Hybrids of Two-Dimensional Ti3C2 and TiO2 Exposing {001} Facets

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Hybrids of Two-Dimensional Ti3C2 and TiO2 Exposing {001} Facets toward Enhanced Photocatalytic Activity Chao Peng, Xianfeng Yang, Yuhang Li, Hao Yu, HongJuan Wang, and Feng Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11973 • Publication Date (Web): 09 Feb 2016 Downloaded from http://pubs.acs.org on February 11, 2016

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ACS Applied Materials & Interfaces

Hybrids of Two-Dimensional Ti3C2 and TiO2 Exposing {001} Facets toward Enhanced Photocatalytic Activity Chao Peng,† Xianfeng Yang,‡ Yuhang Li,† Hao Yu,*† Hongjuan Wang,† and Feng Peng† †

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China ‡

Analytical and Testing Center, South China University of Technology, Guangzhou, Guangdong 510640, P. R. China

ABSTRACT: Effectively harvesting light to generate long-lived charge carriers to suppress the recombination of electrons and holes is crucial for photocatalytic reactions. Exposing the highly active facets has been regarded as a powerful approach to high-performance photocatalysts. Herein, a hybrid comprised of {001} facets of TiO2 nanosheets and layered Ti3C2, an emerging 2D material, was synthesized by a facile hydrothermal partial oxidation of Ti3C2. The in-situ growth of TiO2 nanosheets on Ti3C2 allows for the interface with minimized defects, which was demonstrated by high resolution transmission electron microscopy and density functional theory calculations. The highly active {001} facets of TiO2 afford high-efficiency photogeneration of electron-hole pairs, meanwhile the carrier separation is substantially promoted by hole trapping effect by the interfacial Schottky junction with 2D Ti3C2 acting as a reservoir of holes. The improved charge separation and exposed active facets dramatically boost the photocatalytic degradation of methyl orange dye, showing the promise of 2D transition metal carbide for fabricating functional catalytic materials. KEYWORDS: MXenes; layered Ti3C2; (001) TiO2; Schottky junction; hole trapping *

To whom correspondence should be addressed. [email protected] (H.Y.); Tel. & Fax.: +86-20-8711 4916

1 ACS Paragon Plus Environment


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1. INTRODUCTION In response to the increasing environmental and energy-related concerns, photocatalysis is considered as a promising approach to clean environment and sustainable energy. As an enabling material, titania plays a central role in a variety of photo-/photoelectro-catalytic processes across a wide spectrum from pollutant cleanup1 to water splitting2 and artificial photosynthesis,3 because of its high activity, low cost, environmental benignity and good chemical stability. However, so far, the practical application of TiO2-based photocatalytic processes is still hindered by the low-efficiency caused by the rapid recombination of photogenerated electrons and holes. Coupling TiO2 with foreign metals or semiconductors to form heterojunctions can effectively separate the photogenerated electron-hole pairs and thereby increase the lifetime of charge carriers, through electron trapping,4 proper band alignment5 and plasmonic effect.6 To this end, the interface between two components has to be rationally designed to facilitate the transfer of charge carriers and their spatial separation. In principle, an interfacial engineering should be exerted to select appropriate components,7, 8 maximize the contacting area7,

8

and minimize the interfacial defects,7,

8

where the

recombination usually occurs. Besides, a special attention should be paid to the morphology and contacting pattern of two components. On one hand, the electronic property of lowdimensional materials strongly depends on their configuration. For example, a 2-dimensional (2D) form of carbon, graphene, could display superior performance to its 1D and 0D allotropes, carbon nanotube and fullerene, as constructed composites with TiO2 due to the intimate contact.9, 10 On the other hand, the photoexcitation is structurally sensitive because of the different surface energy and atomic configuration of different crystalline facets, which have been proved on the (001) surface of TiO2

