Hybrids of Two-Dimensional Ti C and TiO Exposing {001

am5b11973 1..10Hybrids 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
*S Supporting Information
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 the 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
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/ photoelectrocatalytic 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. Cou- pling 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 alignment,5
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 area,7,8
and minimize the interfacial defects,7,8 where the recombination usually occurs. Besides, special attention should be paid to the morphology and contacting pattern of two components. On one hand, the electronic property of low-dimensional 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
11−13 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−hole pairs is different;15 and (ii) the different work functions (Φ) of TiO2 facets16 may
Received: December 8, 2015 Accepted: February 9, 2016 Published: February 9, 2016
Research Article
© 2016 American Chemical Society 6051 DOI: 10.1021/acsami.5b11973 ACS Appl. Mater. Interfaces 2016, 8, 6051−6060
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19,20 and MoS2-(001) TiO2. 21
MXenes, a new family of 2D materials composed of transition metal carbides and carbonitrides, have attracted intensive interest since their 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 batteries,27,28 and electrochemical capaci- tors.29−31 The unique morphology, dispersibility, and stability also make Ti3C2Tx attractive as adsorbents for heavy metal ions or dyes32−34 and as supports for catalysts.35,36 Moreover, a recent computational study showed that the huge difference between the hole and electron mobility in a Ti2CO2 bilayer makes 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 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 a 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.
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 60 °C 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 was suspended in 15 mL of 1.0 M HCl containing 0.165 g of 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 °C for 4−32 h (see the Supporting Information for more details). With the 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 of 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 micro- structure 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 C 1s 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. 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
Scheme 1. Schematic Illustration of the Ti3AlC2 Exfoliation Process and the Subsequent Formation of (001)TiO2/Ti3C2 Hybrids
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.5b11973 ACS Appl. Mater. Interfaces 2016, 8, 6051−6060
(Nanjing Xujiang Machine-electronic Plant). Ten 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. Five 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 Photoelectrochem-
ical 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 1.5 cm × 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 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 Semicore 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 self-consistent 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 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
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 8.84° can be clearly observed. The (002) at 9.58°
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 °C 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.
and (004) at 19.17° of Ti3AlC2 were broadened and shifted toward lower angle side, 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 the 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 across the stack of layered Ti3C2 to form heterostructures (see Supporting Information for more details). The HRSEM observations show that the interfacial angle between the {001} and {101} facets of anatase is 68.3° 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 contains 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 and 1628 cm−1 (see Figure S7), owing 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 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 improving the photocatalytic activity. 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
Figure 2. (a) TEM image of TiO2/Ti3C2. (b−e) show the close observations of 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 the 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 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.
group I41/amd, a = b = 0.378 nm, c = 0.951 nm) projected along its [001] direction, namely, exposing {001} facets. A close observation of the interface is shown in Figure 2d and compared with a structure optimized by density functional theory (DFT) calculations with the 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 the atomic level, benefiting 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 originates from TiO2 or −OH, two Ti−O−Ti bonds could be formed, both of which contribute to the interface formation via overlapping of the Ti d-orbital and the 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 a 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 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
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 < 1) or titanium oxycarbides, Ti ions with reduced charge state (TixOy), and Ti ions in valence 4+ (TiO2), respectively.
11,44,45 Obviously, as the hydrothermal reaction proceeds, the TiO2 increases while the other species decreased, indicating that Ti3C2 is consumed and transformed to (001)TiO2 nanosheets. The C 1s spectrum of layered Ti3C2 is fitted by five components located at 281.7, 282.2, 284.8, 286.2, and 288.6 eV, corresponding to C−Ti, C−Ti−Oa, C− C/C−H, C−O, and C−F bonds, respectively (Figure 3b).44
The C−Ti−Oa origins from the adsorbed −OH on the surfaces of Ti3C2. Interestingly, a new peak emerges at 283.5 eV after
Figure 3. (a) Ti 2p, (b) C 1s, and (c) O 1s XPS spectra for Ti3C2 and (001)TiO2/Ti3C2 prepared for different reaction times. CHCl = 1 M, CNaBF4 = 0.1 M, T = 160 °C.
