# Graphene Quantum Dots-Modified Ternary ZnCdS Semiconductor for Enhancing Photoelectric Properties

Authors: Zicong Jiang; Yun Lei; Mingzhen Zhang; Zheng Zhang; Zhong Ouyang
Journal: Journal of Nanomaterials
 (2019)

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## Abstract

A series of graphene quantum dots-modified ZnCdS (ZnCdS/G) composites with different contents of graphene quantum dots (GQDs) were prepared by a solvothermal route and characterized via various measurements. GQDs have a type graphene height of 2 nm and exhibit an excitation-dependent PL behavior. GQDs-modified ZnCdS composites present good lattice fingers that can be assigned to the (110) plane of GQDs and (112) plane of ZnCdS. The effect of different GQDs contents on the photoelectric property of ZnCdS was investigated. The results show that the photocurrent density of ZnCdS/G first increases and achieves a maximum of 11.4μA/cm2 with the addition of 0.06 wt% and then decreases as the GQDs content changes from 0.06 wt% to 0.12 wt%. Photocurrent counts as a function of time present a decrease of 10% and remains stable after 1600 s.


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## Body

## 1. Introduction

In the past, semiconductor materials have captured much attention on environmental cleaning, solar energy utilization, photovoltaic devices, and diode devices due to their great photoelectric properties [1–8]. Among them, II–VI semiconductors, such as ZnS and CdS, have been extensively studied for their proper bandgap and sensitive photoelectric reaction [9–11]. However, the individual ZnS or CdS semiconductor is restricted by its fast recombination rate of photo-generated electron-hole pairs, relatively wide bandgap energy, and photocorrosion reaction [12–15]. In order to conquer these difficulties, ternary semiconductor and carbon modification materials were proposed. Compared to the individual ZnS or CdS semiconductor, the heterostructures display a tunable bandgap which can achieve a significant improvement in the generation, transmission, and lifetime of charge carriers [16, 17]. Therefore, many researches have explored the ternary ZnCdS semiconductor. Recently, Chen et al. prepared ZnxCd1-xS NC nanocrystals to investigate the effect of surface structure on the band edge emission and further drew a conclusion that the surface ligands of the Zn0.8Cd0.2S affect the optical properties and quantum yields [18, 19]. Chen et al. fabricated hollow ZnCdS dodecahedral cages with outstanding photoelectric properties for efficient H2 evolution [20]. Zhang et al. successfully enhanced the photoelectric property of ZnxCd1–xS nanorods via adjusting the composition of the composite [21]. Although these results have demonstrated that a ternary semiconductor provides a promising potential for improving the photoelectric property, there is still plenty of room in exploring novel carbon-based materials for the high performance of a ternary semiconductor.Graphene quantum dots (GQDs), a kind of novel carbon material, are small graphene sheets with the lateral size of less than 100 nm in single- or few-layer graphene [22, 23]. GQDs exhibited unique photoelectronic properties, including the tunable bandgap, unique upconverted photoluminescence behavior, excellent optical stability, and strong visible light absorption due to the quantum confinement and strong edge effects [24–29]. The excellent conductivity and abundant active sites on the surface of GQDs make it an ideal and multifunctional platform for integrating with diverse semiconductor materials [30–32]. Chen et al. developed GQDs-decorated ZnS nanobelts for efficient environmental cleaning via a facile hydrothermal process [18]. Tian et al. prepared the N, S-GQDs-rGO-TiO2 nanotube composites with superior material properties for efficient photocatalytic degradation [33]. Lei et al. developed strongly coupled CdS/GQDs nanomaterials with remarkable photocatalytic hydrogen evolution reactions under the irradiation of visible light [34]. These results suggest that GQDs play an important role in improving the performances of semiconductor materials. Therefore, a GQDs-modified semiconductor seems to be a good way to change the photoelectric properties of the semiconductor. Based on our previous work [35, 36], the combination of GQDs and ZnCdS was further investigated as no related work has yet been reported. Meanwhile, the effect of GQDs on the photoelectric performance of GQDs-modified ZnCdS composites was investigated by a series of contrast experiments.Herein, ZnCdS/G composites with different contents of GQDs were synthesized in which GQDs were prepared by a simple cutting from graphite oxide. Atomic force microscopy, UV-visible spectra, photoluminescence spectra, zeta potential, XRD, HRTEM, and X-ray photoelectron spectroscopy were employed to identify the morphology, structure, surface state, and crystal phase of the prepared samples. The photoelectric properties of ZnCdS modified with different GQDs contents were investigated by the photocurrent-time responses and electrochemical impedance spectra.


