Anatase Heterophase Junctions at High

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Fabrication of TiO(B)/Anatase Heterophase Junction at High Temperature via Stabilizing the Surface of TiO(B) for Enhanced Photocatalytic Activity 2

Zihao Wang, YiLan Wang, Wan Zhang, Zenglin Wang, Yi Ma, and Xin Zhou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09763 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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The Journal of Physical Chemistry

Fabrication of TiO2(B)/Anatase Heterophase Junction at High Temperature via Stabilizing the

Surface

of

TiO2(B)

for

Enhanced

Photocatalytic Activity Zihao Wang1, Yilan Wang1, Wan Zhang1, Zenglin Wang1, Yi Ma1*, Xin Zhou2* 1Key

Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of

Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710119, Shaanxi, China 2College

of Environment and Chemical Engineering, Dalian University, Dalian, 116622, P. R.

China *Corresponding authors: [email protected] (Yi Ma); [email protected] (Xin Zhou)

1

ACS Paragon Plus Environment

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Abstract: In recent years, TiO2(B) has been widely used in photocatalysis, electrocatalysis and lithium-ion battery due to its unique crystal structure. However, as a metastable phase semiconductor, TiO2(B) is prone to transform into the stable anatase or rutile phase at high temperature. Therefore, the low crystallinity of TiO2(B) was usually formed at low temperature. Both of the above two concerns restrict its application in photocatalysis. In this research, we found that an appropriate amount of HF could inhibit the phase transformation process from TiO2(B) to anatase. XRD patterns and Raman spectra showed that when 0.3wt% HF was added prior to the calcination, a large amount of TiO2(B) could be maintained even at 750 oC. Thus, the sample with TiO2(B)/anatase heterophase junction structure was obtained at high temperature, which exhibited the enhanced photocatalytic activity both for pollution degradation and hydrogen production. The phase transformation mechanism was finally revealed by DFT calculations. It was demonstrated that the F anions adsorbed on the surface of TiO2(B) could efficiently decrease the surface energy from 0.63 J/m2 for the clean surface to 0.22 J/m2 for the F anions adsorbed surface, which was the internal factor for the retarded phase transformation process and promoted the fabrication of heterophase junction at high temperature.

2


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1. Introduction Anxiety

about

the

energy

crisis

and

environmental

problem

make

photocatalysis/photo-electrocatalysis a promising technology for solving the urgent issues.1-3 Since the first photoelectrocatalytic water splitting reaction found by Fujishima and Honda,4 enormous efforts have been devoted to develop the efficient and

environmental

friendly

photocatalysts.

Among

various

materials

for

photocatalytic reactions,5-9 many attentions have been focused on TiO2 because of its nontoxicity, low-cost, chemical and biological inertness, resistant for photo and chemical corrosion and availability.10-14 Therefore, great efforts have been made on the improvement of the activity of TiO2-based material not only due to its potential application on fuel generation,15,

16

environmental purification,16,

17

self-cleaning,17

solar cells,18, 19 but also a model photocatalyst for understanding the mechanism of photocatalytsis process.20, 21 The crystal structure of a material is one of the most important issues for photocatalytic reactions, because the physical/chemical properties may be different among the crystal forms. Anatase, rutile, brookite and TiO2(B) are the four typical crystal forms of TiO2 in nature. They all consist of TiO6 octahedra but share edges and corners in different manners. Currently, anatase and rutile have been widely investigated. The research of brookite is limited because of the difficulty in preparation. The fourth crystalline form, TiO2(B), was first synthesized in 1980 by Marchand et al.,22 and exhibits a monoclinic C2/m structure comprised of edge- and corner sharing TiO6 octahedra with an open channel parallel to the b-axis that sits 3



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between axial oxygens. In recent years, TiO2(B) has been usually used as the anode material in the lithium-ion battery,23,

24

while the unique crystal structure with

anisotropy may also show a potential application in photocatalysis. For example, the photo-generated electrons and holes could transport in different directions due to the anisotropic structure, which is beneficial for the charges separation. However, this promising photocatalyst has not been fully investigated yet. Actually, the single phase TiO2 is far from satisfied for the photocatalytic activity due to the lower charge separation efficiency. Zhang and co-workers revealed that anatase/rutile hetero-phase junction formed in mixed phase of TiO2 can greatly enhance the hydrogen production activity due to the promoted charge separation efficiency.25-27

Since

then,

the

mixed

phase

structure

anatase/rutile28-30anatase/brookite,31-33 brookite/rutile,34-36 TiO2(B)/anatase/rutile,37, anatase/rutile/brookite39,

40

of 38

have been widely studied. There are already some

investigations on TiO2(B)/anatase.41-43 Wang et.al reported a novel approach to fabricate hierarchical TiO2 microspheres (HTMS) assembled by ultrathin nanoribbons where the anatase/TiO2(B) heterojunction can be obtained at the temperature of 550 oC.44

