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Preparation and photoelectrochemical performance of visible-light active AgI/TiO2-NTs composite with rich #-AgI Qi Wang, Xiaodong Shi, Enqin Liu, Jianjia Xu, John Crittenden, Yi Zhang, and Yanqing Cong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00883 • Publication Date (Web): 12 Apr 2016 Downloaded from http://pubs.acs.org on April 12, 2016

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Industrial & Engineering Chemistry Research

Preparation and Photoelectrochemical Performance of Visible-Light Active AgI/TiO2-NTs Composite with Rich β-AgI Qi Wang,†, ‡ Xiaodong Shi,† Enqin Liu,† Jianjia Xu,† John C. Crittenden,‡ Yi Zhang,† and Yanqing Cong*,† †

School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou

310018, China ‡

The Brook Byer Institute for Sustainable Systems and School of Civil and Environmental

Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

Corresponding Author:

Prof. Yanqing Cong School of Environmental Science and Engineering Zhejiang Gongshang University Hangzhou 310018, PR China Phone: (+86-571)-28008211 E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: AgI sensitized TiO2 nanotube arrays (AgI/TiO2-NTs) with adjustable β/γ ratio of AgI were prepared by simple dissolution-precipitation-calcination process. The samples were characterized by various techniques, including X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, ultraviolet-visible diffuse reflectance spectroscopy, linear sweep voltammetry, electrochemical impedance spectroscopy and Mott-Schottky plots. We found that calcination temperature (100 oC to 500 oC) had significant effect on regulating the phase of AgI. After calcination at 350 oC, highest β/γ ratio of AgI was achieved. Moreover, greatly enhanced photocurrent response and reduced charge transfer resistance were also observed, which together led to easier generation and separation of photogenerated electron-hole pairs. Thus, for the reduction of Cr(VI) under visible light, significantly enhanced photoelectrocatalytic (PEC) performance was observed using AgI/TiO2-NTs350 as photoanode and Ti foil as cathode, respectively. At very low content of AgI (1.25%), the estimated kCr(VI) (0.0155 min-1) was nearly 5 times that on pure TiO2-NTs350.


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1. INTRODUCTION Chromium-containing wastewaters, arising from dyeing, tanning, electroplating and steel manufacturing industries, have posed serious threat to the environment. Cr(VI) and Cr(III) are primary existing states of chromium,1 among which Cr(VI) was carcinogenic, much more toxic and easier migrated. Therefore, the reduction of Cr(VI) to less toxic and easier precipitated Cr(III) was the key step for the disposal of such wastewaters.2–4 The industrial processes for Cr(VI) reduction involve the use of stoichiometric reducing agents such as SO2, Na2SO3, FeSO4 and Fe.5,6 Moreover, large amounts of sludge were produced in FeSO4 and Fe based processes, which may cause secondary pollution. Photocatalysis, especially TiO2 photocatalysis, as a new promising and clean technology, has gradually attracted much attention in the catalytic reduction of Cr(VI).7–9 Under UV irradiation, electron-hole pairs will be generated, Cr(VI) can be reduced to Cr(III) by capturing photogenerated electrons in the conduction band through successive one electron process.10 However, much efforts still need to be done for better performance and more efficient utilization of solar energy.11 One strategy is to separate the easily coupled electron-hole pairs as long as possible. Furthermore, it is also necessary to extend the response of TiO2 to visible light region since UV light only accounts for less than 5% of solar irradiation. Finally, easier reuse and stable recycle ability of a photocatalyst are also vital for its future practical application. Photoelectrocatalysis (PEC) has been proved to be more efficient than photocatalysis (PC) due to enhanced separation of photogenerated electron-hole pairs via applied bias voltage.12–14 The photogenerated electrons can be continuously extracted and driven to the cathode by an external electrical circuit. Moreover, different from TiO2 nanoparticles, the one-dimensional and highly ordered TiO2 nanotubes (TiO2-NTs) offer an excellent electrical channel for vectorial


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charge transfer and adsorption ability.15–17 Therefore, the photogenerated electron-hole pairs on TiO2-NTs can be more easily separated, leading to greatly enhanced activity. However, pure TiO2-NTs can still only be excited by UV light. Doping of other elements or modifying by a photosensitizer is a feasible way to extend the response of TiO2-NTs to visible light region.18–21 Recently, the highly photosensitive but unstable AgI was successfully applied in the field of photocatalysis as a visible light photosensitizer. When coupled with TiO2 nanoparticles, the decomposition of AgI was drastically inhibited due to matched conduction band (CB) between AgI (ECB = -0.15 V vs SHE) 22 and TiO2 (ECB = -0.05V vs SHE). The photogenerated electrons in the CB of AgI were ultimately transferred to O2 or Cr(VI) (E0Cr(VI)/Cr(III) = 1.35 V vs SHE) rather than Ag+ through the CB of TiO2. In this way, highly efficient and stable activity for the degradation of organic pollutants, killing of bacteria or reduction of Cr(VI) was achieved on AgI/TiO2 under visible light irradiation.23–27 Herein, as an electron mediator, TiO2 itself can not be excited by visible light. Moreover, we have previously reported the dominating effect of specific surface area rather than crystallinity in transferring photogenerated electrons from AgI to Cr(VI).27 However, in the PEC process using TiO2-NTs as support for AgI, applied bias voltage provides additional energy to separate electron-hole pairs and transfer electrons, which may lead to different dominating factors from PC process. Moreover, a mixture of both β- and γ-AgI were identified on AgI/TiO2 prepared at room temperature.23 Since the visible light absorption of pure β-AgI and γ-AgI were different,28 changes in the proportion of β- to γ-AgI may lead to different activity. In the present study, AgI was loaded onto TiO2-NTs film via deposition-precipitation method. The ratio of β- to γ-AgI was adjusted by calcining the as-prepared AgI/TiO2-NTs at different temperatures (100 °C to 500 °C). The PEC properties of the as-prepared AgI/TiO2-NTs


