Electronic Properties and Chemical Reactivity of TiS2 Nanoflakes

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Electronic Properties and Chemical Reactivity of TiS Nanoflakes Clotilde S. Cucinotta, Kapildeb Dolui, Henrik Pettersson, Quentin M. Ramasse, Edmund Long, Sean E. O'Brian, Valeria Nicolosi, and Stefano Sanvito J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 02 Jun 2015 Downloaded from http://pubs.acs.org on June 2, 2015

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The Journal of Physical Chemistry

Electronic Properties and Chemical Reactivity of TiS2 Nanoflakes. Clotilde S. Cucinotta,1,2,* Kapildeb Dolui,1,2 Henrik Pettersson,1,2 Quentin M. Ramasse,4 Edmund Long,1,2 Sean E. O’Brian,1,2 Valeria Nicolosi1,2,3 and Stefano Sanvito.1,2,* 1 Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland. 2 School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland. 3 School of Chemistry, Trinity College Dublin, College Green Dublin 2, Ireland. 4 SuperSTEM Laboratory, SciTech Daresbury Campus, Keckwick Lane, Daresbury WA4 4AD, UK. *corresponding authors: [email protected], [email protected]. KEYWORDS: Transition metal dichalcogenides, TiS2, Two-dimensional nanomaterials, Density Functional Theory, Activated processes, Oxidation, Scanning transmission electron microscopy. ABSTRACT: Transition metal dichalcogenides have a laminar structure, with strongly covalently bonded layers weakly interacting through van der Waals forces. They are of special interest also because of their unique properties once exfoliated in nanoflakes. We analyse the microstructure of oxidised TiS2 nanoflakes with atomically resolved scanning transmission electron microscopy and propose a comprehensive model for their reactivity by means of first principles simulations. In particular we find that reaction to water proceeds from the edges of the flake, while it is thermodynamically possible but kinetically hindered in the middle, unless it is initiated by the presence of a surface vacancy. Importantly O substitution for S allows fine-tuning control of the flake bandgap, paving the way for the use of TiS2-xOx alloys as surface catalysts and photovoltaic materials.

Introduction Transition metal dichalcogenides (TMC) have a laminar structure, where strongly covalently bonded layers couple to each other via weak van der Waals interactions. Due to their technological potential as catalysts and lubricants for the petroleum industry,1 the bulk form of many of them has been widely studied in the last thirty years.2-3 More recently advances in scalable exfoliation methods4-6 have enabled the production of TMC-based nanostructures, composed of a few down to a single layer, and thus have created an essentially new materials platform. High surface to volume ratio, electron confinement, different reactivity, and possible nanostructuring and functionalization underpin completely new functionalities and make exfoliated TMCs radically different from their bulk layered precursors. As a consequence, a range of new applications for these materials is emerging, going from nano-electronics to sensing, to catalysis, to electrochemical energy storage/harvesting and conversion. For instance, layered VS2-based nanostructures have been proposed as promising supercapacitor electrodes.7 Exfoliation of MoS2 to monolayers transforms the band gap from direct to indirect, allowing the fabrication of room-temperature field-effect transistors that can surpass the physical limits of Si in terms of on/off ratio (>108), miniaturization and energy

efficiency.8-9 Functional composites Pt-TiS2 have been proposed for electro-catalytic hydrogen evolution.10 LiTiS2 electrodes are considered very promising for lightweight and high energy-density batteries.11 Interestingly, for some of the TMCs these novel properties are largely unexplored. Thus, developing theoretical models for understanding fundamental processes underlying selected functionalities is crucial for predicting and optimizing the materials performance and their use in applications. For instance, it is remarkable how little it is known about how the 2D nature of the TMCs can affect the material’s chemical reactivity. A primary example is represented by TiS2, where distinct chemical reactions are experimentally observed upon exposure to 1O2 or H2O, with evidence of reactivity at the edge of the nanostructure in the presence of water.12 Understanding and controlling TiS2 surface oxidation, edge passivation and chemical reactivity could enable important applications. To cite one, it has been experimentally shown12 that TiS2/TiO2 hetero-structures can provide a range of solar energy adsorption wider than that of the single component phases, therefore a defective TiOxS2-x nanostructure could be used for photovoltaic applications. By connecting computational and experimental findings it is possible to achieve a better understanding of the

