A novel photoelectrochemical sensor based on g-C3N4@CdS QDs for sensitive detection of Hg2+

Journal Pre-proof A novel photoelectrochemical sensor based on g-C3N4@CdS QDs for sensitive detection of Hg2+

Zhongping Li, Wenxia Dong, Xiaoyu Du, Guangming Wen, Xiujun Fan PII:

S0026-265X(19)31575-9

DOI:

https://doi.org/10.1016/j.microc.2019.104259

Reference:

MICROC 104259

To appear in:

Microchemical Journal

Received date:

26 July 2019

Revised date:

8 September 2019

Accepted date:

11 September 2019

Please cite this article as: Z. Li, W. Dong, X. Du, et al., A novel photoelectrochemical sensor based on g-C3N4@CdS QDs for sensitive detection of Hg2+, Microchemical Journal(2019), https://doi.org/10.1016/j.microc.2019.104259

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© 2019 Published by Elsevier.

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A novel photoelectrochemical sensor based on g-C3N4@CdS QDs for sensitive detection of Hg2+ Zhongping Li, a * Wenxia Dong,a Xiaoyu Du, a Guangming Wena, b * and Xiujun Fan c a

Institute of Environmental Science and School of Chemistry and Chemical Engineering, Shanxi

University, Taiyuan 030006, People's Republic of China School of Chemistry and Chemical Engineering, Jinzhong University, Jinzhong, 030619, China.

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Institute of Crystalline Materials, Shanxi University, Taiyuan 030006, People's Republic of

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China

*Corresponding authors. Tel./fax.: +86-351-3985596 E-mail addresses: [email protected] (Z. Li), [email protected] (G. Wen).

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Abstract Graphite carbonitride (g-C3N4) is one of the ideal metal-free semiconductors for photocatalysis, quantum dots are excellent photoelectrically active materials, and both have broad application prospects in photoelectrochemical (PEC) devices. In this work, a novel photoelectrochemical (PEC) sensor was latest employed based on the complex of g-C3N4 and CdS QD (g-C3N4@CdS) fixated on a tin oxyfluoride (FTO)

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electrode for discovering Hg2+. The PEC system showed a good linear range of

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20-550 nM, with a detection limit of 12 nM. The possible mechanism is signal

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shutdown strategy based on a lower sedimentation equilibrium constant of HgS. Thus,

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the g-C3N4@CdS based PEC sensor had positive effect on solving some issues, such

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as complicated operation, in the detection of Hg2+.

Keywords: Photoelectrochemical, Hg2+, graphite carbonitride (g-C3N4), CdS

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quantum dots (CdS QDs).

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1. Introduction As a highly toxic substance, heavy metal ions mercury (Hg2+) has damage to the environment and humans. Studies have shown that even at low concentrations, Hg2+ is one of the most annoying contaminants that can have serious adverse effects on the brain, kidneys, central nervous system and immune system [1]. Thus, it is important

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to detect Hg2+ with efficient and simple methods. Many conventional methods of colorimetric assay, electrochemistry, fluorescent probe, raman spectrum and UV–vis

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absorption [2-7], have been reported for the detection of Hg2+. Photoelectrochemical

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(PEC) technique as a novel emerging developing analytical technique method has pay

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close attention personality [8, 9]. The conductive material absorbs photons under light

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irradiation, and the electrons on the valence band will transition to the conduction

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band to form electron-hole pairs. In the PEC detection, light is as the excitation source and photocurrent is as the detection signal, photoelectrochemical (PEC) is the

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converse process to electrochemiluminescence (ECL) [10, 11]. PEC has a completely independent excitation source and signal detection system, so PEC has the advantages of low background signal, ultra-sensitive and easy operation [12]. Quantum dots (QD) have the advantages of narrow emission, wide absorption, high quantum yield and good light stability, which have attracted more and more attention in various fields such as optical devices, biosensing and bioimaging [13]. Graphite carbonitride (g-C3N4) is a novel metal-free layered π-conjugated semiconductor material, g-C3N4 has some characteristic, such as low price, good thermal stability, high chemical stability, perfect electrical and optical properties, so it

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is considered to be a promising photoelectrochemical biosensing photoactive material [14]. However, the optical band gap of g-C3N4 is about 2.7 eV, the electrons of g-C3N4 are easy to compound [15]. In order to increase the photoelectric activity of g-C3N4, many methods have been carried out. Such as metal mixing, non-metal mixing [16, 17], shape adjustment [18], and heterogeneity construction between g-C3N4 [19]. CdS

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quantum dots (QD) is a significant II-IV semiconductor and has been widely researched for visible photoactive material for PEC sensor [20]. At the same time, single CdS

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quantum dots also face high recombination rates of photogenerated electron-holes

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[21]. In order to improve this effect, combining with other conductors can reduce the

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charge recombination rate and improve photocurrent.

