Photoelectrochemical Degradation of Organic Compounds Coupled

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Photoelectrochemical Degradation of Organic Compounds Coupled with Molecular Hydrogen Generation using Electrochromic TiO Nanotube Arrays 2

Min Seok Koo, Kangwoo Cho, Jeyong Yoon, and Wonyong Choi Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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Environmental Science & Technology

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Photoelectrochemical Degradation of Organic Compounds

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Coupled with Molecular Hydrogen Generation using

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Electrochromic TiO2 Nanotube Arrays

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Min Seok Koo1, Kangwoo Cho1, Jeyong Yoon2 and Wonyong Choi1* 1

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Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea

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School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742,

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Korea

9

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Submitted to

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(Revised)

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2017

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* To whom correspondence should be addressed (W. Choi)

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E-mail: [email protected]

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Phone: +82-54-279-2283

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1

ACS Paragon Plus Environment


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ABSTRACT

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Vertically aligned TiO2 nanotube arrays (TNTs) were prepared by electrochemical

20

anodization, and then cathodically polarized with dark blue coloration for the dual-functional

21

photoelectrochemical water treatment of organic substrates degradation and accompanying

22

H2 generation. The resulting Blue-TNTs (inner diameter: ~40 nm; length: ~9 µm) showed

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negligible shift in X-ray diffraction pattern compared with the intact TNTs, but the X-ray

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photoelectron spectra indicated a partial reduction of Ti4+ to Ti3+ on the surface. The

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electrochemical analyses of Blue-TNTs revealed a marked enhancement in donor density and

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electrical conductivity by orders of magnitude. Degradations of test organic substrates on

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Blue-TNTs were compared with the intact TNTs in electrochemical (EC), photocatalytic (PC),

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and photoelectrochemical (PEC) conditions (potential bias: 1.64 VNHE; λ > 320 nm). The

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degradation of 4-chlorophenol was greatly enhanced on Blue-TNTs particularly in PEC

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condition, whereas the PC activities of the Blue- and intact TNTs were similar. The potential

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bias of 1.64 VNHE did not induce any noticeable activity in EC condition. Similar trends were

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observed for the degradation of humic acid and fulvic acid, where main working oxidants

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were found to be the surface hydroxyl radical as confirmed by hydroxyl radical probe and

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scavenger tests. H2 generation coupled with the organic degradation was observed only in

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PEC condition, where the H2 generation rate with Blue-TNTs was more than doubled from

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that of intact TNTs. Such superior PEC activity was not observed when a common TiO2

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nanoparticle film was used as a photoanode. The enhanced electric conductivity of Blue-

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TNTs coupled with a proper band bending in PEC configuration seemed to induce a highly

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synergic enhancement.

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Keywords: Titania nanotube, Photoelectrochemical water treatment, Advanced oxidation process,

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Hydrogen production, Water-Energy nexus.

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INTRODUCTION

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The photochemistry and photoelectrochemistry of semiconductor metal oxides have been

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intensely investigated over the past decades to meet the growing needs of controlling

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recalcitrant pollutants by producing active oxidants (e.g., OH radicals), mostly under external

47

input of energy.1-5 In particular, TiO2 has been widely used for photocatalytic or

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electrocatalytic degradation of organic compounds,5-7 mostly thanks to catalytic active sites

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on TiO2 surface, a highly positive potential of the valence band edge, and an excellent

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chemical stability in a large window of potential bias and pH. Low cost and low toxicity are

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additional advantages of titanium oxides for environmental purification purposes.3 Moreover,

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TiO2 has been a key material for storage and conversion of energy, exemplified by its use in

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photocatalytic H2 generation and CO2 reduction as well as Li-ion battery. Among various

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morphologies of TiO2 (such as nanorods,8 films,9 nanoparticles10 and nanotube arrays11)

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investigated for environmental and energy applications, titanium nanotube arrays (TNTs)

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may prove to be an attracting choice, owing to their unique and desirable properties such as

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high specific surface area, open-channel structure that facilitates the mass transfer of

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substrates, and reduced light-scattering loss.12-13 Nevertheless, the applications on TNTs for

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photoelectrochemical (PEC) conversions are still limited by wide band gap, fast charge pair

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recombination, and low electrical conductivity.