11, 12, 13

and the (111) surface of Cu2O.14

Taking TiO2 as an example, the charge separation could be quite different when a heterojunction formed on different surfaces, because: i) the photogeneration rate of electron-


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hole pairs is different;15 and ii) the different work functions (Φ) of TiO2 facets16 may change the direction of charge transfer between two components (electrons/holes usually transfer from low/high to high/low Φ).17,

18

It has been widely documented that coupling TiO2

nanosheeets with {001} facets with other 2D materials can enhance photocatalytic activity effectively, such as graphene-(001) TiO219, 20 and MoS2-(001) TiO2.21 MXenes, a new family of 2D materials composed of transition metal carbides and carbonitrides, have attracted intensive interests since the discovery in 2011,22 because of their excellent structural stability, high electrical conductivity and hydrophilicity.23,

24

Ti3C2Tx

(T=OH, F or O) nanosheets are currently the most studied MXene, which can be readily obtained by selectively etching and exfoliating Ti3AlC2 ceramics with HF.22 Benefiting from their high electrical conductivity and 2D structure, Ti3C2Tx has been regarded as an energy storage material for anodes of Li-ion batteries (LIBs),25, 26 lithium-sulfur batteries27, 28 and electrochemical capacitors.29, 30, 31 The unique morphology, dispersability and stability also make Ti3C2Tx attractive as adsorbents for heavy metal ions or dyes32, 33, 34 and supports for catalysts.35,

36

Moreover, a recent computational study showed that the huge difference

between the hole and electron mobility in Ti2CO2 bilayer make it promising for separating holes and electrons in photocatalysis.37 As a compound of titanium and carbon, Ti3C2Tx affords a natural platform to construct composites of TiO2 and carbonaceous materials, which have been widely recognized as a class of high-performance photocatalyst,9 in which carbonaceous materials may prolong the lifetime of electron-hole pairs, tune the band-gap and adsorb reactants.9, 10 More importantly, the titanium atoms on Ti3C2Tx may act as nucleating sites to allow the growth of TiO2 photocatalysts, thereby an atomic scale interfacial heterojunction between 2D Ti3C2 and TiO2 could be facilely formed to minimize the defect-induced recombination. Hitherto, few reports have been devoted to this topic.38, 39 Naguib et al. have reported that an oxidation of laminar



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Ti3C2Tx resulted in TiO2 nanocrystals decorated on disordered carbon sheets, as a good LIB anode material.38 By a hydrothermal deposition of TiO2, a TiO2/Ti3C2 composite was fabricated for photocatalytic application but with a limited improvement.39 In these preliminary works, the interfaces between the TiO2-Ti3C2 heterojunction have not been properly designed to optimize the photocatalytic activity. Herein, the layered Ti3C2 was used to fabricate a hybrid of TiO2 nanosheets selectively exposing {001} facets and 2D Ti3C2 ((001)TiO2/Ti3C2) through a facile hydrothermal oxidation route without any additional Ti source (Scheme 1). In this design, electrons and holes are photogenerated on the (001) surfaces of TiO2, the most active surface for photocatalysis. 2D Ti3C2(OH)x sheets act as reservoir of the charge carriers to prolong their lifetime, because of the high mobility of charge carriers, especially holes.37 The interfacial bonding was guaranteed under hydrothermal conditions utilizing Ti atoms in Ti3C2 sheets as Ti sources and nucleating centers, which may minimize the interfacial defects. Through this design, the photogenerated charge carriers were effectively separated, and the reaction rate of methyl orange (MO) dye decomposition was improved, showing the promise of MXene as an excellent charge separator for photocatalysis.


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Scheme 1. Schematic illustration of the Ti3AlC2 exfoliation process and the subsequent formation of (001)TiO2/Ti3C2 hybrids.