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In recent years, many photoelectrochemical methods have been used to detect

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Hg2+ [22, 23]. In this task, a novel PEC sensor was fabricated for the detection of Hg2+ based on CdS QD and g-C3N4 heterojunction electrode matrix modified FTO. This

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method can be used to directly detecting Hg2+ without using any biomaterial [24]. It is gratifying that the coupling of CdS QD and g-C3N4 can solve the problem of high electron recombination rate. Due to the excellent secondary stack structure, the photo-generated charge of g-C3N4@CdS can be effectively separated and transferred, resulting in an enhanced and stable photocurrent. Therefore the application of the proposed sensing strategy for rapidly evaluating of Hg2+ was investigated in the real sample.

2. Experimental section

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2.1 Chemicals Thioglycolic acid (TGA) (Shanghai, China), FTO-glass (15x50mm, resistance =10Ω) (Wuhan, China), cadmium chloride (CdCl2·2.5 H2O) (Shanghai, China), Sodium sulfide （Na2S·9 H2O） (Shanghai, China), Urea (AR, 99%) (Chengdu, China), Dimethyl sulfoxide (Shanghai, China), Sodium sulfate solution (Na2SO4 0.1

purification system (≥ 18 MΩ, Milli-Q, Millipore).

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2.2 Apparatus

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M) (Shanghai, China). The ultrapure water used was from a Millipore water

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The UV-vis absorption spectrum was recorded on a U-2910 spectrophotometer

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(Hitachi, Tokyo Japan). X-ray photoelectron spectroscopy (XPS) data is available

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from electronic spectrometer (Shimadzu Japan). Transmission electron microscope

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(TEM) images were produced using a JEM-2100 (JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 200 kV. X-ray diffraction pattern (XRD) was obtained on a

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Rigaku Dmax 2000 X-ray diffractometer with graphite monochromated CuKa radiation (λ = 0.154 nm). Infrared (IR) spectra were recorded at room temperature using a Bruker IR spectrometer. PEC detection was performed on a CHI 830C electrochemical workstation (Shanghai Chenhua Instrument Co., China). A PLS-SXE 300 xenon lamp (Perfect Co., China) (λ > 420 nm) as the irradiation source.

2.3 Preparation of electrode materials The process of preparing CdS QDs is based on previous reports [25]. 250 μL of the thioglycolic acid mix in 50 mL of 0.01 M CdCl2, with N2 saturation for 30 min and add 0.5 M NaOH adjust pH to 11, 5.0 mL of 0.1 M Na2S was added to the mixed

Journal Pre-proof solution. Next, the contents were refluxed at 110 ℃ for 4 hours. Finally, the CdS QDs solution was saved at 4 °C for future use [26]. The urea was heated to 500 ℃ in a furnace at a temperature increase rate of 5 ℃/min and kept for 3 h to prepare the original g-C3N4, and the yellow g-C3N4 product was collected after careful grinding [27]. The g-C3N4 solution was prepared by dissolving 0.1 g of the g-C3N4 in 10 mL of

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dimethyl sulfoxide (DMSO), and g-C3N4@CdS was obtained by ultrasonically mixing CdS QD and g-C3N4 for 30 minutes.

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2.4 Fabrication of FTO modified g-C3N4@CdS electrode

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The FTO conductive glasses were each ultrasonically cleaned in acetone, ethanol

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and water for 30 minutes before using the electrodes. The FTO glass plate was then

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immersed in an ethanol solution for future use. 50 µL of g-C3N4@CdS solution was

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applied to the FTO electrode and dried in an oven at 60 °C for 30 minutes. 0.1M Na2SO4 solution is used as the electrolyte. g-C3N4@CdS/FTO electrode is working

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electrode, the platinum wire is the auxiliary electrode, and Ag/AgCl point is used as the reference electrode, which forms a three-electrode system.