Qiu and coworkers fabricated a TiO2 hierarchical architecture assembled by

nanowires with anatase/TiO2(B) phase-junctions, which was absent at the temperature of 550 oC.45 Cai and coworkers reported a facile hydrogenation process to generate TiO2(B)-anatase heterophase junction in situ with a disordered surface shell, while the structure of anatase/TiO2(B) can be hardly observed at the temperature up to 650 oC.46 It has been found that, although the activity of TiO2(B) is relatively lower than 4


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anatase in most cases, a well matched hetero-phase junction can be formed between anatase and TiO2(B). However, due to the relatively unstable phase structure of TiO2(B), the structure of TiO2(B)/anatase cannot be obtained at higher temperature, which means we could not obtain TiO2(B) with high crystallinity. As is known, the lower crystallinity could result in an inferior charge separation efficiency, which is one of the crucial problems for the unsatisfied activity for TiO2(B)-based photocatalysts. Therefore, how can we improve the crystallinity of TiO2(B) in TiO2(B)/anatase structure becomes an important issue for enhancing its photocatalytic activity. Phase transformation process is the most convenient and effective way for the construction of hetero-phase junction photocatalyst. The atomic-level contact between different phases can be obtained, which is crucial for the fast-speed charge transfer crossing the interface of phase junction. However, as mentioned above TiO2(B) is a metastable phase of TiO2, and the calcination temperature up to 600 oC usually results in a completed phase transformation to anatase. Therefore, the unsatisfied photocatalytic activity of TiO2(B) may due to the current low temperature preparation method, which usually gives a poor crystallinity containing numerous charge recombination centers. Our previous results found that, the phase transformation from TiO2(B) nanowires to anatase was a surface preferred process,47 suggesting that the surface region of the material would transform to anatase faster than that in the bulk region. Inspired from these results, we speculate that, a surface modified TiO2(B) would exhibit a different phase transformation behavior. 5



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Here, we found that a modification of certain amount of HF on the surface of TiO2(B) will greatly inhibit the phase transformation from TiO2(B) to anatase. Quite large amount of TiO2(B) structure can be maintained even at 750

oC.

The

TiO2(B)/anatase hetero-phase junction at higher temperature was successfully constructed and the photocatalytic activity was evaluated both for pollution degradation and H2 evolution reactions. 2. Experimental 2.1 Chemicals and Reagents Titanium (IV) n-butoxide (C16H36O4Ti, 99%) was purchased from J&K Chemical Co., Ltd (Beijing, China). Acetic acid (C2H4O2, ≥99.5%), ethylene glycol (C2H6O2, ≥99.0%), ethanol (C2H6O, ≥99.7%), sodium hydroxide (NaOH, ≥96.0%), hydrochloric acid (HCl, 36.0~38.0%), hexachloroplatinic (IV) acid (H2PtCl6•6H2O, ≥37.0%) ethylenediaminetetraacetic acid (EDTA), tert-butyl alcohol (t-BuOH) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Hydrofluoric acid (HF, 50 wt%) was purchase from Macklin Inc. (Shanghai, China). Rhodamine B (RhB) (≥95%) was obtained from Sigma-Aldrich (USA). 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was purchase from Dojindo Molecular Technolgogies, Inc (Shanghai). All chemicals were used as purchased without further purification. Deionized water was used in all experiments except for mentioned otherwise. 2.2 Catalyst Preparation Hydrogen titanate (HT) was prepared according to the previous report.47 6


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Typically, sodium titanate was firstly prepared via a hydrothermal method. 6 mL titanium (IV) n-butoxide and 3 mL acetic acid were added dropwise to 20 mL ethylene glycol and stirred for 15 min, followed by the addition of 30 mL NaOH solution (15 mol/L) and stirring for another 10 min. Afterwards, the suspension was transferred into a 100 mL Teflon-lined autoclave and maintained at 180 oC for 24 h. After cooling down to room temperature, the sodium titanate was added to a HCl solution (0.1 mol/L) with constant stirring for 12 h. Then, the resulted mixture was centrifuged and washed with deionized water until the filtrate became neutral. Finally, the HT was obtained after drying at 80 oC for 12 h. TiO2(B)/anatase series samples were prepared via a retarded phase transformation process using HF as modifier. Typically, 0.5 g as prepared HT sample was dispersed in a 5 mL aqueous solution containing certain amount of HF and water. The 5 mL water was also used as reference. The resulted slurry sample was centrifuged after stirring for 2 h at room temperature, and then dried at 60 oC for 8 h. Finally, the samples were annealed in a muffle furnace at 750 oC for 2 h. The obtained catalysts were labeled as “x%F-HT”, where x denotes the mass percentage of HF in water. 2.3 Material Characterization X-ray diffraction (XRD) patterns were recorded on a Rigaku MiniFlex 600 (Japan) diffractometer with Cu-Kα radiation (V = 40 kV, I = 15 mA) over a range of 10-70° (2θ) with a step of 0.02°. UV-Raman spectra were recorded on a Renishaw Raman spectrometer (Via + Reflex) system with a 325 nm excitation 7