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microscopic structure and the possible oxidation pathways in TiS2. Aberration-corrected scanning transmission electron microscopy (AC-STEM) can be considered the state of the art technique to investigate a single sheet of TMC atom-by-atom. This is enabled by the fact that the high-angular annular dark-field (HAADF) signal and its intensity is proportional to the atomic number (I∝ Z1.7), so that in TiS2 the Ti atoms will be brighter than the S ones and can be distinguished. In combination with electron energy loss spectroscopy (EELS) the flake chemical composition and thickness can also be determined.13 In this work we carefully analyse the microstructure of an oxidised TiS2 nanoflake with atomic resolved STEM and propose a comprehensive model for the reactivity of TiS2 monolayers, in the presence of water. The model is based on accurate density functional theory14,15 (DFT) calculations at various level of approximation. In the following sections we will study the electronic structure of TiS2 and both the thermodynamics and kinetics of the oxidation at the edge and in the middle of the flake, under exposure to water.i In the last section we will discuss the effects of oxidation on the band-gap. Experimental and Computational Methods Theoretical Calculations The calculation scheme adopted throughout this work is based on density functional theory. (DFT), as implemented in VASP16 and CP2K17 simulation packages. The electronic structure of TiS2 monolayers - clean and incorporating oxygen impurities - has been studied by using the PBE18 (Perdew-Becke-Ernzerhof) exchange and correlation (XC) functional. The projected augmented-waves19 (PAW) approach is used. The electronic wave-functions are expanded using plane waves up to a cut-off energy of 400 eV. Periodic boundary conditions are applied and a vacuum layer of at least 12 Å is placed above the monolayer to minimize the interaction between the adjacent layers. Both atomic positions and cell parameters are allowed to relax until the forces on each atom are less than 0.02 eV/Å. Defective TiS2-xOx monolayers with different oxygen concentration (x) have been modelled by substituting in different n × n supercells one S with an O atom, where n goes from 1 to 7. When n=1, the Brillouin zone is sampled

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In our model the monolayer reacts with a source of oxygen (water), therefore in this article we used the term oxidation (and variation thereof), however the proposed reaction (TiS2 + xH2O → TiS2-xOx + xH2S) is not a classical redox reaction because S does not change its formal oxidation state

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using a (21 × 21 × 1) Monkhorst-Pack k-grid. k-points grids of similar density where used for every cell (e.g. 10 × 10 k points were used for the 2x2 supercell, 4 × 4 k-points for the 5 × 5 supercell and so on). This introduced an error smaller than 1 meV/cell in the evaluation of total energy in every cell. PBE electronic properties have been compared with those obtained at higher levels of accuracy, using the HSE0620 (Heyd-Scuseria-Ernzerhof) hybrid XC and the Green's functions quasi-particle approach GOWO, as implemented in VASP.21 Green's function (Go) and screened interaction (Wo) are calculated as a single shot first order perturbation acting on DFT Kohn-Sham single particle energies and wave-functions. These are obtained using the HSE06 functional. At least 100 unoccupied bands are calculated for the (1 × 1) supercell. Using CP2K, the ideal TiS2 monolayer is modelled using a hexagonal TiS2 (1,1,1) 8 × 8 supercell with 192 atoms and a vacuum region 10.7 Å wide, separating the periodically repeated flakes in z-direction. In order to study the structure and oxidation properties at the edge of a TiS2 nanoflake we have modelled it as periodic in the ydirection and truncated in the x-direction. An orthorhombic supercell is used, obtained by replicating 4×8 times in x and y directions the rectangular unit cell of the monolayer (the primitive cell of this system is hexagonal but our reference is the larger (√3, 1) rectangular unit). We fixed the in plane lattice parameters of the cell to the ab-initio equilibrium value of the infinite monolayer a=3.4 Å. A vacuum region 15.37 (10.73) Å wide separates the periodically repeated flakes in x (z) direction. We assessed that our cell size was sufficiently large to provide a good representation of the Brillouin zone at Γ, verifying that the edge geometries and oxidation energies were very similar if calculated using a 4×8 or a larger 4×20 slab. We finally checked that, by substituting an O atom in the middle of the nanoribbon, the same reaction energy obtained for the infinite surface was recovered. Different reaction pathways leading to TiS2 oxidation are identified and compared, using the CP2K implementation of the nudged elastic band method22,23 (NEB), which provides geometries and energies of the transition states. CP2K code uses Goedecker-type24 pseudopotentials and a mixed Gaussian-type and plane wave basis set to expand the Kohn-Sham orbitals. The atomic orbitals of O, H, S and Ti atoms are expanded in DZVP and DZV Gaussian-type basis sets, respectively. Charge density is expanded in a plane-waves basis set with density cutoff of 800 Ry. Only gamma point is considered for the Brillouin Zone (BZ) integration. NEB-climbing image technique and a spring constant of k=0.09 a.u. are used. A minimization scheme is applied until the residual root mean square and maximum force components acting on each image are smaller than 0.01 and 0.05


