Synthetic Strategy and Structural and Optical Characterization of

Hydrogen-Incorporated TiS2 Ultrathin Nanosheets with Ultrahigh Conductivity for Stamp-Transferrable Electrodes. Chenwen Lin , Xiaojiao Zhu , Jun Feng ...
0 downloads 0 Views 3MB Size
Download PDF
Letter pubs.acs.org/JPCL

Synthetic Strategy and Structural and Optical Characterization of Thin Highly Crystalline Titanium Disulfide Nanosheets Vladimir V. Plashnitsa,*,† Felix Vietmeyer,† Nattasamon Petchsang,† Pornthip Tongying,† Thomas H. Kosel,‡ and Masaru Kuno† †

Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States ‡ Department of Electrical Engineering, University of Notre Dame, 201 Cushing Hall, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Two-dimensional (2D) nanomaterials have recently received significant attention because of their attractiveness for use in many nanostructured devices. Layered transition-metal dichalcogenides are of particular interest because reducing their dimensionality causes changes in their already anisotropic physical and chemical properties. The present study describes the first bottom-up solution-phase synthesis of thin highly crystalline titanium disulfide (TiS2) nanosheets (NSs) using abundant low-cost molecular precursors. The obtained TiS2 NSs have average dimensions of ∼500 nm × 500 nm in the basal plane and have thicknesses of ∼5 nm. They exhibit broad absorption in the visible that tails out into the near-infrared. The obtained results demonstrate new opportunities in synthesizing low-dimensional 2D nanomaterials with potential use in various photochemical energy applications. SECTION: Physical Processes in Nanomaterials and Nanostructures

I

TiS2 nanodisks8,9 as well as spherical fullerene-like nanoparticles.10 The present study describes the first successful bottom-up solution-phase synthesis of crystalline TiS2 NSs using abundant low-cost precursors. The obtained TiS2 NSs have average dimensions of 500 nm × 500 nm with thicknesses between 4 and 6 nm. They show broad absorption between 500 and 650 nm (1.91−2.25 eV), tailing out into the near-infrared. The preparation of high-quality TiS2 NSs entails dissolving a slight molar excess of elemental sulfur in oleylamine (OLA), followed by the introduction of titanium(IV) chloride (TiCl4) at room temperature under inert (N2) conditions. The reaction mixture is subsequently heated to a final growth temperature of 300 °C, whereupon it is kept at this value for 3 h. Additional details about the synthesis can be found in the Experimental Section as well as in the SI (Table S1). Figure 1 shows representative (a) low- and (b, d) highresolution transmission electron microscopy (TEM) images along with (c, e) selected area electron diffraction (SAED) patterns of obtained TiS2 NSs. Additional scanning electron microscopy (SEM) images as well as TEM images can be found in Figures S2 and S3 of the SI. Note that even though many TiS2 NSs are bundled, single ultrathin NSs can readily be observed. This suggests that postsynthesis exfoliation techni-

n recent years, fundamental and industrial interests exist in developing nanostructured layered compounds because reducing their dimensionality leads to corresponding changes of their already anisotropic physical and chemical properties.1,2 This opens up new opportunities for investigating the evolution of two-dimensional (2D) optical and electrical properties in systems beyond graphene.3 Layered titanium dichalcogenides (TiX2, where X = S, Se, and Te) are representative members of an extended family of 2D metal dichalcogenides that exhibit sizable differences in their in-plane versus out-of-plane conductivities (σ||/σ⊥ ≈ 102−103).4 This and other anisotropies arise from the presence of strong, intralayer M−X bonding in contrast to the weak van der Waals (vdW) interactions between layers. In particular, TiS2 crystallizes in a hexagonal layered structure (i.e., ABCABC...), which consists of one hexagonally packed sheet of Ti atoms sandwiched between two hexagonal sheets of sulfur for each monolayer [Figure S1, Supporting Information (SI)]. Conventional TiS2 thin films are prepared using chemical vapor deposition (CVD).5−7 This involves decomposing different titanium and sulfur precursors over a substrate at temperatures exceeding 500 °C. However, this approach is costly, presents limiting opportunities for creating TiS2 nanostructures, and results in wide deviations from the expected TiS2 stoichiometry.5,6 As a consequence, colloidal wet chemical approaches have recently been attempted as a first step toward producing nanocrystalline TiS2. This has yielded © XXXX American Chemical Society