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In order to enhance the activities of TiO2 either as a photocatalyst or an electrocatalyst,

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doping of external elements14-15 has been tried to effectively improve the light absorption and

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electrical conductivity. In addition, a partial reduction of Ti4+ to Ti3+, together with

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intercalation of protons and/or formation of oxygen vacancies, could be a feasible self-doping

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that can be done by hydrogenation, chemical and electrochemical reduction of TiO2.16-19 In

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particular, a cathodization of TNTs proved to a simple and safe procedure for the self-doping

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of TiO2, since the one-dimensional nanostructure of TNTs was suitable for the 3



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electrochemical reduction treatment.20 The so-called electrochemical self-doping would

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change the electronic state in the band gap which in-turn affects the optical properties of TiO2

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(light absorption).21 In addition, an enhanced mobility of charge carriers associated with the

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Ti3+ impurities makes the electrochromic TiO2 suitable for the application to water treatment

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and energy conversion.22

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The PEC approach in which an external potential bias coupled with light irradiation

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effectively separate the charge pairs into a cathode and an anode provides an ideal method for

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achieving various photochemical conversions including the degradation of aqueous

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pollutants,2,23 via the generation of reactive oxygen species and the reduction of proton or

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water to molecular hydrogen. Utilizing solar energy for water treatment with simultaneous

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energy recovery (e.g., H2 generation) is an attractive technology. To this end, it is reasonable

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to propose that the electrochromic TiO2 with an increased charge carrier density and

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electrical conductivity would be desirable for its application to PEC water treatment and

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energy conversion.

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Herein, we investigate an exemplified PEC system for simultaneous water treatment and

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molecular hydrogen production, using electrochromic titania nanotube arrays (denoted as

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Blue-TNTs) as a photoanode. A cathodic polarization of crystalline TNTs was found to

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enhance the degradation of organic compounds and H2 production significantly whereas such

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PEC activities could not be obtained with using intact TNTs or common TiO2 nanoparticle

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film. This investigation demonstrates a good strategy for utilizing immobilized catalysts for

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energy-recovering water purification process without the need of recovery and recirculation

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of catalysts.

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MATERIALS AND METHODS

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Electrodes Preparation and Characterization.

TNTs were fabricated by a two-step 4


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anodization procedure to obtain relatively stable and uniform structure.24-25 The anodization

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process was performed in a single compartment cell with a Ti foil (Aldrich, 3 x 2 cm2, 0.127

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mm thick, 99.7% purity) as a working electrode and a Pt wire as a counter electrode. Before

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the anodization, Ti foils were cleaned with ethanol and deionized (DI) water by

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ultrasonication, and then dried in air. The first anodization was performed at 60 V for 15 min

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in ethylene glycol electrolyte containing 0.5 wt% NH4F (Sigma-Aldrich, 98% purity) and 3

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wt% H2O. The resulting TNTs layer was removed by ultrasonication in a concentrated H2O2

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solution (Junsei, 35% purity), then washed with DI water. The second anodization was done

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at 60 V for 45 min in ethylene glycol electrolyte containing 0.2 wt% NH4F and 1 wt% H2O.

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After washing with ethanol and DI water, the as-formed TNTs were dried in air and annealed

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at 450 °C for 3 h (in air with a ramp rate of 2 °C min-1). In order to prepare Blue-TNTs, a

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constant current (0.017 A/cm2) was applied to the TNTs electrode for 30 s in phosphate buffer

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solution ([KH2PO4]0 = 0.1 M, pH = 7.2 with addition of NaOH) with using a Pt wire as a

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counter electrode.26 The activities of the Blue-TNTs electrode were compared with those of

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the TiO2 nanoparticulate film electrode as a control. For the preparation of TiO2

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nanoparticulate film, commercial TiO2 (P25) of which particle size is about 20 - 30 nm was

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used. P25 was coated on a glass substrate (2 x 2 cm) by a doctor-blade method.27 P25 powder

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was thoroughly mixed with ethanol in a concentration of 0.15 g mL-1. The mixed paste was

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cast on the substrate glass plate, dried under air, and then heated at 200 °C for 1 h to remove

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residual ethanol in the immobilized photocatalyst electrode of which thickness was about ~10

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µm. The TNTs and Blue-TNTs were characterized by diffuse reflectance spectroscopy (DRS,

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Shimadzu UV-2401PC), X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with

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monochromated Al-Kα radiation (1486.6 eV), X-ray diffraction (XRD, Max Science Co.,

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M18XHF) using Cu-Kα radiation, and field emission scanning electron microscopy (FE-SEM,

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JEOL, JSM-7401F). 5



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Electroanalytical Measurements.

