In-situ fabrication of Cu3(MoO4)2(OH)2 films decorated with MO (M=Zn, Cu, and Ni) for CO2 photoconversion into value-added compounds

Article abstract of DOI:10.1016/j.jallcom.2021.159846

CO₂ photoreduction is an emerging technology to produce value-added compounds using abundant resources such as CO₂, H₂O, sunlight, and photocatalyst materials. Here, Cu₃(MoO₄)₂(OH)₂ (CMO) films were proposed as photocatalysts for CO₂ photoconversion into value-added compounds, such as methanol, formic acid, and hydrogen. The CMO films were decorated with simple oxides MO (M=Zn, Cu, and Ni) by an in-situ microwave-hydrothermal method. The CMO films exhibited a bar morphology, high crystallinity, band gap suitable for solar-light activation (2.8 eV), and valence and conduction band potential of 1.3 eV and 1.5 eV vs NHE, respectively. After decoration with the simple oxides, the films retained the bar morphology and resulted in a more efficient charge transfer by forming type-I heterostructure between Cu₃(MoO₄)₂(OH)₂ and CuO; while, type-II heterostructures were produced between CMO and NiO and ZnO. The CO₂ was photoconverted to HCOOH and CH₃OH in the liquid phase using CMO reference and the decorated films as photocatalysts. Cu₃(MoO₄)₂(OH)₂ decorated with CuO (CMO/Cu) favored the production of 155 μmol g^?1 HCOOH, 109 μmol g^?1 for CH₃OH, and 127 μmol g^?1 H₂ after 1 h under visible-light irradiation. The yields obtained were up to 8 times higher than other conventional photocatalysts. The stability and recyclability of the films were demonstrated after three consecutive cycles. The photocatalytic activity for value-added compounds production increased as follows: CMO/Cu > CMO/Ni > CMO/Zn > CMO, associated to a higher CO₂ adsorption, 2D morphology, adequate conduction band potential, and a more efficient charge transfer.

Full text of DOI:10.1016/j.jallcom.2021.159846

Journal of Alloys and Compounds 873 (2021) 159846
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
In-situ fabrication of Cu3(MoO4)2(OH)2 films decorated with MO
(M=Zn, Cu, and Ni) for CO2 photoconversion into value-added compounds
Manuel Alejandro Ávila-Lópeza, E. Luévano-Hipólitob,⁎, Leticia M. Torres-Martíneza,c
a Universidad Autónoma de Nuevo León, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P. 66455 San Nicolás de los
Garza, NL, Mexico
b CONACYT - Universidad Autónoma de Nuevo León, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P. 66455 San
Nicolás de los Garza, NL, Mexico
c Centro de Investigación en Materiales Avanzados, S. C. (CIMAV), Miguel de Cervantes 120 Complejo Ind. Chihuahua, Chihuahua Chihuahua C.P. 31136, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
CO2 photoreduction is an emerging technology to produce value-added compounds using abundant resources
such as CO2, H2O, sunlight, and photocatalyst materials. Here, Cu3(MoO4)2(OH)2 (CMO) films were proposed
as photocatalysts for CO2 photoconversion into value-added compounds, such as methanol, formic acid, and
hydrogen. The CMO films were decorated with simple oxides MO (M=Zn, Cu, and Ni) by an in-situ microwave-
hydrothermal method. The CMO films exhibited a bar morphology, high crystallinity, band gap suitable for
solar-light activation (2.8 eV), and valence and conduction band potential of 1.3 eV and 1.5 eV vs NHE, re-
spectively. After decoration with the simple oxides, the films retained the bar morphology and resulted in a
more efficient charge transfer by forming type-I heterostructure between Cu3(MoO4)2(OH)2 and CuO; while,
type-II heterostructures were produced between CMO and NiO and ZnO. The CO2 was photoconverted to
HCOOH and CH3OH in the liquid phase using CMO reference and the decorated films as photocatalysts.
Cu3(MoO4)2(OH)2 decorated with CuO (CMO/Cu) favored the production of 155 µmol g−1 HCOOH,
109 µmol g−1 for CH3OH, and 127 µmol g−1 H2 after 1 h under visible-light irradiation. The yields obtained
were up to 8 times higher than other conventional photocatalysts. The stability and recyclability of the films
were demonstrated after three consecutive cycles. The photocatalytic activity for value-added compounds
Received 2 February 2021
Received in revised form 30 March 2021
Accepted 4 April 2021
Available online 8 April 2021
Keywords:
Cu3(MoO4)2(OH)2
CO2 photoconversion
In-situ synthesis
Films
Solar fuels
Value-added compounds.
production increased as follows: CMO/Cu
> CMO/Ni > CMO/Zn > CMO, associated to a higher CO2
adsorption, 2D morphology, adequate conduction band potential, and a more efficient charge transfer.
