 shows the SEM images of the BiVO4 sample prepared at different pH. When the
BiVO4 sample is fabricated at pH of 2.05, it looks olive-like structure (Fig. 12(a)), while it is
steadily changing into a spherical structure when the pH value varied from 2.05 to 4.02 (Fig.
12(a-c)) and then into cuboids structure when the pH values altered up to 6.00 (Fig. 12(d-e)). On
the other hand, it is noted that the BiVO4 synthesized without EG has an irregular dendritic
structure (Fig. 12(f)). This specifies the vital role of EG exist in the synthesis precursor for the
creation of the hierarchical architecture. Thus, the BiVO4 with various kinds of hierarchical
architecture can be synthesized efficaciously via template free solvothermal process with an EG-
H2O mixture solvent via regulating the pH of the metal precursor. As a result, they reported that
the olive-like BiVO4 synthesized at pH of 2.05 showed ~95.7% degradation of MB within 1 h
under visible light conditions. This enhancement of photocatalytic performances is mainly
credited to the large surface area (6.01 m2/g) that can make easy the adsorption of more MB
molecules on surface of BiVO4.
BiVO4 single-crystal nanoparticles with polyhedral, rod-like, tubular, leaf-like and
spherical morphologies have been synthesized by Meng et al. [48] using a hydrothermal process
in the attendance of triblock copolymer P123 as a surfactant and their photocatalytic
performances were assessed toward the decomposition of MB under visible-light conditions. Fig.
13 shows the growth mechanisms of the BiVO4 nanoparticles with different morphologies under
dissimilar hydrothermal circumstances. In surfactant-free synthesis method, amorphous
nanoparticles were first formed at the first stage, these primary nanoparticles then accumulated
and self-assembled rendering to the favored orientation at a pH of 1, 6 or 9, and lastly
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crystallized into polyhedral or spherical BiVO4 particles via an Ostwald ripening process after
annealing at 400 oC. While in P123 assisted method, part of the surfactant molecules adsorbed on
the surface of the BiVO4 nuclei to reduce the surface energy of the nanocrystals. The adsorbed
P123 engaged as a capping agent to control the growth rate and thus creating 2D BiVO4
nanosheets. Under dissimilar hydrothermal conditions (pH = 1, 6, 9 or 10), the concurrent co-
operative self-assembly of these 2D nanoentities after the Ostwald ripening route and annealed at
400 oC provided upsurge to BiVO4 single-crystallites with a polyhedral, rod-like, leaf-like,
tubular, hollow spherical or porous spherical morphologies. Among various BiVO4 samples, the
ones derived hydrothermally with P123 at pH=6 or 10 influenced outstanding absorption
behavior and exhibited exceptional photocatalytic efficiency of the addressed reaction. The
higher visible-light activity of rod-like and tubular BiVO4 morphology is related to the greater
surface areas and concentrations of surface oxygen defects, and distinctive particle
morphologies. From the result, it could be observed that the hydrothermal temperature, pH and
surfactant have a significant impact on the morphology of the product.
Sun et al. [49] synthesized BiVO4 quantum tubes with a diameter of 5 nm and thickness
of 1 nm, via a hydrothermal method. The photocatalytic activity of synthesized BiVO4 quantum
tubes was compared with that of bulk BiVO4 for the degradation of RhB under visible light
illumination. After 15 min of illumination, the photodegradation of RhB reached close to 100%
over BiVO4 quantum tubes, which is ten times higher than that of bulk BiVO4. The enhanced
photocatalytic activity of BiVO4 quantum tubes can be credited to its physical properties such as
quantum size effect, BET surface area and morphology. In particular, the ultra-narrow diameter
of 5 nm and ultrathin wall thickness of 1 nm could allow for more efficient transfer of electron-
holes, generated inside the crystal, on the surface. Meanwhile, its extremely large surface area of
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44.2 m2/g (60 times higher than that of bulk BiVO4 (0.7 m2/g)) provides more active sites. Both
enhanced surface area and quantum size effect contribute to the higher photocatalytic efficiency.
Sharma et al. [50] synthesized octahedral shaped BiVO4 by hydrothermal method using SDBS as
a surfactant. The synthesized octahedral shaped BiVO4 showed enhanced photocatalytic activity
under visible light irradiation for degradation of MB and enhanced photoinduced antimicrobial
activity towards Escherichia Coli (E. coli) inhibition. From the results they reported that, after
135 min irradiation around 62% of MB got degraded and also 96% of E.coli growth is inhibited
under 2 h of illumination. In Table 1, we summarized the controlled synthesis of BiVO4 with
various morphologies and its photocatalytic performances.
4. Modification of BiVO4 with metal and non-metal elements
4.1. Surface modification of BiVO4 with graphene
Graphene or reduced graphene oxide (RGO) is a 2D material with carbon atoms
organized in a hexagonal “honeycomb” lattice construction. Graphene is used as a novel
semiconductor supporter, which has various exclusive properties such as large surface area, high
flexibility, high charge carrier mobility and high transparency. Graphitic carbon might improve
the transport of photogenerated charge carriers in semiconductor materials, which is credited to
the great quantity of delocalized electrons from the conjugated sp2-bonded carbon network [59].
The combination of photocatalyst and graphene or RGOs is promising to possess excellent
adsorption property, conductivity and controllability simultaneously, which would facilitate
effective photodegradation of pollutants [26].
