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Investigating the Unrevealed Photocatalytic Activity and Stability of Nanostructured Brookite TiO2 Film as an Environmental Photocatalyst Mingi Choi, Jonghun Lim, Wonyong Choi, Wooyul Kim, and Kijung Yong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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ACS Applied Materials & Interfaces

Investigating the Unrevealed Photocatalytic Activity and Stability of Nanostructured Brookite TiO2 Film as an Environmental Photocatalyst

Mingi Choi,a Jonghun Lim,b Minki Baeka, Wonyong Choi,a,b Wooyul Kim,*c and Kijung Yong*a

a

Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH),

Pohang, 37673, Republic of Korea b

School of Environmental Science and Engineering, Pohang University of Science and Technology

(POSTECH), Pohang, 37673, Republic of Korea c

Department of Chemical and Biological Engineering, College of Engineering, Sookmyung Women’s

University, Seoul, 04310, Republic of Korea

*Corresponding authors: [email protected] (Wooyul Kim), and [email protected] (Kijung Yong)

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Abstract Among three polymorphs of TiO2 the brookite is the least known phase in many aspects of its properties and photoactivities (especially comparable to anatase and rutile) since it is the rarest phase to be synthesized in the standard environment among the TiO2 polymorphs. In this study, we address the unrevealed photocatalytic properties of pure brookite TiO2 film as an environmental photocatalyst. Highly crystalline brookite nanostructures were synthesized on titanium foil using a well-designed hydrothermal reaction, without harmful precursors and selective etching of anatase, to afford pure brookite. The photocatalytic degradation of rhodamine B, tetramethylammonium chloride, and 4chlorophenol on UV-illuminated pure brookite were investigated and compared with those on anatase and rutile TiO2. The present research explores the generation of OH radicals as main oxidants on brookite. In addition, tetramethylammonium, as a mobile OH radical indicator, was degraded over both pure anatase and brookite phases, but not rutile. The brookite phase showed much higher photoactivity among TiO2 polymorphs, despite its smaller surface area compared with anatase. This result can be ascribed to the following properties of the brookite TiO2 film: (i) the higher driving force with more negative flat-band potential, (ii) the efficient charge transfer kinetics with low resistance, and (iii) the generation of more hydroxyl radicals, including mobile OH radicals. The brookitenanostructured TiO2 electrode facilitates photocatalyst collection and recycling with excellent stability, and readily controls photocatalytic degradation rates with facile input of additional potential.

Keywords TiO2 electrode, brookite, anatase, mobile hydroxyl radical, immobilized photocatalyst,

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1. Introduction Titanium dioxide (TiO2) has been thoroughly studied in various applications due to its diverse advantages, such as durability, non-toxicity, and low cost.1–8 Under UV illumination, photogenerated charge carriers (electrons and holes) are trapped in conduction bands (CB) and valence bands (VB), respectively, which subsequently initiate the generation of reactive oxygen species (ROS), such as hydroxyl radicals, superoxide anions, and hydrogen peroxide. These oxidants facilitate various applications, such as the decomposition of organic compounds, organic synthesis, and solar fuel production.1–3,9 The photocatalytic decomposition of numerous pollutants in water using UVilluminated TiO2 has been successfully demonstrated, and is considerably related to the strong oxidation power of the valence band holes (h+VB) and OH radicals (free (•OHf) or surface trapped (•OHs)). In particular, •OHf, which can diffuse from the surface to bulk solution, facilitates the full mineralization of nonspecifically adsorbed (chemisorbed) pollutants because of electrostatic interactions between the charged TiO2 surface, the organic substrates, and their intermediates under specific pH conditions. TiO2 exists as three natural crystalline polymorphs: anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic).10 Numerous studies have investigated photocatalytic properties of anatase, rutile, and TiO2 (anatase or rutile) composites.11–16 Comparatively little is known about the properties and photoactivities of the brookite phase, because it is the rarest phase to be synthesized in the standard environment among TiO2 polymorphs. Fair comparison of the three TiO2 polymorphs is challenging due to their highly different physicochemical properties resulting from different synthetic methods, and the difficulties in obtaining pure phases. The activity of brookite is still disputed, with the activity ratio (activitybrookite/activityanatase or activitybrookite/activityrutile) highly dependent on the target substrates and properties such as surface area and crystallinity. However, in most cases, brookite appears to have better photocatalytic activity per surface area than other TiO2 polymorphs.17– 20

