1. Introduction

    Research and technological development resort to various renewable/non-conventional energy resources become a need at present due to very high environmental impacts and limited stocks of conventional energy resources. It’s predicted that hydrogen will play a key role in shifting the global energy system towards a sustainable energy system by 2050 [1-5]. Hydrogen sources are the hydrocarbons that are produced from fossil fuels[2, 3]. Many industries are using hydrogen such as a catalyst in petroleum processing and petrochemical production, oil and fat hydrogenation, using in fertilizer production, Raney nickel catalyst, oxygen scavenger, in addition as a fuel [6, 7]. The characteristics that make hydrogen the best energy carrier are: high efficiencies of production and conversion into electricity, completely renewable fuel; storage as a gas, liquid, or metal hydride that can easily be transported via pipelines or tankers over long distances; conversion into other forms of energy[1, 6]. The production processes of hydrogen are chemical, biological, electrolytic, thermo- chemical, photolytic, and splitting of water [8, 9].

Hydrogen is produced from the splitting of water through water electrolysis, alkaline electrolysis, polymer electrolyte membrane electrolysis, photocatalysis, photo-biological production, high- temperature water decomposition, and thermo-chemical [10, 11]. Photocatalytic water splitting reactions are categorized into two types: photoelectrochemical and photochemical. Although the underlying principles of photochemical and photoelectrochemical systems are similar, their structural arrangements vary [12]. The difference is that both hydrogen evolution reaction and oxygen evolution reaction half-reactions happen in the suspending photocatalyst particles in the suspension system, resulting in the evolution of a mixture of oxygen and hydrogen. In contrast, water redox reactions in the photoelectrochemical system are performed on two different surfaces (photoanode and photocathode) respectively, which to a large extent simplifies the subsequent gas separation process and promotes the engineering production of hydrogen or oxygen[13, 14]. Photochemical reactions use a semiconductor material that absorbs photons with energies greater than their bandgap energy and light energy is directly used to execute the chemical reaction[10, 12]. In photochemical reactions, the factors that affect the reaction behavior of hydrogen production are crystal structure, operating temperature, bandgap, light intensity, and the pH of the solution. Several essential conditions must be met for the Photochemical water splitting process: (1) strong light absorption; (2) rapid charge separation; (3) low redox potential; and (4) acceptable stability. To now, researchers from several fields throughout the world are investigating novel material systems that meet all of these characteristics [15].

Titanium dioxide is the most common semiconductor used in hydrogen production, however, TiO₂ has rapid recombination of photogenerated electron-hole pairs, wide bandgap limits TiO₂ use in the visible light region, and large over the potential for hydrogen evolution on TiO₂ surface [16-18]. Valery Rosenband et al. presented a parametric investigation of aluminum water reaction to generate hydrogen, using a novel activated aluminum powder. An initial thermo-chemical process involving a small fraction of a lithium-based activator induced a spontaneous reaction of the activated aluminum particles with water [19, 20]. This work aims to generate hydrogen gas by Al₂O₃ NPs. The advantage and the novelty of Al₂O₃ NPs can be used in visible light. Also, it has a high absorption capacity, non- toxic, highly abrasive, thermally stable, and inexpensive. Adding L-methionine as a capping agent improves the properties of Al₂O₃ NPs and provides high hydrogen gas yields more than pure Al₂O₃.

2. Materials and Methods

2.1.   Materials 

Ethanol (95%) was received from Carlo ERBA Reagents Spain. Ammonia solution (28%) was purchased from Chem Solute Germany, L-methionine was supplied by Bio Basic Canada INC. Aluminum chloride hexahydrate was bought from Panreac Quimica SA Barcelona.

