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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Synthesis of ZnIn2S4/CdS Heterostructure Based on Electrostatic Interaction Mechanism for the Indirect Photoelectrochemical Detection of Dopamine Hao Wang, Huili Ye, Bihong Zhang, Faqiong Zhao, and Baizhao Zeng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05287 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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The Journal of Physical Chemistry

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Synthesis of ZnIn2S4/CdS Heterostructure Based on Electrostatic

2

Interaction Mechanism for the Indirect Photoelectrochemical

3

Detection of Dopamine

4

Hao Wang1, Huili Ye1, Bihong Zhang, Faqiong Zhao, Baizhao Zeng*

5

Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of

6

Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan

7

430072, Hubei Province, P. R. China

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E-mail address: [email protected] (BZ Zeng)

9

10

Abstract

11

A ZnIn2S4/CdS photocatalyst was synthesized based on an electrostatic interaction

12

mechanism and used to construct a photoelectrochemical (PEC) sensor for the highly

13

sensitive and selective detection of dopamine (DA). As CdS and ZnIn2S4 possessed

14

overlapped band potentials they could form heterojunction, which was favorable to

15

charge separation and photoelectrochemical conversion. Thus, the ZnIn2S4/CdS

16

composite showed better photoelectrochemical properties and higher photocatalytic

17

activity than ZnIn2S4 and CdS. When it was used for DA sensing in weak alkaline

18

solution, DA turned to polydopamine and acted as effective electron acceptor to make

19

the anodic photocurrent decrease. Owing to adopting this detection strategy the

20

interference of coexistent reductive species (i.e. electron donor) was avoided

21

effectively. Under the optimized conditions, the photocurrent change was linear to

22

DA concentration in the range from 0.3 to 300 µM, and the detection limit was 0.1 1 ACS Paragon Plus Environment

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µM. The ZnIn2S4/CdS/ITO sensor was applied for the indirect detection of DA in

24

urine samples and it presented good performance.

25

1. Introduction

26

As a typical II-III2-VI4 semiconductor, ternary sulfide photocatalyst ZnIn2S4 with

27

various morphologies (i.e. nanowires, flower-like microspheres and nanotubes) has

28

been synthesized by using different methods.1 The photocatalyst is eco-friend and can

29

be irradiated by visible-light, while most metal-oxide photocatalysts, like TiO2 and

30

ZnO, are effective only under ultraviolet (UV) light.2 In addition, ZnIn2S4 is quite

31

stable in comparison with other metal chalcogenide semiconductors.3 For example,

32

CdS is unstable under light irradiation because of the strong oxidation capacity of

33

photogenerated

34

ZnIn2S4-based photocatalyst is applied in many fields, including charge storage,5

35

thermoelectricity6 and photocatalysis.7 However, the efficiency of pure ZnIn2S4 is far

36

from satisfaction due to the high recombination rate of photogenerated electron-hole

37

pairs.

holes,

which

is

known

as

photocorrosion.4

Accordingly,

38

To resolve this issue, a number of ZnIn2S4 based hybrid semiconductor

39

heterojunctions have been synthesized with expectation to improve its photocatalytic

40

performance. For example, Chai’s group fabricated a MWCNTs/ZnIn2S4 composite

41

for hydrogen production under visible-light irradiation.8 Peng et al. developed

42

ZnIn2S4-PVDF-poly-(MMA-co-MAA) composites for the degradation of methyl

43

orange with high photocatalytic activity.9 Liu and co-workers fabricated 2D ZnIn2S4

44

nanosheet/1D TiO2 nanorod heterostructure arrays for the photoelectrochemical (PEC)

45

splitting of water.10 We notice that ZnIn2S4 and CdS have overlapped band potentials,

46

and the ZnIn2S4/CdS heterostructure composite may present better PEC response 2 ACS Paragon Plus Environment

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47

under irradiation of visible light and higher charge separation efficiency than pure

48

ZnIn2S4 and CdS. However, up to now, ZnIn2S4 and its composites have not been

49

applied for the preparation of PEC sensors.

