Accepted Manuscript Title: Photovoltammetric behavior and photoelectrochemical determination of p-phenylenediamine on CdS quantum dots and graphene hybrid film Author: Yuhan Zhu Kai Yan Yong Liu Jingdong Zhang PII: DOI: Reference:

S0003-2670(15)00595-4 http://dx.doi.org/doi:10.1016/j.aca.2015.05.007 ACA 233912

To appear in:

Analytica Chimica Acta

Received date: Revised date: Accepted date:

9-4-2015 5-5-2015 7-5-2015

Please cite this article as: Yuhan Zhu, Kai Yan, Yong Liu, Jingdong Zhang, Photovoltammetric behavior and photoelectrochemical determination of pphenylenediamine on CdS quantum dots and graphene hybrid film, Analytica Chimica Acta http://dx.doi.org/10.1016/j.aca.2015.05.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Photovoltammetric

behavior

and

photoelectrochemical

determination of p-phenylenediamine on CdS quantum dots and graphene hybrid film

Yuhan Zhu, Kai Yan, Yong Liu, Jingdong Zhang*[email protected]

Key Laboratory for Large-Format Battery Materials and System (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, P.R. China

*

Corresponding author. Tel: +86-27-87543032. Fax: +86-27-87543632.

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Graphical Abstract

2

Highlights ► Photovoltammetric behavior of PPD on CdS-GS hybrid film was studied. ► GS doped in CdS greatly improved the photoelectrochemical response of PPD. ► CV of PPD on CdS-GS film became a sigmoidal shape under photoirradiation. ► Novel photoelectrochemical strategy for PPD determination was developed.

3

Abstract

A photoelectroactive film composed of CdS quantum dots and graphene sheets (GS) was coated on F-doped SnO2 (FTO) conducting glass for studying the electrochemical response of p-phenylenediamine (PPD) under photoirradiation. The result indicated that the cyclic voltammogram of PPD on CdS-GS hybrid film became sigmoidal in shape after exposed under visible light, due to the photoelectrocatalytic reaction. Such a photovoltammetric response was used to rapidly optimize the photoelectrocatalytic activity of hybrid films composed of different ratios of CdS to GS toward PPD. The influences of scan rate and pH on the photovoltammetric behavior of PPD on CdS-GS film revealed that although the controlled step for electrochemical process was not changed under photoirradiation, more electrons than protons might participate the photoelectrocatalytic process. Furthermore, the photoelectroactive CdS-GS hybrid film was explored for PPD determination based on the photocurrent response of film toward PPD. Under optimal conditions, the photocurrent signal on CdS-GS film was linearly proportional to the concentration of PPD ranging from 1.010-7 to 3.010-6 mol·L-1, with a detection limit (3S/N) of 4.310-8 mol·L-1. Our work based on CdS-GS hybrid film not only demonstrated a new facile photovoltammetric way to study the photoinduced electron transfer process of PPD, but also developed a sensitive photoelectrochemical strategy for PPD determination.

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Keywords

Photovoltammetric behavior; Photoelectrochemical determination; CdS quantum dots; Graphene; p-Phenylenediamine

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1. Introduction As a newly emerged but promising analytical technique, photoelectrochemical (PEC) detection based on photon-to-electricity conversion has attracted considerable interest due to its high sensitivity as well as simple and cheap instrumentation [1]. To fabricate PEC sensors, various photoactive materials have been explored as transducer species which can convert photoirradiation to electronic response. Among these, CdS quantum dots (QDs) have been extensively employed in visible light-responsive devices because of their narrow band gap and size tunable optical properties [2-5]. Nevertheless, the semiconducting property of CdS is disadvantageous to electron transfer. To improve the PEC response, hybridization of CdS QDs with graphene possessing high electrical conductivity has been proposed in several PEC sensing systems [6-8]. In PEC processes, analytes can be either reduced by photogenerated electrons or oxidized by photogenerated holes, which leads to distinct PEC responses under different conditions [9,10]. Thus, electrochemical analysis under photoirradiation not only promotes the sensitivity of detection, but also provides a useful approach to understanding the photoinduced electron transfer processes. Masaoka et al. have recently investigated the electrochemical responses of metal complexes under photoirradiation by cyclic voltammetry [11]. However, to the best of our knowledge, the voltammetric characteristics in photoirradiation process on photoactive electrode have seldom been discussed specifically.

