Selective and sensitive visible-light-prompt photoelectrochemical sensor of Cu2+ based on CdS nanorods modified with Au and graphene quantum dots

Journal Pre-proof Selective and Sensitive Visible-Light-Prompt Photoelectrochemical Sensor of Cu2+ Based on CdS Nanorods Modified with Au and Graphene Quantum Dots Izwaharyanie Ibrahim (Data curation) (Writing - original draft) (Writing - review and editing), Hong Ngee Lim (Supervision) (Conceptualization), Nay Ming Huang (Visualization) (Investigation), Zhong-Tao Jiang (Software) (Validation), Mohammednoor Altarawneh (Software) (Validation)

PII:

S0304-3894(20)30236-3

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122248

Reference:

HAZMAT 122248

To appear in:

Journal of Hazardous Materials

Received Date:

18 May 2019

Revised Date:

4 February 2020

Accepted Date:

4 February 2020

Please cite this article as: Ibrahim I, Lim HN, Huang NM, Jiang Z-Tao, Altarawneh M, Selective and Sensitive Visible-Light-Prompt Photoelectrochemical Sensor of Cu2+ Based on CdS Nanorods Modified with Au and Graphene Quantum Dots, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122248

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Selective and Sensitive Visible-Light-Prompt Photoelectrochemical Sensor of Cu2+ Based on CdS Nanorods Modified with Au and Graphene Quantum Dots

Izwaharyanie Ibrahim1, Hong Ngee Lim1,2*, Nay Ming Huang3, Zhong-Tao Jiang4, Mohammednoor Altarawneh4

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Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM

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Serdang, Selangor, Malaysia Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology,

Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

School of Energy and Chemical Engineering, Xiamen University Malaysia, Selangor Darul

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University, Xiamen 361005, China

School of Engineering & Information Technology, Murdoch University, Murdoch, WA 6150,

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Australia

Corresponding Author - E-mail: [email protected]

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*

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Ehsan 43900, Malaysia & College of Chemistry and Chemical Engineering, Xiamen

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Graphical abstract

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A triple structure of CdS/Au/GQDs as a photo-to-electron conversion medium for the

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real-time detection of Cu2+ ions. 

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Highlights:

The detection limit of 2.27 nM was obtained, which is 10,000 fold lower than that of WHO’s Guidelines (~30 µM).

The photocurrent reduction was negligible after 30 days of storage, suggesting the high

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stability of the photoelectrode.

The real-time monitoring of Cu2+ ions in real water samples showed satisfactory results.

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The excellent results acquired confirmed the capability of the studied photoelectrode as

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a practical detector for Cu2+ ions.

Abstract

Nowadays, increasing the risk for copper leaching into the drinking water in homes, hotels and schools has become unresolved issues all around the countries such as Canada, the United States, and Malaysia. The leaching of copper in tap water is due to a combination of acidic 2

water, damaged pipes, and corroded plumbing fixtures. To remedy this global problem, a triple interconnected structure of CdS/Au/GQDs was designed as a photo-to-electron conversion medium for a real time and selective visible-light-prompt photoelectrochemical (PEC) sensor for Cu2+ ions in real water samples. The synergistic interaction of the CdS/Au/GQDs enabled the smooth transportation of charge carriers to the charge collector and provided a channel to inhibit the charge recombination reaction. Thus, a detection limit of 2.27 nM was obtained, which is 10,000 fold lower than that of WHO’s Guidelines for Drinking-water Quality (~30

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M). The photocurrent reduction was negligible after 30 days of storage under ambient conditions, suggesting the high stability of photoelectrode. Moreover, the real-time monitoring of Cu2+ ions in real samples was performed with satisfactory results, confirming the capability

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of the investigated photoelectrode as the most practical detector for trace amounts of Cu2+ ions.

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Keywords: Photoelectrochemistry; CdS/Au/GQDs; Photo-to-electron conversion; Copper

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sensing; High stability.

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1.0. Introduction

Ordinarily, there are several methods for copper ion determination, which exploit

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several detection principles, including surface plasmon resonance (SPR) spectroscopy [1], infrared absorption spectroscopy [2], fluorescence based sensing [3], colorimetric

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chemosensing methods [4], electroanalytical sensing [5], and eletrochemiluminescence sensing [6]. Even though the spectroscopy methods permit the effective determination of copper in the presence of other interfering metal ions, they lack precision because they are subject to easy interference from the surrounding atmosphere and consequently exhibit distortion [7]. Additionally, each of these methods suffers from at least one unfavorable limitation, such as an

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intricate laboratory procedure, relatively higher instrumentation cost, and the need for welltrained professionals for its implementation. Sensors based on the photoelectrochemical (PEC) approach are new yet have been shown to represent powerful and advanced techniques in a wide area of study, including biomedical analysis [8], solar cells [9], water splitting [10], and environmental assessment and monitoring [11]. Because of the excitation of electrons upon stimulation by light and the resulting photocurrent signal, sensing based on the PEC approach can offer advantages

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compared to the foregoing electrochemical sensing method, such as sensitivity, robustness, broad applicability, and low undesired background noise [12]. The PEC sensing technique involves the transfer of electrons between an analyte and a semiconductor upon photo-

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irradiation, which subsequently leads to changes in the photocurrent signal [13]. However, a functioning PEC sensor mainly relies on a photoactive metallic semiconductor such as CdS,

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TiO2, ZnO, or SnO2, based on its photoelectric conversion efficiency.