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were evaluated by linear sweep voltammetry (LSV) and the reduction of Cr(VI) under visible light irradiation. The effects of external potential, solution pH, electrolyte concentration and coexisting organic pollutant were studied in detail. Stability of the optimal AgI/TiO2-NTs was also evaluated in cyclic experiments.

2. EXPERIMENTAL METHODS 2.1. Chemicals. Ti sheet (99.7%, 0.35 mm thick) was cut into pieces of 25 mm × 20 mm. Pt sheet (≥ 99.9%) was used as received. Cr(VI) aqueous solution was prepared by using analytical grade potassium dichromate (K2Cr2O7). Silver nitrate (AgNO3), potassium iodide (KI), ethylenediaminetetraacetic acid (EDTA), sodium fluoride (NaF) and sodium sulfate (Na2SO4) were all of analytical grade and used as received without further purification. The pH of the solutions was adjusted with 1 M HClO4 or NaOH. 2.2. Preparation of AgI sensitized TiO2 nanotubes (AgI/TiO2-NTs). TiO2-NTs were fabricated by anodization method according to our previous report.29 Ti sheet (25 mm × 20 mm × 0.35 mm) was progressively polished by 120 mesh, 320 mesh, 800 mesh and 1000 mesh sandpapers, respectively. And then ultrasonically cleaned sequentially with acetone, ethanol and doubly distilled water. For the anodization process, the polished, ultrasonic cleaned Ti sheet was used as anode and Pt sheet as cathode, respectively. The electrolyte containing 0.5 wt% NaF and 0.5 mol/L Na2SO4. The applied anodized voltage was 20 V. After 5 h anodization under constant magnetic stirring, the anodized films were washed with doubly distilled water and dried in air. The as-prepared film was denoted as TiO2-NTs. AgI/TiO2-NTs was prepared through deposition-precipitation method. Briefly, TiO2-NTs was sequentially immersed into 100 mL 1×10-3 mol/L KI and AgNO3 aqueous solution for 2 h,


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respectively. And then washed with distilled water and dried in air. Finally, the as-formed AgI/TiO2-NTs were annealed in a muffle furnace at different temperatures (100 °C, 200 °C, 350 °C and 500 °C) for 2 h. The as-prepared film electrodes were denoted as AgI/TiO2-NTsX, where X represents the calcination temperature. For example, AgI/TiO2-NTs calcined at 350 °C was denoted as AgI/TiO2-NTs350. 2.3 Characterization. The crystal phases of the as-prepared AgI/TiO2 samples were determined using an X-ray diffractometer (XRD, Regaku D/Max-2500, with Cu Kα radiation at 1.5406 Å). The diffuse reflectance spectra (DRS) were recorded on a UV-Vis spectrophotometer with Integrating Sphere (TU-1901, Beijing Purkinje General Instruments Co., Ltd.). Also, AgI/TiO2-NTs350 was characterized by X-ray photoelectron spectroscopy (XPS, ESCA lab 220i-XL spectrometer with Al Kα at 1486.6 eV) and the binding energies of Ag 3d, I 3d, Ti 2p and O 1s were calibrated to the C 1s peak (284.8 eV). The morphology of AgI/TiO2-NTs350 film was characterized using a field emission scanning electron microscope (FE-SEM; Hitachi S4800). Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) measurements were performed using a CHI 660E instrument (Chenhua, Shanghai) in a three-electrode system, with a AgI/TiO2-NTsX film electrode, a saturated Ag/AgCl electrode and a Pt sheet as working, reference and counter electrodes, respectively. The photo-response of the as-prepared AgI/TiO2NTs samples was characterized by linear sweep voltammetry (LSV) method using a potentiostat (CHI 660E). 2.4. Photoelectrochemical (PEC) activity. The PEC activities of the as-prepared AgI/TiO2-NTs films were carried out in a two-electrode glass cell (100 mL) under constant magnetic stirring, using Cr(VI) (8×10-5 mol/L) as model pollutant and Na2SO4 (0.2 mol/L) as electrolyte. The as-prepared AgI/TiO2-NTs films were used as anode and Ti sheet as cathode,


