Cu2O heterostructures for solar photocatalytic production of low-carbon fuels

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Development of Na2Ti6O13/CuO/Cu2O heterostructures for solar 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 LE I N FO

A B S T R A C T

Keywords: Na2Ti6O13 CuO/Cu2O Photocatalytic hydrogen production CO2photoreduction Tunnel structure

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.

1. Introduction Finite fossil fuel, global warming and the exponential increase of the world population have encouraged the development of new technologies 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 hydrogen and reduce CO2 into fuels as formaldehyde, methane or methanol has been considered as a promising approach to address the problems of energy crisis [4–7]. However, CO2 and H2O are extremely stable molecules, which imply that energy input is needed for their transformation [8–11]. 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 reductive 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. In particular, n-type anatase TiO2 has been extensively studied due ⁎

to its excellent stability and suitable potential of the conduction band. However, the photoactivity of TiO2 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 a general formula A2TinO2n+1 (n = 6, 7, 8) exhibit a structure that consists in a 3D arrangement of TiO6 octahedra connected by corners and edges forming a zig-zag structure with rectangular tunnels where alkali metals are located [23]. Due to their physicochemical properties; these materials have been used for several practical applications as reinforcing additives [24], humidity sensors [25], and lithium ion batteries [26]. Also, Na2Ti6O13, K2Ti6O13 and BaTi4O9 have been studied for different photocatalytic applications such as hydrogen production [27–32], degradation 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]. Na2Ti6O13 compound has been modified with different co-catalysts including RuO2 [45] and CuO [46] to improve the performance for hydrogen evolution reaction; however, there are only a few reports for CO2 photo-reduction. Recently, Yoshida et al. prepared Na2Ti6O13 with 1% of Ag nanoparticles through solid-state reaction followed by a

Corresponding author. E-mail address: [email protected] (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

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photo-deposition method, obtaining a CO production of 4.6 μmol h-1 [47]. The reduction potential of CO in NHE scale is -0.51 V, formaldehyde 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. Cu2O and CuO materials have gained great interest in the improvement of photo-reduction 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 naturally abundant on earth [53–56]. Also, several authors had reported an efficiency improvement when they used a mixture of phases CuO/Cu2O. The coupling of two materials with different band gap provides a simultaneous opportunity for higher light absorption and rapid charge separation [57,58]. In the present work, we prepared heterostructures of Na2Ti6O13/CuO/Cu2O to evaluate their performance in the hydrogen evolution reaction and photo-reduction of CO2 into formaldehyde and methanol production. For the first time, in this work the effect of the presence of Cu2O and CuO in different proportions as co-catalysts for the photocatalytic conversion of H2O and CO2 to solar fuels is studied, and a comparative study of the physicochemical and electronic properties influencing the photocatalytic activity of the materials is presented, describing the mechanisms involved in the reactions.

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). 2.3. Electrochemical tests The electrochemical characterization was performed employing a potentiostat/ galvanostat AUTOLAB PGSTAT302 N. A three-electrode electrochemical cell was employed for the analysis: Na2Ti6O13 powders impregnated with co-catalyst (CC) particles were deposited on fluordoped 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. 2.4. Photocatalytic reaction 2.4.1. Hydrogen evolution The photocatalytic test for all samples was carried out by measuring the amount of H2 produced in a 250 mL Pyrex reactor at room temperature. The photocatalyst (100 mg) was dispersed in 200 mL of deionized water. Then, the reactor was kept in the dark, and N2 was bubbled through the solution reaction to remove O2. Afterwards, the photocatalytic reaction system was closed and irradiated with Uv/vis lamp (254 nm, 4400 μW/cm2). The H2 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. Experimental 2.1. Preparation of the photocatalysts 2.1.1. Synthesis of Na2Ti6O13 material The reagents used in the synthesis of Na2Ti6O13 were TiO2 (99.9 %, Anatase-Sigma Aldrich) and Na2CO3 anhydrous (99.8%, Sigma 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. 2.1.2. Impregnation of CuO particles Cupric acetate (98%, Fermont) in different amounts (0.1–5% in weight) was dissolved in ethanol. The respective mass fraction of the photocatalyst was added, and the suspension was kept under continuous stirring for 1 h. After this time, the temperature was kept at 70 °C until complete evaporation. Finally, the samples were thermally treated at 400 °C for 2 h to promote the thermal decomposition of the precursors and to obtain the metal oxides.