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source. The surface morphologies of the photocatalysts were observed using a SU 8020

Field

Emission

Scanning

Electron

Microscopy

(FESEM,

Hitachi

High-Technologies, Tokyo, Japan) operating at 5 kV. Transmission electron microscopy (TEM) images were performed on a JEM-2100 instrument (JEOL, Japan) under an acceleration voltage of 200 kV. Specific surface areas were obtained by acquiring N2 adsorption isotherms under a relative pressure of 10-6 MPa at 77 K on a Micromeritics ASAP2020 HD88 specific area. All the photocatalysts were degassed at 300

o

C for 4 h before adsorption experiments. Electron

paramagnetic resonance spectra (EPR) were recorded under ambiance at room temperature on a Brucker EPR spectrometer (E500-9.5/12). Superoxide radicals (O2•−) and hydroxyl radicals (•OH) was detected by trapping with DMPO using methanol and water as solvent, respectively. The sample was detected under dark or under irradiation with a 300-W xenon lamp (CEL-HXF 300, Beijing China Education Au-Light Co., Ltd). All EPR spectra were recorded under the same experimental conditions: microwave frequency, 9.42 GHz; center field, 3350.4 G; sweep width, 100 G; modulation frequency, 100 kHz; and power, 20.00 mW. X-ray photoelectron spectroscopy (XPS) measurements were carried out using an Axis Ultra DLD XPS instrument (Kratos Analytical Ltd.), and analyzed by the XPSPeak software. Binding energies for the high resolution spectra were calibrated by setting C 1s to 284.6 eV. UV-Vis diffuse reflection spectra were recorded on a UV-3600 spectrophotometer (Shimadzu, Japan) in the range of 200-800 nm. Photocurrent and electrochemical impedance spectroscopy (EIS) were 8


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performed on a CorrTest CS310 electrochemical workstation (Wuhan CorrTest Instrument Co. Ltd., China) in a three-electrode cell with Na2SO4 electrolyte (1.0 mol/L) at room temperature. The photocatalyst loaded thin film electrode, platinum sheet and saturated Ag/AgCl electrode were used as working electrode, counter electrode and reference electrode, respectively. The working electrode was prepared by electrophoresis method on the surface of a FTO glass substrate (1 cm × 0.5 cm), followed by drying at 60 oC and annealing at 400 oC for 2 h in a muffle furnace. The photocurrent measurements (i-t curve) were carried out at 0.3 V (vs. saturated Ag/AgCl electrode) under chopped illumination with a 300 W Xe-lamp as the light source. The EIS data were obtained with a frequency range from 0.01 Hz to 100 kHz at 0.3 V (vs. saturated Ag/AgCl electrode). 2.4 Photocatalytic Activity Assays 2.4.1 Photocatalytic Degradation of RhB In a typical experiment, 50 mg as-prepared photocatalyst was added to a 150 mL RhB aqueous solution (4 mg/L) in the quartz photoreactor. The photocatalytic activity was evaluated under the irradiation of a 300 W Xe-lamp (CEL-HXF 300, Beijing China Education Au-Light Co., Ltd). Before irradiation, the mixture was stirred in dark for 0.5 h to ensure the adsorption equilibrium and formation of homogeneous suspension. The temperature of the reaction system was maintained at ~ 15 oC. During the reaction process, 2 mL reaction solution was taken out every 10 min and diluted to the same volume after the suspended photocatalyst was filtered off with a Millipore filter (pore size 0.22 μm). The solution was then analyzed on a 9