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respectively. Applied voltage was supplied by a DC Constant Current Power (WYL603 type). During a typical PEC test, the anode was irradiated by a Xe lamp (Shanghai. Lansheng Electronic Co., Ltd) fitted with a cut-off filter to achieve visible light (λ > 420 nm). The incident light intensity (ca.100 mW cm-2) was detected by a powermeter (model FZ-A). Before initiating the reactions, the electrode was immersed in the reactant and placed in dark under constant magnetic stirring for 30 min to establish adsorption-desorption equilibrium. The reactor was kept under constant air-equilibrated conditions throughout the experiment. At given time intervals, 2 mL aliquot samples were collected. The concentration of Cr(VI) was measured using the 1,5diphenylcarbazide colorimetric method.30,31,38 In detail, 2 mL aliquot samples were added sequentially by 1 mL mixed acid (1:1 concentrated H2SO4 and H3PO4) and 1 mL chromogenic agent (1,5-diphenylcarbazide dissolved in acetone-water mixed solution). After 5 minutes, the mixture were monitored by detecting the purple complex at 540 nm on a UV-Vis spectrophotometer (Tu-1901, Beijing Purkinje General Instruments Co., Ltd.). The concentration of EDTA was analyzed using HPLC with a Diamonsil C18 column and ultraviolet detection at 210 nm. The mobile phase consisted of 7:3 (V/V) deionized water/acetonitrile, and the flow rate was maintained at 1.0 mL/min.

3. RESULTS AND DISCUSSION 3.1. Structure and morphology of the as-prepared AgI/TiO2-NTs films.


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ο anatase TiO 2 ♦ Ti substrate ·AgI

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β (112) / γ (311)

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(B)

substrate

(200) (105) (211) (204)

♦ Ti

(101)

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β (002) /γ (111)

TiO2 AgI ο · ο

Intensity (a.u.)

(A) Intensity (a.u.)

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οο

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60

70

80

Figure 1. (A) XRD spectra of AgI/TiO2-NTs film electrodes calcined at different temperatures: (a) AgI/TiO2NTs100, (b) AgI/TiO2-NTs200, (c) AgI/TiO2-NTs350 and (d) AgI/TiO2-NTs500; (B) Enlarged XRD spectra of AgI/TiO2-NTs350.

The structure of the as-prepared AgI/TiO2-NTs films was characterized by XRD analysis and the results were shown in Figure 1A. Obvious signals of Ti foil were observed in all the tested samples, since Ti sheet was used as substrate for the formation of TiO2-NTs via anodization. When the calcination temperature increased from 100 °C to 350 °C, obvious diffraction peaks of anatase TiO2 were observed. Relatively weak diffraction peaks of AgI were also observed in all the tested samples. As for AgI/TiO2-NTs350, the enlarged XRD spectrum was depicted in Figure 1B. After comparing with the standard XRD patterns, the existence of AgI was identified as a mixture of β (JCPDS: 09-0374) and γ (JCPDS: 09-0399) phase. For example, 2θ values at 23.7o and 46.4o were ascribed to (002) and (112) crystal planes of β-AgI, respectively, which were reported to be overlapped with (111) and (311) planes of γ-AgI.23,32 Interestingly, small changes in the XRD spectra of AgI were observed in the 2θ range of 21.5o to 24.5o (Figure S1). By comparing the peak intensities centered at 22.3° and 23.7°, respectively, the ratio of β(100) to β(002)/γ(111) can be estimated based on the peak area ratio. Finally, the


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variation in the composition of β-AgI and γ-AgI can be indirectly explored. As shown in Figure S1, the ratio of β-AgI to β(002)/γ(111) increased firstly from 0.52 on AgI/TiO2-NTs100, to 0.78 on AgI/TiO2-NTs200 and 0.95 on AgI/TiO2-NTs350, respectively. And then the value decreased to 0.41 on AgI/TiO2-NTs500. The highest value (0.95) was observed on AgI/TiO2-NTs350, indicting the highest ratio of β to γ-AgI. Since β-AgI nanocrystals was reported to exhibit much higher visible light absorption than AgI colloid (β-AgI and γ-AgI mixture),28 it is possible that the PEC properties of AgI/TiO2-NTs will probably be regulated due to different β/γ-AgI ratio. Besides, peaks corresponding to Ag0 species were not observed by XRD in the present study.33,34 To further corroborate the surface chemical state of AgI, XPS spectra was used to investigate the Ag 3d and I 3d on AgI/TiO2-NTs350. As shown in Figure 2A, two peaks of Ag 3d located at 367.9 eV (Ag 3d5/2) and 373.9 eV (Ag 3d3/2), which were very similar to the reported values for β-AgI.35 As for I 3d (Figure 2B), peaks appeared at 619.1 eV (I 3d5/2) and 630.6 (I 3d3/2) were ascribed to lattice I in AgI.23,28,36 In addition, no peaks corresponding to I2 or Ag0 were observed. Therefore, based on the above analysis, it can be deduced that the as-prepared AgI/TiO2-NTs films were constituted of β phase rich AgI, anatase TiO2 and Ti foil.

