Development of Na2Ti6O13/CuO/Cu2O heterostructures for solar photocatalytic production of low-carbon fuels

Article abstract of DOI:10.1016/j.materresbull.2019.110679

Na₂Ti₆O₁₃/CuO/Cu₂O materials were prepared by solid-state and impregnation method, using copper oxide as cocatalyst (CC, 0.1%–5%). The catalytic activity was evaluated for H₂ evolution and CO₂ reduction. XPS analysis revealed the presence of Cu₂O and CuO in different proportions. Na₂Ti₆O₁₃ impregnated with 0.1% of cocatalyst exhibits majoritary the Cu₂O phase; while Na₂Ti₆O₁₃ with 5% of cocatalyst shows mainly CuO. Electrochemical measurements showed higher photocurrent and lower resistance to charge transference in Na₂Ti₆O₁₃-0.1% CC, associated with better a charge flow. Na₂Ti₆O₁₃-0.1% CC exhibited the highest H₂ production (33 μmol g^-1 h^-1) and Na₂Ti₆O₁₃-5% CC showed the best CO₂ conversion to CH₂O (25 μmol g^-1 h^-1) and CH₃OH (4.6 μmol g^-1 h^-1). A major content of Cu₂O phase favored the H₂ evolution by the formation of a Z-scheme, where the strong negative character of the CB of Cu₂O enhances the kinetics of H₂O reduction, while a higher content of CuO improved CO₂ adsorption and reduction.

Full text of DOI:10.1016/j.materresbull.2019.110679

Materials Research Bulletin xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Development of Na Ti O /CuO/Cu O heterostructures for solar
2
6
13
2
photocatalytic production of low-carbon fuels
Luz I. Ibarra-Rodríguez, Ali M. Huerta-Flores, Leticia M. Torres-Martínez*
Universidad Autónoma de Nuevo León, Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Av. Universidad S/N Ciudad Universitaria, San Nicolás de
los Garza, Nuevo León, C.P. 66455, Mexico
A R T I C L E I N F O
A B S T R A C T
2
Ti 13/CuO/Cu O materials were prepared by solid-state and impregnation method, using copper oxide as
Keywords:
Na
cocatalyst (CC, 0.1%–5%). The catalytic activity was evaluated for H2 evolution and CO2 reduction. XPS analysis
revealed the presence of Cu O and CuO in different proportions. Na Ti 13 impregnated with 0.1% of cocatalyst
exhibits majoritary the Cu O phase; while Na Ti 13 with 5% of cocatalyst shows mainly CuO. Electrochemical
measurements showed higher photocurrent and lower resistance to charge transference in Na Ti 13-0.1% CC,
associated with better a charge flow. Na Ti 13-0.1% CC exhibited the highest H production (33 μmol g h )
and Na Ti 13-5% CC showed the best CO conversion to CH
major content of Cu O phase favored the H evolution by the formation of a Z-scheme, where the strong negative
character of the CB of Cu O enhances the kinetics of H O reduction, while a higher content of CuO improved CO
adsorption and reduction.
2
6
O
Na
CuO/Cu
Photocatalytic hydrogen production
CO photoreduction
Tunnel structure
2 6 13
Ti O
2
O
2
2
6
O
2
2
6
O
2
2
6
O
-1
-1
2
6
O
2
-
1
-1
-1 -1
2
6
O
2
2
O (25 μmol g h ) and CH OH (4.6 μmol g h ). A
3
2
2
2
2
2
1
. Introduction
to its excellent stability and suitable potential of the conduction band.
However, the photoactivity of TiO presents different drawbacks due to
it only absorbs a small fraction of the solar spectrum [17–22]. Related
materials such as alkali-metal titanates with general formula
Ti 2n+1 (n = 6, 7, 8) exhibit a structure that consists in a 3D ar-
rangement of TiO octahedra connected by corners and edges forming a
zig-zag structure with rectangular tunnels where alkali metals are lo-
cated [23]. Due to their physicochemical properties; these materials
have been used for several practical applications as reinforcing ad-
ditives [24], humidity sensors [25], and lithium ion batteries [26].
2
Finite fossil fuel, global warming and the exponential increase of the
world population have encouraged the development of new technolo-
gies to take advantage of renewable resources. Sunlight reaching the
earth surface is unlimited renewable energy, and for this reason, the
scientific community is focused on converting incoming photons in
chemical energy [1–3]. The water splitting reaction to produce hy-
drogen and reduce CO
thanol has been considered as a promising approach to address the
problems of energy crisis [4–7]. However, CO and H O are extremely
a
A
2
n
O
6
2
into fuels as formaldehyde, methane or me-
2
2
2 6 2 6 4 9
Also, Na Ti O13, K Ti O13 and BaTi O have been studied for different
stable molecules, which imply that energy input is needed for their
transformation [8–11].
photocatalytic applications such as hydrogen production [27–32], de-
gradation of dyes [33], chlorophenols [34,35] and volatile organic
compounds [36]. Their unique structure has several advantages which
allow an improved unidirectional flow of electrons and transport of
charges to the material surface. In addition, the presence of long axis
promotes the formation of one dimensional (1D) microstructure such as
tubes [37], belts [38,39], wires [40] and whiskers [41]; which makes
them suitable for being modified with different co-catalysts [42–44].