2.5 PEC detection

Insert the FTO electrode, platinum wire electrode and Ag/AgCl electrode into 0.1 M Na2SO4 electrolyte. The Xe light was turned on/off keeping 30 s and test for 20 s in the presence of the light source. Repeat the above experiment several times until the photocurrent is stable. The aqueous solution of Hg2+ was added dropwise to the electrolyte and deposited for 5 min for detection.

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3. Results and discussion 3.1 Photoelectric sensing mechanism This work developed a simple and sensitive "signal shutdown strategy" PEC method, the three-electrode system is shown in Fig. 1a. As shown in Fig. 1b, composite g-C3N4@CdS is coated on the surface of F-doped tin oxide (FTO)

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electrode to form a g-C3N4@CdS/FTO electrode. When light is irradiated, the photoactive material is under the action of light induction, and excitons are generated

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to form a photocurrent [28]. At the same time, after the photocurrent is formed,

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mercury ions (II) are dropped into the Na2SO4 electrolyte, there is competition

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between Hg2+ and CdS QD, and Hg2+ can be trapped on the surface of the

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photovoltaic material. The sedimentation equilibrium constant of HgS (4×10-53) is

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much smaller than the sedimentation equilibrium constant of CdS (8×10-27). The formation of excitons is suppressed under photoexcitation, thereby preventing

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photoelectron escaping and photoelectron transport, resulting in a decrease in photocurrent. The designed "signal shutdown strategy" has good performance and extends the application of PEC sensing strategies [29]. Scheme 1.

3.2 Compositional characterization The element composition of the g-C3N4@CdS was analyzed using X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum (Fig. S1) indicates that the elements of C, N, Cd, S and O are present in the sample. C 1s has four component peaks (Fig. 1a). The peak located at 284.82 is graphite C atoms [30], 285.5 eV, 288.5

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eV was ascribed to C-N-C, 288.2 eV can be assigned to N-C=N bond [31, 32]. Three peaks centered at 398.56 eV, 398.96 eV, and 400.95 eV can be fixed from the N 1s spectrum (Fig. 1b). 398.96 eV should belong to tertiary nitrogen (N−(C)3), 400.95 eV corresponds to C−N−H. The peak at 398.56 eV is associated with the triazine rings (C−N=C) [33]. Fig. 2c displayed three peaks at 411.47 eV, 404.72 eV and 405.48 eV,

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which are summarized as Cd 3d3/2 and Cd 3d5/2 for Cd2+. the S element at 163.6 eV, 164.73 eV and 161.2 eV, which be part of S 2p1/2 and S 2p3/2 (Fig. 2d) [34].

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Therefore, these XPS results further confirmed the successful formation of the

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g-C3N4@CdS complex.

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Fig. 1.

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3.3 Morphology and spectroscopic characterizations

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Analysis of crystal phase and elemental composition of synthetic materials by X-ray diffraction (XRD) (Fig. 2a). Two diffraction peaks (24.5° and 27.4°)

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correspond to the graphite phase (100) and (002) peaks of g-C3N4, the peaks at 36.5° and 44° point to the (102) and (110) planes of CdS [35, 36]. The TEM analysis indicates that the bigness of CdS QDs about 3-4 nm (Fig. 2b). Fig. 2c reveals the structure of g-C3N4. Thus, the sheet structure of g-C3N4 determines that it can be aggregated on the FTO electrode. This also determines that the wide area of g-C3N4 can raise a valid contact location for CdS nanoparticles. The TEM image of the g-C3N4@CdS complex (Fig. 2d) shows that the CdS QDs were successfully loaded onto the surface of g-C3N4, further demonstrating the formation of g-C3N4@CdS. SEM image of g-C3N4 (Fig. 2e) indicating that the prepared g-C3N4 is a sheet

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structure. The SEM image (Fig. 2f) exhibits that CdS QDs are dispersed on the surface of g-C3N4 . Fig. 2 Fig. 3a shows the electrochemical impedance spectroscopy (EIS) of each step constructed in the presence of [Fe(CN)6]3-/4-. Inset is an equivalent circuit diagram, the

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circuit consists of a double layer capacitor (C) of electrodes and electrolyte, solution resistance (Rs), electron transfer resistance (Ret) and Warburg impedance (Zw). First,

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the EIS of the blank electrode was tested to be about 0.4 kΩ (curve a). When CdS,

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g-C3N4, g-C3N4@CdS was fixed on the FTO electrode, the EIS value dropped to 0.22

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kΩ, 0.17 kΩ, 0.15 kΩ (curve b, c, d). This indicates that g-C3N4@CdS has best

