Monolayer Ti3C2Tx as an Effective Co-catalyst for Enhanced

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Monolayer Ti3C2Tx as an Effective Co-catalyst for Enhanced Photocatalytic Hydrogen Production over TiO2 Tongming Su, Zachary D. Hood, Michael Naguib, Lei Bai, Si Luo, Christopher M. Rouleau, Ilia N. Ivanov, Hongbing Ji, Zuzeng Qin, and Zili Wu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02268 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 12, 2019

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ACS Applied Energy Materials

Monolayer Ti3C2Tx as an Effective Co-catalyst for Enhanced Photocatalytic Hydrogen Production over TiO2

Tongming Su a,b*, Zachary D. Hoodb,d, Michael Naguibc, Lei Baib,f, Si Luob, Christopher M. Rouleaub, Ilia N. Ivanovb, Hongbing Ji a,e, Zuzeng Qina,*, Zili Wub,*

aSchool

of Chemistry and Chemical Engineering, Guangxi Key Laboratory of

Electrochemical Energy Materials, Guangxi University, Nanning 530004, China bCenter

for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge,

Tennessee 37831, USA cDepartment

of Physics and Engineering Physics, Tulane University, New Orleans, Louisiana

70118, USA dElectrochemical

Materials Laboratory, Department of Materials Science and Engineering,

Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA eSchool

of Chemistry, Sun Yat-sen University, Guangzhou 510275, China

fDepartment

of Chemical and Biomedical Engineering, West Virginia University,

Morgantown, West Virginia 26506, USA

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ABSTRACT: Titanium dioxide, TiO2, represents a promising candidate for hydrogen production via photocatalysis. However, its large bandgap and fast charge recombination limits its efficiency. To overcome this limitation, we explored in this work two-dimensional titanium carbide MXene, Ti3C2Tx (Tx = O, OH, F), as feasible co-catalysts for hydrogen production with TiO2 as the photocatalyst. We synthesized a series of Ti3C2Tx/TiO2 composite photocatalysts with monolayer Ti3C2Tx as the co-catalyst to improve the separation of photoinduced electrons and holes. The physicochemical properties of the Ti3C2Tx/TiO2 composites were investigated by a variety of characterization techniques, and the effect of the monolayer Ti3C2Tx on the photocatalytic performance of the Ti3C2Tx/TiO2 composites is elucidated by comparison to the multilayer counterpart. The photocatalytic hydrogen evolution rate of the optimized monolayer Ti3C2Tx/TiO2 composite is over 9 times higher than that of the pure TiO2 and 2.5 times higher than the multilayer counterpart. The significantly enhanced activity is attributed to the superior electrical conductivity of monolayer Ti3C2Tx and charge-carrier separation at the MXene-TiO2 interface. A mechanism of photocatalytic hydrogen evolution over the Ti3C2Tx/TiO2 system is proposed. This work demonstrates the potential of monolayer MXenes as effective co-catalysts for photocatalysis and further broadens the applications of the MXene family of two-dimensional materials. KEYWORDS: monolayer titanium carbide; MXene; titanium dioxide; photocatalysis; hydrogen evolution

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INTRODUCTION Photocatalytic water splitting into hydrogen over semiconductor-based photocatalysts is considered to be an effective technology to convert solar energy to chemical energy1. During the past several decades, various photocatalysts have been exploited for the photocatalytic hydrogen evolution from water, such as metal oxides2, nitrides3, sulphides4, etc. Among them, metal oxide-based semiconductors have been studied most extensively over the years5. For example, titanium dioxide, TiO2 has been considered to be one of the most promising photocatalysts for photocatalytic hydrogen evolution due to its suitable band edge alignment, excellent stability, low cost and nontoxicity6. However, due to the fast recombination of the electron-hole pairs, TiO2-based photocatalysts previously exhibited fairly low efficiency for photocatalytic water splitting. Several strategies have been employed to enhance the separation of electron-hole pairs in TiO2, such as morphological control7, doping with metal and nonmetal8, constructing heterostructures with other semiconductors9, coupling with co-catalysts10, hydrogenation, sensitization, defect engineering, and surface plasmon resonance effect11 and so on. Catalytic reactions occur on the surfaces of the photocatalyst. Therefore, the photocatalytic activity of TiO2 can be tuned by engineering the morphology and surface structure12-13. Doping with nonmetals or metals is an effective means to extend the light absorption range or enhance the charge carrier separation of photocatalysts. For instance, the band gap energy of TiO2 decreases when carbon is doped into the lattice of TiO2, extended the visible light adsorption toward longer wavelength8. Additionally, Rh and Nb co-doped TiO2 are capable for both visible light absorption and efficient separation of photogenerated 3



electrons and holes14. Construction of a heterostructure between two semiconductors can not only extend the light absorption range, but also promote the separation of photogenerated charge carriers. For example, when TiO2 are combined with FeS2, the light absorption range is extended from the ultraviolet-visible to near-infrared (NIR) spectrum, and the charge transfer is enhanced between the FeS2 and TiO2 interface, resulting in an increase in the photocatalytic hydrogen production15. Additionally, introducing Ti3+ defects onto TiO2 was found to strengthen the absorption of visible light and improve the photocatalytic activity16. Loading with co-catalysts is another effective strategy to suppress the recombination of electrons and holes in TiO2. For example, when loading with Pt, Au and Ag, the photocatalytic activity TiO2 can be greatly enhanced due to the large work function of these noble metals17-18. However, the high price and scarcity of these noble metals restrict the large-scale production of noble-metal-containing photocatalysts. In addition to noble metals, MoS219, graphene20, graphene oxide21, Bi2O3 etc.22 can also be used to enhance the separation efficiency of electrons and holes. In the TiO2/MoS2/graphene system, the graphene serves as the electron collector, and the MoS2 acts as the source of active adsorption sites, which dramatically enhances the H2 evolution over TiO223. However, the photocatalytic hydrogen production rate still needs to be improved to meet the requirements for industrial application. Two-dimensional (2D) layered materials have attracted intensive attention due to their extraordinary physical and chemical properties, such as graphene, MoS2, and g-C3N46. MXenes, a novel type of 2D materials discovered in 201124, has recently attracted much interest due to its excellent electrical conductivity, structural stability and hydrophilicity25. Owing to the low Fermi level of metallic MXene compared to typical semiconductors, 4


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MXene can be used as co-catalyst for semiconductor photocatalysts. For example, Ti3C2, the most studied MXene, has been used as co-catalyst to enhance the photocatalytic performance of TiO2, g-C3N4, CdS, ZnS, and ZnxCd1-xS10,

26-28.