Versatile photocatalytic reactions have been undertaken to demonstrate the higher activity of the 3



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brookite phase, including H2 production in the presence of an electron donor,17 O2 production in the presence of an electron acceptor,17 decomposition of various substrates,18–23 photodeposition of metals or metal oxides,21 and CO2 reduction.24 Studies of the photocatalytic activity of pure brookite have largely been conducted in slurry-type reactions, and a detailed mechanism, including specific ROS generation, is still unknown. While some studies have reported the synthesis of brookite films,25–28 the synthesis of pure and high-quality brookite crystallites remains challenging. In conventional TiO2 photocatalysis, the immobilization of nanoparticulate TiO2 on a support in a photoreactor is crucial for its practical use, avoiding a post- separation step of catalysts and preventing catalyst loss and influx of contaminated TiO2 into treated water. The process of fixing nanoparticulate TiO2 to a support can dramatically reduce the number of active sites and increase mass transfer limitations. The immobilized nanoparticle layers have poor charge transfer kinetics between nanoparticles because the nanoparticle interfaces can act as recombination centers.29 Furthermore, the complex process, involving binder mixing, coating, calcining, and more, can cause defects and impurities in the photocatalyst structure. Consequently, improved techniques are needed to coat (or imbed) nanocrystallized TiO2 layers on supports for practical applications. In this study, pure-phase brookite nanostructures grown on titanium foil were synthesized using an environmentally benign hydrothermal method. Their unique nanobullet morphology was confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and their purity was verified using Raman spectra and X-ray diffraction (XRD) analysis. Electrochemical measurements revealed the charge transfer kinetics and flat-band potential of the nanostructured brookite TiO2 film. The photocatalytic activities of brookite were systematically studied in the decomposition of rhodamine B (RhB), tetramethylammonium (TMA), and 4-chlorophenol (4-CP), and compared with those of anatase. Brookite showed highly enhanced photoactivity compared to other TiO2 polymorphs, despite having an approx. 10-fold smaller surface area than anatase. The photocatalytic degradation of 4-CP was repeated up to 10 times to determine photostability, and both 4


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photocatalytic and photoelectrochemical properties with external bias systems were compared to demonstrate potential practical applications.

2. Experimental section 2.1 Materials Titanium foil (0.127 mm thick, 99.7% trace metals basis), sodium hydroxide (NaOH, pellets, semiconductor grade, 99.99% trace metals basis), rhodamine B (RhB, dye content 90%), tetramethylammonium chloride (TMA, reagent grade, 98%), 4-chlorophenol (4-CP, 99%), coumarin (C9H6O2, 99%), sodium perchlorate (NaClO4, ACS reagent, 98%) and hydrochloric acid (HCl, ACS reagent, 37%), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO, sigma), were purchased from Sigma Aldrich. Perchloric acid was purchased from Samchun Chemical. Deionized water used was ultrapure (18 MΩ·cm) and prepared by using a Barnstead purification system.

2.2 Synthesis of TiO2 nanostructures 2.2.1 Brookite TiO2 nanostructure Brookite and anatase TiO2 were synthesized using the reported hydrothermal method.30 To synthesize brookite TiO2 nanostructures, titanium foil (3 × 5 cm2) was cleaned in an ultrasonicator with water, acetone, and ethanol for 10 min, and then placed in a Teflon-lined stainless steel autoclave filled with 0.1 M NaOH aqueous solution (70 mL). The autoclave reactor was heated in an oven at 220 °C for 24 h. After the reaction, the reactor was cooled and the foil containing TiO2 nanoarrays was immersed in 1 M HCl solution for 10 min. After the reaction, the sample was placed in a muffle furnace for heat treatment at 500 °C for 3 h. In this step, the TiO2 nanostructures are composed of a large amount of brookite and a small amount of anatase. To obtain pure brookite, the anatase was selectively etched with 0.25 % HF solution for 60 min.31