50

PEC sensor is a promising electroanalytical technique for low cost, rapid analysis,

51

high sensitivity and simple instrumentation, it has attracted much attention of

52

chemists and biologists.11 In the past few years, a number of PEC sensors were

53

constructed for the detection of DA, and most of them were based on the traditional

54

hole oxidation mechanism.12-14 Namely, DA acted as an effective electron donor for

55

scavenging the photogenerated holes on the surface of semiconductor and inhibited

56

the recombination of electron-hole pairs.15 However, little attention was paid on the

57

construction of PEC sensors based on the interaction between electron acceptors in

58

solution and electrons from the photo-excited semiconductor, especially in PEC

59

sensors based on n-type semiconductors.16 Previously, our group found that the

60

cathodic photocurrent of Z-scheme BiOI-CdS photocatalyst significantly increased in

61

an air-saturated electrolyte (O2 acted as an electron acceptor and facilitated the spatial

62

separation of charge carriers).17 What's more interesting, Wang et al developed an

63

original strategy for DA detection, based on the decreased anodic photocurrent.18

64

They found that the anodic photocurrent of CdS QDs decreased in the presence of DA,

65

due to the polydopamine (PDA), produced by the oxidation of DA, acted as electron

66

acceptor for the photoexcited CdS QDs, inhibiting the electron transfer from CdS

67

toward to the ITO electrode and leading to the decrease of photocurrent. However, the

68

photoelectrode material CdS QDs suffered from the high recombination rate of

69

photogenerated charge carriers and photocorrosion.19 Therefore, it is still necessary to

70

improve the stability of photoactive materials and the separation efficiency of charge

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carriers. 3 ACS Paragon Plus Environment


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Motivated by these facts, a ZnIn2S4/CdS heterostructure composite was prepared

73

based on an electrostatic interaction mechanism. The composite showed better

74

photoelectrochemical property and higher photocatalytic activity than pure CdS and

75

ZnIn2S4. The high photocatalytic activity resulted from the formation of

76

heterogeneous junction, which accelerated the transfer of charge carriers at interface

77

and inhibited their recombination effectively. DA easily turned to PDA in alkaline

78

solution, and PDA could act as effective electron acceptor. Hence, it could inhibit the

79

transfer of photogenerated electrons to the ITO electrode, and made the anodic

80

photocurrent significantly decrease. Thus, the anodic photocurrent of the as-prepared

81

ZnIn2S4/CdS/ITO decreased with increasing DA concentration. Therefore, it could be

82

used for the sensitive and selective detection of DA.

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2. Experimental

84

2.1 Chemicals and apparatus

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Zn(CH3COO)2·2H2O,

InCl3·4H2O,

thioacetamide

(TAA),

CdCl2·2.5H2O,

86

Na2S·9H2O, triethanolamine (TEA) and DA were obtained from Aladdin Chemistry

87

Co., Ltd. (Shanghai, China). Tris(hydroxymethyl)aminomethane, hydrochloric acid,

88

NaOH, uric acid (UA), glucose (Glu), ascorbic acid (AA), glutathione (GSH),

89

L-cysteine (Cys) and other reagents of analytical grade were provided by Sinopharm

90

Chemical Reagent Co., Ltd. (Shanghai, China), used without any further purification.

91

A

92

tris(hydroxymethyl)aminomethane and hydrochloric acid.

10

mM

Tris-HCl buffer

solution

(pH=8.5)

was

prepared

by

using

93

The zeta potential was measured with a NanoZS90 Laser Particle Size and Zeta

94

Potential Analyzer (Malvern, England). Scanning electron microscopy (SEM) images

4 ACS Paragon Plus Environment

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95

were recorded with a field emission SEM (ZEISS SIGMA, Germany). X-ray

96

photoelectron spectra (XPS) were obtained using an ESCALAB 250Xi spectrometer

97

(Thermo Fisher Scientific Inc., USA). X-ray diffraction (XRD) patterns were obtained

98

using a Bruker D8 diffractometer (Germany) with Cu Kα radiation, over 2θ range of

99

15-70° at a scanning rate of 6° min-1. UV-visible diffuse reflectance spectra (DRS) of

100

the samples were recorded with a UV-visible spectrophotometer (UV-3600, Shimadzu,

101

Japan), BaSO4 was used as reflectance standard. The photoluminescence (PL) spectra

102

were measured with a Hitachi F-4600 fluorescence spectrophotometer (Tokyo, Japan)

103

at an excitation wavelength of 340 nm.