2. EXPERIMENTAL SECTION 2.1. Synthesis of layered Ti3C2. The layered Ti3C2 was fabricated by selectively exfoliating the Al layers from Ti3AlC2 with HF(49 wt%) at 60oC for 12 h with stirring. Then the solids were centrifuged, thoroughly rinsed with DI water and dried. 2.2. Synthesis of (001)TiO2/Ti3C2 and p-TiO2/Ti3C2. Typically, 100 mg of the layered Ti3C2 were suspended in 15 mL 1.0 M HCl containing 0.165 g NaBF4. After stirring for 30 min and ultrasonication for 10 min, the suspension was transferred into a 100 ml Teflon-lined stainless steel autoclave for a hydrothermal oxidation at 120-220

o

C for 4-32 h. (see the Supporting Information for more details) With the



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morphology-directing reagent NaBF4, the (001)TiO2/Ti3C2 can be grown, as illustrated in Scheme 1. The particulate TiO2/Ti3C2 (p-TiO2/Ti3C2) was synthesized through a facile hydrothermal method. 100 mg Ti3C2 powder was added to 15 mL deionized water, and was stirred for 30 min followed by additional ultrasonication for 10 min. Thereafter, the suspension was transferred into a 100 ml Teflon-lined stainless steel autoclave for a hydrothermal reaction at 160 °C for appropriate durations. 2.3. Characterizations. Electron probe microanalysis (EPMA-1600, Shimadzu) was used to analyze the contents of various elements. The structure of the materials were investigated by X-ray diffraction analysis (XRD, Bruker D8 Advance, Germany) at 40 kV and 40 mA using Cu-Ka radiation, and performed in an angle range of 5-70°. The Brunauer-Emmett-Teller (BET) specific surface areas (SBET) of the samples were measured by N2 adsorption at liquid N2 temperature in an ASAP 2010 analyzer. The surface morphology and the microstructure of the as-prepared samples were characterized by field emission scanning electron microscopy (FESEM, Zeiss Merlin) at an acceleration voltage of 5 kV. The STEM image, EDX elemental mapping, TEM image and HRTEM images were all taken in a JEOL JEM-2100F at an operating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was carried out in a Kratos Axis ultra (DLD) spectrometer equipped with an Al Ka X-ray source. Binding energies were referenced to the C1s peak of (C-C) bond which set at 284.8 eV. The optical properties of all samples were obtained using a UV-vis diffuse reflectance spectroscope (DRS, Hitachi-U3010) with an integrated sphere attachment. FT-IR spectrometer (Nicolet 6700, USA) of all samples was examined with a slice of powered potassium bromide and sorbent in 400-4000 cm-1. 2.4. Photocatalytic reaction.


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Methyl orange (MO) was selected as a probe chemical to evaluate the photocatalytic activity of the photocatalysts. The photocatalytic reaction was conducted in a cylindrical glass vessel fixed in the XPA-II photochemical reactor (Nanjing Xujiang Machine-electronic Plant). 10 mg catalysts were dispersed in 200 mL MO aqueous solution (20 mg L-1) in the dark with strong stirring for 1 h to achieve adsorption equilibrium. Air was bubbled into the reaction solution at a constant rate. Thereafter it was irradiated by the ultraviolet (UV) light generated from a 300 W mercury lamp. 5 mL solution was taken out every 10 min, centrifuged to remove the catalyst, and then measured on a UV-vis spectrophotometer (Shimadzu UV 2550). In recycling experiments, the used catalysts were washed with deionized water for several times and dried at ambient temperature under vacuum before each test. Isopropanol (IPA), ammonium oxalate (AO), and p-benzoquinone (BQ) were used as scavenger of •OH, h+ and •O2−, respectively, to identify the active species in the photocatlytic reaction. The trapping experiments were carried out under the same conditions as those for photocatalytic reaction, except for adding 1 mM of the trapping reagent. 2.5. Preparation of photoelectrodes and Photoelectrochemical measurement. For the photoelectrochemical measurement studies, the electrodes were prepared by an electrophoretic deposition method. The electrophoretic deposition was carried out by dispersing 40 mg of samples and 10 mg iodine powder in 50 ml of acetone. In this process, acetone reacts with iodine generating H+, so that the photocatalyst particles were positively charged. Under magnetic stirring, two parallel FTO (fluorine doped tin oxide) glasses were immersed in the solution with a 10-15 mm separation, and a 50 V bias was applied using a DC power supply for 10 min. With the DC voltage was applied, the particles were deposited on the cathode. The coated area was about 1.5cm*1 cm and then dried. Photoelectrochemical measurements were measured in a standard three-electrode cell with a quartz window by electrochemical workstation (CHI 660D Instruments Inc., Shanghai). The