TU-1900 UV-Vis spectrophotometer (Beijing Purkinje General Instrument. Co., Ltd.) within the range of 400-700 nm for monitoring the reaction process. In the control experiments, 2 mmol t-butanol, 2 mmol EDTA, and 100 mmol methanol were used in the reaction system, respectively. Photocatalytic degradation of RhB can be approximated as the pseudo-first-order reaction when the initial concentration is relatively low. According to the Langmuir–Hinshelwood kinetic model, the reaction rate constant k (min1) can be obtained by the formula ln(C/C0) = kt, where C and C0 represent the RhB concentrations at time t and initial time. By the linearly fitted slope between ln(C/C0) and t, the value of k can be obtained. 2.4.2 Photocatalytic Hydrogen Production Photocatalytic hydrogen evolution from water was carried out in a CEL-SPH2N photocatalytic activity evaluation system (Beijing China Education Au-Light Co., Ltd) at 15 oC, irradiated by a 300 W Xe-lamp (PLS-SXE300C, Beijing China Perfectlight Co., Ltd). Typically, to a 100 mL aqueous solution with 20 vol% of sacrificial agent, 50 mg photocatalyst was dispersed and followed by addition of 0.1 wt% Pt as cocatalyst (in-situ photo-precipitation). To ensure the reaction system under inert condition, the suspension was degassed with a mechanical pump for 10 min to completely remove the dissolved oxygen before illumination. A magnetic stirrer was located at the bottom of the reactor to maintain good dispersion of the photocatalysts throughout the experiment. The evolved hydrogen was analyzed by gas chromatograph (GC) using a thermal conductivity detector (TCD) with argon as carrier gas. 10


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2.5 Density-Functional Theory (DFT) Calculations All DFT calculations were performed using the VASP program.48, 49 The exchange correlation potential was described by the Perdew-Burke-Ernzerhof functional (PBE) within the generalized gradient approximation.50 The projector-augmented wave method was applied to describe the core-valence electron interaction.51, 52 The valence electronic states were expanded in plane wave basis sets with energy cutoff at 400 eV. Full optimization of cell parameters for bulk TiO2(B) has been carried out by a 375 Monkhorst-Pack type k-point sampling. The calculated lattice parameters, a=12.230 Å, b=3.751 Å, c=6.557 Å, and β=107.03, were in good agreement with experimental data .53, 54 The TiO2(B) (100) surface was constructed from the optimized structure of bulk. The vacuum layer between slabs was 15 Å. A 7  5  1 Monkhorst-Pack k-point mesh was applied on structural calculations of (100) surface. The geometries were considered to be converged until forces were smaller than 0.01 eV/Å.

3. Results and Discussion

3.1 Material Characterizations

3.1.1 Phase Structures of HT Photocatalysts with Various Amount of HF

XRD and UV-Raman spectroscopy are the sensitive technologies for the characterization of the phase structure of TiO2. Here, x%F-HT series photocatalysts were investigated by XRD and UV-Raman spectroscopy. Figure 1a showed the XRD patterns of the x%F-HT photocatalyst. The typical XRD peaks of anatase are located 11



Figure 1. (a) XRD patterns and (b) UV-Raman spectra of the photocatalysts (,Anatase;,TiO2(B)).

at 25.3, 37.8 and 48.0o (red line, JCPDS 21-1272), while the diffraction peaks of TiO2(B), are located at 14.2, 15.2, 24.9 and 28.6o (black line, JCPDS 46-1238). In the absent of HF, the HT (0.0%F-HT) was completely transformed into anatase. When a small amount of HF was added, the diffraction peaks at 14.2, 15.2 and 28.6o were observed, indicating the presence of TiO2(B) in the structure. The highest amount of TiO2(B) was obtained when the amount of HF was at 0.3%. Further increasing the amount of HF, TiO2(B) structure gradually disappeared. The x%F-HT photocatalyst were further investigated by UV-Raman spectra. As shown in Figure 1b, the peaks at 250 and 400 cm–1 are the typical characteristic Raman bands of TiO2(B) and anatase, respectively.55-57 The intensity variation for the band of TiO2(B) (~ 250 cm–1) was not clear as it in XRD patterns. However, the band can still be observed for 0.1%F-HT and 0.3%F-HT, implied that its intensity firstly increased and then decreased. In these spectra, one new peak at 320 cm–1 appeared and it increased with the amount of HF, which may be due to the HF-related structures on the surface of TiO2. Generally, TiO2(B) can be easily obtained by low temperature thermal treatment ( ~ 400 oC) of HT. With increasing the temperature, the phase structure of TiO2(B) will 12


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gradually transform into anatase. Typically, TiO2(B) would not maintain the phase structure up to 700 oC.45, 57 Therefore, 0.0%F-HT, which was obtained at 750 oC, was in 100% anatase structure. However, some interesting results were observed when small amount of HF added in prior to the calcination process, that the metastable phase TiO2(B) was still maintained even the calcination temperature was up to 750 oC. The presence of HF was apparently the crucial factor, which inhibited the phase transformation process of TiO2(B) to anatase. Our precious results revealed that the phase transformation of TiO2(B) to anatase initiated from the surface region. The presence of HF adsorbed on the surface may decreased the surface energy of TiO2(B), which could make a more thermodynamically stable surface and results in the retarded phase transformation process. However, further increased the amount of HF, the corrosion effect became dominant and the destroyed surface of the material might be back to an unstable state.