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As a typical aromatic diamine, p-phenylenediamine (PPD) has been widely used in polymer, textile and hair dye industries [12]. However, PPD can be absorbed percutaneously by living beings, and may lead to significant damage due to the risks of allergenic, toxic, and mutagenic effects [13-15]. Various analytical techniques have been established to determine PPD, including UV spectrophotometry [16], gas chromatography-mass spectrometry (GC-MS) [17,18], high performance liquid chromatography (HPLC) [19,20], capillary zone electrophoresis (CZE) [21], micellar electrokinetic capillary chromatography (MEKC) [22,23], fluorescence [24] and electrochemical analysis [25,26]. In the present work, we systematically investigated the electrochemical behavior of PPD on CdS QDs and graphene sheets (GS) hybrid film under photoirradiation and developed a novel PEC strategy for determination of PPD. Based on promoted PEC performance of CdS QDs by doping appropriate amount of GS, an interesting photovoltammetric response of PPD was observed under visible light irradiation. The CdS-GS film was applied to PPD detection, which provided a linearly enhanced photocurrent response toward PPD in the concentration of range from 1.010-7 to 3.010-6 mol·L-1. 2. Experimental 2.1. Chemicals Cadmium perchlorate hexahydrate (Cd(ClO4)26H2O) was provided by Alfa Aesar Chemical Co. Ltd. (Tianjin, China). Poly(diallyldimethylammonium chloride) solution (PDDA) and Na2S·9H2O were obtained from Aladdin Reagent Co. Ltd.

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(Shanghai, China). Nafion (5% in a mixture of lower aliphatic alcohols and water) was purchased from Sigma-Aldrich. Other reagents of analytical grade were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Doubly distilled water was used throughout the investigation. 2.2. Preparation of CdS QDs and GS Water soluble CdS QDs were synthesized by a hydrothermal method according to our previous work with a little modification [27]. Briefly, mercaptoacetic acid (1.3 mL) was added into 100 mL of 0.2 mol·L-1 Cd(ClO4)26H2O solution, followed by adjusting the solution pH to 10 with 2.0 mol·L-1 NaOH. The obtained solution was loaded into a 250-mL three-necked flask and reacted under 100 °C in a thermostatic bath under constant passage of high purity nitrogen gas. After 30 min, freshly prepared 0.23 mol·L-1 Na2S (90 mL) solution was rapidly added into this mixture. After another 4-h reaction, the product was collected by centrifugation, washed several times with ethanol, and dried at 60 ºC. GS was prepared via liquid-phase ultrasonic exfoliation [28]. In this method, 25 mg graphite powder was added into 50 mL N-methyl-2-pyrrolidone (NMP), and then a grey liquid consisting of a homogeneous phase was obtained after 48-h ultrasonication. The resulting graphene-NMP suspension was centrifuged, washed with plenty of water and ethanol several times to remove NMP solvent, and then dried in vacuum at 60 C for 5 h. 2.3. Modification of electrode The resultant GS powder was dispersed in 0.3% Nafion solution to give a 1

8

mg·mL-1 GS suspension by ultrasonic agitation at least for 1 h. After the addition of CdS QDs, the mixture was ultrasonic for another 0.5 h. The CdS-GS composites were coated on F-doped SnO2 (FTO) conducting glass (Dalian Heptachroma SolarTech Co. Ltd., China). Prior to coating, FTO substrates, cut into small rectangular pieces (1×1.5 cm2), were cleaned by sonication in acetone, ethanol and water for 30 min, and then dried with nitrogen gas. FTO substrates (exposed geometric area of 0.096 cm2) were immersed into 2% PDDA solution containing 0.5 mol·L-1 NaCl for 30 min, and then thoroughly rinsed with distilled water to remove the loosely adsorbed PDDA. After being dried with nitrogen gas, the FTO surface was coated with 5 μL of CdS-GS mixed solution (containing 5 mg·mL-1 CdS and 1 mg·mL-1 GS), and then dried at 60 o

