Simultaneous Oxidation of p-Chlorophenol and Reduction of Cr(VI

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Simultaneous oxidation of p-chlorophenol and reduction of Cr(VI) on fluorinated anatase TiO2 nanosheets with dominant {001} facets under visible irradiation Zhiqiao He, Lixian Jiang, Da Wang, Jianping Qiu, Jianmeng Chen, and Shuang Song Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie503997m • Publication Date (Web): 02 Jan 2015 Downloaded from http://pubs.acs.org on January 6, 2015

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Industrial & Engineering Chemistry Research

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Simultaneous oxidation of p-chlorophenol and reduction of Cr(VI) on

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fluorinated anatase TiO2 nanosheets with dominant {001} facets under visible

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irradiation

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Zhiqiao HE, Lixian JIANG, Da WANG, Jianping QIU, Jianmeng CHEN, Shuang

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SONG*

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College of Biological and Environmental Engineering, Zhejiang University of

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Technology, Hangzhou 310032, People’s Republic of China

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* Corresponding author:

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Shuang SONG

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Tel.: 86-571-88320726; Fax: 86-571-88320276.

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E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT

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A series of fluorinated anatase TiO2 nanosheets with dominant {001} facets were

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synthesized to oxidize p-chlorophenol (PCP) and reduce Cr(VI) simultaneously under

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visible light. The {001}/{101} surface facets ratio of TiO2 was controlled by varying

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the initial HF concentration, and fluorine-free samples were obtained by

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alkaline-washing. A synergistic effect among TiO2, Cr(VI), and PCP was observed,

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which is ascribed to effective trapping of photogenerated electrons and holes by Cr(VI)

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and PCP, respectively. A maximum synergistic effect was obtained at a molar ratio of

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[PCP] to [Cr(VI)] of 1. Using X-ray diffraction, X-ray photoelectron spectroscopy,

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Electron paramagnetic resonance spectroscopy, Field-emission scanning electron

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microscopy, Transmission electron microscopy, and Brunauer–Emmett–Teller

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analyses, the optimum ratio of exposed {001} to {101} facets for TiO2 was

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determined to be 80:20 because of selective transfer and charge balance of

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photogenerated carriers. Surface fluorination facilitates the formation of oxygen

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vacancies and unsaturated Ti atoms, which is useful for visible light activity induction,

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for extending the lifetime of photogenerated electrons-holes pairs, and for enhancing

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the rate of PCP oxidation and Cr(VI) reduction.

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2


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1. INTRODUCTION The discharge of effluent containing heavy metal ions is usually also accompanied

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by aromatic pollutant discharge from various industries involving wood preserving,

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heavy metal finishing, petroleum refining, leather tanning and finishing, paint and ink

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formulation, and automobile part manufacture.1,2 The oxidative and reductive abilities

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of heavy metal ions and aromatic compounds, respectively, allow us to explore

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effective methods for simultaneous heavy metal ion and aromatic compound

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removal.3-6 Heterogeneous photocatalysis, as a promising process for solar energy

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utilization and environmental cleanup, provides a possibility for the complete

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oxidation of organic pollutants with photogenerated holes (h+), and the effective

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reduction of heavy metal ions with photogenerated electrons (e−).7-9 TiO2 is one of the

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most promising photocatalysts because of its biological and chemical inertness, cost

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effectiveness, nontoxicity, and excellent redox ability.10,11

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The photocatalytic activity of TiO2 is very dependent on its bulk and surface

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properties and the nature of the catalytic reaction involved. The equilibrium crystal

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shape for anatase TiO2 is generally a truncated square bipyramid enclosed with two

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flat square surfaces ({001} facets) and eight isosceles trapezoidal surfaces ({101}

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facets),12,13 which exposes the majority {101} and minority {001} facets.14,15 Most

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anatase TiO2 nanocrystals are dominated by the thermodynamically stable {101}

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facets because of their lowest surface energy (0.44 J m–2), rather than the more

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reactive {001} facets with relatively higher surface energy (0.90 J m–2).16,17 Since the

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pioneering work of Lu et al.,18 the morphology and crystal facet-controlled synthesis 3



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of TiO2 has attracted considerable attention to improve its photoelectric and

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photocatalytic properties. Anatase TiO2 with an exposed {001} facet has been applied

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to decolorize dyes, degrade organic chemicals,19 and reduce heavy metal ions and

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CO2.20-23 However, previous work has focused on the single utilization of either

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photo-generated h+ or e−. These studies have rarely been concerned with the

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simultaneous utilization of h+ and e− for effective oxidation and reduction. The {101}

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and {001} facets of anatase TiO2 exhibit different band structures and band edge

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positions based on density functional theory (DFT) calculations.24 The lower

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conduction band potential of anatase (101) compared with that of anatase (001) results

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in a directed transfer of photogenerated electrons and holes to {101} and {001} facets

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during photocatalysis, respectively.25 The ratio of exposed {101} and {001} facets has

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a pronounced effect on the photocatalytic activity of TiO2 enclosed with {101} and

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{001} facets and thus deserves special attention.

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Accompanied with the use of F− as capping group for developing various TiO2

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nanostructures with high reactive facets, increased attention has been given to the role

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of surface fluorination in photocatalytic reactions.26-29 Surface fluorination can be

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achieved easily by a simple ligand exchange reaction between surface hydroxyl

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groups on TiO2 and F−, as described in equation (1): ≡Ti−OH + F− ↔ ≡Ti−F + OH−

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(1)

Surface fluorination can increase photocatalytic activity by enhancing Ti3+

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production (unsaturated Ti atoms), retarding the recombination of

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e−/h+ pairs because of the maximal electronegativity of fluorine, and extending the 4


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optical absorption to the visible light region by producing a sufficient concentration of

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oxygen vacancies (and Ti3+).30-32 However, a negligible or even unfavorable effect of

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surface fluorination was observed for the photocatalytic oxidation of aromatic air

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pollutants such as toluene, benzene, and m-xylene.26,32 The conflicting role of F on

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TiO2 has prompted us to explore the influence of surface fluorination on the

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photocatalytic simultaneous degradation-reduction of organics and heavy metal ions.