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The distinctive features of a nanosize CdS semiconductor have led to its comprehensive study for the development of advanced photovoltaic devices due to its promising performance in the solar energy storage research area [14, 15]. Recently, efforts have been dedicated to

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intensifying its PEC performance by modifying the morphology and structure of CdS [16]. Compared with the modified structure of CdS, a one-dimensional (1D) nanostructure has a

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practical and substantial potential due to its specific directionality for the transportation of

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charges carried along the 1D pathway, and subsequently a decreased probability of the recombination of charge carriers [14]. Comprehensive studies on a narrow band gap semiconductor with 1D nanostructures (e.g., nanotubes, nanorods, nanofiber, and nanowire) have been performed, and the outcomes consistently portrayed a much better PEC performance than their counterpart particulate nanostructures [14].

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Currently, intensive studies focusing on enhancing the sensitivity of a semiconductor by coupling other components are rapidly springing up [17, 18]. There are many extensive studies on nanocomposites consisting of metallic semiconductors and noble metal materials. The goals for such hybrid nanostructures are to prolong the electron lifetime, make charge separation and transportation feasible, and thus intensify the PEC activity upon the detection of the targeted analyte [12, 17, 19]. CdS with surface modification using a noble metal, particularly Au, is a frequently used photo-active material, and it is becoming a flexible method

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for improving the photocatalytic activity of CdS. It was believed that Au nanoparticles (NPs) could serve as electron mediators and efficiently transport the photogenerated charge carriers [18]. QD-based PEC assay is also getting much attention as a vital photoelectrochemically

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active material with a high-efficiency photocurrent conversion performance, a size dependable PEC response, and the production of multiple charge carriers with a single photon [20].

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Therefore, with the discovery of graphene quantum dots (GQDs), the zero-dimensional carbon

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family is seen to have the potential to minimize the electrochemical impedance and ease the charge transport kinetics at the semiconductor-metal interface, which could subsequently reduce the charge recombination rate [11, 21].

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Taking into account the above thinking, a new type of multi-functional hybrid nanostructure of CdS NRs/Au QDs with modified GQDs has successfully been designed.

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Herein, CdS acts as the photocatalytic active center, which is in contact with the charge

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collector. Meanwhile, Au is used as an assistant catalyst, which is endowed with resonant photon scattering, and thus mediates the charge transfer to the photocatalytic active center. More importantly, both the GQDs and CdS play key roles as photosensitizers, where the photoexcited charge carriers are generated. A practical mechanism whereby the CdS/Au/GQDs amplify the PEC performance was proposed after careful characterization and analysis. Because of the fast and facile charge separation that results from the favorable band alignment

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of the energy levels between the GQDs and the semiconductor-metal of CdS/Au, this promising hybrid architecture has potential applications in superior environmental monitoring devices based on the PEC detection of Cu2+.

2.0. Experimental Section 2.1. Materials Water soluble green GQDs were purchased from ACS Material, USA. As indicated by

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the manufacturer, the GQDs had a well-distributed size of <6 nm. The X-ray photoelectron spectroscopy (XPS) measurement of the GQDs based on the C1s core level spectra was deconvoluted into the four components of C-C (sp2), C-C (sp3), C-O, and C=O bonds at 285.0,

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286.5, 287.5, and 289.0 eV, respectively [22]. Copper (II) sulfate pentahydrate (CuSO4.5H2O) and cadmium acetate dihydrate (Cd(CH3COO)2•2H2O) were purchased from Hamburg Thiourea (CH4N2S) was purchased from Fisher Scientific, USA.

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Chemicals, Germany.

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Chloroauric acid (HAuCl4.3H2O) and trisodium citrate (Na3C6H5O7) were purchased from Aldrich, USA. N-butylamine (C4H11N) was purchased from Acros Organics, Belgium. Acetone ((CH3)2CO, 99.5%) was purchased from Friendemann Schmidt, USA. Ethanol (CH3CH2OH2,

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95%) was purchased from Systerm, Malaysia. Potassium chloride (KCl) and triethanolamine (TEA, 99%) were obtained from Merck, USA. Indium tin oxide (ITO) conducting glass slides

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(7 Ωsq-1) were commercially supplied by Xin Yan Technology Limited, China. Each stock

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solution of Ba2+, Co2+, Li2+, Ni2+, Mn2+, K+, Zn2+, Na+, Mg2+, Ag2+, Fe2+, and Fe3+ was composed of an appropriate amount of the dissolved compound (BaCl2.2H2O, CoSO4, Li2SO4, NiSO4, MnSO4.H2O, KCH3CO2, ZnSO4.7H2O, NaCH3OO, MgCl2.6H2O, Ag2SO4, FeSO4, and Fe2(SO4)3) in Milli-Q deionized water with a resistivity of 18.2 MΩ cm. Each sample solution was 10 mL. Deionized water was used in all the experiments. Unless otherwise specified, all

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of the materials and reagents were used as received without conducting any purification process.

2.2. Preparation of CdS NRs The CdS NRs was synthesized Yang et al. (2002) [23], with modifications. In a typical experiment, the solvothermal process for the synthesis of the CdS NRs was carried out by mixing 0.457 g (6 mmol) of CH4N2S powder in 20 mL of n-butylamine. Next, 0.533 g (2 mmol)

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of Cd(CH3COO)2•2H2O was completely dissolved in the mixture. With vigorous stirring, the final mixture was placed in a Teflon autoclave, sealed tightly, and subjected to a hydrothermal reaction at 100 °C for 24 h. The final attained CdS NR precipitate was centrifuged and

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thoroughly washed with ethanol and deionized water. Finally, the obtained product was dried at room temperature for 24 h for further analysis. The yielded yellowish powder was denoted

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as CdS NRs.