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
performance. Some critical requirements of the photocatalyst found
so far are: i) solar-light activation, ii) efficient separation and
Solar-driven heterogeneous photocatalysis is considered an emerging
technology to both mitigate greenhouse concentration in the atmo-
sphere and to produce value-added products, i.e., hydrogen (H2),
synthesis gas (CO/H2), and other compounds as CH4, CH3OH, HCOOH,
HCOH, through H2O and CO2 reduction [1–3]. The main problems of the
photocatalytic reduction of CO2 are related to the low conversion effi-
ciency, lower selectivity, reduced CO2 adsorption on photocatalyst sur-
face, and competition for H2 production. Therefore, it is important to
develop efficient photocatalysts for CO2 reduction, chemically stable, and
selective for solar fuels (CH4 and CH3OH) and value-added products
(HCOH, CO, and HCOOH) [3–5]. The physicochemical properties of the
semiconductor play important roles in determining its photocatalytic
transportation of charge carriers (e-/h+), iii) chemical stability, iv)
homogeneous morphology (1D, 2D, and 3D), v) structural defects
such as oxygen vacancies (OVs), and also other important para-
meters such as low cost and non-toxicity [6–9]. These parameters
are important for the practical application and commercialization of
these materials. The most studied semiconductor photocatalysts for
CO2 reduction and H2 production are TiO2, ZnO, Cu2O, CuO, NiO,
Bi2O3, and WO3 [3,9]. Other semiconductor materials have been also
explored for CO2 photoconversion, such as BiVO4, Bi2WO6, Zn2GeO4,
Zn2GaO4, Zn2SnO4, and AMoO4 (A=Ca, Sr, Ba), among others in the
last years [3,9,10]. Recently, AMoO4 (A=Ca, Sr, Ba) materials has been
proposed as efficient photocatalysts to produce H2 and CH3OH from
CO2 photoreduction [11,12]. In these works, one strategy to increase
the efficiency of the AMoO4 photocatalysts was to immobilized them
as films. The use of photocatalysts as films is associated with a more
efficient charge transfer, rapid charge transport, better surface
⁎
Corresponding author.
E-mail address: edith.luevanohp@uanl.edu.mx (E. Luévano-Hipólito).
https://doi.org/10.1016/j.jallcom.2021.159846
0925-8388/© 2021 Elsevier B.V. All rights reserved.

M.A. Ávila-López, E. Luévano-Hipólito and L.M. Torres-Martínez
Journal of Alloys and Compounds 873 (2021) 159846
exposure, efficient light-absorption, etc. [11,13]. Therefore, the use of
molybdate films is a good alternative for the CO2 photoconversion.
Metal molybdates and their derivatives (hydrates, oxyhydrates, and
polyoxometalates) have interesting properties, i.e., magnetic and
electric, promoting their application in different areas such as pho-
tocatalytic degradation of dyes and lithium-ion batteries [14–16].
Metal molybdates present various oxidation states, making them
promising materials for photocatalytic applications due to the
the CMO films in a 40 mL of 0.06 M solution of Zn2+, Cu2+ and Ni2+
.
The mixture was stirring for 30 min in a Teflon reactor. Then, a
second solution of NaOH (molar ratio Acetate:NaOH 1:2) was dis-
solved in 10 mL of distilled water and then, it was added dropwise
under vigorous stirring. After that, the mixture was exposed to
300 W in a microwave-hydrothermal reactor at 80 °C for 1 h. Finally,
the films were cleaned three times with distilled water and dried at
80 °C for 12 h. The obtained films were labeled as CMO/M, where M
refers to Zn, Cu, or Ni.
e- transfer present in the MoO4 anion [17].
2-
Lindgrenite (Cu3(MoO4)2(OH)2) is a copper molybdate with ap-
plications in some fields, such as catalysis, luminescence, and energy
storage [18–20]. The lindgrenite structure consists of CuØ6 octahedra
(Ø = O or OH) that share edges with the MoO4 tetrahedron [21].
Particularly, lindgrenite defined as 1D and 2D nanostructure has
been used as the lithium storage [22] and pharmaceutical sensor
[23]. The application of Cu3(MoO4)2(OH)2 nanostructured has re-
sulted in good efficiencies due to its ferromagnetic nature and an
efficient charge transfer.
