Facile synthesis of visible-light-driven Cu2O/BiVO4 composites for the photomineralization of recalcitrant pesticides

Article abstract of DOI:10.1039/c7ra08513c

Cu₂O/BiVO₄ composites with different Cu₂O co-catalysts were synthesized by a simple impregnation method at 200 °C for 4 h under N₂ atmosphere. The Cu₂O powder was obtained through the chemical reduction of copper sulfate using different reducing reagents such as ascorbic acid, glucose or fructose. The as-synthesized samples were characterized by X-ray powder diffraction, scanning and transmission electron microscopies, UV-vis diffuse reflection absorption and electrochemical impedance spectroscopy. The photoactivities of the Cu₂O/BiVO₄ composites were evaluated in terms of the mineralization of 4-chlorophenol (4-CP) in aqueous solutions under visible-light irradiation. The role of experimental variables including the preparation method of the co-catalyst, catalyst concentration, and 4-chlorophenol initial concentration in the photocatalytic performance of Cu₂O/BiVO₄ catalysts was analyzed. Likewise, the viability to recover and reuse these photocatalysts was also tested. It was found that the complete abatement of 4-CP led to the formation of chlorocatechol (4-CC) without concomitantly involving the formation pathway of hydroquinone/p-benzoquinone, 4-CC was identified in the light of experimental evidence (UV-vis) and spectral simulation done by using time-dependent density functional theory (TD-DFT) with the polarized continuum model.

Full text of DOI:10.1039/c7ra08513c

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Facile synthesis of visible-light-driven Cu2O/BiVO4
composites for the photomineralization of
Cite this: RSC Adv., 2017, 7, 45885
recalcitrant pesticides†
a
E. Aguilera-Ruiz,a M. de la Garza-Galvan,
P. Zambrano-Robledo,a
´
J. C. Ballesteros-Pacheco,b J. Vazquez-Arenas,c J. Peral
and U. M. Garcıa-
´
d
a
*
Perez
´
Cu2O/BiVO4 composites with different Cu2O co-catalysts were synthesized by a simple impregnation
method at 200 ꢀC for 4 h under N2 atmosphere. The Cu2O powder was obtained through the chemical
reduction of copper sulfate using different reducing reagents such as ascorbic acid, glucose or fructose.
The as-synthesized samples were characterized by X-ray powder diffraction, scanning and transmission
electron microscopies, UV-vis diffuse reflection absorption and electrochemical impedance
spectroscopy. The photoactivities of the Cu2O/BiVO4 composites were evaluated in terms of the
mineralization of 4-chlorophenol (4-CP) in aqueous solutions under visible-light irradiation. The role of
experimental variables including the preparation method of the co-catalyst, catalyst concentration, and
4-chlorophenol initial concentration in the photocatalytic performance of Cu2O/BiVO4 catalysts was
analyzed. Likewise, the viability to recover and reuse these photocatalysts was also tested. It was found
that the complete abatement of 4-CP led to the formation of chlorocatechol (4-CC) without
concomitantly involving the formation pathway of hydroquinone/p-benzoquinone, 4-CC was identified
in the light of experimental evidence (UV-vis) and spectral simulation done by using time-dependent
density functional theory (TD-DFT) with the polarized continuum model.
Received 1st August 2017
Accepted 20th September 2017
DOI: 10.1039/c7ra08513c
rsc.li/rsc-advances
synthesis of quinizarin (dye), clobrate (drug), chlorophenesin
and dichlorophen (fungicides).1
1. Introduction
An extensive amount of research has been carried out in
order to remove numerous compounds of this family by using
different strategies which include physical, biological, and
chemical methods, like absorption onto activated carbon,
microbial degradation and advanced oxidation processes
(AOPs), respectively.3 The use of AOPs in the abatement of
recalcitrant pollutants has been widely known for years, having
the advantages of a potential complete mineralization of
organic pollutants to CO2 achieved under mild conditions. One
of the most efficient AOPs is heterogeneous photocatalysis with
semiconductor materials.
Nowadays, the generation of wastewater from industries and
households is one of the largest environmental problems that
modern society needs to address, since clean water is a non-
renewable resource essential to human life.1 Chlorophenols
are among the most dangerous pollutants to remove from water
due to their high biorecalcitrant character, even at low
concentrations, being extremely dangerous to living organisms.
These compounds are used in different applications including
additives for wood, paint and vegetable ber preservation, or as
disinfectants.2 For example, 4-chlorophenol, is used in the
Since the pioneering work of Fujishima and Honda, back in
1972, TiO2 has been the most used and studied photocatalyst.4–7
Nevertheless, the research about heterogeneous photocatalysis
with other materials has been increasing during the last years;
with the goal of developing new catalytic materials that could be
activated by visible-light irradiation, either through metal or
non-metal doping, surface sensitization, semiconductor
coupling, noble metal deposition, or crystal defect increase.8
Two of those novel semiconductors are cuprous oxide (Cu2O)
and bismuth vanadate (BiVO4), materials that have been widely
studied mainly because their absorption maxima are in the
visible region of the electromagnetic spectrum. Cu2O is a p-type
a
´
´
´
´
´
Universidad Autonoma de Nuevo Leon, Facultad de Ingenierıa Mecanica y Electrica,
´
´
´
´
Centro de Investigacion e Innovacion en Ingenierıa Aeronautica, Carretera a Salinas
Victoria Km 2.3, C. P. 66600, Apodaca, Nuevo Leon, Mexico. E-mail: ulisesma.
