Nanolayered Ti3C2 and SrTiO3 Composites for Photocatalytic

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Nanolayered TiC and SrTiO Composites for Photocatalytic Reduction and Removal of Uranium(VI) Hao Deng, Zijie Li, Lin Wang, Li-Yong Yuan, Jianhui Lan, Zhi-yuan Chang, Zhifang Chai, and Wei-Qun Shi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00205 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Applied Nano Materials

Nanolayered Ti3C2 and SrTiO3 Composites for Photocatalytic Reduction and Removal of Uranium(VI)

Hao Deng†,‡, Zi-jie Li†, Lin Wang†, Li-yong Yuan†, Jian-hui Lan†, Zhi-yuan Chang‡, Zhi-fang Chai†,§, Wei-qun Shi*,†

†Laboratory

of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese

Academy of Sciences, Beijing 100049, China ‡Department

of Radiochemistry, China Institute of Atomic Energy, Beijing 102413,

China §Engineering

Laboratory of Advanced Energy Materials, Ningbo Institute of Industrial

Technology, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China

Corresponding author: *E-mail:

[email protected], Tel: +86-010-88233968, Fax: +86-010-88235294 1

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Given its promising electron transportation ability, excellent electrical conductivity and larger work function (6.2 eV) disclosed by density functional theory calculations, MXene material, O-terminated Ti3C2 has the potential to serve as a perfect co-catalyst. Herein, a novel Ti3C2/SrTiO3 heterostructure based on partly superficial oxidation from precursor multilayered Ti3C2 is developed as a photocatalyst for efficiently photocatalytic reduction and removal of uranyl. Specifically, the composite of 2 wt% Ti3C2/SrTiO3 (0.02Ti3C2/SrTiO3) exhibits an excellent photocatalytic UO22+ removal rate of 77 %, which is nearly 38 times higher than that of the pristine SrTiO3. The enhanced photocatalytic performance of 0.02Ti3C2/SrTiO3 is systematically identified by photoluminescence spectroscopy, UV-vis diffuse reflectance spectroscopy, Raman spectroscopy and electrochemical characterizations. The multi-layered Ti3C2 as a co-catalyst can facilitate the charge transportation and inhibit the recombination of electrons in the conduction band. This work establishes the enticing potential for developing doped perovskite oxide crystals based on the MXene Ti3C2 with sun-light responsivity for improving solar energy utilization. KEYWORDS: Two-dimensional Ti3C2; Strontium titanate; Photocatalytic reduction removal; Uranium(VI); Ti-based interface 1. INTRODUCTION Highly mobile hexavalent uranium U(VI) is released into the environment owing to large scale nuclear industry activities such as mining, spent fuel reprocessing and nuclear waste disposal, making uranium a source of environmental pollution. Conventional ion exchange and adsorption of nanomaterials have some shortcomings in practical applications, such as weak interaction with radionuclides. Uranium is more likely to return to the aqueous solution again, causing secondary pollution.1 Tetravalent uranium U(IV) is commonly supposed to form insoluble species, resulting in the immobilization of uranium under reduction conditions.2 However, most of the reported reduction methods involve large amount of reagent consumption, and easily generate secondary waste.3-4 Semiconductor-based photocatalysis, a green method for photocatalytic reduction and removal of harmful heavy metal ions under light irradiation,5-7 thus brings 2


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a new opportunity to fight against the uranium contamination. Titanium dioxide (TiO2) is of great concern as a candidate for photochemical removal of uranium8 due to its high resistance to photocorrosion and nontoxicity.9 While, its inherent light absorption and charge separation efficiency decides the relatively low photocurrent density of TiO2.10 On the other hand, perovskite oxide strontium titanate (SrTiO3) has a more excellent electron mobility at room temperature (5-8 cm2 V-1 s-1) compared to that of TiO2 (0.1-4 cm2 V-1 s1),11-12

and higher conduction band energy position,12 which is beneficial for the

photocatalytic reduction. Additionally, in the case of epitaxial integration of SrTiO3 with TiO2, the interfacial stress-induced lattice deformation will produce a ferroelectric materials with tremendous electric polarization.13 Currently, some studies based on the time-resolved steady-state spectroscopic technique demonstrated that a number of photogenerated carriers may recombine quickly at the end of light irradiation, correlated with its relatively lower photonic efficiency (< 10%) for most semiconductor-based photocatalytic reactions.9,

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In order to separate the photo-generated e-/h+ pairs

effectively and thereby prolong e-/h+ lifetimes, a series of nano materials10, 15-16 have been coupled with SrTiO3 crystals.17 For instance, Rh doped SrTiO3 (SrTiO3: Rh) exhibited much higher efficiency for H2 evolution under visible light.15 Sun et al.16 incorporated Ba2FeNbO6 with a narrow band gap of ≈ 2.29 eV into SrTiO3 in molten salt media. The color of formed SrTiO3-Ba2FeNbO6 heterostructure can be turned from white to yellow green matching the solar spectrum well. Furthermore, TiO2-SrTiO3 core-shell heterostructure combined with SrTiO3 intrinsically high charge mobility on the basis of ferroelectric polarization leaded to a drastically increased photocurrent density.10 It is obvious that the much stronger combination of heterogeneous materials is favorable to photocatalytic reaction owing to the short diffusion pathway for photo-generated e-/h+ transfer, leading to decreased recombination probability.12 Thus, the contacting pattern between two components needs to be rationally established. The main reason is that most of the current hetero-component as co-catalysts exist: (i) inappropriate components or lack of surficial functionalities to fabricate strong function with photocatalysts;18 and (ii) inefficient electron shuttling, for the rapid interfacial carrier transportation and the extended structural stability.