2.3. Preparation of Au NPs/GQDs

Au NPs were synthesized by the citrate reduction of HAuCl4 [24]. Briefly, 5 mg of

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GQDs was dissolved in 50 mL of a 1 mM HAuCl4 solution and then stirred vigorously for 30 min. The mixture solution was heated until boiling under stirring. Then, 5 mL of 40 mM

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trisodium citrate was quickly added to the boiling mixture. The solution changed color from

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pale yellow to black and finally turned to a red wine color over several minutes. The reflux reaction then occurred for 60 min. After the solution was cooled down, it underwent a centrifugation process to remove the extra free GQDs and trisodium citrate, and was then redispersed in distilled water. For comparison, bare Au NPs were also prepared in a similar way.

2.4. Preparation of CdS NRs/Au NPs/GQDs

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CdS NRs/Au NPs/GQDs were prepared using the reflux approach. Here, 100 ml of the freshly prepared CdS NR powder at 10% w/v was mixed with the Au NP-GQD solution. The mixture was left for 4 h to grow the Au NPs-GQDs over the CdS NRs. Afterward, the solution was placed in a three-necked flask and directly heated at 100 °C in a reflux system in a nitrogen gas atmosphere for 1 h. This step was conducted in order to grow the Au NPs-GQDs on the CdS NR structures. A final dark yellow colloidal solution was obtained. After cooling down, the resultant mixture was centrifuged and washed with ethanol and deionized water three times.

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Lastly, the precipitates were dried at room temperature for 24 h to yield dark yellowish powders of CdS NRs/Au NPs/GQDs. For comparison, CdS NRs/Au NPs and CdS NRs/GQDs were also

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prepared using similar methods.

2.5. Characterization Techniques

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The morphologies of the nanomaterials were characterized using a field emission

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scanning electron microscope (FEI Quanta SEM Model 400 F) equipped with an energy dispersive X-ray (EDX) accessory and transmission electron microscopy (TEM) (Hitachi, HT7700, Tokyo, Japan). The particle size analysis of nanomaterials was measured by using an

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ImageJ software. The surface bonding structures of the samples were examined using XPS (Kratos Axis Ultra XPS spectrometer, Manchester, UK) with Al Kα radiation (hν = 1486.6 eV).

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The crystalline phase of the CdS NRs/Au NPs/GQDs was analyzed using a Philips X’pert

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system X-ray powder diffractometer with Cu Kα radiation (λ = 1.5418 Å), and the optical absorption properties in the spectral region of 200–800 nm were assessed using a UV-Vis spectrophotometer (Thermo Scientific Evolution 300). A photoluminescence analysis was conducted using a Renishaw inVia Raman microscope with laser excitation at λ = 514 nm.

2.6. Preparation of Photoelectrochemical Sensor Electrodes

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The CdS NR/Au NP-GQD-modified ITO sensor electrode was prepared using a simple doctor blade method. First, an ITO glass substrate (2 cm × 2 cm) was cleaned ultrasonically using acetone, ethanol, and deionized water. After that, 0.5 ml of the 50% w/v CdS NR/Au NPs/GQD powder was mixed with a nafion solution (0.1% w/v), and the mixture was ground for several minutes to obtained a uniform CdS NR/Au NP/GQD paste. Then, the CdS NR/Au NP/GQD paste was cast onto the ITO glass by a doctor blade with tape as the spacer. The resultant CdS NR/Au NP/GQD thin film was dried at 70 °C and calcined at 150 °C for 1 h in

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an inert atmosphere. For comparison, CdS NR-, CdS NR/Au NP/ITO-, and CdS NR/GQD/ITOmodified electrodes were also fabricated by following the same procedure.

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2.7. Photoelectrochemical Experiments

Photoelectrochemical detection was performed using a three-electrode system that

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included the active material as the working electrode, a platinum wire as the auxiliary electrode,

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and an Ag/AgCl electrode as the reference electrode. Here, 0.1 M KCl containing 0.5 M TEA as the electron donor was used as an electrolyte. A 150 W low cost solar simulators (Abet Technologies) was used as the light source in this experiment. One sun performance (10 cm

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distance of the photoactive site to the light source) for up to a 35 mm diameter illuminated field with uniformity of +/- 25% can be achieved using this solar simulator. Before the PEC test, the

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samples were carefully washed with 0.1 M KCl and immersed in the electrolyte for 10 min to

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fully remove the absorption molecules. Immediately after the background photocurrent was stabilized, the photocurrent response was recorded while the excitation light was turned on and off. The photocurrent signal was measured and analyzed on a computer-controlled VersaSTAT-3 electrochemical analyzer from Princeton Applied Research. The experimental setup was shown in Figure S1.

