Ternary ZnO/CuO/Zeolite composite obtained from volcanic ash for photocatalytic CO2 reduction and H2O decomposition

Article abstract of DOI:10.1016/j.jpcs.2020.109917

An n-p heterojunction based on ZnO/CuO supported in a zeolitic framework (ZF) was designed to produce solar fuels from H₂O decomposition and CO₂ conversion. ZF was synthesized from volcanic ashes by an alternative microwave-hydrothermal method using a biodegradable template for its formation. The framework resulted in NaAlSiO₄ (NAS) with a high surface area and a morphology composed of circular channels of 50 nm. Incorporating the ZnO/CuO heterostructure in the NAS channels resulted in an improved light-absorption, more efficient charge transfer, nanostructure morphology, and more active sites available for the CO₂ adsorption and photocatalytic reactions. The activities for H₂ and light-hydrocarbons (HCOOH, HCOH, and CH₃OH) evolution were evaluated in the photocatalytic water-splitting and CO₂ reduction under UVA irradiation, respectively. The ZnO/CuO/NAS composite exhibited a remarkably higher H₂ (187 μmol/g) and HCOOH (2721 μmol/g) evolution after 3 h of irradiation. These results were related to the synergistic effect among ZnO, CuO, and NAS framework. A mechanism of the photocatalytic reaction was proposed.

Full text of DOI:10.1016/j.jpcs.2020.109917

Journal of Physics and Chemistry of Solids 151 (2021) 109917
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: http://www.elsevier.com/locate/jpcs
Ternary ZnO/CuO/Zeolite composite obtained from volcanic ash for
photocatalytic CO2 reduction and H2O decomposition
a
b
,
c
,
*
b
´
´
´
E. Luevano-Hipolito , Leticia M. Torres-Martínez
, A. Fernandez-Trujillo
a
´
´
´
Catedras-CONACYT - Universidad Autonoma de Nuevo Leon, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P, 66455,
´
San Nicolas de los Garza, NL, Mexico
b
´
´
´
Universidad Autonoma de Nuevo Leon, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P, 66455, San Nicolas de los Garza,
NL, Mexico
c
´
Centro de Investigacion en Materiales Avanzados, S.C. Miguel de Cervantes 120 Complejo Ind, Chihuahua, Chihuahua, Chih, 31136, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords:
An n-p heterojunction based on ZnO/CuO supported in a zeolitic framework (ZF) was designed to produce solar
fuels from H2O decomposition and CO2 conversion. ZF was synthesized from volcanic ashes by an alternative
microwave-hydrothermal method using a biodegradable template for its formation. The framework resulted in
NaAlSiO4 (NAS) with a high surface area and a morphology composed of circular channels of 50 nm. Incorpo-
rating the ZnO/CuO heterostructure in the NAS channels resulted in an improved light-absorption, more efficient
charge transfer, nanostructure morphology, and more active sites available for the CO2 adsorption and photo-
catalytic reactions. The activities for H2 and light-hydrocarbons (HCOOH, HCOH, and CH3OH) evolution were
evaluated in the photocatalytic water-splitting and CO2 reduction under UVA irradiation, respectively. The ZnO/
Volcanic ashes
Zeolites
Photocatalysis
Solar fuels
Hydrogen production
CO2 reduction
CuO/NAS composite exhibited a remarkably higher H2 (187 μmol/g) and HCOOH (2721 μmol/g) evolution after
3 h of irradiation. These results were related to the synergistic effect among ZnO, CuO, and NAS framework. A
mechanism of the photocatalytic reaction was proposed.