2.4.2. CO2 reduction A batch Pyrex reactor of 250 mL for photo-conversion of CO2 to lowcarbon fuels at room temperature. For a typical run, 100 mg of semiconductor are dispersed in 200 mL of deionized water. Then, the reactor was pressurized at 2 psi with pure CO2. After, the system was irradiated under UV/vis lamp (254 nm, 4400 μW/cm2). For formaldehyde quantification, it was used a spectrophotometric method assisted by microwave oven reported by Andrea C. Gigante et al. in 2004 [59]. In the case of methanol measurements, it was also used as a spectrophotometric 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 0.013 and dwell time of 448.5 s per step. The crystallite size of the 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 obtain 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, 6490 L V) operating in the secondary electron mode coupled with an XRay 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 areas were measured by N2 Physisorption using a Belsorp II mini (Bel Japan), the samples were outgassed at 300 °C for 3 h before the analysis. The optical emission of the materials was studied in a fluorescence spectrophotometer (Agilent Cary Eclipse) at room temperature with an

3. Results and discussion 3.1. X-ray diffraction Fig. S1 shows the diffraction pattern for Na2Ti6O13 pristine powders. The compound crystalized in a monoclinic phase (JCPDS 01-0779461). 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 shows the XRD-patterns of Na2Ti6O13 with different proportions of cocatalyst (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 reflections of hexatitanate powders is observed (Fig. 2a). Na2Ti6O13 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

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Table 1 Physicochemical properties of Na2Ti6O13/CuO/Cu2O materials. Materials

Weight (%)

Band gap (eV)

Crystallite size (nm)

BET (m2/g)

3.61 3.59 3.59 3.58 3.58

32 38 30 25 29

< 20

Na2Ti6O13-CC

0.1 0.5 1 2 5

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 structure 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 Na2Ti6O13-5 % CC as wide and amorphous signals, associated 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 Na2Ti6O13 with 2 and 5 % of CC present slightly smaller crystallite sizes 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 Na2Ti6O13 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. 3 shows the micrographs of the prepared samples; they exhibited characteristic belt morphology with sizes of around 1 μm. This is a common feature for Na2Ti6O13 material because it exhibits a tunnel structure. However, it is possible to observe large particles with a rectangular shape. This fact is related to the synthesis method that used

Fig. 1. XRD patterns of Na2Ti6O13 powders impregnated with different percentages co-catalyst.

Fig. 2. XRD patterns of Na2Ti6O13 powders impregnated with co-catalyst particles: a) Zoom at 11°-15°, and b) 32°-40°. 3

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Fig. 3. SEM images of Na2Ti6O13 powders impregnated with co-catalyst. a) Bare Na2Ti6O3 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

Na2Ti6O13-CC

0.1 0.5 1 2 5

0.1 0.4 0.9 1.6 3.4

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 material. 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 particles appear on the edges of Na2Ti6O13 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 particles of the co-catalyst decorating the edges of Na2Ti6O13 help to increase the photocatalytic activity of the material [64].

Fig. 5. XPS Cu 2p spectra of powders Na2Ti6O13 modified with co-catalyst.

3.3. XPS analysis To obtain chemical information of the metal oxide nanoparticles, XPS analyses were carried out. Fig. 5 shows the XPS Cu 2p spectra for Na2Ti6O13 powders modified with co-catalyst particles. It can be seen

Fig. 4. a) SEM and b) elemental mapping of Na2Ti6O13 powders impregnated with 5% CC. 4

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Fig. 6. XPS Cu 2p3/2 spectra of powders Na2Ti6O13 modified with co-catalyst. a) 0.1% CC, b) 0.5% CC, c) 1% CC, d) 2% CC, e) 5% CC.