2.2.  Preparations of Al₂O₃ NPs

    An ethanolic solution of 0.1 M AlCl₃.6H₂O was prepared by adding 150 ml of ethanol to 3.6 g of AlCl₃.6H₂O gradually in a 250 ml conical flask under magnetic stirring. After that, an ammonia solution of 28% was dropped until the gel was formed under continuous stirring for 3 min. The gel could maturate for 30 hrs at room temperature and then the gel was dried at 100 ^°C for 24 hrs in the oven. The formed gel was calcined for 2 hrs at 1000^°C with a heating rate of 20 ^°C / min. Finally, Al₂O₃ NPS was formed. For preparing capped Al₂O₃ NPS with L-methionine, the same procedure above was carried out by adding 0.1 g of L-methionine as a capping agent to the AlCl₃ solution. The mechanism of formation of Al₂O₃ NPs can be explained by hydrolysis of the aluminum chloride hexahydrate to produce the solution in the presence of ethanol, and then capping agent added before the condensation of hydroxide groups that aggregate together to produce the gel [21].

3. Characterization Techniques

    The structural identifications and the surface modification of L-methionine capped Al2O3 NPs and pure Al2O3 ANPs were characterized by Fourier transform infrared (FTIR) spectrophotometer (Spectrum BX 11- LX 18-5255 Perkin Elmer). The samples were mixed with KBr and this powder were pressed to form a translucent pellet. The spectra were recorded in the wave number range of 4000-400 cm⁻¹. The crystalline structures of the prepared materials were evaluated by XRD analysis (X-ray 7000 Shimadzu-Japan) at room temperature. The Bragg angle (2θ) in the range from 5 to 80 degrees was varied to determine the degree of crystallinity of the prepared samples. The X-ray source was Cu target generated at 30 kV and 30 mA with a scan speed of 4deg/min. The microstructure was investigated using (HR-TEM), JEM-2100. Samples were prepared by dispersing 5 mg of a powder sample in 5 ml of ethanol and sonicated for 10 min. A drop of this colloidal solution was evaporated on a copper grid, which was coated with carbon and investigated. The charge of the prepared materials was measured using a Zetasizer Malvern Nano-ZS. The suspension was placed in a universal folded capillary cell attached to platinum electrodes.

4.  Measurements of Hydrogen Production

    The generation of hydrogen gas from Al₂O₃ NPs and capped Al₂O₃ NPs was measured using two different techniques under dark and illumination. The system consists of a hot plate, condenser, and flask. The first method was adapted from the work of Rosenband and Gany [19, 20]. The hydrogen gas released was passed through a condenser and then measured in an inverted burette by water displacement. The second method is applied by using a hydrogen sensor. The hydrogen sensor module was prepared by coding the Arduino microcontroller, which is utilized to measure the hydrogen in parts per million (ppm). Arduino Uno R3 is the microcontroller programmed to be used as a medium of interaction by receiving input from the hydrogen sensor module and sending output to the laptop for data recording purposes. The coding commands Arduino to display the hydrogen reading in ppm every second. A hydrogen-harvesting jar was built by attaching the MQ-8 hydrogen sensor module, the hydrogen sensor just below the cover, and the Arduino Uno R3 on the top. Then, all the holes of the cover were sealed with hot glue to ensure the most precise reading of hydrogen. This sensitive hydrogen sensor module which can measure from 10 to 1000 ppm is responsible for detecting the hydrogen produced in the hydrogen jar. The produced amount of hydrogen gas value is only taken once the readings from the hydrogen sensor are found to be in a steady state condition. Another setup measurement system was used to estimate the hydrogen evolved by the hydrogen sensor MQ-8- Arduino Uno R3 controller connected to the laptop. The hydrogen sensor is utilized to measure hydrogen in the range from 10 to 10000 ppm [22]. The hydrogen reading is displayed in ppm every second by Arduino coding. The produced amount of hydrogen gas value is only taken once the readings from the hydrogen sensor are found to be in stable condition as a part of the calibration process. This technique was adopted by the work of  Miskon, Thanakodi, Shiema, and Tawil [23].