1. Introduction
Remarkably, the synthesis by the microwave-hydrothermal method
(MWH) promotes a fast heating speed, sensitive reaction, and uniform
In recent years, heterogeneous photocatalysis has represented an
alternative technology for the H2 production via H2O decomposition and
the generation of renewable light hydrocarbons (solar fuels) such as
CH3OH, H2CO, HCOOH, CO, and CH4 from CO2 reduction [1–3]. The
global process requires photocatalyst materials, water, and sunlight to
carry out renewable fuels production. The design of highly active and
selective photocatalysts for the production of solar fuels has experienced
a significant boom due to the awareness of environmental issues and the
continuous search for new and more efficient methodologies to solve the
energy crisis. Among various photocatalysts, zinc oxide (ZnO) is one of
the most competitive materials for this application due to its high pho-
toactivity and stable performance and its harmless nature, easy acces-
sibility, and low-cost [4,5]. ZnO has been prepared by different methods
such as mechanosynthesis, precipitation, sol-gel, emulsion, micro-
emulsion, hydrothermal, microwave method, and recently the combi-
nation of the last two methods has been gained interest [6,7].
heating system, which allows preparing nanoparticles with a narrow
particle size distribution [7]. These characteristics of MWH are inter-
esting to produce ZnO nanoparticles with the required physicochemical
properties for the photocatalytic CO2 reduction.
The use of heterostructures between ZnO and other semiconductors
with matching band structures such as Bi2O3, CdS, RGO, NiFe2O4,
CuFe2O4, and CuO has resulted in high efficiencies for some solar fuels
production such as CH3OH, CO, and H2 [8–12]. Another solar fuel
commonly reported during the photocatalytic CO2 reduction is formic
acid (HCOOH), widely used as feedstock in the chemistry industry. In
this context, some composites have been proposed to produce HCOOH
such as ZnV2O6/RGO (1942
μ
mol/g), TiO2/MgO (2
μmol/g), BiYO3/Cu
(1000 mol/g), and Ru(II)/Ag/CaTaO2N (80
μ
μ
mol/g) [13–16]. Different
factors influenced the photocatalytic efficiency of these composites. For
example, the activity for HCOOH production on TiO2/MgO composite
was improved with low amounts of MgO (1%), since it provides basic
´
´
* Corresponding author. Universidad Autonoma de Nuevo Leon, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P,
´
66455, San Nicolas de los Garza, NL, Mexico.
E-mail addresses: lettorresg@yahoo.com, leticia.torresgr@uanl.edu.mx, leticia.torres@cimav.edu.mx (L.M. Torres-Martínez).
https://doi.org/10.1016/j.jpcs.2020.109917
Received 13 July 2020; Received in revised form 19 September 2020; Accepted 22 December 2020
Available online 28 December 2020
0022-3697/© 2020 Elsevier Ltd. All rights reserved.

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Journal of Physics and Chemistry of Solids 151 (2021) 109917
sites for the adsorption of the CO2 lewis acid [14]. The efficiency of
BiYO3 increased 2.2 times after doping with Cu, which promotes the
formation of oxygen vacancies, suppressing the recombination of the
electron and hole pair [15]. On the other hand, the hybrid CaTaO2N/Ag
with a binuclear Ru(II) complex (as photosensitizer) was used to pro-
duce HCOOH via a two-step photoexcitation mechanism with high
selectivity (99%) [16]. In general, the high photocatalytic performance
of these composites is related to the appropriate band edge alignments,
an enhancement in the visible light harvesting, and a decrease in the
recombination of eꢀ /h+ pair.
volcano to fabricate zeolitic support of the n-p heterojunction ZnO/CuO.
The ternary composite proposed will be tested as photocatalyst in solar
fuels production from CO2 reduction and H2O decomposition under
visible-light at room temperature.
2. Experimental
2.1. Synthesis of n-ZnO/p-CuO heterostructures
ZnO/CuO heterostructures were synthesized by a one-pot micro-
wave-hydrothermal method. The method implies the dissolution of 0.01
mol of Zn(CH3CO2)2⋅2H2O (Aldrich, 99%) in 50 mL of methanol
(Aldrich, 99%). Then, stoichiometric amounts of Cu(CH3CO2)2⋅H2O
(Aldrich, 99%) were added to the formation ZnO/CuO heterostructures
with 5 wt% of CuO. After that, 1 g of NaOH (Fermont, 99%) was added
to the mixture under stirring. The resulting mixture was exposed to
microwave-hydrothermal heating in a reactor model Mars 6, with the
Another strategy to increase the efficiency of the photocatalysts for
solar fuels production is to support them in high surface area com-
pounds, i.e., inorganic porous materials (IPMs) [17]. This alternative is
useful to provide active sites to the adsorption of CO2. For this purpose,
zeolites represent a good candidate to act as a support of photocatalysts.