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eV/Å, respectively. Due to the large size of our simulation cell, we did not perform a harmonic vibrational analysis to assess the order of the saddle points. Extensive consistency tests have been performed on the structures and formation energies obtained with VASP and CP2K. Liquid Phase Exfoliation TiS2 powder (99.9%, Lot #MKBQ6565V) and benzyl alcohol (Lot #SZBC2980V) were purchased from Sigma Aldrich. 100 mg of TiS2 powder was added to 50 ml benzyl benzoate in an argon filled glove box to minimize oxygen contact. The mixture was immediately sonicated at 20 kHz for three hours using a 34 W Fisherbrand ultrasonic dismembrator. The experiment was kept at a constant temperature or 20°C using a circular IsoUK 4100 R20 refrigeration bath and a closed system reaction vessel to hold the dispersions. The resultant dispersion was placed inside a 50 ml centrifuge tube and centrifuged at 1000 RPM for 1 hour using a Heraeus Multifuge X1 centrifuge. The supernatant was immediately pipetted out and stored in the argon filled glove box. Scanning Transmission Electron Microscopy The STEM samples were prepared by drop casting the TiS2 solution on to a 400-mesh holey carbon grid and subsequently baked overnight in a vacuum chamber. For the scanning transmission electron microscopy (STEM) an aberration-corrected 40-100 kV Nion UltrastemTM 100, equipped with a C5 Nion QO corrector, full correction up to six-fold astigmatism C5,6, a cold FEG emitter with 0.3eV energy spread and a UHV Enfina EELS spectrometer, was used and operated at 100 kV.25 Results and Discussion TiS2 Electronic Structure The wide range of existing reported values for the experimental bandgap of TiS2 and the intrinsic gap error in DFT makes any comparison between theory and experiments difficult. Therefore we have computed the TiS2 electronic structure by using progressively more refined descriptions of the DFT exchange and correlation (XC) functional. Figure 1(a) shows the band structure of bulk TiS2 calculated at the generalized gradient approximation (GGA) level with the PBE parameterization. Such XC functional returns TiS2 as a semi-metal with an indirect band-overlap of about 0.12 eV (see Table 1 for calculated bandgap values). The valence band maximum (VBM) is at Γ, while the conduction band minimum (CBM) is at L. The partial density of states projected on the different atomic species (PDOS) shows that the valence and conduction bands mainly originate from the S3p and Ti-3d orbitals, respectively [see Figure 1(b)]. The progressive inclusion of electron correlation in the functional has the effect of opening of the bandgap, while it

changes little the orbital nature of the bands. As shown in Figures. 1(c-d), the hybrid functional HSE06 produces a quasi-rigid upward shift of the CBM and TiS2 becomes a semiconductor with a bandgap of 0.41 eV. Finally, the Green's functions many-body quasi-particle GOWO approach further opens the bandgap to 1.01 eV. Turning now our attention to TiS2 monolayers it is clear that, unlike MoS2, as the number of layers decreases from bulk to the monolayer limit, the bandgap and the nature of the bands do not change significantly (see Figure 2). This property is robust against the computational method used. Interestingly, a very slight opening of the bandgap with respect to the bulk value is predicted with all methods, resulting in a band-overlap of 0.06 eV with GGA and in a bandgap of 0.48 eV and 1.12 eV for HSE06 and GOWO, respectively. This implies that the inter-layer interaction has little effect on the electronic structure of TiS2. Our GGA-PBE results are in agreement with similar previous calculations26 and with other using different semi-local XC functionals,27,28 but they conflict with earlier results,29-30 predicting bulk TiS2 to be a semimetal and the monolayer to be a semiconductor (indirect band-gap 1.0 eV). These differences are likely to arise from the different geometries used in the calculation and from the slightly different parameterizations of the XC functional. Microstructure of Exfoliated TiS2 Nanoflakes Experimental data obtained by AC-STEM on liquid phase exfoliated TiS2 (the process details are presented in the methods section) show that the sample consists of multi-layered sheets (see Figure S1 of the supporting information - SI). The crystal structure was determined to be hexagonal (P\3m1) by fast Fourier transformation (FFT), Figure 3b, of the high-resolution image of Figure 3a. Along the edges of the flakes an amorphous layer approximately 2 nm thick was detected. Electron energy loss spectroscopy (EELS) reveals that this consists of Ti and O. Moreover the EELS show that the centre of the flake is composed of pure Ti and S. However, we also find evidence for the presence of C along the edges and on the top of the flakes. Our STEM images clearly show that the centre of the flake has an overall crystalline structure, although they may also be consistent with a limited number of O mobile species at the top/bottom surfaces and a small number of S vacancies. Unfortunately, the noise level is too high to determine if TiS2 is truly oxidized or if the O atoms belong to the C contamination. An EELS sum-spectrum of the area is presented in Figure 3c showing the presence of the different elements. An EELS thickness measurement25 was also performed and the thickness was estimated to about 10 layers (Figure S5 in SI) for one of the thin protruding sheets.