In the Ti3C2/CdS system, a Schottky

junction is formed between Ti3C2 and CdS, which can serve as an electron trap to efficiently capture the photo-induced electrons. Under illumination, the electrons are excited from the valence band (VB) to the conduction band (CB) of CdS, then photogenerated electrons in the CB of CdS migrate to Ti3C2 due to the low Fermi level of Ti3C2. With the excellent electrical conductivity and suitable H adsorption Gibbs free energy of Ti3C2, the photogenerated electrons were aggregated on Ti3C2 and the protons are efficiently reduced by the photo-induced electrons on Ti3C2 to form hydrogen26. A Schottky junction can also be formed on the In2S3/anatase TiO2@metallic Ti3C2Tx hybrids29. In addition, Ti2C and Nb2C were also used as effective co-catalysts for photocatalysis30-32. Although MXene is considered as an effective co-catalyst for photocatalytic water splitting, the reported MXene co-catalysts for photocatalysis are mostly multilayer or nanoparticles26-27, which suffers from slow charge transfer in the bulk and shows moderate photocatalytic activity. For example, when multilayer Ti3C2Tx was used as the co-catalyst for TiO2, the TiO2/Ti3C2Tx composites show a low hydrogen production (17.8 μmol h-1 gcat-1), which is only 4 times higher than that of TiO231. Furthermore, even when Ti3C2Tx and Cu were simultaneously used as the co-catalysts to improve the photocatalytic activity of TiO2, the hydrogen production rate is still low due to the multilayer character of Ti3C2Tx33. Monolayer co-catalysts not only possess large specific surface area and more exposed active sites than multilayer co-catalysts for photocatalytic reaction, but also shorten the 5



migration distance of photogenerated charges. Hence, monolayer co-catalysts could effectively promote the separation of photogenerated electrons and holes. For example, compared to multilayer MoS2 (layer numbers ≥ 2) co-catalysts, CdS shows the highest hydrogen generation rate when monolayer MoS2 was used as the co-catalyst34. However, monolayer sulfide co-catalysts are not stable during the photocatalytic reaction. Besides, monolayer graphene has also been used as the co-catalyst of photocatalyst, but the photocatalyst showed relative low photocatalytic activity due to the hydrophobic nature and the lack of active site in graphene35. Compare to monolayer sulfide and graphene, the numerous hydrophilic functionalities on the surface of Ti3C2Tx (Tx = O, OH, F) can promote the strong interaction with water molecules36. In addition, the exposed terminal metal sites on the Ti3C2Tx might lead to stronger redox reactivity than that of the graphene26. Herein, we report the synthesis of multilayer and monolayer Ti3C2Tx (Tx = O, OH, F) and their use as co-catalysts for commercial TiO2 P25. Different from the previous report on the multilayer Ti3C2Tx/TiO2 composites31, the high electronic conductivity and ultra-thin nature of the monolayer Ti3C2Tx was found to be beneficial for the migration of photogenerated electrons from TiO2 to Ti3C2Tx, achieving higher separation efficiency of electron-hole pairs and higher photocatalytic hydrogen production than that of multilayer Ti3C2Tx as the co-catalyst. EXPERIMENTAL SECTION Synthesis of multilayer and monolayer Ti3C2Tx The synthesis of MAX phases was similar to that in the previous report24. Briefly, a mixture of commercial Ti2AlC and TiC was heated to 1350 ºC for 2 h under continuous flow of argon gas, Ar, to produce Ti3AlC2. For the preparation of multilayer Ti3C2Tx, the 6


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as-synthesized Ti3AlC2 powder (2.0 g) was immersed in 20 mL of a 48% HF aqueous solution and stirring at room temperature for 15 h to remove Al layer (To avoid initial overheating due to the exothermic nature of the reaction, the Ti3AlC2 powder was slowly added to the HF solution.). Then the suspension was centrifuged to obtain the Ti3C2Tx slurry, and the slurry was washed repeatedly with deionized (DI) water for several times to reach the pH~7. Then the multilayer Ti3C2Tx slurry was centrifuged and dried in a vacuum oven at 80 ºC for 12 h (multilayer Ti3C2Tx denoted by M-Ti3C2Tx). The monolayer Ti3C2Tx (denoted by Ti3C2Tx) was prepared similarly to the previously reported method37. Briefly, 1.0 g LiF was dissolved in 20 mL of 6 M HCl under stirring at room temperature for 5 minutes. After that, 1.0 g Ti3AlC2 powder was slowly added to the LiF/HCl solution. Then the solution was heated to 35 ºC and keep stirring for 24 h. The resulting suspension was centrifuged and washed several times with DI water until the pH was ≥ 6. Then the suspension was centrifuged at 8000 rpm to remove the un-etched Ti3AlC2 and the multilayer Ti3C2Tx, and the obtained supernatant was the colloidal solution of delaminated Ti3C2Tx. After that, the colloidal solution was filtered through a PVDF membrane (0.25 μm pore size, Millipore) and dried in vacuum oven at room temperature for 24 h to collect the stacked monolayer Ti3C2Tx (denoted by S-Ti3C2Tx). Synthesis of Ti3C2Tx/TiO2 composites To prepare the monolayer Ti3C2Tx/TiO2, 80 mg S-Ti3C2Tx was re-dispersed in 40 mL DI water by sonication for 1 h in an Ar atmosphere, the concentration of the monolayer Ti3C2Tx solution was 2 mg mL-1. 0.3 g commercial TiO2 P25 (99.9%, Degussa, Evonik, anatase to rutile ratio = 80:20) was added to 100 mL DI water and sonicated for 30 minutes. Then, 7



