Supporting Information
Zinc oxysulfide ternary alloy nanocrystals: a bandgap modulated
photocatalyst
Shiv K Pandey, Shipra Pandey, Avinash C Pandey, G K Mehrotra
Preferring to a method for solution-synthesis, composition modulated zinc oxysulfide
(ZOS) nanocrystals with binary ZnO and ZnS have been synthesized through single-pot
solution-combustion method. The chemicals used in entire chemical synthesis and
measurements are of analytical grade and used without further purification. In this method, a
fuel mixture of ethanol and ethylene glycol (in a 4:1 ratio) is prepared and stirred at 40 °C for
15 min. A premeditated stoichiometric ratio of zinc acetate for each composition is dissolved
in above fuel mixture solution and stirred for 10 min at 60 °C to get stabilized the solution.
Following this, an amount of thiourea (as a sulfur source), corresponding to requisite
composition of Zn is added to above solution. The solution is again stirred for 30 min at 60
°C to stabilize the zinc, thiourea and fuel solution mixture. The as prepared solution is placed
in a furnace at 350 ºC for ignition. The solution mixture swelled up with liberation of the
large amount of non-toxic fumes prompting highly exothermic combustion process. The
resulting product is formed as fine yellowish-white powder. Similar procedure is followed for
the synthesis of each composition with stoichiometric variation of Zn, O and S to get the
series of compositions of ZOS along with binary oxides and sulfides. Since, this method
provides a simple and easy way to synthesize ternary alloys, therefore, it could be used
efficiently for the cost effective, high yield and less time consuming for the synthesis of other
ternary compositions that require high temperature for its formation. The as synthesized
products are characterized for their formation.
An in-depth analysis is essential to understand alloying mechanism and to employ
better synthetic strategies for these alloy nanomaterials and therefore we attempted to unwind
the probable complexity involved in the synthesis of homogenous alloy compositions of
ZOS. Introducing to the reactivity aspects, Zn prefers oxygen than sulphur due to high electro
negativity and smaller size,1 the occupation of O and S atoms in lattice require high
temperature. Therefore, we betrothed a chemical method that has considerable advantages
over the physical routes in terms of product quality and good yield at potentially lower cost.
To devise a synthetic scheme for homogenous alloying, it is important that growth rate of
constituents must be comparative and growth conditions for one constituent should not
obstruct the uniform growth of other.2 In an experimental process, a ratio of ethanol and
ethylene glycol is homogenized that acts as a fuel mixture. Moreover, reaction medium
accelerates the reaction kinetics and overrides the activation energy barrier in few seconds at
high temperature. It is reported that temperature plays an important role in alloying process. 3
In synthesis, an initial solution of fuel mixture contains ions of Zn and S. As the temperature
rises, S atoms get released into the solution and tended to form ZnS. Meanwhile, temperature
of solution became closure to flash point of the solvent mixture to ignite combustion. Since,
combustion process materializes in the presence of O and the affinity of Zn towards O is
higher, the formation of ZnO competes with ZnS. Nevertheless, the presence of S in the
solution and in its ambience maintains their ratio with zinc. As the temperature become high
enough due to combustion of fuel, the diffusion of Zn, O and S in lattice occurs as a function
of temperature that infuses distortion in alloy lattice. The parameter of homogeneity in alloys
is regarded as an increased rate of mobility of diffused ions at the high temperatures.3
Therefore, the process of transforming binary compounds into ternary alloys is studied by
quantifying the tunable bandgap.
Various characterization techniques have been used for the confirmation of binary and
ternary alloy compositions. X-ray diffraction (XRD) is performed on Rigaku D/max-2200PC
diffractometer using wavelength of 1.5406 Å (CuKα1). Bright-field Transmission Electron
Microscopic (BF-TEM) imaging, High-Resolution TEM and STEM-EDS mapping of Zn, O
and S elements is performed on JEOL, JEM 2100F electron microscope operated at 200 kV
using LaB6 electron source. UV-Visible absorption measurements for photocatalytic analysis
are performed on Shimadzu UV-2450 UV-Vis spectrophotometer. We have purchased the
Degussa P25 nanopowders (CAS Number-13463-67-7) from Sigma-Aldrich having particle
size about 21 nm with BET surface area of 36-65 m2/g and used it without any chemical or
physical modification.
1. X.F. Fan, Z.X. Shen, Y.M. Lu and J.-L. Kuo, New J. Phys., 2009, 11, 093008.
2. J. Akhtar, M. Afzaal, A. Podhorodecki, M. Syperek, J. Misiewicz, U. Bangert, S.J.O.
Hardman, D.M. Graham, W.L. Flavell, D.J. Binks, S. Gardonio, P. O`Brien, J. Am.
Chem. Soc., 2011, 133, 5602-5609.
3. M.D. Regulacio and M.-Y. Han, Acc. Chem. Res., 2010, 43, 621-630.