´
garcia@gmail.com; Fax: +52 81 83 32 09 03; Tel: +52 81 83 29 40 20
b
´
´
´
´
Universidad Politecnica de Lazaro Cardenas, Departamento de Ingenierıa Ambiental,
´
Laboratorio de Sustentabilidad Energetica, Av. Galeana S/N, Col. 600 Casas, C. P.
´
´
´
60950, Lazaro Cardenas Michoacan, Mexico
c
´
´
´
Centro Mexicano para la Produccion mas Limpia, Instituto Politecnico Nacional, Av.
´
´
Acueducto s/n, Col. La Laguna Ticoman, Ciudad de Mexico, 07340, Mexico
d
`
´
Universitat Autonoma de Barcelona, Department de Quımica, 08193 Cerdanyola del
`
Valles, Spain
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra08513c
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semiconductor with direct band gap between 2.0 and 2.2 eV,9,10 structures using this composite, and extending its applicability
and its use has increased in the recent years due to its low towards other contaminants.
toxicity, good photon absorption and photoelectrochemical
hydrogen production from water splitting. This material can be co-catalysts were synthesized by
In this research, Cu2O/BiVO4 composites with different Cu2O
a
simple impregnation
obtained by different synthesis routes, such as chemical method, and their photoactivities were assessed in terms of the
reduction, microwave-assisted precipitation method, hydro- degradation of 4-chlorophenol under visible-light irradiation
thermal method and chemical oxidation.9–14 On the other hand, for the rst time. The inuence of different experimental
BiVO4 is a n-type semiconductor with direct band gap of 2.4 eV conditions (4-CP initial concentration, photocatalyst concen-
(monoclinic phase), that presents good chemical stability and tration and O2 ux) towards the photomineralization of this
photocatalytic properties.15 This material has been extensively organic compound has been investigated in detail. Multiple 4-
considered as binary and ternary heterostructured photo- chlorophenol by-products are detected during the treatment
catalyst for the abatement of recalcitrant compounds due to its using experimental and computed simulated UV-vis spectra, the
adequate crystallinity and narrow band gap.16–18 To this last obtained by applying time-dependent density functional
concern, Guo et al. fabricated BiVO4/InVO4 heterojunctions for theory, and subsequently contrasted with spectra found in the
the degradation of rhodamine B highlighting that the enhanced literature.
photocatalytic activity of this composite was due to the
The reusability of the catalyst was also studied, and a corre-
combined effect created by two different semiconductors.17 In lation is established between its performance and its structural,
the same direction, Lin et al. also synthesized ternary hetero- optical and electrochemical properties.
structured Ag–BiVO4/InVO4 composite which displayed
considerable photocatalytic activity for 4-CP degradation
compared to single and binary photocatalysts. This synergism
2. Experimental
2.1 Synthesis of semiconductors
was assigned to the strong absorption in the visible region
resulting from Ag nanoparticles incorporation, and the low
recombination rate of the electron–hole pairs due to the het-
erostructured combination of Ag, BiVO4 and InVO4.18
Cu2O powders were synthesized through the chemical reduc-
tion of copper sulfate using three different reducing agents:
glucose, fructose or ascorbic acid.
It is well known that the main process in heterogeneous
photocatalysis is the generation of the electron–hole pair. Those
charge carriers are able to react with electron donors and
acceptors that approach the catalyst surface. However, those
charges could recombine and dissipate the energy, and many
research efforts have focused on the reduction of the rate of the
recombination processes by using heterojunctions of semi-
conductors within the frame of a composite.19,20 For example,
Cu2O/BiVO4 composite p–n heterojunction has shown
enhanced visible-light photoactivity for the reduction of inor-
ganic and organic compounds, such as carcinogens (Cr(VI)),
dyes (methylene blue, methyl orange, crystal violet and rhoda-
mine B) and alcohols (phenol).21–25 Wang et al. evaluated the
photoactivity for methylene blue degradation, where the inter-
action between Cu2O and BiVO4 materials narrowed the
bandgap of BiVO4 to incentive the charge transfer across the
interface and spreading the absorption range of visible-light.21
Yuan et al. described the preparation of Cu2O/BiVO4 composites
to eliminate methylene blue and other anionic and cationic
dyes (rhodamine B, crystal violet, methyl orange) under visible-
light irradiation. An enhanced photocatalytic activity was
revealed when Cu2O was incorporated in the optimal 2.5 mol
percentage, since it successfully mitigated the recombination of
photogenerated electrons and holes, compared to single Cu2O
or BiVO4 materials.23
In a typical procedure, 0.0349 mol of CuSO4$5H2O were
dissolved in 50 mL of distilled water. Subsequently, 0.07 mol of
NaOH and 0.035 mol of sodium–potassium tartrate were dis-
solved in 50 mL of distilled water, and this solution was poured
into the copper solution under continuous stirring. Then,
50 mL of the reducing agent aqueous solution (0.7 M of ascorbic
acid and 1.4 M of fructose or glucose) were prepared and added
into the copper–tartrate basic solution, forming a colored
precipitate. The pH of all suspensions was adjusted to 10. The
color of the precipitate changed depending on the reducing
agent; red for glucose, orange for fructose and yellow for
ascorbic acid. The resulting powders were separated by ltra-
tion, washed with distilled water and dried with acetone. The
samples synthesized by using ascorbic acid, fructose or glucose
as reducing agent were named as Cu2O–A, Cu2O–F and Cu2O–G,
respectively. The different synthesis conditions for each method
are shown in Table 1.