Heterogeneous photocatalysis could be a promising solution; when
a semiconductor is illuminated with energy equal or higher than the
energy of band gap; electron and holes are generated, providing re-
ductive and oxidative sites for the interest reactions [12–14]. The
overall efficiency of photocatalytic reactions relays mainly on the
process occurring in the semiconductor, which is influenced by its
structural, surface and electronic properties [15,16]. Semiconductor
oxides have the benefit of optimizing their properties through suitable
design of electronic band structure to achieve the required potential for
the reaction of interest.
Na
including RuO
hydrogen evolution reaction; however, there are only a few reports for
CO photo-reduction. Recently, Yoshida et al. prepared Na Ti 13 with
1% of Ag nanoparticles through solid-state reaction followed by a
2
Ti
6
O13 compound has been modified with different co-catalysts
2
[45] and CuO [46] to improve the performance for
2
2
6
O
In particular, n-type anatase TiO
2
has been extensively studied due
⁎
Corresponding author.
E-mail address: leticia.torresgr@uanl.edu.mx (L.M. Torres-Martínez).
https://doi.org/10.1016/j.materresbull.2019.110679
Received 10 August 2019; Received in revised form 27 October 2019; Accepted 27 October 2019
0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Luz I. Ibarra-Rodríguez, Ali M. Huerta-Flores and Leticia M. Torres-Martínez, Materials Research Bulletin,
https://doi.org/10.1016/j.materresbull.2019.110679

L.I. Ibarra-Rodríguez, et al.
Materials Research Bulletin xxx (xxxx) xxxx
-
1
photo-deposition method, obtaining a CO production of 4.6 μmol h
excitation wavelength of 300 nm. X-ray photoelectron spectroscopy
(XPS) with a monochromated Al Kα (1486.7 eV) and an X-ray source
with a 0.20 eV line width in a dedicated analysis chamber at a base
pressure of < 4.3 × 10−10 mbar. The photoelectrons were separated
with a semi-hemispherical analyzer with a pass energy of 20 eV
(Thermo scientific, Escalab 250 xi, Al anode, 1486.68 eV).
[
47]. The reduction potential of CO in NHE scale is -0.51 V, for-
maldehyde and methanol reduction potentials are -0.48 and -0.38 V
respectively [48–50]; therefore it is possible to obtain these products
using sodium hexatitanates with a suitable co-catalyst. Cu O and CuO
2
materials have gained great interest in the improvement of photo-re-
duction reactions due to the p-type nature and small band gap
(
1.3–2.0 eV) [51,52]. In addition, copper oxides are inexpensive and
2.3. Electrochemical tests
naturally abundant on earth [53–56]. Also, several authors had re-
ported an efficiency improvement when they used a mixture of phases
CuO/Cu O. The coupling of two materials with different band gap
2
The electrochemical characterization was performed employing a
potentiostat/ galvanostat AUTOLAB PGSTAT302 N. A three-electrode
provides a simultaneous opportunity for higher light absorption and
rapid charge separation [57,58]. In the present work, we prepared
2 6
electrochemical cell was employed for the analysis: Na Ti O13 powders
impregnated with co-catalyst (CC) particles were deposited on fluor-
doped tin oxide (FTO) and then used as working electrode, an Ag/AgCl
(3 M KCl) electrode was used as a reference and a Pt wire as the counter
electrode. To prepare the working electrode, the material powders were
mixed with water and ethanol (6:1 volumetric ratio, respectively) to
form an ink, which was deposited into an FTO surface to form a thin
film. A further annealing treatment at 300 °C was applied before the use
of the electrode.
heterostructures of Na
mance in the hydrogen evolution reaction and photo-reduction of CO
into formaldehyde and methanol production. For the first time, in this
work the effect of the presence of Cu O and CuO in different propor-
tions as co-catalysts for the photocatalytic conversion of H O and CO
2 6 2
Ti O13/CuO/Cu O to evaluate their perfor-
2
2
2
2
to solar fuels is studied, and a comparative study of the physicochem-
ical and electronic properties influencing the photocatalytic activity of
the materials is presented, describing the mechanisms involved in the
reactions.