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3.0. Results and Discussion 3.1. Morphology and structural properties In this research, the most important prerequisite for the built-up PEC sensor was to successfully synthesize the photoactive material of the CdS NRs/Au QDs/GQDs with high efficiency for the generation of a photocurrent signal. The morphology and surface structure of the as-synthesized CdS NRs/Au QDs/GQDs were investigated using a transmission electron microscope (TEM) and field emission scanning electron microscope (FESEM). The TEM

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image in Figure 1a shows that the CdS was composed of uniform, dense, and 1D CdS in rodlike structures. Based on the histogram analysis of more than 30 particles, a histogram of the CdS NRs (Figure S2a and S2b) showed a narrow size distribution with average diameter and

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length of ~9.3 and ~27.8 nm, respectively. Moreover, the elemental components of the CdS

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NRs sample were examined using an energy dispersive X-ray (EDX) analysis, and the results are shown in Figure S2c. In addition, the surface structures of the CdS NRs/Au QDs (Figure

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1b) were observed to predominantly consist of nanospheres of Au QDS with a particle size of 4–6 nm, which created an agglomerated nanocomposite of CdS NRs/GQDs (inset of Figure

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1b). After the introduction of GQDs on the CdS NR/Au QD nanocomposite, it displayed highly transparent disk-shaped structures of GQDs on the agglomeration of CdS NRs/Au QDs (Figure

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1c). The transparent structure of the GQDs was hardly visible even after the magnification of the TEM image. TEM images of the Au QDs and GQDs are also shown in Figure S3. In the

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high resolution TEM image of the CdS NR/Au QD/GQD nanocomposite shown in Figure 1d, d-spacing values from the visible lattice fringes of 0.33 nm, 0.24, and 0.17 nm, which were assigned to CdS (002), Au (111), and GQDs (002), respectively. Our results agree with literature values, and gave conclusive evidence that the CdS NRs/Au QDs/GQDs were successfully synthesized using the proposed fabrication approaches [25].

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Figure 1. TEM images of (a) CdS NRs, (b) CdS NRs/Au QDs (inset of size distribution histogram for Au QDs), (c) CdS NR/Au QD/GQD nanocomposite, and (d) lattice fringes of

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CdS NRs/Au QDs/GQDs.

The crystalline phase of the fabricated material was first examined using X-ray

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diffraction (XRD). Figure 2a shows the XRD pattern for the evolution process of the products of pure GQDs, Au QDs, CdS NRs, and the nanocomposite of CdS/Au, CdS/GQDs, and

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CdS/Au/GQDs. The diffraction peak of the GQDs (002) centered at 2θ = 25.2° was broad because of the intercalation of the oxygenated groups [26]. Compared with the characteristic diffraction peak of GQDs, the presence of this peak in the XRD spectra of the as-synthesized CdS/GQD and CdS/Au/GQD nanocomposites confirmed the deposition of GQDs onto the surfaces of CdS and CdS/Au, respectively. Moreover, the XRD profile of CdS NRs revealed the dominant peaks of the pure hexagonal phase of wurtzite CdS. The diffraction peaks 11

corresponding to the hexagonal phase are illustrated in the crystal planes of (100), (002), (101), (102), (110), (103), and (112), which are in good agreement with the JCPDS data (Card No. 65-3414) [27]. In addition, there was no obvious indication of impurities in the phase upon the decoration with Au QDs and GQDs. Evidence of the diffraction signal of the Au QDs was found in the XRD profile of the CdS/Au/GQDs, with three strong diffraction peaks at 39.8°, 66.5°, and 78.6° (red color) attributed to the (111), (220), and (311) crystal planes of the cubic Au (JCPDS Card No. 04-0784) [28], respectively, indicating that Au was successfully

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introduced to the nanocomposites. Vibration spectroscopy is a non-destructive tool for evaluating the crystal phase, and it is also an effective method for determining the true state of carbonaceous materials [29]. Figure

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2b shows the Raman spectra of the GQD, CdS NR, CdS/Au, CdS/GQD, and CdS/Au/GQD nanocomposites. The Raman spectrum of the GQDs shows a major Raman feature with

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significant peaks at 1406.7 and 1641.8 cm-1, parallel to the recorded evidence of the D- and G-

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bands, respectively. The relative intensity of the structural defect D-band to the crystalline Gband (ID/IG) was found to be 1.12, while the reported D-band of the graphene nanoplatelet

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(GNP) was negligible [30]. A comparison of the D-bands of the GQDs and GNP gave a clear indication that the surfaces of the GQDs had many deficiencies with various oxygenated groups

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[31]. Additionally, a 2D peak centered around 2662.5 cm-1 was also present in the GQD spectrum. The intensity ratio of the 2D-band to the G-band was 0.56. This feature showed that

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there were a few layers of GQDs on the surface [32], which plainly highlighted its unique optical and electronic properties. For CdS NRs, three intense and broad peaks were seen to occur at 302.0, 607.1, and 907.8 cm-1, corresponding to the first-, second- and third-order longitudinal optical-phonon (LO) modes of the CdS NRs, respectively. The obtained LO Raman peak was in good agreement with those reported for wurtzite CdS [28, 33]. The existence of D- and G-bands in the CdS/GQD and CdS/Au/GQD spectra emphasized the 12

existence of interaction between the GQDs and wurtzite CdS. GQDs were bound to the CdS NR surface via van der Waals interaction [34]. Therefore, the as-synthesized composite material could work in unison to enhance the PEC performance through the efficient separation of charge carriers. UV-vis absorption spectroscopy was used to examine the optical properties of the assynthesized samples. As depicted in Figure 2c, the GQDs reveal a well-established UV absorption spectrum with a shoulder absorption peak at 262 nm, ascribed to the π–π* transition

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of the C=C sp2 domains [35]. Another absorption peak at 310 nm was observed due to the n– π* transition of several functional groups such as C=O, C−OH, and O−C=O [36]. Moreover, absorption edge of the CdS NRs was found between 510 and 525 nm, which was consistent