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Yu et al. [59] synthesized sphere shaped BiVO4/RGO composites by hydrothermal
process and evaluated the photocatalytic efficiency towards the decomposition of RhB under
regular sunlight irradiation. In comparison with pure BiVO4 the optimized composite displayed
improved photodegradation efficiency for RhB under natural sunlight illumination. After 8 h of
irradiation, optimized composite exhibited about ~69% photodegradation of RhB while pure
BiVO4 showed less than 31% removal. The higher photocatalytic performance of the
BiVO4/RGO composite materials is credited to the arrangement of definite BiVO4/RGO
interfaces, which significantly increase the charge separation efficiency. Yan et al. [60] also
synthesized RGO/BiVO4 composite by a microwave assisted in situ growth method and
evaluated the catalytic efficiency for the decomposition of ciprofloxacin (CIP) under visible-light
conditions. The optimized composite exhibited highly enhanced (~68%) photodegradation of
CIP in 60 min compared with pure BiVO4 (22.7%). The improver of photocatalytic activity of
RGO/BiVO4 photocatalysts can be credited to the successful separation of charge carriers and
enhancement of light absorption properties. A graphene decorated 3D double layer half open
flower BiVO4/rGO was synthesized via the facile solvothermal route by Ou et al. [61] and the
catalytic activity was evaluated towards the photodegradation of the aqueous RhB. Fig. 14 shows
the morphology of rGO, BiVO4 and BiVO4/rGO composite. The SEM images of pure rGO (Fig.
14 (a1,a2)) show the 2D crumpled layered structure and pure BiVO4 exhibits 3D double layer half
open flower superstructure the diameter of the half-open flower is about 4.5 μm and the length of
the rod is about 5 μm (Fig. 14 (b1,b2)). The SEM images of BiVO4/rGO composite (Fig. 14
(c1,c2)), where the rGO sheet can also clearly be seen (marked with red circles). However, when
the GO was introduced, the size of BiVO4/rGO is about 2 times reduced from ~4.5 μm
(diameter) and ~5 μm (length) to ~2 μm and ~3μm, respectively. In contrast with pure BiVO4,
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the BiVO4/rGO composite demonstrated an outstanding photocatalytic performance for the
decomposition of RhB. The BiVO4/rGO composite was exhibiting the best photocatalyst which
decomposed ~94% of RhB dye within 90 min of visible-light illumination.
Li et al. [62] demonstrated that BiVO4 loaded with RGO photocatalyst displayed superior
efficiency for the decomposition of RhB under natural sunlight. The optimized composite
demonstrated ~100% decomposition of RhB in 8 h while the photocatalytic activity of pure
BiVO4 (96%). The enhancement in photocatalytic activity was due to the superior light
absorption efficiency and reduced charge recombination speed within the hybrid structures with
the presence of RGO. Dong et al. [63] fabricated bismuth vanadate (BiVO4) incorporate reduced
graphene oxide (RGO) by the sonochemical approach and the photocatalytic performances were
assessed by the decomposition of the RhB. In contrast with pure BiVO4 the optimized BiVO4
incorporated RGO composite exhibited enhanced photodegradation efficiency for RhB under
natural sunlight irradiation. After 10 h of illumination, optimized composite showed about 99%
photodegradation of RhB while pure BiVO4 showed less than 34% removal. The enhancement in
the photocatalytic efficiency when including RGO with BiVO4 could be credited to the exclusive
morphological structure, superior light absorption efficiency and slow recombination rate of the
composites.
4.2. Modification of BiVO4 photocatalysts with metal and non-metal elements
Semiconductor with metal and non-metal ion doping is one of the strategies to slow down
the recombination rate of photoinduced charge carriers. Generally, a doping material is crucial
components, which may enhance the activity of the photocatalytic activity.
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Doping with non-metals can enhance charge separation, reduce band gap energies and
alter the absorption band in the visible region in semiconductor materials. In 1986, Sato and co-
workers first explored the photocatalytic activity of N doped TiO2 photocatalyst for ethane and
carbon monoxide oxidation. They found that N/TiO2 photocatalyst displayed a superior
photocatalytic activity than pure TiO2 [64]. Thereafter attentions are given to the doping of
BiVO4 with non-metals. Some examples of the non-metal ions doped BiVO4 catalyst used for
different dye degradation studies along with their preparation technique are given in Table 2.
Min et al. [68] prepared La-B/BiVO4 photocatalysts by the sol-gel process. The
optimized La-B/BiVO4 composite (0.075 min-1) showed much higher photocatalytic degradation
rate constant for MO than that of the pure BiVO4 (0.004 min-1) and B/BiVO4 (0.007 min-1) under
visible-light irradiation. The higher photocatalytic performance of the La-B/BiVO4 was credited
to the combined effects of lanthanum and boron efficiently promoting the separation of
photogenerated charge carriers. Chen et al. [69] synthesized Cu-doped BiVO4 by the microwave-
assisted hydrothermal process. The optimized composite was highly active for both UV and
visible-light photodegradation of MB. The possible photodegradation of Cu/BiVO4 system is
shown in Fig. 15. During the irradiation process, photoinduced electrons in the CB and holes in
the VB became adjacent to each other owing to the electrostatic attraction, resulting in the
recombination of charge carriers. The charge carriers were reorganized when the quantity of Cu
doping was suitable, because the work function of Cu was inferior in comparison with BiVO4,
the energy band of a semiconductor can bend downward to create a Schottky barrier. Moreover,
excited electrons do not shift from a higher Fermi level of BiVO4 to a lower Fermi level of metal
until their Fermi energy levels were the similar. Cu2+ became an efficient trap for excitation
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electrons, subsequent in the separation of photogenerated carriers, thus, avoiding the
recombination of carriers, and enhancing the performance of photocatalyst significantly. Zhang
et al. [70] synthesized Au/BiVO4 composite by hydrothermal route and the photocatalytic
performance was explored by the decolorization of MO under visible-light irradiation. 1.48 wt%
Au/BiVO4 composite exhibited a superior photocatalytic activity than other samples. The Au
metal ion plays a significant role in enhancing charge carrier separation and suppressing there
combination effect leading to a higher photocatalytic activity.
