Hydrogen production by ultrasound assisted liquid laser ablation of Al, Mg and Al-Mg alloys in water

Applied Surface Science 478 (2019) 189–196

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Hydrogen production by ultrasound assisted liquid laser ablation of Al, Mg and Al-Mg alloys in water

T

L. Escobar-Alarcóna, , J.L. Iturbe-Garcíab, F. González-Zavalac, D.A. Solis-Casadosd, R. Pérez-Hernándeze, E. Haro-Poniatowskic ⁎

a

Departamento de Física, Instituto Nacional de Investigaciones Nucleares, Carretera México-Toluca S/N, La Marquesa, Ocoyoacac, Estado de México C.P.52750, Mexico Departamento de Química, Instituto Nacional de Investigaciones Nucleares, Carr. México-Toluca s/n La Marquesa, Ocoyoacac, Edo. de México C.P. 52750, Mexico c Departamento de Física, Universidad Autónoma Metropolitana Iztapalapa, Apdo. Postal 55-534 CDMX, Mexico d Universidad Autónoma del Estado de México, Facultad de Química, CCIQS UAEM-UNAM, Paseo Colon esq. Paseo Tollocan S/N, CP 50120 Toluca, Estado de México, Mexico e Departamento de Tecnología de Materiales, Instituto Nacional de Investigaciones Nucleares, Carr. México-Toluca s/n La Marquesa, Ocoyoacac, Edo. de México C.P. 52750, Mexico b

ARTICLE INFO

ABSTRACT

Keywords: Hydrogen Laser ablation Ultrasound Cavitation

The generation of hydrogen upon ablation of Al, Mg and three different Al-Mg alloys targets immersed in water with and without the presence of an ultrasonic field is reported. The effect of the laser fluence used for ablation of each target on the amount of hydrogen produced was investigated. In general terms as the laser fluence increases a higher amount of hydrogen was obtained. It was found that the simultaneous application of an ultrasonic field enhances approximately 100% the hydrogen production. The proposed procedure performed under standard conditions of temperature and pressure consumes only water and very low amounts of material leading to maximum production rates close to 23.2 mmol/min per gr. Additionally, some of the nanostructures produced during the ablation process were characterized by SEM, TEM, XRD and PL. The results reveal that crystalline Mg2Al(OH)7 nanosheets with maximum size close to 100 nm and regular shapes are obtained when the alloy with the maximum aluminum content is ablated at the highest laser fluence under the ultrasonic field.

1. Introduction Nowadays there is an urgent demand for renewable and clean fuel alternatives. Hydrogen has been identified as an option for a future energy supply to substitute the fuels from fossils sources being considered a regenerative and environmentally friendly fuel with high calorific value. However, although hydrogen is the most abundant element in the universe, hydrogen in gas form is hardly found on earth [1]. Therefore, the investigation of processes to release the hydrogen contained in water (H2O), which is the most abundant hydrogen compound on earth, is a key issue. Among the most promising approaches for in situ hydrogen generation are the methods based on water oxidation, in which hydrogen sources such as water and hydrocarbons are usually used as one of the reactants, from which it will be extracted with the help of metals of high activity, such as aluminum, magnesium or silicon [2]. However, an important drawback of this approach is that these elements hardly react with water under standard conditions requiring in some cases elevated temperatures and/or the presence of

⁎

additional chemical compounds as alkalis to activate the reaction. Aluminum and Magnesium are among the metals that produce a violent hydrogen release after the contact between them and water, even under mild conditions, making real-time hydrogen production possible. Additionally, Al and Mg can eliminate the need for hydrogen storage because the hydrogen is transported as water and can be released directly in the place of consumption. Particularly, Al has a high potential for high purity hydrogen generation as it is the second most abundant element in the Earth. Furthermore, Al is electrochemically very active, safe, and cheap [3–5]. On the other side, magnesium can be oxidized relatively easily in presence of acid, while remaining inert in alkali environment. By reacting MgH2 with water or using Mg alone it has been possible to produce hydrogen with good results [6–8]. In order to improve the Al reactivity with water, the use of Al alloys containing some metals such as Zn, Ga, Bi, Pb, Sn, In, and Mg, among others, has been recognized to be one of the most suitable materials applicable for future hydrogen production and there is a trend to use them as energy materials [1,9]. We have chosen the Al-Mg alloy because Mg has a low