Received: April 20, 2012 Accepted: May 23, 2012

1554

dx.doi.org/10.1021/jz300487p | J. Phys. Chem. Lett. 2012, 3, 1554−1558

The Journal of Physical Chemistry Letters

Letter

A key aspect of the NS preparation is the use of amines as a growth solvent, which facilitates single-crystal nanostructure growth. OLA as well as other primary amines react with sulfur to produce sulfur-containing complexes that possess favorable decomposition kinetics. This hypothesis is corroborated by earlier studies, which have reported alkylammonium polythioamine salts12 as well as polythiobisamines13,14 as products stemming from the reaction between primary amines and sulfur. More recently, alkylammonium polysulfides [RR′NH2+)2Sn2−] have been suggested to be the principal reaction product from mixing elemental sulfur with OLA.15 These sulfur-containing complexes all produce in situ H2S when heated, which can subsequently react with available metal precursors to produce desired metal sulfides. In the current synthesis, when TiCl4 is mixed with the above OLA−sulfur precursor, a color change from deep red to black occurs. No precipitate is observed. Only when the mixture is heated to ∼200 °C does an apparent reaction occur as evidenced by the evolution of H2S gas. If the reaction is quenched at this point, a solid black product can be recovered by centrifuging the reaction mixture. Figure 2b shows its X-ray diffraction (XRD) powder pattern. Given the absence of any reflections, we conclude that the material is either amorphous or consists of small molecular clusters. This is supported by TEM observations of the product, which show ill-defined, surfactant-coated aggregates.

Figure 1. (a) Low- and (b) high-resolution TEM images of TiS2 NSs. (c) SAED pattern of TiS2 NSs stacked atop each other. (d) Highresolution TEM image showing the edge of a single TiS2 NS and (e) SAED pattern of a single TiS2 NS.

ques can be used to further optimize the TiS2 NS thickness as well as quality in future studies. Figure 1b,c illustrates the case of several (approximately six) TiS2 sheets stacked atop each other, while Figure 1d,e shows data obtained from a single sheet. Detailed SAED indexing can be found in the SI (Figure S4). Obtained specimens are typically ∼500 × 500 nm (average in-plane size of 550 ± 240 nm; sample size of 230 NSs), with a sizing histogram shown in Figure S5 of the SI. Associated thicknesses are ∼5 nm along the ⟨001⟩ out-of-plane growth axis. The lattice fringes in Figure 1b,d illustrate each NS’s high degree of crystallinity. They arise from an underlying hexagonal lattice characteristic of TiS2. This is further corroborated by the excellent agreement between experimental (100) and (110) dspacings of 0.290 ± 0.008 and 0.170 ± 0.002 nm compared to corresponding literature values of 0.295 and 0.170 nm.11 TiS2 NS stoichiometries have additionally been confirmed using Energy-dispersive X-ray spectroscopy (EDXS) (Figure S6, SI). Quantitative analyses of the Ti/S atomic ratio indicate that the produced TiS2 NSs possess the expected stoichiometry of titanium disulfide (35.6 ± 1.7 atom % Ti; 64.4 ± 1.7 atom % S).