## 2. Experimental

### 2.1. Materials

Graphite powder (chemical purity (CP), 99.85%), ammonia (NH3, 25%), hydrogen peroxide (H2O2, 30%), sulfuric acid (H2SO4, 98%), sodium nitrate (NaNO3), potassium permanganate (KMnO4), zinc acetate (C4H6O4Zn·2H2O), cadmium acetate (C4H6CdO4·2H2O), thiourea (H2NCSNH2), ethylene glycol (HOCH2CH2OH), and ethanol (CH3CH2OH) were commercially available products.


### 2.2. Preparation of GQDs

Graphite oxide (GO, 540 mg) prepared by the modified Hummers method was dispersed into mixtures of 10 ml ammonia and 40 ml hydrogen peroxide. The mixtures were loaded into a Teflon-lined stainless steel autoclave and heated at 180°C for 12 h. The cooling reaction products were centrifuged at 8000 rpm for 10 minutes, and the suspension was filtered with a 0.22μm filter. The final solution was graphene quantum dots.


### 2.3. Preparation of ZnCdS/G Composites

The experiments of ZnCdS/G composites were operated in the one-step solvothermal method illustrated by Scheme1. Briefly, 2.5 mmol cadmium acetate, 2.5 mmol zinc acetate, and 5 mmol thiourea were dissolved in 80 ml ethylene glycol. After that, different contents of GQDs were added to the solutions and sonicated for 30 minutes. In the end, the solutions were loaded into the autoclaves of 100 ml capacity and heated at 140°C for 4 hours. The cooled products were repeatedly washed with ethanol and dried in an oven at 50°C. The ZnCdS/G composites with 0.03 wt%, 0.06 wt%, and 0.12 wt% GQDs were named as ZnCdS/G-0.03, ZnCdS/G-0.06, and ZnCdS/G-0.12, respectively. The pure ZnCdS was prepared by the same procedures without adding the GQDs. To prepare the working electrode, 50 mg composites and 100 mg polyethylene glycol were mixed and grinded with 1 ml ethanol to make a slurry. The slurry was coated onto a clean FTO glass by the doctor blade method. The films on FTO were annealed at 400°C for 30 min to remove the polyethylene glycol in N2 flow.Scheme 1
Schematic diagram of the synthesis of ZnCdS/G composites.


### 2.4. Characterization

Atomic force microscopy (AFM) images were collected by a Bruker MultiMode 8 in the PeakForce Tapping mode of the mica substrate using 2 nm silicon nitride probes. The structural information was characterized by X-ray diffraction (XRD, X’Pert3 Powder) using Cu Kα radiation (λ=1.5406 Å), and the morphological images were observed on a high-resolution transmission electron microscope (HRTEM, JEOL, JEM2100F). X-ray photoelectron spectroscopy (XPS) analysis was conducted using a Thermo ESCALAB 250Xi device. The UV-visible spectra were carried out on a UV-5500 spectrophotometer with the wavelength from 190 to 800 nm. Photoluminescence (PL) spectra were recorded on a Cary Eclipse fluorescence spectrophotometer. Zeta potential was detected by a Brookhaven BI-90 Plus Particle Size Analyzer. Photocurrent-time response as well as the electrochemical impedance spectroscopy measurements was performed on an electrochemical work station (CHI650e) in a three-electrode system. As FTO glass coated with samples was served as the working electrode, a standard calomel electrode and a platinum foil were applied as the reference electrode and the counter electrode, respectively.


## 2.1. Materials

Graphite powder (chemical purity (CP), 99.85%), ammonia (NH3, 25%), hydrogen peroxide (H2O2, 30%), sulfuric acid (H2SO4, 98%), sodium nitrate (NaNO3), potassium permanganate (KMnO4), zinc acetate (C4H6O4Zn·2H2O), cadmium acetate (C4H6CdO4·2H2O), thiourea (H2NCSNH2), ethylene glycol (HOCH2CH2OH), and ethanol (CH3CH2OH) were commercially available products.