3.1.2 Morphology of x%F-HT Series Photocatalysts

Figure 2 showed the SEM images of HT and x%F-HT series samples. The precursor HT was in nanowire structure with regular shape and smooth surface (Figure 2a). After annealing at 750 oC for 2 h (0.0%F-HT, Figure 2b), the nanowires structure of HT began to decompose and a more compact fiber-like structure was obtained with decreased width of ~ 100 nm. However, the resulted fiber-like structure

13



Figure 2. SEM images of (a) HT (b) 0.0%F-HT, (c) 0.1%F-HT, (d) 0.3%F-HT, (e) 0.5%F-HT, (f) 1.0%F-HT, (g) 3.0%F-HT

of 0.0%F-HT was actually constructed with some shorter nanorods. Interestingly, when a small amount of HF was added in (0.1~0.3%HF), the decomposition rate of nanowires was restrained. As shown in Figure 2c and 2d, the nanofibers were in a width of 100 ~ 300 nm and the amount of shorter nanorods obviously decreased, indicating that the phase transformation process was efficiently retarded. Further increasing the amount of HF, the decomposition rate of nanowires seemed accelerated. For 0.5%F-HT (Figure 2e), the nanofiber structure was seriously destroyed with irregular edges appeared and it began to transform into shorter nonarods again. These short nanorods further transformed into nanoparticles in 1.0%F-HT (Figure 2f) and 14


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Figure 3. (a) TEM and (b) HRTEM images of 0.3%F-HT photocatalyst (The yellow circles show the TiO2(B)/anatase heterophase junctions).

accumulated into even larger ones in 3.0%F-HT (Figure 2g). The destroyed surface of HT was also observed in Figure S1. As can be seen, the surface of HT has been seriously destroyed after HF treatment with the smooth surface morphology totally disappeared. Combined with the XRD patterns, it could be speculated that the fiber-like morphology belongs to TiO2(B) while the short nanorods or nanoparticles are in anatase structure. Taking into consideration of XRD (Figure 1a) and SEM (Figure 2), 0.0%F-HT and 3.0%F-HT with 100% anatase were constructed with short nanorods and/or nanoparticles, while 0.3%F-HT in fiber-like structure contained both anatase and TiO2(B). The detailed structure of 0.3%F-HT was further investigated by TEM and HRTEM. Figure 3a showed the TEM image of 0.3%F-HT, the fiber-like structure was TiO2(B) with a width between 100 ~ 200 nm and some anatase nanoparticles was dispersed on the main body of TiO2(B) with a size around 50 nm (in yellow circles). 15



To ensure the presence of phase junction structure in the sample, HRTEM image was performed. As shown in Figure 3b, the top right region with a lattice-spacing value of 3.6 Å corresponded to the (110) planes of TiO2(B) (JCPDS 46-1238), while the dark bottom region with a lattice-spacing value of 3.5 Å belonged to the (101) planes of anatase (JCPDS 21-1272). Both TEM and HRTEM results clearly evidenced that the TiO2(B)/anatase phase junction was formed in 0.3%F-HT. 3.1.3 Porous structure and the surface areas of x%F-HT series photocatalysts The porous structure and the surface areas of x%F-HT series photocatalysts were investigated by the N2 adsorption-desorption test. As shown in Figure 4, no obvious hysteresis loop for 0.0%F-HT and 3.0%F-HT was observed, indicating no porous structure existed in the samples. Other samples with different content of HF showed type-IV isotherms with H3 hysteresis loops, indicating some mesoporous structures existed in these samples. Besides, 0.0%F-HT showed the lowest surface area of only 4.7 m2/g (inset in Figure 4). With increasing the amount of HF, the surface area firstly

Figure 4. Nitrogen adsorption-desorption isotherms and the BET surface areas (inset) of the photocatalysts. 16


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increased and then decreased. The highest surface area was obtained for 0.5%F-HT, which was 4 times higher than that of 0.0%F-HT. Normally, TiO2(B) is apt to containing porous structure while anatase is in more compact structure. Totally phase transformation process will lead to a compact structure of the samples and result in alow surface area. Besides, etching effect of HF on the surface of TiO2 can also promote the increase of the surface area. Therefore, it could speculate that the increase of the surface area for x%F-HT series photocatalysts is due to the intrinsic structure of TiO2(B) together with the etching effect of HF. 3.2 Photocatalytic Performance Photocatalytic activity of x%F-HT series photocatalysts were investigated via degradation reactions using RhB as target molecule. Figure 5a showed the degradation rate of RhB using different samples. Compared to 0.0%F-HT photocatalyst without HF, the degradation rate of RhB over x%F-HT series photocatalysts gradually increased with increasing the amount of HF, and 0.3%F-HT photocatalyst showed the best degradation activity of RhB. Further increase in the amount of HF, the activity of x%F-HT photocatalyst gradually decreased. The reaction rate constant k can be calculated according to the method described in section 2.4.1. As shown in Figure 5b, the reaction rate constants from 0.0%F-HT to3.0%F-HT were 0.038, 0.043, 0.053, 0.039, 0.028, and 0.010 min1, respectively. Among them, 0.3%F-HT exhibited the highest rate constant, indicating that the high content of TiO2(B)/anatase heterophase junctions in 0.3%F-HT could effectively promote the separation of photogenerated charge and increase the photocatalytic activity. The 17