Figure 2. (a) JCPDS powder pattern #88-2479 for hexagonal (P3̅m1) TiS2. Powder pattern for material formed (b) initially after TiS2 growth commences, (c) after heating to 300 °C, and (d) after growing the material at 300 °C for 3 h. Asterisks in (d) denote impurity reflections that are most likely from TiO2 (JCPDS #84-1286). Panels (e) and (f) show closeups of the (001) and (110) reflections. 1555

dx.doi.org/10.1021/jz300487p | J. Phys. Chem. Lett. 2012, 3, 1554−1558

The Journal of Physical Chemistry Letters

Letter

highlights the rapid carrier cooling that occurs following excitation (inset, Figure 3b). Subsequent bleach recovery occurs on a slower ∼15 ps time scale. The strong linear absorption at ∼550 nm as well as the delayed bleach recovery suggests that this feature is the direct M2−−M1+ transition of TiS2.19,20 Samples also absorb across the near-infrared, which, in turn, suggests the absence of a sizable band gap. In particular, absorption is observed for wavelengths up to 1700 nm (Figure S5, SI). This suggests that despite their narrow thicknesses, the current NSs exhibit bulk-like optical properties. While pristine nanosheets exhibit a 1:2 Ti/S atomic ratio, thin and highly crystalline nanobelts (NBs) are occasionally observed in HRTEM images (e.g., Figure 1a; see also Figure S8, SI). They possess Ti/S ratios of ∼1:1.8 to 1:1.7. These stoichiometries are consistent with the partial oxidation of TiS2. The hypothesis is supported by minor TiO2 XRD reflections in Figure 2d, which can be caused either by the formation of nanostructured nonstoichiometric TiO2 domains within the NBs due to impurities in the chemicals used (e.g., OLA) or by their conversion to TiO2 during the XRD experiment. Our attempts to separate pristine TiS2 NSs from the small fraction of NB byproducts have so far been unsuccessful. Nevertheless, this observation suggests that large surface area TiS2 NSs can be attractive precursors for the creation of oxysulfide TiS2−xOx NSs whose favorable electron affinities21 make them candidates for creating next-generation photocatalysts in the visible beyond UV-photosensitive TiO2.22 Recent theoretical calculations suggest that nanostructured TiS2 undergoes a thickness-dependent semimetal-to-semiconductor transition.23−25 Molecular dynamics (MD) and density functional theory (DFT) simulations reveal that the band gap (Eg) of nanostructured TiS2 can be tuned from 1.25 (992 nm) to 1.65 eV (752 nm) once its morphology changes from that of nanotubes23,24 to single-sheet monolayers.24 Predicted Eg values of the latter species show that semiconducting TiS2 NSs should be optically active in the visible. Hence, they are potentially useful for sunlight-driven energy applications, such as solar cells and photochemical water splitting cells. While the current NSs show bulk-like optical properties, their nanometer thicknesses suggest that further optimization may lead to thinner NSs within a size regime where such a semimetal-to-semiconductor transition can be observed. To summarize, high-quality, crystalline TiS2 NSs have been produced for the first time using a facile wet chemistry synthesis. Experimental TiS2 NS structural and optical properties are consistent with thin TiS2 NSs possessing bulklike electronic structure. The optimized synthesis of large surface area TiS2 NSs may in turn lead to the eventual production of semiconducting TiS2 NSs having favorable electron affinities for potential solar-driven energy applications. These TiS2 NSs can additionally be partially oxidized to fabricate nanoscale titanium(IV) oxysulfides (TiS2−xOx), which offer tantalizing opportunities for achieving broad-band optical absorption, high photochemical activity, and stability.

