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Topics in Catalysis https://doi.org/10.1007/s11244-018-0966-6

ORIGINAL PAPER

Mg(OH)2 Films Prepared by Ink-Jet Printing and Their Photocatalytic Activity in  CO2 Reduction and  H2O Conversion

E. Luévano‑Hipólito1 · Leticia M. Torres Martínez2

© Springer Science+Business Media, LLC, part of Springer Nature 2018

AbstractMg(OH)2 films on Al substrates were fabricated by ink-jet printing, and they were applied as photocatalysts in solar fuels production (H2 and CH3OH) from CO2 and H2O conversion. The films were fabricated by means of a deposition of a solution composed of magnesium complex nanoparticles over aluminum foils, which were submitted to a heat treatment to promote the crystallization of Mg(OH)2. The films were characterized by razing incidence X-ray diffraction (GZXD), Fourier-transform infrared spectroscopy (FTIR), Scanning electronic microscopy, X-ray photoelectron spectroscopy (XPS), and N2 physisorp-tion by BET method. The Mg(OH)2 was detected in all the samples synthesized with 1 to 40 layers. According to XPS and FTIR analysis, it was detected the presence of carbonates related to Mg3O(CO3)2 and Al0 and Al3+ due to the substrate. The highest photocatalytic activity was reached using 30 layers of Mg(OH)2 for H2 and CH3OH generation, which it was 268 and 15 µmol g− 1h− 1, respectively. These results were associated to the presence of adequate amounts of MgO and Al2O3 that promote an efficient transfer of the photogenerated electrons between them. Furthermore, the formation of porous structures with high surface area and relative high roughness promoted an increase in the mass transport between the gas and liquid phase, which increase the effectiveness of the photocatalysts.

Keywords Mg(OH)2 · CO2 reduction · Methanol · Hydrogen · Solar fuels

1 Introduction

Heterogeneous photocatalysis is a technology that has been applied to remove organic and inorganic pollutants from water, air, and in recent years, it has been exploited for solar fuels generation in order to mitigate the high dependence of the fossil fuels. This technology involves the use of natural or artificial solar light, water, and a photocatalyst, which it

is generally a semiconductor oxide (as powder or thin film). In this sense, the use of powders and thin films has their own advantages and disadvantages. For example, the use of powders involves an inadequate light diffusion inside the photocatalytic reactor and generally, as the mass of the catalyst is higher than thin films, the efficiency of photo-catalytic reaction increased. In addition, the use of powders in photocatalytic reactions implies a second step to separate them from the solution. Therefore, it is necessary to develop efficient immobilized photocatalyst (thin films) in order to avoid the secondary steps such as the filtering operations. Furthermore, the use of thin films has many advantages such as (i) rapid electron transport, (ii) homogeneous exposure of the photocatalyst, (iii) easy scale-up, (iv) it is possible to deposit the photocatalyst over different substrates, (v) effi-cient absorption of solar energy, and (vi) high surface area [1, 2]. Thus, due to the numerous advantages of the thin films different chemical and physical methods have been proposed such as dip-coating, spin-coating, spray pyroly-sis, sputtering, laser ablation, electrophoretic deposition, chemical vapor deposition, chemical bath, among others [3, 4]. In recent years, the use of ink-jet printing technology to

Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s1124 4-018-0966-6) contains supplementary material, which is available to authorized users.

* Leticia M. Torres Martínez lettorresg@yahoo.com

1 CONACYT - Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Universidad Autónoma de Nuevo León, Cd. Universitaria, 66455 San Nicolás de los Garza, NL, Mexico

2 Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Universidad Autónoma de Nuevo León, Cd. Universitaria, 66455 San Nicolás de los Garza, NL, Mexico

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fabricate thin films of different materials has gained inter-est in the industrial and scientific community since it has numerous advantages such as high speed and low cost of fabrication, and offers the possibility to deposit all kind of materials over different substrates [5–7].