quantitative monolayer Ti3C2Tx solution (2.0, 4.0, 5.0 and 6.0 wt%) was added to the TiO2 suspension and was treated by sonication wave for 30 minutes. The suspension was allowed to stand for 30 minutes and gray precipitate can be observed at the bottom of the beaker (Figure S1). The suspension was centrifuged at 4000 rpm to separate the Ti3C2Tx/TiO2 composite, and transparent supernatant was observed after centrifugation. The Ti3C2Tx/TiO2 composite was dried in the vacuum oven at 80 ºC for 24 h. The obtained Ti3C2Tx/TiO2 composites with different Ti3C2Tx loadings (2.0, 4.0, 5.0 and 6.0 wt%) are denoted as 2-TC/TO, 4-TC/TO, 5-TC/TO and 6-TC/TO. For comparison, 5.0 wt% multilayer Ti3C2Tx/TiO2 (denoted as 5-MTC/TO) was also prepared by the same process by adding 5.0 wt% of multilayer Ti3C2Tx. Photocatalytic hydrogen evolution reaction The photocatalytic hydrogen production reaction was performed in a 65 mL side-irradiation quartz vessel where 30 mg of the photocatalyst was dispersed into 40 mL mixed solution of DI water with 25% methanol as the sacrificial electron donors by ultrasonic. Prior to light irradiation, the system was deaerated with ultrahigh pure Ar for 30 minutes to remove air. The light input was provided by a 200 W Hg lamp with a cutoff filter of 285~325 nm. The solution was stirred during the whole reaction process and the temperature was maintained at 25 °C by the cooling water circulating around the quartz reactor. The gas product was quantified by a gas chromatography compiled with hydrogen calibration plot (Model BUCK 910, equipped with a molecular sieve column and thermal conductivity detector (TCD), with argon as carrier gas).

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Catalysts Characterizations X-ray diffraction (XRD) data was collected on a PANalytical X’Pert MPD Pro powder diffractometer equipped with a Si-based position-sensitive one-dimensional detector and Ni-filtered Cu Kα radiation source. For the measurements, X-rays were generated at 45 kV and 40 mA at a beam wavelength of λ = 1.5416 Å (Cu Kα radiation). Scanning electron microscope (SEM) analysis was performed using a Zeiss Merlin SEM with an acceleration voltage of 20.0 kV. High-resolution transmission electron microscopy (HRTEM) images were obtained on an aberration-corrected FEI Titan S 80-300 STEM/TEM microscope equipped with a Gatan OneView camera at an acceleration voltage of 300 kV. The surface area of catalysts was analyzed by N2 adsorption-desorption isotherms at liquid nitrogen temperature (77 K) with a Quantachrome system, the catalysts were evacuated at room temperature for 1.0 h prior to the measurement. The Brunauer–Emmett–Teller (BET) method was used to estimate the specific surface areas. Raman spectra were collected with an Acton Trivista 555 spectrometer (Princeton Instruments) with laser excitation at 532 nm. UV–vis absorption spectra of the catalysts were collected using a Cary 5000 UV/Vis spectrophotometer equipped with a Praying mantis attachment at room temperature. Photoluminescence (PL) spectra were obtained on a Horiba Jobin Yvon Fluorolog fluorescence spectrometer. The excitation monochromator was set at 325 nm, and the slits on the excitation and detection monochromators were set at 5 nm. X-ray photoelectron spectroscopy (XPS) was analyzed on a Thermo Scientific K-Alpha spectrometer at an operating pressure under 3.0 × 10-7 Pa and a spot size of 400 μm using an Al-Kα microfused monochromatized source (1486.6 eV) with a resolution of 0.1 eV. Atomic force microscopy 9



(AFM) analysis was performed on a Dimension Icon scanning probe microscope with the contact and tapping modes. The AFM data were analyzed using Nanoscope Analysis software. Photoelectrochemical measurements Photoelectrochemical (PEC) measurements were carried out on a BioLogic SP150 electrochemical workstation with a standard three-electrode system. The Ag/AgCl electrode and Pt mesh were used as the reference electrode and the counter electrode, respectively. A 0.2 M Na2SO4 aqueous solution was utilized as the buffer solution. Electrochemical impedance spectroscopy (EIS) was collected in a range from 0.1 kHz to 100 kHz with an AC amplitude of 5 mV. The transient photocurrent was performed in the same three-electrode system with a 200 W Hg lamp (285~325 nm) as the light source. The working electrodes were synthesized as follows: 4.0 mg of the synthesized photocatalyst was first ultrasonically dispersed in the mixture of 250 μL of DI water, 250 μL of ethanol and 20 μL of Nafion® solutions (Sigma Aldrich, 5 wt% in mixture of lower aliphatic alcohols and water, contains 45% water) for 1.0 h. After that, 20 μL of the catalyst suspension was transferred onto the indium tin oxide (ITO) substrate. The ITO substrate was dried at room temperature to obtain the working electrode. In the photoelectrochemical measurements, the potential applied to the working electrode was 0.8 V vs Ag/AgCl.