(A)

(B)

Ag 3d5/2 Ag 3d3/2 373.9 eV

366

368 370 372 374 Binding Energy (eV)

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I 3d3/2

Intensity (a.u.)

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Intensity (a.u.)

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376

615

630.6 eV

620 625 630 Binding Energy (eV)

635

Figure 2. XPS spectra of Ag 3d (A) and I 3d (B) for AgI/TiO2-NTs350.


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The morphology of AgI/TiO2-NTs350 was further characterized by SEM. As shown in Figure 3A, uniform and hollow TiO2-NTs fabricated via anodization were observed. The diameter of TiO2-NTs ranged from 80 nm to 100 nm. Small particles ascribing to AgI were also observed on TiO2-NTs (Figure 3B). Combing with the EDX spectrum which displayed signals of O, Ti, Ag, I and K (Figure 3C), the amount of AgI loaded on TiO2-NTs can be estimated. The atomic ratio of Ag to Ti is 1.25%, indicating very low content of AgI, which is consistent with the weak XRD signals of AgI in Figure 1. Ti

(C)

Element Atom %

O Ti Ag I K O Ti 0

56.55 38.51 0.48 0.52 0.38

Ti 1

2

AgK I 3 4

5

Energy (keV)

6

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Figure 3. SEM image (A and B) and corresponding EDX pattern (C) of the AgI/TiO2-NTs350 film.

3.2. The absorption of the as-prepared AgI/TiO2-NTs films. 2.0

Absorbance

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AgI/TiO2-NTs

1.6 350 °C 200 °C 500 °C

1.2

0.8 100 °C 300

TiO2-NTs350

400 500 600 700 Wavelength (nm)

800

Figure 4. UV-Vis-DRS of TiO2-NTs and AgI/TiO2-NTs calcined at different temperatures.


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The ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis-DRS) of TiO2-NTs and AgI/TiO2-NTs calcined at different temperatures were shown in Figure 4. Compared to pure TiO2-NTs350, all the AgI/TiO2-NTs films exhibited enhanced visible light absorption (λ > 400 nm). Interestingly, calcination temperature greatly affected the visible light absorption of AgI/TiO2-NTs, and 350 °C was found to be the optimal calcination temperature within the tested range. It is reasonable that AgI/TiO2-NTs350 exhibited the highest visible light absorption, since the more visible-light-sensitive β-AgI was observed to be the richest on AgI/TiO2-NTs350 by XRD analysis. Therefore, 350 °C was the optimal temperature in regulating the highest β-AgI content and visible light absorption of AgI/TiO2-NTs, which may finally lead to highly enhanced PEC activity. 3.3. PEC properties of the as-prepared AgI/TiO2-NTs films.

(A)-16 a: AgI/TiO -NTs100 b: AgI/TiO -NTs200 2 2 -14 c: AgI/TiO -NTs350 d: AgI/TiO -NTs500 2 2 c -12

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Figure 5. (A) Photocurrent response of different AgI/TiO2-NTs films in 0.1 M Na2SO4 aqueous solution under chopped visible light irradiation. (B) ESI Nyquist plots of different AgI/TiO2-NTs in 0.5 M Na2SO4 aqueous solution under visible light irradiation with a frequency range of 10-2~105 Hz and a scan rate of 5 mV/s.


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The PEC properties of the as-prepared AgI/TiO2-NTs films were firstly investigated by linear sweep voltammetry (LSV) method in the potential range of 0.2 V to 0.8 V (vs. Ag/AgCl). Under chopped visible light irradiation, huge difference can be observed. As illustrated in Figure 5A, highest photocurrent density was observed on AgI/TiO2-NTs350. Comparing with pure TiO2-NTs prepared at the same condition, the value of photocurrent density on AgI/TiO2-NTs350 was more than 15 times that on TiO2-NTs350 in the whole range of applied voltage (Figure S2). Therefore, it can be concluded that AgI loading led to significantly improved visible light response. Also, the photocurrent densities of AgI/TiO2-NTs initially increased as the film preparation temperature increased from 100 °C to 350 °C, followed by a decline when the temperature increased to 500 °C. Combined with the XRD analysis, calcination from 100 °C to 350 °C will lead to increased crystallinity of anatase TiO2-NTs, indicating the probable importance of crystallinity. However, decreased PEC were observed on AgI/TiO2-NTs500 despite of better anatase crystallinity. Besides, we previously found that amorphous TiO2 nanoparticles were better supports than crystallized ones for loading AgI,27 which is different from the results observed in the PEC test. Therefore, the effect of variation in β/γ ratio of AgI may be considered as the major reason. Herein, EIS Nyquist plots were also carried out to investigate the capacitance and resistance of the as-prepared AgI/TiO2-NTs film electrodes. As shown in Figure 5B, it can be observed that the radius of semi-circle on AgI/TiO2-NTs350 is the smallest under visible light irradiation, corresponding to highest conductivity and smallest resistance. Therefore, for the PEC properties, the joint effect of crystallinity and β-AgI content will lead to different performance of the asprepared AgI/TiO2-NTs films. 3.4. PEC activities of the as-prepared AgI/TiO2-NTs films.