Magnesium hydroxide [Mg(OH)2] is an interesting mate-rial due to its useful applications such as old acidic paper conservative [8], fire retardant [9], CO2 absorber [10], and anticorrosive coating [11]. In addition, Mg(OH)2 powders have been proposed as potential photocatalyst to remove organic dyes in wastewaters, in particular for the photooxi-dation of methyl orange, rhodamine B, and malachite green [12–15]. On the other hand, the use of Mg(OH)2 films as photocatalyst have been not exploited yet, however in lit-erature, there are some reports for the application of films of Mg(OH)2 [16–24]. For instance, several researchers have proposed the use of Mg(OH)2 films as anti-corrosive and flame-retardant coatings, which were obtained by hydro-thermal and electrochemical methods, respectively [17–20]. In recent years, Mg(OH)2 films have been used as electro-catalyst in hydrogen generation [21, 22]. However, in these works the amount of the evolved gas was not reported, which difficult the evaluation of the efficiency of catalysts. In addi-tion, in these works the effect of the substrate (generally glass) has not been investigated. In this work, it is proposed the use of a flexible material (Aluminium foil) to deposit different layers of Mg(OH)2 by ink-jet printing. In this sense, it will be very important to consider the contribution of alu-minum as Al0 or Al3+ in the solar fuels production, consider-ing the mechanism of charge transfer during the excitation of the photocatalyst. In previous reports, it was found a posi-tive effect of doping an adequate amount of Al and Al2O3 with other semiconductors, for example Pei et al. studied the impregnation of TiO2 with Al2O3 and applied them as photo-catalyst in several reactions [25]. They reported a significant enhancement in the photocatalytic activity when Al2O3 was added to as co-catalyst due to a decrease in the recombina-tion of the electron and hole pair. This addition it is also related to an enhancement in the visible light absorption, and an efficient charge transfer of the electrons photogenerated from the conduction band of TiO2 to Al [26]. To the best knowledge of the authors, in general only one work had stud-ied the photocatalytic activity for Mg(OH)2 films for mala-chite green removal in aqueous medium [16]. Thus, the use of Mg(OH)2 films in heterogeneous photocatalysis has not been exploited, and in particular for solar fuels generation, this material has been not investigated yet. Nevertheless, the solar fuels generation using powders of double layered hydroxides as photocatalyst, which are isostructural with the brucite structure of Mg(OH)2 have been reported [27]. These kind of materials favors an efficient CO2 adsorption and its further conversion to solar fuels such as CO, CH4, CH3OH, HCOOH, and HCOH. In this work, we propose

the fabrication of Mg(OH)2 films by ink-jet-printing over aluminum foil as substrate, and their application as photo-catalyst in solar fuels generation. We focus in the quantifica-tion of the products H2 and CH3OH in gas and liquid phase, respectively.

2 Experimental

2.1 Deposition of Mg(OH)2 by Ink‑Jet Printing

Magnesium ink was prepared according to the fol-lowing procedure: first, 1  g of magnesium acetate (Mg(CH3COO)2·4H2O) (Fermont, 99%) was dissolved in 2.5 mL of NH4OH (30% v/v). Then, an appropriate amount of formic acid (HCOOH) (Aldrich, 99%) was added under vigorous stirring for 30 min until it becomes transparent. In order to remove any agglomerate into the solution, it was necessary to filter it through a 0.45 µm mesh size. The vis-cosity of the ink at 25 °C was 7 cP. Commercial aluminum foils (Reynolds, 99%) of 5 × 10 cm were used without addi-tional treatment as substrates for Mg(OH)2 films. Magne-sium inks were deposited on Aluminum foil using a FUJI-FILM Dimatix Materials Printer DMP 2800 of 16 nozzles. The operating conditions in the printer was fixed at 30 °C, 30 V, 10 dpi of resolution, and a line pattern. After the print-ing procedure, the samples were placed in an oven at 200 °C for 5 h. The samples will be identified as AlMgx, where x represents the number of deposited layers of Mg(OH)2.