2.4. Photocatalytic reaction
2. Experimental
2.4.1. Hydrogen evolution
The photocatalytic test for all samples was carried out by measuring
2
2
.1. Preparation of the photocatalysts
the amount of H
perature. The photocatalyst (100 mg) was dispersed in 200 mL of
deionized water. Then, the reactor was kept in the dark, and N was
bubbled through the solution reaction to remove O . Afterwards, the
photocatalytic reaction system was closed and irradiated with Uv/vis
2
produced in a 250 mL Pyrex reactor at room tem-
.1.1. Synthesis of Na
The reagents used in the synthesis of Na
Anatase-Sigma Aldrich) and Na CO anhydrous (99.8%, Sigma
2
Ti
6
O
13 material
2
2
Ti
6
O
13 were TiO
2
(99.9 %,
2
2
3
2
Aldrich). Each reagent was mixed and grounded in an agate mortar in a
stoichiometric ratio; later, they were transferred to a platinum crucible,
and different thermal treatments were applied from room temperature
to 800 °C for 12 h.
2
lamp (254 nm, 4400 μW/cm ). The H produced was analyzed by gas
chromatography in a Thermo Scientific gas chromatograph equipped
with a thermal conductivity detector (TCD) and fused silica capillary
column (30 m x0.53 mm) using nitrogen as the carrier gas. The reaction
products were analyzed at intervals of 30 min over 3 h.
2.1.2. Impregnation of CuO particles
Cupric acetate (98%, Fermont) in different amounts (0.1–5% in
2.4.2. CO
2
reduction
weight) was dissolved in ethanol. The respective mass fraction of the
photocatalyst was added, and the suspension was kept under con-
tinuous stirring for 1 h. After this time, the temperature was kept at
A batch Pyrex reactor of 250 mL for photo-conversion of CO to low-
carbon fuels at room temperature. For a typical run, 100 mg of semi-
conductor are dispersed in 200 mL of deionized water. Then, the reactor
2
7
0 °C until complete evaporation. Finally, the samples were thermally
2
was pressurized at 2 psi with pure CO . After, the system was irradiated
2
treated at 400 °C for 2 h to promote the thermal decomposition of the
precursors and to obtain the metal oxides.
under UV/vis lamp (254 nm, 4400 μW/cm ). For formaldehyde quan-
tification, it was used a spectrophotometric method assisted by micro-
wave oven reported by Andrea C. Gigante et al. in 2004 [59]. In the
case of methanol measurements, it was also used as a spectro-
photometric method using sodium nitroprusside introduced by Yan-Yan
Zhan et al. in 2010 [60].
2.2. Characterization
X-ray diffraction patterns were obtained using
a PANalytical
Empyrean device operating at 45 kV and 40 mA with Cu Ka radiation (λ
1.5406 Å), 2θ values were recorded from 10° to 70° with a step size of
.013 and dwell time of 448.5 s per step. The crystallite size of the
=
0
3. Results and discussion
samples was calculated by the Scherrer equation: L = kλ/βcos (θ),
where L is the crystallite size, k is the Scherrer constant (0.89), λ is the
wavelength of the X-ray radiation (0.15418 nm for Cu Ka), β is the full
width at half maximum (FWHM) of the diffraction peak at 2 theta, and
θ is the diffraction angle. The 5 principal peaks were evaluated to ob-
tain the crystallite size parameter, and an average was obtained. The
morphology and the semi-quantitative elemental analysis of each
sample were determined by scanning electron microscope (SEM-JEOL,
3.1. X-ray diffraction
2 6
Fig. S1 shows the diffraction pattern for Na Ti O13 pristine pow-
ders. The compound crystalized in a monoclinic phase (JCPDS 01-077-
9461). It is possible to observe well defined reflections with a high
intensity related to good crystallinity. This material was impregnated
with an ethanol solution that contains different amounts of cupric
acetate and calcined in air to obtain CuO particles as co-catalyst. Fig. 1
6490 L V) operating in the secondary electron mode coupled with an X-
2 6
shows the XRD-patterns of Na Ti O13 with different proportions of co-
Ray energy dispersive spectroscopy analysis (EDS). The studies of
UV–vis diffuse reflectance for all samples were carried out in a NIR
spectrophotometer (Cary 5000). BET (Brunauer-Emmet-Teller) surfaces
catalyst (CC). It is possible to observe that materials with small amounts
of co-catalyst did not present reflections of copper oxides. In contrast,
powders impregnated with 2 and 5% a slightly change in the main
areas were measured by N
2
Physisorption using a Belsorp II mini (Bel
2 6 13
reflections of hexatitanate powders is observed (Fig. 2a). Na Ti O
Japan), the samples were outgassed at 300 °C for 3 h before the ana-
lysis. The optical emission of the materials was studied in a fluorescence
spectrophotometer (Agilent Cary Eclipse) at room temperature with an
with 2 and 5 % of CC exhibits wide and low-intensity reflections that
could be related to the presence of co-catalyst particles. Several authors
have found changes in the main reflections of tunnel-structured
2

L.I. Ibarra-Rodríguez, et al.
Materials Research Bulletin xxx (xxxx) xxxx
Table 1
2 6 2
Physicochemical properties of Na Ti O13/CuO/Cu O materials.