2.2.2 Anatase and rutile TiO2 nanostructure 5



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For anatase TiO2 nanostructures, the cleaned titanium foil (3 × 5 cm2) was reacted in 0.5 M NaOH aqueous solution at 220 °C for 4–8 h. After the reaction, the reactor was cooled and the foil containing TiO2 nanoarrays was immersed in 1 M HCl solution for 10 min. After the reaction, the sample was placed in a muffle furnace for heat treatment at 500 °C for 3 h. The rutile phase was obtained from phase transformation by annealing the anatase nanowire samples at 1000 °C for 3 h.32

2.3 Characterization of morphology and crystal structure Field-emission scanning electron microscopy (FE-SEM; XL30S, Philips), performed at a beam energy of 5.0 kV, and high-resolution scanning transmission electron microscopy (HR-STEM; JEM2200FS with Image Cs-corrector, JEOL), performed at a beam energy of 200 kV, were used to observe the morphologies of the TiO2 nanostructures. The crystalline structures were confirmed using selected-area electron diffraction (SAED) and fast-Fourier transformation (FFT) patterns of TEM and X-ray diffraction (XRD; D/MAX-2500, Rigaku) results with Cu Kα radiation (40 kV, 100 mA). The XRD spectra were measured in the range of 20–80º with a scan rate of 4º min–1. Raman spectra were measured to characterize the etched brookite and anatase, using a high-resolution confocal Raman spectrometer (Alpha300R, WITec) with an excitation wavelength of 488 nm

2.4 Electrochemical impedance measurements The electrochemical impedance of the TiO2 polymorphs was measured in a three-electrode potentiostat system (Compactstat.e, Ivium Technologies), comprising a TiO2 nanostructure working electrode, Ag/AgCl reference electrode, and Pt counter electrode, incorporating a 1M NaOH electrolyte solution. The working electrodes were illuminated under one-sun conditions at open circuit potential, with a frequency range of 1 MHz to 100 mHz and an amplitude of 10 mV. The measured spectra were fitted using a Z-view software. The Mott–Schottky plot was constructed using voltage and capacitance measurements obtained with the three-electrode system under dark conditions at a frequency of 1000 Hz and amplitude of 10 mV.

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2.5 Time-resolved photoluminescence (TRPL) spectra analysis TRPL of TiO2 nanostructures was measured using a confocal microscope (MicroTime-200, Picoquant, Germany) with a 20x objective. The measurements were performed at the Korea Basic Science Institute (KBSI), Daegu Center, South Korea. A single-mode pulsed diode laser (379 nm with ~30 ps pulse width and ~10 µW laser power) was used as an excitation source with a 375 nm single-mode pulsed diode laser (~100 ps pulse width at 5 MHz repetition rate) excitation. A dichroic mirror (Z375RDC, AHF), a longpass filter (HQ405lp, AHF), a 100 µm pinhole, a band-pass filter, and an avalanche photodiode detector (PDM series, MPD) were used to collect emissions from the samples. Time-correlated single-photon counting technique was used to obtain photoluminescence decay curves, as a function of time with a temporal resolution of 4 and 64 ps. Exponential fittings for the obtained photoluminescence decays were performed by the iterative least-squares deconvolution fitting using the Symphotime software (version 5.3).

2.6 Photocatalytic measurements The TiO2 nanoarrays on titanium foil were immersed in RhB, 4-CP, TMA, and coumarin solutions, and stirred for 30 min before illumination to obtain the equilibrium state for preadsorption. Two reactors (4 mL and 30 mL) with quartz windows were open to the ambient air (air-equilibrated condition), and mixtures were stirred during irradiation. The experiments which needed large sampling volume (i.e., photoelectrochemical (PEC) test and total organic carbon (TOC) removal) were carried out with the 30 mL reactor, and the others were conducted with the 4 mL reactor. A 0.3 mL aliquot of RhB, 4-CP, TMA, and coumarin solutions was obtained every 15 minutes for four times (five times for coumarin solution) by micropipette. A 300-W Xe arc lamp (66902, Newport) with a 10-cm IR water filter (61945, Newport) and λ > 309 nm cut-off filter (FSQ-WG305, Newport) was used as the light source. A typical incident light intensity was measured using a power meter (Newport 1830-C) and determined to be about 200 mW/cm2 in the wavelength range 309-550 nm for all photocatalytic tests. At least duplicate experiments were carried out under the identical condition to 7