Zeolites are porous aluminosilicate materials with both high surface
area and absorption ability related to their porosity [18,19]. The porous
structure can confine small molecules such as CO2 to improve the pho-
tocatalytic reactivity. Hence, zeolite was selected as supporting material
for the heterostructured photocatalysts in this research. Some examples
of zeolitic supports for photocatalytic applications are TiO2/Stellerite
[20], TiO2/Clinoptilolite [21], Ag–TiO2/Zeolite-Y [22], ZnO/Zeolite-Y
[23], and SnO2/Clinoptilolite [24]. In these works, the photocatalytic
activity has been studied for different air and water pollutants abate-
ment with significantly higher efficiencies than bare-materials.
◦
following conditions: 100 W, 100 C, and 1 h. The resulting samples
were washed with distilled water and methanol to remove the by-
products generated during the synthesis.
2.2. Synthesis of zeolite from volcanic ashes
The volcanic ashes (VAs) were taken from the Colima volcano. In the
first stage, VAs were mixed with pellets of NaOH (Fermont, 99%) in a
weight ratio of 1:1.2 in an agate mortar to promote its alkali fusion and
the extraction of the water-soluble sources of Si and Al. Then, VAs were
calcined at 550 ◦C for 2 h in air using a ramp of 10 ◦C/min. After the heat
treatment time elapsed, the sample obtained was mixed with distilled
water and left it to age overnight. The powders were separated from the
solvent by centrifugation, and they were dried at 100 ◦C. Once dried, 1 g
of the product was mixed with 50 mL of distilled water and sodium
citrate (Na3C6H5O7, Fermont 99%) as a biodegradable template at pH 8.
Finally, the sample was calcined at 550 ◦C for 2 h. This sample was
labeled as NAS.
Zeolites can be found in natural deposits and they can be prepared by
different synthesis methods, mainly by the hydrothermal route
(<100 ◦C, 1 bar). Other common methods to synthesize zeolitic frame-
works are sol-gel [25], emulsion [26], ionothermal, solid-state-reaction,
microwave, and sonochemical [27]. These methods can be classified
into four categories according to the use of i) templates, ii) type of silicon
and aluminum sources, iii) solvents, iv) facile-synthesis methods to
shorten the crystallization time with high productive efficiency [27].
The synthetic zeolites can be obtained from different raw materials
such as sodium silicate, fumed silica, tetraethylorthosilicate (TEOS),
colloidal silica (Ludox), aluminum sulfate, among others [28]. Also, to
create microporosity in zeolites, it is necessary to add surfactants of
different chain lengths as templates. Some of them are tetramethy-
lammonium, tetraethylammonium, cetyltrimethylammonium bromide,
chloride, and hydroxide [29,30]. Nevertheless, the use of these com-
pounds is related to high energy consumption during synthesis and a
large amount of waste dumped into the environment. Alternatively,
using eco-friendly and biodegradable templates such as sodium citrate
has not been studied to synthesize zeolites. So far, other green alterna-
tives have been proposed for the synthesis of zeolites, i.e., the use of
natural minerals or industrial by-products as raw materials. For
example, the products of volcano eruptions constitute a natural and
low-cost source of Si and Al that can be used as precursors of zeolites
[31,32]. In Mexico, there are 48 active volcanoes, which the most active
is the Colima volcano (19.5 N, 103.5W) showed in Fig. 1. The emissions
of this volcano are around 400–1600 tons per day [33]. Consequently,
the use of this vast amount of natural waste is of great scientific and
commercial interest. Thus, this work proposes using of ashes of Colima
2.3. Synthesis of ZnO/CuO supported on zeolite
The impregnation of the heterojunction n-p ZnO/CuO on NAS was
carried out by microwave-hydrothermal. For this purpose, 1 g of each
material was mixed in an aqueous solution, and then it was exposed to
microwave irradiation (100 W) for 30 min at 100 ◦C. After treatment,
the samples were washed with distilled water and dried at 100 ◦C
overnight. The sample was identified as ZnO/CuO/NAS. In this case, the
50/50 wt ratio of ZnO/CuO and NAS was used since it presents the
highest photocatalytic activity for H2 evolution, as will be further
discussed.