2
Materials
Weight (%) Band gap (eV) Crystallite size (nm) BET (m /g)
0
0
1
2
5
.1
.5
3.61
3.59
3.59
3.58
3.58
32
38
30
25
29
< 20
2 6
Na Ti O13-CC
materials when they are modified with different compounds. Changes
such as lower intensity, displacement, asymmetrical peaks, among
others [61,62], are reported due to distortions in the crystalline struc-
ture or microstructure. Also, from Fig. 2b it is possible to appreciate
that reflections of CuO monoclinic phase at 2θ = 35.4° and 2θ = 38.8°
are present in Na Ti O13-5 % CC as wide and amorphous signals, as-
2 6
sociated with the presence of co-catalyst as nanoparticles.
In Table 1 are shown the crystallite sizes and surface area values for
each sample obtained after the impregnation process. Samples of
Na Ti O13 with 2 and 5 % of CC present slightly smaller crystallite sizes
2 6
than the rest of the samples. These results could be related to the
changes of main reflections observed in the XRD patterns due to the
presence of co-catalyst particles as it was previously discussed. On the
other hand, surface proprieties did not show significance changes;
commonly Na
2 6
Ti O13 powders calcined at high temperature possess
small surface area, and it is expected that it does not change easily [63].
3.2. Scanning electronic microscopy
Fig. 1. XRD patterns of Na
centages co-catalyst.
2
6
Ti O
13 powders impregnated with different per-
Fig. 3 shows the micrographs of the prepared samples; they ex-
hibited characteristic belt morphology with sizes of around 1 μm. This
is a common feature for Na Ti 13 material because it exhibits a tunnel
2
6
O
structure. However, it is possible to observe large particles with a rec-
tangular shape. This fact is related to the synthesis method that used
2 6
Fig. 2. XRD patterns of Na Ti O13 powders impregnated with co-catalyst particles: a) Zoom at 11°-15°, and b) 32°-40°.
3

L.I. Ibarra-Rodríguez, et al.
Materials Research Bulletin xxx (xxxx) xxxx
Fig. 3. SEM images of Na
2
6
Ti O
13 powders impregnated with co-catalyst. a) Bare Na
Ti
2 6
O
3
and modified with b) 0.1% CC, c) 0.5% CC, d) 1% CC, e) 2% CC, f) 5% CC.
Table 2
Quantitative analysis of the samples by EDS.
Particles deposited Theoretical Weight (%)
Weight (%) determinated by EDS
0
0
1
2
5
.1
.5
0.1
0.4
0.9
1.6
3.4
2 6
Na Ti O13-CC
longer times of synthesis, allowing a major grain growth. It is hard to
appreciate particles of co-catalyst in micrographs; therefore, we carried
out EDS analysis to confirm the presence of the particles on the mate-
rial. Table 2 shows the quantitative results of the chemical analysis by
EDS. It is possible to observe that the values are slightly different from
theoretical values due to mass losses during the impregnation method.
To observe the distribution of co-catalyst particles, an elemental
mapping analysis was carried out for samples with a higher percentage
of co-catalyst (Fig. 4). It is possible to observe that the co-catalyst
Fig. 5. XPS Cu 2p spectra of powders Na Ti O modified with co-catalyst.
2
6 13
2 6
particles appear on the edges of Na Ti O13 belts. It is well known that
edges possess higher energy, which is identified as active sites where
the reaction of interest takes place. Therefore, it is possible that parti-
3.3. XPS analysis
cles of the co-catalyst decorating the edges of Na
crease the photocatalytic activity of the material [64].
2 6
Ti O13 help to in-
To obtain chemical information of the metal oxide nanoparticles,
XPS analyses were carried out. Fig. 5 shows the XPS Cu 2p spectra for
Na Ti 13 powders modified with co-catalyst particles. It can be seen
2
6
O
2 6
Fig. 4. a) SEM and b) elemental mapping of Na Ti O13 powders impregnated with 5% CC.
4

L.I. Ibarra-Rodríguez, et al.
Materials Research Bulletin xxx (xxxx) xxxx
2 6
Fig. 6. XPS Cu 2p3/2 spectra of powders Na Ti O13 modified with co-catalyst. a) 0.1% CC, b) 0.5% CC, c) 1% CC, d) 2% CC, e) 5% CC.
that Cu 2p energy level for all samples presents the characteristic
doublet peaks correspoding to Cu2p1/2 and Cu2p3/2 at ∼952.3 eV and
(OeC]O and CeOH) originated from the copper acetate used as the
precursor of copper oxide [70].
∼
932.7 eV [65]. Also, it is observed shake up satellite structures at
It is possible to appreciate changes in relative areas and satellites
higher binding energies (8 eV) than the main peak Cu 2p. It should be
noted that when increased the amount of co-catalyst added to the
tunnel compound, the satellites are more intense, which could indicate
a major percentage of CuO phase [66]. In order to identify the species
structures that suggest a difference in the amount of CuO and Cu
2
O
phases. To quantify the relative concentration of Cu O and CuO present
2
in the samples, it was used the direct photoemission areas in the fol-
lowing equation [66]:
present over Na
all samples (Fig. 6). Two signals were observed in all materials, first one
between 932.4–932.8 eV associated with the Cu O phase [20,67] and a
second peak at 933.9–934.9 eV related to CuO compound [18,68,69].