Figure 5. Photocatalytic degradation of RhB over x%F-HT series photocatalysts: (a) normalized concentration (C/Co) versus time; (b) pseudo-first-order kinetic plots. Recycle reactions (c) and effect of scavengers EDTA, CH3OH and t-BuOH (d) on the photocatalytic degradation of RhB over 0.3%F-HT photocatalyst.

stability properties of samples were evaluated as shown in Figure 5c. The conversionratio of the reaction at 90 min were 98.4%, 91.1%, 97.5% and 90.1% for the first, second, third and fourth reactions, respectively. It indicated a good stability of 0.3%F-HT photocatalyst for that the degradation rate could still be maintained over 90% after 4 cycles of reactions. Furthermore, 10 cycles of reactions was also performed and the degradation ratio can still be maintained above 80% (Figure S2). The active species for the photo-oxidation reactions mainly include superoxide radicals (O2•−), hydroxyl radicals (•OH), and photo-generated holes (h+).58-60 To 18


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Figure 6. EPR spectra for the reactive species generated from 0.3%F-HT photocatalyst in the presence of DMPO in (a) ultrapure water and (b) methanol. Concentrations: 0.3%F-HT photocatalyst, 2 mg/ml; DMPO, 40 mM; RhB, 4 mg/L.

reveal the predominant active species in this system, control experiments were performed. T-BuOH, EDTA, and methanol were introduced separately as quenchers for •OH, h+, and O2, respectively. As shown in Figure 5d, the addition of t-BuOH carried out using DMPO as trapping agent (Figure 6). No signal was detected in dark showed limited effect on the reaction system and the activity could still be maintained at a high level, indicating that •OH may play a minor role in the oxidation reaction. On the contrary, the reaction was significantly depressed after the addition of EDTA, indicating that h+ is one of the most important active species of this reaction, which may directly react with RhB. In the addition of methanol, the reaction rate has been further decreased, revealed that O2 played a crucial role in the reaction process. To obtain the direct evidence of radical species in this reaction, EPR experiments were condition. A typical •OH signals were detected under irradiation in Figure 6a61, which matched very well with the simulation spectrum (Figure S3a). A typical O2•− signals 19



were detected under irradiation in Figure 6b and it was also matched well with the simulation spectrum (Figure S3b). Both the two signals increased with the irradiation time. Furthermore, in the presence of RhB, the signal intensity of •OH and O2•− obviously decreased, indicating these two radicals could be consumed in the photocatalytic process. O2•− can be generated by dissolved O2 with trapped electron, therefore the presence of O2 is crucial important for the generation of O2•−. Combined with control experiments and EPR characterization, it could be concluded that, O2•− was the predominant radical species in this system; •OH played a minor role in the reaction. Besides, photo-generated h+ may directly react with target molecule and contribute to the oxidation reaction. Photocatalytic performance was also evaluated by H2 evolution reactions. The effects of different sacrificial reagents on hydrogen production were first investigated as shown in Figure 7a. Among all the tested reagents, ethanol showed the highest hydrogen production rate, while EDTA exhibited the lowest hydrogen evolution rate. Alcohols, including methanol, ethylene glycol (EG) and glycerol showed the modest hydrogen production rate. For different alcohols, polyhydric alcohols with high viscosity performed a relatively lower activity because of the restriction of its diffusion. In addition, methanol could easily produce by-product CO in the photocatalytic reactions, thus the poisoned cocatalyst always leads to an unstable activity.62-64 Therefore, hydrogen production rate for the x%F-HT series photocatalysts were evaluated using ethanol as sacrificial reagent. As shown in Figure 7b, in the absent of HF, 0.0%F-HT with 100% anatase showed a fairly well activity. 20


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Figure 7. Photocatalytic H2 evolution rate in the presence of different sacrificial reagents over 0.3%F-HT (a) and over x%F-HT series photocatalysts using ethanol as sacrificial reagent (b).