2.3. Characterization
The crystal structure of the films was characterized by X-ray
diffraction in a Bruker D8 Advance diffractometer in the interval
10–70° and rate of 0.02° for 0.3 s. The crystallite size of the samples
was calculated by the Scherrer equation using the most intense re-
flection (2θ = 25.313°) of the X-ray diffraction patterns. The Scherrer
equation is shown in Eq. (1):
To the best of our knowledge, there are no reports about the
study of Cu3(MoO4)2(OH)2 for any photocatalytic applications
despite its interesting properties. Therefore, based on previous
lindgrenite reports, it seems that Cu3(MoO4)2(OH)2 is a promising
material due to its good performance for energy application, op-
tical and magnetic properties, which indicates that this material
could be used as a photocatalyst [8,19,20,22]. Additionally, the
decoration of lindgrenite with oxides of transition metals such as
Cu, Ni, and Zn could enhance the photocatalytic efficiency to
generate value-added compounds through the formation of new
energy levels in the electronic band structure of the molybdate
[24,25]. The use of ZnO, CuO, and NiO represent another ad-
vantage to the process since they present a good affinity for the
CO2 molecule, which could favor its photocatalytic conversion to
value-added compounds [26–30]. Based on that, here is proposed
the study of a novel material based on Cu3(MoO4)2(OH)2 deco-
rated with simple oxides (ZnO, CuO, and NiO) for the CO2 pho-
toconversion into value-added products, i.e., CH3OH, HCOOH, and
H2 under visible-light.
k
cos
C =
(1)
Where C is the crystallite size in (nm), k is the proportionality
constant (0.9 was selected in this work), λ is the wavelength of Cuα
(1.5405 Å), θ is the angle of the strongest reflection, and β is the
mean width of the reflection with maximum intensity [31]. The
morphology of the films was analyzed by Scanning Electron
Microscopy in a JEOL JSM-6490LV with an acceleration voltage of
20 kV. The samples were analyzed in a Luxo System 273RB-RLI
optical microscope. The band gap energy (Eg) of the materials was
determined using a UV-Visible-NIR spectrophotometer Cary 5000
with an integration sphere. The energy band gap was estimated
using the Kubelka–Munk remission function. The surface of the
films was analyzed through Fourier Transform Infrared spectro-
scopy in
a Thermo Nicolet iS50 equipment in the range of
1000–4000 cm−1, after and before the photocatalyst test. Photo-
luminescence spectra were obtained at room temperature in a Cary
Eclipse spectrophotometer with a Xenon flash lamp. The samples
were excited at 300 nm with an emission slit of 10. Photocurrent
was characterized on a ITO glass. Chronoamperometry was per-
formed in an Autolab with 0.1 M NaOH. The irradiation source was
turned on for 100 s, off for another 100 s, and repeated in an on-
off cycle for 10 min. The film surface was analyzed by X-ray pho-
toelectron spectroscopy (XPS) using a Thermo Scientific Escalab
250 Xi with step energy of 200 eV for survey scans, and 50 eV for
high resolution scans with step sizes of 1.0 and 0.1 eV, respectively.
X-ray photoelectron spectroscopy was used for the determination
of valence band and conduction band.
2. Experimental procedure
2.1. Synthesis of Cu3(MoO4)2(OH)2 films
Lindgrenite (Cu3(MoO4)2(OH)2) films were deposited by an in-
situ microwave-hydrothermal method over glass fibers substrates.
The active area of substrates was 2.73 cm2. The substrate was
cleaned under sonication in three steps using acetone, methanol,
and distilled water, separately. Then, the substrates were dried at
80 °C overnight before their use. Once cleaned, the glass fibers were
placed in a Teflon reactor.
2.4. Photocatalytic experiments
For the lindgrenite synthesis, two 0.25 M solutions of copper
(II) acetate (Cu(CH3COO)2·H2O, Fermont, 99%) and molybdic acid
(H2MoO4, Aldrich, 99%) were prepared separately at 25 °C. The Cu
(CH3COO)2 solution was added dropwise into the H2MoO4 solu-
tion under constant stirring for 30 min at room temperature.
Then, the pH was adjusted at 7 with 40 µL of NH4OH (Fermont,
99%). After 10 min, the suspension was put in a Teflon reactor with
the substrates to microwave irradiation exposure in an equipment
MARS 6 Microwave Digestion System at 120 °C for 1 h and 600 W.