Figure 4. (a) DRS and (b) plots of [F(R∞)hv] 1/2 vs photon energy of (001)TiO2/Ti3C2 prepared for different reaction times. CHCl = 1 M, CNaBF4 =
0.1 M, T = 160 °C.
the hydrothermal oxidation, which could be attributed to the C−Ti−Ob bonding at the interfaces of TiO2−Ti3C2 heterojunctions. This component reaches a maximum at 12 h, indicating the maximized number of heterojunctions at the appropriate oxidation extent. With respect to the O 1s XPS spectra, four peaks at 529.4, 530.4, 531.8, and 533.3 eV have been deconvoluted (Figure 3c), which are ascribed to surface adsorbed O species,46 Ti−O−Ti (lattice O),47 Ti−OH,47 and C−OH species,47 respectively. The Ti−OH species at 531.8 eV unambiguously demonstrate the presence of −OH groups on the surface of Ti3C2. The optical absorption property of the (001)TiO2/Ti3C2
hybrids was characterized by UV−vis DRS (Figure 4a). The hybrids of (001)TiO2/Ti3C2 synthesized for 4 to 16 h exhibit a broad absorption. The light absorption above 400 nm reduces with the increasing reaction time, namely the higher content of TiO2. It could be attributed to the distinct adsorption of carbonaceous materials.48 The band gap energies of two
representative (001)TiO2/Ti3C2 samples, for 12 and 14 h, were determined through the [F(R∞)hv]
1/2 vs photon energy plots (Figure 4b),40 as 1.60 and 2.87 eV, respectively. This result indicates that the band gap widens with increasing the hydrothermal reaction time and the content of (001)TiO2. The band gaps of (001)TiO2/Ti3C2 are narrower compared with TiO2 nanosheets with dominant (001) facets (3.22 eV),40
due to the existence of metallic Ti3C2. 22,25 The narrow band
gap of (001)TiO2/Ti3C2 hybrids may favor optical absorption and photocatalytic reactions. The photocatalytic activity of the (001)TiO2/Ti3C2 hetero-
junction was evaluated by a probe reaction of MO dye elimination under ultraviolet irradiation. As shown in Figure 5a, the degradation rate of MO is maximized at the hydrothermal temperature of 160 °C. This could be explained by the increasing formation rate of TiO2 nanosheets at higher temperatures, which causes the increasing number of heterojunctions at lower temperatures, while a reduction of
Figure 5. (a) The photocatalytic degradation of MO over (001)TiO2/Ti3C2 prepared at different hydrothermal temperatures. The inset of (a) shows the photocatalytic degradation rate constants of (001)TiO2/Ti3C2 prepared at different hydrothermal temperatures. (b) The photocatalytic degradation rate constant (k) of (001)TiO2/Ti3C2 and particulate TiO2/Ti3C2 (p-TiO2/Ti3C2) prepared for different hydrothermal times. The photocatalytic degradation rate constants are normalized by mass of TiO2 (k′) and compared with Degussa P25.
Figure 6. (a) The transient photocurrent of (001)TiO2/Ti3C2 prepared for different hydrothermal times. (b) The partial photocurrent decay of curve (a). (c) Transient open-circuit voltage decay (OCVD) over Degussa P25 TiO2 and the (001) TiO2/Ti3C2 prepared for different reaction times. (d) Average electron lifetimes (τn) of Degussa P25 TiO2 and the (001)TiO2/Ti3C2 reacted for 4 h, 12 h, and 24 h.
heterojunctions at higher temperatures is seen because of the depletion of Ti3C2 (Figure S4). Analogously, the activity is maximized at hydrothermal treatment for 12 h for a similar reason (Figure 5b). The superior activity of TiO2 exposing {001} surfaces to conventional particulate TiO2 is shown in Figure 5b (see Figures S8 and S9 for more details of p-TiO2/ Ti3C2). The two materials have the highest activities at hydrothermal reaction for 12 and 30 h, respectively. The (001)TiO2/Ti3C2 shows about 2.3-fold higher degradation rate constant than p-TiO2/Ti3C2 at their respective maximums, demonstrating the enhanced activity of {001} TiO2, and this agrees with our previous study on the high-activity facets of TiO2.