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confirm the reproducibility. When the detection of OH radical (i.e., DMPO-OH adduct, and coumarinOH adduct) was done, 4-mL Pyrex reactor was open to the ambient air (air-equilibrated condition), and stirred magnetically during irradiation. The RhB concentration was obtained using a UV/vis spectrophotometer (Optizen Pop) by measuring RhB absorbance at 555 nm. Decomposed 4-CP was analyzed by high performance liquid chromatography (HPLC; 1100 series, Agilent) equipped with a diode array detector and a ZORBAX 300SBC18 column. The eluent consisted of 0.1 % phosphoric acid solution and acetonitrile (80:20, v/v). The TMA concentration was measured by ion chromatograph (IC; DX-120, Dionex) equipped with a Dionex IonPac CS 12A column for cations, and a conductivity detector. The eluent solutions were 20 mM methanesulfonic acid. The fluorescence emission intensity of 7-hydroxycoumarin at 450 nm was determined using a spectrofluorometer (FP-8300, Jasco) with an excitation wavelength of 332 nm. The amount of total organic carbon (TOC) in the illuminated 4-CP solution was quantified using a total organic carbon analyzer (Shimadzu TOC-VSH). The degradation of 4-CP in photoelectrochemical (PEC) system with brookite working electrode was measured in a three-electrode potentiostat system (Compactstat.e, Ivium Technologies) with Ag/AgCl reference electrode and Pt counter electrode in 50 mM NaClO4 electrolyte. To provide more convincing evidence of OH radical, the spin trap method was employed with using diamagnetic DMPO (5,5-dimethyl-1-pyrroline-N-oxide) to generate a stable paramagnetic spin-adduct with OH radical. The production of the spin-adduct in UV-illuminated brookite TiO2 film was monitored by electron spin resonance (ESR) spectroscopy. An ESR spectrometer (Jeol JES-FA100) was operated under the condition of field 324 15 mT, power 10 mW, modulation frequency 100 kHz, sweep time constant 0.03 s.

2.7 UV-vis diffuse reflectance spectra

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The UV-vis diffuse reflectance spectra were measured using a UV2501PC (SHI-MADZU) spectrometer with an ISR-2200 integrating sphere attachment.

3. Results and Discussion 3.1 Morphology Brookite and anatase nanoarrays were synthesized on titanium foil using a hydrothermal reaction without a harmful precursor.30 Fig. 1 shows SEM and TEM images of the brookite and anatase nanostructures. The unique bullet-shaped brookite nanostructures are shown in Fig. 1a and 1c. The tilted SEM image in the inset of Fig. 1a shows that the brookite structures are vertically aligned on the titanium foil, with a length of 700–1000 nm, a width of 130–170 nm, and a sharpened tip structure. This nanobullet shape results from the expansion of exposed {210} facets, which have low surface energy to minimize the total surface energy, and a kinetic roughening transition.33–35 The atomic structure obtained by high-resolution TEM is shown in Fig. 1c, with the distance between (020) and (200) plane measured to be 0.283 and 0.469 nm, respectively. A SAED pattern recorded along the [001] zone axis is presented in the inset of Fig. 1c, confirming a brookite crystal phase. The HR-TEM image and clear diffraction pattern suggest that the brookite nanostructures have excellent crystallinity. As shown in Fig. 1b and 1d, the anatase phase composes of nanowire structures with a length of 5 µm and radius of 40 nm. The cross-section of the SEM image, shown in the inset of Fig. 1b, confirms that the anatase nanowires are vertically aligned. The atomic structure and fast-Fourier-transform (FFT) pattern are shown in Fig. 1d. The distance between the (101) plane is 0.355 nm and the FFT pattern, which is recorded along the [010] zone axis, well corresponds with an anatase crystal phase. The SEM image of the rutile structures which were obtained by annealing the anatase sample at 1000 °C is presented in Fig. S1a. Their morphology is changed from wire to rod after annealing.