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MXenes (Mn+1XnTx), a layered carbides or carbonitrides material,19 shows a great potential as an energy storage material in batteries20-21 and electrochemical capacitors.2223

Ti3C2Tx (abbreviated as Ti3C2) nanosheets are an attractive class of materials featuring

hydrophilicity and excellent metallic conductivity.24 T represents a variable surficial termination25 such as O, OH, or F, and x is the number of termination groups. The band structure of F or OH-terminated Ti3C2 has a clear separation of 0.05 - 0.1 eV between valence and conduction bands,19 manifesting its semiconducting nature. Thus, it is plausible to believe that the band structure of Ti3C2 could be finely tuned by varying the terminal groups, which may afford flexible opportunities in special photocatalytic fields, if Ti3C2 was utilized as photocatalysts or co-catalysts. Actually, MXene co-catalyst21, 26 integrated with cadmium sulfide18 has exhibited both excellent photocatalytic performance and attractive quantum efficiency owing to its highly efficient charge separation when exposed to visible light irradiation. Very recently, Ti3C2 derivative from superficial oxidation as a hetero-component20,

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could improve photoelectrical

transformation efficiency owing to the an atomic scale interfacial heterojunction in the Ti-based interface.29 Bao et al. have reported the transformation of Ti3C2 MXene into ultrathin nanoribbons of NaTi1.5O8.3 and K2Ti4O9 as high-performance anode materials for SIBs and PIBs, respectively.20 As far as we know, no reports have addressed the utilization of MXene-derived Ti-based layered materials as a co-catalyst for titanate perovskite oxide photocatalysts. Both exploiting band and interfacial heterostructure to develop and fabricate the hybrid of perovskite oxide and two-dimensional Ti3C2 is interesting. Herein, we constructed a Ti3C2/SrTiO3 heterostructure based on partly superficial oxidation from precursor multilayered Ti3C2, demonstrating greatly increased charge separation and photocurrent density, and enhanced photocatalytic UO22+ reduction. The possibility of MXene as an exceptional co-catalyst was further identified by density functional theory (DFT) calculations. Moreover, the fabrication of this titanate perovskite-type crystals and the utilizing of the heterogeneous photocatalytic reaction adjusted by such perovskite/MXene heterstructure will bring new insights into the applications of photocatalysis and optoelectronics.

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2. EXPERIMENTAL SECTION 2.1. Materials synthesis Multi-layer MXene Ti3C2 from Ti3AlC2 MAX phase materials was fabricated by a chemical etching method with 15 wt % HF solution at 40 oC for 48 h.19, 30 Then Ti3C2 was centrifuged at 5000 rpm, thoroughly rinsed with de-ionized water 6 times to obtain a water suspension, and stored in a refrigerator (0 - 5 oC). The Ti3C2/SrTiO3 composite materials were synthesized by a simple hydrothermal process (Figure 1) generalized from the following two comprehensive steps. Step 1. Firstly, a reagent of 1 mL titanium isopropoxide (≥ 97%) was added to 50 mL of a mixture of ethanol and acetonitrile (3:2 in volume ratio) containing 0.91 g of H2O and 0.38 g of NH3·H2O under vigorous stirring,31 and thoroughly stirring for 6 h aging. A milky suspension of TiO2 was centrifuged, thoroughly rinsed with ethanol for three times, then washed with water for three times and finally dried at 60 ºC for 24 h. Secondly, 0.1 g of dried TiO2 powder, 1:1 molar ratio of Sr(NO3)2 and 0.2 mL of Ti3C2 water suspension (~5 mg) were slowly added into 25 mL of 2 mol L−1 NaOH aqueous solution. The resultant mixture after stirring for 15 min was transferred to a 50 mL Teflon-lined autoclave for 4 h at 140 °C to conduct the crystallization. The final precipitate was centrifuged, thoroughly rinsed and finally dried at 60 ºC. The different loading amount of Ti3C2/SrTiO3, pure SrTiO3 and other different heterogeneous structure-based system were fabricated by the above parallel process, except for the amount of Ti3C2 and metal ions. The mass fractions (wt.%) of Ti3C2 in Ti3C2/SrTiO3 composites were 0, 1, 2, 4, 20 and 100, and these samples obtained were labeled

as

SrTiO3,

0.01Ti3C2/SrTiO3,

0.02Ti3C2/SrTiO3,

0.04Ti3C2/SrTiO3,

0.2Ti3C2/SrTiO3 and Ti3C2, respectively. 2.2. Physicochemical characterization The crystallinity and structure of the synthesized samples were acquired from a wide angle X-ray diffraction system (Bruker D8, Cu Kα radiation, λ = 1.5406 Å). SEM-EDS of all samples were performed on a field-emission scanning electron microscope (Hitachi S-4800) with attached energy-dispersive X-ray spectroscopy (Horiba 7593-H). TEM, the high-resolution TEM and SAED images were done on a transmission electron microscope (JEOL JEM-2100). X-ray photoelectron spectroscopy (XPS) of samples were 5



recorded using a Thermo Scientific ESCALAB250Xi spectrometer equipped with an Al Kα X-ray source. The surface textural characterization was determined by BET and BJH method of the N2 adsorption-desorption isotherms using a Micromeritics ASAP 2020 apparatus. UV-vis absorption spectra of the representative samples were obtained using a Hitachi U-3900 with an integrated sphere using barium sulfate as the reflectance standard. Photoluminescence spectrometry spectra were recorded by a Hitachi F-4600 fluorospectrophotometer under the excitation of 220 nm at room temperature. Raman spectra were collected using a HORIBA scientific spectrometer with a 473 nm laser (Ciel model, Laser quantum Ltd.).

Figure 1. Synthesis diagram of Ti3C2/SrTiO3.

