Interaction mechanism between different facet TiO2 and U(VI): Experimental and density-functional theory investigation

Accepted Manuscript Interaction mechanism between different facet TiO2 and U(VI): experimental and density-functional theory investigation Ke Chen, Changlun Chen, Xuemei Ren, Ahmed Alsaedi, Tasawar Hayat PII: DOI: Reference:

S1385-8947(18)32327-1 https://doi.org/10.1016/j.cej.2018.11.092 CEJ 20394

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

31 August 2018 29 October 2018 11 November 2018

Please cite this article as: K. Chen, C. Chen, X. Ren, A. Alsaedi, T. Hayat, Interaction mechanism between different facet TiO2 and U(VI): experimental and density-functional theory investigation, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.11.092

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Interaction mechanism between different facet TiO2 and U(VI): experimental and density-functional theory investigation Ke Chen,a,b Changlun Chen,*,a,c,d Xuemei Ren,a Ahmed Alsaedi,d Tasawar Hayat,d a

CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute

of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, P.R. China b

School of Chemistry and Materials Science, Hefei National Laboratory for Physical

Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, P.R. China cCollaborative

Innovation Center of Radiation Medicine of Jiangsu Higher Education

Institutions, Soochow University, Suzhou 215123, P.R. China dNAAM

Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia

*Corresponding author. Tel: 86-551-65592788, Fax: 86-551-65591310 E–mail address: [email protected] (C.L. Chen)

1

ABSTRACT TiO2 with different exposed facets often shows different adsorption and photoreduction performances in environmental pollutant treatment, but the latent mechanisms are not fully understood. In this work, {001}, {100} and {101} facetdominated anatase TiO2 was prepared and studied for its effectiveness in U(VI) removal, and the molecular level surface chemistry was studied by density functional theory (DFT) calculations. The experimental results indicated that {001} facet TiO2 exhibited the best adsorption capacity and photoreduction ability compared to {100} and {101} facet TiO2. DFT calculation results showed that the adsorption of U(VI) on the three surfaces leaded to the formation of inner-sphere complexes, in which the monodentate complex was most favored for {001} facet TiO2, whereas bidentate complexes were most favored for {100} and {101} facet TiO2. The results from experimental techniques, such as steady fluorescence emission spectra, time-resolved photoluminescence spectra, photocurrent density, electro-chemical impedance spectroscopy, Mott-Schottky plots suggested that the higher photocatalytic activity could be ascribed to higher electron-hole separation efficiency for three facets TiO2. Furthermore, the dissolved oxygen played different roles in U(VI) photoreduction on the three types of TiO2 in the experiments due to different surface structure arrangement. This work provides knowledge of toxic metal ion removal by facetdependent metal oxides and a basis for the design and synthesis of other reactive facet-dependent materials for environmental management.

Keywords: Facet TiO2; U(VI) removal; Density functional theory; Photoreduction ability

2

1. Introduction Toxic heavy metal ions are widely detected in soils, sediments, surface water, and groundwater and are therefore a severe health threat to the environment and ecosystem [1-3]. The treatment of toxic metal ions has received widespread attention due to their recalcitrance and persistence in the natural environment. Inexpensive, swift, effective and environmentally friendly decontamination technologies are always desired. Several typical methods and technologies, such as adsorption, ion exchange and biological flocculation, exist to cope with heavy metal contaminated wastewater, and all have advantages and disadvantages [4-6]. Nanosized metal oxide nanocrystals (TiO2, Cu2O, α-Fe2O3, and CeO2) are also a hot research area and are being evaluated for their ability to remove heavy metal ions because of their high surface area, specific affinity, nontoxicity, cost effectiveness, and environmentally benign nature [7]. The knowledge of interaction mechanisms of toxic metal ions and radionuclides in the geosphere with environmental media and adsorbent materials is conducive for the assessment of environmental impact and the design of adsorbent materials. An increasing number of researchers are dedicating themselves to the study of facetdependent properties of materials bound by various crystal planes [8], and progress in recent studies confirmed the important role of crystalline facets in toxic heavy metal ion removal because of the different properties of different crystalline facets [8-15]. For example, different exposed facets of Cu2O nanocrystals exhibited a significant difference in catalytic selectivity toward Cr(VI) reduction [9]. The hematite {001} and {110} facets showed inner-sphere monodentate mononuclear and bidentate binuclear configurations for Cr(VI) adsorption, respectively [10]. The adsorption capacity of Co3O4 nanoplates with (111) planes was 19 times higher than that of Co3O4 nanoplates with (001) planes in Pb(II) removal [11]. Bi24Ga2O39 microcubes 3