C. In this process, positively charged PDDA was beneficial to the immobilization of

negatively charged CdS and GS on electrode surface through the electrostatic attraction. 2.4. Apparatus and procedure The morphology was characterized with a Quanta 200 field emission scanning electron microscope (FESEM) (FEI, Netherlands). Cyclic voltammetric (CV) measurements, PEC experiments and electrochemical impedance analysis were performed on a CHI660A electrochemical working station (Shanghai Chenhua Instrument Co. Ltd., China) in a conventional three-electrode system. A CdS-GS film coated electrode, a platinum wire, and a saturated calomel electrode (SCE) were employed as the working, auxiliary and reference electrodes, respectively. A PLS-SXE300 xenon lamp (Perfect Co., China) with an optical filter (λ>420 nm) was

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used as the irradiation source, and the distance between the light source and working electrode surface was 15 cm. 3. Results and Discussion 3.1. SEM characterization of CdS-GS film The surface morphology of GS coated on FTO was observed with SEM. As can be seen (Fig. 1A), the typical sheet-like structure of graphene is formed on the FTO substrate, which is beneficial to provide more adsorption sites for CdS QDs due to the large specific surface area of GS [29]. Fig. 1B shows the SEM image of CdS-GS hybrid film coated electrode. It is clear that many small CdS particles are homogeneously and densely dispersed on the GS surface. The average size of CdS QDs is estimated to be ca. 6 nm. 3.2. Photovoltammetric behavior of PPD on CdS-GS film The electrochemical responses of CdS-GS film coated electrode toward PPD before and after visible light irradiation were studied by CV (Fig. 2A). In the absence of PPD, no faradic current was observed on CdS-GS film in the dark (curve a in Fig. 2A). While the film was irradiated under light, the currents were obviously enhanced (curve b in Fig. 2A), indicating the PEC activity of hybrid film. When 0.1 mmol·L-1 PPD was present in the electrolyte, the current responses on CdS-GS film were significantly increased. In the dark, a pair of well-behaved redox peaks appeared (curve c in Fig. 2A). The redox reaction of PPD, involved two electrons and two protons, can be described as the following processes: (1) PPD is electrooxidized to its oxidation state (diimine) during the positive potential sweep; (2) the resulting diimine

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is electroreduced back to PPD on the electrode surface in the reverse sweep [12,26]. When CdS-GS film was irradiated under visible light (curve d in Fig. 2A), the anodic peak of PPD was markedly increased to reach a limiting current accompanied with the disappearance of cathodic peak. Thus, the cyclic voltammogram exhibited a sigmoidal shape, which was widely observed on the electrochemical responses of redox compounds catalyzed by enzymes [30]. Obviously, in the present case, the response is attributed to photoelectrocatalysis. It is known that diimine could accept the transferred electrons and be reduced back to diamine in weakly acidic solution [26]. The photogenerated electrons on CdS QDs are likely to be transferred to the oxidized form of PPD molecules (diimine) adsorbed on the electrode surface, and then the PPD molecules in the reduced state are continuously oxidized by the electrochemical oxidation process, which results in the appearance of limiting current in the positive scan. Scheme 1 illustrates the proposed photoelectrocatalytic process of PPD on CdS-GS hybrid film. As can be seen, PPD is electrochemically oxidized to diimine on GS, which is then reduced back to PPD by accepting photogenerated electrons from the conduction band of CdS, and thus forming a photoelectrocatalytic cycle. To further observe the influence of photoirradiation on the voltammetric behavior of PPD on CdS-GS film, the light was switched on during either positive or negative scan. It could be seen that the first half of the cyclic voltammogram under photoirradiation showed the limiting current while the last half without irradiation remained the characteristic reduction peak of PPD (curve b in Fig. 2B). This phenomenon means that oxidized form of PPD can be reduced by the electrons