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In this study, the synergistic effect of aqueous ternary systems

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(TiO2/p-chlorophenol (PCP)/Cr(VI)) was validated by comparing the average removal

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rates (ARR) of PCP and Cr(VI) with the binary systems (TiO2/PCP or TiO2/Cr(VI))

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during visible induced photocatalysis. Furthermore, an optimum molar ratio of

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concentration of PCP to Cr(VI) based on the maximum synergistic effect was

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determined. Comparative studies were carried out to determine the role of fluorine on

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the TiO2 surface and the optimal ratio of exposed {001} to {101} facets for PCP

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photo-oxidation and Cr(VI) photo-reduction over TiO2 nanosheets.

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2 EXPERIMENTAL SECTION

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2.1 Materials and reagents

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All major chemicals were of reagent grade or higher, and were used as received

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without further purification. Potassium dichromate (K2Cr2O7, 99.9%) was the Cr(VI)

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source and was purchased from Wuxi Haishuo Biology Co., Ltd., Wuxi, Jiangsu,

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China. PCP (99%) was used as another target compound and was obtained from

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Huadong Medicine Co., Ltd. Hangzhou, Zhejiang, China. Absolute ethanol (99.5%), 5



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Ti(OC4H9)4, and hydrogen fluoride (HF, 40 wt.%) were used to prepare TiO2

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nanosheets, and were obtained from Huadong Medicine Co., Ltd. Hangzhou, Zhejiang,

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China. Concentrated H2SO4 (98 wt.%), 1,5-diphenylcarbazide (DPC, 98%, redox

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indicator), KMnO4 (99.5%) and acetone used to determine the Cr(VI) content were

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purchased from Aladdin Reagent Co., Ltd. Shanghai, China. Distilled water was used

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for all synthesis and photocatalysis processes.

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2.2 Preparation of {001}-faced TiO2

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TiO2 nanosheets were synthesized using a facile hydrothermal route with tetrabutyl

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titanate as titanium source and 40% HF acid solution as the shape-controlling reagent.

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An appropriate amount of concentrated HF solution was added to 50 mL of

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Ti(OC4H9)4 with vigorous stirring at room temperature for 30 min. The resultant

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mixture was transferred to a 200 mL Teflon-lined stainless steel autoclave, followed

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by hydrothermal treatment at 180°C for 24 h. After cooling naturally, the solid product

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was collected by centrifugation and washed several times with absolute ethanol and

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distilled water, and finally dried at 60°C for 10 h. The as-prepared catalyst was

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denoted TiO2-nHF, where n represents the amount of concentrated HF added (in mL).

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The hydrothermal product was treated further with 0.1 M NaOH to replace the F

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groups with hydroxyl groups, washed with distilled water and dried at 60°C. The

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resultant product was termed TiO2-nOH. Control samples were prepared using the

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same conditions but with distilled water instead of HF to yield samples TiO2-H2O.

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2.3 Catalyst characterization The catalyst crystal structure was characterized using a Thermo ARL X-ray

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diffractometer (XRD, model X’TRA 17) with a Cu Kα radiation source operated at 45

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kV and 40 mA for a 2θ from 20° to 80° at 0.01° min−1. To obtain the morphology of

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the TiO2 samples, Field-emission scanning electron microscopy (FESEM) was

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performed with a Hitachi S−4800 instrument with a 10 kV accelerating voltage.

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Transmission electron microscopy (TEM, FEI Tecnai G2 F30) was used to observe

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the catalyst microstructure at an accelerating voltage of 300 kV with a 0.20 nm

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resolution. To analyze the surface-bonded states and elemental composition of the

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photocatalysts, X-ray photoelectron spectroscopy (XPS) was carried out on a

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RBD-upgraded PHI 5000 C ESCA system (PerkinElmer) with Mg-Kα radiation (hν =

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1253.6 eV). After dried in vacuum at 55 °C for 4 h, The TiO2 powders were sprinkled

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onto the surface of sticky carbon tape for analysis. Electron paramagnetic resonance

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(EPR) data were collected at 77 K on a Bruker EMS plus X-band spectrometer

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(microwave frequency: 9.40 GHz, microwave power: 20 mW, modulation amplitude:

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100 kHz). Catalyst Brunauer–Emmett–Teller (BET) surface area was determined by

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analyzing the N2 adsorption–desorption isotherm in the 0.05-0.30 relative pressure

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ranges at 77 K on a Micromeritics ASAP 2010 system.

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2.4 Photocatalytic performance An evaluation of the photocatalytic performance of samples for the synergistic effect of PCP oxidation and Cr(VI) reduction was performed in a 500 mL glass 7



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chamber reactor (10 cm diameter, 11 cm height). The reaction temperature was

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maintained at 20.0 ± 1.0°C by circulating water around the reactor. Cooling water was

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supplied from a thermostat (THD-2015, Tian heng Instrument Factory, Ningbo,

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China). A 500 W Xe arc lamp (Beijing Electric Light Sources Research Institute,

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Beijing, China) with a 420 nm cutoff filter was used as the light source. The distance

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between the xenon lamp and the front surface of the reactor was 20 cm.

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For simultaneous photocatalytic reaction, prior to illumination, known amounts of

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PCP (0.1 mM, 0.2 mM, 0.4 mM, 0.6 mM, 1 mM), 18 mg K2Cr2O7, and 0.3 g catalyst

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were added into 0.3 L distilled water followed by pH adjustment to 3.0 with H2SO4

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solution. Subsequently, the Xenon lamp was turned on to start the photocatalytic

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reaction. A sample of 5 mL suspension was taken at a certain time interval and filtered

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through a 0.45 µm syringe filter to separate the TiO2 particles and then analyzed

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immediately to avoid further reaction. Single degradation of PCP and Cr(VI)

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reduction were conducted under analogous operating procedures.

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The ARR is used to evaluate the activities of catalysts in PCP and Cr(VI) removal,

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which is defined as the decrease in PCP and Cr(VI) concentration per unit time at the

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end of the reaction.