If, however, the reaction is not quenched but is instead heated to temperatures between 250 and 300 °C, the initial product begins to crystallize. Several characteristic XRD reflections emerge at 15.61, 30.84, and 54.10° (Figure 2c). They are associated with the (001), (100), and (110) reflections of hexagonal TiS2 (JCPDS# 88-2479) (Figure 2a). This indicates NS crystallization, induced by the increase of the growth solution’s temperature. Upon prolonged heating at 300 °C for 3 h, the product’s XRD pattern (Figure 2d) fully matches the characteristic reflections of hexagonal TiS2.11 At this stage, what is interesting is that the full width at halfmaximum (fwhm) of the (001) reflection does not significantly narrow from that seen earlier in Figure 2c (see Figure 2e). An estimated domain size using the Scherrer equation is 5 ± 1 nm. Because the (001) intensity and its width reflect growth along the NS c-axis, we conclude that crystallization along [001] is self-limiting. We tentatively suggest that excess OLA in the reaction mixture restricts NS growth along this direction because it has previously been shown to react with sulfur12−15 and can hence passivate terminal S atoms of a given monolayer (Figure S1, SI). OLA has also been suggested to form a soft template16,17 that can similarly yield anisotropic growth. The optical properties of TiS2 NSs have been examined using linear absorption as well as transient differential absorption (TDA) spectroscopy. Given that the average NS thicknesses are comparable to the TiS2 bulk exciton Bohr radius (aB ≈ 2.5 nm), strong confinement effects are not expected.18 Figure 3a shows

Figure 3. (a) Linear absorption and (b) corresponding transient differential absorption (TDA) spectra of TiS2 NSs suspended in toluene (inset: kinetic trace of the bleach growth/recovery taken at 550 nm).

■

EXPERIMENTAL SECTION Titanium(IV) chloride (1 M in toluene, ≥99%), copper(II) sulfate (98%), silver(I) nitrate (0.1 M in acetonitrile, ≥99%), oleylamine (OLA, technical grade, 70%), and tributylphosphine (TBP, 97%) were purchased from Sigma-Aldrich. Sulfur powder (99+%) and trioctylphosphine oxide (TOPO, 99%) were purchased from Strem Chemicals. Dodecylamine (DDA,

an ensemble NS extinction spectrum, revealing a broad absorption between 500 and 650 nm. A full spectrum extending out to 1700 nm can be found in the SI (Figure S7). Figure 3b shows complementary femtosecond TDA traces, which coincide with the peak of the linear absorption. The center of the bleach (λ ≈ 550 nm) grows rapidly within 2 ps and 1556

dx.doi.org/10.1021/jz300487p | J. Phys. Chem. Lett. 2012, 3, 1554−1558

The Journal of Physical Chemistry Letters

Letter

of oxygen-doped TiS2 NBs. This material is available free of charge via the Internet at http://pubs.acs.org.

98%) and 1-hexadecylamine (HDA, 90%) were purchased from Acros Organics. Squalane (≥95%) and tetrabutylammonium hexafluorophosphate (TBAPF6, ≥99%) were purchased from Fluka. Potassium phosphate monobasic (99.6%), isopropyl alcohol, and acetonitrile were purchased from Fischer Scientific. Toluene and methanol were purchased from VWR. Unless otherwise noted, all chemicals were used as received. A representative synthesis of TiS2 NSs is described. More details about the influence of various synthetic parameters can be found in the SI. In a three-neck flask connected to a Schlenk line, elemental sulfur (160.3 mg, 5.0 mmol) is dissolved in OLA (5.0 mL, 10.6 mmol). The color of the OLA solution becomes deep orange, whereupon the reaction mixture is dried and degassed at 100 °C for 1 h. The vessel is then backfilled with N2 and is cooled back to room temperature. Once the temperature has stabilized, TiCl4 (0.4 mL of a 1 M solution in toluene, 0.4 mmol) is injected into the three-neck flask. An immediate color change occurs from deep orange to black. Following injection, the resulting reaction mixture is heated to 300 °C with a temperature ramp rate of ∼10 °C/min. Once at 300 °C, the mixture is kept at this temperature for 3 h. Upon completion of the reaction, it is quickly cooled back to room temperature, followed by the addition of TBP (2.0 mL, 8.1 mmol) to dissolve any unreacted sulfur. Resulting TiS2 NSs are recovered by centrifuging the growth mixture and discarding the supernatant. The recovered black precipitate is then washed multiple times with a 1:1 (by volume) toluene/methanol mixture to remove any excess OLA. Obtained TiS2 NSs are characterized using TEM. To do this, dilute NS suspensions in toluene are dropped onto 200 mesh carbon substrates (Ted Pella). All samples are initially screened with a JEOL 100SX instrument. Low- and high-resolution TEM micrographs are acquired with a Titan 80-300 TEM (FEI) operating at 300 kV. Elemental analyses are conducted with an EDXS attachment (Oxford) to the Titan. SEM images are taken using a field emission scanning electron microscope (Magellan, FEI) operated at 10 kV. Ensemble XRD powder patterns are obtained using a Bruker D8 Discover powder diffractometer (Cu Kα source). Linear absorption spectra of TiS2 NSs suspended in toluene are recorded with a Cary 50 Bio UV/visible (Varian). Additional spectra are acquired with a JASCO V-670 spectrophotometer from 300 to 1700 nm. TDA spectra are obtained using a femtosecond transient differential absorption spectrometer based on a Clark-MXR CPA 2010 Ti:sapphire laser and a commercial transient absorption spectrometer (Helios, Ultrafast systems). The pump wavelength is 387 nm, while the white light probe is generated by passing a fraction of the fundamental (775 nm) through a sapphire plate. The instrument’s temporal resolution is ∼200 fs, with an accompanying spectral resolution of ∼1.5 nm. All measurements are carried out in quartz cuvettes with samples stirred to avoid any precipitation.