RESULTS AND DISCUSSION Morphologies of multilayer and monolayer Ti3C2Tx The morphology of Ti3AlC2, multilayer Ti3C2Tx (M-Ti3C2Tx), stacked monolayer Ti3C2Tx (S-Ti3C2Tx) and monolayer Ti3C2Tx deposited on the Si wafer was elucidated by 10


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SEM. After selective removal of Al from Ti3AlC2 in the HF solution, the bulky 3D Ti3AlC2 particle (Figure 1A) was successfully exfoliated to multilayers of 2D Ti3C2Tx (Figure 1B). In the multilayer Ti3C2Tx, the layers are clearly separated from each other compared to those in Ti3AlC2. The multilayer Ti3C2Tx particles show the clear accordion-like morphology (Figure 1C). To obtain the monolayer Ti3C2Tx, the Ti3AlC2 powder was treated using a LiF/HCl solution. The typical Tyndall effect was clearly observed in an aqueous suspension of monolayer Ti3C2Tx (Figure S2), demonstrating their excellent hydrophilicity and dispersity. The SEM image of the cross section of the S-Ti3C2Tx is shown in Figure 1D. S-Ti3C2Tx is a highly flexible macroscopic film consist of ultrathin Ti3C2Tx flakes (see Figure S3 for the photographs of the S-Ti3C2Tx), which is consistent with previous reports37. From the enlarged SEM image (Figure S4B) of S-Ti3C2Tx, there are irregular spaces among the Ti3C2Tx nanosheets, confirming the weak interaction between the Ti3C2Tx nanosheets. In addition, the top surface of the S-Ti3C2Tx clearly shows irregular Ti3C2Tx flakes that are stacked layer-by-layer after the vacuum-assisted filtration processes (Figure 1E). Generally, van der Waals (vdW) attraction and hydrogen bonding between the nanosheets exist in the multilayer Ti3C2Tx, and the multilayer Ti3C2Tx is unlikely to be exfoliated into monolayer Ti3C2Tx under ultrasonic38. However, the stacked monolayer Ti3C2Tx was obtained by filtration of monolayer Ti3C2Tx colloidal solution and dry at room temperature. In the process of filtration or drying, it is possible to insert water molecules among the monolayer Ti3C2Tx nanosheets, resulting in the loose contact between the monolayer Ti3C2Tx. Moreover, the size of the monolayer Ti3C2Tx nanosheets is different from each other, therefore, the monolayer Ti3C2Tx is a staggered stacking during the filtration, 11



which also leads to the loose contact between the monolayer Ti3C2Tx, as shown in Figure 1E. Thus, it is possible to disperse the monolayer Ti3C2Tx from the S-Ti3C2Tx. The morphology of the Ti3C2Tx flakes can be seen more clearly in Figure 1F. These flakes have a clean surface and the size of Ti3C2Tx flakes was in the range of tens to hundreds of nanometers. The SEM images (Figure S4C, D, E, F) from the other parts of the re-dispersed monolayer Ti3C2Tx deposited on a Si wafer show that most of the Ti3C2Tx are in the nanosheet-shape. Therefore, large amount of high-quality monolayered MXene can be obtained by redispersion of stacked MXene under ultrasonic. The above analysis indicates that the synthetic conditions greatly influenced the shape, size, and morphology of the Ti3C2Tx.

Figure 1. SEM images of Ti3AlC2 (A), multilayer Ti3C2Tx (B, C), S-Ti3C2Tx (D, E) and monolayer Ti3C2Tx deposited on a Si wafer (F). The morphology of the Ti3C2Tx flakes was further studied using TEM. The low-magnification TEM images of Ti3C2Tx show the ultrathin nature of Ti3C2Tx flakes (Figure 2A and B). In addition, the cross-section of Ti3C2Tx flakes shows that the thickness of the monolayer Ti3C2Tx is about 1 nm (Figure 2C), confirming the two-dimensional structure of the Ti3C2Tx flakes39. Moreover, the typical lattice fringes of Ti3C2Tx were observed in the 12


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high-resolution TEM images (Figure 2D). The thickness and shapes of the Ti3C2Tx flakes were further investigated by AFM (Figure 2E). The AFM height profile measured along the red dashed line in Figure 2E shows that the height of Ti3C2Tx flakes is about 1.7 ~ 2.2 nm, which can be identified as monolayers according to the previous reports37,

40.

The same

brightness in the AFM images manifests the same thickness of the Ti3C2Tx flakes, which is in line with the TEM results. Notably, the thickness of the Ti3C2Tx flakes obtained from the AFM height profiles is thicker than that measured from the high-resolution TEM. When we prepared the sample for AFM measurement, the monolayer Ti3C2Tx was deposited on the Si wafer and dried at room temperature. Consequently, it is possible that the surface adsorbates (such as water molecules) present between the Ti3C2 flakes and the Si substrate. Therefore, the monolayer flake is not lying atomically flat on Si surface and the increased thickness compared to TEM measurement is mostly likely due to the surface adsorbates stuck between the Ti3C2Tx flakes and the silicon substrates37.

