(Photo)Electrochemically Active Functional Hybrids of Multilayer CVD Graphene Decorated with Colloidal TiO2 Nanocrystals

A direct and facile solution-based procedure has been implemented for manufacturing optically transparent and UV-light photoactive hybrids based on graphene (from one to 5 layers), grown by Chemical Vapor Deposition (CVD) and decorated with TiO2 nanocrystals (NC) surface functionalized with 1-pyrene butyric acid (PBA). PBA functionalization was obtained by a capping exchange procedure onto pre-synthesized organic-capped NCs [1]. An in-depth characterization by SEM, EDS, AFM, Raman spectroscopy, Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) measurements demonstrated the successful immobilization of the colloidal NCs on the graphene multilayers, which preserves or even enhances the graphene intrinsic structural properties. In particular, we observed an electrical conductivity higher than that measured for bare graphene, which has been accounted for by a p-doping effect of graphene, due to a hole transfer from the nano-objects to graphene, mediated by the short aromatic ligand acting as a charge channel. The hybrids properties are strongly dependent on the number of layers of CVD graphene. The use of two redox mediator probes [inner-sphere, surface sensitive (K4Fe(CN)6) and outer-sphere, surface insensitive (Ru(NH3)6Cl3)], in a synergistic CV and EIS study, allowed to understand these features from a morphological point of view. The results showed a strong difference between the mono-, the bi- and the other multi-layers, in terms of different diffusional mechanism and redox active sites [2-4]. Moreover, the stacked layers of the pyrene-coated TiO2 NCs are found to increase the electroactivity, the capacitive behavior, as well as the photo-electrical response of graphene, concomitantly maintaining its high charge mobility. The photoelectrical conversion of the hybrid is enhanced of 50% with respect to the bare graphene, with a long recombination lifetime of the photogenerated electron-hole pairs. For all the above reasons, the photoactive composite has a great potential as an optically transparent component for manufacturing photoanodes to be integrated in solar cells or photodetectors and in FETs or (photo)electrochemical sensors, also exploiting the possibility of photorenovate the sensor surface [5]. References [1] C. Ingrosso et al., ACS Appl. Mater. & Interfaces, (2015), 7, 4151-4159. [2] C. Ingrosso et al., J. Mat. Chem. A, (2017), submitted, under review. [3] D.A.C. Brownson, D.K. Kampouris, C.E. Banks, Chem. Soc. Rev., (2012), 41, 6944-6976. [4] A. Ambrosi, M. Pumera, J. Phys. Chem. C, (2013), 117, 2053-2058. [5] V. Pifferi et al., Anal. Bioanal. Chem., (2016), 408 (26), 7339-7349. Acknowledgements The authors acknowledge the MIUR National Project PRIN 2012 (prot. 20128ZZS2H).