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2.5 Sample analysis The residual concentration of Cr(VI) in solution was measured using

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spectrophotometry (DPC method).33,34 The measurement of absorbance was carried

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out at a maximum wavelength of 540 nm using a UV–visible spectrophotometer (T6, 8


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Beijing Purkinje General Instrument Co., Ltd., Beijing, China). Changes in PCP concentration were monitored by high performance liquid

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chromatography system (1200 series, Agilent, USA). The instrument was equipped

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with a reverse-phase Zorbax Eclipse XDB−C18 column (150 mm × 4.6 mm, Agilent)

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at 293 K. A mixture of acetic acid/acetonitrile/water (1:35:64, vol %) was used as

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mobile phase at 1 mL min−1 with a total injection volume of 5 mL. The retention time

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was 14 min and the wavelength set for quantification was 280 nm.

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3 RESULTS AND DISCUSSION

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3.1. Catalyst characterization

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3.1.1. XRD analysis

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As shown in Figure 1, characteristic diffraction peaks for TiO2 can be observed at

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2θ of 25.271°, 37.698°, 47.98°, 54.992°, and 62.572°, which are in good agreement

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with the (101), (004), (200), (211), and (204) crystal planes of tetragonal anatase TiO2

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(JCPDS 01-071-1167), respectively. No characteristic impurity peaks were observed

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in the samples, which confirms the formation of the pure anatase phase structure. A

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slight shift of the peaks toward the lower angle (high interplanar distance) with initial

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HF concentration was observed, which indicates the minor expansion of the crystal

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lattice. In contrast, Lu et al.,35 believe that the formation of more oxygen vacancies

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(demonstrated by the XPS analysis) upon fluorination would move the center of the

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diffraction peaks towards a higher angle range. Considering the experimental process,

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the shift in diffraction peaks is complicated depending on the extent of crystal growth 9



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and the amount of defect density formation.36,37 Our results agree closely with those

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of other studies.38 No obvious difference in intensity and narrow full width at half

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maximum (FWHM) was observed between the fluorinated TiO2 and the

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corresponding NaOH-treated samples, which shows that alkaline washing is a

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surface-treatment process and has a negligible effect on crystallographic structure.

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With increasing initial HF concentration, the intensity of the (004) diffraction peak

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(crystal size along [001] direction) decreased, whereas that of the (200) diffraction

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peak (crystal size along [100] direction) increased. This indicates that the TiO2

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nanosheet thickness was reduced and the side was extended by adding HF during the

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preparation process. The thickness and side length of the TiO2 nanosheets can be

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calculated from the FWHM of (004) and (200) based on Scherrer’s equation.39 The

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percentage of exposed {001} facets is calculated according to literature and is listed in

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Table 1.39,40 The percentage of exposed {001} facets increased from 68% to 85% with

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increasing initial HF content from 3 ml to 12 mL, and remained almost unchanged

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before and after NaOH treatment. The TiO2-6HF shows an approximate 80:20 ratio of

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high-energy {001} facets to low-energy {101} facets.

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3.1.2. FESEM and TEM analysis

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Figure 2 shows the SEM micrographs of the fluorinated anatase TiO2 nanosheets

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with different initial HF volume. In the absence of HF during the preparation of TiO2,

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the resulting TiO2 nanocrystals reveal the morphology of large numbers of aggregated

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nanoparticles because of high surface activities and sufficiently strong attractive

10


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forces between the nanoparticles. However, in the presence of HF, the prepared TiO2

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consists of well-defined sheet-shaped structures with an average side length of

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~30–100 nm and a thickness of ~9–18 nm, which confirms that HF serves as the

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facet-controlling reagent in the preparation of high reactivity {001} facets. An

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increased length of TiO2 nanosheets can be achieved by increasing the amount of HF.

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Moreover, washing with 0.1 M NaOH solution appears to alter the area of the (001)

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planes slightly. The top and bottom areas with respect to the total surface area of the

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TiO2 nanosheets can be obtained from a slab model of anatase TiO2 single crystal,41-43

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and the percentage of {001} facets is listed in Table 1. From Table 1, the percentage

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of {001} facets is approximately consistent with the XRD results.

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The typical single-crystal nature of TiO2-6HF is confirmed further by TEM and

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high-resolution TEM (HRTEM) images. The TEM (Figure 3a) showed similar

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morphology to the SEM. From HRTEM (Figure 3b), the lattice spacing parallel to the

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top and bottom facets was determined to be 0.235 nm, which corresponds to the (001)

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planes of anatase TiO2. Lattice fringes with interplanar distances of 0.353 nm were

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observed on the side surface, which are in good agreement with the d-spacing values

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of the thermodynamically stable anatase (101) planes. 43,44 Therefore, the prepared

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TiO2 nanosheets were enclosed solely by high-energy {001} (top and bottom surfaces)

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facets and low-energy {101} (side surfaces) facets.

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3.1.3. XPS analysis XPS analysis was used for the surface characterization of TiO2 samples. The shift 11



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of binding energy was corrected using the C 1s level at 284.6 eV as an internal

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standard. A typical wide-scan XPS survey spectrum of the TiO2 powder is shown in

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Figure 4a. Sharp photoelectron peaks of Ti 2p1/2, Ti 2p3/2, F 1s, O 1s, and C 1s appear

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at binding energies of ~465, ~459, ~684, ~531, and ~284.6 eV respectively. Figure 4b

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shows the high resolution scanning XPS spectra of Ti 2p. Generally, the Ti 2p can be

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de-convoluted into four peaks as Ti3+ 2p1/2, Ti4+ 2p1/2, Ti3+ 2p3/2, and Ti4+ 2p3/2.45 A

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shoulder located between 454–458 eV in the XPS spectra of Ti 2p 3/2 would appear at

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high Ti3+ species concentration. However, no obvious shoulder could be observed in

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this study, which implies a low concentration of Ti3+.45 The binding energy of Ti 2p3/2

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shifts from 458.4 to 458.1 eV with increasing initial HF volume from 3 to 12 mL, and

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implies an increase in percentage of Ti3+ for the fluorinated samples46. Similarly, with

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increasing initial HF concentration, the Ti 2p3/2 peaks shift toward lower binding

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energy for alkaline-treated samples (from 458.7 to 458.3 eV). The origin of the Ti3+

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defects was ascribed to two major factors: one is that the oxygen vacancies at the

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two-fold coordinated bridging sites transfer an extra two electrons to adjacent Ti4+

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sites, and the other is that the added fluorine introduces an electron into the 3d orbital

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of Ti4+ because of charge compensation effects. Ti3+ associated with oxygen defects

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can result in photon excitation in the infrared region whereas F− derived Ti3+ cannot

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cause such excitation.47 Whether the Ti3+ in this study originated from the oxygen

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vacancies can be inferred by fitting the O 1s peaks.