Corresponding author. E-mail address: [email protected] (L. Escobar-Alarcón).

https://doi.org/10.1016/j.apsusc.2019.01.213 Received 30 August 2018; Received in revised form 19 January 2019; Accepted 23 January 2019 Available online 25 January 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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electronegativity (1.2) being then very reactive with oxygen (electronegativity of 3.5). It is worth mentioning that the hydrogen production through the oxidation reactions is mainly performed using Al and Mg being the most promising because they are cheaper and easily available; other metals such as Ni, Mo, Cu [10] and Pt, Y, La, Ce, Yb, have been also employed using different routes to enhance the H2 production [11]. Regarding studies of hydrogen production using laser radiation, it has been reported that powder from charcoal and other forms of carbon mixed with distilled water can release hydrogen when irradiated with 532 nm nanosecond laser pulses. The volume of gas obtained in this case was dependent on the laser power density generating a maximum gas volume close to 1.5 mL after 30 min of irradiation. It was found that the gas generated was composed by hydrogen, carbon monoxide and carbon dioxide, because the carbon powder was oxidized acting as a sacrificial reagent in the photochemical hydrogen generation [12]. The study on the photocatalytic splitting of water induced by laser to produce hydrogen and oxygen using pure Fe2O3 as catalyst by irradiation with a laser beam source of 355 nm has been reported as well [13]. Another approach consists in the laser irradiation of colloidal solutions of Au nanoparticles suspended in water resulting in the formation of breakdown plasmas in liquid and the subsequent emission of H2 [14]. In this case a Nd:YAG laser emitting at a wavelength of 1064 nm and pulse duration of about 10 ns was used for irradiation and it was found that the H2 production depends critically on the energy of the laser pulses and on the concentration of Au NPs in the solution. It is worth mentioning that Barmina et al. have suggested that H2 emission can occur in any experiment on laser ablation in liquids including the ablation of a solid target [14]. The hydrogen production from a mixture of carbonwater and graphite-water by irradiating with nanosecond laser pulses through dehydrogenation and combustion reactions has been reported before [15]. The photocatalytic hydrogen production from methanol aqueous solution containing a mixture of titanium dioxide (TiO2) and graphite silica (GS) is enhanced by laser ablation in liquid [16]. The hydrogen production by plasma electrolysis of water using the second harmonic of a diode pumped solid state laser working with different acids and bases as catalysts has also been reported [17]. In fact, as a result of the investigation about the effect of the presence of persistent bubbles, with lifetimes from milliseconds up to seconds, on the nanoparticle productivity, it has been found that these bubbles are composed by 64% hydrogen and 36% oxygen, very close to the H2O composition [18]. Interestingly, the production of molecular hydrogen during optical breakdown in aqueous media with ultrashort laser pulses suggesting that additionally to the plasma formation, photothermal dissociation of water is also possible [19]. Recently the combination of laser ablation with an ultrasonic field for hydrogen production through a reaction of water with some metals has been reported by our group; in this case, laser radiation is used to remove the native oxide layer of metals very reactive with water favoring displacement chemical reactions releasing hydrogen from water [20]. In this line, a very important issue is to investigate alternative methods for hydrogen production to those widely reported [21–27], without any electrodes and photocatalysts as well as without the generation of greenhouse gases which can be harmful to human beings and the environment. Concerning laser ablation in liquids, this technique has been widely used to produce a variety of nanostructures with different shapes and sizes including metals [28], alloys [29], and oxides [30]. An important effect of the plasma formation as a result of the laser-solid interaction is that some of the plasma surrounding liquid is vaporized producing a cavitation bubble. This bubble expands to its maximum size and then collapses, this implosion occurs very fast at nearly adiabatic conditions generating local temperatures of thousands of Kelvin degrees and pressures of several GPa [31]. Under these extreme conditions if the liquid is water, such bubbles can work as microreactors in which the H2O vapor molecules trapped inside can be dissociated in hydrogen and