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with a three-electrode configuration was employed in this study, which included a working

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electrode (TNTs or Blue-TNTs, 2 x 2 cm2), a stainless steel (SS, 2 x 2 cm2) counter electrode,

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and a Hg/Hg2SO4 reference electrode. The electrode module was connected to a computer-

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controlled potentiostat (Gamry Instruments Reference 600). The distance between anode and

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cathode was 5 mm and the supporting electrolyte for the following electroanalytical

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measurements was 0.1 M Na2SO4 solution. Potentials need to be corrected for the

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uncompensated resistance between the working electrode and the reference electrode. For a

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planar electrode with a uniform current density, the uncompensated resistance (Ru) is given

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by Ru = x/κA, where x is the distance from tip of working electrode to reference electrode

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(cm), A is the area of the working electrode (cm2), and κ is the conductivity of solution.28 The

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measured potentials (reported in this work) should be subtracted by iRu to give the corrected

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potentials and our estimation yielded Ru = 9.6 Ω and iRu = 0.22 V. Linear sweep voltammetry

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(LSV) data were collected in the potential range of -0.5 to +1.3 V (vs. Hg/Hg2SO4) at a scan

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rate of 50 mV s-1. For electrochemical impedance spectroscopy (EIS) Nyquist plot, the

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potential bias was set at open circuit voltage (OCV) with a frequency range of 1 MHz to 0.01

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Hz and alternating current (AC) voltage of 100 mV rms. The Mott-Schottky measurements

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were done at AC potential of 10 mV with a frequency of 100 Hz in the potential range of -0.5

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to +0.2 V (vs. Hg/Hg2SO4). The LSV data and Nyquist plots were also collected under UV

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irradiation, where the light source was a 300-W Xe arc lamp with a UV cutoff filter (λ > 320

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nm).

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Degradation of Organic Substrates Coupled with H2 Generation.

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Sigma), humic acid and fulvic acid (HA, FA, Suwannee River) were chosen as test

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compounds to compare their degradation rates in variable energy input conditions;

A single compartment cell (working volume: 80 mL)

4-chlorophenol (4-CP,

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electrochemical (EC, potential bias only), photocatalytic (PC, irradiation only) and

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photoelectrochemical (PEC, potential bias with irradiation). Coumarin (Sigma) and

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terephthalic acid (TA, Sigma) were used as a trapping reagent of •OH radicals, and tert-butyl

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alcohol (TBA, Sigma) and ethylenediaminetetraacetic acid (EDTA, Sigma) were used as •OH

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and hole scavenger, respectively. The initial substrate concentration was 100 µM for 4-CP

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and 1 ppm for FA and HA, while 0.1 M Na2SO4 was added as a supporting electrolyte (pH 6).

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The degradation of organic substances was conducted in a single compartment cell reactor

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(working volume: 80 mL) with a three-electrode system, which included a working electrode

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(TNTs or Blue-TNTs, 2 x 2 cm2), a counter electrode (SS, 2 x 2 cm2), and a reference

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electrode (Hg/Hg2SO4). The solution in the reactor was air-saturated. The potential bias was

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fixed at +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), and the UV irradiation (λ > 320 nm) source was

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the 300-W Xe arc lamp. The incident light intensity was measured to be 1 W/cm2 (optical

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power meter, Newport 1918-R). For the measurement of molecular hydrogen and oxygen gas

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evolution, the reactor with 0.1 M Na2SO4 solution (with or without organic substrates) was

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initially purged with Ar gas (Linde, 99.9995%) for 1 h to remove the dissolved oxygen.

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During the reaction, gas samples were periodically withdrawn from the headspace (~40 mL)

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with a 100 µL glass syringe (Hamilton 81030).

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Analytical Methods. Analysis of 4-CP and the degradation intermediates of 4-CP was done

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using a high performance liquid chromatograph (HPLC, Agilent 1100). Analysis of anionic

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chlorine species (i.e. Cl-, ClO- and ClO3-) was done by an ion chromatograph (IC, DX-120).

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HA and FA were analyzed by monitoring their fluorescence emission using a

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spectrofluorometer (HORIBA fluoromax-4) under the excitation of 279 nm. H2 in gas

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samples was analyzed by a gas chromatograph (GC, HP6890A) with a thermal conductivity

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detector (TCD) and a 5 Å molecular sieve column. For the comparison of the TNTs and Blue7



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TNTs electrode surface area, the adsorption of methylene blue (MB) dye was measured by

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adsorbing MB on the electrode surface (in 0.5 M MB solution at pH 10 for 3 h) and the

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subsequent desorption of the adsorbed dye at pH 2. The desorbed dye amount was quantified

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spectrophotometrically using a UV/Visible spectrophotometer (Agilent 8453).

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RESULTS AND DISCUSSION

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Physicochemical Properties of the Photoanodes.

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morphologies of intact TNTs (Figure S1 in Supporting Information (SI)) and Blue-TNTs

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(Figure 1a and b) were comparable with each other in the SEM analysis. These nanotubes

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were roughly characterized by an inner diameter of ~40 nm and an average length of ~9 µm.