2.3. Photocatalytic reduction and removal of UO22+ Photocatalytic reduction of UO22+ was carried out in a 100 mL jacketed quartz beaker (cooled by circulation water). Typically, the as-prepared photocatalyst (20 mg) was added into 60 ml of 50 ppm UO22+ aqueous solution by constant stirring. The pH value of the mixture was adjusted with small amounts of NaOH and HCl solutions (∼ 0.1 mol L−1). Then, the suspension was magnetically stirred in dark for 8 h to achieve the adsorption-desorption equilibrium. Before a photocatalytic reaction, reactant solution was purged by N2 for 0.5 h, and any dissolved air was removed, after that the reaction system remained under anaerobic conditions. The purpose of the above experiment is to avoid 6


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the effect of dissolved oxygen on the photocatalytic reduction reaction as much as possible. This mixture was illuminated for 3h using a 300 W Xe lamp (wavelength, 3202500 nm) that offered UV-visible light source as a sun light simulated. After illumination for a given time, aliquots (0.4 mL) of the mixture were pipetted and filtered through 0.45 um Nylon syringe filters. The residual concentrations of uranium were analyzed by a Horiba JY2000-2 inductively coupled plasma optical emission spectrograph (ICP-OES). Residual ratio (%) of UO22+ in solution is calculated with the expression Ct/C0 × 100 %, where C0 and Ct are the concentrations (ppm) of uranium in solution phase at initial (after the dark adsorption) and contact time t (min), respectively. In addition, Ti3C2/SrTiO3 (20 mg) was dispersed into 60 ml of 50 ppm AgNO3 aqueous (including the contrast group of pure AgNO3 aqueous) under the same photocatalytic condition for confirming the photocatalytic active areas. Similarly, after 180 min photo-deposition, the above suspension was filtered, washed with ethanol, and dried in vacuum at 50 ºC for 8 h. The as-obtained materials were used for further characterizations. 2.4. Electrochemical and photoelectrochemical measurements These experiments were carried out in a standard three electrode system with the synthesized samples as the working electrodes, a saturated calomel electrode (SCE) as the reference electrode, and a Pt foil as the counter electrode. The SCE is converted to RHE using the expression: ERHE = ESCE + 0.0591 × pH + 0.2415 V. 0.1 mol L−1 Na2SO4 aqueous solution (pH ≈ 6) was used as the electrolyte in the photoelectrochemical measurements. At first, the cyclic-voltammetry (CV) measurements were employed to the electrode activation of working electrodes within the potential limits of -1 to 1V (vs SCE) including a step size of 0.05 V. The electrochemical impedance spectroscopy (EIS) were measured over a range from 0.01 to 5×105 Hz with an alternating current (AC) amplitude of 0.01 V. Both transient photocurrent responses and open-circuit photovoltage decay measurements were observed in the dark or under illumination. A photocatalytic system containing a 300W Xenon lamp (wavelength, ranging from 320 to 2500 nm) was used as a sun light simulated. Mott-Schottky plots were employed to determine the flat band potential using an alternating current frequency of 2×105 Hz within the potential limits of -1.2 to 0.5 V (vs. SCE). The working electrodes were prepared as follows: 6 mg sample and 100 uL of ethanol were dispersed by sonication. Then the mixture was 7



homogeneously coated onto a 1.2 cm×1.2 cm FTO glass electrode (7 Ω). The obtained electrodes were dried in an oven heated at 60 oC for 0.5 h. 2.5. Theoretical calculations Density functional theory (DFT) calculations were performed by using the Vienna ab initio simulation package (VASP version 5.4.1).32-33 Geometric optimization was performed by generalized gradient approximation (GGA) with the Perdew-BurkeErnzerhof (PBE),34 including the relaxation of atomic coordinates and lattice constants, and total energy calculation. These calculations used an energy cutoff of 400 eV, an energy convergence tolerance of 1.0×10-5 eV, and a 9×9×1 Monkhorst-Pack k-point grid sampling. Hey-Scuseria-Ernzerhof (HSE06) calculations35-36 hybrid functional were performed to get the band gaps and band alignments and density of states (DOS). And the adsorption behavior of UO22+ on hydroxylated Ti3C2 nanosheet has been also systematically investigated by DFT calculations in our previous publication.37 3. RESULTS AND DISCUSSION 3.1. Composition and characteristics Al layers of Ti3AlC2 were selectively exfoliated by a ∼15 wt% hydrofluoric acid (HF) solution to obtain the multi-layer Ti3C2 nanosheets. As a result, F-groups are the predominant surface termination groups. Then, during the subsequent rinsing process with de-ionized water or hydrothermal process in 2 mol L−1 NaOH aqueous solution, a large number of functional groups (-OH or -O) could spontaneously replace F-groups on the surface of Ti3C2, producing excellent hydrophilicity and possible semiconductor properties.19 Figure 2a shows X-ray diffraction (XRD) patterns of Ti3C2 nanosheets, for comparison, the XRD fashion of raw Ti3AlC2 is also presented. After etching Ti3AlC2 with HF treatment, the strongest diffraction peak of Ti3AlC2 at 39.17° disappears. The transformation from Ti3AlC2 to Ti3C2 shows the apparent shift of the (002) reflection to lower angle side at the peak of 9.70° and (004) 19.32°, suggesting the successful exfoliation of the Al layers and the appearance of multilayered Ti3C2 according to the reference.29 A high-magnification SEM image (Figure 2b) revealed an accordion-like architecture and the thickness of the exfoliated MXene sheets ranging from 25 to 45 nm. 8