Figure 2. TEM images (A, B, C, and D), AFM images (E) and height profiles from AFM (F, G) of monolayer Ti3C2Tx. 13



Structural properties of as-prepared samples

Figure 3. XRD patterns of Ti3AlC2, M-Ti3C2Tx, S-Ti3C2Tx, TiO2, 2-TC/TO, 4-TC/TO, 5-TC/TO, 6-TC/TO and 5-MTC/TO (A, B), Raman spectra of TiO2, S-Ti3C2Tx, 2-TC/TO, 5-TC/TO and 6-TC/TO (C, D). The XRD patterns of Ti3AlC2, M-Ti3C2Tx, and S-Ti3C2Tx are shown in Figure 3A. It is obvious that the most intense peak at 2θ = 39.02° corresponds to the (104) diffraction of Ti3AlC224 and no diffraction peak of Ti2AlC (such as 12.97°)41 or TiC (such as 36.0°)42 can be observed in the XRD pattern of Ti3AlC2. Therefore, the precursor Ti3AlC2 is considered to be a single phase. The peak of Ti3AlC2 at 2θ = 39.02° disappeared after etching by HF or LiF/HCl. At the same time, the (002) and (004) peaks are broader and shifted to lower angles. 14


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The elimination of the Ti3AlC2 peaks and the shift of (002) and (004) peaks indicate the removal of interlayer Al and the formation of 2D Ti3C2Tx (MXene) 43. Compared to Ti3AlC2, the (002) and (004) peaks of multilayer Ti3C2Tx shifted from 9.67° and 19.19° to 8.89° and 18.10°, respectively, indicating an increase in the interlayer d-spacing distance. Moreover, the (002) and (004) peaks of S-Ti3C2Tx further shifted respectively to 7.38° and 14.63°, suggesting the interlayer distance in the S-Ti3C2Tx is larger than that in multilayer Ti3C2Tx. From the XRD patterns of the Ti3C2Tx/TiO2 composites (as displayed in Figure 3B), all diffraction peaks of the Ti3C2Tx/TiO2 samples can be assigned to the anatase and rutile phases of the P25. Introducing Ti3C2Tx onto P25 did not change the crystal structure and the phase composition of P2544. In addition, no characteristic diffraction peaks of Ti3C2Tx were found due to its relatively low mass content and high dispersion on P25. The Raman spectra of Ti3C2Tx, TiO2, and Ti3C2Tx/TiO2 composites are shown in Figure 3C. The characteristic bands at 207, 390, and 630 cm-1 can be attributed to Ti3C2Tx45-46. Additionally, the characteristic bands at 147, 400, 519, and 641 cm-1 are assigned to the Eg(1), B1g(1), A1g + B1g(2), and Eg(2) modes of anatase TiO247. No characteristic bands of Ti3C2Tx were found in the Raman spectra of the Ti3C2Tx/TiO2 composites due to the low loading of Ti3C2Tx. However, as shown in Figure 3D, it is worth noting that the band of TiO2 at 147 cm-1 slightly shifted to 151 cm-1 after the loading of Ti3C2Tx, indicating the interaction between TiO2 and Ti3C2Tx. Morphology of Ti3C2Tx/TiO2 composites The SEM images of TiO2 and 5-TC/TO are shown in Figure 4A and 4B. Fine nanoparticles of P25 with uniform size were observed (Figure 4A). In the SEM image of the 5-TC/TO sample, the P25 nanoparticles are loaded on the ultrathin Ti3C2Tx flakes, and no 15



multilayer Ti3C2Tx can be observed, which agrees with the XRD pattern. When the Ti3C2Tx flakes were added to the TiO2 suspension, the TiO2 particles can be adsorbed on the Ti3C2Tx flakes to form the gray precipitate and then sink to the bottom (Figure S1). These results demonstrating that the Ti3C2Tx/TiO2 composites were successfully prepared. With the increase of the content of Ti3C2Tx flakes from 2.0 wt% to 6.0 wt%, more and more Ti3C2Tx flakes can be found in the SEM images of the Ti3C2Tx/TiO2 composites, as displayed in Figure S5. Moreover, the surface area of Ti3C2Tx/TiO2 composites gradually increases with the increase of Ti3C2Tx content due to the high surface area of the Ti3C2Tx flakes (Figure S6 and Table S1). The BET specific surface area of the TiO2, 2-TC/TO, 4-TC/TO, 5-TC/TO, and 6-TC/TO is 48.7, 54.6, 54.8, 58.3, and 61.4 m2 g-1, respectively. On the contrary, as shown in Figure S5D, only P25 nanoparticles can be found in the SEM images of the 5-MTC/TO sample, suggesting that the multilayer Ti3C2Tx was covered by the small TiO2 particles. In addition, the BET specific surface area of the 5-MTC/TO sample (49.5 m2 g-1) is lower than that of 5-TC/TO (61.4 m2 g-1), which is due to the larger surface area of monolayer Ti3C2Tx when compared to that of multilayer Ti3C2Tx. To further analyze the morphology and microstructures of the Ti3C2Tx/TiO2 composites, TEM and HRTEM imaging was carried out for 5-TC/TO. As shown in Figure 4C, the typical size of P25 particles is around 25 nm. From the higher magnification TEM images in Figure 4D, the Ti3C2Tx flakes with thickness of 1 nm can be observed, indicating that the Ti3C2Tx flakes are uniformly located within P25 particles. In the HRTEM images (Figure 4E and 4F), the typical lattice fringes of monolayer Ti3C2Tx can be found in the 5-TC/TO sample, and the measured lattice fringes of 0.36 nm is in agreement with the (101) plane of anatase TiO247. In 16


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addition, the co-existence of TiO2 and Ti3C2Tx can be clearly seen in Figure 4F. Generally, the Ti3C2Tx surface is terminated with O/OH/F groups, which would interact with the O/OH groups on the TiO2 surface via hydrogen bonding. Moreover, Van der Waals can also exist between the TiO2 surface and Ti3C2Tx monolayer. Therefore, the TiO2/Ti3C2Tx composites were constructed by the hydrogen bonding and the Van der Waals interaction. This can be confirmed from Figure 4F that the TiO2 forms intimate connections with the Ti3C2Tx, as well as the observed Raman shift of TiO2 mode in Figure 3D. When the TiO2/Ti3C2Tx is under light irradiation, such intimate contact will facilitate transfer of the photogenerated electrons from the TiO2 to the Ti3C2Tx, resulting in the separation of the photogenerated electrons and holes.