■

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +1 574 631 7441. Fax: +1 574 631 6652. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This work was financially supported by the Center for Sustainable Energy at Notre Dame (CSEND). The authors thank the Center’s Materials Characterization Facility as well as the Notre Dame Radiation Laboratory/Department of Energy, Office of Basic Energy Sciences for use of their facilities and equipment. V.V.P. thanks Dr. Anthony C. Onicha for assistance in taking TEM images as well as Dr. Galyna Krylova and Matthew McDonald for useful discussions.

(1) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451−10453. (2) Raveau, B. Strongly Correlated Electron Systems: From Chemistry to Physics. C. R. Chim. 2011, 14, 856−864. (3) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (4) Schepherd, F. R.; Willaims, P. M. Photoemission Studies of the Band Structures of Transition Metal Dichalcogenides. I. Groups IVA and IVB. J. Phys. C: Solid State Phys. 1974, 7, 4416−4426. (5) Carmalt, C. J.; O’Neill, S. A.; Parkin, I. P.; Peters, E. S. Titanium Sulfide Thin Films from the Aerosol-Assisted Chemical Vapour Deposition of [Ti(SBut)4]. J. Mater. Chem. 2004, 14, 830−834. (6) Peters, E. M.; Carmalt, C. J.; Parkin, I. P. Dual-Source Chemical Vapour Deposition of Titanium Sulfide Thin Films From Tetrakisdimethylamidotitanium and Sulfur Precursors. J. Mater. Chem. 2004, 14, 3474−3477. (7) Zhang, Y.; Li, Z.; Jia, H.; Luo, X.; Xu, J.; Zhang, X.; Yu, D. TiS2 Whisker Growth by a Simple Vapor-Deposition Method. J. Cryst. Growth 2006, 293, 124−127. (8) Park, K. H.; Choi, J.; Kim, H. J.; Oh, D.-H.; Ahn, J. R.; Son, S. U. Unstable Single-Layered Colloidal TiS2 Nanodisks. Small 2008, 4, 945−950. (9) Jeong, S.; Han, J. H.; Jang, J.-T.; Seo, J.-W.; Kim, J.-G.; Cheon, J. Transformative Two-Dimensional Layered Nanocrystals. J. Am. Chem. Soc. 2011, 133, 14500−14503. (10) Prabakar, S.; Collins, S.; Northover, B.; Tilley, R. D. Colloidal Synthesis of Inorganic Fullerene Nanoparticles and Hollow Spheres of Titanium Disulfide. Chem. Commun. 2011, 47, 439−441. (11) Swanson, H. E.; Morris, M. C.; Evans, E. H. Standard X-ray Diffraction Powder Patterns; Natl. Bur. Stand. (U.S.) Monograph 25; U.S. Department of Commerce: Washington, DC, 1966. (12) Davis, R. E.; Nakshbendi, H. F. Sulfur in Amine Solvents. J. Am. Chem. Soc. 1962, 84, 2085−2090. (13) Levi, T. G. Solfuri e Polisolfuri di Basi Organiche. Gazz. Chim. Ital. 1930, 60, 975−987. (14) Levi, T. G. Le N Politioamine Superiori Alle Ditio. Gazz. Chim. Ital. 1931, 61, 286−293. (15) Thomson, J. W.; Nagashima, K.; Macdonald, P. M.; Ozin, G. A. From Sulfur−Amine Solutions to Metal Sulfide Nanocrystals: Peering into the Oleylamine−Sulfur Black Box. J. Am. Chem. Soc. 2011, 133, 5036−5041. (16) Cademartiri, L.; Ozin, G. A. Ultrathin Nanowires  A Materials Chemistry Perspective. Adv. Mater. 2009, 21, 1013−1021.