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In addition, Ti3C2/SrTiO3 was synthesized by partly superficial oxidation from multilayer nanostructure Ti3C2 via a simple hydrothermal crystallization, and their crystallinity and structures were confirmed by XRD analysis. These XRD patterns (Figure S1a) confirm that each Ti3C2/SrTiO3 composite consists of a cubic perovskite phase SrTiO3 (JCPDS No. 35-0734, space group Pm3m). Simultaneously, it can be also easily observed that the partial diffraction peaks of Ti3C2/SrTiO3 are consistent with Ti3C2 sheets and negligible TiO2 phase. By contrast, no additional Sr2+ ions and TiO2 in the same NaOH aqueous solution, the XRD peaks of the new species Ti3C2/NaxTiyOz (labeled as alkalization-Ti3C2) from simultaneous oxidation and alkalization processes (Figure S1b) are also strong and sharp. Simultaneously, no signals associated with TiO2 phase are observed,20 suggesting the dissolution of TiO2 from Ti3C2 superficial oxidation.28 Nevertheless, the phase of TiO2 nanocrystals formed by the surface oxidation from Ti3C2 is not enough to form a large amount of SrTiO3 crystals due to the excellent stability of Ti3C2 in the step of superficial oxidation (Figure S1b). Thus, the primary part of extra additional TiO2 (amorphous phase) is aimed at making these SrTiO3 nanocrystals bigger in hydrothermal crystallization. Of course, tiny amounts of unreacted extra TiO2 will still not form the heterostructure with Ti3C2. In addition, the corresponding results from Ti3C2 to Ti3C2/SrTiO3 was confirmed by the deconvoluted TiO2 peak of Ti3C2 in the X-ray photoelectron spectroscopy (XPS) spectrum (Figure 3a-b). As for the high-resolution O 1s XPS spectra, the numerous O species are present in Ti3C2 and Ti3C2/SrTiO3 (Figure 3a), while the F-terminations is negligible for Ti3C2/SrTiO3 (Figure 3b), indicating the superficial -F groups of Ti3C2 replaced by -O or -OH groups after hydrothermal crystallization. The high-resolution XPS spectrum (Figure 3c) of Ti 2p of Ti3C2 is fitted with four doublets, and can be assigned as TiC, TiOC, TiO2 and TixOy38 which is reasonable for Ti3C2/SrTiO3 composite. Then, the morphology of the Ti3C2/SrTiO3 samples were thoroughly investigated. The representative SEM and TEM images of Ti3C2/SrTiO3 were selected, as summarized in Figure 3d-g. Figure 3d shows that platelike Ti3C2 nanosheets with the size of 3-6 um are covered with the rich SrTiO3 nanospheres, which is quite distinguishable for the topography of pristine SrTiO3 as shown in Figure S2 and S3a-b. The nanospheres or columnar grains with interconnected channels of Ti3C2/SrTiO3 are clearly shown in the enlarged SEM image (Figure 3e) of 9



Figure 3d, and the TEM image of Figure S3c-d. Epitaxial growth of SrTiO3 nanocrystals proceeds on the substrates of Ti3C2, and edge dislocations and vertical grain boundaries leads to a few SrTiO3 nanocrystals of mosaic structure or columnar grains with releasing the high tensile strain.13 Furthermore, TEM analysis of Ti3C2/SrTiO3 in Figure 3f shows a uniform external surfaces of the Ti3C2 covered with small nanospheres (100-200 nm) and the larger nanocubes (200 - 400 nm) of SrTiO3. A detailed observation in Figure 3f, the center and edge (see blue rectangles) were selected for the selective area electron diffraction (SAED) analysis, respectively. Their corresponding phases of SAED are consistent with Michel W. Barsoum et al’s24 and Takashi tachikawa’s reports.39 The representative crystal phases confirm that the Ti3C2/SrTiO3 composites are composed of crystallographically nanocrystals, including the perovskite (SrTiO3, corresponding SAED in Figure S3b and MXenes (Ti3C2, corresponding SAED in Figure S4. The lattice spacing of the nanospheres in a heterointerface is about 0.276 nm, which may be assigned to the (110)-oriented SrTiO3 (Figure 3g). The above results support that the crystal structure of SrTiO3 was not affected in the process of the epitaxial nanocrystal nucleation and subsequent crystal growth. Thus, Ti3C2/SrTiO3 heterostructure could not only obtain of complete crystals of SrTiO3 for photocatalytic reaction, but also build the strong combination based on of Ti-based interface Ti3C2 for photo-generated e-/h+ transfer.

Figure 2. (a) X-ray diffraction (XRD) patterns of Ti3C2 nanosheet and Ti3AlC2 (MXA phase). (b) SEM image of Ti3C2 nanosheet.

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Figure 3. (a-c) The high-resolution XPS spectra of Ti 2p, O 1s and F 1s for Ti3C2/SrTiO3. (d) SEM micrograph of Ti3C2/SrTiO3 and (e) its magnification SEM micrograph. (f) TEM micrograph of Ti3C2/SrTiO3 along with SAED (blue-squared), and (g) HRTEM of the blue-squared section in the image of (f).

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Figure 4. (a) Photocatalytic reduction removal UO22+ of SrTiO3 with different loading amount of Ti3C2. Excitingly, in the presence of 50 ppm UO22+ (pH=4), the performances of all the Ti3C2/SrTiO3 samples are improved in aqueous solution under simulated sunlight irradiation. (b) Removal rate (%) generated by the photocatalysts along with the different sonication times. (c) Removal rate (%) generated by comparing with the amorphous TiO2, alkalization-Ti3C2 and physical compound Ti3C2/SrTiO3. (d) Curve fitted uranium 4f XPS spectrum recorded from the surface of 0.02Ti3C2/SrTiO3 under simulated sunlight illumination. The unlabeled peaks represent satellite peaks of the U4f7/2 and 4f5/2 peaks.

3.2. Photocatalytic activity The photocatalytic activity of a series of SrTiO3 coupled with different amounts of Ti3C2 was systematically investigated. Excitingly, in the presence of 50 ppm UO22+ (pH = 4), the photocatalytic activities of all Ti3C2/SrTiO3 composites are improved when exposed to a sun light simulated. And some static adsorption data of the samples before photocatalytic reaction were shown in Figure S5a. As shown in Figure4a, the uranium removal rate (normalized concentration) by pristine SrTiO3 was almost negligible after 180 min under simulated sunlight illumination. In contrast, a small amount (0.01 wt.%) of Ti3C2 incorporated with SrTiO3 (denoted as 0.01Ti3C2/SrTiO3) significantly enhances the 12