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The MB dye adsorption on each electrode was measured to be almost the same between

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TNTs (9.0(±0.2) nmol cm-2) and Blue-TNTs (9.6(±0.3) nmol cm-2) electrode. Therefore, the

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surface area difference between TNTs and Blue-TNTs can be ruled out as a main factor

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influencing the electrode activity. Blue-TNTs and intact TNTs also exhibited almost identical

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XRD patterns (Figure S2 in SI), primarily related with anatase form, whereas the cathodic

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polarization process turned the color of intact TNTs to dark blue (Figure 1d (inset)). This

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electrochromic phenomenon would arise from the surface defect generation associated with a

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change in the oxidation state of Ti species during the cathodic polarization, which might be

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accompanied with intercalations of monovalent cations (such as H+29). A formation of Ti3+

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along with the intercalation of proton in the TiO2 lattice can be expressed by Eq. 1. 30-31

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The horizontal and cross-sectional

> Ti + e + H → > Ti H

(1)

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The XPS measurement (Figure 1c) confirmed the existence of Ti3+ on the surface of Blue-

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TNTs. For the intact TNTs, typical maxima of Ti4+ were observed at binding energies of

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465.1 (Ti 2p1/2) and 458.9 eV (Ti 2p3/2),32 while the Blue-TNTs peaks were shifted to lower 8


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binding energies, owing to the partial contribution of Ti3+ state. The DRS shown in Figure 1d

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indicated a red-shift of the spectrum and greater absorption of UV light for the Blue-TNTs,

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which manifested the electrochromism. However, this enhanced light absorption of Blue-

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TNTs was found to have an insignificant effect on the photocatalytic activity (vide infra).

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The electrochemical analysis evaluated the charge transfer characteristics under dark and

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light illumination conditions, which was shown as Nyquist and Mott-Schottky plots. In the

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Nyquist plots (Figure 2a) obtained in the dark condition, the behavior of intact TNTs and

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Blue-TNTs followed a transmission line model for TiO2 nanotube system,33 and both

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electrodes exhibited very large charge transfer resistance. Under UV irradiation, on the other

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hand, more distinct semicircular relation was observed in Blue-TNTs, owing to the enhanced

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interfacial charge transfer. The charge transfer resistance of Blue-TNTs under irradiation was

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observed to be far smaller than that of the intact TNTs, based on the low-frequency domain

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intercepts of the arcs. This result indicates more facile electron migration through the

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nanotube network in Blue-TNTs, which was also confirmed by Mott-Schottky plots (Figure

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2b). Blue-TNTs exhibited a markedly flat slope, corroborating an improvement in electrical

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conductivity. The charge carrier density (ND) was estimated through Mott-Schottky equation

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expressed as Eq. 2: 34-35

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=

E − E ! " −

#$

%

(2)

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where Csc is the space charge capacitance (F cm-2); e is elementary charge (1.602 × 10-19 C); ε

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is the relative dielectric constant of electrode material (48 for anatase TiO2; assumed to be

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identical for Blue-TNTs)24, ε0 is the permittivity of vacuum (8.85 × 10-12 N-1 C2 m-2); ES is the

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applied potential (V); EFB is the flat band potential (V); k is the Boltzmann’s constant (1.38 ×

212

10-23 J K-1), and T is the absolute temperature (K). Our estimation suggested the donor

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density of TNTs and Blue-TNTs to be 9.92 × 1020 and 3.63 × 1023 cm-3, respectively. 9



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Consequently, the presence of Ti3+ as a defect in Blue-TNTs notably enhanced the electrical

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conductivity and charge carrier density, which reconfirmed the previous report.26, 36-37 Our

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results further showed that the corresponding improvement in interfacial charge transfer is

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more pronounced in a PEC condition, rationalizing the application of Blue-TNTs as a

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photoanode. More direct evidences are presented in the linear sweep voltammograms (Figure

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2c) obtained with and without UV (λ > 320 nm) irradiation. Both electrodes showed

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negligible dark current, whereas the photocurrent of Blue-TNTs was distinctly higher than

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TNTs under irradiation. The photocurrent of TNTs was almost saturated at the anodic

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potential bias of -0.2 V (vs. Hg/Hg2SO4), whereas that of Blue-TNTs monotonically

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increased up to +1.3 V (vs. Hg/Hg2SO4). At +1.0 V (vs. Hg/Hg2SO4), the photocurrent

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generated with Blue-TNTs (5.7 mA/cm2) was four times higher than that of the intact TNTs

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(1.4 mA/cm2).

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Photoelectrochemical Degradation of Organic Compounds.