The inks and the aluminum substrate were characterized by thermogravimetric and differential analysis (TGA/DTA) in air from ambient temperature until 600 °C with a heating rate of 10°C/min. The particle size distribution of the ink prepared was analyzed by using a Microtrac, nano-flex 180° DLS size. The structural characterization of the Mg(OH)2 films was carried out by X-ray powder diffraction using a Bruker D8 Advance diffractometer with Cu Kα radiation (40 kV, 30 mA). The morphology of the samples was ana-lyzed by scanning electron microscopy using a JEOL 6490 LV. The thickness of the film was estimated by cross-section images of the films. The thickness value reported is an aver-age of the different zones analyzed. The specific surface area of the films was evaluated by N2 physisorption using the BET method by means of a Bel-Japan Minisorp II surface area and pore size analyzer by means of N2 adsorption–des-orption isotherms at − 196 °C. The surface groups in the films were studied by FTIR using a Perkin Elmer FTIR/FIR Frontier with ATR accessory in a range of 500–4000 cm−1. The surface study was performed using X-ray photoelec-tron spectroscopy (XPS) using a Thermo Scientific K-Alpha XPS instrument with monochromatized Al Kα radiation (hν = 1486.68 eV). Binding energies of all peaks were cor-rected using C 1s energy at 284.5 eV corresponding to

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adventitious carbon. The band gap energy of the AlMgx samples was calculated from the diffuse reflectance spectra converted to the Kubelka–Munk function using a UV–Vis NIR spectrophotometer (Cary 5000) coupled with an inte-gration sphere.

The photocatalytic reactions for solar fuels production were carried out in a cylindrical Pyrex batch reactor at 25 °C and 2 psi. Then 100 mL of deionized water was added into the reactor and N2 or CO2 was bubbled into it to remove the oxygen dissolved in water. The reactor was irradiated outside the reactor with two halogen (and heterochromatic) lamps of 20 W each one. The irradiance in the center of the photocatalytic reactor (103 µW cm−2) was measured using a Mannix, UV light-meter. The H2 and CH3OH produced were quantified using a gas chromatograph Thermo Scientific GC Ultra equipped with a thermal conductivity detector (TCD), and by the complexation of methanol with sodium nitroprus-side with UV–Vis spectroscopy, respectively.

3 Results and Discussion

3.1 Ink Characterization

The inks were characterized by a simultaneous TGA/DTA measurements (Fig. 1). The first weight loss (75%) occurred from 100 to 200 °C followed by a second loss of 15% above 400 °C. The first peak below 100 °C is associated to the loss of water from the magnesium acetate precursor, and the solvents used (NH3 and HCOOH). Regarding to DTA analysis, it was observed four peaks related to an endother-mic oxidation of organic matter and the Mg(OH)2 and MgO crystallization. The peak in 170 °C corresponds to the oxida-tion of organic compounds in the inks. The crystallization of Mg(OH)2 and MgO can be associated to the exothermic

peaks in 200 and 420 °C, respectively. Furthermore, in the literature, the compound (Mg3O(CO3)2) associated with CO2 absorption in Mg(OH)2 has been reported. The decarboxyla-tion of this compound occurs at 385 °C as it was corrobo-rated with the endothermic peak at this temperature [28] by DTA analysis shown in Fig. 1.

The ink showed a homogeneous particle size distribu-tion, with a mean of 813 nm, whose data are shown as Sup-plementary Fig. S1. Additionally, some particles with sizes higher than 1 µm were detect them.

3.2 Film characterization

Figure 2 shows the XRD patterns of Mg(OH)2 films over Al substrates. The substrate showed reflections associated with Aluminum in 2θ = 39° and 45°. The presence of Mg(OH)2 was confirmed after the deposition of one layer of the mag-nesium ink, and the reflection intensity was increased as long as the layers was increased in 2θ = 37°. In addition, a third phase was detected in 2θ = 29.6 and 45.3° related to Mg3O(CO3)2 in the samples with 5, 10, 20, 30, and 40 layers of Mg(OH)2. This phase is associated with the presence of residual organic matter due to the low annealing temperature used. The presence of Al2O3 was not possibly to identify by this technique probably due to its low amount in the samples (< 5% wt).