104

PEC measurements were performed with a CHI 832C electrochemical workstation

105

(CH Instrument Co., Ltd, Shanghai, China). A PEC-10W Multifunctional LED Light

106

Source System (Tianjin Brillante Technology Co., Ltd. China) was employed as the

107

irradiation source; the distance was fixed at 10 cm between the light source and

108

electrode surface. Photoelectrochemical tests were carried out at a constant potential

109

of 0 V (vs. Ag/AgCl). The supporting electrolyte was 10 mM Tris-HCl buffer solution.

110

All experiments were conducted at room temperature except mentioned other where.

111

Electrochemical impedance spectroscopy (EIS) experiment was carried out on a CHI

112

604D electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd, China),

113

the electrolyte was 0.1 M KCl containing 5.0 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1),

114

and the frequency range was from 100 kHz to 0.1 Hz at open circuit potential.

115

2.2 Synthesis of lamellar ZnIn2S4

116

The ZnIn2S4 was synthesized by a simple hydrothermal method.4 Briefly, 1 mmol

117

Zn(CH3COO)2·2H2O and 2 mmol InCl3·4H2O were dissolved in 60 mL water, and

118

then 6 mmol TAA was added into the solution. After stirring for 30 min, the mixture

119

was transferred into a 100 mL Teflon-lined autoclave and was maintained at 160 ℃



120

for 12 h. The product was collected by centrifugation and then washed for several

121

times with deionized water, ethanol, and dried at 60 ℃ for 12 h.

122

2.3 Preparation of ZnIn2S4/CdS heterostructure

123

The ZnIn2S4/CdS heterostructure was prepared according to an electrostatic

124

interaction mechanism as reported previously with a little modification.20 In brief,

125

0.0500 g ZnIn2S4 was dissolved in 30 mL water and 15 mL 0.10 M CdCl2 aqueous

126

solution was added drop-wisely under vigorous stirring. After stirring for 30 min, 45

127

mL 0.10 M Na2S aqueous solution was added slowly, stirred for another 30 min. The

128

brown solid product, marked as ZnIn2S4/CdS, was washed for several times with

129

deionized water, ethanol, and dried at 60℃ in an oven overnight (Scheme 1). For

130

comparison, the volume of CdCl2 solution was changed (i.e. 5, 10, 20, 30 mL) and the

131

final products were labeled as ZnIn2S4/CdS-x (x= 1, 2, 4, 5), respectively. Pure CdS

132

was prepared using the same way but without ZnIn2S4.

133 134

Scheme 1. The synthesis of ZnIn2S4/CdS heterostructure by using an electrostatic interaction

135

mechanism.

136

2.4 Fabrication of photoelectrochemical sensor

137

Prior to modification, the ITO glass (1 cm × 2 cm) was cleaned by successive

138

sonication in NaOH solution (1 M), acetone, ethanolandpure water each for 20 min.

139

After being dried at 60 ℃ in an oven, 3M tape with a fixed area of 0.070 cm2 was

140

stuck on the ITO glass.21 Then, a 15 µL (1 mg mL-1) of aqueous dispersion of


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ZnIn2S4/CdS was dropped onto the ITO surface and dried at 60 ℃ to obtain

142

ZnIn2S4/CdS/ITO working electrode.

143

3. Results and discussion

144

3.1 Characterization of ZnIn2S4/CdS heterostructure

145

The obtained ZnIn2S4, CdS and ZnIn2S4/CdS heterostructure were characterized by

146

SEM and TEM (Figure 1). As can be seen, the ZnIn2S4 showed a lamellar structure

147

which was composed of two-dimensional nanosheets (Figure 1A and E), whereas CdS

148

displayed agglomerated-particles shape (Figure 1B and F). As to the ZnIn2S4/CdS,

149

CdS nanoparticles evenly distributed on the surface of ZnIn2S4 sheets (Figure 1C, D

150

and G). The zeta potential of ZnIn2S4 in water was -34.4 mV (Figure S1), indicating

151

that its surface carried negative charges.22 Thus Cd2+ was easily adsorbed by ZnIn2S4

152

and CdS nanoparticles tended to grow on the ZnIn2S4 sheets (Scheme 1).