To clarify each step of the synthesis, Fig. S1 shows a schematic
representation of the ZnO/CuO/NAS composite fabrication.
2.4. Characterization
X-ray powder diffraction was used to perform the structural char-
Fig. 1. Colima volcano located at 19.5 N and 103.5W in Mexico.
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Journal of Physics and Chemistry of Solids 151 (2021) 109917
acterization in a Bruker D8 Advance diffractometer with Cu K
α
radiation
required a mobile phase based on H2SO4 0.005 M-acetonitrile 80:20 v/v
(λ = 210 nm). For formaldehyde measurement, the sample was mixed
with 2,4-dinitrophenylhydrazine and phosphoric acid for a complex
formation (λ = 360 nm). In this case, a mixture of acetonitrile-water
45:55 v/v was used as a mobile phase. On the other hand, CH3OH was
identified by a complexometric method with sodium nitroprusside in a
UV–Vis spectrophotometer Cary 5000 at 481 nm.
(40 kV, 30 mA). The analysis was made using a scan rate of 0.05◦ by 0.5
s. Crystallite size was obtained using the Debye’s-Scherrer equation (1):
Kλ
hklcosθ
D =
(1)
β
Where D is the crystallite size, K is the shape factor (0.94), λ is the
wavelength of Cukα radiation, and βhkl is the instrument broadening,
which it was calculated by using equation (2). In this case, we used the
Al2O3 corundum as reference.
3. Results and discussion
3.1. Structural characterization
1/2
2
2
βhkl = [(βhkl
)
ꢀ (βhkl
)
]
(2)
measured
instrumental
The crystal structure of the pristine and synthesized materials was
investigated by X-ray powder diffraction. Fig. S2 shows the diffracto-
grams of VAs and NAS samples. Pristine ashes (VAs) crystallized in the
triclinic system associated with the presence of albite (NaAlSi3O8,
JCPDS card no. 83–1608) as only crystal phase. NAS sample exhibited a
cubic zeolite NaAlSiO4 structure according to the JCPDS card no.
11–0221. Also, the presence of traces of albite was identified at 2θ =
13.6, 22.8, 28.2, 29.1, 31.7, and 45.4◦. The crystallite size of the VAs and
NAS samples was 52 and 43 nm, respectively.
The chemical composition of the volcanic ash was investigated by X-
ray fluorescence (XRF) using a Panalytical Epsilon Energy Dispersive
Fluorescence equipment. Scanning electron microscopy (SEM) was used
to study the morphology of the samples in a JEOL 6490 LV microscope.
The surface area of the samples was calculated from N2 adsorption-
desorption isotherms obtained in a Bel-Japan Minisorp II analyzer at
ꢀ 196 ◦C. The band gap of the samples was calculated from the diffuse
reflectance spectra obtained in a UV–Vis NIR spectrophotometer (Cary
5000) coupled with an integration sphere. The band gap energy was
calculated using the Kubelka-Munk function, considering a direct
transfer of charge in the samples. Photoluminescence analysis was per-
formed in a fluorescence spectrophotometer Cary Eclipse integrated
with a Xenon flash lamp at 25 ◦C. The samples were excited at 290 nm,
using a scanning speed of 600 nm/min, and an excitation width and
emission slit of 10 nm.
The X-ray diffraction patterns of ZnO, CuO, and the heterostructure
between them are shown in Fig. 3. Bare-ZnO was obtained in pure form
according to the JCPDS card of the polymorph hexagonal No. 36–1451.
The diffractogram of the ZnO/CuO heterostructure did not show re-
flections associated with CuO; however, its crystallite size was larger
(29 nm) concerning ZnO (18 nm). In the diffractogram of the ternary
composite was possible to identify crystalline phases related to ZnO,
NaAlSiO4, and traces of NaAlSi3O8. These two phases are related to the
NAS sample, as it is shown in Fig. S2. The crystallite size of the ternary
composite was 15 nm, as it is shown in Table 1.
