Green Synthesis and Characterization of Gallium(III) Sulphide (α-Ga2S3) Nanoparicles at Room Temperature

Two different batches of Gallium (III) sulphide nanocrystals, (α-Ga2S3)1 and (α-Ga2S3)2 were synthesized at room temperature by the reaction of Gallium (III) chloride with sodium thiosulphate in water for 10 and 20 min respectively. The resultant nanoparticles were characterized by different spectroscopic techniques. TEM micrographs showed well-defined, close to hexagonal particles, and the lattice fringes in the HRTEM images confirmed their nanocrystalline nature. The sizes of (α-Ga2S3)1 and (α-Ga2S3)2 were 12 and 35 nm respectively with similar morphologies. Optical band gap energies (3.43 eV/3.41 eV) and photoluminescence peaks 635/641 nm (red shift) and 414/420 nm (blue shift) of the synthesized α-Ga2S3 nanocrystals suggest that they may be promising photocatalysts. Raman spectra for the α-Ga2S3, shows very sharp bands at 119, 135 and 148 cm-1 due to Ga-S2 scissoring.


Introduction
Synthesis of shape and size control semiconductor nanoparticles has been intensively pursued because of their size-dependent characteristics, and their novel electronic, magnetic, optical, chemical, and mechanical properties that cannot be obtained in their bulk counterparts [1][2][3][4]. A wide range of approaches for synthesis of nanoparticles have been reported, such as the hydrothermal method, solvothermal methods and low temperature synthesis (co-precipitation, sol-gel encapsulation), which are not environmentally friendly [5][6]. Researcher are evaluating different green synthesis routes to synthesize nanoparticles. Group III sulphides are an interesting class of materials with promise in photovoltaic and optoelectronic applications [7]. Gallium sulfide (Ga 2 S 3 ) is an important semiconducting material because of its wide band gap (3.05 eV at 300 K), which leads many valuable properties, making it attractive in photoelectric device, electrical sensor, and nonlinear optical applications. Many synthetic routes have been reported for the synthesis of gallium sulphide, Hu et al. [8] synthesized gallium sulphide nanotubes by annealing the natural lamellar precursor in argon, whereas, Gautam et al., [9] used laser irradiation and thermally induced exfoliation method to get the nanocrystals from bulk gallium sulphide. Ga 2 S 3 nanoclusters encapsulated in Zeolite Y have also been synthesized [10]. Both the groups of Shen and Panda have used thermal evaporation process to synthesized Ga 2 S 3 nanobelts [11]. However, there is no published report or controlled research data on synthesis of Ga 2 S 3 nanoparticles in aqueous medium (green solvent) at room temperature. In this study, we have synthesized α-Ga 2 S 3 nanoparticles at room temperature by the reaction of gallium(III) chloride and sodium thiosulphate without using any toxic organic surfactants and solvents.

Experimental
Gallium (III) chloride and sodium thiosulphate were purchased from Sigma-Aldrich. Deionized water from a MilliQ® system was used in all the experiments. Synthesis was carried out inside a glove box under N 2 atmosphere. An ice cooled aqueous solution of gallium(III) chloride 3.82 g (0.02 mol) and ice cooled aqueous solution of sodium thiosulphate 4.74 g (0.03 mol) were mixed together in a round-bottomed flask, placed in an ice-water bath. After mixing the solution, the round botom flask was removed from the ice-water bath and stirred at room temptarure for 10 min. Resulting light yellow colour colliedal solution was centrifuged at 12000 rpm for 20 min. The light yellow poweder of gallium sulphide (Ga 2 S 3 )1 was washed several times with deionized water and dried in a vaccume oven. A similar method has been used to synthesize (Ga 2 S 3 )2, but here the round bottom flask was stirred for 20 min. The synthesized nanoparticles were characterized by XRD using a PANalytical X'Pert Pro X-ray diffractometer with Cu Kα radiation (λ=1.54 Å) and field emission transmission electron microscope (FETEM-JEM 2100F). Raman spectra over the spectral range 100-800 cm −1 were taken at room temperature using an LABRAM Dilor spectrometer (Jobin Yvon Horiba Group) equipped with a triple monochromator; liquid nitrogen cooled charge-coupled device (CCD) detector. The UV-Vis absorption spectra were recorded using a Perkin Elmer spectrometer. The photoluminescence spectra were measured on a Perkin Elmer LS-55 fluorescence spectrometer.

Results and Discussion
(α-Ga 2 S 3 )1 and (α-Ga 2 S 3 )2 nanoparticles were prepared by the reaction of gallium chloride and sodium thiosulphate for 10 and 20 minuts respectively , at room temperature.  Where K =0.94, λ is the X-ray wavelength and β 2 is the full width at half maximum of the XRD selected diffraction peak and  is diffraction angle. The particle size of the synthesized nanocrystal is found to be 12 nm and 30 nm for (-Ga 2 S 3 )1 and (-Ga 2 S 3 )2 respectively.
The opticl absorption spectra of dispersed (-Ga 2 S 3 )1 and (-Ga 2 S 3 )2 were measured in the region 200-800 nm at room temperature. The optical spectra can be used to calculate an approximate direct band gap using the Tauc relation [14].

Nano Hybrids Vol. 6 α = A(hν -Eg)/2hν
Where α is absorption coefficient, Eg is the absorption band gap, and A is a constant. The optical band gaps were obtained 3.43 eV and 3.41 eV respectively for (-Ga 2 S 3 )1 and (-Ga 2 S 3 )2. The band edges for both samples are blue-shifted in relation to the bulk material and agree with those reported values. The particle size of nanocrystals is also calculated using Brus equation [15]: And , where m e * is the effective mass of the electron (0.19 me), m h * is the effective mass of hole (0.8 mh), R is the radius of the particle, e is the dielectric constant (5.7) and ε 0 is the permittivity of free space. The particle size of the Ga 2 S 3 nanoparticles as estimated using the The photoluminescence spectras of (α-Ga 2 S 3 )1 and (α-Ga 2 S 3 )2 are shown in Fig. 4, and were meausred at the wavelenght range of 300 to 800 nm. For (α-Ga 2 S 3 )1 two broad emission peaks at 414 (blue emission) and 635 nm (red emission) were observed. For (α-Ga 2 S 3 )2 two broad emission peaks were observed at 420 and 641 nm; the red emission is broader than blue emission, and the intensity of this emission peak is higher than red emission. Nano Hybrids Vol. 6

Conclusion
Different techniques used to synthesize nanoparticles such as the hydrothermal method, solvothermal methods and low temperature synthesis (co-precipitation, sol-gel encapsulation), involve the use of toxic, hazardous chemicals (organic solvents, surfactants), high temperature treatment etc., which may pose potential environmental and biological risks. In the present study we synthesized pure α-Ga 2 S 3 nanocrystals by a simple reaction between gallium (III) chloride and sodium thiosulphate in water (green solvent) at room temperature (green environment). As expected, we observed the increasing partcile size with the increasing reaction time. The optical properties of these gallium sulphide nanocrystals are similar to the other reported reported ones in the lietrature synthesized by conventional routes. So, these nano crystals can also be suitbale for optoelectric application.