enclosed

by

{100}

facets

exhibited

higher

photocatalytic

activity

than

microtetrahedrons enclosed by {111} facets toward Cr(VI) reduction [12]. Anatase {001} facet TiO2 showed higher As(III) adsorption and photooxidation capability than {101} facet TiO2 [13]. The {201} TiO2 exhibited best performance for Sb(III) adsorption and photooxidation compared to {001}, {100} and {101} TiO2 [14-16]. Currently, the latent mechanisms are not fully understood. Some studies proposed that the structure-dependent surface active site density and facet orientation-dependent electronic structure of metal oxide crystals greatly influence their application [17, 18]. However, there is no report yet that systematically studies how the density of surface active sites as well as the facet electronic structure affect the removal of toxic metal ions using a detailed sample. Therefore, our study provides an experimental and theoretical approach to verify the above theory using a specific example and to give detailed mechanisms for further application in toxic metal ion removal of facetdependent metal oxides. Different exposed facets of anatase TiO2 crystals highly influence the photo reactivity [8]. Here, we synthesized three types of TiO2 with low-index exposed {001}, {100}, and {101} facets via facile hydrothermal routes. U(VI) was selected to test the adsorption and photocatalytic activity for the three facet TiO2, as U(VI) is chemically toxic and radioactive, causing severe kidney or liver damage and even death [19, 20]. Nowadays, numerous technologies have been employed to remove U(VI) from wastewater [21-23]. However, the material application is limited due to the low efficiency and the high cost. Therefore, the selection of efficient and cheap materials is highly desirable for practical application. TiO2 can be a proper photocatalyst to reduce U(VI), because of the reduction potentials of UO22+/U4+ and UO22+/UO2 are 0.327 and 0.411 V vs SHE, respectively, [24, 25] which is 4

thermodynamically feasible. Simultaneously, radical trapping experiments and experimental photocatalytic characterization were conducted to provide surface information on toxic metal ion removal. At the atomic-scale level, the lack of experimental techniques can be complemented by reliable theoretical calculations, providing further detailed information to uncover the nature of different facet TiO2. Hence, low-index stoichiometric anatase TiO2 adsorption performance and U(VI) photoreduction capacity were systematically investigated using density functional theory (DFT). The optimized structures and electronic properties were studied by the densities of states (DOS), Bader charges, charge density difference, and Gibbs energy.

2. Experimental 2.1 Materials and synthesis All regents and solvents were of analytical reagent grade and were used without further purification. The U(VI) stock solution with a given concentration of 1.0 mmolL-1 was prepared by dissolving specific UO2(NO3)2·6H2O (99.99%) into 250 mL Milli-Q water. {001}, {100}, and {101} facet dominated TiO2 was synthesized [2628], and the detailed synthesis methods for three facet TiO2 were exhibited in the Supporting Information (SI). 2.2 Characterization The morphology of three facets TiO2 was characterized by scanning electron microscopy (SEM) (JSM-6701F, Japan), transmission electron microscopy (TEM) (JEM-2011, Japan) and high resolution TEM (HRTEM) (JEM-2011, Japan). Powder X-ray diffraction (XRD) patterns were recorded using a Rigaku/Max-3A X-ray diffractometer operating with Cu Kα radiation (λ = 1.5418 Å) to investigate the crystal structures. The surface electronic state was recorded via the XPS spectrum (VG 5

Scientific ESCALAB Mark II, UK). The specific surface area and pore size distribution of the TiO2 particles were obtained from Brunauer–Emmett–Teller (BET) and Barrett-Joyner-Halenda (BJH) measurements (Tristar II 3020M, America). The steady fluorescence emission spectra were measured with the excitation wavelength of 312 nm. And the time resolved photoluminescence spectra have been recorded using nano-LED excitation with the wavelength at 370 nm (Fluorolog-3-21, Tempro01, Japan). Room-temperature ESR spectra were obtained at 300 K and 9.063 GHz (JES-FA200, Japan). The optical properties were detected using UV-Vis diffuse reflection spectra (DRS, UV3600-MPC3100, Japan). The photocurrent and electrochemical impedance spectroscopy (EIS) responses were measured by an electrochemical

analyzer

(CHI

660D

electrochemical

workstation

Chenhua

Instrument, Shanghai, China) at 0.4 V bias voltage under illumination of a 300 W Xe lamp (Beijing Perfectlight Technology Co., Ltd). Three electrodes, of which glassy carbon, Ag/AgCl and Pt electrodes were used as working electrode, the counter and reference electrodes, respectively. A 0.5 M Na2SO4 aqueous solution was used as the electrolyte. The Mott–Schottky plots were obtained at a fixed frequency of 100 kHz. 2.3 U(VI) adsorption and photoreduction Adsorption and photocatalysis experiments were conducted to evaluate the effectiveness of the three different facet TiO2 for U(VI) removal under ambient conditions. Batch adsorption experiments of U(VI) on the three facet TiO2 (m/V=0.2 g L-1) were performed using 10 mL polycarbonate tubes at pH = 5±0.1 and I = 0.01 mol L-1 NaNO3 solutions. Briefly, the NaNO3 solutions, three facet TiO2 solution, U(VI) solution and Milli-Q water were mixed in 10 mL polycarbonate tubes at 6