2.5. Photocatalytic activity evaluation
The production of solar fuels was carried out in the experimental set-
up shown in Fig. 2. The set-up consisted of three main parts: i) the
photocatalytic reactor, ii) chromatographs for the quantification of re-
action products, and iii) peripheral equipment (mass flow control,
valves, manometer, moisture trap). The photocatalytic reactor was
constructed of borosilicate glass of 0.25 L. All the tests were evaluated at
25 ◦C. For the experiments, 0.1 g of the photocatalyst was suspended in
distilled water under magnetic stirring. The reactor was purged with N2
(99.9%) for H2 evolution and with CO2 (99.9%) for HCOOH production,
respectively. The suspension was irradiated with two halogen lamps of
20 W with an irradiance of 8 W/m2. The products generated were
identified as follows: H2 in a gas chromatograph Shimadzu GC-2014
equipped with a thermal conductivity detector (TCD), while HCOH
and HCOOH were measured with an HPLC model Prominence-i
LC2030C 3D, Shimadzu equipped with a Shimadzu C18 column using
Also, the investigation of the chemical composition of the pristine
25
μL of the sample. The quantification of formic acid by this technique
Fig. 2. Set-up system used for the photocatalytic experiments.
Fig. 3. Diffractograms of the as-prepared samples.
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Journal of Physics and Chemistry of Solids 151 (2021) 109917
with homogeneous morphology from earth-natural sources.
Table 1
Summary of the physicochemical properties of the samples.
Sample
Crystallite size (nm)
Band gap (eV)
Surface area (m2/g)
3.3. Optical characterization
ZnO
ZnO/CuO
ZnO/CuO/NAS
NAS
18
29
15
43
52
3.1
3.0
2.9
-
6
33
UV–Vis diffuse reflectance spectroscopy was used to investigate the
optical properties of the synthesized samples, as shown in Fig. 5. Bare-
ZnO exhibited the characteristic band edge of the wurtzite, corre-
sponding with a band gap energy of 3.1 eV. The optical absorption edge
shifted from blue to red in the ZnO/CuO and ZnO/CuO/NAS samples
due to the CuO absorption in the visible range (1.4 eV), as it is shown in
Fig. 5. An increase in the absorption light-capacity of the samples was
observed, mainly in the ZnO/CuO/NAS sample. This effect could be
attributed to the light scattering in the porous structure, leading to a
more extended pathway of light for its further utilization [35].
52
121
<1
VA
-
volcanic ash was characterized by XRF. The composition of the VA
sample is shown in Table S1. SiO2, Na2O, and Al2O3 were identified as
principal oxides. Other compounds such as CaO, Fe2O3, and traces of
K2O, MgO, and TiO2 were detected. SiO2/Al2O3 ratio is important to
predict the formation of zeolites from different by-products, i.e., fly ash
or volcanic ash. A SiO2/Al2O3 higher than 2 favors the formation of
zeolitic structures [34]. Thus, according to the XRF characterization,
VAs has a SiO2/Al2O3 ratio of 8, high enough to promote the formation
of zeolitic structures under an adequate alkali activation.
3.2. Morphological characterization
For reference purposes, the morphology of the volcanic ash is pre-
sented in Supplementary Fig. S3. VAs exhibited heterogeneous
morphology composed of big particles (>50 μm) with a rough surface.
SEM micrographs of the as-prepared samples are shown in Fig. 4. The
morphology of the NAS sample was composed of flake-particles inter-
connected among them (Fig. 4a). Also, this sample exhibited notably
circular channels of diameter around 50 nm. Bare-ZnO particles showed
a homogeneous morphology composed of uniformly distributed spher-
ical particles (50 nm), as it can be seen in Fig. 4b. The incorporation of
CuO into ZnO resulted in the formation of bigger and more irregular
particles than the reference (Fig. 4c and d). It was possible to distinguish
CuO on the surface of ZnO as clusters of 1 μm in the ZnO/CuO sample by
EDS elemental mappings (Fig. S4). The ternary composite morphology
resulted in seed-like particles of an average size of 200 nm distributed
homogeneously (Fig. 4e). A closer view of these particles evidenced their
roughness, as it is shown in Fig. 4f. These results indicate the feasibility
of the microwave-hydrothermal method to fabricate nanostructures
Fig. 5. Kubelka-Munk diffuse reflectance spectra of synthesized samples and
the emission of the halogen lamp.