oxygen. Therefore, this can be considered as an alternative to produce hydrogen, in which the only by-product is oxygen. This gas can be removed using as target a metal highly reactive with oxygen as Al and Mg forming oxides; at the same time the metallic nanostructures produced can react also with oxygen and in this way this technique becomes attractive to produce only hydrogen. In previous works, we have reported that the simultaneous application of an ultrasonic field during the ablation in liquid media can help to promote important changes in the morphology and production rate of the generated nanostructures producing for example two dimensional materials in colloidal suspensions [32,33]. It is worth mentioning that ultrasound induced processes have a very interesting parallelism with laser ablation in liquids, because chemical and physical effects of ultrasound arise also from cavitation, that is, the formation, growth, and implosive collapse of bubbles which produces temperatures above 5000 K, pressures exceeding 1000 atm, and heating and cooling rates as higher as 1010 Ks−1 [34]. These two processes are highly nonlinear producing very extreme conditions and can be employed to favor processes that require high temperatures, high pressures, or long reaction times. In this work, it is shown that the use of liquid laser ablation of Al, Mg and three different Al-Mg alloys assisted by an ultrasonic field enhance the splitting of water for hydrogen production with promising results. 2. Experimental procedure Experiments were performed by ablating Al, Mg and three different Al-Mg alloys with different Al/Mg ratios, 0.42, 4 and 9, named alloy 0.42, alloy 4 and alloy 9 respectively hereinafter. The alloy targets, 12x12x20 mm were synthesized by melting following a methodology developed in our Institute (ININ), using a thermal induction oven, working at 4.5 kHz and 8 kW, in an argon atmosphere. Mg and Al with purity of 99.8% were used as raw materials. The obtained ingots were subjected to a homogenization by thermal treatment at 300 °C for 72 h. The target was placed inside of a 20 mL glass flask and afterwards it was filled with triple distilled water and sealed. The flask was placed inside an ultrasonic bath and was connected through a flexible hose to an inverted graduated cylinder filled with water. The volume of the produced gas was determined by the water displacement method measuring directly the volume of water displaced with a resolution of 0.1 mL. The volume measurements were corrected to take into account the vapor pressure of water at 20 °C. Fig. 1a) shows a diagram of the experimental configuration used and Fig. 1b) shows the glass flask containing an Al-Mg target during the H2 production in which the generation of the gas bubbles is observed. A pulsed Nd: YAG laser emitting at the fundamental line (1064 nm) and 5 ns pulse duration at a repetition rate of 10 Hz was used as energy source. The laser beam was directed perpendicular to the target surface passing through a quartz lens with 120 mm of focal length and focused to a spot size close to 0.8 mm, which was measured using an optical microscope after the irradiation of the submerged target by 100 laser pulses. For the experiments under ultrasonic field, an ultrasonic bath working at a frequency of 40 kHz and a power of 70 W was used. Experiments varying the laser fluence from 27 to 77 J/cm2, with and without the presence of the ultrasonic field, were performed. After each experiment the amount of ablated mass was measured using an analytical balance. The ablation time was kept constant at 5 min for all experiments. In order to characterize the produced gas, gas chromatography (GC) and Mass Spectrometry (MS) measurements were performed. A volume of 250 μL of gas obtained after laser ablation was injected repeatedly and analyzed by gas-chromatography (Gow-Mac 580 instrument) equipped with a molecular sieve 5 Å column at 35 °C and an injector controlled by Clarity software V.7.0.01.402 and TCD. Argon was used as carrier gas in the GC at 20 mL/min. The effluent gases were monitored with a portable mass spectrometer (BELMass, MicrotracBEL Corp.). Additionally, some of the obtained nanostructures were characterized by transmission electron microscopy (TEM), for this purpose a drop of 190