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provided by electrode in the absence of light. On the contrary, as illustrated in curve c in Fig. 2B, the first half of the cyclic voltammogram without photoirradiation remained the electrochemical oxidation peak while the last half under irradiation did not exhibit the reduction peak. Obviously, irradiation plays a decisive role in the photovoltammetric response of PPD on CdS-GS film, and only the whole CV process irradiated under light can produce the typical sigmoidal shaped voltammogram. The CV responses of PPD on either GS (Fig. 2C) or CdS QDs (Fig. 2D) coated electrode before and after photoirradiation were recorded. On GS-coated electrode, PPD exhibited two pair of redox peaks (curve a in Fig. 2C). It has been reported that the product electrochemically generated by PPD oxidation, namely diimine, could react with monomer PPD molecules in their reduced state, and the reaction products of diimine with PPD could be further oxidized [12,31], which produces two redox waves on GS-coated electrode owing to the unique electron transfer ability of GS. While the GS-coated electrode was exposed under irradiation, the CV response was almost unchanged (curve b in Fig. 2C), indicating that GS was insensitive to visible light. By contrast, the CdS-coated electrode showed sensitive response to light; nevertheless, the redox peaks of PPD were not observable (curve a in Fig. 2D) due to the low electron transfer ability of semiconducting QDs. And no sigmoidal photovoltammetric response of PPD was found when the CdS-coated electrode was irradiated under light (curve b in Fig. 2D). The photovoltammetric responses of PPD on hybrid films composed of different ratios of CdS to GS were investigated. The hybrid films were prepared by fixing the

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concentration of GS at 1 mg·mL-1 and increasing the concentration of CdS QDs from 1 to 10 mg·mL-1. It was seen that all these hybrid films were effective to enhance the voltammetric response under photoirradiation (Fig. 3). Further observation indicated that the photovoltammetric limiting current was increased with increasing the ratio of CdS to GS from 1:1 to 5:1, but decreased when the ratio of CdS to GS was further increased. It is likely that increasing the amount of CdS is advantageous to enhance the absorption of hybrid film to visible light. However, excessive CdS may decrease the electrical conductivity of film and inhibit the electron transfer on electrode surface. Thus, 5:1 of CdS:GS was selected to prepare the hybrid film in this work except where otherwise indicated. 3.3. Influences of scan rate and pH on the photovoltammetric behavior of PPD The CV responses of PPD on CdS-GS film at different scan rates before and after photoirradiation were recorded (Fig. 4). It was found that the oxidation peak current (ipa) increased linearly with the scan rate () (Fig. 4A). The linear regression equation was expressed as ipa(μA) = 0.082(mV/s)+6.352 (correlation coefficient r=0.993), indicating a surface-controlled electrochemical process. While the CdS-GS film was irradiated under light (Fig. 4B), the oxidation current of PPD was promoted to reach the limiting current (iL), which was also increased linearly with increasing the scan rate. The linear regression equation could be expressed by iL(μA) = 0.101(mV/s)+8.641 (correlation coefficient r=0.989). This result means that the controlled step for electrochemical process is not changed under photoirradiation. On the other hand, the effect of solution pH on the electrochemical and

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photoelectrochemical behavior of PPD on CdS-GS film was investigated (Fig. 5). With increasing the solution pH from 6.0 to 8.0, the anodic peak potential of PPD shifted negatively (Fig. 5A). A linear relationship was found between the anodic peak potential (Epa) and pH, which could be expressed by Epa(V)= -0.065pH+0.604 (correlation coefficient r=0.997). The slope of 0.065 is close to theoretical value of 0.059, indicating the equal number of protons and electrons involved in the process of the oxidation. Under visible light irradiation (Fig. 5B), the potential also shifted negatively from pH 6.0 to pH 8.0. The half-wave potential (E1/2) showed a linear change with pH expressed by E1/2(V) = -0.043pH+0.389 (correlation coefficient r=0.998). The reduced slope (0.043) under photoirradiation may imply that more electrons than protons participate the photoelectrocatalytic process. Meanwhile, the oxidation peak current decreased with increasing the solution pH from 6.0 to 8.0; and PBS buffer solution at pH 6.0 providing the highest current response was selected as the supporting electrolyte in this work. 3.4 Determination of PPD The CV responses of PPD at different concentrations on CdS-GS film were recorded (Fig. 6A). The oxidation peak current of PPD was found to be linearly proportional to the concentration of PPD from 0.7 to 10 μmol·L-1. The linear regression equation was expressed as ipa(μA) = 0.143C(μM)-0.018 (correlation coefficient r=0.996). And the limit of detection (3S/N) was estimated to be 0.16 μmol·L-1. On the other hand, the photovoltammetric responses of PPD at different