Wang et al. [71] prepared nitrogen and samarium co-doped BiVO4 (N-Sm/BiVO4)
nanocomposite by a sol-gel process using the corn stem as a template and explored the
photodegradation of MO under sunlight. The optimized Sm-N/BiVO4 showed a superior
photocatalytic activity compared to the other samples. The enhancement of photocatalytic
activity credited to the combined effects of the nitrogen and samarium ions in the composite
photocatalyst. Bian et al. [72] synthesized Cu/BiVO4 and Ag/BiVO4 composite by hydrothermal
process. The photocatalytic performance was assessed by the decomposition of ibuprofen under
visible-light illumination. The Cu/BiVO4 catalyst showed 89% decomposition efficiency for
ibuprofen and the Ag/BiVO4 catalyst showed the superior performance of 96% degradation
efficiency in 5 h. The enhancement of photocatalytic performances was credited to lower band
gap energies and the higher specific surface areas, which can slow down the recombination of
charge carriers and provide additional active sites for the absorption and exclusion of degraded
constituents. Karunakaran et al. [28] synthesized Tl-doped BiVO4 by hydrothermal process and
evaluated the photodegradation efficiency of MB under visible-light illumination. As a result,
they reported that 19.3 at% Tl/BiVO4 displayed a superior photocatalytic activity than those of
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the other Tl-BiVO4 and pristine BiVO4. The superior photocatalytic performance of the Tl-
BiVO4 was credited to the thallium doping, which reduces the recombination of photogenerated
electron and hole pairs. Wang et al. [73] synthesized cocoon-like hierarchical structure of
Cu/BiVO4 by solvothermal route using EDTA as the chelating agent and utilized as
photocatalyst for the photodegradation of the RhB. The optimized Cu/BiVO4 composite
photocatalyst displayed a higher activity, which were 5 folds higher than that of pure BiVO4. The
improvement of photocatalytic activity was attributed to morphology, improved light absorption,
lower band gap and reduced recombination rate of excited charge carriers. The advanced
photocatalytic activity of Cu/BiVO4 photocatalyst can be implicit according to the proposed
mechanism as shown in Fig. 16. Under visible-light illumination, the Cu/BiVO4 photocatalyst is
activated and electrons (e-) from VB are promoted to the CB, with holes (h+) left in VB. The
electrons trapped by O2 molecules adsorbed on the surface of Cu/BiVO4 acting as reactive sites
to produce O2•- radicals. In the meantime, the photoinduced holes of the VB can react with water
and OH- to produce •OH radicals as strong oxidizing agents. The •OH radicals or photoinduced
holes as active species can attack RhB molecules. Moreover, the excited electrons on the CB
could be arrested by Cu2+ and V4+, resulting in enhanced charge separation efficiency of
photoinduced charge carriers. Due to this phenomenon, more quantities of electrons and holes
get promoted and involved in the photocatalytic reaction, which resulting in a superior
photocatalytic degradation efficiency for RhB.
Geng et al. [74] prepared Co-doped BiVO4 (Co/BiVO4) via a hydrothermal process using
EDTA as the chelating agent and the photocatalytic performances was assessed by the
photodegradation of MB. As a result, they reported that optimized Co/BiVO4 composite exhibit
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superior photocatalytic activity in comparison with pure BiVO4. The enhancement of
photocatalytic performances was attributed to the improved light absorption, the smaller band
gap and the slow recombination rate of excited carriers. Xue et al. [75] synthesized Eu-F-co-
doped BiVO4 microspheres with diameter of ~1-3 µm via a hydrothermal path and utilized it as
visible-light active photocatalyst for the decomposition of RhB as a model organic pollutant. The
Eu-F/BiVO4 microspheres exhibited greatly enhanced photocatalytic performance than that of
bare BiVO4, single Eu or F doped BiVO4 materials. The improved photocatalytic activity was
credited to the co-dopant of Eu and F gave rise to the synergistic effects, viz., the increase in
surface area and large separation efficiency of the photo-induced electrons (e-) and holes (h+).
Regmi et al. [76] synthesized Ni-doped BiVO4 by microwave assisted hydrothermal process and
evaluated the photocatalytic activity toward the degradation of ibuprofen. The optimal 1 wt%
Ni/BiVO4 sample exhibited excellent photocatalytic degradation efficiency (92%) for ibuprofen.
The in-gap energy state and the oxygen vacancies serve as an electron-trapping center that
decreases the migration time of the photogenerated carrier and increases the separation efficiency
of electron-hole pairs. Shi et al. [77] synthesized Pt-doped BiVO4 by the microwave assisted
method and the photocatalytic performance was evaluated by the photodegradation of
ciprofloxacin under visible-light irradiation. 2 wt% Pt/BiVO4 photocatalyst exhibited the higher
degradation efficiency (92%), which is 25% higher when compared with pure BiVO4. The
dopant Pt acts as charge collectors and transporters as well as producing more reactive sites on
the photocatalyst surface which led to improved photodegradation of ciprofloxacin. Yin et al.