The surface groups of Mg(OH)2 films was studied by FTIR (Fig. 3). The presence of Mg(OH)2 in the films was confirmed in all samples by the presence of bands at 660 and 618 cm− 1 [29]. In Fig. 3 bands can be observed at 1570 and 1390 cm−1, which are related to the C=O stretch of bidentate carbonates [30]. The presence of these bands can be associ-ated to the presence of the third phase observed by XRD (Mg3O(CO3)2) or to the residual carbon in AlMgX sam-ples. After performed an analysis in the band that appears at 1390 cm−1, it was observed that this signal was the big-gest in the AlMg30 sample. In this sense, the presence of an adequate amount of carbon can benefit the photocatalytic reaction since it can be act as hole scavenger, which eventu-ally can avoid the recombination of the electron and hole, as will be further discussed. It is noteworthy that due to the chemical nature of carbonate ions, their associated bands are more intense than the corresponding signals of Mg(OH)2 at lower wavenumbers. This fact can be related to the high dipole moment in the compounds associated to C=O cova-lent bonds, which it causes a higher electric field due to the strong interaction between the stretching vibration and infrared radiation. Thus, the signals of Mg–OH ionic bond are less intense.

The SEM images of the AlMgX samples are shown in Fig. 4. The reference sample (substrate) has a smooth surface with some precipitates deposited on it (Fig. 4a). According to an EDS analysis (input Fig. 4a), its chemical composition

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was Al, Si, Cl, Fe, and K, which it may came from the fabri-cation of the foils. Regarding to the AlMgX samples, at low concentrations of Mg(OH)2 some particles with smooth sur-face were observed, and as long as the layer increased from 1 to 10, some spherical particles (< 1 µm) were detected (Fig. 4b-e). In the AlMg20 sample, the morphology changed from spherical to flake-like particles of higher average size (1.2 µm) (Fig. 4f). After deposited 10 layers more (AlMg30), the flakes were accommodated to give rise to the formation of spherical porous structures interconnected between them (Fig. 4g). This fact was more evident at a closer observation of the AlMg30 sample (Fig. 4h). The average size of the spherical particles was about 2 µm. Regarding to the sample with the highest Mg(OH)2 concentration (AlMg40), it has a heterogeneous morphology composed of irregular particles with a wide range of sizes (Fig. 4i). Thus, in general, the particle size tended to develop higher values as long as the

layers number was increased in the films. On the other hand, it was analyzed the thickness of the films by a cross-section analysis, and the results are summarized in Table 1. The increase in the number of deposited layers resulted in an increase in film thickness, as it was expected. The highest thickness (20.8 µm) was obtained in the sample more con-centrated, AlMg40. From SEM images, it was observed an increase in the roughness of the films as long as the concen-tration of Mg(OH)2 was bigger. This fact was more evident in the films with 20 and 30 layers (see Supplementary Fig. S2). After this point, the roughness tended to decreased.

In order to study the composition of the surface of the films, an analysis by XPS was performed in two repre-sentative samples (AlMg30 and AlMg40 samples), and the spectra is shown in Fig. 5. As it can be seen in Fig. 5a, b, the spectra of Mg 2p was deconvoluted in three compo-nents related to MgO in 49.7 eV, Mg(OH)2 in 50.4 eV, and a magnesium carbonate such as Mg3O(CO3)2 in 51.2 eV [31–33]. A deep analysis of the areas of these components showed a higher contribution of MgO and lower amount of Mg3O(CO3)2 in the AlMg30 samples. This fact can be cor-related to the photocatalytic activity, as will be discussed further. The O 1s spectra showed in Fig. 5c, d was deconvo-luted in three components; one of these correspond to car-bonate around 533.9 eV, an oxide in 530.2 eV, and the main peak related to the hydroxide appears in 531.6 eV. These binding energies are in agreement with previous reports in the literature [34–36]. In Fig. 6, it is showed the C1s spectra, where it can be observed the presence of signals associated to C=O and C–O bonds in 289.0 and 285.8 eV, respectively [31, 36, 37]. These results confirm the presence of residual organic compounds (C=O) on the films as was found by XRD and FTIR.