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Blue-TNTs were compared in terms of the degradation of test organic compounds (4-CP, FA

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and HA) under variable energy input conditions (PEC, PC and EC). Figure 3a shows that the

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degradation of 4-CP was negligible in EC condition for both TNTs. In PC condition, both

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TNTs exhibited similar 4-CP degradation efficiency, ca. 25% within 2 hours. An intriguing

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discrepancy was observed in PEC condition which achieved almost complete degradation of

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4-CP with Blue-TNTs within 2 hours, whereas only 45% removal was observed with intact

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TNTs. As a comparison to TNTs electrodes, a mesoporous TiO2 electrode that consisted of

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nanoparticulate (P25) film of which thickness is similar to TNTs was prepared and tested

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under the same conditions (PEC, PC, and EC) as shown in Figure 3b. The TiO2

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nanoparticulate film electrode exhibited much lower activities than TNTs electrodes in PEC

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condition. This clearly shows the superior nature of TNTs as a PEC electrode, which

The activities of intact and

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distinguishes itself from the common nanoparticulate TiO2 film electrode. The TiO2

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nanoparticulate film seems to be much less efficient than TNTs because the diffusion of

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electrolytes and substrate molecules is much hindered within the mesopores of the film.

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Figure 3c depicts the mass balance of chlorine species and H2 generation during the PEC

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degradation of 4-CP with Blue-TNTs. The chloride ion, a degradation product of 4-CP, could

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be sequentially oxidized to hypochlorite (ClO-), chlorite (ClO2-), and chlorate (ClO3-). The

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concentration of ClO2- was below the detection limit owing to the short-lived characteristics

245

(i.e., facile oxidation to ClO3-).38 Therefore, the sum of Cl-, ClO- and ClO3- concentration

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closely matched the reduction in 4-CP concentration which indicates the absence of

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chlorinated organic byproducts. The H2 generation linearly increased with the PEC reaction

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time. Despite the complete removal of 4-CP on Blue-TNTs in 2 h, the degree of

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mineralization (TOC removal) was about 38% in 2 h while a complete mineralization was

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achieved after 6 hours of PEC reaction (Figure 3d). At the same PEC condition, the TOC

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removal efficiencies obtained with intact TNTs were 25% and 42.4% at 2 h and 6 h,

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respectively. Although some intermediates (HQ, CC and BQ) were generated from the

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degradation of 4-CP (see Figure S3), they were eventually mineralized on Blue-TNTs.

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Analogous results were also obtained in the degradation of FA and HA using Blue-TNTs.

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Figure 4 shows the time-dependent variations of FA and HA concentrations based on the

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fluorescence emission intensity (at 451 nm for FA and at 464 nm for HA, Figure S4 and S5 in

257

SI). The removal of the humic substances was again negligible in EC condition, while

258

moderate degradation was observed in PC condition with the pseudo-first-order rate constant

259

of 0.48 and 0.46 h-1 for FA and HA, respectively. These degradation rate constants increased

260

dramatically (10.7 h-1 for FA and 4.6 h-1 for HA) in PEC condition.

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PEC and PC degradations of organic compounds usually involve the surface hydroxyl

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radicals (•OH) generated from water oxidation with holes (Eq. 3) and the direct transfer of

263

holes. h ! + H O or OH " → • OH

264

(3)

265

The results of 4-CP, FA, and HA degradation imply that the facilitated production of •OH as

266

a non-selective oxidant would primarily account for the highly enhanced PEC activities.39 In

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order to specify the role of hydroxyl radicals, we used coumarin and terephthalic acid (TA) as

268

a selective probe reagent for •OH trapping. The reaction of coumarin and TA with •OH

269

produces 7-hydroxycoumarin (7-HC, Eq. 4) and 2-hydroxyterephthalic acid (2-HTA, Eq. 5),

270

respectively. The generation of the hydroxylated products was quantified by monitoring the

271

fluorescence emission (Figure S6 and S7).40

272

• OH

+ coumarin → 7-HC (emission monitored with λex = 332 nm)

(4)

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• OH

+ TA → 2-HTA

(5)

(emission monitored with λex = 315 nm)

274

Figure 5 depicts the time profiles of 7-HC and 2-HTA production whose slope would reflect

275

a pseudo steady-state concentration of OH radicals on the electrode surface. Both TNTs

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produced negligible amount of 7-HC and 2-HTA in EC condition, which implies that OH

277

radicals cannot be generated under the present potential bias condition. On the other hand, the

278

PC condition clearly generated the sign of 7-HC and 2-HTA, which is consistent with the

279

well-known behavior of TiO2 photocatalyst that generates OH radicals under UV

280

irradiation.3-5 In PEC condition, the productions of 7-HC and 2-HTA on Blue-TNTs were

281

almost doubled from that of intact TNTs. This observation is in good agreement with Figure

282

3 and 4, which showed markedly higher PEC activities of Blue-TNTs than intact TNTs. The

283

coumarin and TA tests suggest that the high PEC activities of Blue-TNTs for the degradation

284

of organic compounds should be ascribed mainly to the facile generation of OH radicals.