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more bubbles are produced, they move upward forming bubbles of higher size reaching up to 5 mm as is observed in Fig. 1b). This suggests that such bubbles can interact between them coalescing to form larger ones [18]. After a few seconds they escape from the liquid surface without collapsing behaving as persistent bubbles not as cavitation bubbles and filled with gas. These bubbles are produced where the laser strikes the target surface independently of the presence of the ultrasonic field. However, a clear increase in the number of bubbles formed when the ultrasound is applied becomes evident. At the higher fluences the formation of a colloidal suspension is seen after a couple of minutes of irradiation. The colloidal suspension becomes denser as time goes on impeding eventually that all of the laser pulse energy reaches the target decreasing the production of bubbles on the target surface. The produced gas was characterized by gas chromatography and mass spectrometry. For this purpose, the samples generated under ultrasonic field using each alloy were collected directly into glass vials of 10 mL and were sealed with butyl rubber septa and cap. For the gas chromatography measurements, a gas-tight syringe was used to transfer 200 μL of the sample into the gas chromatographic injection port. Fig. 2a) shows the chromatogram obtained after the injection of each sample at intervals of approximately 4.5 min of the three samples produced. In all cases a single peak at a retention time of 1.5 min corresponding to hydrogen is observed. An additional weak signal is observed at a retention time of 2.3 min revealing the presence of O2 with very low intensity as seen in the inset of Fig. 2a). These results indicate that the gas produced consist of molecular hydrogen with a low amount of molecular oxygen suggesting that this procedure allows the production of practically only molecular hydrogen. The presence of molecular oxygen could be due to air dissolved in water and to the fact that during the dissociation of H2O molecules twice as many hydrogen molecules are created as O2 [18] as well as to the high affinity of the employed metals as scavengers of oxygen to form oxides. Consequently, it is expected that mainly H2 composes the released gas. The three samples were analyzed by mass spectrometry monitoring the signals corresponding to H2 and O2. Fig. 2b) shows the mass spectra in which strong signals due to H2 are observed in good agreement with the previous chromatography results. Signals with very low intensity revealing the presence of O2 are observed. These results show that the proposed experiment produce mainly molecular hydrogen with a very low amount of molecular oxygen. This can be attributed to the extreme conditions reached during the ablation of the target submerged in water in which the formed plasma produce a cavitation bubble which expands to its maximum size and then collapses violently generating local temperatures of thousands of Kelvin degrees and pressures of several GPa [31]. Under these extreme conditions, such bubbles work as microreactors in which the H2O vapor molecules trapped inside are dissociated into hydrogen and oxygen. Probably other reactive species such as %OH, HO2%, H%, O and H2O2 are created from the H2O and O2 dissociation and their associate reactions in the bubble [35]. These chemical products may diffuse out of the bubble and dissolve in the surrounding liquid in which they can react with the metallic target and with the generated metallic nanoparticles allowing the release of only hydrogen. The obtained results of hydrogen production using Al, Mg and their different alloys are shown in Table 1 and Fig. 3. In general terms, when experiments are performed in presence of the ultrasonic field a significant enhancement, approximately of 100% for the alloys, of the H2 production is observed. Fig. 3a) shows that without ultrasound the H2 produced follows linear tendencies as a function of the laser fluence. On the contrary, except for Al, under ultrasonication the data follow a nonlinear monotonic behavior with similar tendencies. Without ultrasound the alloy 0.42 produce the lowest amount of H2, which increases from 44.5 to 133.5 μmol when increasing the laser fluence from 27 to 77 J/ cm2. These values correspond to production rates from 8.9 to 26.7 μmol/min respectively. For the alloy 9 the hydrogen produced increases from 85.4 to 157.5 μmol at the same fluences and the H2

Fig. 1. a) Experimental setup for hydrogen production, b) the glass flask containing an Al-Mg target during the H2 production, the arrows indicate the bubbles formed.

each of the prepared colloids was deposited on TEM 300 mesh copper grids covered with carbon film. These samples were analyzed with a point resolution of 0.19 nm, at an operating voltage of 200 kV (TEM JEOL 2100). Compositional analysis was performed by Energy Dispersive Spectroscopy (EDS) using an EDS probe (Oxford EDS-7274) attached to a scanning electron microscope (JEOL, JSM-5900LV). Structural characterization was carried out by X-Ray Diffraction (XRD) using a Bruker AXS D8-Discover diffractometer with the Cu Kα radiation (λ = 1.5406 Å). Photoluminescence properties of the colloids were studied by PL spectroscopy using a spectrofluorometer (FluoroMax 4, Horiba Jobin Yvon) equipped with a 150 W Xenon lamp as excitation source. 3. Results When the target ablation starts, the appearance of bubbles close to the target surface with sizes in the submillimeter range is evident. As 191

