Highly enhanced dye sensitized photocatalytic oxidation of arsenite over TiO2 under visible light by I− as an electron relay

Electrochemistry Communications 22 (2012) 185–188

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Highly enhanced dye sensitized photocatalytic oxidation of arsenite over TiO2 under visible light by I − as an electron relay Xiang Li, Wenhua Leng ⁎ Department of Chemistry, Zhejiang University, Hangzhou, 310027, China

a r t i c l e

i n f o

Article history: Received 21 May 2012 Received in revised form 17 June 2012 Accepted 19 June 2012 Available online 29 June 2012

a b s t r a c t A highly efficient, regenerated and stable dye-sensitized photocatalytic system for the pre-oxidation of As(III) over nanostructured TiO2 films to less toxic As(V) under visible light (λ ≥ 420 nm) using ruthenium dye as a model sensitizer and I− as an electron mediator is reported. The oxidation rate could be greatly enhanced by I − especially for low concentrations of As(III). © 2012 Elsevier B.V. All rights reserved.

Keywords: Photoelectrochemistry Photocatalysis Arsenite oxidation Dye-sensitization

1. Introduction Arsenic contamination is a serious worldwide environmental problem threatening millions of people's health, [1,2] with which many approaches, such as UV/Fe(III)-complexes,[3] Fe(II)/H2O2 [4], TiO2 photocatalysis [1,2,5–7] and etc., have been taken to cope. Among them, TiO2 photocatalysis has been demonstrated as a promising method for arsenic contaminated water through oxidizing As(III) to less toxic and less mobile As(V) in many studies [1,2,5–7]. Unfortunately, TiO2 cannot directly absorb visible light. An alternative way of achieving utilization of visible light for the decomposition of environment pollutants is through dye-sensitized photocatalytic oxidation (SPCO) over TiO2.[8–10] The self-sensitized degradation of dye pollutants under visible light by electron initiated active oxygen species (such as O2●−) has been widely demonstrated.[8] But such sacrificed SPCO finally stops once the dye is exhausted. [8] Application of regeneration SPCO in wastewater treatment, has received far less attention, [9,10] although it has been applied in dye-sensitized solar cells [11–13], and the oxidation rate of pollutant or the regeneration rate of dye cations (S +) is often limited by the concentration of solute because it is generally very low in water. Here, a new and regenerated SPCO system for the pre-oxidation of As(III) over TiO2 films under visible light (λ ≥ 420 nm) using a popular ruthenium dye (bistetrabutylammonium salt of the complex cis-bis (isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium (II), C58H86O8N8S2Ru) in dye solar cells as a model sensitizer and I − as an electron mediator are described. Notably, the SPCO of As(III) without ⁎ Corresponding author. Tel.: +86 571 87952318; fax: +86 571 87951895. E-mail address: [email protected] (W. Leng). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.06.026

iodide have been reported by Choi et al. [5,10] and us [14], and the SPCO of I − over on K4Nb6O17 to produce H2 has been demonstrated [15]. 2. Experimental section The ruthenium dye (Solaronix) was used as received. All other chemicals and solvents were at least of analytical grade. Milli-Q-Water (Millipore Corp, 18.2 MΩ cm at 25 °C) was used in all experiments. TiO2 and dye/TiO2 film electrodes were prepared on conducting glass as described in refs [16] and [14], respectively. The film thickness was ~10.5 ± 0.5 μm (profilometer, Veeco Dektak 150). The working geometric surface area of TiO2 was 1.0× 1.0 cm and 5.5 cm× 7.0 cm for (photo) electrochemical and SPCO experiments, respectively. Experiments of As(III) oxidation were the same as that described previously [7] except with visible light illumination (500 W Xe lamp equipped with both water and 420 nm glass filters). Briefly, they were conducted in a photo-reactor with 50 cm3 electrolytes that holds a dye/TiO2 film as working electrode and a saturated calomel electrode (SCE) as reference electrode. A large surface area of Pt plate was used as counter electrode. The incident light intensity into the reactor cell was ca. 32.7 mW cm−2 (λ ≥ 420 nm) detected by a photodiode (S370, UDT Instruments, and U.S.A.). The concentration of As(V) and As(III) in the solution was estimated by spectrophotometer described previously [7] and electrochemical detection on Pt electrode[17,18], respectively. The tested solutions of 0.5 mol dm−3 NaClO4 with or without As(III) (pH = 2.0 + 0.1) were employed in all experiments. All experiments were performed on an electrochemical workstation (CHI660a, Shanghai Chenghua Corp., China) and normally repeated at least twice. All potentials in the text are vs. SCE otherwise specified.