BiVO4 powder was synthesized by co-precipitation method
using sodium carboxymethylcellulose (CMC) as template-
structuring agent following the procedure previously
described by our group.26 The nal precursor suspension was
heated to eliminate the solvents and the resulting powder
ꢀ
thermally treated at 350 C for 24 h.
Under this premise and to the best of our knowledge, most
studies conducted with this binary composite have focused on
eliminating dyes while, no prior study has undertaken the
photodegradation of a colorless organochlorine compound (4-
chlorophenol) in aqueous solution using Cu2O/BiVO4
composite as catalyst. It is evident that this information is
critical to gure out the degradation of other chemical
Table 1 Synthesis parameters of semiconductors
Reducing reagent
NaOH/tartrate (mol)
Temperature (ꢀC)
pH
Ascorbic acid
Fructose
Glucose
0.07/0.035
0.105/0.05
0.105/0.05
25
60
60
10
12
11
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LED lamp built with blue, green and yellow LEDs of high power
(3 W). The radiant ux of visible LED-lamp was 3.56 ꢂ 1016
photons per s per cm2, and was measured using Reinecke's salt
as chemical actinometer.24 The emission spectrum and an
image of the lamp are shown in the Fig. S1 (ESI†).
2.2 Synthesis of Cu2O/BiVO4 composites
The Cu2O/BiVO4 composite powders were prepared by the
impregnation method previously reported by our group.24 In
a typical procedure: 2 g of BiVO4 and 0.02 g of Cu2O powders
were mixed mechanically in agate mortar. Such amount of Cu2O
corresponds to a weight ratio of 1%. Then, the resultant mix was
submitted to a thermal treatment at 200 ꢀC for 4 h under
nitrogen atmosphere, in order to obtain the Cu2O/BiVO4
composite photocatalysts. These materials synthesized using
Cu2O–A, Cu2O–F or Cu2O–G powders as co-catalyst were named
as Cu2O/BiVO4–A, Cu2O/BiVO4–F and Cu2O/BiVO4–G,
respectively.
The photocatalytic experiments were carried out by adding
1 g Lꢁ1 of the photocatalyst to 250 mL of a 4-CP solution with an
initial total organic carbon (TOC) concentration of 50 mg Lꢁ1
.
Aer that, the suspension was put in an ultrasonic bath for
3 min to eliminate aggregates. The solution was kept in the dark
for 1 h in order to ensure that adsorption–desorption equilib-
rium of the pesticide on the photocatalyst surface was reached.
Aer that, the light source was turned on. During the reaction,
samples of 9 mL were withdrawn at xed time intervals and
then separated by double centrifugation. The TOC content was
measured using a Shimadzu TOC-L analyzer.
2.3 Characterization of photocatalysts
The structural properties of as-synthesized powders were
The effect of the 4-CP initial concentration was studied, and
the concentrations tested were 50, 100, and 200 mg Lꢁ1. The
effect of the photocatalysts concentration was also studied, and
the experiments were carried out with 1, 3 and 5 g Lꢁ1 of cata-
lyst; another studied factor was the presence of continuous O2
ux during the photomineralization of 4-CP. Finally, the reuse
of the material was tested by using the same material in three
consecutive runs.
Hydroquinone (HQ) was purchased from Sigma-Aldrich
($99%), and used as received without further purication as
standard for its potential detection as intermediate during 4-CP
photodegradation. A 50 mg Lꢁ1 aqueous solution was prepared
to characterize its UV-vis spectrum.
investigated by X-ray powder diffraction (XRD, PANalytical
˚
Empyrean) with Cu Ka (l ¼ 1.5417 A) as radiation source. The
morphology was investigated by scanning and transmission
electron microscopies (SEM, JEOL JSM-6510LV and TEM, JEOL
1400). The optical properties were measured using a diffuse
reectance UV-vis spectrophotometer (DRS, Shimadzu UV-2600)
and the absorption spectra were obtained by using BaSO4 as
reference. The electrochemistry measurements were collected
using a potentiostat/galvanostat Autolab PGSTAT302N.