Figure 4. SEM images of TiO2 (A) and 5-TC/TO (B), and TEM images (C, D, E) and HRTEM image of 5-TC/TO (F). Chemical composition of photocatalysts To analyze the chemical state of the prepared samples, X-ray photoelectron spectroscopy 17



(XPS) spectra of Ti3C2Tx and 5-TC/TO were collected (Figure 5, Table S2, Table S3). As shown in the high resolution XPS of Ti 2p (Figure 5A), several peaks at binding energy of 454.9 (461.3), 455.9 (461.5), 457.0 (462.9), and 460.4 eV (466.2 eV) can be assigned to the Ti (I, II, or IV), Ti2+(I, II, or IV), Ti3+(I, II, or IV), and C-Ti-Fx(III) in the Ti3C2Tx, respectively48. I refers to M atoms bonded to C atoms and one oxygen atom, e.g. Ti3C2Ox, II refers to M atoms bonded to C atoms and an OH group, e.g. Ti3C2(OH)x, III refers to M atoms bonded to C and F atoms, e.g. Ti3C2Fx, IV refers to M atoms bonded to OH-terminations that in turn are relatively strongly physisorbed to water molecules forming OH-H2O complexes, viz. Ti3C2OH-H2O48. The binding energy peaks at 458.6 (464.2) and 459.3 eV (465.3 eV) corresponding to the TiO2 and TiO2-xFx were due to the slight oxidation of the Ti3C2Tx and the

Figure 5. High-resolution XPS peak deconvolution of the Ti 2p (A, C) and C 1s (B, D) in Ti3C2Tx (A, B) and 5-TC/TO (C, D). 18


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residue F- ions from the LiF/HCl solution48. Furthermore, the C 1s XPS spectrum (Figure 5B) exhibited four obvious peaks at 281.5, 284.8, 286.4, and 288.7 eV, which were designated to C-Ti-Tx(I, II, III or IV), C-C, CHx/C-O and COO, respectively49. These four peaks can be ascribed to the Ti3C2Tx and adventitious carbon. In contrast, two main peaks at the binding energy of 458.6 (Ti 2p3/2) and 464.2eV (Ti 2p1/2) can be found in the high resolution XPS of Ti 2p in the 5-TC/TO sample (Figure 5C), which were originated from TiO250. The binding energy of Ti 2p3/2 was 0.5 eV higher than that (458.1 eV) of the pure TiO251, indicating the interaction between TiO2 and Ti3C2Tx. In addition, two small Ti (I, II, or IV) peaks of Ti3C2Tx can also be observed in the Ti 2p XPS spectra, confirming the presence of the Ti3C2Tx in the 5-TC/TO sample. The peaks shown in the C 1s of the 5-TC/TO were similar to those of Ti3C2Tx (Figure 5D). Light-harvesting capability and charge transfer The light harvesting capability of Ti3C2Tx, TiO2 and Ti3C2Tx/TiO2 composites was investigated by collecting ultraviolet-visible absorption spectra. As shown in Figure 6A, the S-Ti3C2Tx exhibits a strong absorption in the entire region (250 - 800 nm), while the TiO2 can only absorb ultraviolet light due to its wide band gap. The light absorption of the Ti3C2Tx/TiO2 composites is obviously increased in the visible light region due to the dark color of Ti3C2Tx. In addition, the light absorption of Ti3C2Tx/TiO2 composites increased with the increase of the content of Ti3C2Tx flakes. Notably, the light absorption of 5-MTC/TO is only slightly increased compared to bare TiO2 and is even lower than that of 2-TC/TO sample. This result suggests that multilayer Ti3C2Tx was partly covered by the TiO2 particles,

19



while the monolayer Ti3C2Tx was dispersed homogeneously in the Ti3C2Tx/TiO2 composites, as verified by the SEM analysis above.

Figure 6. UV-vis absorption spectra (A) of TiO2, S-Ti3C2Tx, 2-TC/TO, 4-TC/TO, 5-TC/TO, 6-TC/TO and 5-MTC/TO, PL spectra (B), Transient photocurrent (C) and EIS spectra (D) of TiO2, 5-TC/TO and 5-MTC/TO. The impact of Ti3C2Tx loading on the charge separation in TiO2 was studied by photoluminescence (PL) measurement (Figure 6B). Generally, the lower the PL intensity is, the lower recombination rate of the electron-hole pairs is52-53. As shown in Figure 6B, the PL intensity decreases after loading with Ti3C2Tx, demonstrating the positive effect of Ti3C2Tx on the separation of the photoinduced electron-hole pairs. Moreover, with the same loading content of 5 wt%, the PL intensity of the 5-TC/TO sample is significantly lower than that of 5-MTC/TO sample, which indicates the superior charge separation effect from monolayer 20


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Ti3C2Tx than the multilayer one. To further clarify the effect of Ti3C2Tx on the separation of electrons and holes in TiO2, the transient photocurrent response (TPC) and electrochemical impedance spectra (EIS) were measured to unravel the charge separation and transfer process. The transient photocurrent of 5-TC/TO and 5-MTC/TO are much higher than that of TiO2, suggesting the enhanced separation of charge carriers in both composites54. Notably, the 5-TC/TO shows the highest transient photocurrent when compared to TiO2 and 5-MTC/TO (Figure 6C). In addition, EIS spectra clearly show that the interfacial charge transfer is improved in 5-MTC/TO and 5-TC/TO when compared to that of TiO2, as the semicircle diameters are smaller for the composite photocatalysts (Figure 6D)10. Moreover, it is obvious that the interfacial charge transfer resistance of 5-TC/TO is smaller than that of 5-MTC/TO. The PL, TPC and EIS analysis clearly indicates that the 2D monolayer Ti3C2Tx is a better co-catalyst than the multilayer Ti3C2Tx for the charge separation in TiO2. Photocatalytic hydrogen evolution To determine the effect of Ti3C2Tx on the photocatalytic performance of TiO2, the photocatalytic hydrogen evolution reaction was carried out under light irradiation over different Ti3C2Tx/TiO2 samples with methanol as the hole scavenger. Figure 7A shows that the amount of generated H2 increases linearly with the reaction time, the average H2 evolution rate was calculated and displayed in Figure 7B. Obviously, the Ti3C2Tx/TiO2 composites show superior photocatalytic activity than that of pure TiO2, due to promotional effect of Ti3C2Tx on the separation of photogenerated charge carriers. In addition, the H2 evolution rate increases with the content of monolayer Ti3C2Tx up to 5.0 wt% and the 5-TC/TO sample 21