5.  Results and Discussion

5.1. Structural Analysis

    The FTIR spectra of the Al₂O₃ NPs and capped Al₂O₃ with L-methionine are shown in Figure 1. The two  spectra nearly have the same absorption peaks. The broad and smooth absorption band in the wavenumber range from 500-1000 cm^-1 reveals the formation of alumina at 567 cm^-1 represents the Al-O-Al bond and the peak recorded at 754 cm^-1 is due to the stretching vibration of Al-O. The absorption band around 1638 cm^-1 indicates the bending mode of water molecules which agreed with the presence of the OH group in the prepared samples [24]. The small absorption peak observed at 2369 cm^-1 is probably owing to the CN stretching. This band has been formed due to the reaction of ethanol and NH₃. The broad absorption band at 3426 cm^-1 is characteristic of the stretching vibration of the hydroxylates (O-H) group that is bonded to Al³⁺. The absorption peak presented at 1388 cm^-1 confirms that there are slight changes in the surface chemistry of Al₂O₃ when the capping agent of L- methionine was added. This band indicates that the surface binding of Al₂O₃ NPs capped with L- methionine is formed via the NH₂ group and it is accepted by Alam [24].

Figure1. FTIR spectra of Al2O3 NPs and capped Al2O3.

    The XRD patterns of the Al2O3 NPs and capped Al2O3 shown in Fig. 2 are analyzed to determine the crystallinity. Figure 2a the pattern of capped Al2O3 NPs which is highly crystalline and is in great agreement with the XRD pattern of the pure α- Al2O3 NPs and γ- Al2O3 NPs powder obtained in a previous work [25, 26]. X-ray patterns of α-Al2O3 NPs with major peaks at 24.8º, 34.5°, 37°, 42.6°, 52°, 56.8°, and 66°, respectively attributed to the (111), (122), (113), (132), (015,) (134), and (060) diffractions. But the peaks of γ-Al2O3 NPs are observed at 2θ values 32.16°, 44.7°, and 76.5° correspond to the diffraction planes of (013), (212), and (253), respectively.

Figure 2b shows the XRD pattern of pure Al2O3 NPs leads to the formation of a mixture of α- and γ- alumina. γ- alumina formation is due to progressive dehydration and surface hydroxyl group desorption and the distorted spinel structure base this phase [27, 28]. The peaks are broad, and profiles are diffused indicating the presence of small crystalline grains and compositional fluctuations. This is consistent with the location of the Al³⁺ ions either by the tetrahedral or octahedral sites within the spinel structure[29]. The α-Al2O3 NPs diffractions planes appear on the patterns at the 2θ values 34.3°, 42.6°, and 66.4° at (013), (132), and (330), respectively. at The γ-Al2O3 NPs diffraction planes appear on the patterns at the 2θ values: 44.6° and 56.7° at (122) and (051) respectively [30]. This indicates the alumina is in the form of an alpha-alumina single-phase (cubic structure) with characteristic peaks at 2 θ equal to 32.8°, 37.0°, 39.6°, 45.3°, 46.7°, 60.9°, and 67.4°.

Figure 2.  XRD pattern of Al2O3 NPs with a) capped Al2O3 NPs and b) Al2O3 NPs.

 5.2.  Morphological Properties

    As shown in the HRTEM images in Figure 3. The prepared pure Al₂O₃ NPs show a lot of aggregations and a high degree of crystallinity as shown in Fig. 3(a, b, c, and d). The particle average sizes of Al₂O₃ NPs were 20 nm. Fig. 3(e, f, and g) shows the HRTEM of capped Al₂O₃ NPs which indicates that the spherical-shaped particles formed are homogenous and have less aggregation because of the proper coverage due to the capping agent proper coverage during the nucleation initial stages which prevents  the particles from coarsening. The particle average size of capped Al₂O₃ NPs with L-methionine was  13 nm [30].

Figure 3. HRTEM image and SEAD of pure  Al₂O₃  NPs (a, b, c and d) capped  Al₂O₃  NPs with L- methionine (e, f, and g).

