Solvothermal synthesis of ternary CdSxSe1−x–graphene composites with improved photoelectric properties

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Solvothermal synthesis of ternary CdSxSe1 x-graphene composites with improved photoelectric properties Yun Lei, Yue He, Rong Li, Feifei Chen, Jun Xu

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Cite this article as: Yun Lei, Yue He, Rong Li, Feifei Chen, Jun Xu, Solvothermal synthesis of ternary CdSxSe1 x-graphene composites with improved photoelectric properties, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.05.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Solvothermal synthesis of ternary CdSxSe1-x-graphene composites with improved photoelectric properties Yun Lei *, Yue He, Rong Li, Feifei Chen, Jun Xu

School of Resource and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070 Abstract: CdSxSe1-x-graphene composites were prepared by a solvothermal process using cadmium acetate as Cd precursors, sulfourea as S precursors and selenium powder as Se precursors. CdSxSe1-x-graphene composites were characterized by X-ray diffraction, scanning electron microscope and ultraviolet-visible absorption spectroscopy. The photoelectric properties of CdSxSe1-x-graphene and bare CdSxSe1-x were investigated by transient photocurrent responses and three-electrode cyclic voltammetry. Compared with bare CdSxSe1-x, CdSxSe1-x-graphene composites show the improved photoelectric characteristics in the presence of graphene. Keywords: CdSxSe1-x-graphene composites; transient photocurrent response; photoelectric characteristics

1

Introduction As an important II-VI semiconductor, CdSxSe1-x ternary semiconductors have excellent

photoelectric properties and can be potentially applied in many fields such as sensors, light detectors, thin film transistors, solar cells, photocatalysts, and so on [1-5]. Some researches have been carried out on the synthesis of CdSxSe1-x particles. Unlu et al. reported a facile method to synthesize highly luminescent colloidal CdSxSe1-x ternary nanoalloys [6]. Jia et al. have grown ternary alloy CdSxSe1-x nanocrystals via a one-pot method [7]. Swafford et al. reported the pyrolytic synthesis of homogeneously alloyed CdSxSe1-x nanocrystals in all proportions [8]. Song et al. successfully synthesized high-quality aqueous CdSxSe1−x QDs sensitizer and effectively deposited on a mesoporous TiO2 film by a one-step hydrothermal method [9]. Since CdSxSe1-x *

Corresponding author. School of resource and environmental engineering, Wuhan University of Technology, 122

Luoshi Road, Wuhan, Hubei, 430070, P.R.China. Tel: 86-27-87882128; Fax: 86-27-87212127 E-mail: [email protected]

particles are unstable and easy to aggregate due to the small size and high surface energy, which results in a reduced surface and a high recombination rate of photoinduced electron-hole pairs. Graphene, as a two-dimensional honeycomb material consisting of a single layer of carbon atoms, has aroused great concern due to the fantastic physical properties, high specific surface area, and excellent electronic transport properties [10-11], which is regarded as a perfect conductive carrier to form hybrid materials [13-14]. Herein, tenary CdSxSe1-x particles were loaded on the surface of graphene via a one-step solvothermal synthesis using cadmium acetate as Cd precursors, sulfourea as S precursors, and selenium powder as Se precursors. CdSxSe1-x-graphene composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet-visible (UV-vis) absorption spectroscopy, and further investigated by transient photocurrent and cyclic voltammetry.

2

Experimental

2.1 Materials The graphite was purchased from Sinopharm Chemical Reagent Co., Ltd. Sulfuric acid (H2SO4), potassium permanganate (KMnO4), sodium nitrate (NaNO3), Hydrogen peroxide (H2O2), barium chloride (BaCl2), hydrazine hydrate (N2H4 • H2O), sodium hydroxide (NaOH), anhydrous ethanol (CH3CH2OH), cadmium acetate (Cd(CH3COO)2•2H2O), sulfourea (H2NCSNH2), selenium powder and anhydrous ethylene glycol (HOCH2CH2OH) were all commercially available products and used without further purification.