2.4 Preparation of photocatalyst lms
All photocatalyst lms were prepared by dip-coating process.
Deposition was performed on uorine-doped tin oxide (FTO)
glass (1 cm2). In a typical process, 30 mg of the photocatalyst
were dispersed in 10 mL of ethanol. Aer that, the suspension
was put in ultrasonic bath for 5 min to eliminate aggregates.
Then, the substrate was sequentially immersed in the suspen-
sion at room temperature for 200 cycles using an immersion
speed of 0.25 mm sꢁ1. Electrochemical impedance spectroscopy
(EIS) of the catalyst lms deposited onto the FTO glass was
carried out under potentiostatic mode (wave amplitude of
10 mV) using a standard three-electrode cell with Ag/AgCl
(3.0 M KCl) and a platinum rod as reference electrode and
counter-electrode, respectively. The electrolyte was a 0.5 M
Na2SO4 aqueous solution.
2.6 Theoretical methods
Calculations were computed in the framework of the density
functional theory (DFT). The optimized structures (i.e. funda-
mental states of 4-CP) and frequency analyses were performed
using the B3LYP hybrid density functional.27 UV-vis spectra
were computed using a time-dependent density functional
theory (TD-DFT) with the polarized continuum model (PCM),28
using the dielectric constant for water. All calculations were
conducted using the 6-311+g(d,p) basis set (split-valence triple-
zeta with polarized and diffuse functions), as implemented in
Gaussian 09, Revision D.01 soware.
The at-band potentials (E) of the semiconductors were
calculated by extrapolation to the x-axis of plots of 1/C2 vs.
potential (V). The potentials were referenced to the normal
hydrogen electrode (NHE) using the following equation:
3. Results and discussions
3.1 Synthesis of semiconductors
The synthesis of the Cu2O co-catalyst using different reducing
agents and potassium sodium tartrate as chelating agent lead to
the formation of powders with different colors, indicating that
their optical properties and particle sizes are different.29 The
chemical reactions occurring during the synthesis are described
below.
Efb ¼ Eext + E0Ag/AgCl + 0.059pH
(1)
where E0Ag/AgCl (3.0 M KCl) ¼ 0.209 V vs. NHE at 25 ꢀC and Eext is
the extrapolated potential to the x-axis.
2.5 Photocatalytic reactions
The synthesis of Cu2O in presence of glucose (reducing
The photoactivities of the samples were tested using the 4- agent) can occur through the reaction of Cu2+ ions and potas-
chlorophenol degradation in a double wall cylindrical photo- sium sodium tartrate in alkaline aqueous media to obtain the
chemical reactor made of borosilicate glass that was kept at bis(tartrato)cuprate(II) complex (reactions (2) and (3)). Subse-
ꢀ
25 C. The visible-light source of the experiments was a 54 W quently, this reacts with glucose in alkaline media, oxidizing the
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aldehyde group contained in the sugar to carboxylic acid and
The use of potassium sodium tartrate in the synthesis of
reducing the Cu2+ ions to Cu+ to produce a red precipitate of Cu2O avoided the reaction between Cu2+ and ꢁOH ions to form
Cu2O (reaction (4)).
Cu(OH)2 (reaction (7)), which is an undesirable product because
it can dissociate into CuO in basic aqueous solutions (reaction
(8)).31
CuSO4 + H2O / Cu2+(aq.) + SO4
(2)
2ꢁ
(aq.)
(3)
(4)
Cu2+ + 2OHꢁ / Cu(OH)2
Cu(OH)2 / CuO + H2O
(7)
(8)
Cu2O precipitation using fructose (reducing agent) can be
explained according to the Lobry de Bruyn–van Ekenstein
Alberda transformation (LDB–AvE),30 where fructose is iso-
merized in a basic medium to a mixture of aldoses (glucose and
mannose) via a tautomeric enediol intermediate (reaction (5)).
Then, the reaction (4) takes place forming an orange Cu2O
precipitate.
3.2 Structural characterization
Fig. 1 shows the XRD patterns of the as-synthesized Cu2O using
different reducing reagents. The characteristic diffraction peaks
(5)
Cu2O formation using ascorbic acid (reducing agent) occurs of the cubic phase were observed at 2q ¼ 29.55, 36.41, 42.29 and
through the oxidation of the ascorbic acid enediol group into 61.34ꢀ (JCPDS card no. 65-3288), which correspond to the crystal
a diketone to produce dehydroascorbic acid, thereby reducing planes (110), (111), (200) and (220) of crystalline Cu2O, respec-
the Cu2+ to Cu+ with the formation of a yellow Cu2O precipitate, tively. No other diffraction peaks were detected for impurities
as shown in reaction (6).
such as Cu or CuO. The crystallite size of the Cu2O powders was
(6)
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XRD patterns of the as-prepared BiVO4 and Cu2O/BiVO4
composites are shown in Fig. S2.† The observed BiVO4 diffrac-
tion peaks can be associated with the monoclinic phase (JCPDS
card no. 14-0688) at 2q ¼ 18.67, 18.98, 28.8, 28.95, 30.52, 34.49,
35.21, and 39.77ꢀ. These diffraction peaks indicated that the
impregnation of 1 wt% of Cu2O did not make any change in the
monoclinic crystal structure of the BiVO4. Likewise, no diffrac-
tion peak of the oxide was observed in the composite XRD
patterns, presumably due to its low concentration.