[78] synthesized hierarchical butterfly wing structure of C-doped BiVO4 by a sol-gel method
using Papilio pairs butterfly wings as a template and utilized it as visible-light assisted
photocatalyst for the decomposition of MB. As a result, they reported that the optimized
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C/BiVO4 photocatalyst exhibited an enhanced photocatalytic performance. The superior
photocatalytic performance was credited due to the improved separation of photogenerated
charge carriers.
Chen et al. [79] synthesized hollow and porous structures of Ag-doped BiVO4 composite
by a sol-gel method using L-lysine as a surfactant. The catalytic activity of the Ag/BiVO4
composite was assessed for the photodegradation of MB and RhB. The 6.5 wt% Ag/BiVO4
catalyst demonstrated superior photocatalytic activity than BiVO4. The admirable photocatalytic
activity of 6.5 wt% Ag/BiVO4 is due to its hollow and porous structure, large specific surface
area. Xu et al. [80] demonstrated that BiVO4 loaded with Ag nanoparticles exhibited high
efficiency for the decomposition of MB. From Fig. 17(a), it can be evidenced that the DRS
spectrum of Ag doped BiVO4 photocatalyst showed a red shift when compared with pure BiVO4.
Fig. 17(b) shows the calculated band gap values of pure BiVO4 and Ag/BiVO4 samples to be
2.39 eV and 2.30 eV, correspondingly. The VB edges of pure BiVO4 and Ag/BiVO4 samples
were calculated to be 1.94 eV and 1.51 eV (Fig. 17(c)) and the conduction band (CB) of the
Ag/BiVO4 sample was shifted to a more negative position when compared to that of pure BiVO4
(Fig. 17(d)). These behaviors caused in a distinctive electronic structure that increased the period
of photocatalysis and enhanced the photo-oxidation capability of the photogenerated charge
carriers, encouraging visible-light photocatalysis. As a result, the photocatalytic activity and
oxidation ability of the Ag/BiVO4 toward MB were drastically improved.
Chala et al. [81] synthesized pure BiVO4 and Fe-loaded BiVO4 photocatalysts by
hydrothermal process and the photocatalytic performances were assessed towards the
photodegradation of MB. Fe/BiVO4 sample showed an outstanding photocatalytic activity
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compared to pure BiVO4. An optimal iron loading was found to be 5.0 mol% showing the best
photodegradation efficiency (81%) under visible-light illumination. The improved photocatalytic
activity upon iron loading is probably owing to the more efficient electron trapping by Fe3+ ions,
resulting an enhanced electron-hole separation. Recently, Regmi et al. [82] synthesized Fe-doped
BiVO4 photocatalyst by microwave assisted hydrothermal process and employed in the
photodegradation of ibuprofen under irradiation with visible-light. XRD (Fig. 18(a)) data show
that the addition of Fe3+ resulted in the appearance of characteristic peaks for the tetragonal
BiVO4 at 24.5o and 32.3o. This result suggests that the presence of Fe3+ ions might induce the
formation of monoclinic and tetragonal heterostructures. In addition, the change in the diffraction
peak position also revealed that the attachment between the Fe3+ and BiVO4 in the composite
(Fig. 18(b). The optimal 1 wt% Fe-doped BiVO4 showed the superior photocatalytic activity,
showing 80% ibuprofen degradation within 180 min of irradiation with visible-light. The
formation of a heterostructures promotes the interfacial charge transfer between phases and slow
down the internal recombination emphasizes the photoinduced charge carriers, which facilitated
the higher photocatalytic activity.
Visible-light driven Pd-doped BiVO4 photocatalyst was synthesized Ge et al. [83] by the
impregnation process and the photocatalytic activity assessed towards the photodegradation of
MO. From Fig. 19(a), 1.0 wt% Pd/BiVO4 exhibited highly enhanced (~100%) photodegradation
of MO in 15 h compared with pure BiVO4 (~42%). From Fig. 19(b), the PdO particles can act as
electron traps encouraging the charge carrier separation and significantly improve the
photocatalytic activities of Pd/BiVO4 sample.
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Luo et al. [84] synthesized Gadolinium-doped BiVO4 (Gd/BiVO4) composite by
microwave assisted hydrothermal method and explored its photocatalytic activity towards
photocatalytic degradation of RhB under visible light irradiation. XRD (Fig. 20) result shows
that the pure BiVO4 (PDF 75-1866) crystallized in the monoclinic system, whereas all Gd/BiVO4
composite exhibits the characteristic diffraction peaks of tetragonal BiVO4 phase (PDF 14-0133).
This result inferred that Gd doping alter the phase of BiVO4 from monoclinic to tetragonal and
promote the formation of BiVO4 tetragonal phase. The optimal 10 at% Gd/BiVO4 showed the
best photocatalytic activity (~96% degradation of RhB) under 120 min of sunlight illumination.
Gao et al. [85] prepared Neodymium-doped BiVO4 (Nd/BiVO4) composite with rod-like
morphology by hydrothermal method using SDBS as a surfactant. They reported that the doping
of Nd amount did not change the crystallinity of BiVO4 and the photocatalytic performance was
evaluated for the degradation of phenol and desulfurization of the thiophene under visible light
irradiation. The optimal 0.8 wt% Nd/BiVO4 composite showed the highest photocatalytic
activity for both the photodegradation of phenol and desulfurization of thiophene, which was 14
times higher than that of pure BiVO4. Zhang et al. [86] synthesized Europium-doped BiVO4
(Eu/BiVO4) composite by hydrothermal method and employed it as visible light driven
photocatalyst for the degradation of MO. After 180 min of illumination, the optimal 1.46 wt%
Eu/BiVO4 composite degraded almost 100% of MO solution. This enhanced photocatalytic
activity could be due to the absorption of light in the visible region and lowering the
recombination rate between the photogenerated charge carriers.