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a

3. Results and discussion

[As(V)] (µM)

400

300

TiO2/dye TiO2/dye+50 mM ITiO2/dye+100 mM I-

200

Air only IDye+ITiO2+I-

100

0 0

60

120

180

240

t (min)

b 50 mM I-

20

Initial rate (µM min-1)

No I-

16

12

8

4

0 10

100

1000

[As(III)] (µM)

c

-

[As(III)]0 = 500 µM; 50 mM I

500

open circuit

0.0 V+ N2

400

[As(V)] (µM)

The data displayed in Fig. 1a clearly indicate that the formation of As(V) was mainly attributed to photoreaction over the dye/TiO2 because it was much smaller when using I −, dye + I −, or TiO2 + I − alone. Conversion of ~ 95% of 500 μM As(III) took only ~ 90 min with 100 mM I − while needed ~ 240 min without I − [14]. The photo-oxidation rate over the dye/TiO2 increased almost linearly with the concentration of As(III) with or without I −, but the later is greater for all concentrations of As(III) studied (Fig. 1b), particularly more evident for low concentrations of As(III) where the rates are more than doubled. Notably, the light absorption of the dye/TiO2 film with and without I − is almost the same (not shown). All these results indicate that I − facilitated the SPCO of As(III) under open-circuit conditions. Furthermore, highly enhanced SPCO rates or apparent rate constants (both are equivalent due to an observed apparent zero-order reaction; see the insert of Fig. 2b) by I − were also found without oxygen but with an anodic bias for all concentrations of As(III) studied (Fig. 2); for instance, it was increased by ~ 9 times for 500 μM As(III) at 0.4 VSCE. Since no other oxidant except S + was produced here (no active oxygen and I − not being oxidized or reduced, Fig. 3a), the enhanced oxidation should result only from more S + involved in the oxidation either directly or indirectly via the products of S + reacting with I − (denoted as Iinterm), in consistent with that a much higher photoanodic current was observed with I − (Fig. 3b). The columbic efficiency, η, defined as the ratio of As(V) and interfacial transferred charge,[7] as shown in Fig. 2, is almost 100% for [As(III)]0 ≥ 300 μM at which the adsorption of As(III) on the dye/ TiO2 is saturated [14], regardless of with or without I −, suggesting that nearly all the separated S + and Iinterm serve to the As(III) photo-oxidation and that both species seem to be completely regenerated; while for [As(III)]0 b 300 μM, thatηS+ b 100% is most probably because the surface concentration of As(III) is too low to cause the active species to be deactivated. The steady-state photocurrent increased sharply with I − initially and then appears to reach a plateau (insert of Fig. 3b). Since the oxidation rate of As(III) is proportional to the photocurrent or passed charge, it can be highly promoted by increasing the concentration of I −, particularly for low concentrations of As(III) where the oxidation is limited by its reaction with S + whereas it can be enhanced by increasing the concentration of I −. Moreover, the incident photon conversion efficiency of the dye/TiO2 film with I − is higher than its absence (Fig. 4) from 420 nm up to 650 nm, in the range of which rare inorganic photocatalyst can be so photoactive at 0.0 VSCE, to our knowledge. The lower efficiencies compared to those [13] of non-aqueous dye solar cells may be due to electrolyte diffusion limitation in the porous TiO2 film [19]. The mechanism of SPCO of As(III) without I − involves the generation of active oxygen by injected electrons [7] and S + (Figs. 1 and 2) as illustrated in the Table of Contents (TOC) graphic. I − functioning as an electron donor in dye solar cells to regenerate S + has been well-documented, [11–13] which is also the present case supported by the observation that the photocurrent increased with its concentration without As(III) (insert of Fig. 3b). Such regeneration may occur through one-electron reaction producing I● and/or two-electron reaction forming I2●− which was found able to quantitatively disproportionate to yield I3− in acetonitrile [12,13]. At pH=2, these three species (E0(I●/I−)=+1.33 VNHE; E0(I2●/2I−)=+1.03 VNHE; E0(I3−/I−)=+0.54 VNHE [12]) can spontaneously oxidize As(III) in the thermodynamically if the potential of S+ still holds in water (E0(S+/S)=+1.10 VNHE in acetonitrile [11]), considered that the major species of As(III) and As(V) in acid aqueous solution are HAsO2 (H3AsO3 or As(OH)3) and H2AsO4−, respectively, [1] and that the reduction potential of this redox in acid solutions is ca 0.5 VNHE (H2AsO4− +3 H+ +2e− =HAsO2 +2H2O, E0 =0.67 VNHE [20]). I● is unlikely to be the main oxidant of As(III) in our system, because the potential