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uranium removal rate to 20%. With the continuous increase of Ti3C2 dosage, the photocatalytic removal rate of UO22+ ions in the Ti3C2/SrTiO3 compounds is gradually enhanced. As for 0.02Ti3C2/SrTiO3, the photocatalytic UO22+ removal rate achieved at the maximum of 77 %, which is 38.5 times higher than that of pristine SrTiO3. This is reasonable because the quick migration of the photo-induced holes to OH– (Eo (·OH/OH–) = 1.58 V vs. NHE) or H2O (Eo(·OH/H2O) = 1.92 V vs. NHE)40 adsorbed on the surface of photocatalyst generate ·OH.41 However, further increase of Ti3C2 dosage incorporated with SrTiO3 leads to the drastic decrease of photocatalytic reduction removal performance for UO22+. This is mainly due to the excessive amount of the black Ti3C2 increasing the opacity, which hinders the light absorption of SrTiO3. The above similar results were reported by Jingrun Ran et al18 and Quanjun Xiang et al.42 In spite of this, a little more (0.04 wt.% Ti3C2) of 0.04Ti3C2/SrTiO3 still retains a photocatalytic UO22+ removal rate of about 38 %. However, pure Ti3C2 shows almost no activity for photocatalytic uranium removal, which is consistent with Jingrun Ran et al’s report.18 Indeed, the remarkable increase in photocatalytic activity of Ti3C2/SrTiO3 is obviously attributed to the Mexene as a co-catalyst. We also examined the effect of Ti3C2 thickness on the photocatalytic activity of the 0.02Ti3C2/SrTiO3 sample by intentatively grinding and sonicating the Ti3C2 precursor for different time. It was found that the uranium removal continuously decreases with the decrease of Ti3C2 thickness, thereby illustrating the essentiality of multi-layered structure of Ti3C2 (Figure 4b). It is well known that thin layered Ti3C2 is more chemically active and probably had been completely transformed to SrTiO3. As a result, Mexene has lost its unique essential characteristics for photogenerated e-/h+ transfer (Figure S5b). Furthermore, the photocatalytic activity of 0.02Ti3C2/SrTiO3 was further evaluated by comparing with the amorphous TiO2 (raw material), alkalization-Ti3C2 (intermediate product of multi-layer Ti3C2 in the hydrothermal process), and physical compound Ti3C2/SrTiO3 (Figure 4c), and it can be argued that the photocatalytic reduction removal of 0.02Ti3C2/SrTiO3 is more excellent than the amorphous TiO2, alkalization-Ti3C2 and physical compound Ti3C2/SrTiO3. It indicates that an important application in the field of photocatalytic reduction removal for UO22+ is displayed in the photocatalytic reaction of 0.02Ti3C2/SrTiO3. To identify distribution of the oxidation state of the uranium element, the high-resolution spectra of 13



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U 4f was obtained by XPS analysis (Figure 4d). According to the corresponding results from Wersin et al’s report,43 the U(VI) (at ∼382.0 eV) and U(IV) (at ∼380.8 eV) components are derived from a spectral fit of the U 4f spectra. In the other hand, the pH influence of the UO22+ reduction (Figure 5a) demonstrates the acidity dependency for the photocatalytic efficiency of 0.02Ti3C2/STiO3. Along with increasing pH from 2.0 to 4.0, the photocatalytic removal rate of U(VI) on 0.02Ti3C2/SrTiO3 significantly increased. Because of the approximate point of zero charge Ti3C2 (~ 2.5),30,

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the electrostatic

function between Ti3C2 and positive charged U(VI) species (i.e., UO22+, UO2OH+, (UO2)2(OH)2+ and (UO2)3(OH)5+) could enhance the photocatalytic reduction removal of U(VI) in such pH value. In our previous work, we also illustrate an affinity of Ti3C2 for U(VI) species through a hydrated intercalation strategy.30 However, the lower removal rate of U(VI) on 0.02Ti3C2/SrTiO3 was obtained at pH > 6.5 owing to the hydrolytic precipitate. Combined with the theoretical distribution fraction (%) of U(VI) species in the aqueous solution (conditions: 50 ppm, T = 25 ± 1 oC) (Figure 5b), the hydrolytic precipitate species (UO2(OH)2·H2O(s)) were confirmed at pH > 4.5. The recyclable utilization of the 0.02Ti3C2/SrTiO3 was investigated by photocatalytic reduction removal of U(VI) for a three-time cycles. After every photocatalytic reaction, this reacted 0.02Ti3C2/SrTiO3 were filtrated and washed by stirring in a 0.1 mol L-1 Na2CO3 solution2 for 8 h in air (the deposited uranium on the surface of 0.02Ti3C2/SrTiO3 was desorbed) and subsequently rinsed with de-ionized water. Finally, after drying at 60 °C in a vacuum oven for 12 h, and then the residual 0.02Ti3C2/SrTiO3 was conducted to next reductively remove U(VI). In Figure 5c, the U(VI) removal rate of the three cycles is 76.1%, 75.6%, and 74.8%, respectively, indicating no significant decrease in the photocatalytic activity of 0.02Ti3C2/SrTiO3. In addition, XRD analysis of the residual 0.02Ti3C2/SrTiO3 indicates that it is structurally intact after photocatalytic reaction. (Figure S5c). These experimental results prove that the heterostructure 0.02Ti3C2/SrTiO3 exhibits excellent stability for the photocatalytic reduction removal of U(VI).

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Figure 5. (a) The pH influence for photocatalytic removal U(VI) of 0.02Ti3C2/SrTiO3. (b) The distribution of U(VI) species, C0 = 50 ppm, T = 25 ± 1 oC. (c) Recyclable stability of 0.02Ti3C2/SrTiO3.