3.3 Morphological characterization
Fig. 2 shows the SEM images of the Cu2O/BiVO4 composites
synthesized by impregnation method at 200 ꢀC for 4 h under N2
atmosphere, and using Cu2O–F, Cu2O–G and Cu2O–A as co-
catalyst. All the composites exhibited morphologies very
similar to that of pure BiVO4. Meanwhile, the pure Cu2O
particles showed spherical morphology with different particle
sizes depending on the reducing agent. The particle size
decreased in the following order: Cu2O–G > Cu2O–F > Cu2O–A.
Fig. 2c shows that the Cu2O–A composite presents a quasi-
spherical morphology similar to pure BiVO4, where it was not
possible to differentiate between both morphologies due to the
nanometric quasi-spherical shape of pure Cu2O–A.
Fig. 1 XRD patterns of the as-synthesized Cu2O powders using
different reducing reagents.
calculated according to Debye–Scherrer equation from the full
width at half maximum height of the diffraction peaks.32 The
crystallite size values were 9, 11 and 13 nm for the as-
synthesized Cu2O–A, Cu2O–F and Cu2O–G, respectively.
Fig. 2 SEM images of Cu2O/BiVO4 composites using different Cu2O-co-catalyst; (a) Cu2O/BiVO4–F, (b) Cu2O/BiVO4–G and (c) Cu2O/BiVO4–A.
(d) TEM image of Cu2O/BiVO4–A composite. The insets in (a–c) show corresponding images of pure Cu2O particles. The inset in (d) shows the
energy dispersive X-ray spectrum for the Cu2O/BiVO4–A composite.
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The formation of
a Cu2O/BiVO4 heterostructure was
conrmed in the Cu2O–A composite by TEM (Fig. 2d). This
image revealed that BiVO4 particles were covered by Cu2O
nanoparticles of ca. 50 nm, forming agglomerates. The
elemental composition of Cu2O/BiVO4 was examined by energy-
dispersive X-ray spectroscopy (inset Fig. 2d). Bi, V, Cu, O
elements were all present in the Cu2O/BiVO4 composite
photocatalyst.
3.4 DRS UV-vis results
The DRS UV-vis spectra indicate that all as-synthesized photo-
catalysts possessed strong absorption in UV and visible-light
regions, as shown in Fig. S3 and S4 (ESI†). The results showed
that Cu2O samples exhibited absorption edges at 572, 650 and
652 nm for Cu2O–A, Cu2O–F and Cu2O–G, respectively
(Fig. S3†). A blue-shi in optical absorption edge is observed as
particle size decreases. The band gap values were calculated by
using Tauc plot,33,34 and the obtained values were 2.36, 2.19 and
1.99 eV for Cu2O–A, Cu2O–F and Cu2O–G, respectively. These
results revealed that the reducing agent used in the synthesis
process played an important role in the morphology and the
optical properties of the as-synthesized Cu2O, presumably,
providing changes in the catalyst crystallinity which affects the
bandgap.
The UV-vis spectra obtained for the composites showed
a slight blue-shi from 515 to 510 nm with respect to pure
BiVO4 (516 nm), as shown in Fig. S4.† The band gap values of
the composites were 2.49, 2.50 and 2.51 eV for Cu2O/BiVO4–A,
Cu2O/BiVO4–F and Cu2O/BiVO4–G, respectively. The size of the
band gap was mainly dominated by the optical features of
BiVO4, since the Cu2O ratio present in the composite was only
around 1%.
Fig. 3 Mott–Schottky plots of the pure BiVO4 and composites-series.
Inset shows the measured donor density (ND) of each material.
Cu2O/BiVO4–F and Cu2O/BiVO4–G, respectively. The E of pure
BiVO4 and Cu2O–A lms were also measured, as shown in Fig. 3
and S5,† resulting in ꢁ0.46 and ꢁ0.57 V (vs. Ag/AgCl at pH 6.1),
respectively.
Considering that the difference between the at-band
potential and the bottom edge of the conduction band or the
top edge of the valence band is negligible for a n-type or p-type
semiconductor, respectively,36 the energy levels of the conduc-
tion band for pure BiVO4 and the valence band for pure Cu2O
could be estimated, as described in Fig. 4.
Based on the at-band potentials (or apparent Fermi levels)
and the optical band gap energy values estimated above, it is
possible to construct the potential energy diagram for all the as-
synthesized samples, as illustrated in Fig. 4. According to the
schematic diagram, before contact of materials, the conduction
band of Cu2O–G lm is more positive than that of Cu2O–A and
Cu2O–F lms. When the two semiconductors are in contact, the
conduction band of Cu2O/BiVO4–A and Cu2O/BiVO4–F
composites powders are negatively shied ca. 127 and 106 mV,
respectively, with respect to that of BiVO4, indicating that only
in these composites it is possible the p–n junction formation
and the injection of free electrons from BiVO4 into Cu2O, thus
providing an enhanced charge-separation process. Meanwhile,
in the Cu2O/BiVO4–G composite the valence band of Cu2O is
more negative than the conduction band of BiVO4, thus the
electron transfer is not possible.