Fig. 4. SEM micrographs of a. NAS, b. ZnO, c-d. ZnO/CuO, and e-f. ZnO/CuO/NAS samples.
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Journal of Physics and Chemistry of Solids 151 (2021) 109917
Fig. 5 also shows the emission spectra of the lamp used as irradiation
source for the photocatalytic experiments. As it can be seen, the main
emission peak of the lamp corresponds to the absorption edge of the
samples. Also, the lamp shows two more emissions at 3.1 and 2.8 eV,
which energy can be absorbed by the ZnO/CuO and ZnO/CuO/NAS to
promote the generation of the electron and hole pair.
compounds investigated.
3.5. Renewable solar fuels production
a) Hydrogen production
To investigate the effect of the n-p heterojunction formation on the
emission characteristic of ZnO, photoluminescence spectra were recor-
ded. Fig. 6 shows the emission spectra of ZnO, ZnO/CuO, and ZnO/CuO/
NAS. The spectra of the samples exhibit a defined peak at 390 nm. This
emission (3.2 eV) is ascribed to near-band emissions due to the excitonic
shift from localized regions below the conduction band to the bottom of
the valence band [36]. This emission is usually related to the free charge
carriers’ recombination through an exciton-exciton collision process
[37]. As can be seen, the intensity of emission at 390 nm decreases after
the formation of the n-p heterojunction. This effect was more notable in
the ternary composite ZnO/CuO/NAS, which evidences an enhancement
in the charge transfer in the material fabricated. It is also important to
note that no emission in the visible-light was observed in the PL spectra.
Fig. 8 shows the hydrogen evolution using the as-prepared and the
reference samples under UV light. As it can be seen, the highest photo-
catalytic activity of the as-prepared samples was obtained with the
ternary composite. After 3 h, 187
μ
mol/g of hydrogen were generated,
mol/g of H2
whereas with the n-p heterojunction ZnO/CuO only 50
μ
were detected. These results indicated that the photocatalytic perfor-
mance for H2 production with the ZnO/CuO/NAS was 3.7 and 18 times
higher than those ZnO/CuO and ZnO samples, respectively. The high
photocatalytic efficiency of the ZnO/CuO/NAS composite was associ-
ated to the combination of its physicochemical properties such as both
high porosity and surface area, enhanced light absorption, and a lower
recombination of the photogenerated electron and hole pair.
For reference purpose, the activity of NAS and VAs was evaluated
under the same experimental conditions, which results are shown as
input image in Fig. 8. The reference samples showed a low photo-
3.4. Textural characterization
catalytic activity (<1 μmol/g).
On the other hand, the effect of different amounts of NAS in the
ternary composite was studied for H2 evolution in Fig. S5. According to
the results, the composite with low (x = 0.3) and high (x = 0.8) amounts
of NAS did not favor higher yields of H2 production regarding the
reference sample (x = 0.5). These results could be attributed to the
following reasons:
Fig. 7 shows the N2 isotherms of the synthesized samples. The NAS
sample exhibited combined type-I and IV isotherms, as well as type H4
hysteresis loop. On the other hand, ZnO and the n-p heterostructure
showed a type-II profile related to macroporous samples. When the
heterostructure was supported on the NAS sample, the samples showed a
mixed profile related to meso-macroporous materials (type-IV), which
indicates the presence of porosity in the ternary composite. This prop-
erty was evidenced by the hysteresis in the interval 0.75–0.95 of P/P0, as
shown in Fig. 7a. Barrett-Joyner-Halenda (BJH) method was performed
to evidence the pore size distribution in the NAS and ternary composite,
which results were included in Fig. 7b. The average pore radius of NAS
sample was 1.8 nm, while the ternary composite exhibited a similar
value (1.6 nm). These results were slightly higher than previous reports
for nanostructured zeolites [38,39].