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The formation of the TiO2 along the rim of the flake is as described by Han et al. in Ref. 12, i.e. it is in an early stage, where only the rim has been oxidized but the diffusion of Ti4+ has not yet started (no holes in the flake were observed). From the high-resolution STEM images (Figure 4), although not entirely conclusive, we cannot find evidence of any large amorphous TiO2 areas in the middle of the flakes’ surface. TiS2 Monolayers: Structure and Thermodynamics of Oxidation The optimized lattice parameters (a=b=3.414 Å; c=5.705 Å; the Ti-S bond length is 2.425 Å) for bulk and TiS2 monolayers are reported in Table 1. These values are in good agreement with those reported in previous theoretical31 and experimental32-33 studies. Oxidised TiS2 monolayers are modelled by replacing S with the valence isoelectronic element oxygen. As the main reactant for our exfoliated flakes is water we propose the following oxidation model, (1) TiS2 + xH2O → TiS2-xOx + xH2S thus that the formation energy of the oxidized monolayer is defined as Eform=[E(TiS2-xOx)+xE(H2S)]– [E(TiS2)+xE(H2O)}, (2) where E(TiS2-xOx) is the total energy of the flake obtained in a calculation with an O atom substituted in place of S, E(TiS2) is the total energy of a clean TiS2 monolayer, E(H2S) and E(H2O) are the energies of the H2S and H2O molecules, respectively. Hereafter positive formation energies correspond to endothermic processes. The optimized lattice constant and the formation energy, Eform, of oxidized flakes are listed in Table 2 for different O concentrations. The table shows that upon oxidation the lattice constant decreases from that of pristine TiS2. This is because the S ionic radius (1.84 Å) is larger than that of oxygen (1.40 Å). Interestingly, Eform is always negative (i.e. the process is always exothermic) and independent of the O concentration. Full substitution of S with O results into the formation of a 2D TiO2 hexagonal phase with octahedral symmetry, which is therefore thermodynamically more stable than the parent 2D-TiS2 structure. Only kinetic reasons hinder its formation.34 Previous theoretical calculations have shown that such 2D TiO2 phase is metastable against various 3D forms,35 but to our knowledge an isolated 2D-TiO2 nanostructure has never been synthetized. Using a relatively large 6 × 6 supercell (and a grid of 4 × 4 k-points to sample the Brillouin zone) we have also evaluated the pairing energy between couples of O substituents. This is defined as the energy difference, ∆E, between the configuration where two O atoms are placed in nearest neighbour positions, and the one where they are as far as possible in the supercell. We find that the situation where two O atoms sit close to each other is energetically preferable by about 20 meV/supercell.

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Based solely on their pairing energy, O-dopants thus tend to form clusters, although the cluster formation is generally controlled by the O diffusion energy barrier. The chemical reactivity of a TiS2 nanoflake is governed by its microscopic structure thus it can be different at its edges or in the middle. We have considered edges of Titerminated nanoribbons with 0%, 50% and 100% Spassivation (labelled as S0, S50 and S100, respectively). The S100 edge terminates with S buckled dimers facing each other. The structure of the S50 edge is characterized by a zigzag motif with S-up and S-down atoms lying across the Ti terminal row. This zigzag line is rigidly tilted toward one side of the plane defined by Ti atoms, leaving an open structure on the other side (See Figure S2 in SI). The S0 structure terminates with an unreconstructed Ti row. In Table 3 we show the dependence of the oxidation energies, Eform, on the nature (edge, surface or vacancy) of the oxidation site. TiS2 oxidation at the edge turns out favourable (exothermic) every time the conformational freedom for positioning the O atom is high, i.e. when the edge is not fully passivated. Thus, whilst the oxidation of the S100 edge is endothermic by about 1 eV, for S50 and S0 nanoribbons oxidation is an exothermic process both at their edge and in the middle. Eform ranges between 0.2 eV and 0.4 eV, depending on the oxidation site. Thermodynamically, oxidation in the middle of the nanoflake is equally favorable at a vacancy or a pristine site. In Table 3 a comparison of the energies calculated with the CP2K and VASP codes is also shown (the two codes implement different basis sets). In general the oxidation energies at the S100 edge are very similar, while those at the S50 edge are slightly different. This has to be ascribed to the slightly different atomic arrangements obtained after relaxation with the two codes. We also note that whilst the CP2K-calculated energies for oxidation at the defective S50 edge or in the middle of the monolayer are very similar, about Eform= 0.2 eV, VASP returns the oxidation process to be slightly more favorable at the edge than in the middle for both S50 and S0 nanoribbons. These differences are however minor and do not change our conclusions. Oxidation of TiS2 Monolayers: Reaction Pathways Our thermodynamic analysis has revealed that oxidation takes place at the flake edges only when this is not fully passivated by S. Therefore, in order to study the reaction pathways leading to oxidation we use the S50 nanoribbon. Our reaction model is described in equation (1) and involves a single water in gas phase at 0 Kelvin, which is a good approximation since intermolecular interac-