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Another oxidant that might be involved in this system is sulfate radical that can be

286

generated from the oxidation of sulfate electrolyte. We additionally carried out 4-CP

287

degradation experiment in inert NaClO4 electrolyte as a control, which was compared with

288

that in Na2SO4 electrolyte to estimate the possible role of sulfate radicals (see Figure S8). The

289

degradation efficiency of 4-CP was moderately reduced when using NaClO4 as an inert

290

electrolyte, which implies that some sulfate radicals are generated from sulfate ions and

291

involved along with the hydroxyl radical in the degradation of organic compounds.

292

Superoxide anion radicals can be also generated on the SS counter electrode through the

293

reduction of dissolved O2 but their oxidation power is not strong enough to degrade

294

recalcitrant organic substrates.

295

To further investigate the role of •OH as an oxidant on Blue-TNTs photoanode, TBA and

296

EDTA were used as scavengers for free and surface-bound •OH and holes in the PEC

297

degradation of 4-CP (see Figure S9). 4-CP was fully degraded without scavenger reagents in

298

2 h PEC reaction, whereas, in the presence of either TBA or EDTA, the PEC degradation of

299

4-CP was significantly retarded, but not completely inhibited. On the other hand, the

300

degradation of 4-CP could be almost completely prohibited in the presence of both TBA and

301

EDTA. This indicates that both OH radicals and holes are responsible as main oxidants of 4-

302

CP in the PEC degradation using Blue-TNTs.

303

In dark condition (EC), the potential applied in this study (+1.64 VNHE) seemed to be

304

insufficient to overcome the kinetic barrier of •OH generation for both intact TNTs and Blue-

305

TNTs. We further carried out an additional EC test at a higher voltage (+2.75 VNHE) and

306

found that the EC system at 2.75 VNHE still exhibited a lower activity than the PEC system at

307

1.64 VNHE for the degradation of 4-CP (see Figure S10). That is, PEC requiring a lower

308

voltage is more efficient than EC requiring a higher voltage (by 1.11 V), which demonstrates

309

the merit of PEC system. Although a PEC system needs light irradiation which is an extra 13



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energy input, the solar application should make PEC systems cost effective. On the other

311

hand, the PC activities of intact TNTs and Blue-TNTs exhibited little difference although

312

Blue-TNTs has higher light absorption in the UV region (see Fig. 1d). Possible explanations

313

include: (i) the Ti3+ as a surface defect site could also serve as a recombination center41 and

314

(ii) the enhanced electron mobility in Blue-TNTs would not effectively increase the

315

photocatalytic activity, since the sluggish interfacial electron transfer steps (e.g., reduction of

316

dissolved oxygen or molecular hydrogen evolution) are rate-limiting. The results in this study

317

clearly demonstrated that the positive effect of electrochemical self-doping in TNTs is

318

particularly outstanding in PEC condition. The band bending under an anodic potential bias

319

in synergy with an enhanced electrical conductivity of Blue-TNTs would facilitate the excited

320

electrons to migrate towards the cathode and effectively overcome the fast recombination

321

process.

322

Molecular Hydrogen Generation.

323

TNTs or Blue-TNTs in 0.1 M Na2SO4 solution with organic substrates (4-CP, HA, and FA) in

324

three conditions (EC, PC, and PEC). In PC and EC conditions, the hydrogen evolution was

325

absent with both TNTs and Blue-TNTs, corroborating that the interfacial electron transfer is

326

sluggish rate-determining step in PC condition. On the contrary, a distinct activity for H2

327

production was observed in PEC condition where the transfer of excited electrons in TNTs

328

and Blue-TNTs to a SS counter electrode could be facilitated under a potential bias for

329

generating H2 (Eq. 6). The hydrogen evolution amount did not depend on the kind of organic

330

substrates (4-CP, HA, and FA) in the PEC reactor.

331

Figure 6a compares the production of H2 with either

2e + 2H → H

(6)

332

The rate of hydrogen generation was observed to be far greater with Blue-TNT (72 µM cm-2

333

h-1) than intact TNTs (19 µM cm-2 h-1). Consequently, the superior activity of Blue-TNTs was 14


Page 15 of 30


334

fully demonstrated in PEC condition, with respect to H2 evolution as well as organic

335

degradation, owing to more efficient migration of charge carriers. When the quantified

336

amounts of H2 were compared with the theoretical ones based on the measured current

337

(dashed lines in Figure 6), the average Faradaic efficiency of H2 evolution was close to unity

338

for both TNTs and Blue-TNTs.