## 2.2. Preparation of GQDs

Graphite oxide (GO, 540 mg) prepared by the modified Hummers method was dispersed into mixtures of 10 ml ammonia and 40 ml hydrogen peroxide. The mixtures were loaded into a Teflon-lined stainless steel autoclave and heated at 180°C for 12 h. The cooling reaction products were centrifuged at 8000 rpm for 10 minutes, and the suspension was filtered with a 0.22μm filter. The final solution was graphene quantum dots.


## 2.3. Preparation of ZnCdS/G Composites

The experiments of ZnCdS/G composites were operated in the one-step solvothermal method illustrated by Scheme1. Briefly, 2.5 mmol cadmium acetate, 2.5 mmol zinc acetate, and 5 mmol thiourea were dissolved in 80 ml ethylene glycol. After that, different contents of GQDs were added to the solutions and sonicated for 30 minutes. In the end, the solutions were loaded into the autoclaves of 100 ml capacity and heated at 140°C for 4 hours. The cooled products were repeatedly washed with ethanol and dried in an oven at 50°C. The ZnCdS/G composites with 0.03 wt%, 0.06 wt%, and 0.12 wt% GQDs were named as ZnCdS/G-0.03, ZnCdS/G-0.06, and ZnCdS/G-0.12, respectively. The pure ZnCdS was prepared by the same procedures without adding the GQDs. To prepare the working electrode, 50 mg composites and 100 mg polyethylene glycol were mixed and grinded with 1 ml ethanol to make a slurry. The slurry was coated onto a clean FTO glass by the doctor blade method. The films on FTO were annealed at 400°C for 30 min to remove the polyethylene glycol in N2 flow.Scheme 1
Schematic diagram of the synthesis of ZnCdS/G composites.


## 2.4. Characterization

Atomic force microscopy (AFM) images were collected by a Bruker MultiMode 8 in the PeakForce Tapping mode of the mica substrate using 2 nm silicon nitride probes. The structural information was characterized by X-ray diffraction (XRD, X’Pert3 Powder) using Cu Kα radiation (λ=1.5406 Å), and the morphological images were observed on a high-resolution transmission electron microscope (HRTEM, JEOL, JEM2100F). X-ray photoelectron spectroscopy (XPS) analysis was conducted using a Thermo ESCALAB 250Xi device. The UV-visible spectra were carried out on a UV-5500 spectrophotometer with the wavelength from 190 to 800 nm. Photoluminescence (PL) spectra were recorded on a Cary Eclipse fluorescence spectrophotometer. Zeta potential was detected by a Brookhaven BI-90 Plus Particle Size Analyzer. Photocurrent-time response as well as the electrochemical impedance spectroscopy measurements was performed on an electrochemical work station (CHI650e) in a three-electrode system. As FTO glass coated with samples was served as the working electrode, a standard calomel electrode and a platinum foil were applied as the reference electrode and the counter electrode, respectively.