When the amount of HF was increased to 0.3%, 0.3%F-HT with a mixed phase structure exhibited an increased hydrogen production rate up to 1170 μmol/g/h. Further increasing the amount of HF, the content of TiO2(B) in the samples dramatically decreased and the hydrogen evolution activity decreased obviously. It can be confirmed that the heterophase junction formed in the catalyst can significantly promote the separation of photo-generated electrons and holes. Generally, the photocatalytic activity will be enhanced as the number of heterophase junction increases. Therefore, 0.3%F-HT with the largest amount of heterophase junctions showed the highest activity both in degradation and hydrogen evolution reactions. Besides, photoelectrochemical (PEC) measurements were carried out using the photocatalysts loaded thin film electrodes. As shown in Figure S4a, 0.0%F-HT and 0.1%F-HT showed the highest photocurrent, other electrodes showed the similar lower photocurrent. The EIS data in Figure S4b showed that, 0.3%F-HT exhibited the largest radius of the Nyquist-plot semicircle, which reflected a slower faradaic charge transfer at the interface. The results in PEC measurements seemed contradictory to the 21



photocatalysis process. 0.3%F-HT exhibited the best photocatalytic activity but the worst PEC performance. In fact, the heterophase junction beneficial for photocatalytic activity could show a negative effect in PEC performance. It is due to much more severe interfacial recombination occurs in the PEC reaction when charge carriers migrate across semiconductor particles to reach a conducting substrate.65 The more heterophase junction exists, the severer interfacial recombination occurs. 3.3 Insight into the Function of HF in the Photocatalytic System 3.3.1 Chemical States of the Catalysts with the Presence of HF As discussed in section 3.1, the addition of HF can retard the phase transformation of TiO2(B) and promote the formation of TiO2(B)/anatase heterophase junction. However, the chemical state of F element and whether it can influence the Ti and O elements in the catalyst are still unclear. Thus, the XPS spectra were performed. Figure 8 showed the bonding energy of Ti 2p, O 1s and F 1s for 0.0%F-HT and 0.3%F-HT before and after the reaction. Ti 2p spectra (Figure 8a) for the three samples were located at around 458.0 and 463.7 eV, which are assigned to the Ti 2p3/2 and 2p1/2 peaks of Ti4+ in TiO2.66 The three samples showed the negligible difference of binding energy on this position, indicating the similar chemical state of Ti. Figure 8b showed the O 1s spectra of the samples. The spectra was deconvoluted into three peaks, wherein the highest peak at ~529.4 eV represented the lattice oxygen in TiO2,46 the broader peak at ~531.4 eV was ascribed to the Ti-OH at the surface related with the oxygen vacancy, and the smallest peak at ~532.9 eV was attributed to the adsorbed O267 or free-OH species.68 Apparently, for 0.0%F-HT the intensity of the 22


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Figure 8. Ti 2p (a), O 1s (b) and F 1s (c) XPS spectra for 0.0%F-HT and 0.3%F-HT before and after the reaction.

peak at ~ 533.0 eV was the smallest. After HF was introduced, a slightly increased intensity of this peak was obtained. These results indicated that the addition of HF on the catalyst could promote the adsorption of O2 on the surface of the catalyst, which was beneficial for the photo-oxidation reaction. After the reaction, both the peaks ascribed to the Ti-OH (~ 531.4 eV) and adsorbed O2 or free-OH species (~ 532.9 eV) increased, which was due to the interaction between catalysts and water in the reaction system. The bonding energy of F 1s for 0.3% F-HT before and after the reaction was also analyzed in Figure 8c. The F 1s spectrum was located at 688.6 eVbefore the reaction, while this peak was totally absent after the reaction. It can be speculated that the F element in the catalyst could exist as physically adsorbed F anions on the surface of TiO2 rather than doping into TiO2 forming a fixed Ti-F bond, 23



Figure 9. Photocatalytic degradation of RhB (a) and hydrogen evolution (b) over 0.0%F-HT photocatalyst with/without presence of HF in the reaction solution.

therefore it can be easily exfoliated from the surface of the catalyst after the reaction. Apparently, the adsorbed F anions on the surface of TiO2 also have little influence on the bandgap of the x%F-HT series photocatalysts as evidenced in Figure S5. As the F anions were only adsorbed on the surface of TiO2, the contribution of the adsorbed F anions for the photocatalytic activity should be clarified. Figure 8 shows the photocatalytic reactions over 0.0%F-HT photocatalyst with/without the presence of HF in the reaction solution, including RhB degradation (Figure 9a) and hydrogen production (Figure 9b). In the control experiments, HF was directly dropped into the reaction system prior to the reaction with the HF amount equal to that in 0.3%F-HT. It was clearly shown that, no big difference was obtained between 0.0%F-HT and the control experiments in the two types of photocatalytic reactions. It was confirmed that the adsorption of F anions on the surface of TiO2 did not contribute to the photocatalytic activity, while the phase structure especially the heterophase junction formed in 0.3%F-HT could be the crucial factor for the high activity. 24