3.5 Electrochemical characterization
The at-band potential of the composites was measured by
electrochemical impedance spectroscopy. Fig. 3 shows the
Mott–Schottky (M–S) plots for the pure BiVO4 and composites
lms.
The M–S plots of the as-synthesized composites showed the
presence of both positive and negative slopes, demonstrating
the n-type and p-type electronic nature of the materials. These
results conrm that the as-synthesized Cu2O/BiVO4 composites
are heterostructured materials with a p–n junction, a feature
that should enhance photogenerated charge separation and
improve the photocatalytic activity.
It is well known that the slope of M–S plots is associated with
the donor density of semiconductors.35 The as-synthesized
composites showed lower slopes in the Mott–Schottky plots
compared to pure BiVO4, indicating a higher donor density in
3.6 Photocatalytic reactions
the composites, and conrming that the heterojunction In order to investigate the inuence of different Cu2O co-
generates a lower charge recombination process. catalyst on the photocatalytic activity of BiVO4, the photo-
The M–S plots also allow determination of E by extrapola- activities of the as-synthesized Cu2O/BiVO4 composites were
tion of the positive and negative slope plots to the x-axis. The evaluated towards the degradation of 4-CP under visible-light
values for the positive slopes were ꢁ0.59, ꢁ0.57 and ꢁ0.36 V (vs. irradiation, as shown in Fig. 5. The results show that the
Ag/AgCl at pH 6.1), and for the negative slopes were ꢁ0.18, prepared Cu2O/BiVO4–A composite produced the best organic
ꢁ0.30 and ꢁ0.04 V (vs. Ag/AgCl at pH 6.1) for Cu2O/BiVO4–A, mineralization, 27% in 240 min, while the low activity of the
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Fig. 4 Schematic diagram of the band energy of the as-synthesized Cu2O-series, BiVO4 and Cu2O/BiVO4 composites.
The effect of photocatalyst concentration was evaluated in
order to improve the 4-CP photomineralization using Cu2O/
BiVO4–A composite, using the following concentrations 1, 3 and
5 g Lꢁ1, as illustrated in Fig. 6.
It is clear that the increase of the Cu2O/BiVO4–A loading in
the reaction played an important role on the mineralization
degree of 4-CP. The increase of the photocatalyst implies
a major surface area where the organic molecule can be
adsorbed, and a larger number of active sites. The best miner-
alization degree was 44% aer 240 min with the 5 g Lꢁ1
concentration, whereas only 32% of mineralization was reached
in the same time with the 3 g Lꢁ1 concentration.
Under the tested experimental conditions, the results indi-
cate that the 4-CP photomineralization increases with catalyst
Fig. 5 Mineralization degree of 4-CP by Cu2O/BiVO4 composites with
different Cu2O co-catalysts under visible-light irradiation. Inset shows
the effect of the initial 4-CP concentration on TOC removal during 4-
CP removal using Cu2O/BiVO4–A catalyst under visible-light
irradiation.
Cu2O/BiVO4–G photocatalyst was attributed to its minor charge
density, as aforementioned.
The effect of 4-CP initial concentration on the mineralization
degree was also studied (see inset of Fig. 5). The best photo-
activity was observed at low 4-CP initial concentration. Cu2O/
BiVO4–A composite reached ca. 27% mineralization degree at
50 ppm of 4-CP, while at higher concentrations of 100 and
200 ppm the mineralization degree was 10 and 5%, respectively.
However, the removed amount was almost the same in terms of
TOC measurement, indicating that, for the level of 4-CP
concentrations herein studied, the surfaces of the three cata-
lysts should be saturated, and their kinetic data do not depend
Fig. 6 Effect of the dosage of Cu2O/BiVO4 on TOC removal during 4-
CP removal using Cu2O/BiVO4–A catalysts under visible-light
on the 4-CP concentration, thus following similar reaction rates. irradiation.
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ꢁ
loading, and no light scattering by the outer layers of catalyst in
the reactor or any other screening effect are enough to nega-
tively affect the photoactivity of Cu2O/BiVO4–A composite at
high concentrations.
ꢁ
)
Cu2O(e
+ cO2 + H+ / Cu2O + H2O2
(13)
(14)
CB
ꢁ
Cu2O(e
+ H2O2 / cOHfree +
ꢁOH + Cu2O
)
CB
It is well know that cOH and cO2ꢁ radicals have an important
role in the oxidation of organic molecules, and the addition of
a permanent O2 ux can signicantly increase the number of
radicals cOH during the reaction.37 Fig. 7 reveals that the
mineralization degree of 4-CP rises to 76% aer 240 min using
Cu2O/BiVO4–A, in the presence of O2 (ow of 250 mL hꢁ1) under
visible-light irradiation. Thus, molecular oxygen is an electron
scavenger assisting the stabilization of holes and enhancing the
reaction rate.