Additionally, it was study the Al2p spectra in order to investigate the stability and composition of the substrate

Fig. 2 Grazing incidence X-ray diffraction patterns (GIXRD) of AlMgX films

Fig. 3 FTIR spectra of AlMgX films

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after the thermal treatment. As it is shown in Fig. 7, the Al2p spectra presents two signals in 72.2 and 74.2 eV, which correspond to Al0 and Al2O3, respectively [31, 37, 38]. Thus, these results suggest the oxidation of substrate by the thermal treatment in spite of it’s the low tempera-ture used. Regarding to the relation between the Al3+/Al0, it was higher in AlMg30 sample.

The specific surface area and the pore volume values of AlMgX are shown in Table 1. These values tended to increase as long of the layers of magnesium hydroxide increased in the films. The highest surface area obtained was 50 m2 g−1, and it was associated to the deposition of 30 layers of Mg(OH)2 (AlMg30). This sample showed a morphology composed of porous particles interconnected between them, which coincides with its high surface area. The pore volume showed a similar tendency, reaching its highest value (1.72 cm3 g−1) when 20 layers of mag-nesium was printed onto the substrate. From this point, the surface area and the pore volume decreased to 25 m2 g−1 and 0.63 cm3 g− 1, respectively. The profile of N2 iso-therms of AlMgX films showed a similar trend (Fig. 8), which it is associated to type-III that is characteristic of macroporous materials with low energy of adsorption. As it can be seen in Fig. 8, the sample with the highest sur-face area (AlMg30) showed the highest volume adsorbed of N2 molecules.

In order to estimate the band gap of the films, it was analyzed the diffuse reflectance spectra of the samples, which results were converted to the Kubelka–Munk remis-sion fuction, and the results are shown in Supplementary

Fig. 4 SEM images of AlMgX films

Table 1 Physical properties of the AlMgX samples

Sample Thickness (μm) Surface area (m2 g−1)

Pore volu-men (cm3 g−1)

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Al – 4.0 0.17 –AlMg1 3.8 4.5 0.15 4.7AlMg3 6.7 4.7 0.20 4.6AlMg5 6.3 8.4 0.24 4.6AlMg10 9.3 12.1 0.32 4.3AlMg20 10.1 44.3 1.72 4.2AlMg30 12.2 50.2 1.14 4.2AlMg40 20.8 25.5 0.65 4.4

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Fig. 5 XPS of Mg2p in (a) AlMg30 and b AlMg40 films and O1s signal in (c) AlMg30 and d AlMg40 films

Fig. 6 XPS of C1s of a AlMg30 and b AlMg40 films

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Fig. S3, and in Table 1. In general, the band gap values obtained were similar among them, and were higher than 4 eV, which implied their activation with UV light. The band

gap obtained were around 4 eV, which is in agreement with previous reports in literature [39].

3.3 Solar Fuels Production

Figure 9 shows the photocatalytic hydrogen and methanol production using AlMgX films, after 3 h of continuous irra-diation. For comparative purposes, the photocatalytic activ-ity of the substrate was studying, and it was not found a significant amount of products (H2 and CH3OH) after 3 h of reaction.

Regarding to AlMgX samples, it can be seen that the number of layers affect the photocatalytic activity, in particu-lar as long as the layers increased, the production of the solar fuels growths. The highest photocatalytic activity in both cases was obtained with the AlMg30 sample, which pro-duced 268 and 15 µmol g−1h− 1 of H2 and CH3OH, respec-tively. It seems that from this value of layers, the activity tended to decrease. The samples with 1, 3, 5, and 10 layers of Mg(OH)2 did not produce enough methanol to be identi-fied and detect it with the method employed, which detection

Fig. 7 XPS of Al2p of a AlMg30 and b AlMg40 films

Fig. 8 N2 isotherms of AlMgX films

Fig. 9 Solar fuels generation from H2O and CO2 reduction: a H2 and b CH3OH using AlMgX films after 3 h of continuous irradiation

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limit was 0.05 ppm. The reaction yields are summarized in Table 2, considering an average mass of 0.1 g.