## 3. Results and Discussion

The morphology of the GQDs was observed on AFM, and the result is shown in Figure1. GQDs are evenly dispersed with the lateral size of 50 nm and thickness of 2 nm. UV-visible absorption spectra and PL spectra were used to evaluate optical properties of GQDs as shown in Figure 2. For the UV-visible spectra, there is a strong peak at 240 nm derived from the π-π∗ transition of the C=C bond and a weak one signal ascribed to the n-π∗ transition of the C=O bond that suggest the presence of carboxyl groups on the surface of GQDs [37]. In the PL spectra of GQDs, the PL intensity orderly increases as the excitation wavelength increases from 320 nm to 420 nm, exhibiting an excitation-dependent PL behavior. The PL peaks around 501 nm show a green emission due to the efficient edge groups of carboxyl, which is consistent with the result of UV-visible absorption spectra of GQDs [38]. There is nonshifting of the GQDs attributed to the high-oxidation degrees of the surface oxidation [39].Figure 1
AFM images of GQDs.
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(b)Figure 2
UV-visible (a) and PL (b) spectra of GQDs.
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(b)The zeta potential was tested to analyze the surface charge of GQDs, ZnCdS, and ZnCdS/G-0.06 in an aqueous solution of pH 7. The results are presented in a histogram as shown in Figure3. GQDs exhibit a negative zeta potential of -15.33 mV due to the existing oxygen-containing functional groups, which is consistent with the result of UV-visible absorption and PL spectra of GQDs, while the pure ZnCdS shows a positive zeta potential of 14.39 mV. The modification of ZnCdS with GQDs causes a decrease in the zeta potential from 14.39 mV to 9.29 mV, indicating that the GQDs link together with ZnCdS by electrostatic attraction.Figure 3
Zeta potential of GQDs, ZnCdS, and ZnCdS/G-0.06 under the condition of pH 7.The crystal structure of the products was characterized by XRD analysis as shown in Figure4. The main peaks of 2θ at 25.5°, 27.0°, 28.6°, 44.3°, 48.3°, and 52.4° can be assigned to the (100), (002), (101), (110), (103), and (112) crystal faces of hexagonal ZnCdS (JCPDS no. 89-2943), respectively. When GQDs are incorporated with ZnCdS, the diffraction peaks of ZnCdS present no obvious shift, which means that the introduction of GQDs has no effect on the lattice structure of the ZnCdS semiconductor. There are no characteristic peaks of GQDs observed in the GQDs-modified ZnCdS composites due to the low content and weak diffraction intensity of GQDs.Figure 4
XRD patterns of pure ZnCdS and ZnCdS/G composites with different amounts of GQDS: (a) XRD patterns of2θ between 10° and 65°; (b) XRD patterns of 2θ between 20° and 35°.
(a)
(b)HRTEM was further applied to investigate the microstructures of the ZnCdS/G-0.06. The present lattice fringes of GQDs-modified ZnCdS composites can be assigned to GQDs and ZnCdS. As shown in Figure5, the lattice spacing of 0.21 nm corresponds to the (100) plane of GQDs. The identified interplanar spacing of 0.175 nm corresponds to the (112) lattice plane of the ZnCdS phase which is in agreement with the result of XRD.Figure 5
HRTEM image of ZnCdS/G-0.06.To further investigate the chemical states of ZnCdS/G-0.06, XPS analysis was performed. Figure6 reveals the presence of Zn, Cd, S, and C in the composites. Zn 2p is divided to Zn 2p3/2 and Zn 2p1/2 that are located at bonding energies of 1021.4 eV and 1044.5 eV, respectively, while Cd 3d is divided to Cd 3d5/2 and Cd 3d3/2 located at around 404.4 eV and 411.1 eV, respectively. According to the literature data [9, 12], the binding energies of Zn 2p and Cd 3d are in line with Zn2+ and Cd2+, indicating the fact that Zn and Cd exist in the form of Zn2+ and Cd2+ in the ZnCdS/G-0.06. The peak of S 2p is fitted with two peaks at approximately 161.2 eV and 162.4 eV. It indicates that sulphur exists in the form of S2-. The detected C element originated from GQDs. The high-resolution spectrum of the C 1s region can be fitted by three main peaks, indicating the presence of carbon existing in three different chemical environments. The binding energy peak at 284.4 eV, 285.6 eV, and 288.3 eV corresponds to the sp2 C peak of C=C bonds, C-OH bond, and C=O bond, respectively.Figure 6
XPS spectra of ZnCdS/G-0.06: (a) Zn 2p XPS spectra; (b) Cd 3d XPS spectra; (c) S 2p XPS spectra; (d) C 1s XPS spectrum.
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(b)