I. Low loads of NAS did not contribute significantly with active sites
to carry out the photocatalytic reaction. However, the efficiency of H2
evolution was 1.9 times higher than the ZnO/CuO heterostructure.
II. High amounts of NAS (x = 0.8) in the composite did not favor the
H2 production since the zeolitic framework does not have photocatalytic
activity. In this scenario, it is necessary to add more ZnO/CuO photo-
catalyst to carry out the H2O reduction.
Thus, the ternary composite with half of the zeolitic support was
chosen for CO2 reduction experiments.
The specific surface area values of the samples are listed in Table 1.
NAS sample exhibited the highest surface area (121 m2/g), which pro-
vide a high number of active sites for the photocatalytic reaction.
The ternary composite ZnO/CuO/NAS synthesized exhibited higher
surface area (52 m2/g) in comparison with bare-ZnO and the ZnO/CuO
heterostructure, which evidences the synergy effect among the
b) Photocatalytic CO2 reduction
The references and the as-prepared samples were evaluated as pho-
tocatalyst in the CO2 reduction in the liquid phase to produce HCOOH,
HCOH, and CH3OH formation, as it is shown in Fig. 9. HCOOH was the
main product of the CO2 reduction, while HCOH and CH3OH were
detected in minor amounts. The ZnO/CuO/NAS composite exhibited
remarkably photocatalytic activity compared with the reference samples
for HCOOH production, reaching a production up to 2721 μmol/g in 3 h.
This efficiency was 3.8 times higher than that of the heterojunction
ZnO/CuO, and up to 500 times higher than the reference ZnO.
Regarding to HCOH and CH3OH production, the photocatalytic effi-
ciency of the ternary composite was also higher than those of the ZnO/
CuO and ZnO samples, as it can be seen in Fig. 9b.
The higher production of HCOOH compared to that of HCOH and
CH3OH can be related to kinetic aspects. Equations (3)–(6) show the
reactions and products of the H2O decomposition and CO2 reduction. As
it can be seen, HCOOH and H2 production is kinetic favored since only
two electrons and protons are required for their formation (equations (3)
and (4)), in spite of its higher thermodynamic potential (ꢀ 0.61 V). In
contrast, the formation of HCOH and CH3OH required 4 and 6 electrons
and protons, respectively at lower potentials (equations (5) and (6)) at
pH 7 [40].
2H2O + 2eꢀ ⇄H2 + 2OHꢀ E◦ = ꢀ 0.41⋅V
(3)
CO2 + 2eꢀ + 2H+⇄HCOOH E◦ = ꢀ 0.61⋅V
(4)
Fig. 6. Room temperature photoluminescence emission spectra of the as-
prepared samples.
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Journal of Physics and Chemistry of Solids 151 (2021) 109917
Fig. 7. a. N2 isotherms of the synthesized samples and b. BJH pore size distribution of porous samples.
Fig. 8. Photocatalytic H2 evolution from H2O decomposition using the as-prepared and reference samples. Input image: H2 evolution of reference materials.
CO2 + 4H+ + 4eꢀ ⇄HCHO + H2O E◦ = ꢀ 0.48⋅V
CO2 + 6eꢀ + 6H+⇄CH3OH + H2 E◦ = ꢀ 0.38⋅V
(5)
(6)
synergistic effect created at the interface of the n-p heterojunction and
the pore walls of the NAS sample. Furthermore, the presence of Al3+ ions
in the NAS sample exhibits a Lewis acidity [42–46], which provides
unoccupied orbital as an electron trap. This Lewis acid site improves the
separation efficiency of photogenerated charges. The photocatalytic
mechanism proposed is shown in Fig. 10. In this mechanism, ZnO is
excited with UV light with the promotion of an electron from the valence
band (VB) to the conduction band (CB) of this semiconductor. Then, the
electrons can be transferred to the CB of CuO, which interacts with the
water molecules to produce H2 or react with the adsorbed CO2 molecule
to produce mainly formic acid. Additionally, the NAS support provides
Lewis acid site (marked as squares) to trap the photogenerated electrons
that can be reacted with the CO2 adsorbed on the support.