From C1s spectra (Fig. S2), it should be noted signals of residual carbon
2 6
Ti O13 surface, Cu2p3/2 signals were decomposed for
A
CuO + Asatellites
+
2
%Cu
=
=
A
CuO + ACu2O + Asatellites
(1)
(2)
2
ACu2O
+
1
%Cu
ACuO + ACu2O + Asatellites
5

L.I. Ibarra-Rodríguez, et al.
Materials Research Bulletin xxx (xxxx) xxxx
Table 3
Quantitative analysis of copper species by XPS.
Samples
Binding Energy CuO/Cu
2
O (eV)
FWHM CuO/Cu
2
O(eV)
2
Relative concentration/CuO/Cu O (%)
Na
2
Na
2
Na
2
Na
2
Na
2
Ti
6
Ti
6
Ti
6
Ti
6
Ti
6
O13-0.1%CC
O13-0.5%CC
O13-1%CC
O13-2%CC
O13-5%CC
934.9/932.6
934.7/932.6
933.9/932.5
934.4/932.8
934.5/932.4
1.5/2.3
3.4/2.8
3.5/2.8
3.6/2.5
3.6/3.3
6/94
47/53
48/52
54/46
60/40
Curve fitting parameters and the calculated relative concentration are
presented in Table 3. It is possible to observe that Na Ti 13 powders
impregnated with a small proportion of co-catalyst present in major
perceantage the Cu O phase; meanwhile, Na Ti 13 samples with a
2
6
O
2
2
6
O
major proportion of co-catalyst exhibit CuO in major percentage. This
behavior could be associated with an improvement in the oxidation
process due to a major amount of acetate precursor on the exposed
surface. It is possible to observe that in SEM micrographs particles of
co-catalyst, appears in edges of belts of Na
particles of acetate precursor could be hidered by large particles of
Na Ti 13 powders, causing a parcial oxidation process.
2 6
Ti O13 particles. The smaller
2
6
O
3.4. Optical properties (UV–vis and photoluminescence spectroscopy)
The diffuse reflectance spectra of Na Ti 13 modified with co-cat-
2
6
O
alyst nanoparticles are presented in Fig. 7a. To determine the band gap
values of these compounds, (F(R)*hv) versus hv were plotted, where F
2
Fig. 8. - Photoluminescence spectra of Na
Cu O/CuO co-catalyst.
2 6
Ti O13 powders impregnated with
(
R) is the Kubelka-Munk function and hv is the incident photon energy.
2
The band gap (Eg) of the materials was determined by the extrapolation
method (Fig. 7b). In these figures, the absorption range of the materials
is observed at 350 nm, which corresponds to a UV absorption. In all
the presence of both phases (CuO/Cu O) in impregnated samples as it
was previously discussed in Section 3.3.
2
samples, Eg values become slightly lower when Na
2
Ti
6
O
13 is modified
The absorption band near to 600 nm appears when Cu O is loaded
x
with co-catalyst nanoparticles associated with a better visible light
absorption capacity. Transition metal oxides promoted an absorption in
the visible range due to the nature of their electronic configuration and
their characteristic d→ d transitions.
into Na Ti O13, and it is attributed to the interband transition 2 Eg →
2
6
2
6 13
2T2g in the Cu(II) nanoparticles deposited in the surface of Na Ti O
[11].
To study the efficiency of separation of charges and the effect of co-
From UV–vis spectra (Fig. 7a), the band gap values were calculated
catalyst particles deposited over the surface of Na Ti O material,
2
6 13
(
Table 1). It should be noted that Na
absorption band around 380 nm. The appearance of this shoulder is
associated with the presence of traces of remaining TiO which possess
2
Ti
6
O
13 pristine present small
photoluminescence analysis at 254 nm wavelength of excitation was
carried out. From Fig. 8, it is possible to observe that the emission is
2
lower when the compound is modified with CuO/Cu O particles. This
2
different absorption region edges [71,72]. In the case of the rest of the
samples, it is possible to appreciate a displacement of this absorption
band. Close to this region, around 370 nm, is commonly reported the
implies that co-catalyst particles are promoting a better charge se-
paration and transport in the interface of titanates and oxide copper
particles.
transitions of Cu-O-Cu in Cu
could be caused by a mix of both signals (TiO
hand, a broad absorption in the region of 600−800 nm is associated
2
O species. The displacement observed
It is important to mention that the edges of belts-microstructure of
Na Ti O possess higher energy due to truncated bonds which favor
the deposition of fine co-catalyst particles [38,53]. The differences in
energy in the different edges of the materials promotes an easier
2
and Cu O). On the other
2
2
6 13
+
2
with electron d-d transitions of Cu
in distorted octahedral sur-
roundings by oxygen in CuO particles [73–75]. These results confirm
2 6
Fig. 7. a) Uv–vis and b) Tauc’s plot of Na Ti O13 powders impregnated with co-catalyst.