different component concentrations. The pH values of the suspension were adjusted by adding a negligible volume of 0.01-1.0 mol L-1 HNO3 or NaOH solution. Subsequently, the suspension was shaken for 24 h to achieve full adsorption equilibrium. A blank experiment of U(VI) without TiO2 was conducted under the same experimental conditions to exclude its adsorption on the walls of the polycarbonate tube. As shown in Fig. S1, the photoreactor was a 100 mL jacketed quartz beaker with magnetic stirring, and cooled by a circulating water jacket to maintain constant temperature. Photoreduction experiments were conducted with 0.1 mmol L-1 U(VI) in suspension containing 0.2 g L-1 TiO2 and 0.01 mol L-1 NaNO3 at pH 5.0. The suspension was magnetically stirred without light for 24 h to achieve the adsorption-desorption equilibrium before illumination with a Xenon lamp with a 420 nm cutoff filter providing ultraviolet light irradiation. Furthermore, control experiments for the abovementioned operation were performed in the dark. A volume of 2 mL liquid from the suspension was withdrawn at a specific time to monitor the photocatalytic progress. The solid powder was removed by centrifuging at 15000 rpm for 5 min, then the suspension was acquired and analyzed by UV–Vis spectrophotometer at a wavelength of 669 nm. The adsorption capacity for U(VI) was expressed according to Equation (1) [16]:

(1) where m (g) is the mass of the adsorbent, V(L) is the volume of the suspension, and C0 and Ce (mmol L-1) are the initial and equilibrated U(VI) concentrations, respectively. The photoreduction process was expressed by a first-order kinetic model after irradiation according to Equation (2) [24]:

ln

𝑐 =‒ 𝑘𝑡 𝑐0 7

(2)

where k is the first-order constant, and C0 and C (mmolL-1) are the U(VI) concentrations at times 0 and t, respectively. 2.4 Computational details Spin-polarized calculations were performed with the VASP 5.4.4 code [29-31]. The plane-wave basis set was applied in the framework of the projector augmented wave (PAW) method [32]. The exchange-correlation energy was determined using the generalized-gradient approximation (GGA) defined by Perdew, Burke, and Ernzerhof [33]. Valence electrons included Ti 3s2sp63d24s2 and O 2s22p4. The uranium atom was described by using a pseudopotential that explicitly considered the 14 electrons (6s26p67s25f36d1). The anatase surface was constructed using a repeated slab geometry from a relaxed unit cell of TiO2 ((a=b=3.886 Å, c/a=2.556)). To construct {001}, (3×3) supercells were built, and (2×2) supercells were built for {100} and {101} surfaces. Constructed slabs with a vacuum thickness were tested as depicted in Fig. S2, and ~15 Å was selected because it sufficiently suppressed the interaction between adjacent slabs. The surface of TiO2 consisted of 135 atoms for {001} planes, 120 atoms for {100} planes, and 144 atoms for {101} planes [14, 34]. Additionally, 3×5×1, 4×3×1, and 5×5×1 Γ-centered k-point Monkhorst-Pack meshes were used for {001}, {100} and {101} planes, respectively. All structures were fully relaxed until the residual force converged to less than 0.02 eV/Å. The energy cutoff was set was to 400 eV. The total electronic energy converged to less than 10-4 eV. Furthermore, we assessed the performance of DFT-GGA by comparing the U(VI) adsorption structures and adsorption energies with those obtained using the DFT+U method with the U values U{Ti3d}=4.2 eV [34] and U(U5f)=4.0 eV [35]. The DOS was obtained using the tetrahedron method with Blöchl corrections for accuracy [36]. The work function was calculated as the difference between the average electrostatic 8

potential in the vacuum and the Fermi energy of the slab [37]. Charge transfers were calculated based on Bader charge analysis [38]. For the photoreduction of U(VI) on different facet TiO2, we evaluated the change in Gibbs free energies for one and two electron U(VI) reductions [39].

3. Results and discussion 3.1 Characterization of three TiO2 We synthesized three anatase TiO2 with different percentages of {001}, {100}, and {101} facet in the present study. Representative SEM images of the products showed that each type of TiO2 have predominantly special facet as depicted in Fig. 1. TEM analysis showed an average size of 40 nm, and thickness of ~6nm of nanosheets TiO2 (Fig. 1a and b). A high-magnification TEM image and its corresponding selected-area electron diffraction (SAED) pattern exhibited the diffraction spot of the (001) zone and the (200) atomic lattice spacing of ~1.9 Å. The SEM image of Fig. 1c displayed that the product of {001} facet TiO2 consisted of well-defined sheet-shaped structure. On the basis of the above structural information, the geometry can be considered as well-shaped nanosheets with dominant (001) facet, and the percentage highly reactive (001) facets in the TiO2 nanosheets was estimated to be ~89% on average [28].