Regmi et al. [87] prepared visible light active Co/BiVO4 photocatalyst by microwave
assisted hydrothermal method. Then the synthesized photocatalyst was explored for the
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degradation of malachite green and inactivation of Escherichia Coli (E. coli) and
Chlamydomonas pulsatilla bacterial cell growth. Under 90 min of illumination, around 99% of
malachite green was degraded and the pure BiVO4 degrade only 61% under 130 min of
illumination. Also, the inactivation of E.coli reaches 81.3% in 5 h and Chlamydomonas pulsatilla
reaches 65.6% in 1 h irradiation to visible light. The enhanced photocatalytic activity is credited
to be due to the enhanced light absorption in the visible region and narrow band gap energy.
Booshehri et al. [88] synthesized Ag/BiVO4 composite and their photocatalytic disinfection
activity were evaluated towards Escherichia Coli (E. coli) under visible light illumination. Under
3 h of illumination, around 100% of E.coli bacterial growth was inhibited in presence of
Ag/BiVO4 composite photocatalyst. This enhancement in the photocatalytic activity of
Ag/BiVO4 was attributed due to the effect of metallic silver nanoparticles, which act as an
electron trap on the surface of BiVO4 and enhance the separation of photogenerated charge
carriers for the generation of reactive oxygen species. In Table 3, we summarize the doping with
metal or non-metal elements into BiVO4 photocatalyst for the decomposition of organic
contaminants.
5. BiVO4 based composites
The construction of semiconductor composites is a successful strategy to enhance the
charge separation and slows down the recombination rate of photoinduced electron and hole
pairs, which has been broadly studied for the last decades. It was established that BiVO4 could be
coupled with various semiconductors, such as TiO2 [93], Cu2O [94-96], g-C3N4 [97,98], Bi2WO6
[99-101,15] and BiOCl [102].
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For example, Zhang et al. [93] synthesized TiO2/BiVO4 composite photocatalyst via
single step microwave assisted hydrothermal process and evaluate the photocatalytic
performance towards the degradation of RhB under UV and sunlight irradiation. Fig. 21
illustrates the photodegradation of RhB under UV-light and natural sunlight irradiations. From
the Fig. 21(a,b), the optimal 20% TiO2/BiVO4 composite exhibited a superior photocatalytic
performance in comparison with pure BiVO4. The higher photocatalytic activity of the
TiO2/BiVO4 was credited to the increasing separation efficiency of photoinduced charge carriers.
Wang et al. [94] used hydrothermal method for the fabrication of Cu2O/BiVO4
composite. In comparison with pure BiVO4 the optimized composite exhibited enhanced
photodegradation efficiency for MB and phenol under visible-light illumination. After 4 h of
irradiation, optimized composite exhibited about 97.3% photodegradation of MB while pure
BiVO4 showed less than 78.3% removal. At the same time Wang et al. also constructed
Cu2O/BiVO4 composite by solvothermal method. The optimized composite was highly efficient
under visible-light for degradation of MB. The rate constant of photodegradation of MB over the
optimized composite reached 0.00918 min-1 which was two-fold enhancement than that of pure
BiVO4 [95]. Similarly, a simple impregnation method was utilized by Ruiz et al. [96] for the
fabrication of Cu2O/BiVO4 composite and the photocatalytic performance was assessed towards
the decomposition of the MO solution in the presence of visible-light. The optimized
Cu2O/BiVO4 composite exhibited highly enhanced (~86%) photodegradation of MO in 4 h
compared with pure BiVO4. This superior photocatalytic performance might be due to the
successful charge separation of electron-hole pairs owing to the creation of a heterojunction
between Cu2O/BiVO4 nanoparticles. The probable degradation mechanism for the MO over
Cu2O/BiVO4 heterojunction under visible-light irradiation is portrayed in Fig. 22.
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Tian et al. [97] synthesized g-C3N4/BiVO4 heterojunction by a simple hydrothermal
process and the catalytic activity was assessed towards the decomposition of MB under visible-
light illumination. The degradation rate constant for the optimized composite reached 0.02985
min-1 which was 3.5 and 2.8 times superior compared with pure g-C3N4 and BiVO4. The
remarkable improvement in catalytic activity of the composite was due to the photoinduced
electrons on the CB of BiVO4 are capable to decrease the adsorbed O2 molecule on the surface of
the semiconductor to produce superoxide radical (O2•-), that degraded the dye molecules due to
the CB standard redox potential of BiVO4 is more negative than the standard redox potential
(O2/O2•-(-0.28 eV)). The photogenerated hole in the VB of C3N4 cannot oxidize H2O to form •OH
owing to its deficient oxidizing ability (H2O/•OH (2.27 eV)). Therefore, the main reactive specie
of g-C3N4/BiVO4 heterojunction could be photogenerated O2•- for the decomposition of MB. The
probable degradation mechanism for the MB over g-C3N4/BiVO4 heterojunction under visible-
light illumination is depicted in Fig. 23. Similarly, Guoet al. [98] synthesized g-C3N4/BiVO4
nanocomposite by a simple hydrothermal process and make use as a visible-light response
photocatalyst for the photodecomposition of the RhB. The optimized nanocomposite
demonstrated ~100% removal within 40 min that was faster than pure g-C3N4 and BiVO4.