The improved photocatalytic performance of the ZnO/CuO/NAS
composite in the CO2 reduction can be associated to a more efficient
charge transfer among the components, enhanced light-absorption, and
a higher surface area that contributes to the fixation of CO2. The
improvement in the CO2 adsorption could be attributed to the presence
of the zeolite. Particularly, the excellent capacity (1.4 mmol/g) of
NaAlSiO4 for CO2 adsorption was evidenced under similar experimental
conditions (PCO =0.15 bar, T = 40 ◦C) by Bae et al. [41]. Thus, this factor
2
could be critical to assure high efficiencies for HCOOH production. Once
the CO2 is adsorbed on the surface of photocatalyst is necessary to
provide an enough number of electrons and protons to carry out is
conversion to light-hydrocarbons. In this context, the strategy of sup-
porting the n-p ZnO/CuO heterojunction on the zeolite also favored the
charge transfer and delay the recombination of the electrons and holes,
as it was evidenced by PL analysis. This effect could be related to the
The production of formic acid in this work using the ZnO/CuO/NAS
composite was notably high (2.7 mmol/g) and competitive with previ-
ous works [11–14]. Also, the ternary composited proposed produced
solar fuels with high efficiencies without sacrificial agents at near
ambient conditions. These results are important since both solar fuels
(H2/HCOOH) generated can be used as chemical feedstock in many
6

´
´
E. Luevano-Hipolito et al.
Journal of Physics and Chemistry of Solids 151 (2021) 109917
Fig. 9. Solar fuels generation via CO2 reduction: a. HCOOH and b. HCOH/CH3OH production.
CRediT authorship contribution statement
´
´
E. Luevano-Hipolito: Methodology, Formal analysis, Funding
acquisition, Supervision, conceived of the present idea. proposed the
utilization of volcanic ashes to fabricate inorganic framework to support
Cu2O. verified the analytical methods and supervised the findings of this
work. Leticia M. Torres-Martínez: Funding acquisition, Supervision,
conceived of the present idea. proposed the use of the material for CO2
utilization to produce solar fuels such as formic acid. erified the
analytical methods and supervised the findings of this work. A.
´
Fernandez-Trujillo: Funding acquisition, Supervision, synthesized the
materials and performed the evaluation for hydrogen evolution. verified
the analytical methods and supervised the findings of this work. All
authors discussed the results and contributed to the final manuscript.
Fig. 10. Photocatalytic mechanism proposed for solar fuels production with the
Declaration of competing interest
ZnO/CuO/NAS composite.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
industrial process. Nowadays, these solar fuels are produced in the in-
dustry using coal and natural gas as raw materials, which makes the
actual process high polluting. Thus, the design of new and advanced
photocatalytic materials such as ZnO/CuO/NAS with high efficiencies to
obtain clean and renewable solar fuels is vital to migrate to cleaner
technologies to supply energy needs.
Acknowledgments
The authors wish to thank CONACYT for financial support for this
´
research through the following projects: Catedras CONACYT 1060 and
4. Conclusions
CONACYT–FC–1725. Also, the authors want to thank Programa Delfín
´
for its support for the scholarship to A. Fernandez-Trujillo. In addition,
Ternary ZnO/CuO/NAS composite was designed to act as an efficient
catalyst to produce value-added compounds such as hydrogen and light-
hydrocarbons from water splitting and CO2 reduction, respectively. The
composite was synthesized by a two-step route of preparation of NiAl-
SiO4 (NAS) from volcanic ash and assembly of the n-p heterojunction
based on ZnO/CuO inside of NAS. SEM analysis evidenced that ZnO/
CuO nanoparticles were embedded in the circular channels of the NAS
framework. This closer contact favored in the composite an increase of
the surface area light-absorption, and a more efficient charge transfer.
ZnO/CuO/NAS composite showed remarkably photocatalytic activ-
the authors want to thank to Nayely Pineda Aguilar from CIMAV for her
help with the SEM characterization.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jpcs.2020.109917.
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