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concentrations on CdS-GS film were measured (Fig. 6B). Although the oxidation currents of PPD on CdS-GS film were improved under photoirradiation, the shape of photovoltammetric curve was influenced by the concentration of PPD. For example, the oxidation current increased continuously with potential and could not reach the limiting state when the concentration of PPD was too low ( 20 μM) to consume the photogenerated carriers. On the other hand, both the oxidation and reduction peaks appeared under photoirradiation when the concentration of PPD was so high ( 600 μM)

that

photogenerated

carriers

were

not

enough

to

participate

the

photoelectrocatalytic reaction. Therefore, quantitative determination of PPD directly using the limiting current of photovoltammetric response seems to be not feasible. To avoid such a problem and effectively apply the photoelectrocatalytic activity of CdS-GS film to PPD determination, the photocurrent responses upon “on-off” irradiation cycle were carried out in 0.1 mol L-1 PBS (pH 6.0) containing different amounts of PPD. As shown in Fig. 7, the current was rapidly increased and then leveled off to a steady state photocurrent value when the lamp was switched on. While the lamp was switched off, the current was quickly decreased to a low dark-current value. The photocurrent difference (PI) before and after adding PPD was found to be linearly proportional to the concentration of PPD ranging from 0.1 to 3 μmol·L-1. The linear regression equation was expressed as ΔPI/μA=0.271C/μM+0.032 (correlation coefficient r=0.997). The detection limit (3S/N) was estimated to be 0.043 μmol·L-1, which was almost 4-fold lower than the value obtained by CV in the absence of light as mentioned above. Moreover, compared with many previously reported methods for

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PPD determination (Table 1), the present photoelectrochemical detection shows higher sensitivity. Therefore, such a CdS-GS hybrid film can be used as the highly sensitive photoelectrochemical sensor for PPD. The reproducibility of such a photoelectrochemical sensor was investigated by recording the photoelectrochemical responses of twelve independently prepared CdS-GS films in 0.1 mol L-1 PBS (pH 6.0) containing 5.0×10-7 mol·L-1 PPD. The relative standard deviation (RSD) was 2.0%, showing a good reproducibility. The RSD of the sensor for 10 successive measurements was 1.5%, indicating a good repeatability. The stability of the CdS-GS film, which was stored in 4 oC if not used, was evaluated by recording the photocurrent response every five days. The result indicated that the response did not show obvious change after twenty days, demonstrating a good stability. Moreover, the photoelectrochemical sensor was applied to the determination of PPD in water samples collected from three lakes in Wuhan City using the standard addition method. As can be seen (Table 2), the recoveries of the proposed method were in the range from 99.2% to 103.2%, indicating the feasibility of the photoelectrochemical sensor for the determination of PPD in water samples. Conclusions In this work, we prepared CdS QDs and GS hybrid film and investigated the photovoltammetric behavior of PPD. Due to the high photocatalytic activity of CdS and excellent electron transfer ability of GS, an interesting voltammetric behavior of PPD was observed on CdS-GS hybrid film under photoirradiation. By contrast, either

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CdS or GS coated electrode did not show such a photovoltammetric response to PPD. The CdS-GS ratio, scan rate, solution pH and PPD concentration showed obvious influences on the photovoltammetric behavior of PPD. Using the as-prepared CdS-GS film, a photoelectrochemical strategy for the determination of PPD was developed, which displayed a high sensitivity, good reproducibility and high stability. The photoelectrochemical method was successfully applied to PPD detection in water samples with suitable precision and accuracy. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 61172005). The authors thank the Analytical and Testing Center of HUST for the help in the characterization of synthesized materials.