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tions and entropic contribution (translational, rotational and vibrational) to reaction energy (approximately 0.05 eV/molecule) are typically significantly smaller than the reaction barrier calculated here..ii In Figure 5 the potential energy profile along the oxidation process at the S50 edge is displayed together with snapshots of the atomic configurations of the various steps. The energy reference (E=0) is set to the energy of the configuration where a water molecule and a TiS2 nanoflake are at an infinite distance from each other. Then the oxidation reaction from water proceeds through four fundamental steps. 1. In the first oxidation step (insets 1 and 2 of Figure 5) the water molecule approaches the open side of the TiS2 edge, and physisorbes at a distance of 2.26 Å from the terminal Ti row. The physisorption energy amounts to 0.76 eV. 2.In the second oxidation step (2 → 3 in Figure 5), one H atom is exchanged from water to a neighbouring S-up atom, thus forming a thiol group (SH) and leaving an hydroxyl group (OH) adsorbed on Ti. This process has activation energy of 0.95 eV and reaction energy of 0.24 eV. The relative transition state is 0.19 eV above the energy zero. 3. In the next oxidation step (3 → 6 in Figure 5) a second H atom moves from OH to the opposite SH group, forming a H2S molecule. This detaches from the Ti row and ends up in a physisorbed state. Such step is prepared by subsequent rearrangements of the neighbouring OH and SH groups. Although these rearrangements (3 → 4 and 4 → 5) are endothermic processes, characterised by reaction (activation) energies of 0.39 (0.50) eV and 0.12 (0.17) eV, respectively, the overall transition state energy for the H transfer is lower than that for the direct transfer from configuration 3 to 6. The final 5 → 6 step represents the overall rate-limiting stage of the reaction and the transition state is 1.1 eV above the energy of the initial configuration 2. The final configuration is characterized by an O atom adsorbed at the edge in place of an S one. 4. The final oxidation step consists in the desorption of the H2S molecule from the edge (6 → 7, ∆E = 0.12 eV), which overall results in a configuration 0.18 eV lower in energy than the zero of our energy scale. From the thermodynamic point of view we also know that water oxidation can take place not only at the edge but also in the middle of the nanoflake, with very similar reaction energy (0.18 eV). In order to study TiS2 oxida-

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As such it provides an evaluation of the energetic and kinetic cost involved in the first step of TiS2 oxidation. Reactions involving the concerted contribution from more than one water molecule have not been considered here.

tion far from the edge we have used a model for an infinite and periodic monolayer (supercell). The calculated reaction pathway is presented in Figure S3 of the SI and it is characterized by a reaction trajectory rather similar to that found for oxidation at the edge. In general there are three main reaction steps: 1. Firstly, a water molecule forms a weakly physisorbed state on the TiS2 surface, -0.01 eV above the energy of the configuration where H2O is at an infinite distance from the flake (Figure S3, inset 1). 2. Then, an intermediate metastable configuration forms by moving one H atom from the water molecule to the neighbouring S atom. This is characterized by an OH group adsorbed on Ti and a SH group physisorbed on the surface. Such configuration is reached after overcoming a barrier of 2.2 eV and is less stable by 1.1 eV than our reference energy configuration (Figure S3, inset 2). 3. Finally, a physisorbed state is created where the H2S molecule is on the surface, at -0.21 eV with respect to the energy zero. This is the result of a second H transfer from the surface OH group to the physisorbed SH one (Figure S3, inset 3). The very high barrier needed to reach the, already unfavourable, first intermediate state indicates that this reaction step is very unlikely, being both thermodynamically and kinetically unfavourable. As a consequence only oxidation at the edge should be experimentally observed on defect-free nanoflakenanoflakes, since oxidation on the surface is both thermodynamically and kinetically hindered. As a final possibility we have investigated whether oxidation on the TiS2 surface can be initiated at a defective site, in particular at a S vacancy. This reaction is again thermodynamically favourable and here we study its kinetic. The reaction path and its energetics are reported in Figure S4 of the SI. Also for this process we find reaction intermediate steps rather similar to those found for the other pathways, namely 1. Firstly, the water molecule forms an initial adsorbed state in the vacancy site at -0.93 eV from the reference energy zero (Figure S4, inset 1) 2. Then, a second stable intermediate configuration is created at -1.15 eV, where a surface SH group faces an OH molecule adsorbed at the vacancy site. This configuration is reached after the transfer of an H atom from the water molecule to an adjacent S atom, overcoming an energy barrier of 0.7 eV (Figure S4, inset 2). 3. A physisorbed state for a H2S molecule is formed at 0.61 eV after a second H transfers from the OH to the SH group. The transition state for this step is at about 0.24 eV above the energy zero and represents the overall rate-limiting step of the oxidation process. 4. Finally, the H2S molecule is released (Figure S4, inset 3), with an overall thermodynamic energy balance of