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Figure 4c shows the O 1s peak deconvoluted using Shirley backgrounds and Voigt

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(mixed Lorentzian–Gaussian) functions.48 In the fitting procedure, the O 1s spectrum 12


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is decomposed into three peaks with peak positions at 529.8, 531.2, and 532.6 eV, and

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the FWHM values are derived from literature (Figure 4c).49,50 The 529.8 eV can be

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assigned to the lattice oxygen Ti−O−Ti, 531.2 eV is related to the formation of surface

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OH species Ti−O−H, and the 532.6 eV corresponds to H−O−H bonds of chemisorbed

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H2O. Calculated surface atomic concentrations are summarized in Table 2. With the

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increase in initial HF concentration, the calculation of O atomic percentages on the

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TiO2-nHF and TiO2-nOH surface indicates an increase of surface OH groups

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(Ti−O−H) content and a corresponding decrease of lattice oxygen (Ti−O−Ti) content.

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Moreover, the content of Ti−O−H for the surface-fluorinated catalyst samples was

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consistently higher than that for the corresponding alkali-washed samples.

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Additionally, the atomic ratio of lattice oxygen to Ti of the prepared catalysts was

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found to be lower than two, which is predominantly responsible for the Ti3+ detected.

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The oxygen vacancy can also be found from UV−visible diffuse reflectance spectra.

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The high-resolution XPS spectra of the F 1s region are illustrated in Figure 4d. For

271

all fluorinated samples, the XPS F 1s spectra show a peak at 683.8–684.3 eV,51 which

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corresponds to surface fluoride (≡Ti−F) generated by ligand exchange between F− and

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surface hydroxyl groups on TiO2. No signal at 688.5 eV was detected, which indicates

274

an absence of lattice F. The Ti−F bond can reduce the surface energy of the

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high-reactive {001} facets greatly, making them more stable than {101} facets. As a

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result, anatase TiO2 single crystals with a large percentage of reactive {001} facets

277

were formed. The F 1s XPS peak of TiO2 nanosheets almost disappears with alkali

278

treatment of TiO2-nHF in an aqueous solution of NaOH, which further confirms that 13



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the F is bonded with Ti on the TiO2-nHF surfaces.

280 281 282

3.1.4. EPR analysis EPR is an effective tool to investigate the microscopic nature of paramagnetic

283

defects in solids, and is used to identify the presence of Ti3+ species in TiO2.52 Figure

284

5 shows EPR spectra of representative samples of TiO2-H2O, TiO2-6OH, TiO2-6HF,

285

and TiO2-12HF. Strong EPR signals were observed for samples TiO2-6OH, TiO2-6HF,

286

and TiO2-12HF, whereas no obvious signal existed for the TiO2-H2O. The relatively

287

strong paramagnetic signal at g = 1.984 matches well with the Ti3+ centers, and the

288

signal at g = 2.028 is assigned to superoxide radicals adsorbed on Ti4+ centers or

289

oxygen vacancies.53-56 The relative intensity of signals increased as TiO2-12HF >

290

TiO2-6HF > TiO2-6OH, which indicates that surface fluorination enhances the

291

generation of Ti3+ and the surface fluorinated catalyst has a relatively higher Ti3+

292

concentration than the alkali-treated samples.

293 294 295

3.1.5. BET analysis Nitrogen adsorption–desorption isotherms were measured to determine the specific

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surface areas of the TiO2 samples before and after alkaline washing. The BET

297

measurements are shown in Table 1, in which the change in TiO2 surface areas caused

298

by fluorination can be observed. The as-prepared TiO2-H2O powders show the largest

299

BET value of 170 m2 g–1, and an increase in initial HF volume corresponds to a

300

decrease in BET surface area. The BET surface areas of alkali-treated samples were

301

~5% larger than the corresponding fluorinated one. This is because pores in the TiO2 14


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were cleaned, and accompanied by fluorine removal.

303 304 305

3.1.6. UV−visible diffuse reflectance spectroscopy The light absorption properties of TiO2-nHF, TiO2-nOH, and TiO2-H2O were

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analyzed by UV−visible diffuse reflectance spectroscopy. All samples showed almost

307

the same overlapped intrinsic absorption edge with a threshold of ~400 nm (Figure 6).

308

An obvious absorption beyond 400 nm is visible for samples TiO2-nHF and

309

TiO2-nOH, and the intensity of absorbance in the visible region increases with

310

increasing initial HF concentration. The TiO2-nHF shows a stronger absorption of

311

visible light than the corresponding base-treated samples. This result implies that the

312

band structures of TiO2-nHF and TiO2-nOH were modified by hydrothermal treatment

313

in aqueous HF solution. TiO2 is prone to oxygen deficiency with surface fluorination

314

because of the lack of neutral oxygen atoms. Excess electrons were therefore

315

redistributed among the nearest neighbor Ti atoms around the oxygen vacancy sites,

316

and formed a shallow donor level just below the conduction band minimum of the Ti

317

3d orbital. Electrons are excited to the oxygen vacancy states from the valence band

318

with visible light energy, and a higher number of oxygen vacancies results in stronger

319

optical absorption across the visible light region.

320 321

3.2. Photocatalytic activity

322

3.2.1. Role of PCP and Cr(VI) in the synergistic system

323

To explore catalyst visible light activity, we compared the photocatalytic removal of

15



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

324

PCP and/or Cr(VI) over {001} facets-dominated TiO2 nanosheets in binary and

325

ternary systems, and their disappearance by direct photolysis and dark adsorption

326

(Figure 7). The contribution of direct photolysis appears to be negligible in the overall

327

reaction process, since only 3.2% PCP is removed and there is a 4.1% reduction of

328

Cr(VI) in the absence of TiO2 after 5 h of irradiation. We also neglect the direct

329

interaction between Cr(VI) and PCP, which is justifiable for a very low change in

330

concentration in an additional experiment. The fact that the PCP and Cr(VI)

331

concentrations decreased insignificantly for dark adsorption (Figure 7a,c) further

332

indicates that the chemical reaction hardly contributed to pollutant removal.