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Scheme and Figure captions Scheme 1. Photogenerated electron transfer and electrochemical process of PPD on CdS-GS film. Fig. 1. SEM images of (A) GS and (B) CdS-GS films. Fig. 2. (A) Cyclic voltammograms of CdS-GS film (a, c) before and (b, d) after photoirradiation in 0.1 mol·L-1 PBS solution (pH 6.0) in the absence (a, b) and presence (c, d) of 0.1 mmol·L-1 PPD. (B) Cyclic voltammograms of CdS-GS film in 0.1 mol·L-1 PBS solution (pH 6.0) containing 0.1 mM PPD under different irradiation conditions: (a) both positive and negative scans are under irradiation, (b) only the positive scan is under irradiation, and (c) only the negative scan is under irradiation. Cyclic voltammograms of (C) GS and (D) CdS QDs coated electrodes in 0.1 mol·L-1 PBS solution (pH 6.0) containing 0.1 mM PPD (a) before and (b) after photoirradiation. Scan rate: 50 mV s-1. Fig. 3. Cyclic voltammograms of CdS-GS films prepared with different ratios of CdS to GS: (A) 1:1, (B) 3:1, (C) 5:1, (D) 7:1, and (E) 10:1 in 0.1 mol·L-1 PBS solution (pH 6.0) containing 0.1 mM PPD (a) before and (b) after photoirradiation. (F) Comparison of photovoltammetric responses of PPD on CdS-GS films prepared with different ratios of CdS to GS: (a) 1:1, (b) 3:1, (c) 5:1, (d) 7:1, and (e) 10:1. Scan rate: 50 mV·s-1. Fig. 4. Cyclic voltammograms of CdS-GS film in 0.1 mol·L-1 PBS (pH 6.0) containing 0.1 mM PPD (A) before and (B) after photoirradiation at different scan rates. Insets: linear plots of anodic peak or limiting current vs. scan rate. Error bars are derived from the standard deviation of five measurements. 23

Fig. 5. Cyclic voltammograms of 0.1 mol·L-1 PBS at various pH values containing 0.1 mM PPD on CdS-GS film (A) before and (B) after photoirradiation. Scan rate: 50 mV·s-1. Insets: linear plots of anodic peak or half-wave potential vs. pH value. Error bars are derived from the standard deviation of five measurements. Fig. 6. (A) Cyclic voltammograms of 0.1 mol·L-1 PBS (pH 6.0) containing (a) 0, (b) 0.7, (c) 1, (d) 3, (e) 5, (f) 7, and (g) 10 μmol·L-1 PPD on CdS-GS film. (B) CVs of 0.1 mol·L-1 PBS (pH 6.0) containing (a) 0, (b) 0.5, (c) 1, (d) 3, (e) 5, (f) 10, (g) 20, (h) 100, and (i) 600 μmol·L-1 PPD on CdS-GS film under photoirradiation. Inset of (B): enlarged cyclic voltammograms corresponding to curves from a to g. Scan rate: 50 mV·s-1. Fig. 7. Photocurrent responses of 0.1 mol·L-1 PBS (pH 6.0) containing (a) 0, (b) 0.1, (c) 0.3, (d) 0.5, (e) 0.7, (f) 1, (g) 3 μmol·L-1 PPD on CdS-GS film at +0.4 V. Inset: calibration curve for PPD on CdS-GS film. Error bars are derived from the standard deviation of three measurements.

24

Table 1 Comparison of different methods for PPD determination.

Method

Linear range (mol·L−1)

Detection limit (mol·L−1)

Reference

Capillary zone electrophoresis

2.0×10−6 - 2.0×10−4

1.57×10−6

[21]

Micellar electrokinetic chromatography

5.0×10−5 - 1.0×10−2

1.97×10−7

[23]

Fluorescence sensor

3.0×10−5 - 4.0×10−4

3.0×10−5

[24]

Square wave voltammetry

2.0×10−6 - 2.0×10−5

6.0×10−7

[25]

Chronoamperometry

2×10−7 - 1.5×10−4

5.0×10−8

[26]

Photoelectrochemistry

1.0×10−7 - 3.0×10−6

4.3×10−8

This work

 

25

 

Table 2 Determination of PPD in water samples by photoelectrochemical sensor (n=5).