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(d)The optical properties of all ZnCdS/G samples with different GQDs contents were characterized by the UV-visible absorption spectra and PL spectra. As shown in Figure7(a), the UV-visible absorption spectra of pure ZnCdS show an efficient absorption edge to 459 nm, corresponding to the specific absorption peak of ZnCdS. Moreover, the absorption edges of ZnCdS/G composites had an obvious red shift with the increase of GQDs content. The significant red shift was due to the decrease of the bandgap energy when the GQDs were incorporated in ZnCdS crystals. The PL spectra were conducted under a 220 nm emission as shown in Figure 7(b). It reveals that the PL intensity decreases with the increase of the GQDs content. The decreased PL intensities of ZnCdS/G composites indicate that the GQDs facilitate the charge transfer and separation.Figure 7
UV-visible absorption spectra and PL spectra of pure ZnCdS (A), ZnCdS/G-0.03 (B), ZnCdS/G-0.06 (C), and ZnCdS/G-0.12 (D).
(a)
(b)The effect of different GQDs additions on photoelectric properties was investigated by photocurrent-time responses. As shown in Figure8, all the curves present an obvious anodic photocurrent spike at the initial time of irradiation, which resulted from the separation of the electron-hole pairs [40]. After that, the photocurrent intensity continuously decreases until it reaches stability, suggesting that recombination process occurs. When the light irradiation is off, the charge carriers are released from the traps, resulting in the photocurrent decaying to zero [41]. Compared with the pure ZnCdS, the anodic photocurrent spikes of ZnCdS/G-0.06 are obvious, and the photocurrent intensity achieves a maximum of 11.4 μA/cm2 which is 10 times as high as that of pure ZnCdS. The mechanism can be explained by Figure 9. It is not difficult to understand that the GQDs act as an influential role in the conductive channel of electrons, which can accelerate the separation efficiency of photo-generated electron-hole pairs and make it possible for more electrons to be transferred to the external circuit. It is clear that the photocurrent density decreases when the GQDs content increases to 0.12%, which is ascribed to the reason that superabundant GQDs particles are anchored on the ZnCdS particles resulting in a low light absorption efficiency. Figure 10 shows photocurrent counts as a function of time under intermittent illumination. The photocurrent density of ZnCdS/G-0.06 composites presents a decrease of 10% and remains stable after 1600 s.Figure 8
Photocurrent-time response of pure ZnCdS (A), ZnCdS/G-0.03 (B), ZnCdS/G-0.06 (C), and ZnCdS/G-0.12 (D).Figure 9
Schematic illustration of photo-generated electron transmission in ZnCdS/G composites.Figure 10
Photocurrent-time response of ZnCdS/G-0.06 at 1600 seconds.To further study the role of GQDs in promoting electron transfer on the contact surfaces, electrochemical impedance was performed at an applied bias voltage of 500 mV. The Nyquist plots and an equivalent electrical circuit are shown in Figure11. The Rs, Rw, and CPE denote the electrolyte resistance, Warburg impedance, and associated double-layer capacitance, respectively. The Rct denotes the charge transfer resistance corresponding to the semicircle in the high-frequency region [42]. As seen, the Rct value of pure ZnCdS is 56 Ω, while that of ZnCdS/G-0.06 decreases to 49 Ω. The apparent decreased Rct value for ZnCdS/G-0.06 shows a lower charge transfer resistance, which indicates that the presence of GQDs promotes electron transfer in the ZnCdS thin film. The result shows a minimum of electrochemical impedance at the addition of 0.06 wt% GQDs, which is consistent with the results of photocurrent-time response.Figure 11
Electrochemical impedance spectra of pure ZnCdS (A), ZnCdS/G-0.03 (B), ZnCdS/G-0.06 (C), and ZnCdS/G-0.12 (D).


## 4. Conclusions

A series of different contents of GQDs-modified ZnCdS composites were successfully synthesized using the solvothermal route. With optimized GQDs decoration, the photoelectric and electrochemical performances of the composites have an obvious improvement to those of pure ZnCdS. The photocurrent density first increases and then decreases with the contents of GQDs changing from 0.03 wt% to 0.12 wt%. The composites at 0.06 wt% GQDs show a minimum of electrochemical impedance and achieve a maximum photocurrent value of 11.4μA/cm2 which is 10 times as high as that of pure ZnCdS. The enhanced photoelectric and electrochemical performances can be attributed to the high conductivity of GQDs which not only accelerate the separation efficiency of photo-generated electron-hole pairs but also induce the reduction of the interfacial electronic resistance. Consequently, the present work offers a new insight into the fabrication of GQDs and ternary semiconductor composites for enhancing the semiconductor properties.



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