The obtained film was cleaned three times with distilled water
and dried at 80 °C overnight. The films obtained were labeled
as CMO.
The production of solar fuels was evaluated in a batch quartz
reactor at room temperature. For these experiments, the films
(2.73 cm2) were placed inside the reactor with 150 mL of distilled
water under magnetic agitation. For the photocatalytic experi-
ments, the reactor was bubbled with N2 or CO2 for 30 min to re-
move the oxygen dissolved in the H2O for H2 evolution and CO2
reduction, respectively. The reactor was irradiated with 2 LED
lamps of 20 W each, with a broad emission from 443 to 535 nm.
Different techniques monitored the reaction products. H2 was
monitored using a Shimadzu GC-2014 chromatograph equipped
with a thermal conductivity detector. The reaction products (HCOH
and HCOOH) were monitored with high performance liquid chro-
matography (HPLC). The HCOOH measurement was carried out by
injecting 15 µL of previously filtered reaction liquid sample. On the
other hand, the HCOH quantification was carried out injecting 25 µL
of the liquid sample into the system, with a Fenomenex C-18
2.2. Decoration of Cu3(MoO4)2(OH)2 films with MO (M= Zn, Cu, and Ni,)
For CMO decoration, Zn(CH3COO)2·H2O Cu(CH3COO)2·H2O, and Ni
(CH3COO)2.4H2O were used. The procedure consisted on immersed
2

M.A. Ávila-López, E. Luévano-Hipólito and L.M. Torres-Martínez
Journal of Alloys and Compounds 873 (2021) 159846
column, using a 50:50 v/v acetonitrile-water mixture as mobile
phase. Previously, the liquid sample was derivatized forming a
complex product of the reaction with 2–4, dinitrophenyl hydrazine
(Sigma Aldrich) and phosphoric acid (Fermont) [32]. Additionally,
CH3OH was determinate by gas chromatography with a Flame
Ionization Detector (FID).
crystal structure of the base material. A closer analysis of the films
evidenced the presence of the hexagonal phase of ZnO at 34.43 and
36.25° (JCPDS card 00–036–1451), monoclinic CuO at 35.43 and
38.47° (JCPDS:00–045–0937), and the monoclinic NiO phase at
43.23° (JCPDS card 01–089–7131), respectively. The corresponding
reflections are shown in the input display in Fig. 1b–d. All the films
showed a crystalline size in a range of 45–64 nm.
SEM images of the Lindgrenite and decorated films are shown
in Fig. 2. The morphology of the CMO films exhibited a hetero-
geneous morphology, which consisted of bars (2.6 µm) with some
small irregular particles deposited on them (Fig. 2a, b). After
decoration, the films showed a homogeneous distribution with a
lower particle size than bare-CMO film. The CMO/Zn morphology
resulted in flake-like particles of an average length of 540 nm
(Fig. 2c, d). The CMO/Cu and CMO/Ni films exhibited a bar mor-
phology of different lengths. The bar size in these samples was
810 nm length and 75 nm width for CMO/Cu (Fig. 2e,f); while for
CMO/Ni sample, some tiny bars of 500 nm length and 32 nm width
were observed (Fig. 2g,h).
Elemental mappings of the films were performed through EDS
spectroscopy to corroborate the presence of Zn, Cu, and Ni (Fig. S1).
According to these results, Zn, Cu and Ni were identified in the de-
corated films, which indicates the presence of these metal oxides.
The weight percentage of each oxide on CMO was around 20%.
Fig. 3a shows the diffuse reflectance spectra converted with the
remission function of K-M of all the samples. Reference
Cu3(MoO4)2(OH)2 film exhibited a band gap of 2.8 eV. The decoration
of the Cu3(MoO4)2(OH)2 film with ZnO, NiO, and CuO resulted in a
slightly red shift from 2.8 to 2.7, 2.6, and 2.5 eV, respectively. Ad-
ditionally, in the CMO/Ni and CMO/Cu films, a second band gap was
observed around 2.4 and 1.5 eV characteristic of NiO and CuO in the
K-M spectrum, respectively. These results indicate that these mate-
rials are suitable for visible-light activation. Also, CMO/Zn presented
an additional band gap at 3.1 eV related to the presence of ZnO.
Fig. 3b includes the images of each film obtained with the optical
microscope.