(OeC]O and CeOH) originated from the copper acetate used as the precursor of copper oxide [70]. It is possible to appreciate changes in relative areas and satellites structures that suggest a difference in the amount of CuO and Cu2O phases. To quantify the relative concentration of Cu2O and CuO present in the samples, it was used the direct photoemission areas in the following equation [66]:

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 ∼932.7 eV [65]. Also, it is observed shake up satellite structures at 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 present over Na2Ti6O13 surface, Cu2p3/2 signals were decomposed for all samples (Fig. 6). Two signals were observed in all materials, first one between 932.4–932.8 eV associated with the Cu2O 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 5

% Cu+2 =

ACuO + A satellites ACuO + ACu2O + Asatellites

(1)

% Cu+1 =

ACu2O ACuO + ACu2O + Asatellites

(2)

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Table 3 Quantitative analysis of copper species by XPS. Samples

Binding Energy CuO/Cu2O (eV)

FWHM CuO/Cu2O(eV)

Relative concentration/CuO/Cu2O (%)

Na2Ti6O13-0.1%CC Na2Ti6O13-0.5%CC Na2Ti6O13-1%CC Na2Ti6O13-2%CC Na2Ti6O13-5%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 Na2Ti6O13 powders impregnated with a small proportion of co-catalyst present in major perceantage the Cu2O phase; meanwhile, Na2Ti6O13 samples with a 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 Na2Ti6O13 particles. The smaller particles of acetate precursor could be hidered by large particles of Na2Ti6O13 powders, causing a parcial oxidation process. 3.4. Optical properties (UV–vis and photoluminescence spectroscopy) The diffuse reflectance spectra of Na2Ti6O13 modified with co-catalyst nanoparticles are presented in Fig. 7a. To determine the band gap values of these compounds, (F(R)*hv) 2 versus hv were plotted, where F (R) is the Kubelka-Munk function and hv is the incident photon energy. 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 samples, Eg values become slightly lower when Na2Ti6O13 is modified 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. From UV–vis spectra (Fig. 7a), the band gap values were calculated (Table 1). It should be noted that Na2Ti6O13 pristine present small absorption band around 380 nm. The appearance of this shoulder is associated with the presence of traces of remaining TiO2 which possess 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 transitions of Cu-O-Cu in Cu2O species. The displacement observed could be caused by a mix of both signals (TiO2 and Cu2O). On the other hand, a broad absorption in the region of 600−800 nm is associated with electron d-d transitions of Cu+2 in distorted octahedral surroundings by oxygen in CuO particles [73–75]. These results confirm

Fig. 8. - Photoluminescence spectra of Na2Ti6O13 powders impregnated with Cu2O/CuO co-catalyst.

the presence of both phases (CuO/Cu2O) in impregnated samples as it was previously discussed in Section 3.3. The absorption band near to 600 nm appears when CuxO is loaded into Na2Ti6O13, and it is attributed to the interband transition 2 Eg → 2T2g in the Cu(II) nanoparticles deposited in the surface of Na2Ti6O13 [11]. To study the efficiency of separation of charges and the effect of cocatalyst particles deposited over the surface of Na2Ti6O13 material, photoluminescence analysis at 254 nm wavelength of excitation was carried out. From Fig. 8, it is possible to observe that the emission is lower when the compound is modified with CuO/Cu2O particles. This implies that co-catalyst particles are promoting a better charge separation and transport in the interface of titanates and oxide copper particles. It is important to mention that the edges of belts-microstructure of Na2Ti6O13 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

Fig. 7. a) Uv–vis and b) Tauc’s plot of Na2Ti6O13 powders impregnated with co-catalyst. 6

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Fig. 9. Mechanism of the enhanced photocatalytic activity over Na2Ti6O13/CuxO.

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 materials; it is possible to observe that the sample with 0.1% of CC particles 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. However, they present a current fluctuation during illumination time; only Na2Ti6O13-0.1% 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 0.1% CC show the minor charge transfer resistance. These results are in agreement with current observed in the cronoamperometry analysis. The addition of co-catalyst allowed the formation of an internal field between Na2Ti6O13 and CuO/Cu2O due to the different nature of the

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. Moreover, the good interdispersion of n-type Na2Ti6O13 and p-type CuxO favors an adequate charge transfer in the interface of the semiconductors, reducing also the recombination and promoting a more efficient use of the photogenerated charges. This mechanism is illustrated in Fig. 9. 3.5. Electrochemical characterization The electrochemical response of the materials by the effect of UV 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

Fig. 10. – Electrochemical characterization of Na2Ti6O13-% CC a) OCP, b) Photocurrent measurements, and c) Nyquist plots. 7