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as-prepared electrode was used as the working electrode, and a platinum net and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The photocurrents were measured on a CHI 660D electrochemical station (Shanghai Chenhua, China), a 300 W Xe arc lamp (Beijing Changtuo, China) equipped with a visible cut off filter to provide ultraviolet light. 0.1 M Na2SO4 aqueous solution (pH ≈ 6) was used as the electrolyte for photocurrent measurement. 1.0 M NaOH aqueous solution (pH = 13.6) was used as electrolyte for transient open-circuit voltage decay (OCVD) (i.e., photovoltage-time) curves. 2.6. Computational Methods. To assess the interaction in the TiO2/Ti3C2 interface layer, density functional theory (DFT) calculations were performed with DMol3 Package in Materials Studio. Because the Ti3C2 layer can be regarded as substrate, we fixed the atomic position of Ti3C2 and only relaxed the atoms in TiO2. Exchange-correlation functions were described by generalize gradient approximation (GGA) with Perdew-Burke-Emzerhof (PBE). DFT Semi-core Pseudopots (DSPPs) was adopt as core treatment. Double numerical plus polarization (DNP) was employed as the basis set, which is comparable to 6-31G** in Gaussian. The orbital cutoff of 5.5 Å was assigned to global atoms. In addition, Grimme method for the long range interaction correction was applied throughout calculations. The convergence tolerance of selfconsistent field (SCF) calculations was 1.0×10-5 Ha and the energy convergence for geometry optimizations was 1.0×10-5 Ha. The k-point in the first Brillouin zone was set to 12 × 12 × 1, which kept same to density of states (DOS) and partial density of states (PDOS) calculations. Ti3C2 is one of hexagonal cells and we extracted one layer to investigate the interface properties. The exposed titanium atoms on the monolayer surface were saturated with hydroxyls. The optimized lattice parameters are 3.0712 × 3.0712 × c, where c represents the direction of vacuum layer. The TiO2-anatase cell belongs to tetragonal system and we directly used the structure file in Materials Studio library. In order to fit the Ti3C2 substrate, the


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following operations were prepared for the calculation system. We converted the imported TiO2-anatase cell into primitive cell, and cleaved (001) surface. Then adjust γ-angle to 120° and the lattice length to 6.1424 Å, which is equal to the twice length of Ti3C2 lattice. During the adjustment of parameters, fractional coordinates of atoms should be kept fixed. Eventually, make a (2 × 2 × 1) supercell of Ti3C2 and combine two modified structures as the TiO2/Ti3C2 interface layer.

3. RESULTS AND DISCUSSION

Figure 1. (a) XRD patterns of Ti3AlC2, Ti3C2 and (001)TiO2/Ti3C2. FESEM images of (b) Ti3C2 with layered structure and (c) (001)TiO2/Ti3C2 hybrid hydrothermally prepared at 160 oC and for 12 h. The inset images of (c) show the enlarged HRSEM image of the TiO2-Ti3C2 heterojunctions in the area highlighted by the green square, and a schematic diagram of an anatase TiO2 crystal exposing {001} and {101} facets. (d) STEM image and EDX elemental mapping of the (001)TiO2/Ti3C2.

