Optical properties of zinc peroxide and zinc oxide multilayer nanohybrid films Da ´ niel Sebo ˝ k a , Tama ´ s Szabo ´ a , Imre De ´ ka ´ny a,b, * a Department of Colloid Chemistry, University of Szeged, H-6720 Szeged, Aradi v.t.1., Hungary b Supramolecular and Nanostructured Materials Research Group of the Hungarian Academy of Sciences, H-6720 Szeged, Aradi v.t.1., Hungary 1. Introduction ZnO 2 and ZnO have recently become widely known and exploited. Zinc peroxide may be used in certain processes of rubber industry for the improvement of wear resistance [1,2], and as an oxidant mainly in mixtures containing explosives [3]. Zinc oxide has been promoted to be one of the most promising materials of semiconductor industry. This is attributed to its wide band gap (E g  3.2–3.3 eV, l g  375–387 nm) that makes it feasible for optoelectronic devices [4,5], solar cells [6], light emitting devices [7,8], transistors [9], and photocatalysts [10] operating in the UV range. On the other hand, similarly to other semiconductor nanoparticles, its band gap energy as well as its optical properties can be controlled in a certain size range by the dimensions [11,12], the morphology [13,14], and surface treat- ment [15] of the particles. ZnO particles were synthesized in aqueous and ethanol media by Bahnemann et al. [12]; quantum size effects have been observed upon particle growth. Spanhel and Anderson [13] prepared ZnO (wurtzite) clusters which, in a self-induced process, grew to crystallites with diameters up to several nanometers. They were then deposited on porous and non-porous supports and calcined for fabrication of ZnO ceramic membranes or thin layers. ZnO 2 can be prepared by the reaction of ZnO [16] or zinc acetate [17,18] with H 2 O 2 . A large number of methods are known for the preparation of ZnO. These include the metalor- ganic chemical vapor deposition [19], laser ablation [20], and the molecular beam epitaxy [21], which all require high temperature and vacuum. On the contrary, wet-chemical methods do not require extreme temperature and pressure conditions. Some typical examples are the electrodeposition [22], sol–gel process [23–25], hydrothermal process [26–28], and syntheses with precipitation from aqueous and alcohol solutions in the presence of stabilizers [29,30] or from water-in- oil microemulsions [31,32]. We report on the synthesis of ZnO 2 sol from zinc acetate. The ZnO 2 nanocrystals are then layer-by-layer (LbL) self-assembled either with a clay mineral of lamellar structure (hectorite) or with a polyelectrolyte (polystyrene sulphonate; PSS) [33–37,43,45–48] according to the deposition protocol schemed in Fig. 1. The as- prepared hybrid multilayered thin films are calcined at 400 8C to obtain ZnO/hectorite films. The optical properties of the thin layers are then determined by numerous methods and a model calculation relying on the modification of the Bruggeman effective medium theory enables to get a deeper insight on the structure and properties of the assemblies. We will especially focus on the effect of using lamellar (hectorite) and polymeric (PSS) sticking materials and Applied Surface Science 255 (2009) 6953–6962 ARTICLE INFO Article history: Received 17 December 2008 Received in revised form 3 March 2009 Accepted 5 March 2009 Available online 17 March 2009 Keywords: ZnO 2 nanoparticles ZnO nanohybrid films Optical properties Optical interference Band gap energy Self-assembly ABSTRACT Zinc peroxide and zinc oxide nanoparticles were prepared and self-assembled hybrid nanolayers were built up using layer-by-layer (LbL) technique on the surface of glass substrate using the layer silicate hectorite and an anionic polyelectrolyte, sodium polystyrene sulfonate (PSS). Light absorption, interference and morphological properties of the hybrid films were studied to determine their thickness and refractive index. The influence of layer silicates and polymers on the self-organizing properties of ZnO 2 and ZnO nanoparticles was examined. X-ray diffraction revealed that ZnO 2 powders decomposed to ZnO (zincite phase) at relatively low temperatures (less than 200 8C). The optical thickness of the films ranged from 190 to 750 nm and increased linearly with the number of layers. Band gap energies of the ZnO 2 /hectorite films were independent from the layer thickness and were larger than that of pure ZnO 2 nanodispersion. Decomposition of ZnO 2 to ZnO and O 2 at 400 8C resulted in the decrease of the band gap energy from 3.75 to 3.3 eV. Concomitantly, the refractive index increased in correlation with the formation of the zincite ZnO phase. In contrast, the band gap energies of the ZnO 2 /PSS hybrid films decreased with the thickness of the nanohybrid layers. We ascribe this phenomenon to the steric stabilization of primary ZnO 2 particles present in the confined space between adjacent layers of hectorite sheets. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author at: Department of Colloid Chemistry, University of Szeged, H-6720 Szeged, Aradi v.t.1., Hungary. E-mail address: i.dekany@chem.u-szeged.hu (I. De ´ka ´ ny). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.03.020