FULL PAPER DOI:10.1002/ejic.201201344 Organic-Free Self-Assembled Copper Sulfide Microflowers Baskaran Ganesh Kumar [a] and Krishnamurthi Muralidharan* [a] Keywords: Self-assembly / Nanostructures / Reaction mechanisms / Copper / Sulfur The hexamethyldisilazane-assisted synthesis of organic-free copper sulfide microflowers is described herein. The syn- thetic method was developed in such a way as not to leave any trace of organic material in the copper sulfide nanopar- ticles after purification. The flowers were constructed by the self-assembly of nanoflakes. Copper sulfide was formed via Introduction The synthesis of nanoparticles (NPs) is considerably en- riched by a variety of excellent synthetic strategies. [1,2] It is also recognized that the shape as well as assembly of NPs will play a major role in future optoelectronic devices. [3] Nevertheless, the synthesis of assembled functional nano- structures with a pre-designed shape remains a formidable task. [4] Consequently, research in this area affords an op- portunity to enhance the scope of the nano regime. In addition to the size and shape, the existence of various stable stoichiometric species of a compound is an additional tool for tuning its optical and electronic properties. Copper chalcogenides (Cu x S) are among these materials, [5] and hence many procedures have been developed for their syn- thesis. [6] Moreover, copper chalcogenides have the following advantages: 1) they are more abundant than conventional inorganic optoelectronic materials, [7] 2) they exhibit metal- like conductivity [8] and 3) optical properties (plasmonic NIR absorption and optical limiting nature). [9] Herein, we report the synthesis of copper sulfide micro- flowers by the 1,1,1,3,3,3-hexamethyldisilazane [HN- (SiMe 3 ) 2 , HMDS]-assisted synthetic methodology. We chose this approach because hexamethyldisilazane is per- ceived to function both as a reductant and capping agent. Note that this HMDS-assisted synthesis does not leave any residues or side-products to modify the properties of the resulting NPs. [10] The synthesized microflowers have dia- meters in the range 100 nm to 3 μm and their petals are constructed of well-defined nanoflakes. The nanoflakes formed have an average width of 12.6  3.4 nm and lengths [a] School of Chemistry, University of Hyderabad, Hyderabad, India E-mail: kmsc@uoh.ernet.in Homepage: http://chemistry.uohyd.ernet.in/~km/ Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201201344. Eur. J. Inorg. Chem. 2013, 2102–2108 © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2102 an S–N polymeric intermediate, which drives the formation of copper sulfide by elmination of trimethylsilyl chloride as a key product. A reaction mechanism has been proposed based on the results of time-dependent NMR studies. The self-as- sembly and stability of the microflowers are explained on the basis of analytical data. of a few microns. The construction of the flowers is driven by the intricate self-assembly of numerous nanoflakes. Results and Discussion Synthesis The methodology used for the synthesis of metal chalco- genides was based on the affinity of halides towards electro- positive silicon centres and the formation of trimethylsilyl halide. Even though the fluoride has a higher affinity than other halides towards cationic silicon, CuCl 2 was preferred as the copper source because of its greater atmospheric sta- bility compared with the fluoride. As expected, the forma- tion of trimethylsilyl chloride (TMSCl) drives the formation of copper sulfide (as evidenced by NMR studies, see Fig- ure 7). A 17-fold excess of HMDS with respect to CuCl 2 was required for complete reaction. The advantage of this syn- thetic method was that it did not require any other organic solvent. This may be attributed to the multiple roles of HMDS in the synthesis, namely as reducing agent, capping agent and solvent. Moreover, excess of the capping agent will also provide uniform surface capping of the growing NPs. Even though flowers were formed in a shorter reaction time (3 h) and with smaller amounts of HMDS, the best quality microflowers were obtained under the optimized re- action conditions. Variation of the amount of other starting materials (CuCl 2 and S) did not yield any other stoichio- metric form of copper sulfide. The formation of flowers was not correlated to the amount of capping agent or starting materials used. Characterization of the Microflowers The PXRD pattern (Figure 1) of the product clearly shows the formation of phase-pure copper sulfide (CuS).