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0.21 eV, very similar to that found for oxidation at the edge or in the middle of a defect free monolayer. We note that the transition state energy is about 1.2 eV higher than that of the adsorbed state for water. This is again very similar to what happens at the edge. Being characterized by similarly viable reaction pathways we can predict that the oxidation process occurs also far from the narrow edge of the TiS2 nanoflake if a surface S vacancy is present. We note that the metastable state with SH and OH facing each other (Figure S4, inset 2) is even more stable than the absorbed state for water. This means that in some experimental conditions the system might be partially blocked in this configuration, and the reaction would be slowed down. The reaction process can then start when a S vacancy is formed at the surface. The energy cost to remove a S atom from the TiS2 surface amounts to 2.36 eV. The vacancy formation energy is obtained as ∆E = [E(TiS2)- E(TiS2-x+xV) + xE(S2/2)], where we have used the normal reference state of a S2 molecule in the gas phase. Importantly, even if the energy cost to initiate the oxidation is pretty high, every oxidation process ends up with the formation of a new S vacancy next to the adsorbed O atom. This is available for reaction with another incoming water molecule. An oxidation chain reaction can then start once the first S vacancy is present on the TiS2 surface. Assuming that one can control the initial S vacancy density by a suitable choice of exfoliation process parameters, our result suggests that controllable oxidation of TiS2 may be possible. Modulation of the TiS2 Gap by O Substitution We have just demonstrated that doping a TiS2 monolayer with O will be a viable process, if it is properly initiated by the generation of surface vacancies or it is nucleated from the edges of a flake. Important technological applications may be enabled by this possibility, since it does allow a fine-tuning control of the bandgap of the material. In Figure 6 the variation of the bandgap against O concentration is presented. In order to evaluate such behaviour, we have used a 3 × 3 supercell and varied the O concentration by substituting the appropriate number of S atoms with O. For instance, for a TiS2-xOx composition with x=1 half of the S atoms (namely, 9) are replaced by O. This particular x=1 configuration is shown in Figure 6(b). Interestingly, the band-gap increases monotonically with O concentration. This can be explained by recalling that the CBM and the VBM are dominated by Ti-3d and S-3p bonding and anti-bonding states. Since the 2p shell of oxygen is located below the 3p of S, the bandgap opens monotonically when the O concentration increases. The largest bandgap-opening is expected when all the S atoms are replaced by O, namely for TiO2. Notably, the bandgap increases linearly with

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O concentration up to x=1, then, upon further O doping, the trend remains linear but the slope becomes larger. Such slope transition arises because at x≈1.0 the nature of the bandgap changes from indirect, from the Γ to the M point, to direct at Γ (see Figure S5 in SI). We have further studied the dependency of the bandgap on the specific arrangement of the substitutional O atoms. In particular we have considered as an illustration the case of x=1 and found the bandgap ranging in the interval 0.44-0.54 eV depending on the actual position of the O and S atoms, namely the bandgap appears not to be very sensitive to the specific microscopic configuration. Since all the results presented so far have been obtained with the GGA, which typically underestimates the bandgap, as a final step we have re-calculated the TiS2xOx bandgap for x=0, x=1 and x=2 by using HSE06 and GOWO. We find that even if the absolute values of the gap calculated with different methods differ, the slope of the bandgap variation with x is robust against the computational approach. In conclusion a simple way to engineer the gap in TiS2 has been devised. This consists in controlling the amount of O substitution, which ultimately may be obtained with a fine optimization of the exfoliation parameters. Conclusions By using a combined computational and experimental approach, we have studied the oxidation of TiS2 nanoflakenanoflakes, in terms of microstructure, electronic properties, formation energies and reaction pathways. Even if the electronic properties of TiS2 are sensitive to the level of theoretical description the general features and trends of the electronic structure do not depend on the approach used. DFT at the GGA level predicts that TiS2 bulk and monolayers are both semimetallic with a band overlap of 0.12 eV and 0.06 eV, respectively. Both TiS2 forms turn out to be semiconductor with an indirect bandgap of 0.41 eV (bulk) and 0.48 eV (monolayer) when one uses the HSE06 functional and finally GOWO predicts bandgaps as large as 1.01 eV (bulk) and 1.12 eV (monolayer). Interestingly for all methods used, the opening of the bandgap is small (≈ 0.06 eV) when the thickness changes from bulk to the single layer, implying that the interlayer interaction has a limited effect on the electronic structure of this material. AC-STEM data show for the first time the geometric microstructure of an oxidised TiS2 nanoflakewith atomic resolution. We observe that the centre of the TiS2 flake has an ideal crystalline P\3ml structure and that oxidation has occurred at the edge of the nanoflakenanoflake, forming an amorphous TiO2 structure consistently with an early stage oxidation involving water. Finally, we have modelled the oxidation pathways of TiS2 nanoflakenanoflakes, finding that this is an edge selective process. After oxidation a S atom is replaced by the iso-electronic element O. This is a multistep reac-