3. Results and discussion
3.1. Films characterization
Lindgrenite decorated films CMO/M (M= Zn, Cu, or Ni) were ob-
tained by in-situ microwave-hydrothermal method. The crystal
structure of the obtained films reference and decorated were ana-
lyzed by X-ray diffraction, as is shown in Fig. 1. The reflections of the
XRD pattern of the reference lindgrenite films corresponded to the
monoclinic Cu3(MoO4)2(OH)2 phase according to JCPDS card
01–075–1438 (Fig. 1a). Other reflections associated with mo-
lybdenum or other copper compounds were not observed in the
CMO sample. On the other hand, the diffraction patterns of the de-
corated films CMO/M are present in Fig. 1b–d. All the films retain the
Fig. 4 shows the PL spectrum of the CMO reference and decorated
films at the excitation wavelength of 300 nm. A very strong emission
band at ~420 nm (~2.8 eV) is observed in the PL spectra, which it
could be related to the band gap emission of the exciton on
Cu3(MoO4)2(OH)2. The additional peak observed at 443 nm could be
attributed to the electron-transfer mechanism involving the O-2p
orbital and the Mo-4d orbital within the molybdenum ion complex
[16]. Photoluminescence (PL) emission can be used to study the
carrier transfer since photogenerated charges recombination causes
the emission. Lower PL emission intensity on CMO/Cu suggests a
lower recombination rate in this sample, which it could impact its
photocatalytic activity. This result could be associated with the CMO/
Cu morphology since this morphology exhibits a more efficient
charge transfer through a long axis, promoting a unidirectional flow
of electrons in the structure [33].
The photoelectrochemical tests of the as-prepared samples
were further investigated by means of chronoamperometric per-
formance, as shown in Fig. 5. All the samples showed considerable
photoresponse and presented stability throughout the test. CMO
films present a positive response characteristic to n-type material.
Similarly, CMO/Zn and CMO/Ni samples had a moderate positive
photocurrent response. At the same time, CMO/Cu showed a
higher photocurrent response. CMO/Cu was the only material that
shows a p-type behavior with a negative photocurrent response;
while the rest of the materials exhibited a positive photoresponse
Fig. 1. X-ray diffraction patterns of the as-prepared films. Input a comparative be-
tween a) CMO and b-d) CMO/M (M= Zn, Cu, or Ni).
corresponding to n-type material. CMO/Cu showed
a higher
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M.A. Ávila-López, E. Luévano-Hipólito and L.M. Torres-Martínez
Journal of Alloys and Compounds 873 (2021) 159846
Fig. 2. SEM images of: a-b) CMO, c-d) CMO/Zn, e-f) CMO/Cu, and g-h) CMO/Ni films.
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M.A. Ávila-López, E. Luévano-Hipólito and L.M. Torres-Martínez
Journal of Alloys and Compounds 873 (2021) 159846
Fig. 3. a) K-M spectrum for molybdates films deposited on glass fiber and b) optical images of the CMO films.
Fig. 4. Photoluminescence (PL) emission spectra of CMO/M films.
Fig. 5. Chronoamperometric response of the CMO/M films.
photocurrent of 250 µA cm−2 followed by CMO/Zn (18 µA cm−2),
CMO/Ni (11 µA cm−2), and CMO (2.9 µA cm−2).
The electron current density in the CMO/Cu sample could be
attributed to the presence of p-n heterostructure that promote a
more efficient transference between n-type Cu3(MoO4)2(OH)2 and p-
type CuO, which it is beneficial for moving the photogenerated
electrons from the CB of Cu3(MoO4)2(OH)2 to the CB of CuO, as will
be further discussed.
XPS investigated the surface compositions and elemental valence
states of the CMO and CMO/M films. The survey spectra of both films
are shown in Fig. S2. The elemental information of the CMO/M films
indicated the presence of C, O, Si, Na, Mo, Zn, Cu, and Ni. Fig. 6 shows
the deconvolution spectra of Cu 2p and Mo3d on the reference film.
Two peaks at 933.0 and 953.0 eV are attributed to the binding energy
of Cu 2p3/2 and Cu 2p1/2, respectively (Fig. 6a). These signals match
with the literature value of Cu2+ state. Also, Fig. 6b shows two peaks
at 232.1 and 235.1 eV, which could be assigned to Mo 3d5/2 and Mo
3d3/2. According to the literature, both peaks correspond with
Cu3(MoO4)2(OH)2 [34].