During the hydrothermal crystallization, the surface area (SBET) of pristine SrTiO3 gradually decreases from 107.17 to 33.71 m2 g-1 of 0.02Ti3C2/SrTiO3 compound (Figure S6a), demonstrating that the particles overlapping and fusion each other on the surface of Ti3C2.45 The results show that the enhancement of photocatalytic activity of 0.02Ti3C2/SrTiO3 is not due to the increase of SBET after combination. In addition, the remarkable photocatalytic activity of 0.02Ti3C2/SrTiO3 were systematically demonstrated by Photoluminescence spectroscopy, UV-vis diffuse reflectance spectra, Raman spectroscopy and electrochemical methods (Figure S6b and Figure S7). As shown in Figure 6a, comparing with the pristine SrTiO3, the light absorption of 0.02Ti3C2/SrTiO3 in region of 300-700 nm is slightly increased, which can be related to the light adsorption of the black MXenes materials.46 Compared with the pristine SrTiO3, there is no 15



significant change in the band gap of 0.02Ti3C2/SrTiO3 (Figure S7a), indicating the Ti, C, O elements of Ti3C2 are not affected into the formation of SrTiO3 crystal,18 which is consistent with XRD analysis. The lower PL intensity under 220 nm laser irradiation (Figure 6b), a considerably smaller interfacial charge-transfer resistance and an enhancement of transient photocurrent density (Figure 6c) directly suggest an improved charge separation efficiency in the 0.02Ti3C2/SrTiO3. It is believed that Ti3C2 will effectively transfer the photo-generated electrons and thereby suppress the charge recombination in the photocatalyst composite.

Figure 6 (a) The light absorption edge of SrTiO3, 0.02Ti3C2/SrTiO3 and 0.2Ti3C2/SrTiO3. (b) PL spectrum of SrTiO3 and 0.02Ti3C2/SrTiO3. (c) EIS Nyquist plots of STO and 0.02Ti3C2/SrTiO3 on FTO-coated cover glasses in 0.1 mol L-1 Na2SO4 solution under simulated sunlight without an applied potential. The imbedded graph exhibits the transient photocurrent density of SrTiO3 and 0.02Ti3C2/SrTiO3 in 0.1 mol L-1 Na2SO4 solution under UV-visible light without an applied potential.

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To clarify the lifetime of photo-generated e-/h+, the charge-transfer kinetics of the 0.02Ti3C2/SrTiO3 composite could be further estimated by the decay of photovoltage at the end of light irradiation.47 Figure 7a shows typical responses of the open-circuit voltage (Voc) of pure SrTiO3 and 0.02Ti3C2/SrTiO3 to light irradiation followed by end of light irradiation. In our work, this Voc (vs SCE) values of the samples show the deviations between themselves and SCE electrodes in Fermi level. Before the illumination, the corresponding open-circuit voltage is described by a redox equilibration. At the beginning of illumination, the photocatalyst is excited and the accumulation of photogenerated e- will cause a shift of its Fermi level to a more negative potential, leading to an increase in Voc. The Voc at time t after stopping the illumination decays and then the data can be analyzed using the Bisquert method47 to yield the average lifetime of the photogenerated e- (τn). The corresponding equation was illustrated below:

k T  dV   n   B  oc  q  dt 

1

where τn (s) is the average e- lifetime, q (1.6×10-19 C) is the charge of an electron, kB (1.38×10-23 J/K) is the Boltzmann constant, and T (K) is the temperature of the three electrode system. The relationship between the e- lifetime of pholocatalyst and the Voc is presented in the inset figure of Figure 7a. The e- lifetime of 0.02Ti3C2/SrTiO3 is in the range from 101 to 103 s. After the termination of light irradiation, the synthetic pristine SrTiO3 by the same hydrothermal condition shows the relatively transient lifetime distribution (τn ~100 s). This yield of 0.02Ti3C2/SrTiO3 is 100 times higher than that of pristine SrTiO3, thereby strongly suggesting a reduced recombination rate for the composites. It is generally believed that the above average e- lifetime (τn) represents a significant symbol of the expeditious recombination from photo-generated e- and h+.47-49 The photo-generated carriers transport along with the heterostructure from trapping /detrapping mechanism,45 prolonging the τn of carriers and making efficient electron trapped by Ti3C2. This may be the main reason why 0.02Ti3C2/SrTiO3 exhibits a higher photocatalytic activity compared to pure SrTiO3.

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To deep reveal the excellent performance of the heterogeneous structure of Ti-based interface, UO22+ reduction was investigated for the other titanate photocatalysts50-51 incorporated with Ti3C2. The successful preparation of pure CaTiO3, 0.02Ti3C2/CaTiO3, pure BaTiO3, and 0.02Ti3C2/BaTiO3 were demonstrated by the XRD results (Figure S8). It was noted that the XTiO3 (X = Ca, Ba) after the incorporation of 0.02 wt% Ti3C2 exhibits a superior photocatalytic activity to reduce U(VI) (Figure 7b). The UO22+ removal rate (%) of compounds obtained under simulated sunlight irradiation were 22 % and 34 %, respectively, while that of their pure phases were almost negligible. Thus, the establishing of Ti-based interface will motivate one to use the excellent heterogeneous structure to enhance photocatalytic performance. However, the relationship between the heterogeneous structure and photo-generated carriers dynamics remains unresolved. Curiously,

SrTiO3

exhibited

much

lower

photocatalytic

removal

rate

than

0.02Ti3C2/SrTiO3 despite the higher specific area on itself (Figure S6a). We applied photo-depositions of metals on the surface of 0.02Ti3C2/SrTiO3 (Figure 7c-d) to reveal its active regions in the photocatalytic process and illustrate the key function of heterogeneous structure.52-53 Figure 7c shows SEM images of 0.02Ti3C2/SrTiO3 under simulated sunlight irradiation using AgNO3 as precursors. XRD spectra (Figure S9) indicates that the deposited Ag particles on the surface of 0.02Ti3C2/SrTiO3 are existed in metallic form. That is, the photo-generated e- of 0.02Ti3C2/SrTiO3 are readily available for the reduction reaction on such Ti-based interface.

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Figure 7 (a) Transient open-circuit voltage decay (OCVD) from STO and 0.02Ti3C2/SrTiO3 under simulated sunlight irradiation without an applied current. The inset shows average electron lifetimes (τn) of STO and 0.02Ti3C2/SrTiO3. (b) Photocatalytic reduction removal UO22+ of the excellent materials-based system. (c) SEM image of Ag/0.02Ti3C2/SrTiO3 under simulated sunlight illumination. (d) The EDS spectrum at 1 and 2 points in c.