333

Figure 7a,c also shows the time dependence of pollutant concentration during

334

binary/ternary photocatalytic removal of PCP and Cr(VI) reduction. The TiO2-6HF

335

displayed low activities toward single PCP degradation (Figure 7a, curve TiO2-6HF,

336

binary) and single Cr(VI) reduction (Figure 7c, curve TiO2-6HF, binary) in water

337

under visible light irradiation, with an ARRs of 5.82 mg L−1 h−1 and 2.38 mg L−1 h−1,

338

respectively. However, in the simultaneous ternary system, the PCP and Cr(VI)

339

removal increased markedly under the same operating conditions. The ARRs for the

340

photocatalytic destruction of PCP and Cr(VI) were estimated to be 10.23 mg L−1 h−1

341

and 4.24 mg L−1 h−1, which are approximately 1.76 and 1.78 times as fast as those in

342

individual systems.

343

Cr(VI) reduction and PCP oxidation may therefore be more efficient only with the

344

presence of all three entities (TiO2, Cr(VI) and PCP) in the reaction system. Choi and

345

Kim have reported that the TiO2 displayed visible photocatalytic activity toward PCP 16


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346

degradation alone.57 They proposed that a surface complexation was generated

347

between TiO2 and phenolic compounds through a phenolate linkage. The

348

complexation enables visible light absorption via ligand-to-metal charge transfer

349

between the substrate and Ti(IV) sites on the TiO2 surface.58 The surface complex

350

formation between TiO2 and PCP can be hindered by TiO2 surface fluorination.

351

According to Choi and Kim,57 the prepared surface fluorinated TiO2-6HF should be

352

resistant toward visible induced degradation of PCP. However, the degradation of PCP

353

over TiO2-6HF under visible light irradiation was still observed in this investigation.

354

This could be because of reasons other than the surface-complex-mediated

355

mechanism.

356

Recently, Ti3+ or oxygen vacancies in TiO2 have exhibited visible light absorption.59

357

Electrons can be excited under visible light from the valence band to some oxygen

358

vacancy states which are located 2.05–2.45 eV above the valence band in anatase

359

TiO2,60,61 or the lower-energy anatase lattice trapping sites.62 In the absence of Cr(VI),

360

molecular oxygen acts as an acceptor of photogenerated electrons in the entire organic

361

photooxidation process, and prevents electron-hole recombination through the

362

formation of reactive oxygen species. The low activity of TiO2-6HF for PCP

363

photo-oxidation is ascribed to electrons created in the interior of TiO2 that cannot be

364

transferred efficiently to surface O2 because of a relatively insufficient concentration

365

of active oxygen vacancies on the particle surfaces.2 In the presence of Cr(VI), PCP

366

served as the photogenerated holes scavenger and was oxidized, whereas Cr(VI) acted

367

as the photogenerated electrons acceptor and was reduced. This is because the 17



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

368

standard redox potential (versus NHE) of Cr2O72−/Cr3+ and O2/O2− is 1.33 V and 0.7 V,

369

respectively. Furthermore, Cr(VI) can adsorb easily on the TiO2 surface whereas O2

370

requires defects for adsorption.63-65 Thus, electrons excited to the oxygen vacancy

371

states can easily transfer to Cr(VI) and trigger the synergistic effect.

372

To achieve a maximal synergistic effect, PCP and Cr(VI) concentration reduction

373

during photocatalysis under visible light was investigated as a function of

374

[PCP]/[Cr(VI)] molar ratio (Figure 8a,b). The photocatalytic activities of TiO2-6HF

375

varied with change in molar ratio of [PCP]/[Cr(VI)]. An increase in [PCP]/[Cr(VI)]

376

ratio from 0.25 to 2.5 increased Cr(VI) removal significantly, and corresponded to an

377

increase in ARRs of Cr(VI) from 3.05 mg L−1 h−1 to 10.58 mg L−1 h−1 at the end of the

378

reaction. The degradation rate of Cr(VI) decreased obviously ,when PCP was removed

379

completely for each run. Holes are therefore prone to oxidize PCP compared with its

380

degradation intermediates. Similarly, the complete removal of Cr(VI) would lead to a

381

decrease in PCP degradation. The synergistic effect therefore occurs only when Cr(VI)

382

and PCP are present simultaneously in aqueous solution; when the Cr(VI)

383

concentration is unchanged, a higher PCP concentration is expected to correspond to a

384

greater probability to react with holes, which results in more photocatalytic electrons

385

for reaction with Cr(VI).

386

Typically, PCP concentration profiles nearly overlap with those of Cr(VI) during

387

synergistic treatment of PCP and Cr(VI) in TiO2 suspensions when the initial molar

388

ratio of [PCP]/[Cr(VI)] is 1 (Figure 8a,b). This ratio can be regarded as the optimal

389

ratio to achieve a maximal synergistic effect. The above mentioned rates decrease did 18


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390

not result in a cease in reaction. The photocatalytic reaction actually proceeded at a

391

slow rate as a result of insufficient oxygen vacancies to transfer electron carriers.

392

Photocatalytic activity depends on many factors such as particle size, specific

393

surface area, crystallization, phase composition, surface states, defects, morphology,

394

and photocatalytic reactor design and reaction conditions.66-68In this work, we

395

investigate the effect of facet percentage and surface fluorination of TiO2 on visible

396

light catalysis activity of TiO2 during simultaneous oxidation of PCP and reduction of

397

Cr(VI). To exclude the effect of other variables, we control the preparation of catalysts

398

and photocatalytic reaction conditions carefully.

399 400 401

3.2.2. Dependence of catalytic activities on facet percentage The facet percentage of TiO2 has a pronounced effect on photocatalytic activity.

402

The photocatalytic activity of samples prepared with various initial HF volumes

403

followed by alkaline treatment was observed by degradation of a 0.4 mM PCP

404

solution and reduction of a 0.4 mM Cr(VI) under visible light. Fluorine-free samples

405

were selected over fluorinated samples as comparable catalysts because the

406

interference of surface fluorination in the evaluation of facet percentage dependence

407

should be excluded.