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fluences respectively. When the ultrasonic field is applied, the lowest H2 production was obtained using Al, from 67.5 to 162.0 μmol, corresponding to production rates from 13.5 to 32.4 μmol/min for fluences of 27 and 77 J/cm2 respectively. Mg produces values from 101.0 to 201.0 μmol for the same fluences. The alloy 0.42 produce amounts of H2 from 75.6 to 289.2 μmol whereas ablating the alloy 9 leads to the highest H2 production from 169.0 to 302.5 μmol increasing the laser fluence from 27 to 77 J/cm2. An interesting result is that the metals alone under ultrasound produce lower amounts of H2 than the production of alloys. It is worth mentioning that ultrasonic excitation alone does not lead to detectable gas production in the present experimental conditions, although it has been reported that the use of ultrasonic excitation allows splitting of water [36]. Concerning the effect of the dissolved gases in water, N2 and O2, are present in percentages close to 78% and 21% respectively. From the Henry's Law [37] the concentration of N2 dissolved in water at 25 °C and 1 atm is 5.3 × 10−4 mol/L, therefore the total amount of N2 in 20 mL of water (the volume used in our experiments) is equal to 10.6 μmol. This value is approximately 23% of the lowest amount of H2 produced and only 3.5% of the highest amount of H2 produced in our experiments. Therefore, the dissolved gases present in the water do not have an important contribution on the reported amount of hydrogen. In order to explain the obtained results, it is important to point out that the proposed procedure for hydrogen generation is based on two physical processes that produce very extreme conditions of temperature and pressure. In fact, in both cases the physical mechanisms behind are not well understood yet. At this moment we can give a tentative explanation as a first approximation which still requires a long term and deeper investigation. To the best of our knowledge at least the influence of the following factors should be considered on the hydrogen production: a) the composition of the target, b) the characteristics of the produced plasma, c) the effects produced by ultrasound, d) the combined effects due to b) and c), e) the chemical reactions between the metals and water, f) the chemical and photochemical reactions between the nanostructures produced and water and g) the photothermal dissociation of water due to the laser pulse. According to data presented in Fig. 3, the alloy containing more Al produces higher amounts of hydrogen whereas the alloy with the lower Al content produces the lowest amount of H2, no matter if the ultrasound is present or not. The results for Al and Mg show a very different behavior, without ultrasound Mg produces approximately 50% more H2 than Al being even better than the alloy 9 whereas under ultrasound the H2 production of both metals remains below the results obtained with the alloys. Therefore, the effect of the composition of the target is evident at least qualitatively. The effect of the produced plasma seems to be also clear because in general terms as the laser fluence increases the plasma density and the plasma temperature increases too, consequently an increase in water dissociation is expected. It is worth mentioning that hydrogen production increases linearly as a function of the irradiation time for all metals at the different laser fluences. On the other side, the enhancement hydrogen production when the ultrasound is applied might be explained in terms

a)

H2 14

O2

Response (arb. units)

12 10 8 6 4 2 0

0

200

400

600

800 1000 1200 1400 1600

Time (sec) b) Fig. 2. a) Gas chromatogram and b) mass spectra of the gas samples produced by laser ablation of the three Al-Mg alloys.

production rate varies from 17.0 μmol/min at the lowest fluence to 31.5 μmol/min at the highest fluence. Intermediate values were obtained with the alloy 4 whereas aluminum produces the lowest amounts of H2. In this case, the highest H2 production was reached using only Mg, the amounts of H2 were 162 to 335 μmol for the lowest and highest

Table 1 Amount of hydrogen produced using the different targets as function of the laser fluence. Laser fluence (J/cm2)

Hydrogen produced (μmol) Alloy 0.42

Alloy 4 Ultrasound

27 40 55 65 77

44.5 88.9 111.2 124.5 133.5

75.6 177.9 244.7 266.9 289.2

Alloy 9 Ultrasound

78.5 103.7 120.5 131.7 142.9

Al Ultrasound

133.4 231.3 275.8 289.2 298.1

85.4 114.8 133.5 146.8 157.5

192

169.0 262.4 289.2 298.1 302.5

Mg Ultrasound

58.5 72.0 90.0 105.5 123.5

67.5 96.5 112.5 135.0 162.0

Ultrasound 81.0 114.0 142.0 153.0 167.5

101.0 152.5 183.0 194.0 201.0

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180

Without ultrasound

Alloy 0.42 Alloy 4 Alloy 9

3.5 3.0

120

Ablated mass (mg)

