Aqueous based reflux method for green synthesis of nanostructures: Application in CZTS synthesis

Graphical abstract

At higher temperatures the Cu-rich phases are prone to form, leading to a greater number of defected particles. Tin sulphide is volatile and goes out of the system at temperatures above 450 8C [17].
Higher the temperature, more the defected particles and at temperatures above 550 8C, CZTS decomposes. Thus synthesis of CZTS at lower temperatures is preferable. Therefore, reflux method would be ideal as it can supply continuous energy for long time periods at a constant temperature.

Method description
A typical reflux setup is shown in the schematic[ 8 _ T D $ D I F F ] (Fig. 1). The reaction vessel (usually a round bottom flask) is fitted with a Liebig or Vigreux condenser which prevents the solvent vapours from escaping the system. The condenser has an outer jacket through which a coolant (usually chilled water) is circulated. The circulating water takes the temperature off the solvent vapours and in turn the vapours condense and fall back into the reaction vessel. Thus the total volume of the solution remains same and therefore can be heated over long time periods. In the reflux method the reaction temperature is the boiling point of the solvent used. Therefore, based on the need, the solvent can carefully be chosen to suit the reaction. If higher temperatures are needed an oil bath or a sand bath is employed. Often times a magnetic stirrer is used to stir the contents of the reaction vessel in order to uniformly distribute the heat.
The starting materials used are CuCl 2 , ZnCl 2 , SnCl 2 Á2H 2 O and thio-urea. All the materials are pure and used without any further purification. Separate aqueous solutions of 1 molar Cupric Chloride, 0.5 molar zinc chloride, and 0.5 molar stannous chloride were prepared. Two molar aqueous solution of thio-urea is taken into the rbf and to it the tin chloride solution was added and stirred well till the solution turned clear. To it the Cu and Zn solutions were added simultaneously. The solution was refluxed for 8 h and was allowed to gradually cool to room temperature. Initially the solution was milky white and upon heating turned greyish-black. The products were filtered, washed with distilled water and ethanol several times. Then the obtained powder was dried under vacuum.
The reaction mechanism:

Validation of the method
In order to confirm the formation of CZTS and in turn validate the method, characterization of the material was carried out. Phase of the material is established by X-ray diffraction analysis and Raman spectroscopy. The bandgap of the material is determined from the UV-Vis absorption studies. The size and the morphology of the synthesized sample is seen from the scanning electron micrograph. The elemental composition is given by the EDX analysis[ 3 _ T D $ D I F F ] .
Phase Identification: Phase identification was done by using a Panalytical X-ray diffractometer with Cu Ka radiation l = 1.5406 A8, step size = 28/min. The XRD pattern of the as synthesized powder is shown in Fig. 2. The peaks at 2 theta 28.548 33.768, 47.58 and 56.248 correspond to the (112), (200), (220) and (312) planes of the kesterite CZTS (JCPDS card no #26-0575) which is in tune with the literature [18]. From the XRD data the average crystallite size is calculated using the Scherrer Formula: where D is the average crystallite size (diameter), l is the wavelength of the incident radiation, u is the Bragg angle and b is the full width (in radians) of the peak at half the maximum intensity.
The average crystallite size calculated from the above formula is 2.5 nm. Since the XRD patterns of CZTS and ZnS are similar, the formation of the phase needed further confirmation, which was achieved by the Raman spectroscopy.
For the Raman spectra, a Horiba iHR 550 Raman spectrophotometer illuminated with a 532 nm laser beam was used. The peaks at 307 cm À1 [ 2 _ T D $ D I F F ] and 339 cm À1 respectively, correspond to the CZTS phase [19]. Fig. 3 shows the Raman spectrum of the as synthesized sample.

Band gap calculations
The optical band gap studies were performed with a Shimadzu UV 2450 uv-vis spectrophotometer. The absorbance spectrum was recorded and Fig. 4a shows the UV-Vis absorbance spectrum. The bandgap value is obtained from the tauc plot where (ahn) 2 is plotted as a function of incident photon energy; hn. The linear part of the curve is extrapolated and the intercept is the bandgap of the material. Fig. 4b shows the Tauc plot. Upon extrapolation, the band gap is found to be 1.56 eV, which is consistent with the literature [20][ 4 _ T D $ D I F F ] .

FESEM and EDX
The electron micrographs were taken using a ZEISS scanning electron microscope fitted with an Energy dispersive X-ray detector. The elemental ratios as obtained from the EDX detector were presented in Table 2. The corresponding electron micrograph is shown in Fig. 5. The particle size analysis from the SEM micrograph is done by using ImageJ software. From the SEM image, the morphology is known to be spherical with an average particle size of 29 nm. The EDX results indicate that the elements in the sample are stoichiometric indicating a pure phase (Fig. 6).