The Zeta potential of capped Al₂O₃ NPs with L-methionine equals - 24.5 mV. The particles' surface charge is completely negative which means more stable. The electrostatic stability of suspension is due to strong repulsive force which reduces coalescing probability among particle charges which makes the suspension more stable in the alkaline media. This is in good agreement with the work of Zawrah [30]. The Zeta potential of pure Al₂O₃ NPs equals - 21.4 mV and it is less stable than capped Al₂O₃ NPs.

5.3. Photocatalytic Activity    

Figure 4 presents the different concentration effects of capped Al₂O₃ NPs with L- methionine (0.1, 0.2, 0.3, and 0.4 g) suspended in 50 ml H₂O under indoor natural light for 30 min at 75°C at a pH of 7.5 on the hydrogen evolved and measured by burette. It is noted that after 30 min, the values of hydrogen evolved from 0.1, 0.2, 0.3, and 0.4 g were 60.5, 60, 36.5, and 20.5 ml, respectively. The rate of the reaction depends on the capped Al₂O₃ NP concentrations [23]. The rates of hydrogen revolved from 0.1, 0.2, and 0.4 g were 2.02, 2.07, and 0.7 ml/min, respectively. The rates of hydrogen revolved from 0.3 g were 1.9, and 0.88 ml/min, respectively. The rate of hydrogen evolved from 0.2 g is the highest yield especially after 15 min due to its low concentration which increases specific surface and makes the reaction faster as shown in Figure 4 [17]. The hydrolysis process that occurred can be expressed as shown in the following equations 1 and 2 [30]:

𝐴𝑙₂ 𝑂_{3 +} 𝐻₂𝑂 → 2 𝐴𝑙𝑂𝑂𝐻                                      (1)

𝐴𝑙𝑂𝑂𝐻 → 𝐴𝑙₂ 𝑂₃ + 1⁄2𝑂₂  + 𝐻₂                            (2)

Figure4.  Amount of hydrogen produced from a different concentration of capped Al2O3 NPs with L- methionine.

Figure 5 compares the hydrogen evolved from a concentration of 0.2 g of capped Al₂O₃ NPs with L- methionine and pure Al₂O₃ NPs suspended in 50 ml H2O under indoor natural light for 30 min at 5°C at and pH of 7.5. It is noticed that after 5 min capped Al₂O₃ NPs have the same yield as pure Al₂O₃ NPs. It is noted that there are two rates of reaction. These rates of hydrogen evolved from capped Al₂O₃ NPs are 3.5 and 0.47 ml/min, respectively. However, the rates of hydrogen evolved from pure Al₂O₃ NPs are 2.75 and 0.82 ml/min, respectively. The effect of the capping agent on the hydrogen evolved improving the particles to become finer, less agglomerated, and more uniform as presented in HRTEM in Figure 3.

Figure5. Amount of hydrogen produced from 0.2 g of pure and capped Al2O3 NPs.

 

The hydrogen evolved amounts from 0.2 g of capped Al₂O₃ NPs with L- methionine suspended in 50 ml H2O under indoor natural light and dark for 25 min at 75° C and pH of 7.5 are shown in Figure 6. The rates of the amount of evolved hydrogen of capped Al₂O₃ NPs under indoor natural light and dark were 2.5 and 2.4 ml/min, respectively. When the temperature of 75° C was used under indoor natural light and dark, it was observed that the rate of evolved hydrogen under indoor natural light was nearly the same as under dark. As a result of the presence of visible light, a little acceleration of the reaction was observed. It can be concluded that this reaction is independent of light. This result is in agreement with the work performed by Deng [30]. Figure 7 presents the amount of hydrogen evolved from 0.2 g of capped Al₂O₃ NPs with L- methionine suspended in 50 ml H₂O under indoor natural light for 40 min at 75^° C and pH of 7.5 measured using hydrogen sensor. From the figure, the maximum peak appeared at 866 ppm after 9.5 min. This result is in good agreement with the work performed by Miskon [22].

Figure 6. Amount of hydrogen produced from 0.2 g of Al₂O₃ NPs under indoor natural light and dark.

Figure 7. Amount of hydrogen sensed from 0.2 g of Al₂O₃ NPs using hydrogen sensor.