3.3 Theoretical calculations The terminated groups tends to stay on the topmost sites of Ti atoms54 and therefore are intentatively distributed on the surface of Ti3C2 in Figure 8a. To find out whether specific groups-terminated Ti3C2 is an excellent co-catalyst to improve the photocatalytic performance of SrTiO3, thus density functional theory (DFT) calculations by the Vienna Ab-initio Simulation Package (VASP)32-33 are performed. The photo-generated e- and h+ from a photocatalyst can be not only effectively separated,55 but also be rapidly transferred to the surface areas to effectively catalyze the UO22+ reduction. The calculated band structure of pure Ti3C2 exhibits a typical metallic nature with a successional density of states at the Fermi level (EF) (Figure 8b-c). In contrast, the band structure of Oterminated Ti3C2 shows decreased number of states at the EF, with a clear separation 19



between valence and conduction bands of 0.5 eV and -1.0 eV (Figure 8b and 8d), indicating a similar semiconducting character (lower conductivities).19 Remarkably, a few continuous valence and conduction bands of O-terminated Ti3C2 at EF indicate that their capability for charge transfer are still excellent. Thus, O-terminated Ti3C2 retains its outstanding electrical conductivity and exceptional capability to transport photogenerated electrons for further photocatalytic reduction. In addition, the EF value of Oterminated Ti3C2 and pure Ti3C2 have been calculated to be 1.88 V and -0.05 V (vs SHE), respectively.18 For a large number of noble metal based co-catalysts, platinum (Pt) is the most attractive co-catalyst in light of its largest work function (W ， 5.65 eV) and minimum overpotential for H2 production.56 By comparison, O-terminated Ti3C2 could be used as an extremely perfect co-catalyst, including its promising electron transportation ability, excellent electrical conductivity and larger work function (O termination always increases W, about 6.2 eV)57 than Pt metal. That is to say, O-terminated Ti3C2 shows an optimum parameter of W and EF, indicating its sufficient ability to admit photo-generated e- from a pristine photocatalyst into itself for the photocatalytic reduction removal of UO22+. In general, the overall UO22+ reduction process can be divided into three stages, in which uranium with U(VI)/(IV) oxidation states is involved, that is, the initial simple adsorption of UO22+, an intermediate state UO22+/U(IV) or UO22+/UO2 (0.327 and 0.411 V vs SHE, respectively3,

58),

and a final product U(IV) or UO2. In our previous

publication,37 the adsorption behavior of UO22+ on hydroxylated Ti3C2 nanosheet has been clearly investigated by DFT calculations. The results indicated that UO22+ with 5fold coordination prefers to binding with the deprotonated O adsorption site (Oterminated) rather than the protonated O (H) (OH-terminated) during the adsorption process. From the lamellar Ti3C2 with an accordion-like architecture, the main anatase phase TiO2 nanocrystals28 were formed by the surface oxidation from Ti3C2. Despite of their different crystallographic structure at the interface of Ti3C2/TiO2, the Ti3C2 and TiO2 combine seamlessly by an in-plane lattice mismatch between (103) plane from Ti3C2 and (11-1) from of TiO2.29 When the Sr2+ ions of strontium hydroxide suspension are reacted with the above anatase phase in 2 mol L-1 NaOH aqueous solution, thus the epitaxial 20


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nucleation of SrTiO3 nanocrystals occurs at such Ti-based heterointerface by the dissolution-precipitation processes.59 With the addition of extra TiO2 to the same alkaline solution, the SrTiO3 grains on the surface of Ti3C2. As such, the lamellar Ti3C2 are coated by the external SrTiO3 nanocrystals at the interface of Ti3C2/SrTiO3. The orientation relationship between (100) cubic SrTiO3 (0.39 nm) bulk crystal and (001) tetragonal TiO2 (0.3785 nm) crystal is in a parallel manner, thus forming a similar orientation of the TiO6 octahedra and a very small mismatch at either side of the interface. Partial O atoms in the (100) plane of SrTiO3 could be bridged to Ti atoms in the (001) plane of TiO2 via Ti3C2 oxidation, while Ti atoms of SrTiO3 could be also be bridged to the O atoms of TiO2.39, 60 On the basis of the aforementioned results, it is believed that the chemical bonds formed by the structural distortion of TiO6 octahedra at the misfit dislocation of TiO2/SrTiO3 are important for determining the final structure of perovskite SrTiO3.39, 60 From the unique structural characteristics of Ti3C2/SrTiO3 hybrid system, the schematic band alignments and photocatalytic mechanism for the UO22+ reduction are proposed in Figure 8e. Under simulated sunlight illumination, the semiconductor SrTiO3 is induced to produce photo-generated e- and h+. Because the actual EF of pristine SrTiO3 (a little below its conduction band of -0.65 V vs SHE12) is much lower than that of Oterminated Ti3C2 (1.88 V vs SHE18). Thus, the difference of EF in the heterointerface will result in the photo-generated e- transport from SrTiO3 to Ti3C2, so that the Fermi level of Ti3C2 is higher than the reduction potential of U(VI)/U(IV) and the equilibrium of EF is established in Ti3C2/SrTiO3 under simulated sunlight irradiation. The Schottky barrier formed in Ti3C2/SrTiO3 heterointerface represents the ability to suppress the photogenerated e- returning to SrTiO3, thereby achieving spatial separation of the e-/h+ pairs. The above similar mechanism was reported by Jakob et al.61

21



Figure 8. a) Top view and side view of the O-terminated Ti3C2 4×4×1 supercell model. The C, O, Ti and Sr atoms are marked by grey, yellow, green and brown spheres, respectively. b) The calculated band structures of O-terminated and no termination Ti3C2 show a change from metal to semiconductor. The total density of states (TDOS) and partial density of states (PDOS) for pure Ti3C2 (c) and O-terminated Ti3C2 (d). e) The photocatalytic mechanism and charge transfer processes of the Ti3C2/SrTiO3 hybrid system under simulated sunlight irradiation. The photo-generated electrons and holes are marked by red (-) and blue (+) spheres, respectively.