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with the absorption edge of hexagonal wurtzite CdS [37]. The nanocomposite of the CdS/Au

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and CdS/GQDs demonstrated an almost similar absorption edge with bare CdS NRs. The UVvis absorption spectrum of the CdS/Au/GQDs displayed an apparent absorption edge at 537

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nm, which was red-shifted compared to the plasmonic absorption band of Au QDs (520 nm). The CdS/Au/GQDs showed a very broadened absorption due to the closer proximity of the CdS

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NRs and Au/GQDs, which was attributed to the combination of the enhanced plasmon Au and unique optical properties of GQDs [14]. Hence, the UV-vis result of the CdS/Au/GQDs

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demonstrated a practical utilization of solar energy, which consequently intensified the photoelectrical related performances. By extrapolating the absorption curve using a linear fitting

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method, the band gap energy can be evaluated using Equation (1): (αhν)n = A(hν − Eg)

Equation (1)

where α is the absorption coefficient, hν is the photon energy, A is a constant characteristic of the material, Eg is the band gap energy, and n is either 1/2 for an indirect transition or 2 for a direct transition. Herein, CdS is known to be a direct transition of n-type semiconductor.

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Referring to the Tauc plot shown in Figure 2d, the band gaps of the samples were calculated to be 3.708, 2.461, 2.409, 2.449, and 2.355 eV, corresponding to GQDs, CdS NRs, CdS/Au, CdS/GQDs, and CdS/Au/GQDs, respectively. The increase in the band gap for the GQDs compared to the other samples (inset Figure 2d) was due to the quantum confinement effects of the QDs material [38]. Moreover, the narrower band gap for the nanocomposite of CdS/Au/GQDs compared to pure CdS NRs was apparently due to the ternary hybrid affecting the crystal lattice and decreasing the band gap energy [39]. Because the CdS/Au/GQD

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nanocomposite was used as a photoactive material for a PEC Cu2+ sensor, the narrower band gap may notably enhance the photo-to-electric conversion efficiency [40].

Photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) was further used

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to examine the electron-hole recombination rate through the surface structure and excited state

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of the samples, the electronic structure and elemental composition of the CdS/Au/GQD nanocomposite. Characterization is provided in more detail in Figure S4 and Figure S5,

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respectively.

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Figure 2. (a) XRD patterns, (b) Raman spectra, (c) UV-vis diffuse reflectance spectra, (d)

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corresponding plots of (αhν)2 vs. hν.

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3.2. PEC Characterization of the Fabricated Photoelectrodes The PEC activities of the fabricated Au QD, GQD, CdS NR, CdS/Au, CdS/GQD, and

CdS/Au/GQD photoelectrodes were first analyzed using linear sweep voltammogram (LSV) curves under light illumination. As demonstrated in Figure 3a, the scans of all samples were recorded in the potential range of -1.0 to 0.7 V vs. Ag/AgCl in a 0.1 M HCl and 0.5 M TEA solution. The LSV curves for the bare and nanocomposite of CdS NRs demonstrated enhanced 15

photocurrent responses under the stimulation of light. In contrast, the LSV scan recorded without light illumination showed an imperceptible background current under the same potential (Figure S6a). Moreover, the pure CdS NRs and their nanocomposite showed increases in the photocurrent density with increases in the forward bias potential (~0.4 V vs. Ag/AgCl), indicating that it was a typical n-type semiconductor [41]. After loading the Au QDs, there was an apparent increase in the photocurrent for the CdS/Au of 1.4 times over the CdS NRs. The enhancement could be ascribed to the fact that the Au QDs helped to improve the

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separation of electron-holes from the Au to CdS and hence further increased the total photoresponse of the electrode [42]. To further enhance the PEC performance of the CdS/Au, GQDs were introduced to the nanocomposite surface. The outcomes showed that the loading

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of GQDs could greatly boost the photocurrent density by 2.3 times compared to the CdS/Au. This could be attributed to the high charge carrier mobility of the GQDs, which subsequently

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reduced the electrochemical impedance of the nanocomposite [43]. It can be seen that the

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CdS/Au/GQDs showed a significantly enhanced and stable photocurrent signal of 0.83 mA cm(0 V vs. Ag/AgCl), which was substantially superior to those of the blank Au QD (0.12 mA

cm-2), GQD (0.17 mA cm-2), CdS NR (0.26 mA cm-2), CdS/Au (0.36 mA cm-2), and CdS/GQD

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(0.30 mA cm-2) photoelectrodes at a 0 bias potential vs. Ag/AgCl. It is noteworthy that when the light was alternated on-off in 30 s intervals, a comparable and instantaneous photocurrent

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response under light and dark conditions was observed due to the PEC effect on the

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CdS/Au/GQD photoelectrode (Figure S6b) [31]. A study of the open-circuit voltage (VOC) was conducted to consider the interfacial kinetics between the photoactive materials and electrolyte, and the lifetime of the charge carrier (Figure S7). The cyclic voltammogram in Figure 3c reveals clear anodic and cathodic peaks for each of the prepared electrodes, measured in a 5 mM K3[Fe(CN)6]- solution containing 0.1 M KCl. The analysis results for the photocurrent densities had the following sequence: GQDs < Au 16

