β-Ga2O3 nanostructures for photocatalytic degradation of red amaranth toxic dye

Beta gallium oxide (β-Ga2O3) microstructures composed of ∼50 nm nanoparticles were synthesized by the hydrothermal method. Using the Tauc plot method a value of ∼4.9 eV was obtained for the optical band gap of β-Ga2O3. TEM and XRD analyses revealed high crystallinity of the β-phase of gallium oxide nanostructures. Since there are few publications for the photocatalytic properties of β-Ga2O3 the obtained results contribute to better understanding of the photocatalytic effect of this material on toxic dye red amaranth. Moreover, it is shown that β-Ga2O3 is a very efficient photocatalyst leading to high percentage degradation of dyes for relatively short periods. For example, the degradation of red amaranth and rhodamine B toxic dyes under UV light irradiation reached 97% and 100% after 165 and 120 min, respectively.


Introduction
The textile industry uses approximately 10,000 tons of synthetic dyes per year [1].These synthetic dyes are toxic and discharged as effluent into the aquatic environment can cause the release of contaminated wastewater, which contains significant concentrations of pollutants [2].The molecular structure of these toxic dyes is very stable, which makes their degradation difficult in conventional water treatment plants [3].Most dyes used in the textile industry are AZO type (N=N), which are characterized by being water-soluble, stable, and resistant to reactions with chemical agents [4].Among this large family of AZO-type compounds is red amaranth (RA).Recent studies reveal the toxic effects associated with exposure to this dye, among which are tumors, allergies, and carcinogenic and mutagenic problems [5].Since this dye has significant negative consequences on the environment and human health, its elimination of industrial effluents is a priority task.
In recent years, various technologies for the treatment of wastewater containing textile toxic dyes have been investigated.Some examples of advanced oxidation methods are ozonation, electrochemical oxidation, Fenton, photo-Fenton, and photocatalysis [6].Photocatalysis is one of the most promising advanced oxidation processes for wastewater treatment and degradation of toxic dyes [7].In general, the photocatalysis reaction involves the use of a semiconductor material.When irradiated with visible and/or ultraviolet light, a process of photon absorption occurs in the semiconductor, causing electron transfer from the valence band to the conduction band, creating electron-hole pairs (e − -h + ).In an aqueous medium, the photocarriers interact with the water molecules, generating reactive oxygen species such as (OH•) radical groups, which degrade the organic molecules until they are mineralized to CO 2 , H 2 O, and inorganic ions [8].The generation of charge carriers requires the energy of the radiated light to be greater than the semiconductor band gap.A wider band gap may result in a higher electron reduction potential and/or higher hole oxidation potential, which can lead to a greater catalytic capacity [9].For this reason, wide band gap semiconductor materials, such as SiC [10], BN [11], GaN [12], NiO x [13], β-Ga 2 O 3 [14], are promising candidates for applications in photocatalysis.
Beta gallium oxide (β-Ga 2 O 3 ) is a semiconductor oxide material that has recently attracted much attention because of its electrical and optical properties, as well as its excellent stability at high temperatures [15].A band gap between 4.5 and 4.9 eV has been reported for β-Ga 2 O 3 , depending on the technique and conditions of synthesis [15].Ga 2 O 3 has been synthesized using various physical and chemical methods; however, the physical methods require ultrahigh vacuum environments and are very expensive.In comparison to the other chemical methods, the hydrothermal method is a low-cost chemical-based alternative that allows synthesis of high purity materials with good control of the chemical composition, morphology, shape and size of the synthesized material [16].Various applications of β-Ga 2 O 3 have been reported, such as UV detectors [17], power electronics [18], gas sensors [19], Schottky barrier diodes [20], field effect transistors [21], water splitting [22] and, recently, in photocatalysis [23].Although β-Ga 2 O 3 has been used as a photocatalytic material for the degradation of some dyes, such as methyl orange [24], methyl blue [25][26][27] malachite green [28] and rhodamine B [29][30][31] more studies are required, in particular for the photodegradation of AZO toxic dyes using β-Ga 2 O 3 .To the best of our knowledge there is no information reported in the literature for the photodegradation of red amaranth toxic dye using β-Ga 2 O 3 .However, degradation of RA toxic dye has been reported using ZnO [32,33] and CuO [34].
In this work, we report the photocatalytic properties of β-Ga 2 O 3 nanostructures for the photo degradation of RA toxic dye in wastewater of the textile industry.

β-Ga 2 O 3 Synthesis
The precursors solution was prepared by dissolving 1.28 g of gallium nitrate hydrate [Ga(NO 3 ) 3 • x H 2 O] (Sigma Aldrich) of 99.9% purity in 5 ml of deionized water (H 2 O DI).After that, 0.83 ml of ammonium hydroxide (NH 4 OH) at 29% was added to the above solution with stirring.The solution was transferred to a 25 ml autoclave reactor, sealed, and heated at 90 °C for 2 h.The Ga(OH) 3 precipitates were washed with deionized water several times and dried at 80 °C for 12 h.Finally, Ga(OH) 3 was calcined at 1000 °C in a muffle furnace for 1 h in air atmosphere to obtain microparticles of β-Ga 2 O 3 [35].Figure 1 depicts the flowchart for β-Ga 2 O 3 synthesis via hydrothermal method.