H2 produced ( mol)

150

90 Alloy 9 Alloy4 Alloy 0.42 Al Mg

60

2.5 2.0 1.5 1.0

30 30

40

50

60

70

80

0.5

2

20

Laser Fluence (J/cm )

30

40

50

60

70

80

2

Laser fluence (J/cm )

a)

a)

With ultrasound

350 300

H2 produced ( mol)

H2 produced ( mol)

300 250 200 150 Alloy 9 Alloy4 Alloy 0.42 Al Mg

100 50 30

40

50

60

70

250 200 150

50 0.5

80

2

Laser fluence (J/cm )

Alloy 0.42 Alloy 4 Alloy 9

100

1.0

1.5

2.0

2.5

3.0

3.5

Ablated mass (mg)

b)

b)

Fig. 3. Hydrogen produced as a function of the laser fluence used to ablate the Al, Mg and the three different Al-Mg targets without (a) and with (b) the presence of ultrasound.

Fig. 4. a) Ablated mass as a function of the laser fluence and b) H2 production as a function of the Al-Mg alloys ablated mass.

of a combined effect in which the formation of ablation cavitation bubbles is increased by the simultaneous application of the ultrasonic field. Additionally, ultrasound could cause effects such as the activation of the highly reactive nanomaterials of the ablated metals (Al or Mg) formed as a result of the target ablation process in water which can act as catalysts to produce hydrogen. It is well known that Al, Mg and its alloys react with water through hydrogen displacement chemical reactions producing as products H2 and oxides (hydroxides), however, such reactions do not occur because these metals are passivated by an oxide layer on their surface. This protective layer can be removed also by laser ablation at low fluences, below the threshold for plasma formation, in combination of ultrasound effects allowing these chemical reactions occur releasing H2. Some experiments in this line have been performed with good results producing roughly four times more hydrogen in the same time [20]. Among the advantages of the proposed procedure the following can be mentioned: firstly, the hydrogen source is water which is abundant on earth, consuming aluminum, magnesium or their alloys which are cheaper raw materials than other compounds used for hydrogen generation by other methods; secondly the simplicity of the procedure and

the very low amounts of material required. Fig. 4a) shows the Al-Mg ablated mass as a function of the laser fluence. As it is expected, with the three alloys a monotonically increasing behavior is observed. The interesting point here is that the amount of ablated mass is very low, ranging from 0.7 to 3.2 mg. This is the amount of the Al-Mg alloy required to produce in 5 min from 75.6 to 289.2 μmol of H2 under ultrasound ablating the alloy 0.42. In fact, Fig. 4b) shows the dependence of the H2 production on the ablated mass. It is seen that for ablated masses lower than 2.1 mg the H2 production increases at higher rates close to 146.8 μmol/mg for the alloy 0.42 whereas for ablated masses greater than 2.1 mg the H2 production increases at a lower rate close to 40.0 μmol/mg for the same alloy. These results suggest that higher laser fluences produces less H2 which can be attributed to the higher amount of ablated material dispersed in the water, forming the colloidal suspension, that blocks partially the laser beam diminishing the amount of energy incident onto the target. Considering that the alloy 9 produces a maximum of 302.5 μmol with 2.6 mg of material in 5 min, this can generate approximately 23.2 mmol/min/g which seems to be potentially very attractive. This amount of hydrogen corresponds to approximately 500 mL/min per gram; this value is greater than the values reported using Al or its alloys using other methods [38,39]. However, it 193