339

On the other hand, the accompanying evolution of O2 in the PEC reactor was also

340

measured and compared with H2 evolution in Figure 6b. The production of O2 was lower than

341

that expected from the stoichiometric water splitting (1/2 of H2 production), which implies

342

that holes were consumed not only in water oxidation to O2 (2H2O + 4h+ → O2 + 4H+) but

343

also in hydroxyl radical generation (H2O + h+ → HO• + H+), sulfate radical generation (SO42−

344

+ h+ → SO4•−), and direct oxidation of organic substrate (4-CP + h+ → 4-CP•+). In addition, it

345

should be noted that the evolution of H2 and O2 was little affected by the presence and

346

absence of 4-CP, which indicates that the direct hole consumption by organic substrates is

347

negligible compared to other hole consuming reactions.

348

Stability.

349

dark ambient condition. However, the electrochromic reaction given by Eq. 1 may be

350

reversible under either potential bias or irradiation so that the backward reaction (Ti3+H+ →

351

Ti4+ + e- + H+) might deteriorate the PEC activity (see Figure S11). In order to check the

352

stability of self-doping in Blue-TNTs, the degradation of 4-CP was repeated up to five cycles

353

in the PEC condition (Figure 7). The pseudo-first order rate constants of 4-CP degradation

354

ranged from 1.8 h-1 to 1.4 h-1 for Blue-TNTs without a significant variation, while the intact

355

TNTs consistently showed much lower activity (0.40 ~ 0.25 h-1). In addition, the rate of H2

356

generation was also maintained during the repeated tests (72 ~ 66 µM cm-2 h-1) with Blue-

357

TNTs in PEC condition. The PEC activities of the electrochromic TNTs (Blue-TNTs) were

For virgin Blue-TNTs, the dark blue color was maintained for several weeks in a

15



Page 16 of 30

358

maintained through multiple uses despite a shift in XPS Ti 2p band after PEC experiment

359

(Figure S11). This implies that the self-doping coupled with proton intercalation and/or

360

oxygen vacancy formation induced a permanent structural distortion to make Blue-TNTs

361

stable enough.42

362

Environmental Implications.

363

photocatalysts for water treatment application would be a requirement of additional

364

separation step for reuse of catalysts as well as effluent clarification. Therefore, much

365

research efforts have been given to the development of immobilized photocatalysts, which

366

were prepared by template synthesis, sol-gel binding and direct growth on supports.43-44 On

367

the other hand, the activities of the immobilized photocatalysts would be hampered by a loss

368

in effective surface area and the diffusion limitations across the immobilized catalyst layers.

369

To this end, it has been proposed that the self-organized TNTs directly grown on Ti substrate

370

by anodic oxidation have an open channel structure which is suitable for efficient diffusion of

371

reactants and dioxygen molecules.12 More importantly, this study clearly demonstrated that a

372

PEC configuration could boost the photoactivity of the prepared electrochromic TNTs for

373

organic compounds degradation and simultaneous H2 generation. The cathodic polarization of

374

TNTs highly enhanced the electrical conductivity to induce much higher photocurrents under

375

a potential bias. At the same time, the photocurrent can be utilized for H2 production on an

376

inexpensive counter electrode. In a practical perspective, various renewable energy sources

377

such as solar cells can provide the relatively low potential bias for the PEC condition. Such

378

combination of a PEC reactor and solar cells can provide an ideal solution for water treatment

379

and energy recovery in remote areas that are not connected to a power grid system. The

380

electrochemical self-doping of TNTs is a simple and versatile method to prepare an

381

immobilized photoanode which shows high activity and durability in PEC condition. The

One of the major limitations of heterogeneous

16


Page 17 of 30


382

properties of Blue-TNT might be further tuned by the adjustment of cathodization conditions

383

for more desirable band bending and visible light adsorption, which requires further

384

investigation.

385

386

ACKNOWLEDGMENT.

387

This work was supported by the Global Research Laboratory (GRL) Program (NRF-

388

2014K1A1A2041044) and KCAP (Sogang Univ.) (No. 2009-0093880) funded by the Korea

389

government (MSIP) through National Research Foundation of Korea (NRF) and, in part, by

390

KIST-UNIST partnership program (2V05120/1.160097.01).

391 392

Supporting Information Available.