On the other hand, the XPS spectra of the decorated films are
shown in Fig. 7. The lower intensity compared of CMO/M with the
CMO reference in the Cu2p and Mo 3d spectra could be attributed to
the good dispersion of the simple oxides on its surface (Fig. 7a, b,
d-g). In the CMO/Zn, the Zn2p spectrum shows two peaks at binding
energies of 1044.4 and 1021.2 eV that can be assigned to the Zn2p1/2
and Zn 2p3/2 peaks, corresponding to ZnO (Fig. 7c) [35]. The Cu2p
spectrum of CMO/Cu (Fig. 7d) showed two Cu2+ peaks and their
respective satellites (Cu2p3/2 and Cu2p1/2) appeared at 934.6, 954.6,
962, and 962.5 eV for CMO/Cu [36]. This signal is related to the
presence of CuO in the CMO/Cu surface. The Ni2p spectrum in Fig. 7h
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M.A. Ávila-López, E. Luévano-Hipólito and L.M. Torres-Martínez
Journal of Alloys and Compounds 873 (2021) 159846
Fig. 6. XPS spectra of CMO film: a) Cu 2p and b) Mo 3d.
displays the two main peaks located at 855.5 and 873.3–861.7 eV
with its respective satellite peaks. Also, the peaks located at 861.7
and 879.6 eV were attributed to the Ni2p3/2 and Ni2p1/2 spin-orbit
levels of NiO [37]. These results corroborate the presence of the
simple oxide decorations on the Cu3(MoO4)2(OH)2 films. Also, these
results confirm that doping was not present in the heterostructures
since the simple oxides were found on the CMO surface as a
decoration.
XPS valence band spectroscopy was carried out to elucidate the
diagram band of the CMO films. The valence band potential was
obtained by linear extrapolating the low binding energy edge in-
tersecting with the binding energy axis, as shown in Fig. 8a. The
reference Cu3(MoO4)2(OH)2 film showed a valence band maximum
(VB) of 1.5 eV and therefore, a conduction band (CB) of − 1.3 eV
(Fig. 8b). These values can thermodynamically favor the CO2 pho-
toconversion to different value-added products [38].
conditions without CO2 to evaluate the photocatalytic performance
of the CMO/M films. The CMO exhibited low activity for H2 pro-
duction (2.5 µmol g−1), as is shown in Fig. 10a. However, after the
decoration with simple oxides, a considerable increase in the H2
production was observed (Fig. 10b). The best result for H2 production
was obtained with the CMO/Cu sample, which it was up to 50 times
higher (127 µmol g−1) than the reference sample. This result could be
attributed to a more efficient charge transfer in this sample in the p-
n heterostructure.
To elucidate the photocatalytic mechanism on each hetero-
structure, band diagrams were constructed. According to the
conduction band potential of each material, two types of hetero-
structures were identified: i) type-I in CMO/Cu and ii) type-II for
CMO/Zn and CMO/Ni samples, as is shown in Fig. 11. In all the
cases, the conduction band (CB) potential of CMO was more ne-
gative than the corresponding values for simple oxides. After the
photoactivation, the electrons on the CB of CMO are transferred to
the CB of the simple oxides (ZnO, NiO, and CuO). On the contrary,
the holes in the valence band (VB) of the simple oxides (ZnO and
NiO) travels to the VB of CMO because of the difference in the
band edges positions (Case-I). This effect inhibits the photo-
generated pair recombination, enhancing the charge separation,
and therefore, the photocatalytic activity. On the other hand, in
the CMO/Cu occurs a different mechanism. Since the band gap of
the CMO/Cu (2.5 eV) is smaller than CMO, the band arrangement
favored the formation of a type-I (p-n) heterostructure (Case-II).
In this case, the holes could be accumulated in the CuO; however,
the presence of Na2O in the substrate act as a hole scavenger fa-
voring the electron transfer, as PL and chronoamperometric
characterization evidenced it. This sample exhibited more affinity
for CO2, which in this case could act as an electron scavenger that
promotes its conversion to value-added products. The lower band
gap of CMO/Cu leads to higher light absorption and high photo-
catalytic activity.
3.2. Photocatalytic experiments
The production of value-added products from the photocatalytic
CO2 reduction are shown in Fig. 9. All the films exhibited photo-
catalytic activity to produce HCOH, HCOOH, and CH3OH after 1 h
under visible-light irradiation. The highest production of HCOOH was
obtained with the CMO/Cu film, which production was around
155 µmol g−1 followed by CMO/Ni (118 µmol g−1
) and CMO/Zn
(82 µmol g−1). Moreover, it was possible to generate CH3OH with the
CMO/Cu (109 µmol g−1), CMO/Ni (60 µmol g−1), CMO/Zn (55 µmol g−1),
and CMO (53 µmol g−1) films. Also, traces of HCOH were identified in
the liquid phase (<0.5 µmol g−1).