153 154

Figure 1. SEM images of (A) ZnIn2S4, (B) CdS and (C) ZnIn2S4/CdS heterostructure; (D)

155

enlarged SEM image of ZnIn2S4/CdS heterostructure. TEM images of (E) ZnIn2S4, (F) CdS and

156

(G) ZnIn2S4/CdS heterostructure.



157

Figure 2A displayed the XRD patterns of ZnIn2S4, CdS and ZnIn2S4/CdS

158

heterostructure. The major diffraction peaks of pure ZnIn2S4 could be indexed to a

159

hexagonal phase of ZnIn2S4 (ICDD-JCPDS card NO.72-0773) with (006), (102), (108)

160

and (110) crystal planes at 2θ of 21.2°, 27.6°, 39.8° and 47.1°, respectively.23 All the

161

diffraction peaks of the prepared CdS matched well with that of cubic CdS phase

162

(JCPDS no. 65-2887), with scattering angles of 26.5°, 44.2° and 52.2°, corresponding

163

to crystal planes (111), (220) and (311), respectively.24 The diffraction peaks of

164

ZnIn2S4 and CdS could be found in composite materials, but their intensity was weak,

165

indicating that their crystallinity decreased due to the influence by each other. It

166

meant that ZnIn2S4/CdS was not a simple mixture of ZnIn2S4 and CdS, but formed

167

heterojunction.25,26 The XPS analysis of ZnIn2S4/CdS heterostructure was performed

168

to investigate the elemental composition and the surface chemical states (Figure 2B).

169

As a result, elements Zn, In, Cd, and S were observed clearly. The measured binding

170

energies corresponding to Cd 3d5/2 and Cd 3d3/2 were 405.5 and 411.6 eV (Figure 2C)

171

respectively, meaning Cd was present as divalent ion. The peaks centered at 160.8 and

172

162.3 eV could be assigned to S2- (Figure 2E).27 For the In 3d spectra (Figure 2D), the

173

binding energies around 444.9 and 452.6 eV could be assigned to In 3d5/2 and In 3d3/2,

174

respectively. In addition, the high-resolution Zn 2p spectra (Figure 2F) displayed

175

characteristic peaks at 1021.2 and 1045.1 eV, corresponding to Zn 2p3/2 and Zn 2p1/2,

176

respectively.28


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177 178

Figure 2. (A) XRD patterns of pure ZnIn2S4, CdS and ZnIn2S4/CdS heterostructure. XPS patterns

179

of the ZnIn2S4/CdS heterostructure: typical XPS survey (B), Cd 3d (C), In 3d (D), S 2p (E), and

180

Zn 2p (F) spectra.

181

Figure 3A revealed the UV-vis DRS of the as prepared samples, and they showed

182

strong photoabsorption to UV-visible light. ZnIn2S4 displayed a wider absorption

183

edge than CdS. The color of the ZnIn2S4, CdS and ZnIn2S4/CdS powders was shown

184

in Figure S2. The absorption edge of the ZnIn2S4/CdS was between those of CdS and

185

ZnIn2S4. The band gap (Eg) of ZnIn2S4/CdS was also between those of pure ZnIn2S4

186

and CdS, indicating the formation of heterostructure.29 The band gap of crystalline

187

semiconductor could be calculated from the classic Tauc approach by using the

188

following equation:30

189

(αhν) = A(hν- Eg)n/2

(1)

190

Where α, h, ν, A and Eg represent the absorption coefficient, Planck constant, light

191

frequency, proportional constant and band gap energy, respectively. The value of n is

192

a characteristics number of the transition in a semiconductor (for direct transition, n=1,

193

and for indirect transition, n=4). As reported, for the ZnIn2S4 semiconductor n=431 9 ACS Paragon Plus Environment


194

and for CdS n=132. Herein the Eg values of ZnIn2S4 and CdS are calculated as 2.08

195

and 2.20 eV (Figure 3B), respectively. The CB and VB potentials of ZnIn2S4 and CdS

196

were estimated according to the empirical equations:33

197

EVB = X-E0 + 0.5 Eg

(2)

198

ECB = EVB - Eg

(3)

199

Where X is the absolute electronegativity of the semiconductor (Table S1a), Eg is the

200

band gap of the semiconductor and E0 is the energy of free electrons on the hydrogen

201

scale (about 4.5 eV). The X values for ZnIn2S4 and CdS are 4.86 and 5.19 eV,

202

respectively. The calculated values of VB and CB for ZnIn2S4 and CdS are listed in

203

Table S1b. The VB edge of CdS (1.79 eV) is more positive than that of ZnIn2S4 (1.40

204

eV), while the CB edge of ZnIn2S4 (-0.68 eV) is more negative than that of CdS

205

(-0.41 eV). This indicates ZnIn2S4 and CdS semiconductors possess overlapped band

206

potentials (Figure 3C) and they can form an effective heterojunction.34 Generally

207

speaking, a heterojunction of different semiconductors with matched energy levels is

208

favorable to charge separation and photoelectrochemical conversion.