In particular, the Bi2WO6/BiVO4 nanocomposite photocatalysts were extensively
investigated by varying the synthesis methods and conditions. For instance Geng et al. [99]
synthesized BiVO4/Bi2WO6 composite via a hydrothermal process. The photocatalytic behavior
ofBiVO4/Bi2WO6 composite was assessed by photodegradation of MB. For the optimized
composite, the degradation rate constant was 0.0055 min-1 which was about 4.0 and 2.3 times
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higher than that of the pure Bi2WO6 and pure BiVO4, correspondingly. Chaiwichian et al. [100]
also worked on the same photocatalyst system, a heterojunction of BiVO4 and Bi2WO6. The
aggregated nanoparticles of BiVO4 were deposited onto the surface of Bi2WO6 nanoplate by a
hydrothermal treatment process. The photocatalytic activity of nanocomposite was evaluated for
the photocatalytic degradation of MB under visible-light illumination. In comparison with pure
BiVO4 and Bi2WO6, the optimized heterojunction was highly active for the photodegradation of
MB. Chen et al. [15] fabricated self-assembled hierarchical BiVO4/Bi2WO6 nanocomposite
through template free solvothermal method and utilized as a visible-light reactive photocatalyst
for the decomposition of MB. After 7 h of irradiation, the photocatalytic activity of the optimized
nanocomposite exhibits higher efficiency, this was about 3-fold enhancement in comparison with
pure BiVO4 and Bi2WO6. This improved photocatalytic activity due to the matching band energy
level created the heterojunction which could successfully improve the separation efficiency of
photoinduced charge carriers for the degradation of organic contaminations. A likely
photocatalytic phenomenon for the photodegradation of MB dye over BiVO4/Bi2WO6
heterojunction depicted in Fig. 24. Zhang et al. [101] also work the same Bi2WO6/BiVO4
composite system for the visible-light driven photodegradation of phenol. As a result, they
reported that the optimized composite demonstrated ~83.5% decomposition of phenol in 6 h
while the photocatalytic activity of pure BiVO4 (47.8%) was very low.
Song et al. [102] fabricated BiOCl/BiVO4 heterojunction by hydrothermal process and
the effects of hydrothermal temperature on the morphology and photocatalytic performance of
BiOCl/BiVO4 were also studied. In contrast with pure BiOCl and BiVO4 the optimized
composite exhibited enhanced photodegradation efficiency for RhB. After 4 h of illumination,
optimized heterojunction exhibited about 96% photodegradation of RhB which was two-fold
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enhancement in comparison with the pure BiVO4. Fig. 25 depicted the schematic photocatalytic
mechanism of the p-n heterojunction BiOCl/BiVO4 photocatalysts under visible-light
illumination. From the Fig. 25 it can be observed that the BiVO4 is doped with BiOCl, the band
arrangement of the composite is distorted, creating a p-n heterojunction with an interior electric
field at the boundary among the p-type BiOCl and n-type BiVO4 semiconductors. Under visible-
light irradiation, the BiVO4 is excited, resulted in the carrier generation in their surface. The
photogenerated holes are then transported to the VB of BiOCl, at the same time the electrons in
the VB transferred to the CB of BiVO4 catalyst surface. Concurrently, the photosensitization
influence of RhB on BiOCl effects in the migration of the photoexcited electrons from RhB
molecules to the CB of BiOCl. Then, the excited electrons tend to transfer into the CB of BiVO4
motivated by the force of the internal electric field. At last, the photoexcited charge carriers are
separated effectively credited to the generated p-n heterojunction among BiOCl and BiVO4,
diminishing the recombination of charge carriers and extending the lifetime of the carriers.
Natarajan et al. [103] synthesized a nanoplate GeO2/BiVO4 composite via a hydrothermal
technique. The composite material was utilized for the decomposition of RhB under direct
sunlight irradiation. The optimized composite demonstrated ~92.2% degradation of RhB in 4 h
while the photocatalytic performance of pure BiVO4 (~40.9%) and GeO2 (~19.9%) was very
low. The improvement of photocatalytic efficiency mainly due to the effective trapping of BiVO4
surface electrons by germanium of GeO2 as well as the charge transfer from BiVO4. Fan et al.
[13] fabricated visible-light active BiFeO3/BiVO4 heterojunction by hydrothermal process. The
catalytic activity was tested for the photodegradation of the RhB. The optimized heterojunction
exhibited the highest rate (0.0146 min-1) of RhB degradation which was 29.2 and 7.3 times
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higher compared with pure BiFeO3 and BiVO4, respectively. Mao et al. [7] constructed porous
Bi2O3/BiVO4 p-n heterojunction composite micro-rods by solvothermal process followed by
annealing treatment. The optimized composite was highly efficient for photodegradation of
phenol under visible-light irradiation. For 60 min illumination, Bi2O3/BiVO4 exhibited 96.3%
degradation while pure BiVO4 showed very low (0.6%) efficiency. This superior photocatalytic
behavior was explained by the following reasons, based on the band gap energy of BiVO4 and
Bi2O3, and the radical trapping experiments results (Fig. 26) a probable direct Z-scheme system
is projected to describe the improved photocatalytic activity of the Bi2O3/BiVO4 which is
systematically demonstrated in Fig. 26(b). BiVO4 is a n-type semiconductor with a band gap of
2.15 eV, and its calculated conduction band (CB) and VB energy levels are 0.63 and 2.78 eV.