The best performance to value-added products generation of
the CMO/Cu could be attributed because CuO provides enough H+
for the CO2 reduction due to its p-type nature. It is important to
mention that the Cu3(MoO4)2(OH)2 film decorated with CuO
showed production around three times higher to produce HCOOH
and CH3OH. Also, the higher production of value-added products
obtained by CMO/Cu could be associated with the higher affinity
for CO2 of CuO, and, subsequently, promoted photocatalytic re-
duction [39,40].
All the heterostructures were deposited on glass fiber that it has
already proven to be an effective option as a substrate. This fact has
been related to its chemical composition, i.e., SiO2, CaO, and Na2O.
Mainly, the presence of Na2O could act as a hole scavenger pre-
venting the re-oxidation of the reaction products and contributing to
the CO2 adsorption on the surface to promote its subsequent
The hydrogen evolution experiments are shown in Fig. 10. These
experiments were conducted under the same experimental
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M.A. Ávila-López, E. Luévano-Hipólito and L.M. Torres-Martínez
Journal of Alloys and Compounds 873 (2021) 159846
Fig. 7. XPS spectra of CMO/M films: a-c) CMO/Zn, d-e) CMO/Cu, and f-h) CMO/Ni.
7

M.A. Ávila-López, E. Luévano-Hipólito and L.M. Torres-Martínez
Journal of Alloys and Compounds 873 (2021) 159846
Fig. 8. a) XPS valence band spectra and b) band diagram of CMO.
photocatalytic reduction [40]. The effect of Na2O as hole scavenger is
shown in Fig. 11.
3.3. Stability of the photocatalysts
The films surface was studied after and before the photocatalytic
performance by FTIR spectroscopy. The spectra obtained for the
films are shown in Fig. 12. The FTIR spectra of CMO films before the
photocatalytic performance (Fig. 12a) present
a band around
3340 cm−1 related to the infrared vibration bands of O–H presented
in Cu3(MoO4)2(OH)2. Additional bands located between 960 and
800 cm−1 related to the vibrational of Mo=O in the MoO4 were ob-
served [17,22]. Other bands at 700–900 cm−1 were identified in the
FTIR spectra, which are associated with the bending modes of
Cu–OH [41,42]. Fig. 12b shows the FTIR spectra of the CMO/M films
after its photocatalytic application. It was possible to observe an
increase of the intensity of vibration bands of O-H at 3340 and
1660–1650 cm−1, which are related to the symmetric stretching and
bending vibrational modes of absorbed water and hydroxyl groups
[41,43]. The CMO/Cu and CMO/Ni films presented a series of bands at
1560 and 1410 cm−1, associated to the symmetrical vibrational
modes of O–C–O of monodentate carbonate species with linear
Fig. 9. CO2 photocatalytic reduction to value-added compounds using the as-pre-
pared CMO and CMO/A films after 1 h of visible-light.
Fig. 10. a) Kinetics of H2 production and b) yield obtained after 1 h using the as-prepared CMO and CMO/M films under visible-light.
8

M.A. Ávila-López, E. Luévano-Hipólito and L.M. Torres-Martínez
Journal of Alloys and Compounds 873 (2021) 159846
Fig. 11. Proposed band diagram for a) CMO/Zn and CMO/Ni, and b) CMO/Cu samples.
coordination [44,45]. Also, bidentate carbonates were identified at
1340 and 1020 cm−1 with lower intensity [46]. These bands are
characteristics of the intermediate precursors of the value-added
products. Therefore, the CO2 affinity was confirmed in the CMO/Cu
and CMO/Ni films. In this process, materials with an affinity for CO2
molecule are required, which is directly reflected in more efficient
production of solar fuels [5,40,47].
Lastly, the stability of all the samples was explored over three
consecutive cycles. After three cycles, the production of the films
continues to present activity to produce the value-added products.
Fig. 13 shows that all the films exhibit activity for CO2 reduction after
two consecutive cycles; however, after the third cycle, they present a
reduction in their photocatalytic performance. This fact could be
related to the formation of hydroxides and partial carbonation of the
films’ surface. The reference sample (CMO) showed a notable re-
duction in its photocatalytic activity after the third cycle, suggesting
its carbonation (Fig. 13a). The CMO/Zn and CMO/Ni films showed a
decrease in their photocatalytic activity, which could be related to
the photocorrosion and the presence of mono and bidentate species
that could block the active sites for CO2 reduction (Fig. 13b,c) [48,49].