408

Usually, an increase in specific surface area enhances the specific photocatalytic

409

activity. Consequently, a comparison of ARR per unit surface area (ARR/S) allows

410

one to determine the difference in catalytic activity caused by facet percentage. Figure

411

7 and Table 1 show that the ARR/S of Cr(VI) reduction and PCP oxidation increases 19



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

412

with an increasing percentage of {001} facets when the facet percentage is less than

413

80%. The ARR/S decreases with further increase in {001} facets percentage.

414

Therefore, the photocatalytic activity of TiO2 can be enhanced by adjusting the {001}

415

facets ratio to an optimum value of 80%.

416

Heterogeneous photocatalytic reaction usually occurs on the photocatalyst surface.

417

Thus, adsorption plays a significant role in the photoreaction process. Cr(VI)

418

adsorption on the TiO2 surface via the interactions between surface hydroxyl groups

419

(≡Ti−OH) and Cr2O72−, and PCP adsorption on TiO2 is a condensation reaction

420

between a surface and a substrate hydroxyl group. The ≡Ti−OH plays an active role in

421

determining the catalyst ability during the simultaneous reduction of Cr(VI) and the

422

oxidation of PCP in aqueous solution. For fluorine-free samples, an increase in initial

423

HF volume results in an increase in the percentage of {001} facets, which is

424

beneficial to the generation of ≡Ti−OH by dissociation of a H2O molecule. As shown

425

in Table 1, an increase in the percentage of {001} facets from 11% to 85% leads to an

426

increase in equilibrium adsorption capacity of Cr2O72- (from 9.62 to 12.38 mg g-1).

427

Nevertheless, the catalysts displayed negligible adsorption of PCP in the simultaneous

428

ternary system. This is because the TiO2 surface prefers adsorbing Cr(VI) rather than

429

PCP.2 Since Cr(VI) is reduced photocatalytically on the catalyst surface, greater

430

adsorption is expected to accelerate the reduction of Cr(VI) by providing a

431

high-concentration reaction environment. But, the ARR/S from Cr(VI) reduction and

432

PCP oxidation did not increase consistently with an increase in initial HF content in

433

this work. This finding is generally in agreement with other studies and may be 20


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434

explained by differences in surface energy levels of the conduction and valence bands,

435

as well as surface structures.12,69

436

DFT predicts that the conduction band potential of anatase (101) is slightly lower

437

than that of anatase (001). For a TiO2 nanocrystals enclosed with {101} and {001}

438

facets, the photogenerated electrons migrate selectively and concentrate to the {101}

439

anatase crystal facets thereby creating reduction sites, whereas the photogenerated

440

holes transfer preferentially and store on the {001} anatase crystal facets to produce

441

oxidation sites. Yu et al. demonstrated that the surface heterojunction between

442

co-exposed {001} and {101} facets facilitates the separation and consequent selective

443

transfer of photogenerated electrons and holes.24 The inherent properties that {101}

444

facets of TiO2 crystals have a higher photocatalytic reduction activity than the {001}

445

facets has been imaged by a single-molecule, single-particle fluorescence

446

approach.70-72 Direct evidence can be obtained by irradiating decahedral anatase TiO2

447

in H2PtCl6 and Pb(NO3)2 aqueous solution. Pt particles were observed mainly on the

448

{101} facets, whereas PbO2 particles were deposited on the {001} facets.69

449

It is expected that an increase in {001} and {101} facets facilitate the oxidation of

450

PCP and enhance the reduction of Cr(VI), respectively. However, reduction and

451

oxidation occur simultaneously in the photocatalytic process and photogenerated

452

electrons and holes are consumed equally in the redox reactions. A balance between

453

the redox reactions must be achieved for optimal photocatalytic activity. Since an

454

increase in the area of exposed {001} facets corresponded to a loss of {101} facets, an

455

appropriate ratio of {001} to {101} facets attributed to well-balanced charge carriers 21



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

456

is required to produce superior photocatalytic activity. On the basis of the results

457

obtained above, we conclude that the optimal ratio of {001}:{101} facets is 80:20 in

458

terms of the rates of Cr(VI) reduction and PCP oxidation, which is an index of the

459

charge separation rate.

460 461

3.2.3. Effect of surface fluorination of TiO2 on visible light catalysis activity of

462

TiO2 nanosheets

463

To determine whether surface fluorination affects catalytic activities with

464

simultaneous Cr(VI) reduction and PCP oxidation in water under visible light, a

465

surface ligand exchange procedure was applied to remove F− on the TiO2 surface by

466

alkaline washing. The XRD and FESEM results demonstrate that the TiO2

467

morphology and the percentage of {001} facets remained almost unchanged before

468

and after alkali washing. Figure 7a illustrates the effect of surface fluorination on TiO2

469

photocatalytic activity for Cr(VI) transformation with PCP. The reaction rate was

470

improved by fluorinating the TiO2 surfaces with HF under hydrothermal conditions.

471

The catalytic activities of the fluorinated TiO2 were consistently higher than the

472

corresponding fluorine-free samples. For example, as listed in Table 1, the ARRs of

473

PCP and Cr(VI) were 10.23 mg L−1 h−1 and 4.24 mg L−1 h−1 over TiO2-6HF under

474

visible light, respectively, which are 11.43% and 6.29% higher than the rates attained

475

with TiO2-6OH.

476 477

Two hydroxyl groups were formed after reaction of the water molecule with bridging-oxygen vacancies by the dissociative adsorption of water molecules at defect 22


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478

sites.73,74 Since the electronegativities of F and O are 4.0 and 3.44,75,76 respectively,

479

the electron cloud of the Ti−O bonds tends to shift toward the F atoms on the

480

fluorinated TiO2 surface. The presence of F weakens the Ti−O bond and enhances the

481

generation of oxygen vacancies. As mentioned above, surface hydroxyl groups act as

482

effective adsorption sites for PCP and Cr(VI). Surface fluorination strengthens the

483

interaction between TiO2 and reactants, and thus is helpful in the photocatalytic redox

484

reaction. Besides the above advantages, surface ≡Ti−F groups can act as

485

photogenerated electron-trapping sites by holding trapped electrons tightly.77 The

486

subsequent recombination of charge carriers in TiO2 was suppressed effectively.

