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.