exhibits the highest hydrogen evolution rate (2.65 mmol h-1 gcat-1), which is 9.1 times higher than that of the pure TiO2 (0.29 mmol h-1 gcat-1), due to the promoted separation of the photoinduced electrons and holes in TiO2 as verified by the PL, TPC, and EIS results. The performance of TiO2/Ti3C2Tx was compared with other reported TiO2/MXene, TiO2 and MXene-based photocatalysts in Table S5. Although the testing conditions vary among each study, the performance of the present catalyst (5% monolayer Ti3C2Tx/TiO2 (P25)) is among the best photocatalysts for hydrogen evolution. Further increasing the content of monolayer Ti3C2Tx leads to the deterioration of the photocatalytic performance. This result is likely due to the light shielding effect of the dark Ti3C2Tx, which will block the light absorption of the TiO2. Besides, with an excess of Ti3C2Tx content, it is possible that the Ti3C2Tx/TiO2 composites become too conductive to have efficient charge separation.

Figure 7. Photocatalytic hydrogen evolution over TiO2, 2-TC/TO, 4-TC/TO, 5-TC/TO, 6-TC/TO and 5-MTC/TO as a function time on stream (A); and average hydrogen evolution rate (B). Interestingly, with the same loading amount of 5.0 wt%, the 5-TC/TO sample shows ~ 3 22


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times higher photocatalytic activity than that of 5-MTC/TO sample (0.92 mmol h-1 gcat-1). This can be attributed to the following reasons; (1) the surface area of 5-TC/TO is higher than that of 5-MTC/TO due to the 2D nature of the monolayer Ti3C2Tx and more active sites can be exposed on the monolayer Ti3C2Tx; (2) the light absorption capacity of the 5-TC/TO is larger than that of 5-MTC/TO, (3) a larger contact interface between Ti3C2Tx and TiO2 can be formed in 5-TC/TO than that in the 5-MTC/TO. Based on the above analysis, monolayer Ti3C2Tx should be a superior co-catalyst for the photocatalyst compared to the multilayer Ti3C2Tx. Furthermore, the apparent quantum yield (AQE) of the TiO2, 2-TC/TO, 4-TC/TO, 5-TC/TO, 6-TC/TO and 5-MTC/TO was calculated to be 1.7%, 7.6%, 12.0%, 15.8%, 10.1% and 5.5%, respectively (Table S6). In order to confirm the role of Ti3C2Tx in the Ti3C2Tx/TiO2 composites, 5.0 wt%-Ti3C2Tx/Al2O3 was used as the photocatalyst for the photocatalytic hydrogen evolution reaction. The SEM images of the Al2O3 and 5.0 wt%-Ti3C2Tx/Al2O3 can be found in Figure S7. No hydrogen was obtained after reaction under the light irradiation with 5.0 wt%-Ti3C2Tx/Al2O3 as the photocatalyst, suggesting that the Ti3C2Tx is not a photocatalyst due to its metallic character, but rather a co-catalyst in the Ti3C2Tx/TiO2 composites. To figure out whether the enhanced light absorption in the visible region can arouse the visible light activity of the Ti3C2Tx/TiO2 composites, visible light with a band-pass filter (＞400 nm) was used as the light source for the photocatalytic hydrogen evolution reaction. After reaction for 4 h, no hydrogen was detected, indicating the photoinduced electrons and holes can only be produced under the ultraviolet light. Photocatalytic stability measurement 23



The stability of photocatalysts under light irradiation is very important to their practical application. Therefore, the photocatalytic stability of the 5-TC/TO sample was investigated. After 16 h (4 cycles) reaction, the photocatalytic hydrogen production rate decreased from 2.65 mmol h-1 gcat-1 to 2.30 mmol h-1 gcat-1, as displayed in Figure 8A. Generally, Ti3C2Tx is unstable in the aqueous solution in the presence of dissolved oxygen. However, the Ti3C2Tx solution could be well-preserved when the dissolved oxygen was removed from the solution by Ar gas and well-protected from the dissolved oxygen55. In our study, prior to the photocatalytic hydrogen evolution reaction, the solution was deaerated with ultrahigh pure Ar to remove air and any dissolved oxygen. 25% methanol was used as the sacrificial electron donors to consume the holes, hardly any oxygen can be produced and exist in the solution. Furthermore, the photogenerated electrons is likely to restrain the oxidation of Ti3C2Tx. Therefore, the Ti3C2Tx should be comparatively stable in such reaction condition. To find out the reasons of the reduced photocatalytic activity, the crystal structure, light absorption, surface area, morphology, and chemical states of the 5-TC/TO after reaction for 4 cycles were investigated. As shown in Figure 8B, there is no obvious change in the XRD patterns of the used 5-TC/TO, indicating that the crystal structure remains unchanged after the photocatalytic reaction. However, the light absorption of the used 5-TC/TO decreased slightly compare to the fresh sample, which could be due to the TiO2 particles became detached from the Ti3C2Tx flakes during the photocatalytic reaction (Figure 8C). From SEM analysis (Figure 8D), the Ti3C2Tx flakes are still present in the used 5-TC/TO and in contact with the TiO2 particles, indicating the stability of the Ti3C2Tx flakes. In addition, the surface area of the 5-TC/TO shows no significant change before (58.3 m2 g-1) and after (58.4 m2 g-1) reaction 24


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(Figure S8).