487

Fluorine on the TiO2 surface increases the rate of photocatalytic reduction of Cr(VI)

488

and oxidation of PCP compared with TiO2-nOH catalysts.

489

Furthermore, theoretical calculations show that a high vacancy concentration could

490

induce a vacancy band of electronic states,73 behaving as a shallow donor level just

491

below the conduction band of TiO2. As a result, light absorption was enhanced and the

492

trapping of photogenerated carriers was accelerated. In addition to the presence of

493

oxygen vacancies, the existence of surface fluorine on TiO2 is indispensable to change

494

atomic coordination numbers and the bonding length of the Ti−O−Ti network in

495

oxygen-deficient anatase TiO2 sheets.44 Fluorination reconstructs the exposed Ti

496

atoms by lowering the coordination number, and generates unsaturated Ti atoms that

497

are coordinated by four O atoms.20,44 In contrast, the increased hydroxylation of the

498

TiO2 surface after alkaline treatment decreases the amount of surface fluorine and

499

saturates Ti moieties. The unsaturated Ti is a favorable site for photocatalytic reaction. 23



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

500

This theory is confirmed by the fact that the surface fluorination of anatase TiO2

501

nanosheets favors the production of high concentration oxygen vacancies and

502

unsaturated Ti atoms and promotes the photocatalytic rate of the reaction system

503

involving Cr(VI), PCP, and TiO2 suspension.

504 505

4 CONCLUSIONS

506

Fluorinated TiO2 nanosheets with a high percentage of exposed {001} facets are

507

beneficial to reduce Cr(VI) and oxidize PCP simultaneously. The coexistence of TiO2,

508

Cr(VI), and PCP in aqueous solution triggers significant synergistic effects for Cr(VI)

509

reduction and PCP oxidation under visible light. The appropriate exposure of {001}

510

facets and effective surface fluorination of TiO2 facilitated the photocatalytic redox

511

reaction of PCP and Cr(VI). Among the catalysts prepared with different percentages

512

of {001} facets and extent of fluorination, anatase TiO2-6HF with an optimal

513

{001}:{101} ratio of 80:20 had the highest photocatalytic activity, with an ARR for

514

PCP removal and Cr(VI) disappearance of 10.23 mg L−1 h−1 and 4.24 mg L−1 h−1,

515

respectively. The optimal facet ratio is beneficial for effective separation of e−/h+ pairs

516

to balance photo-induced charges for PCP oxidation on highly reactive {001} facets

517

and Cr(VI) reduction on {101} facets. Surface fluorination promotes the generation of

518

active sites including oxygen vacancies and unsaturated Ti atoms on the catalyst, and

519

the separation of photoexcited electrons and holes under irradiation of aqueous

520

suspensions of TiO2. This study has motivated us to improve catalyst preparation

521

conditions and photocatalytic system operating parameters to ensure catalysts have 24


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522

promising applications in the removal of other water pollutants.

523 524

ACKNOWLEDGEMENTS

525

This work was supported by the Program for Changjiang Scholars and Innovative

526

Research Team in University (Grant IRT13096), the National Natural Science

527

Foundation of China (Grants 21076196, 21177115 and 21477117), and the Zhejiang

528

Provincial Natural Science Foundation of China (Grants LR13B070002 and

529

LR14E080001).

530 531

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Page 37 of 47

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


766

Figure captions

767

Figure 1. XRD patterns for TiO2-H2O, TiO2-nHF, and TiO2-nOH.

768

Figure 2. FE-SEM images of TiO2-H2O, TiO2-nHF, and TiO2-nOH.

769

Figure 3. TEM (a) and HRTEM (b) images of TiO2-6HF.

770

Figure 4. XPS spectra (a) of TiO2-H2O, TiO2-nHF, and TiO2-nOH, as well as XPS

771

spectra in the core levels of (b) Ti 2p, (c) O 1s, and (d) F 1s.

772

Figure 5. EPR spectra of TiO2-12HF, TiO2-6HF, TiO2-6OH, and TiO2-H2O.

773

Figure 6. UV−vis diffuse reflectance spectra of TiO2-H2O, TiO2-nHF, and TiO2-nOH.

774

Figure 7. Removal of PCP and Cr(VI) over (a and c) TiO2-nHF in ternary system

775

[TiO2, PCP, Cr(VI)] and binary system [TiO2, PCP or Cr(VI)], and over (b

776

and d) TiO2-nOH in ternary system [TiO2, PCP, Cr(VI)], as well as (a and c)

777

direct photolysis of PCP and Cr(VI) in the absence of catalyst and in the

778

presence of TiO2-6HF in darkness. The initial concentration of PCP and

779

Cr(VI) were 0.4 mM and 0.4 mM, respectively, the catalyst dose was 1.0 g

780

L−1, and the reaction temperature was 20 °C.

781

Figure 8. (a) PCP removal and (b) Cr(VI) reduction in the presence of various mole

782

ratio of [PCP]/[Cr(VI)]. The initial concentration of Cr(VI) was 0.4 mM, the

783

catalyst dose was 1.0 g L−1, and the reaction temperature was 20 °C.

784

37



(204)

(211)

(004)

(200)

Fig. 1 (101)

785

TiO2-12OH TiO2-9OH

Intensity (a.u.)

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

TiO2-6OH TiO2-3OH TiO2-12HF TiO2-9HF TiO2-6HF TiO2-3HF TiO2-H2O

20

786

30

40

50

60

70

80

2Theta (degree)

787

38


Page 38 of 47

Page 39 of 47

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


788

Fig. 2

789 790

39



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

791

Fig. 3

792 793

40


Page 40 of 47

Page 41 of 47

794

Fig. 4 Ti 2p3/2 458.3

TiO2-12OH

(a)

TiO2-9OH TiO2-6OH

TiO2-12OH

TiO2-3OH TiO2-12HF TiO2-6HF

Intensity (a.u.)

TiO2-3HF

Intensity (a.u.)

