MoS2 Nanoclusters Grown on TiO2: Evidence for New Adsorption

Subscriber access provided by WEBSTER UNIV

C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

2

MoS Nanoclusters Grown on TiO: Evidence for New Adsorption Sites at Edges and Sulfur Vacancies Randima Piyumalie Galhenage, Hui Yan, Takat B Rawal, Duy Le, Amy J. Brandt, Thathsara D. Maddumapatabandi, Nhat Nguyen, Talat S. Rahman, and Donna A. Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00076 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Figure 1, high resolution, width should be 4.45" with aspect ratio unchanged

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 2 of 63

Page 3 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60







Page 4 of 63

Page 5 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60







Page 6 of 63

Page 7 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60







Page 8 of 63

Page 9 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9, high resolution image, width should be 2.25" with aspect ratio unchanged





Page 10 of 63

Page 11 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60







Page 12 of 63

Page 13 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60







Page 14 of 63

Page 15 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Content Image, high resolution image, width should be 3.25" with aspect ratio unchanged



MoS2 Nanoclusters Grown on TiO2: Evidence for New Adsorption Sites at Edges and Sulfur Vacancies Randima P. Galhenage1, Hui Yan2, Takat B. Rawal3, Duy Le4, Amy J. Brandt,1 Thathsara D. Maddumapatabandi,1 Nhat Nguyen,1 Talat S. Rahman*4 and Donna A. Chen*1 1Department

of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208 Phone: 803-777-1050, Fax: 803-777-9521, Email: [email protected]

2Department 3Center

of Chemistry, University of Louisiana at Lafayette, Lafayette, LA 70504

for Molecular Biophysics, Oak Ridge National Laboratory, Oak Ridge, TN, 37830 and

the Department of Biochemistry and Cellular and Molecular Biology, University of Tennessee, Knoxville, TN, 37996 4Department

of Physics, University of Central Florida, Orlando, FL 32816

Phone: 407-823-1480, Fax: 407-823-5112, Email: [email protected] *Corresponding Authors

1 ACS Paragon Plus Environment

Page 16 of 63

Page 17 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Abstract MoS2 clusters have been grown on a TiO2(110) substrate in order to provide a model surface for better understanding the adsorbate interactions and chemical activity on titania-supported MoS2 clusters. Scanning tunneling microscopy experiments show that clusters with elongated shapes and flat tops are formed, and the long axes of the clusters have specific orientations with respect to the [001] direction on TiO2(110). In contrast, deposition of Mo in the absence of H2S results in a high density of smaller, round clusters that cover the majority of the surface. The morphologies of the MoS2 clusters do not change after exposure to various gases (D2, CO, O2, H2O, and methanol) in ultrahigh vacuum. However, exposure to higher pressures of O2, H2O or methanol (10 Torr), as well as exposure to air, causes the clusters to disintegrate as Mo in the clusters becomes oxidized. Temperature programmed desorption studies with CO on the MoS2 clusters show a distinct desorption peak at 280 K, which is not observed on metallic Mo or titania. Density functional theory calculations suggest that these new adsorption sites for CO are at the edges of the elongated MoS2 clusters, particularly along the (1010) edge containing sulfur vacancy sites.



Introduction Nanostructured MoS2 layers and clusters have attracted interest for a variety of applications, including in heterogeneous catalysis,1,2 electrocatalysis,2-4 photocatalysis,2,5 electronic devices4,6,7 and lithium ion batteries.4,8 For example, MoS2 is a key component of the industrial hydrodesulfurization (HDS) catalyst used for the removal of sulfur in crude oil processing.9-14 Furthermore, MoS2 catalysts have more recently gained attention as low cost, earth-abundant catalysts for the hydrogen evolution reaction (HER) in electrochemical cells for water splitting, 15-19

in which MoS2 is reported to have activity that is nearly as high as that of Pt.20 In addition,

low-dimensional MoS2 exhibits unusual electronic properties, given that MoS2 single layers have a direct band gap, in contrast to the bulk behavior.21-25 Although the basal plane of MoS2 is catalytically inert,9,26,27 nanostructured MoS2 is active for HER, with the active sites located at the MoS2 edges.15-17,19,28-31 Likewise, HDS activity is proposed to be at the undercoordinated edge sites,32-36 where hydrogen dissociation and recombination,37 as well as C-S bond scission,38 occur. Furthermore, density functional theory (DFT) based computational studies predict activation of the MoS2 basal plane in the presence of sulfur vacancies39 and adsorbed transition metal nanoparticles.40 The sulfur vacancy induced activation of the basal plane has been observed in HER, in which sulfur vacancies serve as active sites,18,41 and in some cases the sulfur vacancies are reported to have higher activity than edge sites.18,39,41,42 However, sulfur-vacancy defects at edge sites are not required for activity in HDS reactions,38 based on the atomically resolved scanning tunneling microscopy (STM) studies of molecules adsorbed on single-layer MoS2 on Au(111) carried out by Besenbacher and coworkers.1,43-45 Specifically, STM images of the MoS2 clusters show bright "brim" states, which appear ~0.4 Å higher than the cluster tops and are known to have 1D metallic character.24,46 These brim states facilitate adsorption and dissociation of molecules in hydrotreating reactions.36,38,47,48 For example, thiophene and dibenzothiophene adsorb on fully sulfided edges of single-layer MoS2.1,45,49 A number of studies have shown that titania50-53 or mixed titania-alumina54,55 supports provide enhanced activity for MoS2 clusters in HDS reactions compared to alumina and silica. It is proposed that the increased activity on titania is due to electronic effects resulting from the presence of the reduced Ti3+ species;53 alternatively, MoS2 on the TiO2 support is proposed to have higher dispersion, thereby increasing the number of active edge sites for MoS2.50


Page 18 of 63

Page 19 of 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Moreover, TiO2/MoS2 hybrid structures have shown promise for photocatalytic splitting of water,56 as well as other photocatalytic reactions.57-59 DFT calculations indicate that the Au/MoS2 interface is distinct from the Au/TiO2 interface with respect to the location of the active sites;60 while the latter is the active region for reactions such methanol oxidation,61 the active sites for Au/MoS2 appear to be on the nanoparticle itself rather than at the interface. Given these applications involving the MoS2/TiO2 interface, it is valuable to establish a better understanding of the growth and activity of MoS2 clusters on TiO2. In the work reported here, MoS2 clusters have been grown on a TiO2(110) surface in ultrahigh vacuum and characterized by STM and X-ray photoelectron spectroscopy (XPS). Changes in cluster morphology and oxidation states were investigated after exposure to various adsorbate gases (D2, CO, O2, H2O, and methanol). In addition, temperature programmed desorption (TPD) studies of CO on MoS2 clusters exhibited evidence for a unique adsorption site, most likely at the edges of the MoS2 clusters, as shown by accompanying DFT based calculations. These calculations also predict a hierarchy of adsorption sites for CO and indicate that the preferred binding site is at the (1010) edge with sulfur vacancies. Additionally, strong binding of sulfur atoms with underlying oxygen atoms from the TiO2 support may lead to formation of oxygen vacancies. Although atomic-resolution STM studies of MoS2 on TiO2(110) have been previously reported,62,63 to our knowledge this is the first investigation of adsorption and reaction on these well characterized, TiO2-supported MoS2 clusters. Experimental Section All experiments were carried out in two ultrahigh vacuum (UHV) chambers, which have been described in more detail previously.64-68 The first chamber has a base pressure of

MoS2 Nanoclusters Grown on TiO2: Evidence for New Adsorption

Recommend Documents