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To compare the activities per surface area, the approximate surface areas of the brookite and anatase nanostructures were estimated. The surface area of TiO2 nanostructures can be extrapolated by adsorption and desorption amount of dye.36–39 The brookite and anatase samples were immersed in 0.3 mM N719 (C58H86O8N8S2Ru) solution for 24 hours. After absolute adsorption, the samples were rinsed by ethanol, and immersed in the 0.1M NaOH solution for desorption. The absorption spectra of desorbed dye solution are shown in Fig. S3, and from the figure we can see that the amount of dye adsorbed on anatase nanostructures is five times greater than that of brookite. The approximate surfaces areas of two samples were presumed based on the morphologies obtained by SEM and TEM. The anatase nanostructures are considered as cylinders with a height of 5 µm and radius of 40 nm. The brookite nanostructures are considered as rectangular pyramids with a width of 140 nm and height of 180 nm on a rectangular prism with a width of 140 nm and height of 720 nm. Assuming the unit area is closely packed with brookite or anatase, the anatase nanostructures have about 5.2 times larger active surface area (details in the supporting information).

3.2 Crystal structure Many of the previous studies on brookite have reported difficulties in obtaining pure brookite.25,40– 44

To confirm the purity of the brookite nanostructure, XRD and Raman analyses were conducted. Fig.

S4a compares the Raman spectra of brookite before and after HF etching treatment. Most brookite peaks remain after etching, but some peaks originated from anatase are diminished after HF etching reaction. The results obtained from Raman spectra of brookite and anatase using a 488-nm laser are shown in Fig. 2a. The brookite and anatase phases have different Raman spectra due to their distinct crystalline polymorphs. A variety of studies have clearly established that anatase (tetragonal, space group: I41/amd) structures have six Raman-active modes (A1g + 2B1g + 3Eg) and brookite (orthorhombic, space group: Pbca) structures have 36 Raman-active modes (9A1g + 9B1g + 9B2g + 9B3g).19,45–47 As shown in Fig. 2a, the anatase nanostructures have typical Raman peaks at 149 (Eg), 201 (Eg), 401 (B1g), 519 (B1g + A1g), and 640 cm–1 (Eg), while the brookite nanostructures have strong 10


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Raman peaks at 134 (B1g), 150 (A1g), 158 (B1g), 201 (A1g), 221 (B1g), 252 (A1g), 292 (B3g), 327 (B1g), 373 (B2g), 400 (B2g), 414 (B1g+A1g), 467 (B2g), 510 (B1g), 549 (B3g), 588 (B2g), and 643 cm–1 (A1g). These well-defined, sharp Raman peaks from anatase and brookite indicate that they both have excellent crystalline qualities. The purity of brookite nanostructure is also confirmed by XRD. Several studies have reported that the purity of brookite can be confirmed by ratio of (121) and (120) peak intensity.19,48–51 The (121) brookite peak at 2θ = 30.8° is only found for brookite structure, and the (120) brookite peak and the (101) anatase peak overlap at 2θ = 25°. The amount of brookite and anatase can be estimated by

/ + ratio. The ideal brookite should have ratio of ~0.9, and brookite/anatase mixture should have the low ratio. Our as-prepared brookite samples have a rather low ratio of ~0.56, which increases to 0.91 after HF treatment. This result implies that the anatase phase is selectively removed and pure brookite nanostructures are obtained with HF etching. As shown in Fig. S4b, the anatase peaks diminish, while the intensities of the brookite peaks do not change after HF treatment. The remained peaks at blue dots after HF treatment are overlapped peaks of brookite. As presented in Fig. S5, the unique nanobullet morphology of brookite is unchanged after HF treatment. From the Fig. 2b we can see that anatase (JCPDS no. 21-1272) and brookite (JCPDS no. 29-1360) have many common XRD peaks. However, the (121) peak at 2θ = 30.8° is only found for brookite, and can be used as a brookite fingerprint. Other minor brookite XRD peaks for (032), (113), and (161), at 2θ = 46.1°, 57.2°, and 65.9°, respectively, are also observed. For the wire structures, (101) and (200) peaks are clearly observed at 2θ = 25.3° and 48.1°, respectively, which are distinctive of the anatase phase. Fig. S1b presents the XRD results of a phase transformed rutile (JCPDS no. 211276) from the anatase nanowire samples. Its peaks including strong (110) peak at 2θ = 27.5° are well consistent with reference peaks.