[As(III)]0 = 500 µM

500

300

200

1st 2 3

1st times 2nd

100

0 0

60

3rd

5 10

5th

15th

120 180

t (min)

0

20

40

60

t (min)

Fig. 1. (a) Formation of As(V) under illumination and open circuit at different conditions; (b) As(III) concentration dependence of initial photooxidation rate (illumination for 10 min); (c) repeat photooxidation of As(III) on the dye/TiO2 film under open circuit and at 0.0 V. Air-saturated.

of S+ is not large enough to drive oxidation of I− to I●. I3− has been demonstrated to be able to oxidize As(III) at pH=3 aqueous solutions, but the rate was only ~10 μM min−1 even if I3− =100 μM [21], much less than the maximum value we observed here. In addition, I3− or I2 was not detectable spectrometrically during the SPCO until when nearly all As(III)

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a) Dark

a) 500 µm As(III) 120 30

0 100 air+50 mM I-

80

20

η

kapp

-

-

Light+N2+I

With I -

2

No I

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60

Light+N

J st (µA)

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η (%)

kapp (µM min-1)

-200

N2+50 mM I-

-600

N

2

40

Dark+air+IDark+air

air

-800

20

[As3+]= 500 µM

-1000

dark+air+ I-

0 0.6

0 -0.4

-0.2

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0.4

-1200 -0.3 -0.2 -0.1 0.0

25

with I-

with I-

no I-

no I-

6000 140

20

80 10

0.3 mM+ I-; 1 mM + I-; Linear fitted

[As(V)] (µM)

1200

60

0.3 mM 1 mM

η (%)

15

J st ( µ A)

100

kapp (µM min-1)

4000

120

Photocurrent (µA cm-2)

η

kapp

600

200

0

0.5

60

[I- ] / mM

60

1500 2000 2500

air+50 mM I-

0 [As3+]= 500 µM

-0.2

120

0 3000

[As(III)]0 (µM) Fig. 2. Apparent rate constants (kapp) and columbic efficiency (η) of oxidation of As(III) over the dye/TiO2 versus applied potential (a) and concentration (b) with and without 50 mM I−. Illumination time normally b60 min.

was converted to As(V), from which further irradiation made the solutions get yellow (I − may be recovered via I2 when As(III) is exhausted). Hence, I3− is also unlikely to be the oxidant of As(III) in our system. So, the main oxidant appears to be I2●− (see TOC graphic). The results of dark oxidation of As(III) by the electro-generated active oxygen indicate that addition of I− decreased the η of active oxygen (Fig. 2a) though increased the dark current (Fig. 3a). The reason is not very clear at present. Such a slightly negative effect of I − on the active oxygen-induced oxidation of As(III) partly offsets the positive effect of S+-induced oxidation. This may be the main reason why the accelerated oxidation at open-circuit is not as significant as that under an anodic bias and without O2. The repeat experiments of SPCO of As(III) over an identical piece of dye/TiO2 film under both open-circuit and potential bias showed that the rate was almost the same for several runs (up to 15 h, Fig. 1c), demonstrating that the electrode has excellent stability against dye desorption or degradation under the present conditions, in consistent with ~100% of columbic efficiency. The current strategy is rather unique as it firstly employs I\I bonds as a buffer of light energy and then efficiently release to As(III).

N2+50 mM I-

air

0.0

0.2

0.4

Potential (V vs SCE) Fig. 3. Steady-state current-potential curves of the dye/TiO2 electrode in the dark (a) and under illumination (b) saturated with air or N2, respectively. Current data were recorded for 2 min.