X-ray diffraction (XRD) patterns of the materials are shown in Figure 1a. After Ti3AlC2 was etched by HF, a (002) reflection of Ti3C2 at 8.84o can be clearly observed. The (002) at 9.58o and (004) at 19.17o of Ti3AlC2 were broadened and shifted toward lower angle side,



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indicating the removal of interlayer Al and the formation of Ti3C2 nanosheets, as shown in Figure 1b. After the hydrothermal oxidation, the anatase TiO2 (JCPDS No. 21-1272) phase emerged, meanwhile, the further shift of (002) peak of Ti3C2 to low angle side suggested the further delamination of Ti3C2 nanosheets. Figure 1c displays a typical SEM image of the TiO2/Ti3C2 hybrid. Square nanosheets in width of ~400 nm and thickness of ~50 nm were sideling inserted cross the stack of layered Ti3C2 to form heterostructures (see Supporting Information for more details). The HRSEM observations show that the interfacial angle between {001} and {101} facets of anatase is 68.3o on average, agreeing well with the anatase TiO2 crystallite exposing a large proportion of {001} planes.11 According to the width and thickness of (001)TiO2 naosheets observed by SEM, the proportion of {001} facets can be estimated as about 77.5 % from a geometric model of anatase TiO2.40 The formation of TiO2 was further verified by STEM and EDS mapping of Ti, O and C. As shown in Figure 1d, by spotlighting the area containing a square thin sheet on a large 2D substrate, it was clearly revealed that the large 2D substrate can be attributed to titanium carbide and the oxygen-containing small sheet is TiO2. It should be noted that the carbide sheet contain considerable oxygen because of hydroxyl groups terminating the surfaces. The formation of –OH terminated Ti3C2 can be supported by FTIR spectroscopy, showing the broad bands at 3431 cm-1 and 1628 cm-1 (see Figure S7), owning to the stretching vibration of –OH. In our previous work, the {001} surface exposed TiO2 can be formed by the hydrothermal conversion of TiN powders in the presence of NaBF4,41 producing random aggregates of TiO2 sheets. In this work, by controlling the oxidation extent through reaction duration and temperature, the TiO2 nanosheets can be homogeneously distributed around the layered Ti3C2 to provide improved accessibility to light and reactants (see Supporting Information for the effects of temperature and duration). More importantly, at an appropriate extent of oxidation, the formation of TiO2/Ti3C2 heterojunctions would be


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maximized, where 2D Ti3C2 sheets traverse TiO2 nanocrystals at the most active {001} facets, as shown in Figure 1c. The intimate contact between these two phases might facilitate the separation of charge carriers photogenerated on the {001} surfaces, thereby improve the photocatalytic activity.

Figure 2. (a) A TEM image of TiO2/Ti3C2. (b-e) show the close observations to the areas highlighted by yellow boxes in (a). (b) and (c) show the intimate contact of layered Ti3C2 with a TiO2 crystal. (e) shows a growth of TiO2 perpendicular to the [001] direction of Ti3C2 sheet. (f) shows a tiny TiO2 crystal formed near the edge of Ti3C2. The insets (f1) and (f2) show the lattice fringes of TiO2 and Ti3C2, respectively. The



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inset of (c) shows the indexed FFT image of TiO2. The interface structure is shown in (d). (g) The initial and the final optimized interface structures simulated by DFT. The black, red, cyan and green balls stand for C, O, Ti and H elements, respectively. Two Ti-O-Ti bonds are labeled as Ti1-O1-Ti2 and Ti3-O2-Ti4, whose PDOS analyses are shown in (h) and (i), respectively.

