Creating self-assembled arrays of mono-oxo (MoO3)1 species on TiO2(101) via deposition and decomposition of (MoO3)n oligomers

Nassar Doudin; Greg Collinge; Pradeep Kumar Gurunathan; Mal-Soon Lee; Vassiliki-Alexandra Glezakou; Roger Rousseau; Zdenek Dohnálek

doi:10.1073/pnas.2017703118

New Research In

Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved November 2, 2020 (received for review August 26, 2020)

Significance

The design and synthesis of hierarchically ordered oxides remains a critical challenge in material science and catalysis. Here, we demonstrate that well-ordered homotopic arrays of mono-oxo (MoO₃)₁ can be easily prepared on anatase TiO₂(101) via the deposition of (MoO₃)_n oligomers. As revealed by our combined experiential and theoretical studies, the oligomers spontaneously decompose and self-assemble into chemically identical and thermally stable monomers. The oligomer decomposition is permitted at room temperature due to the dynamic coupling of decomposition steps to the lattice phonons of TiO₂. We identify transient mobility of the oligomers as key to the self-assembly of the complete overlayer. The ease of preparation and thermal stability of this atomically precise system makes it highly suitable for a broad range of applications.

Abstract

Hierarchically ordered oxides are of critical importance in material science and catalysis. Unfortunately, the design and synthesis of such systems remains a key challenge to realizing their potential. In this study, we demonstrate how the deposition of small oligomeric (MoO₃)_1–6 clusters—formed by the facile sublimation of MoO₃ powders—leads to the self-assembly of locally ordered arrays of immobilized mono-oxo (MoO₃)₁ species on anatase TiO₂(101). Using both high-resolution imaging and theoretical calculations, we reveal the dynamic behavior of the oligomers as they spontaneously decompose at room temperature, with the TiO₂ surface acting as a template for the growth of this hierarchically structured oxide. Transient mobility of the oligomers on both bare and (MoO₃)₁-covered TiO₂(101) areas is identified as key to the formation of a complete (MoO₃)₁ overlayer with a saturation coverage of one (MoO₃)₁ per two undercoordinated surface Ti sites. Simulations reveal a dynamic coupling of the reaction steps to the TiO₂ lattice fluctuations, the absence of which kinetically prevents decomposition. Further experimental and theoretical characterizations demonstrate that (MoO₃)₁ within this material are thermally stable up to 500 K and remain chemically identical with a single empty gap state produced within the TiO₂ band structure. Finally, we see that the constituent (MoO₃)₁ of this material show no proclivity for step and defect sites, suggesting they can reliably be grown on the (101) facet of TiO₂ nanoparticles without compromising their chemistry.

Footnotes

↵¹Present address: Department of Chemical & Environmental Engineering, Yale University, New Haven, CT 06437.
↵²N.D. and G.C. contributed equally to this work.
↵³To whom correspondence may be addressed. Email: roger.rousseau{at}pnnl.gov or zdenek.dohnalek{at}pnnl.gov.

Author contributions: R.R. and Z.D. designed research; N.D., G.C., P.K.G., and M.-S.L. performed research; N.D., G.C., P.K.G., M.-S.L., and Z.D. analyzed data; and N.D., G.C., P.K.G., V.-A.G., R.R., and Z.D. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2017703118/-/DCSupplemental.