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conductivity. When g-C3N4@CdS /FTO was immersed in an electrolyte containing

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Hg2+ for 5 minutes, the EIS value was increased to 0.55 kΩ (curve e), it indicates that the conductivity of Hg2+/g-C3N4@CdS/FTO increases, this hinders the interface

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electron transfer and is consistent with our experimental results. CdS QDs, g-C3N4, and g-C3N4@CdS composites of FT-IR are shown in Fig. 3b. Pure g-C3N4 has three characteristics at 3176.8 cm-1, 1200-1700 cm-1, and 811.2 cm−1. The peak at 3176.8 cm-1 is assigned to the telescopic vibration of the N-H bond. The peak at 1200-1700 cm-1 are ascribed to CN heterocycles in g-C3N4, the peaks at 811.21 cm-1 interrelate the triazine ring structure is found. For CdS QDs, two broad peaks around 1402.54 cm-1and 1136.27 cm-1 are belonged to Cd-S bond. The CdS QDs exhibited symmetric of COO- at 1578.26 cm-1. The peak at 3417.4 cm−1 could be defined as the water molecules of surface-adsorbed. In the FT-IR spectrum of the

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g-C3N4@CdS composite, all IR bands of CdS QDs were present, while the peaks of g-C3N4 were not as pronounced. The results may be attributed to the relatively high percentage of CdS QDs present in the composite resulting in an IR absorption intensity of CdS QDs greater than g-C3N4 [37, 38]. UV-visible absorption spectroscopy was used for characterizing the optical

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properties of g-C3N4, CdS and g-C3N4@CdS nanocomposites (Fig. 3c). As can be seen, the absorption of pure g-C3N4 is enhanced at 460 nm. Meanwhile, the absorption

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of pure CdS at 500 nm is enhanced. When g-C3N4 and CdS QD were coupled together,

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the absorption is significantly wider than that of pure material [39]. As shown in Fig.

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3d, the photocurrent of pure CdS is 0.43 µA, and the photocurrent of pure g-C3N4 is

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0.55 µA. Composite g-C3N4@CdS has higher photocurrent than pure material, this

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result indicates that the composite has a better light response than a pure substance. Fig. 3

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The modified electrode was prepared by a drop coating method, then was dried in an oven. The temperature of the baked electrode was investigated. As shown in Fig. S2a, the photocurrent is the highest when the best baking temperature is 60 ℃, When the baking temperature is too low or too high, the electrode material cannot be stably applied to the electrode, which affects the further experiments. In order to study the influence of electrode materials on photoelectric response, the photocurrents of CdS and g-C3N4 at different ratios were studied. It can be found from Fig. S2b. When the mass rate of CdS to g-C3N4 is 2:1, the photocurrent is the largest, indicating that the proper mass rate of 2:1 is the best condition. The possible reason is that CdS can

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supply more holes to join in photoelectrochemical processes, but a large amount of CdS blocks the transfer of photogenerated electrons on its surface.

3.4 PEC characterization Under optative conditions, the g-C3N4@CdS/FTO electrode showed an excellent photocurrent response that was reduced by mercury ion induced exciton trap formation (Fig. 4a). As the concentration of mercury ions increases, the photocurrent

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decreases. The curves of the photocurrent with different concentrations of Hg2+ were

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shown in Fig. 4b, inset showed that the photocurrent value appeared a well linear

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relationship with the logarithm of Hg2+ concentrations ranging from 20-550 nM

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(R2=0.9917), and the detection limit (LOD) is 12 nM (S/N=3).

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Stability is a very important factor for PEC sensors to consider in practical

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applications. When the excitation light is repeatedly turned on and off, its photocurrent is studied as shown in Fig. 4c. No change in photocurrent was observed

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for 15 minutes, indicating the perfect stability of the PEC sensor. Fig. 4d shows the effect of normal heavy metal ions such as Pb2+, Zn2+, Mn2+, Fe2+, K+, Ca2+, Na+, Ni2+, Co2+, Cr3+, Cd2+ ,Cu2+, Ag+ on Hg2+ detection, which manifest that these heavy metal ions even at a high capacity have small influence on the discover of Hg2+. But copper ions and silver ions have a more obvious reaction in this system than other metal ions. However, their effects are as positive as other metal ions, which was different to detecting of Hg2+ on the reduction of photocurrent in the system. Therefore, we believe that the proposed PEC sensor has good selectivity for the detection of Hg2+, Where I-I0 is the photocurrent difference between ion/g-C3N4@CdS/FTO(I) and