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The PEC activities of the as-prepared AgI/TiO2-NTs films were investigated using Cr(VI) as model pollutant. AgI/TiO2-NTs and Ti sheet were used as anode and cathode, respectively. As shown in Figure 6A, huge difference can be observed among the tested samples. The highest Cr(VI) reduction rate was observed on AgI/TiO2-NTs350. By pseudo-first-order kinetic fitting (Figure 6B), the rate constant (kCr(VI)) for single Cr(VI) reduction was estimated to be 0.0155 min-1 on AgI/TiO2-NTs350, which was about 3 times that on AgI/TiO2-NTs100. Since amorphous TiO2 and better anatase crystalinity were confirmed on AgI/TiO2-NTs100 and AgI/TiO2-NTs500 by XRD, respectively, the greatly enhanced PEC activity can not be simply attributed to the phase transition of TiO2. The most probable reason was the variation of AgI phase. For comparison, TiO2-NTs350 without the loading of AgI was also tested. The kCr(VI) was calculated to be 0.0032 min-1. Therefore, it is AgI rather than TiO2 plays vital role for the PEC reduction of Cr(VI). (A)1.0

(B)1.2

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0.4

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Figure 6. PEC dynamics (A) and pseudo-first-order kinetics (B) for Cr(VI) reduction using different AgI/TiO2-NTs electrodes; The reduction dynamics (C) and pseudo-first-order kinetics (D) of Cr(VI) using AgI/TiO2-NTs350 electrode under different processes. External potential: 2.0 V, Electrolyte: 0.2 M Na2SO4, pH = 3.0.

Using AgI/TiO2-NTs350 as anode and Ti sheet as counter cathode, the reduction of Cr(VI) was compared in PC, electrocatalytic (EC) and PEC processes, respectively. Moreover, as a control experiment, the reduction of Cr(VI) was also carried in the photolysis process without a photoelectrode. As illustrated in Figure 6C, negligible activity was observed by photolysis. In single PC or EC process after 70 min reaction, the removal efficiency of Cr(VI) (ηCr(VI)) was only 8.6% or 28.5%, respectively. Whereas, a value of 66.5% was obtained in the PEC process. After curve fitting, the calculated pseudo-first-order kinetic constants for the above three processes (PC, EC and PEC) were estimated to be 0.0013 min-1, 0.0049 min-1 and 0.0155 min-1, respectively. Obviously, there is a synergistic effect in the PEC process. For quantitative evaluation of the synergistic effect, synergistic factor (denoted as SF) was used according to the following equation37:


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SF =

(1) +

Where kPC, kEC and kPEC represent the rate constant for Cr(VI) reduction in the PC, EC and PEC processes, respectively. A SF value of 2.5 was calculated in the PEC process for Cr(VI) reduction. Therefore, it can be inferred that the separation efficiency of photogenerated electronhole pairs can be greatly enhanced under the conditions of applied bias voltage.11 In the PEC process, the photogenerated electrons were attempted to be driven to the Ti cathode by applied external potential, leaving the oxidative holes on anode. Therefore, coexisting organic pollutant will probably improve the electron-hole separation. As a chelating agent to heavy metal, EDTA has been extensively applied in the field of photography, detergents, pulp, paper agrochemicals and printed circuit board, which cause EDTA pollution in such wastewater.38–40 Herein, the effect of coexisting EDTA on the reduction of Cr(VI) was investigated. The concentration of Cr(VI) and EDTA were measured using the 1,5diphenylcarbazide colorimetric method and HPLC, respectively (see details in Experimental section). As shown in Figure 7A (Left), the value of ηCr(VI) after 70 min PEC reaction increased from 66.5% to 95.2% when 1.0 mM EDTA was coexisted with Cr(VI). Meanwhile, EDTA was simultaneously degraded. As shown in Figure 7A (Right), the removal efficiency of EDTA reached 84.8% at the same time. Thus, simultaneous reduction of Cr(VI) and degradation of EDTA was achieved in the PEC process with fast reaction rate.


15


60

0.6 0 mM EDTA

40

0.4 1.0 mM EDTA

0.2 0.0

0


60

20 70

0

(B) 0.05 y = 0.0167 + 0.0255x -1

kCr(VI) (min )

80

0.8

EDTA Removal efficiency (%)

100

(A) 1.0

Cr(VI) C/C0

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 16 of 32

0.04

2

R = 0.989

0.03 0.02 0.01

0.0 0.2 0.4 0.6 0.8 1.0 EDTA concentration (mmol/L)

Figure 7. (A) The effect of EDTA concentration on the PEC reduction of Cr(VI) (Left) and simultaneous removal efficiency of 1.0 mM EDTA (Right ) using AgI/TiO2-NTs350 electrode; (B) Relationship between apparent rate constant kCr(VI) and coexisting EDTA concentration. External potential: 2.0 V. Electrolyte: 0.2 M Na2SO4, pH = 3.0.