4-CP + cOHfree/ads // 4-CC //
HHQ + HBQ // degraded products
(15)
(16)
(17)
HQ + cOHfree/ads /// intermediates ///
degraded products
BQ + cOHfree/ads /// intermediates ///
degraded products
Once the charge carriers are photogenerated by visible-light
(reaction (9)), the conduction band (CB) electrons of BiVO4 are
transferred to the valence band (VB) of Cu2O resulting in the
presence of free holes on the VB of BiVO4 and free electrons on
the CB of Cu2O.
The mineralization degree achieved in this work for 4-
chlorophenol is better than that one reported in previous
studies using visible-light-driven semiconductors materials
such as N–TiO2 (ca. 60% aer 360 min), Eu2O3–TiO2 (ca. 50%
aer 240 min), CuS (ca. 60% aer 300 min), TiO2/WO3 (ca. 40%
aer 180 min).38–41
The visible-light photodegradation of 4-chlorophenol by
Cu2O/BiVO4 composites can be achieved due to the improve-
ment of the separation of photogenerated charge carriers on the
composite. The proposed pathway for the photocatalytic
degradation of 4-chlorophenol using Cu2O/BiVO4 composite is
described with the following equations:
On other hand, the free holes on the VB of BiVO4 react with 4-
chlorophenol or hydroxide anion either free or adsorbed
(reactions (10) and (11)), to produce the direct oxidation of
organic molecules or the generation of cOH radical, respectively.
The free electrons on the CB of Cu2O reduce the oxygen mole-
cule resulting in the generation of cOH radical by a series of
reactions ((12)–(14)). Thus, the generation of cOH radical is
favored by the oxygen presence due to its role as electron
scavenger. The oxidation reactions of 4-chlorophenol and
intermediates, such as, hydroquinone (HQ) and benzoquinone
(BQ), are carried out by the cOH radical (reactions (15)–(17)) as
reported in previous studies using different photocatalyst
materials.1,18
+
VB
ꢁ
)
Cu2O/BiVO4 + hv / BiVO4(h
+ Cu2O(e
(9)
)
CB
+
)
BiVO4(h
+
ꢁOH / cOHads + BiVO4
(10)
VB
+
BiVO4(h
+ 4-CP // intermediates //
)
VB
In order to evaluate the reproducibility of the Cu2O/BiVO4
composite performance towards the 4-CP photomineralization,
which is an important factor for applications related to water
disinfection, reuse of the composite material was also investi-
gated under visible-light irradiation (Fig. 8). Aer three cycles,
degraded products + BiVO4
(11)
(12)
ꢁ
ꢁ
Cu2O(e
+ O2 / Cu2O + cO2
)
CB
Fig. 7 Effect of continuous O2 flux during the mineralization of 4-CP Fig. 8 Recycling uses of Cu2O/BiVO4–A for degrading 4-CP under
by Cu2O/BiVO4–A under visible-light irradiation and different photo- visible-light irradiation, condition used: 25 mg Lꢁ1 of 4-CP initial
catalyst dosage.
concentration and 5 g Lꢁ1 of photocatalyst.
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the photoactivity of the composite did not exhibit any remark- other hand, it was not possible to dissolve 4-CC in water in
able loss, conrming that the composite is stable during the a concentration range detected by UV-vis spectroscopy, neither
photocatalytic reaction studied. The chemical stability of Cu2O/ to nd experimental data in literature concerning UV-vis spec-
BiVO4 composite was also conrmed by XRD (Fig. S6†), trum for this aromatic compound.
demonstrating that the monoclinic and cubic crystal structure
The UV-vis spectrum of 4-CC was simulated (Fig. 9) using
of BiVO4 and Cu2O, respectively, remains unchanged. These time-dependent density functional theory (TDDFT), in order to
results showed the feasibility of organic photocatalytic miner- confront it against the data shown in Fig. S7.† As observed in
alization by using Cu2O/BiVO4 composite catalyst.
this gure (t ¼ 20 min), a small peak at 279 nm reveals the
remaining 4-CP in solution, while the band at 253 nm can be
assigned to the formation of 4-CC intermediate, as conrmed in
the simulated spectrum described in Fig. 9. Note that this
absorption band is located relatively far away from the HQ peak
(296 nm, refer to Fig. 9),48 which also precludes detection of p-
benzoquinone (ꢃ244 nm).49 Accordingly, 4-CC production is
favored under the experimental conditions herein used. When
the treatment reaches 40 min, the presence of 4-CP vanishes
from the UV-vis spectrum (Fig. S7†) and only 4-CC is registered
in the analysis, conrming the complete destruction of the
original aromatic compound. Subsequently, abatement of 4-CC
can be observed up to 240 min. It has been reported that the
reaction mechanism for 4-CC degradation can involve the
generation of malic, maleic and fumaric acids.50 However,
absorption bands at wavelengths higher than 320 nm, the
typical absorption ranges for these compounds, were not
detected. Another possibility is that these compounds could be
present in concentrations lower than the detection limit of the
spectrophotometer. A forthcoming study devoted to account for
the reaction mechanism of 4-CP photomineralization with
Cu2O/BiVO4 composite catalysts will involve detailed analysis of
intermediates using high-performance liquid chromatography-
mass spectrometry (HPLC-MS).