The sample with the highest photocatalytic activity (AlMg30) showed interesting properties such as: (i) high surface area, (ii) a morphology composed of notably porous particles, and (iii) relative high roughness. In this sense, these properties are directly related to an increase in the mass interfacial transport in the heterogeneous reaction and a light harvesting that promote multiple reflection inside the pores, and ventually a higher photon absorption [40]. This fact was corroborated with the reflectance spectra of some films, which it shows a significant decrement in the reflec-tance (higher absorbance since 350–200 nm) in AlMg30 film (see Supplementary Fig. S3). Another factor that can affect the photocatalytic activity is the composition of the films. As it was shown in the XPS analysis, this sample showed a higher contribution of MgO, which has a more negative potential of its conduction band (BC) (− 1.4 eV) than Mg(OH)2 (0.10 eV), which values were calculated by the procedure suggested by Schoonen [7]. Also, the pres-ence of the inorganic carbonate material (Mg3O(CO3)2)

can participate in the reaction since its CB energy level is − 0.43 eV, which is more positive than MgO and more nega-tive than Mg(OH)2. Thus, it is proposed the reaction scheme shown in Fig. 10, which involves an electron transfer since MgO to Mg(OH)2, to delay the recombination of the pho-togenerated charge. This fact feature the reduction of more H+ and CO2 molecules to H2 and CH3OH from a thermo-dynamically standpoint. In addition, since the adsorption of CO2 is favored in Mg(OH)2, the gas can react with electrons and protons to produce methanol. In addition, the presence of an adequate amount of the residual carbonated matter seems to benefit the reaction yield since it can act as hole scavenger reducing the recombination of the photogenerated electron and hole [41].

In the photocatalytic reduction of CO2, the competition for the photogenerated electrons by water reduction occurs when this compound is used as electron donor. This is due to that the reduction of water is a relative easy process in term of kinetics and thermodynamics. For instance, the reduction potential of H2O to H2 is 0 V at pH 0, which is lower than the required potential for CO2 reduction to CH3OH (− 0.38 V) [42]. Also, from a kinetic standpoint, water reduction is a 2-electrons process, which is more facile than the 6 required electrons for CH3OH generation. Additionally, the conver-sion of CO2 is also limited by its low solubility in water, thus the chance for electrons to meet and react with water is much higher than with CO2. However, in spite of these issues, the production reported in this work is higher than previous reports in CO2 conversion with photocatalytic coat-ings of different materials. For instance, Wu et al reported TiO2 coatings over optical fibers for CO2 photoconversion to methanol, and they obtained a production of 0.45 µmol g−1 h−1 [43]. On the other hand, Liou et al reported a methanol generation of 0.16 µmol g−1 h−1 from CO2 photoconversion using a monolith coated with NiO/InTaO4 [44]. Thus, the results obtained in this work are promising in the searching of new strategies in order to mitigate the high dependence of the fossil fuels.

The recycle capability is important in the practical application of photocatalysts. For this reason, it was study the stability of the films after three consecutive experi-ments, and the results obtained are shown in Fig. 11. For these experiments, the films were recover and dried at 80 °C for 2 h, and then they were reused as photocatalyst for solar fuels generation. As it is shown in Fig. 11, the AlMg30 film exhibited good stability for up three con-secutive cycles. The net efficiency decreased only 10%. On the other hand, the decreased in the methanol gen-eration was more affected after the consecutive experi-ments. After three experiments, the methanol production decreased more than a half of its initial value. In order to study the cause of the decreased in the photocatalytic activity, it was analyzed the composition of the films by

Table 2 Reaction yields obtained for H2 and CH3OH generation from H2O and CO2 reduction, respectively

ND refers to not detected

Sample Solar fuels production (μmol g−1 h−1

H2 CH3OH

Al 4.5 NDAlMg1 15.1 NDAlMg3 47.6 NDAlMg5 124.7 NDAlMg10 166.7 NDAlMg20 215.4 1.8AlMg30 267.9 14.5AlMg40 104.1 11.4