4. Conclusions A new regenerated dye sensitization system using I − as an electron mediator for efficiently oxidizing As(III) to As(V) has been realized. The rate can be highly accelerated by I − under both open-circuit and potential bias without O2. The current study of

TiO2

2.5

Dye/TiO2 Dye/TiO2+I-

2.0

IPCE (%)

0

120

N2

Time / min

1000

0.4

0

2000

20 0

500

0.3

[As(III)] / mM 0 0.1 0.2 0.5 1 5

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40

0

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b) Light

b) E = 0.2 V

5

0.1

Potential (V vs SCE)

E (V vs SCE)

[As3+]= 500 µM E = 0.0 V

1.5

1.0

0.5

0.0 450

500

550

600

650

Wavelength (nm) Fig. 4. Incident photon conversion efficiency (IPCE) of the dye/TiO2. N2-saturated; other experimental conditions were the same as those in Ref [7].

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using an electron relay for achieving efficient regeneration of S + and oxidation of target pollutants will provide a new strategy for large scale solar energy remediation technologies (may also apply for other dyes), and will be also relevant for large scale solar hydrogen production in As(III)-rich contaminated wastewaters. Acknowledgments This project was supported by the National Science Foundation Council of China (NSFC, Grant No. 50971116) and the National Basic Research Program of China (Grant No. 2011CB936003). References [1] H. Lee, W. Choi, Environmental Science and Technology 36 (2002) 3872. [2] S.-H. Yoon, S.-E. Oh, J.E. Yang, J.H. Lee, M. Lee, S. Yu, D. Pak, Environmental Science and Technology 43 (2009) 864. [3] B.D. Kocar, W.P. Inskeep, Environmental Science and Technology 37 (2003) 1581. [4] S.J. Hug, O. Leupin, Environmental Science and Technology 37 (2003) 2734. [5] J. Ryu, W. Choi, Environmental Science and Technology 38 (2004) 2928. [6] S.H. Yoon, J.H. Lee, Environmental Science and Technology 39 (2005) 9695.

[7] H. Fei, W. Leng, X. Li, X. Cheng, Y. Xu, J. Zhang, C. Cao, Environmental Science and Technology 45 (2011) 4532. [8] C. Chen, W. Ma, J. Zhao, Chemical Society Reviews 39 (2010) 4206. [9] W. Zhao, Y. Sun, F.N. Castellano, Journal of the American Chemical Society 130 (2008) 12566. [10] Y. Park, S.-H. Lee, S.O. Kang, W. Choi, Chemical Communications 46 (2010) 2477. [11] G. Boschloo, A. Hagfeldt, Accounts of Chemical Research 42 (2009) 1819. [12] J.G. Rowley, B.H. Farnum, S. Ardo, G.J. Meyer, Journal of Physical Chemistry Letters 1 (2010) 3132. [13] A.Y. Anderson, P.R.F. Barnes, J.R. Durrant, B.C. O'Regan, Journal of Physical Chemistry C 115 (2011) 2439. [14] X. Li, W. Leng, Journal of Physical Chemistry C 116 (2012), submitted for publication. The data without I- were from there. [15] Y.I. Kim, S. Salim, M.J. Huq, T.E. Mallouk, Journal of the American Chemical Society 113 (1991) 9561. [16] W.H. Leng, P.R.F. Barnes, M. Juozapavicius, B.C. O'Regan, J.R. Durrant, Journal of Physical Chemistry Letters 1 (2010) 967. [17] G. Hignett, J.D. Wadhawan, N.S. Lawrence, D.Q. Hung, C. Prado, F. Marken, R.G. Compton, Electroanalytical 16 (2004) 897. [18] P. Tomčík, S. Jursa, Š. Mesároš, D. Bustin, Journal of Electroanalytical Chemistry 423 (1997) 115. [19] C. Law, S.C. Pathirana, X. Li, A.Y. Anderson, P.R.F. Barnes, A. Listorti, T.H. Ghaddar, B.C. O'Regan, Advanced Materials 22 (2010) 4505. [20] In: A.J. Bard, R. Parsons, J. Jordan (Eds.), Standard Potential in Aqueous Solution, Marcel Dekker, New York, 1985, p. 166. [21] J. Yeo, W. Choi, Environmental Science and Technology 43 (2009) 3784.