QDs < CdS NRs < CdS/GQDs < CdS/Au < CdS/Au/GQDs. These results demonstrated that the ternary hybrid of CdS/Au/GQDs exhibited the fastest electron transfer, which was contrary to the photocurrent responses of the other modified electrodes. Nevertheless, the acquired results suggest that the CdS NRs gave a better electron transfer with GQDs than when standing alone, proving that the GQDs facilitated the electron mobility and thus enhanced the photocurrent density of the CdS/GQD electrode. As expected, the triplet interconnected structure of CdS/Au/GQDs showed a pronounced and well-defined redox peak, indicating that

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the plasmonic Au QDs possessed good conductivity and accelerated the electron transfer. To investigate the interface behavior of the synthesized materials, electrochemical impedance spectroscopy (EIS) measurements were carried out under light stimulation. As

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presented in Figure 3d, the charge transfer resistance (Rct) values were 1383.57, 983.61, and

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630.83 Ω for blank GQDs, Au QDs, and CdS NRs, respectively. In general, a smaller Rct value is usually attributed to the effective separation of photogenerated charge carriers and a faster

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interfacial charge transfer process. Once Au QDs were assembled on the CdS NRs, the Rct value of the CdS/Au was substantially decreased to 356.23 Ω, owing to the SPR effects localized on

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the Au QDs efficiently enhancing the interfacial transfer of the charge carriers. This could be further explained by the generation of hot holes in the Au QDs when they were excited, which

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hence activated the surface of the Au QDs [42]. In addition, a rapid decrease in the Rct value was observed when the GQDs were adsorbed onto the surface of the CdS/Au, which was

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ascribed to a faster charge transfer at the nanocomposite/electrolyte junction. Conclusively, the smaller Rct value of the CdS/Au/GQDs (87.84 Ω) implied a prolonged lifetime for the photogenerated charge carrier, where a bode-phase plot explained in detail the correlation between the Rct value and electron lifetime. In the bode-phase plot (Figure 3e), the average electron lifetime can be calculated using Equation (2): 17

1

Ʈe = πfmax 2

Equation (2)

where fmax is the peak frequency in the bode-phase plot, which is closely related to the migration ability and interface electron transfer between the photoactive material and electrolyte. As depicted in Figure 3e, the characteristic frequency peak for the CdS/Au/GQDs was shifted to the lower frequency region of 19.95 Hz (Ʈe = 7.98 ms), compared to other photoactive materials. The bode-phase plot peak frequencies of the Au QDs, GQDs, CdS NRs, CdS/Au,

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and CdS/GQDs were found to be 398.10, 2511.89, 63.10, 25.12, and 63.09 Hz, respectively, and correlated with the electron lifetimes of 0.40, 0.06, 2.52, 6.34, and 2.52 ms. It is worth mentioning that the longer free carrier lifetime of the CdS/Au/GQDs further confirmed the excellent performance of the hybrid material, which could greatly improve the transportation

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of charge carriers and lowering of the recombination rate of the photo-induced electron-hole

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pairs. Therefore, the smaller Rct value and higher Ʈe of the CdS/Au/GQDs could further manifest the close interaction of both measurements in providing a clear explanation of the

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interfacial charge transfer process.

A chronoamperometry on-off curve further corroborated the aforementioned

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observation (Figure 3f). The current-time curve of the fabricated photoelectrode recorded at 0 V vs. Ag/AgCl showed that the photocurrent increased steeply to a saturated value when the

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light was on and decayed to its original value when the light was off. The consistency and

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instantaneous current generation revealed the existence of an efficient charge transport directionality due to the favorable architecture of the CdS in the NR structure. Furthermore, it is interesting to report that introducing the Au QDs onto the CdS NR surface showed a comparable increase in the photocurrent response to 1040 mA cm-2, which was 1.8 times higher than that of the bare CdS NRs (581.09 mA cm-2). This phenomenon was attributed to the role of the Au QDs, which not only induced the SPR effect and increased the photocurrent output

18

due to the higher generation of photoelectrons, but could also be utilized as electron sinks to scavenge the charge recombination in the CdS/Au nanocomposite [44]. Moreover, GQDs were decorated on the CdS/Au with the aim of further amplifying the PEC performance. Comparing the photocurrent response without GQD loading, an approximately one order of magnitude increase was obtained with the CdS/Au/GQDs (1250 mA cm-2). This enhanced performance of the CdS/Au/GQDs was attributed to the facile and higher carrier mobility of the GQDs, which subsequently prolonged the lifetime of the charge carriers (7.98 ms). Conclusively, the results

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on the current-time curve provide explicit evidence that the hybrid of these three components (CdS NRs, Au QDs, and GQDs) was beneficial and responsible for the overwhelming performance of the LSV, VOC, and bode-phase measurements. In addition, the appropriate

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architecture of the CdS NRs itself was also responsible for the notable enhancement in the PEC performance. In contrast with the CdS NPs, the PEC performance of the CdS NRs revealed a

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tremendous accomplishment, as shown in Figure S8. This occurred because the NR structure

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provided a much better charge transport channel along the 1D pathway to facilitate the charge collection by the supporting substrate, and finally reduced the charge carrier recombination

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ur

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[14].

19

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Figure 3. PEC characterizations of Au QDs, GQDs, CdS NRs, CdS/Au, CdS/GQDs, and CdS/Au/GQDs: (a) LSV curves recorded at scan rate of 5 mV s-1, (b) CV responses, (c) EIS analysis results, (d) bode-phase plots measured in 0.1 M KCl aqueous solution containing 5 mM K3[Fe(CN)6]- under light stimulation, and (e) amperometric current-time curves recorded at 0 V vs. Ag/AgCl under “on-off” illumination.