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were constructed in agreement with the conduction and valence band potentials reported in the literature [17,56,80,81]. The obtained photocatalysts exhibit a typical tunnel structure, which promotes an enhanced 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 separation in the interface of the heterostructure Na2Ti6O13/CuO/Cu2O 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 electronhole pairs occurs. Considering that Cu2O and CuO exhibit conduction band potentials more negative than Na2Ti6O3, the electrons in the CuxO cocatalyst are transported towards Na2Ti6O3 to perform the reduction of CO2 and H2O. Meanwhile, the holes move in the contrary direction and carry out the oxidation of H2O. As it was discussed, the samples exhibited different behavior for H2 production and CO2 reduction. The sample with the highest content of Cu2O (Na2Ti6O13 -0.1% CC) showed the highest hydrogen production. As in this sample, the cocatalyst is mainly composed by Cu2O (94%) and a negiglible amount of CuO (6%), we propose the formation of a Zscheme between Cu2O and Na2Ti6O13 (Fig. 12a), where electrons migrated from sodium hexatitanate to Cu2O to perform the photo-reduction reaction of H+/H2. Meanwhile, in Na2Ti6O13, the photo-oxidation 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. Regarding the CO2 photoreduction, the samples with a higher content of CuO (2% CC and 5% CC) showed the highest formation of formaldehyde and methanol, which is adjudicated to the better adsorption capacity of CuO, in addition to the enhanced charge transfer promoted by the formation of a p-n heteroestructure between CuO/ Cu2O and Na2Ti6O3 materials, where electrons flow from the conduction band of CuO/Cu2O phases to the conduction band of pristine tunnel material. In this way, the conduction band potential of Na2Ti6O13 compound is suitable positionated for the reduction of CO2 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 potential 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 diference, the formaldehyde formation occurs first, and if additional electrons are available, they are used for the formation of methanol [82]. These results highlighit the fact that CO2 photoconversion involves several factors that influence the efficiency of the photo-reduction process, such as the CO2 molecule affinity over the photocatalyst surface, which is an important step to initiate the photocatalytic reduction of CO2 molecule. Table 4 shows a summary of the activities for CO2 reduction and H2 evolution over the materials prepared in this work, compared to related materials. Na2Ti6O13 Na2Ti6O13/CuO/Cu2O powders show comparable hydrogen evolution rates, while in the case of CO2 photoreduction products, the materials prepared in this work show higher activites compared to K2Ti6O13 and extensively studied TiO2. These results highlight the use of copper oxides as a promising alternative to enhance the photocatalytic production of solar based fuels. Particularly, our materials exhibited superior formaldehyde production (25 μmolg-1 h-1) compared to previous reports. Accordingly, the photocatalysts prepared in this work are promising materials for CO2 photoconversion process.

Fig. 11. Summary of the products of photo-reductionof CO2 over Na2Ti6O13CuO/Cu2O materials.

band gap. These characteristic improve the charge transferend over surface increasing the probabilities to interact with the medium. 3.6. Photocatalytic tests for solar fuels production In Fig. 11, it is possible to observe the formaldehyde and methanol production for the materials studied in this work. Na2Ti6O13 pristine powders and samples with 0.1%, 0.5%, and 1% CC present low production (< 1 μmolg-1). Meanwhile, samples with 2% CC and 5% CC show the significant formation of formaldehyde and methanol. Na2Ti6O13-5%CC exhibits the highest formaldehyde production (25 μmolg-1 h-1) and methanol production (4.6 μmolg-1 h-1). Even though that all samples present a mixture of the phases CuO/ Cu2O, the sample with 5 % CC shows a major perceantage of CuO phase compared with the rest of materials. These results could be associated with the presence of CuO phase. Several athours have modified diferent photocatalyts with CuO improving the yield of several CO2 photo-reduction products [77]. Wan Nor Rosman et al. reported that CuO is much better CO2 adsorbent compared with Cu2O in terms of propierties and adsorption ability. Also, the high electronegativity of CuO compared with Cu2O promotes the formation of CuOeOeC]O bonds on the metal surface, which increases the reactivity and conversion of CO2 [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 CuCO3.Cu(OH)2, which then facilitated photo-reduction reactions [79]. According to this, the presence of CuO phase in the samples is a determinant factor in the photoreduction of CO2. 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 UVlight (Fig. 12). In this case, the sample Na2Ti6O13-0.1% CC showed the 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 promoted by the formation of a heterostructure between Na2Ti6O13CuO/Cu2O, which reduces the recombination process and improves the charge flow in the interface of the semiconductors. In all cases, the addition of the co-catalyst in different percentages increased the hydrogen production, and the sample with a higher content of Cu2O (0.1 % CC) showed the highest yield. This could be attributed to the more negative character of the conduction band of Cu2O, accelerating the kinetics of the reduction reaction of H+ to H2. Fig. 13 shows a suggested mechanism for the photocatalytic conversion of H2O and CO2 molecules. For this, schematic band diagrams