393

SEM image of intact TNTs (Figure S1); XRD patterns of TNTs and Blue-TNTs (Figure S2);

394

HPLC analysis of 4-CP degradation intermediates (Figure S3); Fluorescence emission spectra

395

of FA (Figure S4) and HA (Figure S5); Fluorescence emission spectra of 7-HC (Figure S6)

396

and 2-HTA (Figure S7) generated on TNTs and Blue-TNTs; Effects of electrolytes on the

397

PEC degradation of 4-CP (Figure S8); Effects of TBA and EDTA on PEC degradation of 4-

398

CP (Figure S9); Comparison of PEC and EC degradation of 4-CP with different bias

399

potentials (Figure S10); XPS Ti 2p band of Blue-TNTs before and after PEC experiment

400

(Figure S11).

401 402

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of

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an

self-doping

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reduced

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TiO2

nanotube

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22


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Figure 1. (a) The horizontal and (b) cross-sectional images of Blue-TNTs, together with (c) the difference XPS spectra of Ti 2p band between Blue-TNTs and intact TNTs. (d) DRS spectra of intact TNTs and Blue-TNTs. The inset of (d) shows the variation of apparent color (electrochromism) during the cathodic polarization of intact TNTs.

23



528 529 530 531 532 533 534 535 536

Figure 2. (a) Nyquist plots of intact TNTs and Blue-TNTs with and without light irradiation. Nyquist plots were obtained in the range from 1 MHz to 0.01 Hz at OCV under UV irradiation. The inset figures show the magnification of the regions of interest. (b) MottSchottky plots in electrochemical impedance spectroscopy. The Mott-Schottky measurements were done at AC potential of 10 mV with a frequency of 100 Hz in the potential range of -0.5 to +0.2 V (vs. Hg/Hg2SO4). (c) Linear sweep voltammograms of intact TNTs and Blue-TNTs with or without light irradiation (λ > 320 nm). 24


Page 24 of 30

Page 25 of 30


537 538 539 540 541 542 543 544

Figure 3. (a) Time profiles of 4-CP degradation with intact and Blue-TNTs in PEC, PC and EC conditions. (b) Time profiles of 4-CP degradation with TiO2 nanoparticulate (P25) film electrode (with a thickness similar to TNTs) in PEC, PC, and EC condition. (c) Mass balance of chlorine species concentrations and H2 generation during the time course of 4-CP degradation with Blue-TNTs in PEC condition. (d) Time profiles of TOC removal with intact and Blue-TNTs. ([4-CP]0 = 100 µM, pH 6, +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), λ > 320 nm).

25



Page 26 of 30

545 546 547 548 549 550

Figure 4. (a) Time-dependent variation of the fluorescence emission intensity (λex = 279 nm) of fulvic acid (FA: emission at λ = 451 nm) and (b) humic acid (HA: emission at λ = 464 nm) in PEC, PC and EC conditions. ([FA]0 and [HA]0 = 1 ppm, pH 6, +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), λ > 320 nm).

26


Page 27 of 30


551 552 553 554 555 556 557

Figure 5. Time profiles of fluorescence emission intensity that indicates the generation of (a) 7-HC (emission at λ = 456 nm) and (b) 2-HTA (emission at λ = 425 nm) as an indicator of •OH generation on intact TNTs and Blue-TNTs in PEC, PC and EC conditions. Excitation wavelength was 332 nm (coumarin) and 315 nm (TA). [Coumarin]0 = 1 mM, [TA]0 = 500 µM in 2 mM NaOH, +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), λ > 320 nm.

27



Page 28 of 30

558 559 560 561 562 563 564 565 566

Figure 6. (a) Production of H2 in PEC, PC and EC conditions with intact TNTs and BlueTNTs. The dashed lines represent the theoretical amount of H2 evolution based on the measured current with assuming Faradaic efficiency of unity. The reactions were conducted in 0.1 M Na2SO4 electrolyte. (b) Production of H2 and O2 with and without 4-CP in PEC condition with Blue-TNTs. The dashed line represents the stoichiometric amount of O2 that is expected from the stoichiometric water splitting (1/2 of H2 production). ([4-CP]0 = 100 µM, pH 6, +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), λ > 320 nm).

28


Page 29 of 30


567 568 569 570 571 572

Figure 7. The PEC degradation of 4-CP on intact TNTs and Blue-TNTs and the concurrent H2 generation on Blue-TNTs for five repeated runs. For the H2 generation, the reactions were conducted in 0.1 M Na2SO4 electrolyte (with 4-CP). ([4-CP]0 = 100 µM, +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), λ > 320 nm).

29



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Table of Content (TOC)

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Photoelectrochemical Degradation of Organic Compounds Coupled

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