And Bi2O3 is a p-type semiconductor with a band gap of 2.50 eV, with the calculated CB and VB
values are -0.46 eV and 2.04 eV, respectively. After these two component materials get into
interaction, a p-n heterojunction will be created when their Fermi levels attained an equilibrium
state and an inner electric field, directing from the BiVO4 side to Bi2O3, is made across the
interface. The Fermi level of Bi2O3 tends to move up, at the same time BiVO4 tends to move
down to attain the equilibrium state, resulting in that the total energy band of Bi2O3 is raised up
and BiVO4 is moving down. Under visible-light illumination, both BiVO4 and Bi2O3 are excited
creating photoinduced charge carriers. The photoinduced electrons on the CB of Bi2O3 possess a
greater reducibility and photoinduced holes on the VB of BiVO4 have a greater oxidizability
activity in this heterostructured system. According to the classical double charge migration
mode, the photogenerated electrons on the CB of Bi2O3 migrate to the CB of BiVO4, while holes
on the VB of BiVO4 migrate to the VB of Bi2O3. Conversely, since O2•- is one of the major
reactive species, but the CB potential of BiVO4 is lower than the standard reduction potential. It
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is not possible to generate the main reactive species O2•- through double charge migration
mechanism in this photocatalytic process (Fig. 26(a)). Due to this phenomenon, a direct Z-
scheme electrons and holes migrate mode is planned to describe the improved photocatalytic
behavior. In this heterostructure system, both BiVO4 and Bi2O3 can be photoinduced under the
visible-light, the holes on the VB of Bi2O3 and the electrons on the CB of BiVO4 recombination
straight at their interface region. The electrons on the CB of Bi2O3 are arrested by the surface O2
to give O2•- and the residual holes in BiVO4 can directly oxidize phenol. Herein, the
Bi2O3/BiVO4 is a category of direct Z-scheme photocatalysts candidates and the photocatalytic
activity is drastically enhanced based on this charge transfer efficiency.
Huang et al. [14] synthesized BiIO4/BiVO4 composite by facile hydrothermal method and
utilized as a visible-light active photocatalyst towards the photodegradation of the RhB. The
optimized composite encouraged the separation of photoinduced charges and thus enhanced the
degradation efficiency of RhB under visible-light illumination. A reaction rate constant of 0.28 h-
1 was exhibited by the optimized composite, a ten-fold enhancement over that of pure BiIO4 and
pure BiVO4. The greater photocatalytic performance of the BiIO4/BiVO4 composite was due to
the efficient interfacial charge migration of photogenerated charge carriers. Fang et al. [104]
fabricated CdS/BiVO4 composite via a simple solvothermal process. The photocatalytic activity
was assessed towards the decomposition of malachite green (MG) under visible-light
illumination. The synthesized 1.5-CdS/BiVO4 nanocomposites had noticeably improved
photocatalytic activity for the decomposition of MG. The enhanced light absorption properties of
CdS/BiVO4 composite led to the formation of more photoinduced electron-hole pairs, thus
improve the photocatalytic activity. The optimized composite demonstrated 98.3% removal
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within 120 min that was faster than the pure BiVO4 nanoparticles. A hydrothermal method was
adopted by Li et al. [105] to prepare BiVO4/SrTiO3 composite and evaluated the photocatalytic
activity for the photodegradation of sulfamethoxazole. After 60 min irradiation, ~91% of the
sulfamethoxazole was degraded over the optimized BiVO4/SrTiO3 composite while only about
54% and 23% degrade on a pure BiVO4 and SrTiO3, respectively. Zhao et al. [106] used in-situ
hydrothermal method for the synthesis of a BiVO4/MoS2 p-n heterojunction and utilized as a
visible-light active photocatalyst for the photodegradation of crystal violet (CV) and chromium
reduction Cr6+. The optimized composite displayed 76.5% reduction of Cr6+ within 90 min and
69.2% degradation of CV within 60 min. Under the similar conditions, pure BiVO4 reached only
12.1% and 22% reduction of Cr6+ and degradation of CV. The improvement of photocatalytic
activity was credited due to the reduction of charge recombination, the greater specific surface
area and strong adsorption ability towards the degradation of pollutant molecules. Li et al. [107]
synthesized BiVO4/FeVO4 heterojunction by hydrothermal method and utilized as a visible-light
active photocatalyst for the decomposition of metronidazole (MNZ). In BiVO4/FeVO4
heterojunction a huge number of FeVO4 particles are gathered around BiVO4 nanoparticles,
which was studied by TEM (Fig. 27(a)). From the HR-TEM images as presented in Fig. 27(b,c)
expose the ideal crystal structure and clearly differentiate the (121) and (112) lattice plane of
monoclinic-structured BiVO4 and triclinic-structured FeVO4 with interplanar spacings of 0.31
nm and 0.32 nm. After 4 cycles run the optimized catalyst for the degradation of metronidazole
showed a high stability (Fig. 27(d)). Yan et al. [108] synthesized a monoclinic-tetragonal BiVO4
heterojunction by microwave assisted method and utilized as visible-light active photocatalytic
performances for the decomposition of tetracycline. The M-T BiVO4 heterojunction exhibited
enhanced photodegradation efficiency for tetracycline. The M-T BiVO4 composite showed
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80.5% degradation of tetracycline within 60 min while the photocatalytic activity of pure M-
BiVO4 (60.2%) and T-BiVO4 (17.3%) was very low. Fig. 28 shows the photocatalytic
degradation mechanism of M-T BiVO4 heterojunction in the presence of visible-light
illumination. From Fig. 28, the heterojunction structure between T-structure and M-structure of
BiVO4, electrons on T-structure CB can easily transfer into the CB of M-structure at the similar
time holes on T-structure VB can also transfer into M-structure VB, which successfully slow
down the recombination of electron-hole pairs and promote photocatalytic activity.