209

Electrochemical impedance spectroscopy (EIS) is widely applied to investigate the

210

electron-transfer efficiency.35 In this case, it was also used to characterize the

211

electrode change. Figure 3D presented an overview of the semicircular Nyquist plots

212

for the CdS/ITO and ZnIn2S4/CdS/ITO. The EIS of CdS/ITO showed big diameter,

213

reflecting the poor electrical conductivity of CdS. As to the ZnIn2S4/CdS, its

214

charge-transfer resistance was much smaller than that of CdS. The reason was that the

215

amount of CdS was relative less in ZnIn2S4/CdS.36, 37


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216 217

Figure 3. (A) UV-Vis diffuse reflectance spectra of ZnIn2S4, CdS and ZnIn2S4/CdS; (B) plots of

218

(αhν)1/2 vs. photon energy (hν) for ZnIn2S4 and (αhν)2 vs. photonenergy (hν) for CdS; (C) band

219

structure diagram for ZnIn2S4/CdS; (D) EIS of CdS/ITO and ZnIn2S4/CdS/ITO.

220

3.2 Possible mechanism of charge transfer

221

The possible mechanism for the PEC sensing of DA is displayed in Figure 4, which

222

is different from the traditional hole oxidation mechanism. Under irradiation (LED,

223

490-500 nm), electrons are excited from the occupied VB to the empty CB of ZnIn2S4,

224

leaving holes in the VB. The photogenerated electrons in the CB of ZnIn2S4 transfer

225

to CB of CdS and produce anodic photocurrent. Meanwhile, holes are transported in

226

the opposite direction to the heterojunction interface. Thus, the efficient spatial

227

separation of carriers is achieved. The addition of TEA can scavenge photogenerated

228

holes and amplify the anodic photocurrent signal. In addition, after adding DA in this

229

weakly alkaline solution (Tris-HCl, pH = 8.5), the formed PDA with abundant BQ

230

groups can act as electron acceptor and inhibit charge separation, hence the anodic

231

photocurrent significantly decreases. The inhibition by BQ may be related to the 11 ACS Paragon Plus Environment


232

formation of recombination complex, in which photo-induced electrons transfer from

233

the CB of the photocatalyst to the LUMO of BQ.27, 38 The photocurrent change (∆I:

234

Isample–Iblank) depends on the amount of PDA produced by DA oxidation. Therefore,

235

this enables the indirect detection of DA.

236 237

Figure 4. Possible charge separation diagrams and the response mechanism of ZnIn2S4/CdS

238

heterojunction to DA.

239

3.3 PEC activity and PEC response of ZnIn2S4/CdS/ITO sensor

240

Linear sweep voltammogram (LSV) was recorded in 0.10 M Na2SO4 aqueous

241

solution with light on and off, and the ZnIn2S4/CdS photoanode exhibited higher

242

photocurrent than pure ZnIn2S4 and CdS under visible light illumination (Figure 5A).

243

This was attributed to the inhibition of CdS immobilized on the surface of ZnIn2S4 to

244

the recombination of electron–hole pairs.

245

The photocurrent responses of the electrodes under LED illumination were

246

explored (Figure 5B). We can see that all electrodes responded rapidly under the

247

irradiation of visible light. The ZnIn2S4/CdS/ITO showed enhanced photocurrent

248

response than pristine ZnIn2S4/ITO and CdS/ITO. The photocurrent density was

249

further enhanced by the addition of TEA because of its hole-trapping ability (Figure

250

5C). As a result, there was a large scale left for the photocurrent response of DA. The 12 ACS Paragon Plus Environment

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251

PEC responses of ZnIn2S4/ITO and CdS/ITO to 30 µM DA were shown in Figure 5D,

252

the ∆I were 0.25 and 1.05 µA cm-2, respectively, which were smaller than that of

253

ZnIn2S4/CdS/ITO (i.e. 5.12 µA cm-2). That was to say, the ZnIn2S4/CdS composite

254

had more favorable and sensitive response to DA.