Moreover, the effect of EDTA concentration on PEC reduction of Cr(VI) was also studied in detail and the results were depicted in Figure 7B. It can be observed that the reduction dynamics for Cr(VI) increased gradually with increasing EDTA concentration. And the estimated kCr(VI) exhibited good linear correlation with EDTA concentration. Therefore, Cr(VI) is more readily reduced when coexisting with organic pollutant. Since external potential was very important in the PEC process, the effect of external potential on the PEC reduction of Cr(VI) was studied both in the absence and presence of a hole scavenger (EDTA). Different external potentials ranging from 0.5 V to 2.5 V were tested, and the results were shown in Figure 8. In the absence of EDTA, the reduction of Cr(VI) followed first order kinetics (Figure 8A). Enhanced PEC activity were obtained at high external potential. Increasing pseudo-first-order rate constants (kPEC) can be observed with increasing external potentials


16

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(Figure 8B). Moreover, the extent of the electrochemical enhancement (E, in %) can be estimated from the following equation:41,42

E=(

-

) × 100% (2)

As shown in Figure 8B without the presence of EDTA, the value for E started at 18.8% with 0.5 V external potential, while the value drastically increased to 58.1%, 82.9%, 91.6% and 96.1% with increasing external potential of 1.0, 1.5, 2.0 and 2.5 V, respectively. However, little difference for E were obtained when the external potential was larger than 2.0 V. Taking into account the economic benefits and energy saving reasons, it is better to chose 2.0 V in the following studies.

in the absence of EDTA

in the absence of EDTA

(B) 0.03 -1

kPEC (min

0.6

0.02

0.5 V 1.0 V 1.5 V 2.0 V 2.5 V

0.4 0.2 0.0

80

)

0.8

100

0

10

60

E (%)

(A) 1.0

Cr(VI) C/C0

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


40

0.01 20


70

0.00

0.5

1.0 1.5 2.0 Applied Potential (V)

2.5

0


17


in the presence of EDTA

(C) 1.0

in the presence of EDTA

(D) 0.08

100 80

0.4

0.5 V 1.0 V 1.5 V 2.0 V 2.5 V

0.2 0

10

E ( %)

kPEC (min

0.6

0.0

0.06

-1

)

0.8

Cr(VI) C/C0

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 18 of 32

60

0.04

40 0.02


70

0.00

20 0.5

1.0 1.5 2.0 Applied Potential (V)

2.5

0

Figure 8. The effect of external potential on the PEC reduction of Cr(VI) on AgI/TiO2-NTs350 under visible light irradiation in the absence of EDTA (A) and (B), in the presence of 0.1 mM EDTA (C) and (D). Electrolyte: 0.2 M Na2SO4, pH = 3.0.

Moreover, the effect of external potential on the PEC reduction of Cr(VI) was also investigated in the presence of 0.1 mM EDTA. As shown in Figure 8C, similar trend was observed with increasing external potential. Moreover, the E value increased drastically from 42.0% at 0.5 V to 80.2% at 1.0 V. When the external potential was larger than 1.0 V, the trend for E increasement slow down. At 2.0 V, the value of E was estimated to be 93.6% in the presence of 0.1 mM EDTA, while the value was 91.6% in the absence of EDTA. Considering the little difference of E in the absence/presence of EDTA at 2V, the following experiments were conducted in the absence of EDTA to further investigate the effect of solution pH and electrolyte concentration.


18

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(B) 1.0

(A) 1.0

Na2SO4 concentration 0.8

0.6

Cr(VI) C/C0

0.8

Cr(VI) C/C0

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


0.6

pH 7.0 pH 5.0 pH 4.0 pH 3.0 pH 2.0

0.4 0.2 0

10

0M 0.2 M 0.5 M 1.0 M 1.5 M

0.4 0.2


70

0

10


60

70

Figure 9. (A) The effect of pH on the PEC reduction of Cr(VI) in 0.2 M Na2SO4 in the absence of EDTA; (B) The effect of electrolyte concentration on the PEC reduction of Cr(VI). Reaction condition: AgI/TiO2-NTs350, pH = 3.0.

pH was always recognized as an important influence factor on the reduction of Cr(VI). Proton supply to the TiO2/solution interface was reported to play a vital role in the UV irradiated system.9 Therefore, the influence of initial solution pH in the present PEC process was studied from pH 2.0 to pH 7.0. As shown in Figure 9A and Table S1 (entry 1-5), the PEC dynamics for Cr(VI) reduction on AgI/TiO2-NTs350 were dependent of pH under visible irradiation. The reduction of Cr(VI) became significantly slower at higher pH values. For example, at pH ≥ 5, the reduction efficiency after 70 min PEC reaction was less than 15%, while at pH 2, the value was 81.3%. Apparently, low pH is beneficial to Cr(VI) reduction. Since CrO42- (pH > 6) and HCrO4(pH 1–6) are predominant existing states of Cr(VI) at medium to low pH values, the above phenomenon can be explained by the following equation due to the consumption of H+: CrO42- + 8H+ + 3e- → Cr3+ + 4H2O