43 The excellent activity of hybrids can be further demonstrated by the rate constant normalized by the mass of TiO2 (k′), which can be determined through the elemental analysis by EPMA (see Tables S1 and S2 in the Supporting Information for details). As seen in Figure 5b, the maximum k′ value of (001)TiO2/Ti3C2, 18.8 min−1 g−1, exceeds that of Degussa P25 TiO2, 15.7 min−1 g−1. In addition, the k′ values of (001)TiO2/Ti3C2 are also higher than that of pure TiO2 with exposed {001} and {101} facets reported in our previous work obtained under similar conditions (1.86 min−1 gTiO2
−1),43
confirming that the 2D Ti3C2 can improve the photocatalytic property of TiO2. Figure 6a shows the photocurrent test of the samples
synthesized at 160 °C for different times under ultraviolet light irradiation (<400 nm, 100 mW/cm2) by electrodepositing the catalysts on a FTO (fluorine doped tin oxide) substrate (see Figure S11 for the photoelectrochemical tests of samples at different reaction temperatures). The photocurrent shows a similar tendency with the MO degradation test. The (001)TiO2/Ti3C2 sample for 12 h of hydrothermal oxidation has the highest photocurrent, even comparable with that of Degussa P25 TiO2 (Figure S12), because of the good compromise between the content of active TiO2 and the number of (001)TiO2/Ti3C2 heterojunctions (see Table S2). It should be noted that the photocurrent decays are very different among the heterojunctions with different hydrothermal durations, suggesting the different separation and recombination of photogenerated electron−hole pairs.49,50 As shown in Figure 6b, the samples for 4, 8, and 12 h display much slower photocurrent decays featured by the long tails, while the samples for 16, 24, and 32 h display rapid decay as the irradiation was turned off . The slow reversible photoresponse is indicative of long charge carrier lifetimes and metastable donor states,51,52 which have been observed in a variety of elaborate heterojunction structures, e.g. tunable bulk/surface
defects of TiO2, 49,51 Ti3+ doped TiO2,
52 TiO2/CNTs, 53 TiO2/
GR,54 and TiO2−Pd@Pt hybrid structures.55
= * −
=
0 (1)
where q is the unsigned charge of an electron (1.6 × 10−19 C), kB is the Boltzmann constant (1.38 × 10−23 J/K), T is the temperature in K of the three-electrode system, and n and n0 are the photocarrier density in the semiconductor nanostruc- ture under nonequilibrium state and equilibrium state, respectively.18,56−58 The temporal profiles of VOC of three representative samples were monitored in the three-electrode system under UV (λ < 400 nm) light to examine the dynamics of electrons within the TiO2 CB. As shown in Figure 6c, the steady photovoltages of the (001)TiO2/Ti3C2 samples for different hydrothermal durations are in the same order with the transient photocurrent, i.e. 12 h > 24 h > 4 h. After turning off the light, the (001)TiO2/Ti3C2 samples at hydrothermal durations for 4 and 12 h exhibit extended VOC transient lasting more than 500 s, indicating the slow recombination of electrons and holes and prolonged charge carrier lifetime. While increasing the hydrothermal reaction to 24 h, it was observed that the photovoltage was promptly established and faded back to the equilibrium value in the dark, as the irradiation was turned on/off. This behavior can be rationalized by the reduced number of Ti3C2/TiO2 heterojunctions, as afore-described. The average electron lifetime (τn) can be quantitatively
calculated through the VOC decay by the equation:18,56−58
τ = − −
(2)
where τn represents the average electron lifetime and Voc is the open-circuit voltage at time t.18,56−58 As shown in Figure 6d, the electron lifetime in TiO2 CB is in the range from 100 s to 104 s. The (001)TiO2/Ti3C2 for 4 h hydrothermal oxidation shows the longest short-lived distribution of τn at ca. 100 s, which appeared at the moment lighting was turned off. The short-lived τn decreases with the hydrothermal reaction
Figure 7. (a) The effects of scavengers on the degradation of MO with (001)TiO2/Ti3C2-160 °C-12h under UV light irradiation. (b) The recycling test of (001)TiO2/Ti3C2-160 °C-12h for the degradation of MO under UV light irradiation.
