Creating Self-Assembled Arrays of Mono-Oxo (Moo3)1 Species on Tio2(101) Via Deposition and Decomposition of (Moo3)N Oligomers
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Creating self-assembled arrays of mono-oxo (MoO3)1 species on TiO2(101) via deposition and decomposition of (MoO3)n oligomers Nassar Doudina,b,1,2, Greg Collingea,b,2, Pradeep Kumar Gurunathana,b, Mal-Soon Leea,b, Vassiliki-Alexandra Glezakoua,b, Roger Rousseaua,b,3, and Zdenek Dohnáleka,b,c,3 aPhysical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99354; bInstitute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99354; and cVoiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99163 Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved November 2, 2020 (received for review August 26, 2020) Hierarchically ordered oxides are of critical importance in material homotopy at this length scale. However, while most SACs are science and catalysis. Unfortunately, the design and synthesis of composed of single metal sites, few have functional groups such such systems remains a key challenge to realizing their potential. as oxo ligands of the monodispersed oxide clusters produced here. In this study, we demonstrate how the deposition of small This necessitates a need for hierarchically ordered oxide-on-oxide — oligomeric (MoO3)1–6 clusters formed by the facile sublimation of materials (2, 13, 14). — MoO3 powders leads to the self-assembly of locally ordered ar- Our particular interest in TiO2-supported MoO3 is primarily rays of immobilized mono-oxo (MoO3)1 species on anatase derived from molybdenum oxide’s ability to act as both a Lewis TiO2(101). Using both high-resolution imaging and theoretical cal- acid and base (29–31), as well as a redox-active promoter culations, we reveal the dynamic behavior of the oligomers as (32–34). Many important reactions such as hydrodesulfurization, they spontaneously decompose at room temperature, with the selective oxidation, metathesis of olefins, and selective NOx re- TiO2 surface acting as a template for the growth of this hierarchi- duction are catalyzed and or promoted by molybdenum oxides cally structured oxide. Transient mobility of the oligomers on both (2, 35–43). Further, titanium dioxide is often used as a reducible bare and (MoO ) -covered TiO (101) areas is identified as key to 3 1 2 oxide support material which itself is capable of acid/base and CHEMISTRY the formation of a complete (MoO3)1 overlayer with a saturation redox chemistries. As such, understanding factors that control coverage of one (MoO3)1 per two undercoordinated surface Ti the growth, aggregation, and stability of metal oxide clusters is of sites. Simulations reveal a dynamic coupling of the reaction steps fundamental importance (44, 45) and allows us to explore the to the TiO2 lattice fluctuations, the absence of which kinetically relation between cluster size and activity (18, 46–48). In this respect, prevents decomposition. Further experimental and theoretical previous studies of (WO3)3 supported on rutile and other oxide characterizations demonstrate that (MoO3)1 within this material supports are foundational (2, 8, 46, 49, 50), but similar studies on are thermally stable up to 500 K and remain chemically identical well-defined anatase surfaces are scarce (6, 51). None of these with a single empty gap state produced within the TiO2 band structure. Finally, we see that the constituent (MoO3)1 of this ma- terial show no proclivity for step and defect sites, suggesting they Significance can reliably be grown on the (101) facet of TiO2 nanoparticles without compromising their chemistry. The design and synthesis of hierarchically ordered oxides re- mains a critical challenge in material science and catalysis. Here, hierarchical oxides | molybdenum trioxide | oxide clusters | TiO2(101) | we demonstrate that well-ordered homotopic arrays of mono- self-assembly oxo (MoO3)1 can be easily prepared on anatase TiO2(101) via the deposition of (MoO3)n oligomers. As revealed by our com- bined experiential and theoretical studies, the oligomers spon- xide clusters supported on oxide substrates are of great taneously decompose and self-assemble into chemically identical interest due to their importance in materials science and O and thermally stable monomers. The oligomer decomposition is heterogeneous catalysis (1–5). The nature and strength of their permitted at room temperature due to the dynamic coupling of interactions not only determine their structure, distribution, and decomposition steps to the lattice phonons of TiO . We identify stability but ultimately also their overall activity (6–8). Under- 2 transient mobility of the oligomers as key to the self-assembly of standing the mechanism and dynamics of their formation at an the complete overlayer. The ease of preparation and thermal atomistic level is, therefore, critical to tailoring oxide on oxide stability of this atomically precise system makes it highly suitable systems with desired properties. For example, in materials sci- for a broad range of applications. ence, the self-assembly of oxides into ordered structural motifs is one of the critical challenges for the design of hierarchical ma- Author contributions: R.R. and Z.D. designed research; N.D., G.C., P.K.G., and M.