(3)

HCrO4- + 7H+ + 3e- → Cr3+ + 4H2O

(4)


19


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Page 20 of 32

Moreover, the valence band of the photoanode will shift to more negative potential together with the conduction band at higher pH, leading to decreased oxidative ability. The redox potential of Cr2O72-/Cr3+ shifts -138 mV/pH with the increase in pH, while the conduction band edge of a semiconductor shifts -59 mV/pH.9,43 Though Cr(VI) was reduced by capturing electrons on the Ti cathode, a drop of 79 mV per pH increment may lead to decreased thermodynamic driving force for Cr(VI) to capture the photogenerated electrons isolated by electric field. Besides, Cr(III) will be precipitated onto the surface of TiO2 at high pH, which may covered the active sites leading to decreased reduction efficiency. Above all, the pH of the reaction solutions was adjusted to pH 3 in the whole study unless otherwise mentioned. The present study has verified the synergistic effect in the PEC process. Comparing with single PC process, an applied external potential will be beneficial for the separation of photogenerated electron-hole pairs on AgI/TiO2-NTS350 anode. The photogenerated electrons will be driven to the Ti cathode through an external circuit, leaving holes on anode. The conductivity as well as PEC activity will be affected by the concentration of electrolyte.44,45 Herein, the effect of electrolyte (Na2SO4) concentration on the PEC reduction of Cr(VI) was also studied, and the results were shown in Figure 9B and Table S1 (entry 6-10). Enhanced PEC activity can be observed with increasing Na2SO4 concentration. For example, the reduction efficiency of Cr(VI) after 70 min PEC reaction was 66.5% in 0.2 M Na2SO4 solution, while the value increased to 81.6% in 1.5 M solution. The estimated pseudo-first-order rate constants kCr(VI) exhibited good linear correlation with electrolyte concentration (Figure S3). Therefore, it can be inferred that the as-prepared AgI/TiO2-NTS350 anode is very suitable for handling high salinity wastewater containing Cr(VI).


20

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1.0

nd

st 1 run 2

0.9

th

rd th run 3 run 4 run 5 run

0.8

Cr(VI) C/C0

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


0.7 0.6 0.5 0.4 0.3

0

50

100 150 200 250 300 350 Irradiation time (min)

Figure 10. Reuse of AgI/TiO2-NTs350 for PEC reduction of Cr(VI) under visible light irradiation at an external potential of 2.0 V. Electrolyte: 0.2 M Na2SO4, pH = 3.0.

The stability of the preferred AgI/TiO2-NTs350 film electrode in cyclic runs was another important factor for future practical application. Herein, five cyclic runs were conducted by just taking the AgI/TiO2-NTs350 film out of solution and washed for the next successive run. It can be observed from Figure 10 that the PEC activities were barely declined. After five cycling runs, the reduction efficiency of Cr(VI) was 62.5%, while the value was 66.5% in the first run after 70 min PEC reaction. Therefore, the present PEC system presents superior visible light PEC performance and recyclable stability, which is a promising technology in Cr(VI)-containing wastewater treatment. 3.5. Proposed mechanism. In order to ascertain the mechanism for PEC reduction of Cr(VI), experiments were carried out in two separate glass cells (100 mL) using salt bridges as connection (Figure 11A). AgI/TiO2-NTs350 film and Ti sheet were used as anode and cathode in separate glass cells, respectively. The initial Cr(VI) concentration was the same in the two glass cells. As shown in


21


Figure 11B, after 70 min PEC reaction, about 90% Cr(VI) was reduced in the cathode cell, while the value was 12% in the anode cell. It should be noted that the reduction of Cr(VI) in the anode cell was due to the effect of EDTA, since about 11% Cr(VI) was reduced in Cr(VI)-EDTA mixture without the presence of a photoanode (control experiment, data not shown). Therefore, it can be inferred that the reduction of Cr(VI) occurs on the cathode surface, which is dramatically different from the reduction of Cr(VI) on the surface of a photocatalyst in the PC process. Herein, photogenerated holes (h+) were left on the anode. The ability of h+ in the oxidative decomposition of H2O to O2 or degradation of organic pollutants will probably influence the reduction dynamics on the cathode.

(B) 1.0

Anode cell

0.8

Cr(VI) C/C0

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 22 of 32

0.6 0.4

Cathode cell

0.2 0.0

0

10


60

70

Figure 11. (A) Schematic process and (B) reduction dynamics for the PEC reduction of Cr(VI) in two separate glass cells using salt bridges as connection. 80 uM Cr(V), 1mM EDTA, pH 3.