ASSOCIATED CONTENT

S Supporting Information *

Schematic view of TiS2 layering; effect of growing solvent and temperature on nanostructured TiS2 morphology; SEM images; low- and high-resolution TEM images, detailed SAED indexing, size distribution histogram, EDXS spectrum, quantitative EDXS analysis and UV−visible linear absorption spectrum of the obtained TiS2 NSs; and low- and high-resolution TEM images 1557

dx.doi.org/10.1021/jz300487p | J. Phys. Chem. Lett. 2012, 3, 1554−1558

The Journal of Physical Chemistry Letters

Letter

(17) Huo, Z.; Tsung, C.-K.; Huang, W.; Zhang, X.; Yang, P. Sub-Two Nanometer Single Crystal Au Nanowires. Nano Lett. 2008, 8, 2041− 2044. (18) Let, A. L.; Mainwaing, D.; Rix, C.; Murugaraj, P. Synthesis of Optical Properties of TiS2 Nanoclusters. Rev. Roum. Chim. 2007, 52, 235−241. (19) Myron, H. W.; Freeman, A. J. Electronic Structure and Optical Properties of Layered Dichalcogenides: TiS2 and TiSe2. Phys. Rev. B 1974, 9, 481−486. (20) Beal, A. R.; Knights, J. C.; Liang, W. Y. Transmission Spectra of Some Transition Metal Dichalcogenides: I. Group IVA: Octahedral Coordination. J. Phys. C: Solid State Phys. 1972, 5, 3531−3539. (21) Umezawa, N.; Janotti, A.; Rinke, P.; Chikyow, T.; Van de Walle, C. G. Optimizing Optical Absorption of TiO2 by Alloying with TiS2. Appl. Phys. Lett. 2008, 92, 041104. (22) Wang, H.; Lewis, J. P. Second-Generation Photocatalytic Materials: Anion-Doped TiO2. J. Phys.: Condens. Matter 2006, 18, 421−434. (23) Teich, D.; Lorenz, T.; Joswig, J.-O.; Seifert, G.; Zhang, D.-B.; Dumitrica, T. Structural and Electronic Properties of Helical TiS2 Nanotubes Studied with Objective Molecular Dynamics. J. Phys. Chem. C 2011, 115, 6392−6396. (24) Ivanovskaya, V. V.; Seifert, G.; Ivanovskii, A. L. Electronic Structure of Titanium Disulfide Nanostructures: Molayers, Nanostripes and Nanotubes. Semiconductors 2005, 39, 1058−1065. (25) Liu, Y.-H.; Porter, S. H.; Goldberger, J. E. Dimensional Reduction of a Layered Chalcogenide into a 1D Near-IR Direct Band Gap Semiconductor. J. Am. Chem. Soc. 2012, 134, 5044−5047.

1558

dx.doi.org/10.1021/jz300487p | J. Phys. Chem. Lett. 2012, 3, 1554−1558