Fig. 10 Scheme of the photocatalytic mechanism proposed

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XRD and FTIR, which results are shown in Fig. 12. The analysis by XRD indicated the presence of oxidation of the substrate after the third experiment from Al to Al2O3, according to the apparition of a signal in 2θ = 35° (JCPDS Card 46-1212). Also, it is important to note that it was not observed the presence of additional reflections associated to the most common compounds reported when powders of Mg(OH)2 are used as CO2 sorbent, such as MgCO3, MgCO3∙3H2O, MgCO3∙5H2O, Mg5(CO3)4(OH)2·4H2O, (Mg5(CO3)4(OH)2·5H2O [45], which it suggests an alter-native mechanism for the adsorption of CO2 over Mg(OH)2 films. On the other hand, this fact was corroborated by FTIR analysis, in where it was noted a wide band around 500 cm−1 related to Al2O3 (in an Al–O stretching mode)

[46]. Djebaili et al. proposed that this band of is related to an amorphous structure or disordered defects [47], which it can affect the photocatalytic activity, as it was discussed. Thus, it seems that the substrate will be develop an eventually oxidation after the consecutive steps. This phenomenon did not affect significantly the hydrogen pro-duction, probably because the samples still had the poten-tial to reduce H+ to H2, but not the required input energy to reduce CO2 to CH3OH.

On the other hand, it was explore the photocatalytic activity of the films using only UV lamp by means of a monochromatic lamp of 254 nm, which results are shown in Fig. 13. According to the results, the activity for H2 generation was slightly higher (11%) using the UV lamp in comparison with the heterochromatic lamp. However, the methanol generation decreased more than a half (64%) using the UV lamp as irradiation source. Thus, it seems that the contribution of the visible and infrared irradia-tion in the halogen lamp play an important role in the conversion of CO2. In this sense, Schlögl et al. studied the effect of the methanation of CO2 at different conditions, and they found the importance of the infrared contribution (provided by a xenon lamp) in this reaction [48]. Amin et al. study the effect of the temperature in the CO2 photo-conversion, and found an increase in this value as long as the temperature increased from 50 to 120 °C [49]. They attributed this enhancement in the activity to a promo-tion in desorption of the products from the photocatalyst surface and to an increased in the collisions between the charges and the CO2 molecule. Thus, for the generation of solar fuels from CO2 photoconversion, it is important provide adequate amounts of thermal energy in order to promote the activation of the molecule and the photocata-lyst surface.

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Fig. 12 Characterization of the AlMg30 film after the photocatalytic experiments by a GIXRD, and b FTIR

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4 Conclusions

Mg(OH)2 films over aluminum foils were obtained by ink-jet printing, and they were used as catalyst in solar fuels gen-eration (H2 and CH3OH). The high photocatalytic activity reached with the Mg(OH)2 films over aluminum was asso-ciated with the presence of high surfaces areas, adequate pore volumes, and relative high roughness that increase the active sites and the diffusion of CO2 inside the films. These properties were confirmed by SEM results, in where it was observed a clear porous network in the sample with the highest photocatalytic activity. Another factor that played an important role in the photocatalytic activity was the composition of the films. It was found that the sample with a higher contribution of MgO promoted a higher reaction yield, which it was associated to the development of more negative potential of its conduction band that delays the recombination rate of the photogenerated charges. In addi-tion, the presence of an adequate amount of Al2O3 played an important role in decrease the recombination of the pho-togenerated pair, which eventually provides a higher number of electrons in the conduction band available to reduce CO2 and H2O. The analysis of stability of the films indicated that the production of methanol was decreased on a half of its initial value probably because a partial oxidation of the sub-strate from Al to the Al2O3, with the not enough potential to reduce CO2 to CH3OH. According to the results, the H2O reduction was more favorable kinetic and thermodynami-cally than the CO2 conversion since the first reaction requires less photogenerated electrons and a lower energy than the second one.

Acknowledgements The authors wish to thank CONACYT for financial support for this research through the following projects:

Cátedras CONACYT 1060, CONACYT-CB-2014-23704, CONACYT-PDCPN-2015-487, CONACYT-NRF-2016-278729, and SEP-INTE-GRACIÓN DE REDES TEMÁTICAS 2015-CA-244.

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Fig. 13 Comparison of the photocatalytic activity of the AlMg30 film using different irradiation sources

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