Figure 8. Stability of photocatalytic hydrogen production over 5-TC/TO, every 4 h the reaction system was purged with Ar for 30 min to remove H2 (A), XRD (B), UV-vis absorption spectra (C) and SEM images (D) of 5-TC/TO before and after reaction. Furthermore, the XPS spectra of the used 5-TC/TO are similar to that of the fresh 5-TC/TO, indicating the chemical states is unchanged after the photocatalytic hydrogen evolution (Figure S9, Table S4). According to the above analysis, since the Ti3C2Tx/TiO2 composites are prepared by mixing the TiO2 particles and Ti3C2Tx flakes together, it is likely that some TiO2 particles can become detached from the Ti3C2Tx flakes, which results in some loose contact and thus the decreased activity of the Ti3C2Tx/TiO2 composites. On the other hand, most TiO2 particles are still in close contact with the Ti3C2Tx flakes, accounting for the still high activity of the Ti3C2Tx/TiO2 composites after several cycles. 25



Proposed reaction mechanism

Figure 9. Proposed mechanism of photocatalytic hydrogen evolution over the Ti3C2Tx/TiO2 system. Based on above experimental results and discussions, the photocatalytic mechanism of Ti3C2Tx/TiO2 system for photocatalytic hydrogen evolution is proposed and illustrated in Figure 9. Firstly, the photoinduced electrons and holes are produced on the CB and VB of TiO2 under light irradiation, respectively. Secondly, due to the excellent electrical conductivity of the monolayer Ti3C2Tx and the contact between the monolayer Ti3C2Tx and TiO2, the photogenerated electrons can rapidly transfer from the CB of TiO2 to the Ti3C2Tx. Thus the holes are left on the VB of TiO2, and these holes was consumed by the hole scavenger (CH3OH). Thirdly, the photogenerated electrons accumulated on Ti3C2Tx could react with H+ to generate H2. Furthermore, the ultrathin nature of 2D Ti3C2Tx shortens the transfer distance of charge carriers to the Ti3C2Tx surface, and the Schottky junction formed at the Ti3C2Tx/TiO2 interface facilitates the charge transfer26, 31. In addition, the photocatalytic reaction might also be promoted by the photothermal effect originated from the light absorption of Ti3C2Tx10, 56. 26


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Compared with multilayer Ti3C2Tx, more exposed surface-active sites on the monolayer Ti3C2Tx and higher density of the Ti3C2Tx/TiO2 interfaces are beneficial to the photocatalytic performance. Also, some of the Ti3C2Tx monolayers have smaller size than the multilayers as show by the SEM images, which can be beneficial to photocatalysis since it is well known that the catalytic property can be influenced by the size of the single-layer flakes based on literature work. On one hand, the size can have a great effect on the Fermi level of the Ti3C2, which influences the photocatalytic performance of the photocatalyst 57. On the other hand, if the catalytic active site is on the edge of the single-layer flakes, the smaller the size, the more exposed edges, which is beneficial to the catalytic property. In this case, the size of the flakes has a great effect on the catalytic property58. The location of active site on the Ti3C2Tx photocatalytic hydrogen production is still not clear at present and further studies are warranted. CONCLUSIONS In summary, the monolayer Ti3C2Tx/TiO2 composites were successfully fabricated by a simple impregnation method. The photocatalytic activity of the Ti3C2Tx/TiO2 composites for hydrogen evolution showed a volcano relationship with the content of the monolayer Ti3C2Tx flakes. The optimum loading amount of the Ti3C2Tx flakes is 5.0 wt%, which shows 9.1 times higher hydrogen production rate than bare TiO2. The enhanced catalytic activity is attributed to the large quantity of surface sites on the monolayer Ti3C2Tx/TiO2 composites and the efficient separation of charge carries at the heterojunctions formed between Ti3C2Tx and TiO2. The enhanced separation efficiency of photoinduced charge carries and improved transfer rate of electrons at the interface were confirmed by PL, TPC and EIS. Furthermore, we found that 27



the monolayer Ti3C2Tx is much better than multilayer Ti3C2Tx as a co-catalyst in improving the photocatalytic activity of TiO2 due to its 2D character. This study highlights the potential application of 2D MXenes as efficient co-catalysts, and presents a promising strategy to enhance the photocatalytic performance of semiconductor-based photocatalysts. Also, this work proves that in order to achieve an ultimate performance of MXene in certain applications, their 2D monolayer nature should be utilized rather than using the multilayer materials. It is noted in the current study that some of the TiO2 particles are detached from the Ti3C2Tx flakes during the photocatalytic reaction, resulting in decreased photocatalytic activity. To improve the stability, the Ti3C2Tx/TiO2 composites via in situ TiO2 growth onto Ti3C2Tx can be a potential approach and warrant further studies for the photocatalytic hydrogen evolution. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Supplemental characterization of photocatalysts by XRD, SEM, XPS and N2 adsorption-desorption, Comparison of the hydrogen evolution rate for TiO2-based photocatalysts. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (T. Su) *E-mail: [email protected] (Z. Qin) *E-mail: [email protected]. Tel: +1(865)576-1080 (Z. Wu) 28


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported and conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. TMS acknowledges the support from China Scholarship Council. ZDH gratefully acknowledges a Graduate Research Fellowship award from the National Science Foundation (DGE-1650044). LB acknowledges financial support from National Science Foundation supplemental intern funding 1511818.

Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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Monolayer Ti3C2Tx as an Effective Co-catalyst for Enhanced

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