255 256

Figure 5. (A) Linear sweep voltammograms of (a) CdS/ITO, (c) ZnIn2S4/CdS/ITO and (d)

257

ZnIn2S4/ITO, (b) ZnIn2S4/CdS/ITO in dark. Solution composition: 10 mM Tris-HCl buffer

258

solution (pH = 8.5) containing 0.050 mM TEA; (B) PEC response curves of (a) ZnIn2S4/ITO, (b)

259

CdS/ITO, (c) ZnIn2S4/CdS/ITO in Tris solution, and (d) ZnIn2S4/CdS/ITO in Tris solution

260

containing 0.050 mM TEA; (C) PEC response curves of ZnIn2S4/CdS/ITO in 10 mM Tris-HCl

261

buffer solution (pH = 8.5) containing 0.050 mM TEA (a), 0.050 mM TEA and 30 µM DA (b), 0

262

mM TEA and 0 µM DA (c), and 30 µM DA (d). Bias potential: 0 V (vs. Ag/AgCl), with on-off

263

illumination of 490-500 nm LED light; (D) PEC response of ZnIn2S4/ITO (a, b) and CdS/ITO (c, d)

264

in blank solution (a, c) containing DA (b, d); (E) Illuminated open circuit potential of CdS/ITO

265

and ZnIn2S4/ITO in 0.10 M Na2SO4 solution; (F) Decay of open circuit potential in dark.

266

In order to make the electron transport mechanism clear, the open-circuit

267

voltage-decay process was recorded (Figure 5D). The electron-hole pairs

268

recombination caused Voc decay. The decay lifetime of the accumulated electrons can

269

be related to the drop in the potential using the following equation:39

270

τ = (kBT/e)(dVoc/dt)-1

(4)



271

Where kBT is the thermal energy, e is the positive elementary charge, and dVoc/dt is

272

the first-order time derivation of the Voc. Figure 5E shows the calculated τ as a

273

function of Voc. It is clear that the ZnIn2S4/CdS heterostructure has longer electron

274

lifetime in comparison with the pure ZnIn2S4. This proves that the introduction of CdS

275

promotes the interfacial charge transfer efficiently and makes the photogenerated

276

electron lifetime increase. It is consistent with the observation in PL spectra (in Figure

277

S3). The ZnIn2S4/CdS displays lower PL emission than the pristine ZnIn2S4,

278

indicating that the modification of CdS enables the effective separation of

279

photogenerated charge carriers. This contributes to the good PEC and photocatalytic

280

performance.10

281

3.4 Influence of CdS content

282

To evaluate the influence of CdS content, the SEM images and transient

283

photocurrent responses of the ZnIn2S4/CdS and ZnIn2S4/CdS-x (x=1, 2, 4, 5) modified

284

electrodes were recorded (Figure S4 and Figure S5). It could be observed that the

285

amount of CdS particles on the surface of ZnIn2S4 sheets gradually increased with

286

increasing the precursor concentration of CdS, and at last most of the ZnIn2S4 sheets

287

were covered. The photocurrent increased significantly with increasing the content of

288

CdS (curves a-c), and the ZnIn2S4/CdS/ITO displayed the highest photocurrent

289

response. When the content of CdS was further increased, the photocurrent declined

290

significantly (curves c-e). When the CdS content was too low, the charge separation

291

was poor and photocurrent was small. When it was too high the transport of carriers

292

was hindered and attenuation occurred due to the charge recombination, thus the

293

overall photocurrent decreased. In this case, the ZnIn2S4/CdS heterostructure

294

displayed best PEC performance and was chosen as electrode modification material.

295

3.5 Photoelectrochemical detection of DA


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296

The ZnIn2S4/CdS heterostructure modified ITO electrode was applied to the

297

quantitative determination of DA. As shown in Figure 6A-B, the photocurrent

298

decreased with increasing DA concentration. The photocurrent change was

299

proportional to the logarithm of DA concentration in a wide range of 0.3 to 300 µM,

300

the linear regression equation was ∆I/µAcm-2 = - 5.158﹣3.019 log c/µM with a

301

correlation coefficient of 0.9982. The detection limit was 0.1 µM. Some reported DA

302

PEC sensors were listed in Table 1 for comparison, and the linear range and detection

303

limit of the sensor could meet the practical needs.