Sun et al. [109] synthesized BiVO4 quantum tubes/graphene nanocomposite and explored
its photocatalytic activity for the degradation of the RhB, MB and MO dye solution and the
obtained results are shown in Fig. 29. It can be observed that the BiVO4 quantum tubes/graphene
nanocomposite exhibits a superior photocatalytic activity compared to that of bulk BiVO4 and
graphene, which is 20 times higher than that of commercial P25 or bulk BiVO4 and 1.5 times
higher than that of m-BiVO4 quantum tubes. The improved visible light driven photocatalytic
properties of ordering BiVO4 quantum tubes/graphene nanocomposite is ascribed due to the
synergistic effect between the m-BiVO4 quantum tubes and graphene sheets.
Ju et al. [110] synthesized nest-like Bi2WO6/BiVO4 composite photocatalyst by one step
hydrothermal method and evaluated its photocatalytic antifouling activity under visible light
irradiation. They reported that among various composites the Bi2WO6/BiVO4-1 composite
exhibited the highest photocatalytic antifouling activity when compared with pure Bi2WO6 and
BiVO4. The Bi2WO6/BiVO4-1 exhibited the best photocatalytic antifouling performance and
about all (100%) of Pseudomonas Aeruginosa (P. aeruginosa), Escherichia Coli (E. coli) and
Staphylococcus Aureus (S. aureus) bacterial growth was inhibited within 30 min. This
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enhancement of photocatalytic activity was attributed due to the development of heterojunction,
which enhance the separation of photogenerated charge carriers. Xiang et al. [111] synthesized
AgI/BiVO4 composite photocatalyst by hydrothermal method and the photocatalytic activity was
evaluated for the photocatalytic degradation of RhB and inhibition of Pseudomonas aeruginosa
(P. aeruginosa) bacterial growth under visible light irradiation. 20% AgI/BiVO4 showed higher
photodegradation and photocatalytic antibacterial activity than pure AgI and BiVO4. 20%
AgI/BiVO4 composite showed the superior photocatalytic performance. Close to 100%
degradation of RhB was observed within 150 min and the antibacterial activity could be achieved
over 100% after 30 min photocatalytic antibacterial reactions. In Table 4, the BiVO4 based
composite photocatalysts for degradation of organic pollutants have been summarized.
Conclusions and prospective
A comprehensive overview of the developments over the past few decades of visible-
light driven BiVO4 based photocatalysts is provided. As a photocatalyst, BiVO4 has many
advantages, such as narrow band gap, low production cost, non-toxic and high photostability
which could find promising applications in the field of visible-light active photocatalysis. We
focused on the morphology control, improvement of photocatalytic efficiency, and applications
of BiVO4 based photocatalyst for wastewater treatment process. The main observations are
summarized as follows:
The photocatalytic efficiency of BiVO4 could be improved by morphology control. In
this strategy various additives, surfactants and templates are used to fabricate a BiVO4 with a
controlled morphology. The synthetic methods and the photocatalytic behavior of numerous
morphologies were introduced systematically. It is found that nanotube and porous morphologies
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of BiVO4 always showed a superior photocatalytic performance, which could be credited to the
larger effective surface area, rapid and effective diffusion of pollutant molecules into nanotube
and porous structures which provides the more active sites for photocatalytic reaction.
The strategies for enhancing the photocatalytic activity of BiVO4, such as surface
modification with metal, non-metal and semiconductor coupling were also reviewed in detail.
The doping of metal and noble metal on the photocatalyst has been confirmed to be a successful
method to increase the photocatalytic activity of BiVO4. Because of metal and noble metal act as
an electron pool and catalyzes O2 reduction by a multi electron reduction process from H2O2 and
H2O instead of the traditional single electron reduction process. Moreover, the coupling of
graphene or rGO could considerably improve the photocatalytic performance of BiVO4. The
enhanced photocatalytic activity was credited to the greater specific surface areas, outstanding
charge migration and electron-hole separation ability of the graphene/BiVO4 composites. The
coupling of BiVO4 with other semiconductor materials with dissimilar energy level creates an
ultimate system to provide a faster charge separation, reduced recombination rate of
photogenerated charge carriers leading to an improved photocatalytic activity for the remediation
of organic pollutant present in the aqueous environment. From the literature, it is found that the
crystallinity, pH of the precursor solution, product morphology, surfactant, surface area and
calcination temperature of the final products have strong effects on the photocatalytic activity.
Also, the construction of p-n heterojunction and Z-scheme heterojunction provides a more
effective charge separation and a rapid charge transfer, which facilitate the higher photocatalytic
activity.BiVO4 could be a potential material for water treatment and environmental remediation
in the coming decades.
Conflicts of Interest
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The authors declare no conflict of interest.
Acknowledgements
The authors Ms. A. Malathi and Dr. J. Madhavan are grateful to the authorities of
Thiruvalluvar University for their support.
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