Found (mol·L-1)

RSD (%)

Recovery (%)

1

0 5.0×10-7 1.0×10-6

0 5.16×10-7 9.92×10-7

1.6% 1.5%

103.2% 99.2%

2

0 5.0×10-7 1.0×10-6

0 5.04×10-7 1.03×10-6

1.1% 1.3%

100.8% 102.7%

3

0 5.0×10-7 1.0×10-6

0 5.12×10-7 9.94×10-7

1.2% 1.8%

102.4% 99.4%

Samples

Spiked (mol·L-1)

26

Scheme 1. Photogenerated electron transfer and electrochemical process of PPD on CdS-GS film.

27

A

B

Fig. 1.

28

20

A

40

d

B

16

I / A

I / A

30

c

12 8 4

c b

0

a

20

a

10

b

0 -4 -0.2

0.0

0.2

0.4

0.6

-0.2

0.0

Potential /V

4

0.2

0.4

C

b

a

8

D b

2 6

I / A

I / A

0.6

Potential /V

0

-2

4

a 2

-4

0

-0.2

0.0

0.2

0.4

0.6

-0.2

Potential /V

0.0

0.2

Potential /V

Fig. 2.

29

0.4

0.6

16

B

A

12

b

b

12

8

I / A

I / A

8

4

4

a

a

0

0

-4

-4 -0.2

0.0

0.2

0.4

-0.2

0.6

0.0

20

16

C

16

0.2

0.4

0.6

Potential /V

Potential /V

D

b

b

12

I / A

I / A

12 8

8 4

4

a

a 0

0

-4

-4 -0.2

0.0

0.2

0.4

-0.2

0.6

0.0

16

0.4

20

E 12

0.6

b

c d e

F

16

b

a

12

I / A

8

I / A

0.2

Potential /V

Potential /V

4

a

8 4

0

0 -4

-4 -0.2

0.0

0.2

0.4

-0.2

0.6

Potential /V

0.0

0.2

Potential /V

Fig. 3.

30

0.4

0.6

200 mV/s 150 mV/s 100 mV/s 50 mV/s 30 mV/s 10 mV/s 5 mV/s

A

25

24

Current /

A

20

I / A

15

20

16

12

10

8

0

50

5

100

150

200

Scan rate / (mV/s)

0 -5 -0.2

0.0

0.2

0.4

0.6

Potential /V

B

32

30

I / A

16

Current / A

25

24

20

15

10

0

50

100

150

200

Scan rate / (mV/s)

200 mV/s 150 mV/s 100 mV/s 50 mV/s 30 mV/s 10 mV/s 5 mV/s

8

0 -0.2

0.0

0.2

Potential /V

Fig. 4.

31

0.4

0.6

6.0

0.24

pH

A

Peak potential

12

0.16

0.12

0.08

8.0

6.0

4

6.5

7.0

7.5

pH 6.0 pH 6.5 pH 7.0 pH 7.5 pH 8.0

0

-0.2

8.0

pH Value

pH

I / A

8

0.20

0.0

0.2

0.4

0.6

0.8

1.0

Potential /V

.0

20

pH 8

0.14

5

Half-wave potential

I / A

10

.0

pH 6.0 pH 6.5 pH 7.0 pH 7.5 pH 8.0

pH 6

B 15

0

0.12 0.10 0.08 0.06 0.04 6.0

6.5

7.0

pH Value

-5 -0.2

0.0

0.2

0.4

Potential /V

Fig. 5.

32

0.6

7.5

8.0

A

2.0

g

Current/ A

1.5 1.0 0.5

a 0.0 -0.5 -0.2

0.0

0.2

0.4

0.6

Potential / V

50

B 9

30 20

Current/ A

Current/ A

40

i

g

6

a

3

0

-0.2

0.0

10

0.2

0.4

0.6

Potential / V

a

0 -10 -0.2

0.0

0.2

Potential / V

Fig. 6.

33

0.4

0.6

4.0

1.00

0.75

g

PI/ A

Photocurrent / A

3.2

2.4

0.50

0.25

0.00

1.6 0.0

0.6

1.2

1.8

2.4

3.0

CPPD/ M

0.8

a

0.0 0

20

40

Time / s

Fig. 7.

34

60