304 305

Figure 6. (A) Photocurrent response of the ZnIn2S4/CdS/ITO in the presence of 0, 0.3, 1.0, 3.0, 10,

306

30, 100, 300 µM DA (from a to h); (B) linear relationship between response photocurrent and

307

logCDA; (C) photocurrent response stability of the ZnIn2S4/CdS/ITO in 10 mM Tris-HCl buffer

308

solution (pH=8.5) containing 0.050 mM TEA; (D) comparison of the photocurrent responses of 30

309

µM AA, UA, Cys, GSH, Glu and DA. Applied potential: 0 V (vs. Ag/AgCl).

310

To evaluate the reproducibility, five electrodes were prepared under the same

311

conditions and 50 µM DA was determined. A relative standard deviation (RSD) of 5.3%

312

(n=5) was obtained. The repeatability was investigated by monitoring 50 µM DA

313

using one modified electrode, and the RSD of the photocurrent was 4.1% (n=5). The 15 ACS Paragon Plus Environment


Page 16 of 24

314

stability of the PEC sensor was examined, as shown in Figure 6C, no obvious change

315

of photocurrent response was observed in 400 s, indicating the chemical and optical

316

properties of the sensor kept almost unchanged. The photocurrent responses of foreign

317

species, such as AA, UA, Cys, GSH and Glu, were recorded and compared with that

318

of DA (Figure 6D). It was interesting, these foreign species presented very low PEC

319

response under the same conditions. The high stability and selectivity should be

320

ascribed to the good photoactive material used and the unique response mechanism.

321

Finally, the ZnIn2S4/CdS/ITO was applied to the detection of DA in human urine

322

samples. The determination results are listed in Table S2. As can be seen, the

323

recovery and precision are satisfactory, indicating that the PEC sensor is promising in

324

the indirect detection of DA in real samples.

325

Table 1 Comparison of various DA PEC sensors.

PEC sensors

Role of DA

Linear range(µM)

LOD (µM)

Reference

G-C3N4/TiO2

electron donor

0.1-50

0.02

40

CuTsPc/TiO2

electron donor

4-810

0.5

41

TiO2 NPs

electron donor

0.002

42

WO3-ITO

electron donor

53-80 85-155

0.3

14

ZnIn2S4/GR

electron donor

0.01-20

0.001

43

0.4-10

0.17

18

0.3-300

0.1

This work

0.2-200 200-5000

PDA as electron CdTe-FTO

ZnIn2S4/CdS

acceptor PDA as electron acceptor

326 327

4. Conclusion

328

A ZnIn2S4/CdS heterostructure with good PEC properties was synthesized, based

329

on an electrostatic interaction mechanism. The overlapped band potentials and fast 16 ACS Paragon Plus Environment

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330

interfacial charge transfer made the photogenerated electron lifetime increase. Thus,

331

the resulting ZnIn2S4/CdS/ITO showed good PEC and photocatalytic performance.

332

When it was used for the PEC sensing of DA, it showed high sensitivity and

333

selectivity. During the detection process, DA turned to PDA and acted as effective

334

electron acceptor to inhibit the charge transfer and make the anodic photocurrent

335

decrease. The PEC sensor could detect 0.1 µM DA with a linear range of 0.3 to 300

336

µM.

337

338

Author Information

339

Corresponding Author

340

*E-mail address: [email protected] (BZ Zeng)

341

Author Contributions

342

1

343

the final version of the manuscript.

344

Supporting information

345

The Zeta potential of ZnIn2S4, digital pictures of products, PL spectra, SEM images of

346

ZnIn2S4/CdS-x, photoelectrochemical response curves of ZnIn2S4/CdS-x.

347

Acknowledgement

H.W. and H.Y. contribute equally to this work and all authors have given approval to

348

The work was supported by the National Natural Science Foundation of China

349

(Grant No. 21675117). The authors appreciate the Analytical and Testing Center of

350

WHU for the help in material characterization. 17 ACS Paragon Plus Environment


351

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