6

L.I. Ibarra-Rodríguez, et al.
Materials Research Bulletin xxx (xxxx) xxxx
2 6 x
Fig. 9. Mechanism of the enhanced photocatalytic activity over Na Ti O13/Cu O.
separation of the photogenerated charges, and the performing of the
oxidation and reduction photocatalytic reaction in separated places,
which reduces the recombination process and increases the conversion
efficiency.
illumination was turned on. This behavior is characteristic of n-type
semiconductors, and it is related to the negative charge accumulation in
the conduction band [76].
Fig. 10b shows the photocurrent measurements of the interest ma-
terials; it is possible to observe that the sample with 0.1% of CC par-
ticles showed a better response when the illumination was turned on.
The materials showed an efficient photocatalytic response since they
generated and eliminated the photocurrent in a very short time. How-
ever, they present a current fluctuation during illumination time; only
Moreover, the good interdispersion of n-type Na
2 6
Ti O13 and p-type
Cu
x
O favors an adequate charge transfer in the interface of the semi-
conductors, reducing also the recombination and promoting a more
efficient use of the photogenerated charges. This mechanism is illu-
strated in Fig. 9.
2 6
Na Ti O13-0.1% CC shows stability. To determinate the charge transfer
resistance of the studied materials, the Nyquist plots were analyzed.
These plots are shown in Fig. 10c, where it is possible to observe the
characteristic capacitive semicircle for each material. The powders with
3
.5. Electrochemical characterization
The electrochemical response of the materials by the effect of UV
0.1% CC show the minor charge transfer resistance. These results are in
light illumination was evaluated by measuring the OCP as a function of
time. Fig. 10a shows the diagrams of the OCP analysis for the study of
the material in this work. It is possible to observe that all the powders
shifted their initial potential value to more negative ranges when the
agreement with current observed in the cronoamperometry analysis.
The addition of co-catalyst allowed the formation of an internal field
between Na Ti O13 and CuO/Cu O due to the different nature of the
2 6 2
Fig. 10. – Electrochemical characterization of Na
2
Ti
6
O13-% CC a) OCP, b) Photocurrent measurements, and c) Nyquist plots.
7

L.I. Ibarra-Rodríguez, et al.
Materials Research Bulletin xxx (xxxx) xxxx
were constructed in agreement with the conduction and valence band
potentials reported in the literature [17,56,80,81]. The obtained pho-
tocatalysts exhibit a typical tunnel structure, which promotes an en-
hanced charge separation and transport through the one-dimensional
structure due to the internal electric field and electron confinement
favored in this type of structure. Moreover, the improved charge se-
2 6 2
paration in the interface of the heterostructure Na Ti O13/CuO/Cu O is
attributed to the appropriate band alignment that allows an efficient
electron and holes migration and reduces the recombination, promoting
a higher photocatalytic efficiency.
After the photocatalyst irradiation, the generation of the electron-
2
hole pairs occurs. Considering that Cu O and CuO exhibit conduction
band potentials more negative than Na
cocatalyst are transported towards Na
2
Ti
Ti
6
O
O
3
, the electrons in the Cu
x
O
2
6
3
to perform the reduction
of CO
and carry out the oxidation of H
As it was discussed, the samples exhibited different behavior for H
production and CO reduction. The sample with the highest content of
Cu O (Na Ti 13 -0.1% CC) showed the highest hydrogen production.
As in this sample, the cocatalyst is mainly composed by Cu O (94%)
and a negiglible amount of CuO (6%), we propose the formation of a Z-
scheme between Cu O and Na Ti 13 (Fig. 12a), where electrons mi-
grated from sodium hexatitanate to Cu O to perform the photo-reduc-
tion reaction of H+/H
. Meanwhile, in Na Ti 13, the photo-oxidation
2
and H
2
O. Meanwhile, the holes move in the contrary direction
2
O.
2 2 6 13
Fig. 11. Summary of the products of photo-reductionof CO over Na Ti O -
2
CuO/Cu
2
O materials.
2
2
2
6
O
2
band gap. These characteristic improve the charge transferend over
surface increasing the probabilities to interact with the medium.
2
2
6
O
2
3.6. Photocatalytic tests for solar fuels production
2
2
6
O
process of water is carried out. In this way, the reactions took place in
separated spaces improving the inner electrical field and reducing the
recombination process.
In Fig. 11, it is possible to observe the formaldehyde and methanol
production for the materials studied in this work. Na
2 6
Ti O13 pristine
powders and samples with 0.1%, 0.5%, and 1% CC present low pro-
duction (< 1 μmolg ). Meanwhile, samples with 2% CC and 5% CC
show the significant formation of formaldehyde and methanol.