The detailed crystallographic relationship between TiO2 and Ti3C2 was revealed by HRTEM. Figure 2a clearly shows the growth of TiO2 between two layers of Ti3C2, whose interfaces can be distinguished in Figures 2b and c. In Figure 2b, the dashed line represents the interface between TiO2 and Ti3C2. The typical layered structure of Ti3C2 can be observed above the interface. The FFT pattern in Figure 2c confirms that the crystal between two Ti3C2 sheets is an anatase TiO2 (space group I41/amd, a=b=0.378 nm, c=0.951 nm) projected along its [001] direction, namely, exposing {001} facets. A close observation to the interface is shown in Figure 2d and compared with a structure optimized by density functional theory (DFT) calculations with DMol3 Package in Materials Studio. In this heterojunction, the lattice of Ti3C2 agrees well with the unit cell parameters of Ti3C2 provided by Naguib et al.22 The interlayer distance, 0.55 nm, can be ascribed as the two adjacent -OH groups terminating Ti3C2 sheets. At the interface, the Ti3C2 and TiO2 joint seamlessly on atomic level, benefited from the small mismatch between {103} of Ti3C2 and {11-1} of TiO2, despite their different crystallographic form. A DFT calculation suggested that chemical bonding can be generated at the interface through Ti-O-Ti. Depending on whether the oxygen atom origins from TiO2 or –OH, two Ti-O-Ti bonds could be formed, both of which contribute to the interface formation via overlapping of Ti d-orbital and O p-orbital (Figures 2h,i). The structures in Figures 2e and 2f may help to rationalize the growth mechanism of the heterojunction. Figure 2e shows a case of TiO2 embedded in a Ti3C2 nanosheet, which might represent the initial stage of TiO2 crystallization without forming special orientation. Figure 2f displays a small TiO2 crystal embedded in a crack of Ti3C2 nanosheets, implying the nucleation of TiO2 may occur at the defective sites of Ti3C2. These results may rationalize a


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plausible growth mechanism of (001)TiO2/Ti3C2 hybrids. Under the acidic hydrothermal conditions, the titanium atom of Ti3C2 may transform to hydrated Ti3+ ions42 that can be oxidized to TiO2 at the defects of Ti3C2. Assisted by the directing reagent NaBF4, the formation of high-energy {001} facets was enhanced during the sequential crystal growth, because of the lower energy of {001} planes adsorbing F-.43

Ti3C2

TiO2 TixOy Ti-X

Ti-C

(b)

Ti3C2

C-C C-F

(001) TiO2/Ti3C2- 4 h

(001) TiO2/Ti3C2-24 h

(001) TiO2/Ti3C2-32 h

468

464 460 456 452 Binding energy (eV)

(001) TiO2/Ti3C2-4 h

Intensity (a.u.)

(001) TiO2/Ti3C2-12 h

C-O

Ti-C

(c)

Ti3C2

C-Ti-Oa

(001) TiO2/Ti3C2-24 h

Ti-O-Ti adsorb O

(001) TiO2/Ti3C2-4 h

C-Ti-Ob

(001) TiO2/Ti3C2-12 h

Ti-OH

C-OH

Intensity (a.u.)

(a)

Intensity (a.u.)

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(001) TiO2/Ti3C2-12 h

(001) TiO2/Ti3C2-24 h

(001) TiO2/Ti3C2-32 h

(001) TiO2/Ti3C2-32 h

290 288 286 284 282 280

536

Binging energy (eV)

534

532

530

528

Binding energy (eV)

Figure 3. (a) Ti 2p, (b) C1s and (c) O 1s XPS spectra for Ti3C2 and (001)TiO2/Ti3C2 prepared for different reaction time. CHCl = 1 M, CNaBF4 = 0.1 M, T = 160 °C.

The formation of (001)TiO2/Ti3C2 heterojunctions was further confirmed by XPS. As shown in Figure 3a, The Ti 2p core level is fitted with four doublets (Ti 2p3/2−Ti 2p1/2) with a fixed area ratio 2:1 and doublet separation of 5.7 eV.44 The Ti 2p3/2 components centered at 454.8, 455.7, 457.2, and 459 eV can be assigned as Ti−C bond, Ti−X from substoichiometric TiCx (x

Hybrids of Two-Dimensional Ti3C2 and TiO2 Exposing {001} Facets

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