Both the SAED pattern and HRTEM images (Fig. 1e, f) revealed that

the growth direction of {100} facet TiO2 nanorods was along the [001] zone axis. The SEM and TEM images exhibited an average length of ~500 nm and a width of ~80 nm with 83% exposure of {100} facets, as depicted in Fig. 1 g [27]. As show in Fig. 1i, j, k, the SEM and TEM images displayed the particle’s mean center diameter of about 500 nm, and the length of about 1 μm TiO2. The lattice spacing of 3.5 Å can be noted from TEM image, which can be indexed to {101} facets. Fig. 1k showed that the as-prepared sample exhibited spindle-like morphology. And the calculated 9

percentage of exposed {101} facets was nearly 92% [26]. A sketch schematic descriptions of the three facets TiO2 crystals after the measured images have been provided in the last column in Fig. 1d, h, l. The XRD patterns of the {001}, {100}, and {101} facet TiO2 in Fig. 2a can coincidently be indexed to the pure anatase TiO2, and it was further analyzed in SI. The N2-BET surface areas of the {001}, {100}, and {101} facet TiO2 were measured as 23.221, 14.069 and 24.148 m2/g, respectively (Fig. 2b). Although the sample with the {001} predominant facet possessed average dimensions of 40 nm, it has approximately the same surface areas as the spindle-shaped particles, which were 1 micron in length. As shown in Fig. 2b, the {001} facet TiO2 possessed larger macropores (>50 nm), which can be ascribed to the pore of sample stacking according to the analysis of SEM and TEM, whereas the {101} facet TiO2 mainly had mesopores (<15 nm). The surface area was reasonable, accounting for the analysis of the distribution of pore diameters. The zeta potential values of the three samples were measured and shown in Fig. 2c, and the points of zero charge (pHpzc) of {001}, {100}, and {101} facet TiO2 were 6.7, 5.8, 5.9, respectively. Interestingly, the adsorption experiment was conducted at pH 5.0, where the values of zeta potential were similar for three samples. The similarities of the surface area and surface charge distribution at pH 5.0 allowed for the accurate analysis of the facet-dependent adsorption and photocatalytic reduction. Furthermore, the XPS survey in Fig. S3 showed that no fluorine or residual organics existed in the three TiO2 samples. 3.2 U(VI) adsorption on the three TiO2 Fig. 2d illustrated the adsorption isotherms based on the specific surface area 10

normalized results of U(VI) on {001}, {100}, and {101} facet TiO2 at 298 K and pH 5.0. The adsorption data were fitted using the Langmuir and Freundlich models [19]. Table S1 exhibited the corresponding parameters. The Langmuir model fitted the experimental data better than the Freundlich model. The normalized adsorption capacity qe (mmol/m2) followed the order {001} (0.011) > {100} (0.009) > {101} (0.005). Notably, the adsorption capacity was not proportional to SBET, and similar results were also reported for arsenic and antimony adsorption [13, 15]. These results indicated that the facets of crystals have a great influence on their adsorption capability. To explore the atomic-scale difference of the facets, DFT calculations were conducted to study the different adsorption performances for the {001}, {100}, and {101} facet TiO2. [UO2(H2O)5]2+ preferentially pentacoordinated in its equatorial plane has already be reported by experimental and theoretical studies[40-42]. Surface atomic structures of three facets TiO2 were shown in Fig. S4. The U(VI) maybe interacted with the TiO2 surface via chemical bonding to form inner-sphere complexes or via hydrogen bonding to form outer-sphere complexes [43, 44]. The unsaturated O2c sites are generally predicted to be the chemically active sites, and this was confirmed by preliminary calculations. Thus, the non-equivalent inner-sphere and outer-sphere configuration for three facets TiO2 interacted with U were studied, and the most stable configurations were reported here, as shown in Fig. S5, 6, and 7 for {001}, {100} and {101} facet TiO2, respectively. The key geometrical parameters and binding energies were collected and shown in Table S2. According to the energy analysis, the monodentate structure was favored for {001} facet TiO2, whereas bidentate structures were more stable for {100} and {101} facet TiO2. However, due to the U(VI) bonding with surface active O2c sites, the density of O2c is 14.46, 18.53 11

and 19.91Å2/Osurf_O2c, in consistent with our qe results that {001} facet TiO2 adsorbed more U(VI) than the other two samples. Although the most stable geometry of U(VI) adsorbed on {001} was a monodentate structure, the adsorption energy of the bidentate structure was higher than that of the other two facet TiO2 for adsorption. To compare the intrinsic adsorption mechanism, the most stable bidentate complex of the optimized structures of U(VI) adsorbed on {001}, {100}, {101} facet TiO2 were displayed in Fig. 3a, b, and c, respectively. The adsorption energy of three bidentate complex were decreased and followed the order of {001} (-4.68 eV) > {100} (-3.66 eV) > {101} (-2.94 eV), and the increase in the distances between the U atom and Osurf atom, U-Osurf, were 2.217/2.295, 2.376/2.420, and 2.523/2.535 Å for {001}, {100}, and {101} facet TiO2, respectively. To gain additional insight into the interaction between U(VI) and the three facet TiO2, the difference in the charge density adsorbed on the three facet TiO2 was presented in Fig. 3d, e, and f, all at their relative position in the adsorption system.