3.3 Charge transfer kinetics

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To initiate the photocatalytic reactions, efficient charge transfers of photogenerated carriers from a conduction band (or valence band) to the active trap site on the surface is required. To assess the charge transfer kinetics of brookite and anatase, electrochemical impedance spectroscopy (EIS) measurements were conducted. Fig. 3a shows the electrical circuit scheme, comprising two RC (a parallel resistor and capacitor) elements, where Rsc is the resistance of the semiconductor, CFEsc is the space charge capacitance, Rct is the semiconductor/electrolyte charge transfer resistance, and CFEH is the Helmholtz capacitance.52,53 Nyquist plots and parameters obtained from EIS under one-sun illumination at open circuit potential are presented in Fig. 3b and Table 1. In the Nyquist plot, two charge transfer modes are present; the first through the nanostructured semiconductor (at high frequency), and the second through semiconductor/electrolyte interface (at low frequency).52 Table 1 shows that the charge transfer resistance within the brookite semiconductor (Rsc = 103 Ω) is lower than that of anatase (603 Ω), meaning that efficient charge separation occurs in the brookite. Effective charge separation in the brookite phase is attributed to its excellent crystallinity, which prevents recombination of photogenerated electron–hole pairs. In previous research, brookite was also shown to have a longer electron life time and slower recombination rate than anatase, as confirmed by opencircuit voltage decay.30 Charge transportation at the semiconductor/electrolyte interface (Rct) is also affected by semiconductor conductivity, with brookite reported to have higher conductivity than anatase.54 Therefore, brookite allows faster transport of photogenerated electrons to counter electrodes, and easier transfer of residual holes at the semiconductor interface to the electrolyte. The light absorption properties of the TiO2 samples were obtained using UV-vis diffuse reflectance spectroscopy with integrating sphere (Fig. S6). The onset of reflectance spectra of the samples which are necessary for time-resolved photoluminescence (TRPL) spectra was measured. The information about the recombination processes of TiO2 photocatalyst can be obtained by time-resolved photoluminescence (TRPL) spectra.55–57 Fig. S7 provides the normalized TRPL spectra at 500 nm of brookite and anatase nanostructures excited with 375 nm laser. The decay process of the brookite is slower than that of anatase, which means photogenerated carriers of brookite have slower 12


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recombination rate and longer life time. Because of efficient charge separation, the electron and hole are transferred to the conduction band and valence band, respectively, minimizing recombination and resulting in the generation of more reactive radicals. The Mott–Schottky plots obtained from the capacitance and potential, as determined by EIS measurements, are presented in Fig. 3c. The flat-band potential and donor density can be calculated from the Mott–Schottky plots using the following equation:58,59

=

− −

(1)

where C is the capacitance, e is the electron charge, ε is the dielectric constant, ε0 is the vacuum permittivity, Nd is the donor density, E is the potential, Efb is the flat-band potential, k is the Boltzmann constant, and T is the temperature. The flat-band potentials which can be obtained from the intercept on the X axis of Fig. 3c of anatase and brookite are –0.89 V and –1.06 V vs. SCE, respectively. In agreement with previous studies, the brookite phase has a more negative flat-band potential, resulting in a conduction band higher on the electrochemical scale.23,60,61 Due to more negative flat-band potential of brookite, the electron in conduction band can transfer to O2 preferentially, and it results in the efficient charge transfer, and less recombination, which agree with the results from EIS and TRPL.62 The slope of the brookite phase is much smaller than that of the anatase, which also indicates the brookite phase has a much greater carrier concentration.

3.4 Photocatalytic activity: Hydroxyl radical generation To compare the photocatalytic activities of anatase, rutile, and brookite TiO2, we measured the photocatalytic degradation (PCD) rates of Rhodamine B (RhB), tetramethylammonium (TMA), and 4chlorophenol (4-CP), which have been frequently studied for their photocatalytic conversion using TiO2. Fig. 4 shows the time profiles for photocatalytic decomposition of RhB, TMA, and 4-CP under UV-irradiated TiO2. All TiO2 polymorphs effectively degrade RhB, TMA, and 4-CP under UV 13