On the contrary, the CMO/Cu film’s photocatalytic activity was stable
after three consecutive cycles, as is shown in Fig. 13d. The slight
decay could be attributed due to the partial carbonation and for-
mation of hydroxides in the decorates of CMO. This suggests CuO
switches into Cu(OH)2 and CuCO3, and the presence of O–H groups
and carbonate species could partially reduce the activate of the
photocatalytic material [5,39].
The films were characterized by XRD to investigate its chemical
stability. Fig. 14 shows the diffractograms of films after the photo-
catalytic test. The presence of additional phases such as carbonate
phases was not observed at the level of resolution of this technique,
which confirms the stability of crystal structure of the films.
It is worth mentioning that if the CO2 reduction process is
scale, separation units should be included to obtain pure products.
In this case, a column destination should be incorporated after the
reactor outlet. Considering the products obtained: CH3OH
(Tb = 65 °C), HCOOH (Tb = 100.8 °C), and H2 (Tb = −253 °C), at least
three steps should be added [50]. Also, no-reacted CO2
(Tb = −57 °C) and H2O (Tb = 100 °C) should be considered for the
column's future design.
Fig. 12. FTIR spectra of CMO/M films a) before and b) after performing the CO2
photocatalytic test.
9

M.A. Ávila-López, E. Luévano-Hipólito and L.M. Torres-Martínez
Journal of Alloys and Compounds 873 (2021) 159846
Fig. 13. Stability of the CMO films after three cycles of photocatalytic evaluation under visible-light: a) CMO. b) CMO/Zn, c) CMO/Cu and d) CMO/Ni. N.D. refers to no detected.
4. Conclusions
Cu3(MoO4)2(OH)2 films decorated with ZnO, CuO, and NiO were
grown on flexible substrates through a rapid and low energy de-
mand microwave-hydrothermal method. The films exhibited good
activity for CO2 photoconversion to value-added compounds, i.e.,
CH3OH, HCOOH and H2 under visible-light irradiation.
The incorporation of ZnO, CuO and NiO on Cu3(MoO4)2(OH)2
promoted a more efficient charge separation and enhanced the light-
absorption, which resulted in an increase of the production of value-
added compounds. The production of value-added products fol-
lowed the tendency: CMO/Cu > CMO/Ni > CMO/Zn > CMO.
The best photocatalytic performance of the CMO/Cu film was
associated to the formation of type-I heterostructure between
Cu3(MoO4)2(OH)2 and CuO, which promoted a more efficient charge
separation and the accumulation of enough electrons for the CO2
conversion to CH3OH, HCOOH in liquid phase and H2 in gas phase.
This result was attributed to the 2D morphology and a higher CO2
adsorption. The stability of the films was evaluated after three
consecutive cycles for the photocatalytic application. After three
cycles, the films exhibited activity to produce value-added com-
pounds, also their crystalline phase was not modified. All results
indicated that these materials could be scale up and design for ap-
plication in low cost photocatalysts to produce value-added products
an under sunlight irradiation.
CRediT authorship contribution statement
Leticia M. Torres-Martínez: Original idea, Discussion,
Supervision, Writing
- review & editing. E. Luévano-Hipólito:
Original idea, Data analysis, Discussion, Supervision, Writing - re-
view & editing. Manuel Alejandro Ávila-López: Writing, metho-
dology, Data analysis, Results.
Fig. 14. X-ray diffraction patterns of the a) CMO and b-d) CMO/M (M= Zn, Cu, or Ni)
after photocatalytic test.
10

M.A. Ávila-López, E. Luévano-Hipólito and L.M. Torres-Martínez
Journal of Alloys and Compounds 873 (2021) 159846
Declaration of Competing Interest
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The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have influ-
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Acknowledgments
catalysis, photocurrent response and lithium storage, RSC Adv.
12015–12024, https://doi.org/10.1039/C4RA15725G
5 (2015)
The authors wish to thank CONACYT for financial support for this
research through the following projects: Cátedras CONACYT 1060
and CONACYT-FC-1725. Manuel Alejandro Ávila López wants to
thank CONACYT for the Ph.D. scholarship, CVU 70725. In addition,
the authors want to thank to M.C Luis Gerardo Silva Vidaurri from
CIMAV for his help with the XPS analysis.
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