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according to the 00-048-0601 JCPDS card (magenta lines). The presence of bands instead of sharp peaks reveal the nanoscale dimensions of the obtained material. The average crystallite size was estimated using the Scherrer equation, D = 0.9λ/β cos θ [42]. The obtained results, using the peak at 23.1° attributed to the (006) plane of Mg2Al (OH)7, reveal a crystallite size of 7.6 nm. An interesting feature of the colloids obtained at the higher laser fluences (65 and 77 J/cm2) under ultrasonic field is the intense luminescence that they present. We are interested in the luminescent response of this material because its effective atomic number (Zeff = 9.9) is very close to the one of biological tissue making this material potentially attractive in personal dosimetry. It is important to point out that samples prepared without and with ultrasonic field at laser fluences lower than 65 J/cm2 did not show luminescence. As shown in Fig. 6a), the colloidal solution prepared at 65 J/cm2 exhibits a high intensity emission in a broad excitation range from 270 to 350 nm. The maximum emission occurs at an excitation wavelength of 303 nm in a broad wavelength

Table 2 Elemental content of the nanomaterials formed at different fluences. Fluence (J/cm2)

Al (at.%)

27 40 55 65 77

12.8 12.7 13.8 16.7 18.0

± ± ± ± ±

0.9 0.9 1.2 0.8 0.9

Mg (at.%)

O (at.%)

34.0 34.1 32.8 29.0 27.4

53.2 53.2 54.6 54.3 53.4

must be pointed out that higher values have been reported [40,41]. Compared with methods employing laser radiation, in general terms, our approach seems to generate higher amounts of hydrogen [12–17]. The present approach to generate hydrogen can be considered as a bifunctional procedure because at the same time it can be used to produce nanomaterials with different shape and size. In this case, some of the obtained nanostructures using the Al-Mg alloy 0.42 were characterized by the EDS, SEM, PL and TEM techniques. In order to determine the composition of the obtained nanostructures, a drop of each of the prepared colloids was deposited on pieces of silicon wafers for EDS and SEM measurements. The EDS measurements were carried out at 5 points in each sample reporting the average of the obtained values. Table 2 shows the atomic content of aluminum, magnesium and oxygen in the samples as a function of the laser used fluence. The oxygen content remains almost constant around 53 at.%. For fluences lower than 55 J/cm2 the Al and Mg contents are approximately constant whereas for higher fluences the Al content decreases in approximately 20% and the Mg increases slightly. The target and the obtained nanostructures were characterized by X-ray diffraction. The diffraction pattern of the target included in Fig. 5a) shows peaks at 20.6, 23.8, 31.7, 34.0, 36.1, 38.2, 40.1, 42.0, 43.7, 47.2, 48.8, 50.4, 52.0, 53.5, 55.0, 56.5, 58.0, 59.4, 60.8, 62.2, 64.9, 66.3 and 67.6° (green circles) corresponding to the cubic crystalline structure of the Al12Mg17 intermetallic alloy (01-073-1148 JCPDS card). In addition of these reflections, six peaks (red stars) can be clearly observed with good intensity at 32.2, 34.4, 36.6, 47.8, 57.4, 63.1 and 68.6° attributed to metallic Mg (00-035-0821 JCPDS card). Therefore, the alloy used as target is composed by a mixture of Al12Mg17 and Mg. The diffraction pattern of the nanostructures obtained at a laser fluence of 65 J/cm2 is shown in Fig. 5b). For this purpose, the colloidal solution was centrifugated and afterwards dried in order to obtain the material in powder form. This diffraction pattern shows bands peaking at 23.1, 34.7, 38.9, 45.1, 46.2, 61.2 and 62.4° corresponding to a Magnesium Aluminum Hydroxide, Mg2Al(OH)7,

emission (

= 300 nm)

exc

excitation (

= 357 nm)

em

PL intensity (arb. units)

357 nm

303 nm

2

F = 65 J/cm

250

300

350

400

450

Wavelength (nm) a)

With ultrasound Without ultrasound

PL intensity (arb. units)

357 nm

2

F = 65 J/cm

332 nm H2O

x3 406 nm

350

400

450

500

550

Wavelength (nm) b) Fig. 5. XRD pattern of the a) Al-Mg alloy 0.42 used as target, b) NPs obtained by laser ablation in liquid media using the Al-Mg alloy 0.42.