20

3.3. Analytical performance of PEC sensor To investigate the PEC sensing ability of the CdS/Au/GQD photoelectrode, we investigated the PEC responses in the absence and presence Cu2+ ions. The results are illustrated and discussed in Figure S9. Meanwhile, the relationship between the photocurrent response and Cu2+ ion concentration is shown in Figure 4. As depicted in Figure 4a, the photocurrent intensity decreased upon an increase in the concentration of Cu2+ ions. This signifies that the fabricated photoelectrode exhibited presentable and gratifying sensing properties, which could

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be utilized in real-time sensing applications for heavy metal Cu2+ ions. Scheme 1 schematically displays the PEC photon-to-electron process and charge transfer mechanism for the CdS/Au/GQD photoelectrode. The ternary hybrid of CdS/Au/GQDs was strongly believed to

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exhibit a better photocurrent performance than the CdS/GQDs [1] and CdS/Au [28], which has

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been reported in our previous works. This phenomenon could be attributed to the incorporation of plasmonic Au QDs, which effectively improved the photocatalytic properties for the

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following reasons. First, the noble metal incorporated on the semiconductor possessed a Schottky barrier, which was favorable for the separation of charge carriers, and consequently

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inhibited the recombination of electron-hole (e--h+) pairs [45]. Second, the SPR excitation of Au generated a hot electron, which then improved the photocatalytic performance of the

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photocatalyst. Last, the conductivity of the photoelectrode was remarkably enhanced by the excellent charge transport properties. In addition, the introduction of GQDs which acted like a

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semiconductor with a smaller band gap energy, enabled photoexcitation under light stimulation, and thus generated the e--h+ pairs. Simultaneously, it should be noted that the CdS NRs could also be photoexcited under the illumination of light. The conduction band (CB) of the GQDs was more negative than those of the CdS NRs and Au QDs, indicating a favorable charge transfer and increased intimacy of the GQDs-to-CdS/Au interfacial interaction. Therefore, it resulted in the efficient separation of the photogenerated charge carriers and significantly 21

prolonged the lifetime of the e--h+ pairs [46]. The charge transfer processes of the CdS/Au/GQDs under the illumination of light can be separated into five steps: (1) excitation of electrons from the valance band (VB) to the CB of the GQDs and leaving holes in the VB; (2) electron injection from the CB of the GQDs into the Au QDs; (3) plasmonic Au induced hot electrons injected into the CdS NRs; (4) CdS NRs simultaneously exciting electrons to the CB and holes in the VB; and (5) electrons driven to the ITO electrode and the generation of a photocurrent due to the lower energy level of the CB ITO. Meanwhile, the generated holes in

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the VB of the GQDs and CdS NRs were transferred to the electrolyte containing TEA (sacrificial electron donor), as depicted in Scheme 1a. Evidently, the existence of Cu2+ ions in the CdS solution greatly decreased the photocurrent signal. This decrease in the photocurrent

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intensity was triggered by the binding of S2- to Cu2+, forming CuxS (x = 1,2) on the CdS surface due to the chemical displacement of Cd2+ by Cu2+. Moreover, under light stimulation, a rapid

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reduction of Cu2+ to Cu+ also occurred. The assembly of Cu+ or CuxS (x = 1, 2) on the CdS led

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to the emerging of a new band gap energy below the CB of CdS. Therefore, the formation of lower energy levels promoted a new channel for the e--h+ recombination, at these recombination centers (Cu2+ or CuxS). As a consequence, the photocurrent signal was noticeably decreased in

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the Cu2+ solution, and the working mechanism for the PEC Cu2+ detection based on the

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CdS/Au/GQD photoelectrode is displayed in Scheme 1a.

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Scheme 1. Schematic representation of CdS/Au/GQDs in (a) absence and (b) presence of Cu2+

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under light stimulation.

Moreover, the standard calibration curve for Cu2+ detection showed a good linear

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relationship from 0.1 to 290 nM with a correlation coefficient of 0.9998 (Figure 4b). The calculated detection limit (LoD) was 2.27 nM (S/N = 3). The as-prepared CdS/Au/GQD

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photoelectrode exhibited a comparably lower detection limit with wider linear responses, as compared to our previous work for the PEC sensing of Cu2+ ions (Table S1). It is worth

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mentioning that the structure and size of the CdS particles in this current work were significantly reduced, which thus intensified the PEC performance. The smaller particles

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immobilized on the ITO substrate were observed to have a shorter interparticle distance, and favorable contact between the particles and ITO substrate, which resulted in an excellent charge transport ability and efficient electron collection at the ITO substrate. As anticipated, the promising visible-light-activated CdS/Au/GQD photoelectrode could directly be used as a probe for other PEC cells. Based on our previous study reported in Ibrahim et al. (2016) [28, 31], Ibrahim et al. (2018) [47] and Ibrahim et al. (2019) [48] on the comparison of various 23

detection methods, active materials and also signaling principle, our approached revealed the lowest detection limit with an acceptable linear range, simple signaling principle, higher stability, remarkable selectivity and cost-effective since the only minimum amount of Au NPs was used to fabricate the active material, yet enabling an outstanding performance in the overall PEC Cu2+ detection. Therefore, this confirmed the capability of the fabricated photoelectrode as the most practical detector for trace amounts of Cu2+ ions. The PEC responses of the CdS/Au/GQDs with various metal ions were measured to