The

yellow/blue

lobe

enclosed

regions

represented

the

density

reduction/increase upon adsorption. Such reductions were clearly present on both Osurf atoms along the Osurf-U direction, which was attributed to chemical bonds between the U atom and Osurf atoms. By calculating the Bader’s decomposition of charge density [38], all atom charges were obtained. Electron was transferred from the TiO2 to the U(VI) as expected, and from simple electronegativity considerations, △QM gradually decreased and followed the order {001} (|0.94|e) > {100} (|0.86|e) > {101} (|0.81|e), indicating that more electron transfers would be favorable for U(VI) adsorption. To verify the chemical bonding between the U atom and Osurf atom of the three different facet TiO2, the projected densities of states (PDOS) for the three adsorption configurations were analyzed. The PDOS of the U atom and Osurf atom at the 12

adsorption sites in bidentate adsorption configurations were plotted in Fig. 3g, h, and i. The strong hybridization between U 5f orbitals and O 2p orbitals was noted as the deep dark green region, which further showed the formation of chemical bonds during the adsorption process. 3.3 U(VI) photoreduction on the three facet TiO2 At first, the U(VI) photolysis was negligible under the current experimental condition in the absence of TiO2 (Fig. S8). The concentration change of U(VI) in solution with the {001}, {100}, and {101} facet TiO2 before and after UV irradiation was illustrated in Fig. 4a, b, and c, respectively. Upon UV illumination, U(VI) was completely removed in 150 min for {001} (Fig. 4a) and 180 min for {100} facet TiO2 (Fig. 4b), and only 50% of U(VI) was eliminated by {101} facet TiO2 during the 180 min irradiation (Fig. 4c). Fig. 4d represented the fitting results of the U(VI) photoreduction on the three types of TiO2 with a first-order kinetics model, and the rate constant k followed the order {001} (0.023 min-1) > {100} (0.011 min-1) > {101} (0.002 min-1). According to the U(VI) removal efficiency, {001} facet TiO2 exhibited best performance for U(VI) adsorption and photoreduction. As shown in Fig. S9, the XRD pattern of representatively {001} facet TiO2 after irradiation still had clearly defined peaks, indicating the stability of this material. To further validate the valence states of U, the photoreduction products of U on the three facet TiO2 were characterized by XPS. Fig. 5 showed the curve-fitted U 4f photoelectron spectra recorded from the three facet TiO2 after reacted with U(VI). The XPS spectra of U 4f located at the regions of 377–387 and 388–398 eV correspond to the U 4f7/2 and its satellite peak, respectively. The U 4f7/2 signal can be deconvoluted into two components centered at approximately 380.8 and 382.0 eV, ascribed to the characteristic signals of the U(IV) and U(VI) species, respectively [4]. According to 13

the peak area analysis, the ratio of U(IV)/U(VI) were 0.34967, 0.2869 and 0.2452 for the {001}, {100}, and {101} facet TiO2, respectively. Thus, XPS results indicated that the photocatalytic activity sequence of TiO2 was in the order {001} > {100} > {101}. Some reports ascribed the discrepant reduction or oxidation efficiency to photoinduced carrier separation or reactive oxygen species [12, 24, 45]. The different efficiencies of U(VI) removal on the three facet TiO2 led us to further study the intrinsic mechanism. 3.4 Photocatalytic mechanism To evaluate the photoreduction mechanism, active species were first detected. Both hydroxyl radical (˙OH) and superoxide radical (O2˙−) existed in our TiO2 samples according to our ESR detection, as shown in Fig. S10. Generally, the greater the formation rate of ˙OH radicals, the higher separation efficiency of electron-hole pairs [46]. The formation of ˙OH radicals on the three facet TiO2 were analyzed by PL (excitation at 312 nm) using a previous method [47]. As shown in Fig. S11 a, b, and c, with increasing illumination time, the fluorescence intensity at approximately 440 nm gradually increased for the {001}, {100}, and {101} facet TiO2, and Fig. S11d shows the plots with an almost linear increase, where the slope of these lines represents the formation rate of the ˙OH radicals. Therefore, the formation rate of the three facet TiO2 were ordered as {001} > {100} > {101}. Furthermore, as shown in Fig. 6a, the UV-Vis diffuse reflectance spectra demonstrated that the {001} facet TiO2 had a smallest bandgap. Combined with the X-ray photoelectron valence band spectra (Fig. 6b), the scheme of the band structure was exhibited in Fig. S12. Moreover, the work function expressed the difficulty of losing one electron from surface. From our 14