Regarding the CO
2
photoreduction, the samples with a higher
-
1
content of CuO (2% CC and 5% CC) showed the highest formation of
formaldehyde and methanol, which is adjudicated to the better ad-
sorption capacity of CuO, in addition to the enhanced charge transfer
promoted by the formation of a p-n heteroestructure between CuO/
2 6
Na Ti O13-5%CC exhibits the highest formaldehyde production
-
1
-1
-1 -1
(
25 μmolg h ) and methanol production (4.6 μmolg h ).
Even though that all samples present a mixture of the phases CuO/
Cu
tion band of CuO/Cu
tunnel material. In this way, the conduction band potential of
Na Ti 13 compound is suitable positionated for the reduction of CO
2
2
O and Na
2
Ti
6
O
3
materials, where electrons flow from the conduc-
Cu
2
O, the sample with 5 % CC shows a major perceantage of CuO phase
2
O phases to the conduction band of pristine
compared with the rest of materials. These results could be associated
with the presence of CuO phase. Several athours have modified diferent
2
6
O
photocatalyts with CuO improving the yield of several CO
duction products [77]. Wan Nor Rosman et al. reported that CuO is
much better CO adsorbent compared with Cu O in terms of propierties
and adsorption ability. Also, the high electronegativity of CuO com-
pared with Cu O promotes the formation of CuOeOeC]O bonds on
the metal surface, which increases the reactivity and conversion of CO
78]. Moreover, André E. Nogueira et al. discussed the role of CuO in
photoreduction reactions, suggesting that the main step to produce
ligth hydrocarbons is related to the formation of CuCO .Cu(OH)
which then facilitated photo-reduction reactions [79]. According to
this, the presence of CuO phase in the samples is a determinant factor in
2
the photoreduction of CO . For this reason, the samples with a higher
percentage of CuO (2% CC and 5% CC) exhibited higher formaldehyde
and methanol productions.
Additionally, the photocatalytic activity of the samples was tested
towards the water conversion reaction to produce hydrogen under UV-
2
photo-re-
to formaldehyde en methanol. As the formadehyde production requires
a lower amount of electrons, this product is thermodinamically favored,
and its production is higher compared to methanol [77]. Also, the po-
tential required for formaldehyde (0.48 V vs NHE) is lower compared to
the required for methanol (−0.38 V vs NHE), due to this potencial di-
ference, the formaldehyde formation occurs first, and if additional
electrons are available, they are used for the formation of methanol
[82].
2
2
2
2
[
3
2
,
These results highlighit the fact that CO
several factors that influence the efficiency of the photo-reduction
process, such as the CO molecule affinity over the photocatalyst sur-
face, which is an important step to initiate the photocatalytic reduction
of CO molecule.
Table 4 shows a summary of the activities for CO
evolution over the materials prepared in this work, compared to related
materials. Na Ti 13 Na Ti 13/CuO/Cu O powders show comparable
hydrogen evolution rates, while in the case of CO photoreduction
products, the materials prepared in this work show higher activites
compared to K Ti 13 and extensively studied TiO . These results
2
photoconversion involves
2
2
2
reduction and H
2
2
6
O
2
6
O
2
light (Fig. 12). In this case, the sample Na
2
Ti
6
O13-0.1% CC showed the
2
highest hydrogen evolution. This sample showed the lowest charge
transfer resistance and tne highest photocurrent in the electrochemical
tests. The enhanced charge transport, compared to bare hexatitanate, is
2
6
O
2
highlight the use of copper oxides as a promising alternative to enhance
the photocatalytic production of solar based fuels. Particularly, our
promoted by the formation of a heterostructure between Na
2 6 13
Ti O -
-
1
-1
CuO/Cu O, which reduces the recombination process and improves the
2
materials exhibited superior formaldehyde production (25 μmolg h )
charge flow in the interface of the semiconductors. In all cases, the
addition of the co-catalyst in different percentages increased the hy-
compared to previous reports. Accordingly, the photocatalysts prepared
2
in this work are promising materials for CO photoconversion process.
drogen production, and the sample with a higher content of Cu
2
O (0.1
%
CC) showed the highest yield. This could be attributed to the more
4. Conclusions
negative character of the conduction band of Cu
2
O, accelerating the
+
kinetics of the reduction reaction of H to H
2
.
Herein, the performance of Na
evaluated for hydrogen evolution and CO
observed that formaldehyde and methanol products where favored
2
Ti
6
O
13/CuO/Cu
2
O materials was
Fig. 13 shows a suggested mechanism for the photocatalytic con-
version of H O and CO molecules. For this, schematic band diagrams
2
photo-conversion. It was
2
2
8

L.I. Ibarra-Rodríguez, et al.