-S.L. terials (9–12). Many approaches, including the formation of or- performed research; N.D., G.C., P.K.G., M.-S.L., and Z.D. analyzed data; and N.D., G.C., dered porous networks (9–11), self-assembly of nanocrystals P.K.G., V.-A.G., R.R., and Z.D. wrote the paper. (12–14), and patterning of structural motifs (15, 16), have been The authors declare no competing interest. devised for creating ordered structures at the nanometer and This article is a PNAS Direct Submission. longer length scales. Yet, despite its importance (2, 17, 18), or- Published under the PNAS license. – dering at subnanometer length scales remains challenging (19 21). 1Present address: Department of Chemical & Environmental Engineering, Yale University, Order at the subnanometer length scale is crucial in chemistry New Haven, CT 06437. applications such as catalysis. To be a good catalyst, catalyst 2N.D. and G.C. contributed equally to this work. materials need to contain a high density of homotopic active 3To whom correspondence may be addressed. Email: [email protected] or zdenek. centers so that their catalytic activity is not only maximized but [email protected]. well-characterized and controlled. The current focus on single- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ atom catalysts (SACs), which exhibit a broad range of unique doi:10.1073/pnas.2017703118/-/DCSupplemental. chemical properties (22–28), demonstrates the significance of Published January 20, 2021. PNAS 2021 Vol. 118 No. 4 e2017703118 https://doi.org/10.1073/pnas.2017703118 | 1of11 Downloaded by guest on September 23, 2021 studies have produced an oxide-on-oxide structure with a subnanometer scale order. In this study, we follow the deposition of well-defined small (MoO3)n oligomers (n = 1 to 6) (2, 45) on anatase TiO2(101) surface. Using high-resolution imaging via scanning tunneling microscopy (STM), spectroscopic characterization by X-ray photoelectron spectroscopy (XPS), deposited mass quartz mea- surements with crystal microbalance (QCM), and with theoreti- cal studies via density functional theory (DFT) we find that at 295 K the (MoO3)1–6 oligomers fall apart into (MoO3)1 mono- meric units, with all Mo ions being in the (6+) oxidation state. The (MoO3)1 monomers from each oligomer remain in close proximity, forming groupings that at low coverages reflect the distribution of gas-phase (MoO3)n oligomers. The higher sta- bility of (MoO3)1 is confirmed by DFT and a conceivable, low- energy dissociation path is derived. At intermediate coverages, the groupings increase in size, indicating that the (MoO3)n oligomers are transiently mobile before they fall apart. Ulti- mately, due to such mobility, a complete layer with relatively high order of (MoO3)1 can be prepared. At all coverages, the (MoO3)1 overlayers exhibit thermal stability all the way to ∼500 K, where the onset of reduction and disordering is observed. Overall, this work demonstrates that vapor deposition of oxides onto oxide substrates can produce stable, subnanometer ordered arrays of oxide monomers. Fig. 1. (A) The large-scale STM image of clean anatase TiO2(101) (175 × 175 Results and Discussion 2 nm , sample bias, Vs = +1.2 V, tunneling current, It = 60 pA). (B) High- × 2 = = Cluster Deposition and Dissociation at Room Temperature. As out- resolution STM image (5 5nm,Vs +0.9 V, It 120 pA) of TiO2(101). lined in the Introduction above and Methods, the sublimation of The approximate positions of O2c and Ti5c atoms is marked by red and blue dots, respectively. (Inset) A ball-and-stick top view of the surface. Red and MoO3 powders leads to the formation of gas-phase oligomeric purple spheres represent O3c and O2c, respectively; dark and light gray (MoO3)n clusters (n = 1 to 6), with the distribution dominated by spheres represent Ti6c and Ti5c, respectively. The rectangular surface unit cell (MoO3)3. This cluster size distribution was determined previ- × × – (10.24 Å 3.78 Å, DFT: 10.46 Å 3.83 Å) (50) and primitive unit cells are ously using infrared reflection absorption spectroscopy (IRAS) marked in green and yellow, respectively. (C) Large-scale STM images (100 × 2 (52) but has not been confirmed via direct high-resolution STM 100 nm ,Vs = 1.9 V, It = 40 pA) after the deposition of 0.03 ML of (MoO3)1 imaging. The results of such experiments on anatase TiO2(101) equivalents at 295 K. Here 1 ML is defined relative to the coverage of Ti5c are presented in Fig. 1. atoms. Several groupings of three neighboring bright features typically The images shown in Fig. 1 A and B illustrate the clean ana- observed at low coverages are highlighted by the dotted pink circles.