Fig. 6. a) PL excitation and emission spectra of the Mg2Al(OH)7 nanostructures, b) PL emission of the sample obtained at 65 J/cm2 under excitation at 303 nm. 194

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range, from 330 to 400 nm, with its maximum at 357 nm. The PL spectra corresponding to samples upon excitation at 303 nm with and without ultrasonic field are shown in Fig. 6b). Photoluminescent emission is observed around 357 nm for the sample with ultrasound whereas signals at 332 and 406 nm, attributed to the water of the colloidal solution, are detected for samples without ultrasound. The origin of this behavior is unclear at present, we may speculate with the presence of size and shape effects affecting the processes behind the absorption and emission wavelengths and additional work is underway to investigate this property. Fig. 7a) shows the SEM micrographs of a sample prepared with ultrasound at 65 J/cm2. The presence of structures with sheet-like appearance and regular shape is revealed. Additionally, the presence of some scattered spherical nanoparticles is observed. These results suggest that AlMg oxide flakes are obtained with ultrasound. Fig. 7b) shows a TEM image of the nanostructures obtained when laser ablation in liquid media is performed in presence of the ultrasonic field in which 2D nanostructures are clearly detected. In this case nanosheets have sizes ranging from 10 to 100 nm roughly and exhibit regular shapes. Additionally, this TEM image reveals the presence of regions in which several layers are overlapped. The inset in Fig. 7b) displays the electron diffraction pattern of the TEM image showing the typical ring diffraction pattern characteristic of a polycrystalline material revealing that the 2D nanostructures are crystalline. Fig. 7c) shows a TEM image of the nanostructures obtained without ultrasound, in this case agglomerated nanostructures are present in which some nanoflakes are observed, suggesting that ultrasound favors the 2D nanostructures formation in agreement with previous reports [32,33]. 4. Conclusions The combination of liquid laser ablation with an ultrasonic field for hydrogen production from water splitting using Al, Mg and some Al-Mg alloys was successfully implemented. The gas chromatography and mass spectrometry techniques allowed the detection and identification of hydrogen as the main produced gas with lower amounts of O2 indicating that hydrogen of high purity is obtained. According to our measurements the dissolved gases present in the water do not contribute in an important manner to the amount of hydrogen production reported here. The results reveal that the presence of the ultrasonic field enhances approximately by 100% the hydrogen production. This is attributed to a combined effect by the simultaneous laser and ultrasonic cavitation bubbles generation. An important advantage of the proposed H2 production procedure is the low amount of mass consumed which leads to maximum production rates close to 23.2 mmol/min/g, comparable or even higher than previously reported values. The produced nanomaterials in colloidal form were characterized revealing the formation of Mg2Al(OH)7 nanostructures, in some cases luminescence emission from the ablated material was found. Acknowledgments This work was partially supported by the CONACYT, Mexico Project 240998. The authors thank Arturo Olalde and Albina Gutierrez for their technical assistance. F. González-Zavala thanks to SEP-PROMEP of México for its support. References [1] H.Z. Wang, D.Y.C. Leung, M.K.H. Leung, M. Ni, A review on hydrogen production using aluminum and aluminum alloys, Renew. Sust. Energ. Rev. 13 (2009) 845. [2] O.V. Kravchenko, L.G. Sevastyanova, S.A. Urvanov, B.M. Bulychev, Formation of hydrogen from oxidation of Mg, Mg alloys and mixtures with Ni, Co, Cu and Fe in aqueous salt solutions, Int. J. Hydrog. Energy 39 (2014) 5522. [3] Mei-Qiang Fan, Fen Xua, Li-Xian Sun, Studies on hydrogen generation characteristics of hydrolysis of the ball milling Al-based materials in pure water, Int. J. Hydrog. Energy 32 (2007) 2809. [4] O.V. Kravchenko, K.N. Semenenko, B.M. Bulychev, K.B. Kalmykov, Activation of aluminum metal and its reaction with water, J. Alloys Compd. 397 (2005) 58.

Fig. 7. a) SEM micrograph of sample prepared with ultrasound at 65 J/cm2, b) TEM image of the nanostructures obtained in the presence of ultrasound, the inset shows the electron diffraction pattern of the TEM image revealing a ring diffraction pattern, c) TEM image of the nanostructures obtained without the presence of ultrasound. 195

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