Characterization of β-Ga 2 O 3
The optical band gap of β-Ga 2 O 3 was determined by UV-Vis spectroscopy using a Cary model 50 spectrometer.Particle size and morphological characterization was performed by means of high-resolution transmission electron microscopy (HR-TEM) using a JEOLTM JEM 2010 microscope at 200 keV.
Structural and crystallographic information were obtained by x-ray diffraction analysis (XRD) using a Panalytical Empyrean diffractometer with Cu Kα radiation source (λ = 0.15406 nm) and a real-time multipass detector X´Celerator.The synthesized powder was scanned over the 2-theta range 20-80°at room temperature.
x-ray photoelectron spectra of β-Ga 2 O 3 powder were obtained by x-ray photoelectron spectroscopy (XPS) using a SPECS spectrometer equipped with a PHOIBOS ® 150 WAL hemispherical electron analyzer and an Al Kα x-ray source.The XPS raw data were fitted by an iterative procedure using CasaXPS peak fitting software.The

Photocatalytic degradation test
Degradation test was carried out on amaranth dye (Sigma -Aldrich A1016 -100 G) and rhodamine B dye (Sigma -Aldrich 234141-25 G) using photochemical reactor Rayonet, model RPR 100, equipped with 16 lamps of 35 W each and 253.7 nm wavelength.A 250 ml flask was filled with a solution of RA or RhB dyes at 20 ppm in water, to which 10 mg of photocatalytic material was added.The degradation experiment was carried out at a   temperature of 20 °C, monitored by a K-type thermocouple, and controlled by a closed-loop using a Peltier as a cooling element.Before turning on the UV lamps, air, as oxygen source, was bubbled through the solution.The experiment was carried out by sampling every 15 min, for a total time of 165 min for RA solution and 120 min for RhB.Degradation of the dyes was determined from the intensity of the characteristic absorption peaks at 520 nm for RA and 556 nm for RhB, respectively.Sampling was carried out using a UV-Vis spectrometer, Cary model 50.

Optical, Morphological, structural, and chemical composition properties
The optical band gap of the β-Ga 2 O 3 was estimated by the Tauc plot method from the absorption measurements.The Tauc expression relates optical band gap, E g , and absorption coefficient, α, by the following expression.
where hv is the photon energy, and n is a number that depends on the nature of the electronic transitions responsible for the absorption.For a semiconductor with a direct band gap, which is the case of β-Ga 2 O 3 [36], n equals 2. E g was determined by fitting the linear part of (αhν) 2 versus hν dependence, extrapolated to the energy axis (figure 2).The optical band gap obtained for β-Ga 2 O 3 was ∼4.9 eV, which corresponds to values reported in the literature for this material [37].TEM images in figure 3 are representative of studied samples.Figure 3 204) and (421) All the diffraction peaks obtained are in good agreement with the values reported in the literature [25][26][27] and Inorganic Crystal Structure Database (ICSD no.98-008-3645) [39].No other significant peaks were found in the diffractogram, thus excluding the existence of other phases.Table 1 shows the crystallite size of β-Ga 2 O 3 powder calculated using the Scherrer formula: Here, D is the crystallite size, λ is the x-ray wavelength, β is the full width at half maximum (FWHM) of the most intense peak and θ is the Bragg -angle in radians.Figures 5(a) and (b) Ga 2p 1/2 , Ga 2p 3/2 peaks that representing the Ga-O bonding and O 1 s XPS spectra of β-Ga 2 O 3 .The peaks at 1144.7 eV and 1117.7 eV were assigned to Ga 2p 1/2 and Ga 2p 3/2 respectively [40].To visualize the different oxygen species, present in the nanostructure material, the O 1 s peak was deconvoluted.A Voigt function fit with Gaussian-Lorentzian curves and a background type Shirley-Sherwood were used.The O 1 s region (which describes the oxygen state in metal oxides) was fitted with two partially overlapped peaks related to Ga 2 O 3 and oxygen vacancies (Ov).The first peak at 531.5 eV corresponds to Ga-O bond and the second peak at 532.8 eV was assigned to Ov [40,41].From the XPS spectra, percentage atomic concentrations were estimated from the integrated peak areas using sensitivity factors offered by CasaXPS Processing Software for analysis techniques XPS, AES, SIMS.The obtained results confirmed the formation of Ga 2 O 3 with an O/Ga atomic ratio of ∼1.5±10% from studied samples.From the XPS results it may be concluded that β-Ga 2 O 3 with a high degree of purity was obtained.