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illumination, except for rutile, which fails to decompose TMA (discussed later). The photocatalytic activities of TiO2 increase in the order of rutile < anatase < brookite, although the activity ratio (activitybrookite/activityanatase or activitybrookite/activityrutile) highly depends on the kinds of substrates. Fair comparison of the photoactivity of TiO2 phases is difficult because the morphologies of each phase on titanium foil are quite different: nanowires for anatase, nanobullets for brookite, and rods for rutile. However, given that the anatase specific surface area is approx. 10 times larger than that of brookite (see supporting information), there is a marked difference between the photocatalytic activities of brookite and anatase (normalized by surface area). The enhanced performance of brookite could be ascribed to the higher conduction band potential60,61 and/or improved charge transport properties due to low resistance (see section 3.3).30 However, it is difficult to fully explain why the phasedependence of TiO2 photocatalytic activity also varies with the organic substrates using only these intrinsic properties. To understand the substrate-specific properties of the TiO2 polymorphs, monitoring the relative generation of the main oxidants, such as •OHf, •OHs, and h+VB, on each TiO2 phase is key, as the majority of photocatalytic applications are related to oxidative conversion. To confirm the direct evidence of OH radical generation under UV-irradiation, the DMPO (5,5-dimethyl-1-pyrroline-Noxide) spin-trapping experiment was carried out. The typical ESR spectrum of DMPO-•OH adduct with a quartet signal (intensity ratio of 1:2:2:1) was observed in the UV illuminated brookite TiO2 film (see Fig S8). To measure the generation of OH radicals (without distinguishing between •OHf, and •OHs), coumarin is used as a probe radical-trap reagent (see reaction 1). The resultant oxidized product, a coumarin-OH adduct (7-hydroxycourarin), can be quantified by fluorescence spectroscopy.63,64

•OH + coumarin 7-hydroxycoumarin

14


(2)

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In Fig. 5, the time-profiles of 7-hydroxycoumarin production from the reaction with OH radicals generated by brookite and anatase under UV irradiation are compared. The brookite phase shows enhanced production of 7-hydroxycoumarin compared with anatase, indicating that brookite facilitates the generation of OH radicals under UV illumination. In particular, among the three organic compounds, cationic TMA would be electrostatically repelled by the positively charged TiO2 surface (point of zero charge in TiO2: 5.5–6.8)

65

in acidic solution (pH 3), resulting in the complete no

adsorption of TMA.66 Therefore, TMA is reported to indicate mobile OH radicals, which reveals that mobile OH radicals are generated by anatase TiO2, but not rutile.67 In this study, the fact that both brookite and anatase induce measurable TMA decomposition at pH 3, while rutile does not (due to electrostatic repulsion) implies that the generation of mobile OH radicals occurs on the anatase phase, in addition to brookite. In particular, the amount of mobile OH radicals produced is clearly higher for brookite than anatase. Notably, the common observation that brookite has activity comparable with, or higher than, anatase in many photooxidation reactions is attributed to the facile generation of •OHf on both brookite and anatase (but not rutile). The fact that no mobile OH radical production occurs on UV-illuminated rutile film is consistent with our previous results using rutile nanoparticles in a slurrytype system.67 Further experimental studies are underway to determine the mechanism of mobile OH radial generation on anatase and brookite TiO2.

3.5 Practical use: photostability and photoelectrochemical properties To test the photostability of brookite TiO2 film, the PCDs of 4-CP were repeated using the same catalyst, as shown in Fig. 6. The photoactivity of brookite is maintained without marked deactivation for up to 10 repeat cycles. The total organic carbon (TOC) removal of 4-CP was measured to confirm that the brookite nanostructures not only remove 4-CP, but also its photooxidative intermediates, such as benzoquinone, hydroquinone, and fumaric acid.68 Table 2 presents experimental data from the TOC removal of 4-CP on brookite, showing 80 % and 95 % TOC removal after 12 and 24 h, respectively.

15



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Facile control of additional potential to induce high efficiency is required for practical use. Herein, to accelerate the PCD rate of 4-CP, an external bias was applied to the highly crystalline brookite electrode. The initial PCD rate of 4-CP in the photoelectrochemical (PEC) system is clearly enhanced (by approx. 44% compared with the photocatalytic system) under biased conditions (at 1 V vs. SCE), which is not enough potential for only electrochemical decomposition of 4-CP (Fig. 7). The external potential makes electrons transfer to the counter electrode, and facilitates efficient charge separation of electron–hole pairs, which results in enhanced activity for substrate degradation.