duration to ca. 50 and 2 s for the samples at 12 and 24 h, respectively. It is widely accepted that the relatively short-lived distribution of τn is an important signal of the rapid recombination of photoexcited electrons and holes.18,55−58
The almost 2 orders of magnitude change of electron lifetime strongly suggests that the recombination rate can be significantly reduced by forming heterojunctions between TiO2 nanosheets and layered Ti3C2. Additionally, it should be noted that the (001)TiO2/Ti3C2 for 4 and 12 h display slower OCVD and longer electron lifetimes than Degussa P25 TiO2, as shown in Figure 6c,d, suggesting the improved charge separation over the hybrids of (001)TiO2 and 2D Ti3C2. To understand the photocatalytic mechanism of (001)TiO2/
Ti3C2, the effects of reactive oxygen species (ROS), including •OH and •O2
−, and h+ were investigated by introducing isopropanol (IPA), ammonium oxalate (AO), and benzoquinone (BQ) as the scavengers, respectively.59,60 As shown in Figure 7a, the degradation efficiency significantly decreases with N2 bubbling, indicating that the degradation of MO is a photocatalytic oxidation (PCO) process. Three scavengers can quench the reaction in the order: IPA > AO > BQ, indicating that the ROS contributes more than h+ in the MO degradation. According to these results, it can be concluded that •OH is the most important reactive species in the TiO2-catalyzed PCO process of MO, being consistent with Hirakawa T. et al.61
The stability and reusability of (001)TiO2/Ti3C2 hybrid photocatalyst were tested to degrade MO for four runs under UV light (Figure 7b). The photocatalytic activity of (001)TiO2/Ti3C2-160 °C-12h for MO degradation was well maintained after 4 runs with 4.9% decrease of MO degradation from 97.4% to 92.5%, suggesting that (001)TiO2/Ti3C2 hybrids can be used as a stable catalyst for the photodegradation process. On the basis of the above experimental results, a possible
photocatalytic mechanism for the degradation reaction with (001)TiO2/Ti3C2 was proposed. As Scheme 2a shows, the −OH terminated Ti3C2 sheets are considered metallic because of its very narrow band gap and high carrier mobility.22,25
Under ultraviolet irradiation, the exposed {001} facets of TiO2 are excited to produce electrons (e−) and holes (h+). The transfer of electrons is forbidden at the interfaces between TiO2 {001} surfaces and Ti3C2, because of the more negative Fermi level of Ti3C2 than the CB of TiO2.
37 The −OH terminated Ti3C2 sheets have recently been discovered as an ultralow work function material with a work function ca. 1.8 eV by theoretical calculations.62 This work function is much lower than that of TiO2 (4.924 eV for (001) surface, 6.578 eV for (101)
surface).16 From the viewpoint of work function, a Schottky barrier may be established through their interfaces. Because the work function of Ti3C2−OH is lower than that of the TiO2(001) surface, the photogenerated holes, instead of electrons, could be injected from TiO2 to Ti3C2−OH. The 2D structure and high mobility of the hole make Ti3C2−OH a huge reservoir of holes. The Schottky barrier at Ti3C2−TiO2 interfaces could effectively prevent the holes from flowing back to TiO2 (Scheme 2b); thereby, a spatial separation of photogenerated electron−hole pairs is achieved, as revealed by the substantially prolonged electron lifetimes. In this interpretation, the enriched electrons on TiO2(001) may react with the dissolved oxygen to yield superoxide radical anions (•O2
−), which further react with H+ and e− to generate highly reactive hydroxyl radicals (•OH).
4. CONCLUSIONS In summary, we have achieved the synergetic utilization of Schottky-junction trapping holes and active facets exposed to enhance the photocatalytic performance by fabricating an atomic scale interfacial heterojunction composed of (001) TiO2 nanosheets inserted in the layered Ti3C2. The heterostructure was synthesized by a hydrothermal oxidation of layered Ti3C2 assisted by NaBF4. Ti3C2 not only afforded the Ti source of the heterojunction; more importantly, it also form a Schottky- junction with the {001} surfaces of n-type semiconductor TiO2. Under UV irradiation, the ultralow-work-function −OH terminated Ti3C2 acts as a reservoir of holes through the hole trapping by the Schottky-junction, whereby the spatial separation of photogenerated electrons and holes was achieved. The (001) TiO2−Ti3C2 hybrid simultaneously overcomes the problems of fast recombination of photogenerated electrons and holes, and the utilization of the highly active {001} facets of TiO2, therefore, enabled the significantly improved photocatalytic degradation of MO. This work opens a new window exploiting the MXenes for high-performance photocatalysts.
ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11973.
More experimental results on the effects of NaBF4 concentration, temperature, and reduction duration on the synthesis of (001)TiO2/Ti3C2; characterizations of p- TiO2/Ti3C2; method determining the content of TiO2 in heterojunction samples; photocatalytic degradation
Scheme 2. (a) The charge-transfer process over (001)TiO2/Ti3C2. (b) Schematic band alignments and charge flows at {001} TiO2−Ti3C2 interfaces
AUTHOR INFORMATION
ACKNOWLEDGMENTS
This work was financially supported by the National Natural Science Foundation of China (No. 21273079), the Guangdong Provincial Natural Science Foundation (Nos. S20120011275, 2014A030312007), Program for New Century Excellent Talents in University (NCET-12-0190), and the Fundamental Research Funds for the Central Universities of China (No. 2015PT012). We also acknowledge the high-performance computing support by the Information and Network Engineer- ing and Research Center, SCUT.
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Hybrids of Two-Dimensional Ti C and TiO Exposing {001