Materials Research Bulletin xxx (xxxx) xxxx
-1
-1
2 6 2
Fig. 12. a) Hydrogen production over Na Ti O13 powders impregnated with co-catalyst versus time, b) Summary of the H production in μmol g h .
when CuO phase increased in the samples prepared. This phase ex-
hibited a better CO adsorption capacity and allowed the formation of a
p-n heterostructure.Therefore, the sample with the highest CuO con-
tent, Na Ti 13- 5% CC, showed the highest production of for-
maldehyde (25 μmol g h ) and methanol (4.6 μmol g h ). In the CO
photo-reduction reaction, the affinity of CuO to CO molecule enhanced
, promoting a higher pro-
revising it critically for important intellectual content; and (c) approval
of the final version.
2
2
6
O
-
1
-1
-1 -1
Declaration of Competing Interest
2
2
The authors confirm that:
the adsorption and photoconversion of CO
duction of methanol and formaldehyde.
2
All authors have participated in (a) conception and design, or
analysis and interpretation of the data; (b) drafting the article or re-
vising it critically for important intellectual content; and (c) approval of
the final version.
This manuscript has not been submitted to, nor is under review at,
another journal or other publishing venue.
The authors have no affiliation with any organization with a direct
or indirect financial interest in the subject matter discussed in the
manuscript
On the other, in the hydrogen evolution reaction, the sample with
the highest content of CuO, Na Ti 13-0.1% CC, showed the major H
production (33 μmol g h ), which corresponds to more than 3.3 times
the activity of Na Ti 13 pristine powders. These results were asso-
ciated with the favorable negative character of the Cu O conduction
2
6
O
2
-
1
-1
2
6
O
2
band and to the better charge transport in the material surface due to
the construction of Z-scheme, where photo-reduction and photo-oxi-
dation processes take place in separated places and reduce the re-
combination process.
Even thought is hard to compare products of CO
due to the diversity of methods and reaction conditions, Na
CC prepared in this work exhibit a promising activity for HCOOH and
CH OH production compared to similar photocatalysts modified noble
metals and other cocatalysts. Also, the use of copper oxide based ma-
terials developed herein allow the obtention of higher efficiencies
through the incorporation of low-cost and abundant cocatalysts.
All authors have participated in (a) conception and design, or
analysis and interpretation of the data; (b) drafting the article or
2
photo-reduction
Acknowledgments
2
Ti 13-5%
6
O
The authors wish to thank CONACYT for financial support for this
research through the following projects: CONACYT-CB-2014-237049,
CONACYT-FC-1725, CONACYT-PDCPN-2015-487, CONACYT-NRF-
3
2016-278729, PAICYT IT1021-19, PAIFIC/2018-5, UANL-CA-244
PROFIDES Desarrollo de materiales ambientales ID 63185. Luz I. Ibarra
Rodriguez acknowledges CONACYT for the Ph.D. scholarship. FIC-
UANL supported this research.
2 2 2 6 2
Fig. 13. – Mechanism of the photocatalytic conversion of H O and CO over Na Ti O13/CuO/Cu O heterostructured materials. a) Z-scheme, b) p-n heteroestructure.
9

L.I. Ibarra-Rodríguez, et al.
Materials Research Bulletin xxx (xxxx) xxxx
Table 4
2
Summary of related materials reported activities for hydrogen evolution and CO photo-conversion.
Main products (μmol g h-
-
1
Reference
Photocatalyst
Irradiation source
Experimental conditions
1
)
K
Pt-K
2
Ti
6
O
Ti
13
UV lamp 254 W
150 W Hg-lamp
Catalyst 0.1 g, water DI 200 mL, 2 Psi CO
Catalyst 0.15 g Pt-K Ti 13 and 0.15 Cu/ZnO, 77 kPa CO
mL,
2
HCHO (8.3)
[83]
[84]
2
6
O
13 coupled Cu/ZnO
2
6
O
2
, water
H
2
(17)
4
4
CH (0.27)
HCHO (0.56)
HCOOH (3)
Cu doped-TiO
2
monoliths
200 W Hg-lamp
Monoliths coated with Cu-TiO
water vapor was introduce (4 mL min
Catalyst 1 g, water 300 mL
2
, Ultrapure CO
2
saturated with
H
CH
H
2
(12.55)
[85]
-1
)
3
OH (3.92)
(265)
Na
2
2
Ti
6
O13/2% RuO
2
-Zr
400W high-pressure
mercury lamp
Uv lamp 254 W
2
[45]
Na
Ti
6
O
13
Catalyst 0.1 g, water DI 200 mL
H
2
(9)
Current work
Na
Cu
Na
2
Ti
O
6
O
O
13/0.1% w CuO/
13/5% w CuO/Cu
,HCHO (< 1), CH
(< 1)
3
OH
2
2
Ti
6
2
O
H
2
(33.3)
HCHO (< 1),CH
< 1)
(10)
3
HCHO (25),CH OH (4.6)
,
3
OH
(
H
2
,
Appendix A. Supplementary data
Glycerol:Water Mixtures, J. Phys. Chem. C 114 (2010).
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