theoretical calculation, the sequence of surface work function as displayed in Fig. S12 followed the order of {001} < {100} < {101}, similar to a previous report [37]. A lower work function corresponds to a lower energetic barrier for donating electrons from the surface of the catalyst to the adsorbed oxygen or adsorbates, which lead to the formation of OOH˙ species or direct charge transfer to adsorbates [48]. Fig. 6c showed the PL spectra of the three types of TiO2. The weak emission peak suggested a decreased recombination efficiency of the photoradiation charge carrier. From Fig. 6c, we noted that {001} facet TiO2 had the best performance for charge separation and recombination efficiency. To further understand the photoexcited charge carriers, the time-resolved PL spectra at the wavelength of its maximum emission was recorded as shown in Fig. 6d. The decay behavior of the samples can be well-fitted to a biexponential function in the form of f(t)=A1exp(-t/τ1) + A2exp(-t/τ2) [49, 50]. The average PL lifetime (τ) was deduced by the following equation: τ=(A1τ12+A2τ22)/ (A1τ1+A2τ2) [51, 52]. Generally, the longer the lifetime, the faster the separation of electron-hole pairs. Thus, the decay time constants were calculated by multiexponential fitting and showed in Table S3. It can be noted that PL lifetime (τ) was 27 ns for {001} facet TiO2, which was 1.076 and 1.499 times longer than that of {100} and {101} facet TiO2, respectively. It confirmed that photogenerated carrier separation was on the order of {001} > {100} > {101}. What’s more, the photocurrent, EIS and Mott-Schottky plots were also recorded to further understand the electronic properties. As the higher of photocurrent intensity, the higher the charge separation[53], and the photocurrent intensity of three samples 15

as displayed in Fig. 6e exhibited the order of {001} > {100} > {101}, indicating the highest separation efficiency of photogenerated electron- hole pairs in {001} facet TiO2. The EIS exhibited in Fig. 6f, showed the decreased electron-transfer resistance in {001} facet TiO2 with the smallest diameter of the semicircular Nyquist plots. Mott-Schottky plots were displayed in Fig. 7 with all positive slopes, indicating the ntype semiconductor. Importantly, the {001} facet TiO2 showed substantially smaller slopes of the Mott–Schottky plot compared to {100} and {101} facet TiO2, suggesting a faster charge transfer. What’s more, the carrier density Nd can be calculated from the slope of the Mott–Schottky plot using Nd=(2e0ε ε 0)[d(1/C2)/dv]-1, where e=1.6×10−19 C, ε0=8.86×10−12 F/m, and ε=48 for anatase TiO2 [54], the Nd values of three samples were determined as 9.08× 10-18, 6.06× 10-18, 1.79× 0-18 cm−3 for {001}, {100} and {101} facet TiO2, respectively. Based on the analysis of PL spectra, timeresolved PL, photocurrent response, EIS and Mott–Schottky analysis, we can conclude that photogenerated hole-electron charge separation efficiency followed the order: {001}> {100}> {101}, and the photogenerated hole-electron recombination rate of the three samples followed the order of {001} < {100} < {101}. In addition, the photogenerated electron (e-) and O2˙− are commonly accepted as the main species determining the photoreduction of heavy metal ions. Hence, the radical trapping technique was used to investigate the contributions of photogenerated electron (e-) and O2˙− to U(VI) photoreduction. EDTA-2Na (40 mM) was used as the h+ scavenger, and the formation of O2˙− was suppressed by purging high-purity argon. To compare the contribution of reactive species, the radical trapping technique was 16

investigated with EDTA and N2 purging. The results in Fig. 4a, b, and c showed that the presence of EDTA notably improved the reaction rate at an early time point for the three facet TiO2. Finally, approximately 70-80% of the U(VI) can be removed by the three facet TiO2, and an obviously enhanced reduction ability of the {101} facet TiO2 compared with the control experiment can be noted, which indicated the lack of enough electrons to reduce U(VI) in the control experiment. EDTA was a very effective hole acceptor in the photocatalytic system, which could provide more electrons to the photoreduction process [25, 55]. This result indicated that photogenerated electrons dominated U(VI) photoreduction on TiO2. The limited reductive deposition of U(VI) may be because the single decarboxylation of EDTA results in tricarboxylic acid, which cannot be easily oxidized further, thereby halting any further reductive process of U(VI) on the TiO2 surface [25]. Then the increasing of concentration of U(VI) can be noted and maybe ascribed to the reoxidation of reduced uranium species due to the existence of dissolved oxygen. The high-purity purging gas argon was used to eliminate dissolved oxygen. Fig. 4a and b showed that the photoreduction of U(VI) by the {001} and {100} facet TiO2 was suppressed by 25% and 32%, respectively. Notably, Fig. 4c showed that the removal efficiency of U(VI) notably increased, leading to U(VI) being completely removed after 150 min of UV illumination for the {101} facet TiO2 with the purging of argon gas compared to that without argon gas. Thus, dissolved oxygen had different roles in U(VI) photoreduction on the {001}, {100} and {101} facet TiO2. 17

