Aluminum Oxide Nanoparticles as a Photocatalyst for Water Splitting

12 Among various photoinduced hydrogen gas production techniques, photochemical catalytic water 13 splitting is promising and an ideal future energy source because of the low cost, stability, and high 14 sustainability of the reaction system features. Aluminum oxide nanoparticles ( NPS) as a photocatalyst 15 for the splitting of water into hydrogen gas using solar energy is one of the noble missions of material 16 science. In this work, pure aluminum oxide and L-methionine capped aluminum oxide NPs have been 17 synthesized using the sol-gel method. The pure Al 2 O 3 and capped Al 2 O 3 NPs with L-methionine were 18 investigated by Fourier transform infrared (FTIR), X-ray diffractometer (XRD), high-resolution 19 transmission electron microscopy (HRTEM), and Zeta potential. It was found that the average particle 20 sizes of Al 2 O 3 NPs and capped Al 2 O 3 NPs with L-methionine were 13 and 20 nm, respectively. L-21 methionine capped Al 2 O 3 NPs had a higher negative charge with a potential of - 24.5 mV. The capping 22 agent slightly improved the production of hydrogen from 44 to 60 ml at 75° C after 30 min under 23 illumination.


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][2][3][4][5].Hydrogen sources are the hydrocarbon that is 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 specific characteristics that make hydrogen the best energy carrier such as high efficiencies of production and conversion into electricity, completely renewable fuel, stored as a gas, liquid, or metal hydrides that can easily to transport via pipelines or tankers over long distances and conversion into other forms of energy, friendly environmental, low production cost, and low carbon content [1,6].The production processes of hydrogen are chemical, biological, electrolytic, thermo-chemical, photolytic, and splitting of water [8,9].
Hydrogen produces 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 is categorized into two types: photoelectrochemical and photochemical.Photochemical reactions using a semiconductor material that absorbs photons with energies major 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.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 [13][14][15].
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 [16,17].This work aims to generate hydrogen gas by Al2O3 NPs.The advantage and the novelty of Al2O3 NPs can use in visible light.Also, it has a high absorption capacity, non-toxic, highly abrasive, thermal stability 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.

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 AlCl3.6H2Ogradually in a 250 ml conical flask under magnetic stirring.After that, ammonia solution 28% was dropped until the gel was formed under a 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 sol in the presence of ethanol, and then capping agent adding before the condensation of hydroxide group's that aggregate together to produce the gel [18].

Characterization techniques
The structural identifications and the surface modification of L-methionine capped Al2O3 NPs and pure Al2O3 were NPs characterized by Fourier transform infrared (FTIR) spectrophotometer (Spectrum BX 11-LX 18-5255 Perkin Elmer).The samples were mixed with KBr and this powder was pressed to form a translucent pellet.The spectra were recorded in the wavenumber 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 scan speed 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 is 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.

Measurements of hydrogen production
The generation of hydrogen gas from Al2O3 NPs and caped Al2O3 NPs was measured using two different techniques as shown in figure (1) under dark and illumination.The system consists of a hot plate, condenser, flask.The first method was adapted by the work of Rosenband and Gany [16,17].
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 to detect 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 stable condition as a part of the calibration process.Another setup measurement system was used to estimate the hydrogen evolved by 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 [19].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 [20].

Structural analysis
The FTIR spectra of the Al2O3 NPs and capped Al2O3 with L-methionine are shown in Fig. 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 [21].The broad absorption band at 3426 cm -1 is characteristic of the stretching vibration of hydroxylates (O-H) group that is bonded to Al 3+ .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 [21].The XRD patterns of the Al2O3 NPs and capped Al2O3 shown in Fig. 2 are analyzed to determine the crystallinity.Fig. 2a the pattern of capped Al2O3 NPs which is highly crystalline and it is in great agreement with the XRD pattern of the pure α-Al2O3 NPs and γ-Al2O3 NPs powder obtained by the previous researcher [22,23].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) reflections.But the peaks of γ-Al2O3 NPs are observed at 2Ɵ values 32.16°, 44.7°, and 76.5° correspond to the diffraction planes of (013), (212), and (253) respectively Fig. 2b shows the XRD pattern of pure Al2O3 NPs leads to the formation of a mixture of α-and γalumina.γalumina formation is due to progressive dehydration and surface hydroxyl groups desorption and the distorted spinel structure base this phase [24,25].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 Al 3+ ions either by the tetrahedral or octahedral sites within the spinel structure [26].The α-Al2O3 NPs reflection 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 reflection planes appear on the patterns at the 2θ values: 44.6° and 56.7° at (122) and ( 051) respectively [27].This indication the alumina is in the form of an alpha-alumina single-phase (cubic structure) with characteristic peaks at 2Ɵ equal to 32.7677, 37.0404, 39.5720, 45.2826, 46.7282, 60.8500, and 67.4371.

Morphological properties
As shown in the HRTEM images in Fig. 3.The prepared pure of Al2O3 NPs show a lot of aggregations and a high degree of crystallinity as shown in Fig. 3a The Zeta potential of capped Al2O3 NPs with L-methionine equals -24.5 mV.The particles' surfaces charge is completely negative which means more stable.The electrostatic stability of suspension is due to strong repulsive force which reduces coalescing probability among particles charges which makes the suspension more stable in the alkaline media.This is in good agreement with the work of Zawrah [27].The Zeta potential of pure Al2O3 NPs equals -21.4 mV and it is less stable than capped Al2O3 NPs.a)

Photocatalytic activity
Fig. 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 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 NPs concentrations [20].The rates of hydrogen revolved from 0.1, 0.2, 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, 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 which is shown in Fig. 4 [14].The hydrolysis process that occurred can express as shown in the following equations 8 and 9 [27] : NPs.It is noted that there are two rates of reaction.These rates of hydrogen evolved from capped Al2O3 NPs is 3.5 and 0.47 ml/min, respectively.But the rates of hydrogen evolved from pure Al2O3 NPs is 2.75 and 0.82 V/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 Fig. 3.   Fig. 7 Amount of hydrogen sensed from 0.2 g of Al2O3 NPs using hydrogen sensor.

Conclusions
Al 2O3 NPs used as a photocatalyst in the production of hydrogen under illumination and darkness.The prepared Al2O3 NPs were polycrystalline, and the particle average sizes of capped Al2O3 NPs with Lmethionine were 13 nm and pure was 20 nm.Capped Al2O3 NPs had higher stability and yield compared to pure Al2O3 NPs, due to the presence of a capping agent which improved the stability of

Fig. 3
Fig. 3 HRTEM image and SEAD of a) pure Al2O3 NPs and b) capped Al2O3 NPs with Lmethionine.

Fig. 4
Fig. 4 Amount of hydrogen produced from a different concentration of capped Al2O3 NPs with Lmethionine.

Fig. 5
Fig. 5 Amount of hydrogen produced from 0.2 g of pure and capped Al2O3 NPs.

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