4. Conclusions TiO2 photocatalyst is of paramount importance as an essential element of solar photoenergy utilization for a practical use. Diverse approaches are being made to deeply understand the TiO2 polymorphs and in search of active photocatalysts. In this study, pure-phase brookite nanostructures were synthesized on Ti foil using an environmentally benign hydrothermal method. Electrochemical measurements revealed the intrinsic properties of highly crystalline brookite TiO2, which have a higher driving force with more negative flat-band potential, and efficient charge transfer kinetics. We tested and compared the photocatalytic activities of brookite and anatase TiO2 films for the degradation of several organic compounds in water. Previous studies on brookite TiO2 have revealed its higher photoactivity among TiO2 polymorphs, but most were conducted in slurry-type reactions and provided little information on the role of major photooxidants.17–20 Herein, we demonstrate that the superior photoactivity of the brookite phase for versatile organic pollutant degradation stems from its OH radical generating properties, especially mobile OH radicals, and other intrinsic properties. The role of OH radicals in decomposition reactions on brookite TiO2 film depends on the type of substrate. Based on its stability tests, facial input of an additional bias, high mineralization efficiency, and ability to generate mobile OH radicals under UV illumination, nanostructured brookite TiO2 film is a practical environmental photocatalyst for wastewater purification. The development of brookite TiO2 film provides an alternative approach in the conventional anatase and rutile TiO2 application. More studies on TiO2 16


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polymorphs are needed for practical applications.

Acknowledgements This work

was supported

by the National Research

Foundation

of Korea (NRF-

2016R1A4A1010735).

Supporting Information SEM and XRD data, An approximate surface area, data of brookite before and after HF treatment (Raman, XRD, and SEM), UV-vis diffuse reflectance spectra, Time-resolved photoluminescence spectra, ESR spectrum of DMPO-•OH adduct, Control test of RhB for self-sensitized degradation.

17



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Mechanistic Study. Langmuir 1996, 12 (26), 6368–6376.

23



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Fig. 1. Morphologies of TiO2 nanostructures. Top-view SEM images of (a) brookite and (b) anatase; insets show the tilt-view SEM images. HR-TEM images of (c) brookite and (d) anatase; insets show their diffraction patterns and TEM images.

24


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Fig. 2. (a) Raman spectra of brookite and anatase using a 488-nm laser. (b) XRD patterns of brookite and anatase.

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Fig. 3. (a) Scheme of electrical circuit of brookite nanobullet structure and anatase nanowire structure. (b) Nyquist plots of EIS results of brookite and anatase obtained under one-sun illumination conditions at open circuit potential. (c) Mott–Schottky plot of anatase and brookite.

26


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Fig. 4. Time profiles for the degradations of (a) RhB, (b) TMA, and (c) 4-CP with rutile, anatase, and brookite. Experimental conditions: [RhB] = 5 µM, [4-CP]0 = 100 µM, [TMA]0 = 50 µM, pHi = 3.0, λ > 310 nm, air-equilibrated for 30 min before irradiation. 27



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. Fig. 5. Time profiles for generation of the coumarin-OH adduct of brookite and anatase. Experimental conditions: [coumarin]0 = 1 mM, pHi = 3.0, λ > 310 nm, air-equilibrated for 30 min before irradiation.

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Fig. 6. Brookite recycling test in the degradation of 4-CP. Experimental conditions: [4-CP]0 = 100 µM, pHi = 3.0, λ > 310 nm, air-equilibrated for 30 min before irradiation.

29



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Fig. 7. Time profiles for the degradation of 4-CP with brookite in 50 mM NaClO4 electrolyte with or without light at 1 V vs. SCE. Experimental conditions: [4-CP]0 = 100 µM, pHi = 3.0, λ > 310 nm, airequilibrated for 30 min before irradiation.

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Table 1. Parameters of anatase and brookite determined by fitting the electrical circuit model to EIS data.

Sample

Rs [Ω]

Rsc [Ω]

CPEsc [µF]

Rct [Ω]

CPEH [µF]

Anatase

6.61

603.1

415.24

5831

62.48

Brookite

10.19

103.2

918.52

4233

62.91

Table 2. TOC removal (%) of 4-CP using brookite.

TOC

0h

12 h

24 h

0

79.0 ± 1.0

94.7 ± 0.3

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