4. Conclusions Herein, the performance of Na2Ti6O13/CuO/Cu2O materials was evaluated for hydrogen evolution and CO2 photo-conversion. It was observed that formaldehyde and methanol products where favored 8

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Fig. 12. a) Hydrogen production over Na2Ti6O13 powders impregnated with co-catalyst versus time, b) Summary of the H2 production in μmol g-1 h-1.

revising it critically for important intellectual content; and (c) approval of the final version.

when CuO phase increased in the samples prepared. This phase exhibited a better CO2 adsorption capacity and allowed the formation of a p-n heterostructure.Therefore, the sample with the highest CuO content, Na2Ti6O13- 5% CC, showed the highest production of formaldehyde (25 μmol g-1 h-1) and methanol (4.6 μmol g-1 h-1). In the CO2 photo-reduction reaction, the affinity of CuO to CO2 molecule enhanced the adsorption and photoconversion of CO2, promoting a higher production of methanol and formaldehyde. On the other, in the hydrogen evolution reaction, the sample with the highest content of CuO, Na2Ti6O13-0.1% CC, showed the major H2 production (33 μmol g-1 h-1), which corresponds to more than 3.3 times the activity of Na2Ti6O13 pristine powders. These results were associated with the favorable negative character of the Cu2O conduction band and to the better charge transport in the material surface due to the construction of Z-scheme, where photo-reduction and photo-oxidation processes take place in separated places and reduce the recombination process. Even thought is hard to compare products of CO2 photo-reduction due to the diversity of methods and reaction conditions, Na2Ti6O13-5% CC prepared in this work exhibit a promising activity for HCOOH and CH3OH production compared to similar photocatalysts modified noble metals and other cocatalysts. Also, the use of copper oxide based materials 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

Declaration of Competing Interest The authors confirm that: All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising 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

Acknowledgments 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-NRF2016-278729, 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. FICUANL supported this research.

Fig. 13. – Mechanism of the photocatalytic conversion of H2O and CO2 over Na2Ti6O13/CuO/Cu2O heterostructured materials. a) Z-scheme, b) p-n heteroestructure. 9

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Table 4 Summary of related materials reported activities for hydrogen evolution and CO2 photo-conversion. Photocatalyst

Irradiation source

Experimental conditions

Main products (μmol g-1 h)

Reference

HCHO (8.3) H2 (17) CH4 (0.27) HCHO (0.56) HCOOH (3) H2 (12.55) CH3OH (3.92) H2 (265)

[83] [84]

H2 (9) ,HCHO (< 1), CH3OH (< 1) H2 (33.3) ,HCHO (< 1),CH3OH (< 1) H2 (10) ,HCHO (25),CH3OH (4.6)

Current work

1

K2Ti6O13 Pt-K2Ti6O13 coupled Cu/ZnO

UV lamp 254 W 150 W Hg-lamp

Catalyst 0.1 g, water DI 200 mL, 2 Psi CO2 Catalyst 0.15 g Pt-K2Ti6O13 and 0.15 Cu/ZnO, 77 kPa CO2, water 4 mL,

Cu doped-TiO2 monoliths

200 W Hg-lamp

Na2Ti6O13/2% RuO2-Zr

400W high-pressure mercury lamp Uv lamp 254 W

Monoliths coated with Cu-TiO2, Ultrapure CO2 saturated with water vapor was introduce (4 mL min-1) Catalyst 1 g, water 300 mL

Na2Ti6O13 Na2Ti6O13/0.1% w CuO/ Cu2O Na2Ti6O13/5% w CuO/Cu2O

Catalyst 0.1 g, water DI 200 mL

Appendix A. Supplementary data

[85] [45]

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