3.7 Identication of 4-CP intermediates
The photodegradation of 4-CP was conducted during 240 min to
analyze the degradation capacity of the Cu2O/BiVO4 composites
under visible-light irradiation. Fig. S7† shows the UV-vis spec-
trum of 4-CP at t ¼ 0 min (red continuous line) under the
experimental conditions proposed in the present study. As
observed, a characteristic band is obtained at approximately
279 nm, which is typical for aqueous media.42 When the pho-
tocatalytic treatment is conducted for 20 min, a remarkable
decrease is registered for the main peak, suggesting the 4-CP
degradation into intermediate compounds. Multiple photo-
catalytic studies have successfully reported the mineralization
of 4-CP into CO2, however, while some of those studies report
the predominance of hydroquinone (HQ),43–45 others identify
the predominant formation of 4-chlorocatechol (4-CC),46,47
without discussing the inuence of catalyst or particular
experimental condition favoring the formation of a determined
intermediate.
Under this premise, the formation of any of these aromatic
compounds was explored during the course of photocatalytic
degradation (i.e. t ¼ 20 min). Fig. 9 exhibits an UV-vis spectrum
that was carried out to detect HQ in aqueous solution
(50 mg Lꢁ1). The single peak recorded at ꢃ296 nm, which agrees
with the literature,48 is an indicator of the HQ presence. On the
4. Conclusions
Cu2O/BiVO4 composites catalysts with different Cu2O co-
catalysts were synthesized by a simple impregnation method.
The variation of the reducing reagent during the synthesis of
Cu2O played an important role in the optical, crystallographic
and electrochemical properties of the Cu2O/BiVO4 composite
catalyst, resulting in a different photocatalytic performance
towards the mineralization of 4-CP in aqueous solutions under
visible-light irradiation. The EIS study conrmed the p–n elec-
tronic behavior of the composites and demonstrated that
a larger amount of charge carriers exist in Cu2O/BiVO4
composite catalyst with respect to the pure materials. The study
of some experimental variables on the photocatalytic perfor-
mance of Cu2O/BiVO4 catalysts, such as preparation method of
co-catalyst, catalyst concentration, 4-CP initial concentration
and oxygen ux showed their inuence to enhance the photo-
catalytic activity of the material for 4-CP mineralization under
visible-light irradiation. The complete destruction of 4-CP pro-
ceeded throughout the formation of 4-CC without involving the
parallel generation of hydroquinone/p-benzoquinone, accord-
ing to experimental and computed UV-vis spectra determined
for pure intermediate compounds. From its performance and
chemical stability and reusability, the Cu2O/BiVO4 composite
Fig. 9 Experimental UV-vis spectrum collected for standard hydro-
quinone (HQ, continuous line) in a 50 mg Lꢁ1 aqueous solution, and
theoretical UV-vis spectrum computed for 4-chlorocatechol (4-CC,
dashed line) using time-dependent density functional theory (B3LYP/
6-311+g(d,p)).
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presented an evident potential for its use as catalyst for water
remediation under visible-light irradiation.
copper foils with water vapor, Thin Solid Films, 2012, 520,
2679–2682.
12 Q. Zhu, Y. Zhang, J. Wang, F. Zhou and P. K. Chu, Microwave
synthesis of cuprous oxide micro-/nanocrystals with
different morphologies and photocatalytic activities, J.
Mater. Sci. Technol., 2011, 27, 289–295.
Conflicts of interest
There are no conicts to declare.
13 Q. Zhu, Y. Zhang, F. Lv, P. K. Chu, Z. Ye and F. Zhou,
Cuprous oxide created on sepiolite: Preparation,
characterization, and photocatalytic activity in treatment of
red water from 2,4,6-trinitrotoluene manufacturing, J.
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14 X. Zhang, J. Song, J. Jiao and X. Mei, Preparation and
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Acknowledgements
The authors acknowledge the nancial support provided by the
CONACYT (Grant No. 178895), the PAICYT, PRODEP, and the
Spanish “Ministerio de Ciencia e Innovacion”, through project
HIDROPILSOL (CTQ2013-47103-R). The authors thank Dr
Felipe Montoya for a critical reading of the manuscript and for
stimulating discussions.
´
´
´
´
´
´
15 U. M. Garcıa Perez, S. Sepulveda-Guzman, A. Martınez-de la
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Cruz and U. Ortiz Mendez, Photocatalytic activity of BiVO4
nanospheres obtained by solution combustion synthesis
using sodium carboxymethylcellulose, J. Mol. Catal. A:
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