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3.3.2 The Surface Energy of TiO2(B) Influenced by the Present of F As mentioned in 3.1.1, the present of HF may change the surface energy of TiO2(B) and finally results in a retarded phase transformation process. To confirm our hypothesis, DFT calculations on the surface energy of TiO2(B) with and without the F anions adsorption were performed. The surface energy was calculated by the following equation,  = (Eslab – NETiO2 – NFEF )/2A, where Eslab is the total energy of the surface slab, ETiO2 is the energy per unit of TiO2, N is the total number of unit TiO2 contained in the slab, EF = 1/2EF2, EF2 indicates the total energy of F2, NF is the number of adsorbed F atoms, and 2A is the total exposed area of the two identical sides of the slab. The calculated  values are 0.63 J/m2 for the clean surface and 0.22

Scheme 1. Surface energy variation of TiO2(B) with/without the F anions adsorption. The insets are the relaxed structures of TiO2(B) (100) surface with and without adsorbed F anions. The grey, red and light blue balls represent Ti, O and F atoms, respectively.

25



J/m2 for fluorine anions adsorbed surface, which indicates that the adsorption of F anions remarkably stabilizes (100) surface (Scheme 1). 3.3.3 Retarded Phase Transformation Mechanism of TiO2(B) According to the above structural characterizations and discussions, the phase transformation of TiO2(B) with absence or presence of HF was represented in Scheme 2. Typically, HT will transform to TiO2(B) at a low temperature around 400 oC. If the annealing temperature was up to 700 oC, 100% anatase could be obtained with fiber-like morphology (Scheme 2a). In the presence of a small amount of HF, the surface energy of TiO2(B) was dramatically decreased and the more stable surface would retard the phase transformation process. In this case, major part of TiO2(B) would be maintained with minor amount of anatase islands dispersing on the bulk of TiO2(B) (Scheme 2b). Further increase the amount of HF, the corrosion effect of HF

Scheme 2. Schematic representation of phase transformation of TiO2(B) with/without the presence of HF

26


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became dominant and the destroyed surface of HT or TiO2(B) would totally transform into anatase with the fiber-like morphology collapsed (Scheme 2c). Besides, the largest surface area was obtained for the sample with proper amount of HF, which was not only related to the content of TiO2(B) but also due to the moderate etching effect of HF.

27



Conclusion: In summary, we have successfully fabricated TiO2(B)/anatase heterophase junctions at high temperature via a retarded phase transformation process. Appropriate amount of HF (0.3wt%) could obviously delay the phase transformation process of TiO2(B) to anatase and result in a mixed phase structure. As demonstrated by DFT calculations, the F anions adsorbed on the surface of TiO2(B) could efficiently decrease the surface energy from 0.63 J/m2 for the clean surface to 0.22 J/m2 for the F anions adsorbed surface, which was the internal factor for the retarded phase transformation process. Photocatalytic reactions showed that, 0.3%F-HT with the largest amount of TiO2(B)/anatase heterophase junctions showed the best activity both in pollution degradation and hydrogen production reactions. This work provides an effect approach for the control of the phase structure of TiO2(B) at high temperature and will definitely extend the application of TiO2(B)-related material on photocatalysis, electrocatalysis and lithium-ion battery in the future. ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications website at DOI: SEM image of HT treated with 3.0%HF; Recycle reactions of photocatalytic degradation of RhB over 0.3%F-HT photocatalyst; EPR spectra and corresponding simulation spectra for the reactive species generated from 0.3%F-HT photocatalyst; Photocurrents and Nyquist plots of different samples; UV-Vis diffuse reflectance spectra of the photocatalysts(PDF)

ACKNOWLEDGMENT 28


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This work was supported by the National Natural Science Foundation of China (21603134 ， 21473183), Young Talent Fund of University Association for Science and Technology in Shaanxi, China (20150104), Natural Science Basic Research Plan in Shaanxi Province of China (2016JQ2023), Open Fund of State Key Laboratory of Catalysis (N-17-06), and the Fundamental Research Funds for the Central Universities (GK201603032). REFERENCES (1) Ma, W. G.; Wang, H.; Yu, W.; Wang, X. M.; Xu, Z. Q.; Zong, X.; Li, C., Achieving Simultaneous CO2 and H2S Conversion via a Coupled Solar-Driven Electrochemical Approach on Non-Precious-Metal Catalysts. Angew. Chem. Int. Ed. 2018, 57, 3473-3477. (2) Xiao, F. X.; Miao, J. W.; Tao, H. B.; Hung, S. F.; Wang, H. Y.; Yang, H. B.; Chen, J. Z.; Chen, R.; Liu, B.,

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Anatase Heterophase Junctions at High

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