Photocatalytic degradation of toxic dyes
The photocatalytic properties of β-Ga 2 O 3 have been mainly studied using rhodamine B dye.For this reason, we performed photodegradation not only of RA but also of RhB to compare the properties of our photocatalyst to those reported in the literature.The photocatalytic activity was tested by monitoring the UV-vis absorbance spectra of RA and RhB solutions without β-Ga 2 O 3 (figures 6(a), (c)) and of RA and RhB solutions with 10 mg of β-Ga 2 O 3 photocatalyst (figures 6(b), (d)).The most intense absorbance bands of RA and RhB dye solutions are observed at wavelengths of 520 nm and 556 nm, respectively.The absorption peaks at these wavelengths gradually decrease with time due to degradation of the dyes.The degradation percentage, D % ,was determined using the follow equation: where A 0 corresponds to the initial absorbance of the dye mixture and A t , to the absorbance after t minutes.Both values are related to the initial concentrations, C 0 , and the concentration at time t, C t , which varies according to the Lambert-Beer law [42].The degradation percentage of RA and RhB dye solutions without and with β-Ga 2 O 3 photocatalyst are shown in figures 7(a) and (b).The obtained results show that by action of radiation, in absence of photocatalyst, only ∼28% of the RA was decomposed, while in the presence of 10 mg of β-Ga 2 O 3 the degradation of RA was ∼97%.These degradations were achieved in an estimated time of 165 min.The degradation of RhB without and with β-Ga 2 O 3 were ∼20% and 100% for time interval of 120 min.
Corina et al (2016) reported photocatalytic degradation of RA molecule by breaking the N=N AZO bond due to interaction with reactive radical species OH• [43].Then aliphatic carboxylic acids and sulfated phenols are generated and transformed into CO 2 and H 2 O.The degradation process is schematically presented in figure 8.The constant recirculation of air during the reaction supplies oxygen, which ensures that the photogenerated pairs (e − -h + ) do not recombine in Ga 2 O 3 [44].Moreover, the oxygen forms superoxide radicals (O 2 •− ) by interaction with electrons generated in the conduction band of β-Ga 2 O 3 .These superoxide radicals are of utmost importance since they have the ability to generate more OH• radicals, essential for the oxidation process [44].The photocatalytic reactions mechanisms are summarized as follows: In table 2, the photocatalytic activity obtained in this work is compared with results reported for Ga 2 O 3 -based catalysts for dyes degradation.It may be concluded that the photocatalytic properties of the synthesized here β-Ga 2 O 3 nanostructures are better than previously reported for degradation of RhB.Furthermore, it is shown that these nanostructures can degrade RA.The high photocatalytic activity of the nanostructures is attributed to the excellent crystalline properties of β-Ga 2 O 3 leading to a low recombination rate of the photogenerated e − -h + pairs and high electron mobility in the conduction band [45].
In addition, the comparison of the results obtained in this work for photodegradation of RA toxic dye with photodegradation caused by ZnO [32,33] and CuO [34] shows that the β-Ga 2 O 3 is more efficient photocatalyst.
Further research in the effect of the morphology of the gallium oxide microstructures on the optical properties must be addressed as a future study of this material as a catalyst.

Conclusions
β-Ga 2 O 3 microstructures were synthesized by the hydrothermal method.TEM images showed that the microstructures are composed of nanoparticles with size of about 50 nm.In aqueous solution, the size of the microstructures was reduced and individual nanoparticles of ∼50 nm were formed.XRD analysis confirmed the high crystallinity of the material and the formation of β-phase gallium oxide.XPS analysis revealed а high purity and near-stoichiometric composition of the material.The value of the optical band gap of β-Ga 2 O 3 obtained by the Tauc plot method was 4.9 eV.The photocatalytic efficiency of the β-Ga 2 O 3 nanostructures under UV light irradiation reached 97% and 100% for degradation of RA and RhB toxic dyes after 165 and 120 min, respectively.The fast photocatalytic degradation of RA and RhB can be attributed to the high crystallinity of the β-Ga 2 O 3 nanoparticles, their high purity and low density of defects.All these factors lead to high electron mobility and as a result to better photocatalytic efficiency.The obtained results show that the synthesized here β-Ga 2 O 3 nanostructures are very promising for applications in photodegradation of AZO type toxic dyes such as red amaranth.

Figure 2 .
Figure 2. UV-vis spectrum and optical band gap energy (E g ) of β-Ga 2 O 3 determined from Tauc plot method.

Figure 4 .
Figure 4. X-ray diffraction patterns of the annealed sample showing characteristic peaks of β-Ga 2 O 3 .
(a)  shows morphology of the synthesized β-Ga 2 O 3 , while a nanocluster conformed by nanoparticles with size less than 50 nm is clearly visible in figure 3(b).In figure3(c) a high-resolution image of a nanoparticle confirms the crystalline structure of β-Ga 2 O 3 .Interplanar distance of 0.28 nm was measured on (002) plane (figure3(d)), in accordance with results in[38,39].

Table 2 .
Parameters comparison of the photocatalytic activity of this work with the reported Ga 2 O 3 -based catalysts for dye degradation.