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tion, passing through (1) a physisorbed state for water on the surface, (2) the formation of OH and SH groups, adsorbed on the surface, (3) the generation and subsequent release of a H2S molecule. The reaction energy (barrier) amounts to ≈ 0.2 (1.1) eV. The same reaction steps characterize both oxidations at the edge and in the middle of TiS2 nanoflakenanoflakes. Far from TiS2 edges, oxidation is globally thermodynamically allowed but kinetically hindered, unless the reaction is initiated by the presence of a surface vacancy. Although the energy cost to remove a S atom from the TiS2 surface is pretty high (2.36 eV), once a vacancy is formed, a chain oxidation reaction can be sustained. This suggests that oxidation is viable also far from the edges and a suitable selection of the experimental technique can result in controlling the process. Crucially, when S atoms are replaced by O, the bandgap opens, thus that a fine-tuning control of the gap of TiS2 monolayers is made possible by O substitution. Important technological applications for surface catalysis and photovoltaics are enabled by this possibility.



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FIGURES

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Figure 1. Electronic structure of bulk TiS2: (a) band structure calculated at the GGA level; density of states calculated by using (b) GGAPBE, (c) HSE06 (c) and (d) GOWO. In panel (a) the valence and conduction bands are represented with a green and red line, respectively. The VBM is aligned with the energy zero and it is highlighted by a dashed blue line.

Figure 2. Electronic structure of a TiS2 monolayer: (a) band structure calculated at the GGA-PBE level; (b) band structure calculated at the HSE06 level; density of states calculated by using (c) GGA-PBE, (d) HSE06 and (e) GOWO. The valence and conduction bands are represented with a green and red line, respectively. The VBM is aligned with the energy zero and it is highlighted by a dashed blue line.


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Figure 3. High-resolution image of a TiS2 flake edge showing an extended crystalline region in the upper part. The bright dots in the image correspond to Ti atoms surrounded by S atoms. Below the dotted line an area of amorphous TiO2 is indicated. In the magnified HAADF image (blue box in the image) the Ti and S atom columns are well separated. Beside the HAADF image we present the corresponding Electron Energy Loss (EEL) spectrum images (elemental maps) of S, C, Ti and O respectively (left to right) for the same region. These elemental maps confirm that the bright areas in the HAADF STEM image are indeed Ti atoms, surrounded by S. In the part of the The O signal is predominantly localised at the edge (dotted region). Some C contamination can be seen, especially at the edge. b) FFT of Survey image in (a) (gamma set to 0.4 to suppress the background) and c) the EELS sum-spectrum used to produce the elemental maps presented in (a). The low signal to background ratio at the O edge explains the noise in the O-map..



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Figure 4. High-resolution image of the flake centre. In this image bright spots correspond to Ti columns, while darker spots correspond to S columns. The intensity profiles along the four indicated lines show very small intensity fluctuations in the S columns. These variations are of the same order of noise level. Substitutions of a S with an O would give detectable lower intensity (surely above noise level); whilst, detecting only one/few O atoms on top of a S column would give a non-detectable fluctuation like the one experimented in this case.). The intensity fluctuations in the Ti columns could however be attributed a missing Ti atom in the column.

Figure 5. Oxidation path for a water molecule at the edge of a TiS2 nanoflake (50% S coverage). Together with the energy diagram we present a number of balls and sticks panels representing all the intermediate steps along the reaction path. The numbers label the various reaction intermediate configurations. Yellow, pink, red and white spheres represent S, Ti, O and H atoms, respectively.


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Figure 6. Variation of the TiS2-xOx electronic structure with O concentration. The upper panels show the top and side views of a (a) clean and (b) 50% O-substituted monolayer modelled by using a (3×3) supercell. The bandgap variation against O-concentration is presented in panel (c). Results are shown for calculations performed with GGA-PBE (blue line), HSE06 (red line) and GOWO (green line).

