Article pubs.acs.org/JPCB
Growth and Stability of Titanium Dioxide Nanoclusters on Graphene/ Ru(0001) Ryan T. Frederick,† Zbynek Novotny,‡,⊥ Falko P. Netzer,§ Gregory S. Herman,*,† and Zdenek Dohnálek*,‡,∥ †
School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, United States Fundamental and Computational Sciences Directorate and Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99354, United States § Surface and Interface Physics, Institute of Physics, Karl-Franzens University, A-8010 Graz, Austria ∥ Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99163, United States ‡
ABSTRACT: Titanium dioxide/graphene composites have recently been demonstrated to improve the photocatalytic activity of TiO2 in visible light. To better understand the interactions of TiO2 with graphene we have investigated the growth of TiO 2 nanoclusters on single-layer graphene/Ru(0001) using scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Deposition of Ti in the O2 background at 300 K resulted in the formation of nanoclusters nucleating on intrinsic defects in the graphene (Gr) layer. The saturation nanocluster density decreased as the substrate temperature was increased from 300 to 650 K, while deposition at 700 K resulted in the significant etching of the Gr layer. We have also prepared nanoclusters with Ti2O3 stoichiometry using lower O2 pressures at 650 K. Thermal stability of the TiO2 nanoclusters prepared at 300 K was evaluated with AES and STM. No change in oxidation state for the TiO2 nanoclusters or etching of the Gr layer was observed up to ∼900 K. Annealing studies revealed that cluster ripening proceeds via a Smoluchowski mechanism below 800 K. Above 800 K, the changes in cluster shapes indicate an onset of diffusion within the clusters. At even higher temperatures, the nanoclusters undergo reduction to TiOx (x ≈ 1−1.5) which is accompanied by oxidation and etching of the Gr. Our studies demonstrate that highly thermally stable TiOx nanoclusters of controlled composition and morphology can be prepared on Gr supports.
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INTRODUCTION Titanium dioxide (TiO2) is a prototypical transition metal oxide that has been extensively investigated.1 Much of the interest is due to the application of TiO2 as a photocatalyst for a wide range of applications1−3 that include photoassisted degradation of organic molecules, water purification,4 antibactericidal coatings,5 self-cleaning windows,6 and selective photoinduced eradication of cancerous T24 cells.7 Significant effort has been invested to optimize its photocatalytic properties at visible wavelengths to improve performance for these applications and expand to other solar applications.3,8−10 TiO2 has also been used for sensing NOx,11,12 CO,13,14 SO2, and O2.14 Graphene (Gr) is a promising inert support for catalytically active materials. Studies have demonstrated that photocatalytic CO2 reduction15−17 and selective oxidation of organic compounds18−26 are enhanced when TiO2 is supported on Gr. A number of mechanisms have been proposed to explain this increased activity.19,25,27−29 Furthermore, TiO2-Gr has enhanced photoabsorption in the visible region.19,28,30 This is likely due to the formation of a new “visible-band” state that emerges below the TiO2 conduction band. The formation of © 2017 American Chemical Society
this band has been attributed to various mechanisms in the TiO2-Gr composites, including down-shifting of Ti 3d orbital energies.28,30 In this study, we have investigated the growth of TiO2 nanoclusters on a single-layer-Gr supported on Ru(0001). This system has been selected due to its potential as a model system for catalytic and photocatalytic studies. The Gr films have been grown via chemical vapor deposition (CVD) which allows for repeated, in situ creation of reproducible Gr layers with wellcontrolled average densities of defects, that are utilized as TiO2 nanocluster nucleation sites. Further, Ru(0001) was selected as the metal support, as it is one of the most widely investigated Gr supports with a large superstructure that can be employed as a template.31−33 STM has been employed to follow the cluster density, morphology, and size distribution as a function of coverage, and deposition and annealing temperature. AES was used to determine the cluster stoichiometry. Special Issue: Miquel B. Salmeron Festschrift Received: June 5, 2017 Revised: July 29, 2017 Published: August 9, 2017 640
DOI: 10.1021/acs.jpcb.7b05518 J. Phys. Chem. B 2018, 122, 640−648
Article
The Journal of Physical Chemistry B
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EXPERIMENTAL SECTION All experiments were performed in an ultrahigh vacuum (UHV) system with a base pressure