Ti 2p1/2

(b)

TiO2-9OH

TiO2-9HF

TiO2-H2O

TiO2-6OH

458.7 458.1


458.4

TiO2-6HF TiO2-3HF TiO2-H2O 1200 1000 800

600

400

200

0

470 468 466 464 462 460 458 456 454

Binding energy (eV)

Binding energy (eV)

(c)

TiO2-12OH TiO2-9OH

TiO2-9OH

TiO2-6OH

TiO2-6OH


TiO2-12HF TiO2-9HF TiO2-6HF

TiO2-3HF

TiO2-3HF TiO2-H2O

TiO2-H2O

795

TiO2-3OH

TiO2-6HF

532

(d)

TiO2-12OH

Intensity (a.u.)

Intensity (a.u.)

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


531

530

529

528

690

Binding energy (eV)

688

686

684

682

Binding energy (eV)

796

41


680


797

Fig. 5

g = 1.984

g = 2.028

TiO2-12HF

TiO2-6HF

Intensity (a.u.)

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

Page 42 of 47

TiO2-6OH TiO2-H2O 2.05

2.03

2.01

1.99

1.97

g value 798 799

42


1.95

Page 43 of 47

800

Fig. 6

(a)

120

(b)

100

120 100

80

Absorbance (a.u.)

Absorbance (a.u.)

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


TiO2-12HF TiO2-9HF

60

TiO2-6HF TiO2-3HF

40

TiO2-H2O

20

80 60 TiO2-12OH TiO2-9OH

40

TiO2-6OH TiO2-3OH

20

0

0 300

400

500

600

700

800

300

400

500

600

Wavelength (nm)

Wavelength (nm)

801 802

43


700

800


803

Fig. 7 direct photolysis TiO2-12HF, ternary

P25, ternary TiO2-H2O, ternary

TiO2-12OH, ternary

TiO2-9HF, ternary

TiO2-6HF, in dark

TiO2-6HF, ternary

TiO2-6HF, binary

TiO2-6OH, ternary

TiO2-9OH, ternary

C/C0 of PCP

0.8

0.6

0.4

0.6

0.4

0.2

0.2

0.0 1.0

(d) 1.0

0.8

0.8

0.0

C/C0 of Cr(VI)

C/C0 of PCP

TiO2-6OH, in dark

(b) 1.0

0.8

(c)

P25, ternary TiO2-H2O, ternary

TiO2-3OH, ternary

TiO2-3HF, ternary

(a) 1.0

C/C0 of Cr(VI)

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

Page 44 of 47

0.6

0.4

0.2

0.6

0.4

0.2

0.0

0.0 0

60

120

180

240

300

0

Time (min)

60

120

180

Time (min)

804 805

44


240

300

Page 45 of 47

Fig. 8

(a) 1.0

0.1 mM PCP 0.2 mM PCP 0.4 mM PCP 0.6 mM PCP 1.0 mM PCP

0.8

PCP concentration

0.6 0.4

(b) 1.0

0.1 mM PCP 0.2 mM PCP 0.4 mM PCP 0.6 mM PCP 1.0 mM PCP

0.8

C/C0 of Cr

806

C/C0 of PCP

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


0.2

Cr(VI) concentration

0.6 0.4 0.2

0.0

0.0 0

60

120

180

240

300

0

Time (min)

50

100

150

200

Time (min)

807 808

45


250

300


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

Page 46 of 47

Table 1. Summary of physicochemical parameters of various photocatalysts and the average removal rate for the reduction of Cr(VI) and oxidation of PCP Catalysts

SBET

Crystallite

Percentage of ARRCr(VI)

(m2 g−1)

phase (%)

{001}a (%)

Equilibrium adorption

(mg L−1 h−1) capacityb (mg g−1) Cr2O72-

PCP

ARRPCP

ARRCr(VI)/SBET

ARRPCP/SBET

(mg L−1 h−1)

(mg L−1 h−1 m−2)

(mg L−1 h−1 m−2)

TiO2-H2O

170

A100

(11) 8

2.46

9.62

negligible

6.15

0.0362

0.0145

TiO2-3HF

144

A100

(68) 53

3.68

10.95

negligible

7.84

0.0544

0.0256

TiO2-6HF

127

A100

(80) 77

4.24

11.85

negligible

10.23

0.0806

0.0334

TiO2-9HF

125

A100

(84) 80

4.12

12.08

negligible

9.84

0.0787

0.0330

TiO2-12HF

119

A100

(85) 81

3.89

12.38

negligible

8.96

0.0753

0.0327

TiO2-3OH

151

A100

(65) 50

2.66

10.74

negligible

6.14

0.0396

0.0176

TiO2-6OH

134

A100

(78) 74

3.99

11.06

negligible

9.18

0.0642

0.0298

TiO2-9OH

131

A100

(83) 76

3.40

11.22

negligible

7.67

0.0586

0.0260

TiO2-12OH

125

A100

(84) 78

3.04

11.48

negligible

7.07

0.0584

0.0243

a

Data inside and outside parentheses were obtained from XRD and FESEM measurements, respectively.

b

In ternary system.

46


Page 47 of 47

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


Table 2. Results of fitting data for the high-resolution spectra of the O 1s regions Catalysts

(Ti-O-Ti)/Ti

Ti-O-Ti

Ti-O-H

H-O-H

BE (eV)

area (%)

BE (eV)

area (%)

BE (eV)

area (%)

TiO2-H2O

2.10

529.6

89.3

531.1

7.6

532.3

3.1

TiO2-3HF

1.74

529.9

77.9

531.1

18.1

532.6

4.0

TiO2-6HF

1.72

529.7

73.3

531.5

22.5

532.8

4.2

TiO2-9HF

1.70

529.6

71.2

531.5

24.4

532.6

4.4

TiO2-12HF

1.68

529.7

69.0

531.1

26.1

532.8

4.9

TiO2-3OH

1.95

529.7

78.2

531.1

16.6

532.6

5.2

TiO2-6OH

1.83

529.4

77.3

531.1

20.0

532.7

2.7

TiO2-9OH

1.82

529.8

73.9

531.1

22.5

532.7

3.6

TiO2-12OH

1.68

529.9

72.4

531.1

25.1

532.6

2.5

47


Simultaneous Oxidation of p-Chlorophenol and Reduction of Cr(VI

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