4. CONCLUSIONS As noted above, a novel Ti3C2/SrTiO3 heterostructure is successfully fabricated by partly superficial oxidation from precursor multilayered Ti3C2 as an excellent co-catalyst using a simple hydrothermal crystallology. By combining the first principle calculations and means of photoelectrochemical measurements and photo-deposition at active areas, we could suggest that the heterogeneous structure formed in Ti3C2/SrTiO3 has great advantages in prolonging the carriers lifetimes due to its excellent ability of charge separation and electron transfer, which significantly improves photocatalytic performance. There is no doubt that a well-defined heterostructure could transfer photo-generated carriers more pervasive than an unordered structure such as scattered nanoparticles. Thus, 22


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it is very meaningful for achieving solar energy utilization. The synthetic strategy of titanate perovskite-type crystals and the in-depth study of the mechanism of heterogeneous photocatalysis will bring new insights into the application of photocatalysis and optoelectronics. Furthermore, this study could also pave the way for developing doped perovskite oxide crystals based on the MXene with sun-light responsivity for improving solar energy utilization.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD patterns of SrTiO3 with different loading amounts of Ti3C2 and Ti3C2/NaxTiyOz (Figure S1), SEM-EDS of pure SrTiO3 (Figure S2), TEM and SAED patterns of SrTiO3 and Ti3C2 (Figure S3 and S4), the static adsorption data of the part samples (Figure S5), XRD patterns of multilayer and thin layer Ti3C2/SrTiO3 compounds obtained by sonication (Figure S5), N2 sorptiondesorption isotherm, Raman spectroscopy, Kubelka-Munk vs photon energy plots and MottSchottky plots of SrTiO3 and Ti3C2/SrTiO3 (Figure S6 and S7), XRD patterns of CaTiO3, 0.02Ti3C2/CaTiO3, BaTiO3 and 0.02Ti3C2/BaTiO3 (Figure S8), XRD patterns of Ag particles deposited on 0.02Ti3C2/SrTiO3 (Figure S9) (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Tel: +86-010-88233968, Fax: +86-010-88235294

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grants no. 11675192, 21577144 and 91326202). The Science Challenge Project (JCKY2016212A504) is also acknowledged. The authors are thankful to Dr. D.D. Shao at Institute of Plasma Physics, Chinese Academy of Sciences, for his help with XPS measurements. REFERENCES (1) Jiang, X.-H.; Xing, Q.-J.; Luo, X.-B.; Li, F.; Zou, J.-P.; Liu, S.-S.; Li, X.; Wang, X.-K. Simultaneous Photoreduction of Uranium(Vi) and Photooxidation of Arsenic(Iii) in Aqueous Solution over GC3n4/Tio2 Heterostructured Catalysts under Simulated Sunlight Irradiation. Appl. Catal. B: Environ. 2018, 228, 29-38. (2) Li, Z. J.; Huang, Z. W.; Guo, W. L.; Wang, L.; Zheng, L. R.; Chai, Z. F.; Shi, W. Q. Enhanced Photocatalytic Removal of Uranium(Vi) from Aqueous Solution by Magnetic Tio2/Fe3o4 and Its Graphene Composite. Environ. Sci. Technol. 2017, 51, 5666-5674. (3) Lu, C. H.; Zhang, P.; Jiang, S. J.; Wu, X.; Song, S. Q.; Zhu, M. S.; Lou, Z. Z.; Li, Z.; Liu, F.; Liu, Y. H.; Wang, Y.; Le, Z. G. Photocatalytic Reduction Elimination of Uo22+ Pollutant under Visible Light with Metal-Free Sulfur Doped G-C3n4 Photocatalyst. Appl. Catal. B: Environ. 2017, 200, 378-385. (4) Salomone, V. N.; Meichtry, J. M.; Zampieri, G.; Litter, M. I. New Insights in the Heterogeneous Photocatalytic Removal of U(Vi) in Aqueous Solution in the Presence of 2Propanol. Chem. Eng. J. 2015, 261, 27-35. (5) Velegraki, G.; Miao, J. W.; Drivas, C.; Liu, B.; Kennou, S.; Armatas, G. S. Fabrication of 3d Mesoporous Networks of Assembled Coo Nanoparticles for Efficient Photocatalytic Reduction of Aqueous Cr(Vi). Appl. Catal. B: Environ. 2018, 221, 635-644. (6) Wang, X. L.; Pehkonen, S. O.; Ray, A. K. Photocatalytic Reduction of Hg(Ii) on Two Commercial Tio2 Catalysts. Electrochim. Acta 2004, 49, 1435-1444. (7) Canterino, M.; Di Somma, I.; Marotta, R.; Andreozzi, R. Kinetic Investigation of Cu(Ii) Ions Photoreduction in Presence of Titanium Dioxide and Formic Acid. Water Res. 2008, 42, 44984506. (8) Evans, C. J.; Nicholson, G. P.; Faith, D. A.; Kan, M. J. Photochemical Removal of Uranium from a Phosphate Waste Solution. Green Chem. 2004, 6, 196-197. (9) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding Tio2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114, 99199986. (10) Wu, F.; Yu, Y.; Yang, H.; German, L. N.; Li, Z.; Chen, J.; Yang, W.; Huang, L.; Shi, W.; Wang, L.; Wang, X. Simultaneous Enhancement of Charge Separation and Hole Transportation in a Tio2 Srtio3 Core-Shell Nanowire Photoelectrochemical System. Adv. Mater. 2017, 29, 1701432. (11) Wrighton, M. S.; Ellis, A. B.; Wolczanski, P. T.; Morse, D. L.; Abrahamson, H. B.; Ginley, D. S. Strontium-Titanate Photoelectrodes - Efficient Photoassisted Electrolysis of Water at Zero Applied Potential. J. Am. Chem. Soc. 1976, 98, 2774-2779. 24


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Nanolayered Ti3C2 and SrTiO3 Composites for Photocatalytic

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