However, previous studies reported that oxygen participation decreases the reduction efficiency of U(VI) due to competition for electrons with U(VI) [25, 55]. Studies also reported that O2˙− plays a critical role for U(VI) reduction as an electron shuttle between TiO2 and U(VI) (x O2˙− + U6+ → U(6 − x)+ + xO2) [56]. From above analysis, it seems that electrons and oxygen were both responsible for U(VI) reduction at {001} and {100} facet TiO2, whereas {101} facet TiO2 was mainly dominated with electron for U(VI) reduction. For the photogenerated electrons dominated U(VI) photoreduction, the XPS analysis provided evidence for the reduction of U(VI) to U(IV) [57, 58]. Two electrons were required for this redox reaction, and major changes to the coordination shell of U are expected upon reduction to U(IV). However, a one electron reduction should also be considered for U(V), which is extremely unstable and can easily disproportionate to U6+ and U4+ and rarely detected by XPS. Thus, by adding 2.0 extra electron carriers to the U(VI) adsorbed on the TiO2 system, we modeled the distribution of photogenerated charges. The charge density difference between the origin system and electron added system was exhibited in Fig. S13a, b and c, and the electrons were mainly located at the Ti and U atoms. For the three facet TiO2 photoreduction, one electron and two electrons were both considered when evaluating the reaction free energy. As shown in Fig. S13d and f, all steps proceed downhill in energy for two electron photoreduction, whereas the energy was uphill for the {001} one electron reduction to U(V) but downhill when one more electron participated in the photoreduction. The two electron reduction was more favorable than the one electron reduction, and the {001} facet TiO2 exhibited high favorability of the two electron reduction, which is in consistent with the photoreduction experiments. 18

To further examine the mechanism of enhanced photocatalysis with and without oxygen, electron transfer processes were investigated. Fig. S14 showed that the electron was located mainly at the subsurface Ti6c for {001} facet, whereas it was located at the surface Ti5c for the {100} and {101} facets. As reported, the adjacent trapping sites as holes mainly localize on surface O2c. The electrons localize on surface Ti5c atoms for {101} facet, leading to easier charge carrier recombination, in consistent with our photocatalytic characterization [30]. In order to clarify the different role of oxygen, DFT calculations were performed with molecular O2 associated with the Ti5c atom on the three facets. The optimized geometries were depicted in Fig. 8a, b, and c, the affinity of O2 adsorption on three facets TiO2 was on the order of {001} (-1.38 eV) > {100} (-1.27 eV) > {101} (-0.84 eV), and the charge transfer also followed this trend. Fig. 8d, e, and f exhibited the charge density difference for O2 adsorbed on the three facet TiO2. We can compare the distribution of electron charges for {001} facet located at the surface layer with Fig. S14, and the comparison is consistent with some studies that O2 adsorption could promote the transfer of electrons from the subsurface atomic layer to the outermost atomic layer for the {001} facet, as shown in Fig. 8d [13]. TiO2+hv→h++e-

(3)

UO22++2e-→UO2 (reductive deposition)

(4)

UO22++e→UO2+

(5)

UO2++4H++e→U4++2H2O

(6)

O2+e→O2-

(7) 19

nO2-+U6+→nO2+U(6-n)+

(8)

From the adsorption energy and charge transfer analysis, we supposed that the oxygen on {001} was still adsorbed on the surface after accepted electrons, the same with {100} facet TiO2. Whereas oxygen would dissociate in the solution after accepted electrons for {101} surface due to the weaker interfacial interaction. Thus, the existed oxygen adsorbed on surface can be synergetic with O2C of surface TiO2 to interact with U(VI), whereas the dissociation negatively oxygen just acted as one role to compete electrons with U(VI) for {101} facet TiO2, as shown in Fig. 9a. Therefore, the mechanism can be suggested and illustrated in Fig. 9b. After UV illumination, the photogenerated holes left in the valence band (VB) TiO2 can directly oxidize H2O or OH-. The reduction was induced from the electron transfer from the conduction band (CB) to O2 or U(VI). The O2 can promote the reduction of U(VI) for {001} and {101} facet TiO2, whereas hinder the U (VI) reduction for {101} facet TiO2.