To evaluate the oxidative ability of AgI/TiO2-NTs, the valence band position (EVB) should be estimated, which can be indirectly obtained from the band gap (Eg) and conduction band (ECB) via the following equation:

EVB = Eg + ECB (5)


22

Page 23 of 32

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


ECB can not be directly measured, but flat band potential (Efb) was always used to evaluate the conduction band edge. Herein, Mott-Schottky (M-S) analyses were carried out to measure Efb and the results were shown in Figure S4. It can be seen that the Efb shifts positively from ca. 0.45 V (vs Ag/AgCl) for AgI/TiO2-NTs100 to -0.26 V (vs Ag/AgCl) for AgI/TiO2-NTs350, then increased to -0.17 V (vs Ag/AgCl) for AgI/TiO2-NTs500. Since the CB of n-type AgI or TiO2 shifts 59 mV/pH towards lower pH, the Efb values at pH 3 were further estimated for consistency of experimental condition. For example, the Efb of AgI/TiO2-NTs350 was -0.024 V vs Ag/AgCl (0.173 V vs SHE) at pH 3. Combined with the XRD and UV-Vis-DRS analysis, crystallinity of anatase TiO2-NTs and Efb value were both increased after calcination (from 100 °C to 500 °C). Meanwhile, AgI/TiO2NTs350 displayed the highest visible light absorption, corresponding to smallest Eg. According to equation (4), decreased Eg will be partially offset by increased Efb (ECB). Therefore, the oxidative ability of photo-generated holes in VB can be well maintained on AgI/TiO2-NTs350. Moreover, AgI/TiO2-NTs350 also exhibited highest photocurrent density, greatly reduced charge transfer resistance, and easily transferred photogenerated electron–hole pairs (Figure 5). Therefore, due to the combined effect, the optimal activity was achieved for simultaneous oxidative reaction on AgI/TiO2-NTs350 photoanode and reduction of Cr(VI) on cathode.


23


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Page 24 of 32

Figure 12. Schematic illustration for the PEC reduction of Cr(VI) under visible light irradiation.

Based on the above analysis, a proposed mechanism can be illustrated in Figure 12. Under visible light irradiation, e–- h+ pairs can be generated on AgI/TiO2-NTs. With the help of external potential, the photogenerated e– can be more efficiently separated and transferred to the cathode. Finally, Cr(VI) will be reduced by capturing e– (including photogenerated and electrogenerated) on the cathode, while h+ will attack H2O/organics in the absence/presence of organics.

4. CONCLUSIONS The present study reported the facile preparation of visible-light-active AgI/TiO2-NTs film with rich β-AgI and highly enhanced PEC performance. Simple calcination led to adjustable β/γ ratio of AgI. After calcination at 350 oC, highest β/γ ratio of AgI as well as visible light absorption was achieved on AgI/TiO2-NTs350. Moreover, greatly enhanced photocurrent response and reduced charge transfer resistance were also observed under visible light irradiation. Due to easier generation and separation of electron-hole pairs, the PEC reduction of Cr(VI) was significantly enhanced using the optimal AgI/TiO2-NTs350 as photoanode and Ti foil as cathode, respectively. Moreover, good stability of AgI/TiO2-NTs350 was also observed in cyclic runs.


24

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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


Therefore, this study provides an efficient and convenient way for the disposal of Cr(VI)containing wastewater.

Supporting Information Supporting information includes enlarged XRD spectra (Figure S1), LSVs photocurrent (Figure S2), effect of electrolyte concentration (Figure S3) and M-S plots (Figure S4). Influence of various factors on the reduction kinetics and removal efficiencies of Cr(VI) were also presented in Table S1.

ACKNOWLEDGEMENT This work was financially supported by Zhejiang Provincial Natural Science Foundation of China (LY14B070002, LY14E080002), National Natural Science Foundation of China (21477114, 21576237), and Talented Youth Fund Projects (QZ13-7) of Zhejiang Gongshang University.


25


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degradation of Acid Orange 7 azo dye using a highly stable TiO2 photoanode synthesized by atmospheric plasma spray. Appl. Catal. B: Environ. 2013, 132–133, 142. (43) Wang, X.; Pehkonen, S.O.; Ray, A.K. Removal of aqueous Cr(VI) by a combination of photocatalytic reduction and coprecipitation. Ind. Eng. Chem. Res. 2004, 43, 1665. (44) Hao, F.; Lin, H.; Zhang, J.; Zhuang, D.; Liu, Y.; Li, J. Influence of iodine concentration on the photoelectrochemical performance of dye-sensitized solar cells containing non-volatile electrolyte. Electrochim. Acta 2010, 55, 7225. (45) Chen, J.; Xia, Y.; Dai, Q. Electrochemical degradation of chloramphenicol with a novel Al doped PbO2 electrode: Performance, kinetics and degradation mechanism. Electrochim. Acta 2015, 165, 277.


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Table of Contents

AgI/TiO2-NTs350

exhibited

greatly

enhanced

photocurrent

density

and

photoelectrocatalytic reduction of Cr(VI) under visible light irradiation. The reason was ascribed to increased portion of β-AgI with high visible light absorption, which led to more efficient separation and easier transfer of photogenerated electron-hole pairs.


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Preparation and Photoelectrochemical ... - ACS Publications

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