TABLES. Table 1. Unit-cell parameters and band-gaps for TiS2 bulk and monolayers, calculated using GGA-PBE, HSE06 and GOWO. ∆Eg represents the bandgap. Negative bandgaps indicate an overlap between the valence and the conduction band (the material is a semi-metal).

Experiment Bulk Monolayer

PBE

HSE06

a=3.41 Å a=3.414 Å a=3.41 Å c=5.69 Å c=5.709 Å c=5.69 Å ∆Eg=-1.5-2.5 eV ∆Eg=-0.12 eV ∆Eg=0.41eV

GoWo

∆Eg=1.01 eV

a=3.41 Å a=3.407 Å ∆Eg=1.12eV ∆Eg=-0.06 eV ∆Eg=0.48 eV

Table 2. Optimized lattice constants, formation energies and bandgaps for oxidized systems with different O concentrations. In order to model different concentrations a S atom is replaced by an O atom in n × n supercells of different size, with n running from 1 to 7.

S:O ratio Cell (n × n) 1×1 1:1 2×2 7:1 3×3 17:1

Lattice constant (Å) Eform (eV) ∆Eg (eV) 3.18 3.35 3.39

-0.332 -0.319 -0.326


0.54 0.21 0.08


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4×4 5×5 6×6 7×7

31:1 49:1 71:1 98:1

3.40 3.405 3.41 3.41

-0.319 -0.328 -0.303 -0.302

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-0.02 -0.04 -0.05 -0.05

Table 3. Formation energies, Eform, in eV for O substitution at the edge or in the middle of a TiS2 nanoflakenanoflake. The oxidation process considered is described by equations (1) and (2) in the main text. S100, S50 and S0 represent nanoflakenanoflakes with 100%, 50% and 0% S passivated edges, respectively. In S50-line system the terminal row of S atoms is aligned. In the S50 system the S atoms form a zigzag line. Up and down refer to the oxidation site in the S50-zigzag geometry. TiS2 represents our model for the infinite ideal monolayer modelled by using a 8×8 supercell. -vac indicates the presence of a S surface vacancy (when a S vacancy initiates oxidation, O adsorbs at the vacancy site and the process ends up in a configuration with a S vacancy next to the adsorbed O atom). Values labelled with a star are obtained using the CP2K code, while the others are obtained with VASP.

S100 S100* S50-line S50* S0 TiS2 TiS2-vac* TiS2*

Edge 1.03 1.00 -0.40 -0.18 (up) -0.22 (down) -0.47

Middle -0.31 -0.32 -0.32 -0.30 -0.21 -0.18

ASSOCIATED CONTENT

Funding Sources

Supporting Information. Figure S1: STEM Ronchigram overviewing a flake; the area, where the low-loss EELS spectrum was collected and the thickness of the flake. Figure S2: Zigzag structure of the 50% S saturated edge of a TiS2 nanoflake. Figure S3: Oxidation path for a water molecule reacting in the middle of a TiS2 nanoflake. Figure S4: Oxidation path for a water molecule reacting in the middle of a TiS2 nanoflake at a S vacancy site. Figure S5: GGA-bandstructure (left panel) and the PDOS (right panel) of a TiSO monolayer. This material is available free of charge via the Internet at http://pubs.acs.org.

ERC Consolidator (QUEST); SFI-AMBER; SFI PIYRA 13/YI/I2601; ERC 2DNanoCaps Grant Agreement N° 278516; FP7 MoWSeS Grant agreement No: 317451. SS and CSC have been supported by the European Research Council (Quest project). VN wishes to thank the support of: the SFI PIYRA grant, the European Research Council (2DNanoCaps project) and the EU ITN MoWSeS project within the FP7 framework.

AUTHOR INFORMATION Corresponding Authors * [email protected]; [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. CSC and KD wrote the first draft of the manuscript and performed the calculations; HP and QMR acquired the AC-STEM and EELS; SOB prepared the samples, HP and EL performed the microscopy data analysis. SS and CSC planned the calculations. VN planned the experiments.

ACKNOWLEDGMENT SS and CSC have been supported by the European Research Council (Quest project). All calculations were performed on the Parsons cluster maintained by the Trinity Centre for High Performance Computing, under project id: HPC_12_0722. This cluster was funded through grants from Science Foundation Ireland. VN wishes to thank the support of: the SFI PIYRA grant, the European Research Council (2DNanoCaps project) and the EU ITN MoWSeS project within the FP7 framework. SuperSTEM is the U.K. National Facility for Aberration-Corrected STEM, supported by the Engineering and Physical Sciences Research Council (EPSRC). Dr. Evie Doherty is acknowledged for reading the manuscript.


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TOC

For Table of Contents Only



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256x144mm (150 x 150 DPI)


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Electronic Properties and Chemical Reactivity of TiS2 Nanoflakes

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