4. Conclusion The recalcitrant and persistent toxic heavy metal ions have different performances in facet-dependent metal oxide removal. In this study, the combined experimental and theoretical studies revealed that the {001} facet TiO2 had much higher adsorption and photocatalytic activity than the {100} and {101} facet TiO2 for U(VI) deposition. DFT calculations showed that monodentate complexes were most favored for {001} facet TiO2 for U(VI) adsorption, and bidentate complexes were most favored for {100} and {101} facet TiO2. Theoretically, electronic calculations revealed that {001} transferred more electrons from the surface to U(VI), which may be more favorable for U(VI) adsorption. The PL, time-resolved PL, photocurrent 20

density, EIS, Mott-Schottky plots indicated that {001} facet TiO2 possessed highest electron-hole separation efficiency, which lead to the quickest U(VI) photoreduction. The radical scavenging experiments revealed different roles for dissolved oxygen in U(VI) photoreduction, which can be ascribed to the difference of surface structures. The combination of experimental and theoretical measurements in this study can provide guidance for toxic heavy metal removal in selecting high-efficiency facetdependent metal oxides and for the design of facet-dependent materials used for environmental remediation.

Acknowledgements Financial support from the National Natural Science Foundation of China (21477133), the Anhui Provincial Natural Science Foundation (1608085QB44), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, the Priority Academic Program Development of Jiangsu Higher Education Institutions, the CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, and the support of the theoretical calculations by the University of Science and Technology of China Supercomputer Centers are acknowledged.

Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at http://dx.doi.org/xx.xxxx/j.cej.xxxx.xx.xxx

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29

Fig. 1. HR-TEM (1st column, 2nd columns) images, and SEM, schematic illustrations (3rd column) for {001} (1st row), {100} (2nd row), {101} (3rd row) facet TiO2. Insets in the HR-TEM images show the measured lattice spacing.

30

Fig. 2. (a) XRD patterns of the three facet TiO2 (JCPDS No. 21-1272). (b) N2 adsorption-desorption isotherm of the three different facet TiO2, and the distribution of the pore diameter of the three types of TiO2 (c) Zeta potential of the different facet TiO2 as a function of pH in 0.01 molL-1 NaNO3 solution. (d) Normalized adsorption isotherm of U(VI) for the different facet TiO2: pH = 5±0.1, m/V = 0.2 g L-1, and I = 0.01 mol L-1 NaNO3.

31

Fig. 3. The most stable bidentate configuration of U(VI) adsorbed on the (a) {001} facet TiO2, (b) {100} facet TiO2, and (c) {101} facet TiO2. The charge density difference resulting from the adsorption of U(VI) on (d) {001}, (e) {100} and (f) {101} facet TiO2. The PDOS with PBE+U approach for adsorbed sites of O1 and O2 2p orbitals and the U 5f orbital for (g) {001}, (h) {100} and (i) {101} facet TiO2 adsorption. 32

Fig. 4. Variation of U(VI) concentration before and during UV irradiation for (a) {001}, (b) {100}, (c) {101} facet TiO2: initial U(VI) = 0.1 mmol L-1, m/V = 0.2 g L-1, and pH =5.0. (d) The fitting results of U(VI) photocatalytic reduction on the three TiO2 with first-order kinetics.

Fig. 5. Curve-fitted U 4f XPS spectra recorded from U(VI) adsorption on the {001}, {100}, and {101} facet TiO2. 33

Fig. 6 (a) UV-Vis diffuse reflectance spectra of the three facet TiO2. The inset shows typical plots of (αhv)1/2 vs hv for the band gap energies. (b) Valence-band XPS spectra of the three facet TiO2. (c) The steady fluorescence emission spectra of the three types of TiO2. (d) Time-resolved PL spectra of {001}, {100} and {101} facet TiO2. (e) Transient photocurrents of {001}, {100} and {101} facet TiO2. (f) EIS curves of {001}, {100} and {101} facet TiO2.

34

Fig. 7. Mott–Schottky plots of {001}, {100} and {101} facet TiO2.

Fig. 8. Optimized geometries of an O2 molecule adsorbed at the Ti5c site on the (a) {001}, (b) {100} and (c) {101} facet TiO2. The charge density difference of O2 adsorbed on (d) {001}, (e) {100} and (f) {101} facet TiO2.

35

Fig. 9. (a) Possible electron and hole transfer paths. (b) Possible U(VI) reduction paths and schematic diagram of the proposed photocatalytic reduction mechanism of the TiO2 photocatalyst under ultraviolet light irradiation.

36

Graphic Abstract Facet dependence of interaction mechanism between TiO2 with U(VI)

 

Highlights 

{001} TiO2 exhibited the best adsorption capacity and photoreduction ability compared to {100} and {101} TiO2.



The mechanisms of U(VI) adsorption on {001} {100} and {101} TiO2 are different



{001} TiO2 possesses the highest electron-hole separation efficiency



The dissolved oxygen promotes U(VI) photoreduction on {001} and {100} TiO2, but inhibits on {101} TiO2

37