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g-C3N4@CdS/FTO(I0). Fig. 4

3.5 Actual sample detection and comparison of different methods Therefore, in order to verify the practicability of the PEC sensor, Hg2+ was added to tap water for detected Hg2+. At the same time, we also detected Hg2+ in the lake

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water. Then record the photocurrent and calculate the concentration of Hg2+ (repeated three times for each sample), the result is shown in Table 1. It was found that the

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tested water sample of Hg2+ concentration was the same as the actual added Hg2+

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concentration, the calculated recovery rate was 96%-103%, and the relative standard

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deviation (RSD) was also maintained in a small range. The results show that the PEC

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sensor has potential application value for the detection of Hg2+ in water.

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Compared with the Photoelectrochemical reported methods for detecting Hg2+, as shown in Table 2. They have a good performance in the linear range or detection

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limit, and our method prepared a novel photoelectrochemical sensor for detecting Hg2+, and with excellent linear range [40-42]. Table1. Table2.

4. Conclusion In summary, a simple and novel PEC sensor was constructed by detecting the mercury ions by attaching the photovoltaic material g-C3N4@CdS to the FTO electrode. By introducing CdS quantum dots on the sensing interface, non-toxic CdS QDs were employed as a S source and the detection signal of mercury ions is amplified, which effectively improves the sensitivity of the sensor. PEC has good

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sensitivity, selectivity and stability.

Acknowledgments We gratefully acknowledge the National Natural Science Foundation of China (21575083), the Natural Science Foundation of Shanxi Province (201801D121042), Transformation of Scientific and Technological Achievements Programs of Higher

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Education Institutions in Shanxi (2019KJ008) and the State Key Laboratory of Analytical Chemistry for Life Science，Nanjing University (SKLACLS1911) for the

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financial support on this work.

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Scheme 1. (a) Sensor schematic. (b) Schematic illustration of photoelectrochemistry for sensing of Hg2+. Fig. 1 XPS spectra of g-C3N4@CdS: (a) C 1s, (b) N 1s, (c) Cd 3d, and (d) S 2p. Fig. 2 XRD pattern of g-C3N4@CdS (a). TEM images of CdS QDs (b), g-C3N4 (c), g-C3N4@CdS (d) and SEM images of g-C3N4 (e), g-C3N4@CdS (f). Fig. 3 (a) EIS (a: FTO, b: CdS, c: g-C3N4, d: g-C3N4@CdS/FTO, e:

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Hg2+/g-C3N4@CdS/FTO), Inset: equivalent circuit diagram; (b) FT-IR spectrum of

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CdS, g-C3N4, g-C3N4@CdS; (c) UV-vis spectrum of CdS, g-C3N4, g-C3N4@CdS; (d)

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Photocurrent response diagrams for different materials.

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Fig. 4. (a) Photocurrent response at different contents of Hg2+. (b) Linear calibration

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curve of photocurrent. Inset: the linear calibration curve of photocurrent. (c) Sensor

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stability. (d) Under the same experimental conditions, the effect of adding other ions on the detection of mercury ions in the presence of divalent mercury ions. (Error bars

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represent three independent measurements).

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Spiked (nM)

Found (nM)

Recover (%)

Recovery (%)

Running water

80

82.72

96.71

2.05

150

155.06

99.96

1.67

380

384.01

98.9

1.78

80

83

96.4

1.58

160

166

96.4

1.16

350

343

102.0

1.55

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Lake water

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Table2. Photoelectrochemical methods were used to determine of Hg2+. Methods

WO3/Au ZnS QDs Fe3+/ZnO-Ag g-C3N4@CdS

Photoelectrochemical Photoelectrochemical Photoelectrochemical Photoelectrochemical

Linear range 4.2-840 pM 10-11-10-6 M 0.5-100 nM 20-550 nM

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Materials

Detection limit 2 pM 2 pM 0.1 nM 12 nM

Refs 38 39 40 This work

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Highlight 

For the first time, g-C3N4@CdS QD is used to detect Hg2+.

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The g-C3N4 monolayer and the CdS monolayer form a novel complex.

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An internal electric field is formed between the g-C3N4 and CdS monolayers, which effectively suppresses recombination of photogenerated electrons and

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improves photoelectric response.

Figure 1

Figure 2

Figure 3

Figure 4