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examine the selectivity of the as-synthesized photoelectrode, and the results are shown in Figure 4c. It should be noted that the concentration of Cu2+ (0.29 μM) was 13.8 times lower than the concentration of other ions (4.0 μM) for the sake of easy comparisons. Compared to

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Cu2+, slight decreases in the photocurrent density were found for Ba2+, Co2+, Li+, Ni2+, Mn2+, K+, Zn2+, Na2+, Ag+, and Fe2+. Even though a much lower concentration of Cu2+ was added, the

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CdS/Au/GQDs photoelectrode exhibited a higher sensitivity to Cu2+ compared to the other

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metal ions. This was due to the lower solubility product constant (Ksp) of CuS compared to those of FeS, ZnS, MnS, NiS, and so on. Ag2S had a lower Ksp than the CuS, but no obvious

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change in the photocurrent was observed, showing the non-existence of interaction between the CdS and Ag+. Hence, the obtained results were probably due to the fact that the Ag+ solution

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was prepared in HCl to avoid hydrolysis. As a result, our approach for detecting Cu2+ ions reveals a higher selectivity, proving that the present PEC sensor exhibited a remarkable

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application potential for the quantitative analysis of Cu2+ ions in real samples. To obtain firm results for the selectivity and sensitivity of the CdS/Au/GQD

photoelectrode, a DPV analysis was conducted because it permits a better analytical signal by eliminating the non-faradaic current compared to LSV. The simultaneous determination of all the ions in the mixture of Cu2+, Ba2+, Co2+, Li+, Ni2+, Mn2+, K+, Zn2+, Na2+, Mg2+, Ag+, and Fe2+ was feasible, with up to a ten times smaller concentration of Cu2+ than the other ions (Figure 24

4d). The DPV peak current at around 0.2 V for the solution with the various concentrations of ions was observed. This well-defined peak corresponded to the Cu2+ peak [49]. The peak current of Cu2+ at 0.2 V evidently decreased with an increase in the concentration of ions in the mixture solution. However, a menial and weak DPV peak centered at around 0.7 V appeared, corresponding to the Fe2+ peak. It clearly showed that the peak was inconsistent upon an increase in the concentration of ions, indicating some tendency of Fe2+ to bind to the CdS/Au/GQDs surface. Fortunately, none of the other metal ions revealed a peak in the DPV

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measurements, even when a wide potential was applied, showing that the modified photoelectrode was selective for Cu2+ with very little interference from Fe2+. The ultra-high sensitivity of the CdS/Au/GQDs to Cu2+ was calculated to be 1.4828 μA nM-1 cm-2, indicating

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that the triplet interfacial structure had a strong synergistic effect for the interaction with Cu2+ ions, and thus helped to strengthen the detection ability in a real sample. To analyze the

re

reliability and validity of the proposed PEC sensor, the inter-assay precision of three modified

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ur

na

(Figure S10 and Table S2).

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CdS/Au/GQD photoelectrodes was investigated and discussed in Supplementary Material

25

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Figure 4. (a) Photocurrent vs. time of CdS/Au/GQDs photoelectrode for consecutive additions

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of Cu2+ in concentrations of 0.1–290 nM at 0 V vs. Ag/AgCl, (b) calibration curve between ∆I and [Cu2+], (c) effect of various heavy metal ions on changes in photocurrent intensity, (note:

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concentration of Cu2+ ions: 0.29 μM; concentration of other ions: 4 μM), and (d) simultaneous

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detection of Cu2+, Ba2+, Co2+, Li+, Ni2+, Mn2+, K+, Zn2+, Na2+, Mg2+, Ag+, and Fe2+ at different concentrations.

4.0. Conclusion A facile and economic sensing platform using CdS/Au/GQDs for the visible-lightprompt, sensitive, and selective PEC sensing of Cu2+ ions was constructed. Because of the 26

synergic interaction between the CdS/Au and GQDs, this triply interconnected structure exhibited several notable advantages such as an enhanced light absorption ability; better charge separation, which subsequently inhibited the recombination of charge carriers; the effective reduction of background interference; a fast response; simplicity; and cost-effectiveness for practical applications. This promising method enabled the precise and rapid recognition of Cu2+ in a wide linear range (0.1–290 nM), with a low limit of detection (2.27 nM). Furthermore, the DPV peak response of the CdS/Au/GQDs was unaffected by the presence of several metal ions

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in the electrolyte, showing the high selectivity of the fabricated photoelectrode to Cu2+. Even after the photoelectrode was stored for a month under atmospheric conditions, the photocurrent response in the presence of Cu2+ was maintained at up to 91% of the initial response, revealing

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the stability of the photoelectrode. Hence, the fabrication and investigation of the CdS/Au/GQD nanocomposite provided a new paradigm for the construction of an unconventional visible-

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light activated photoelectrode, and further established a facile and advanced strategy for

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designing a photoelectrode with high selectivity, sensitivity, low cost, and high reliability for the PEC dynamic sensing of Cu2+ in real samples.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.

Credit Author Statement Izwaharyanie Ibrahim: Data curation, Writing- Original draft preparation, WritingReviewing and Editing Hong Ngee Lim: Supervision, Conceptualization 27

Nay Ming Huang: Visualization, Investigation. Zhong-Tao Jiang: Software, Validation Mohammednoor Altarawneh: Software, Validation

Acknowledgment This research was supported by FRGS MRSA (UPM/700-2/1/FRGS/MRSA/5524986) from

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the Ministry of Education of Malaysia.

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