2.2 Preparation of graphite oxide Graphite powers were oxidized to graphite oxide by modified Hummers method [15, 16]. At first, 5 g graphite powers were gradually added with 10 mL sulfuric acid and 5 mL nitric acid, and stirred for 60 min to prepare acidification graphite. Secondly, 5 g acidified graphite was mixed with 110 mL sulfuric acid, 3 g sodium nitrate and 15 g potassium permanganate. The mixture was heated at 32

-38

for 30 minutes for further oxidation of the graphite intercalation compound.

Lastly, the solution was added with 30% H2O2, and heated at 70

-100

for 15-20 min.

2.3 Synthesis of CdSxSe1-x-graphene composites At first, light-brown GO dispersion with the concentration of 1.2 mg/mL was obtained by mild sonication of the as-prepared graphite oxide (48 mg) in 40 mL distilled water for 1 h. Then 0.532 g cadmium acetate was dissolved in the 50 mL ethylene glycol (as solution A), 0.228 g thiourea and 0.079 g Se powder are ultrasonically dispersed in 10 mL hydrazine (as solution B). Subsequently, GO solution was mixed with solution A and solution B uniformly. Lastly, the mixed solution was placed into a Teflon-lined stainless steel autoclave at 120

for 8 h. The

products were centrifuged and washed with water and ethanol, and then dried in a vacuum oven at 45

. The CdSxSe1-x-graphene films were prepared as follows: 200 mg of CdSxSe1-x-graphene was

grinded with 100 mg polyethylene glycol (PEG, molecular weight: 5000) and 0.5 mL ethanol to make a slurry. The slurry was coated onto a FTO glass by the doctor blade method. The films were annealed at 450

℃ for 30 min in N flow. 2

2.4 Characterization X-ray diffraction (XRD) was performed on an X Pert PRO DY2198 diffractometer using the monochromatized X-ray beam from Cu Kα radiation. The morphology of CdSxSe1-x-graphene composite was observed on a scanning electron microscopy (SEM). Ultraviolet-visible (UV-vis) absorption spectra were collected with a UV-vis spectrophotometer (Lambda 750 S). Cyclic voltammetry (CV) measurements were carried out on the CHI650E electrochemical working station with a three-electrode system, which was equipped with a working electrode, a platinum foil counter electrode, and a standard calomel electrode (SCE) reference electrode. The transient photocurrent responses were recorded for typical on/off cycles of intermittent visible-light irradiation (60 s) at a bias potential of 0.4 V.

3

Results and discussion

3.1 XRD analysis Fig. 1a shows the XRD pattern of GO. The feature diffraction peak appeared at 10.2° is ascribed to the (001) lattice plane of GO. As a comparison in Fig. 1b, the diffraction peak at 10.2°

disappears and a broad diffraction peak at 23.4° appears, indicating that GO has been exfoliated and reduced to graphene with oxygenated functional groups removed. As can be seen from Fig. 1c, the peak appeared at 2θ around 26.32° is assigned to (100) reflections of hexagonal CdS0.75Se0.25 (JCPDS49-1459). In addition, the peak position of CdSxSe1-x is located somewhere between those of hexagonal CdS (JCPDS77-2306) and CdSe (JCPDS08-0459), indicating that the formation of the alloyed CdSxSe1-x via intermixing binary CdS and CdSe, rather than the formation of the independent CdS and CdSe.

3.2 SEM analysis SEM images of pure graphene and CdSxSe1-x-graphene composites are shown in Fig 2. As can be seen from 2a, pure graphene has the two-dimensional structure of graphene sheets with micrometers-long wrinkles. Fig 2b shows the morphology characterization of CdSxSe1-x-graphene composites. It can be seen that CdSxSe1-x particles are arranged uniformly on the surface of graphene nanosheets, which play an important role in preventing CdSxSe1-x particles from aggregating in CdSxSe1-x-graphene composites. The graphene sheet serves as the supporting matrix and electron acceptor for CdSxSe1-x particle, which may be beneficial to provide a conductive path for the photogenerated electron and hence enhance the charge transfer process.

3.3 Ultraviolet-visible absorption spectra The UV-visible absorption spectra of GO, graphene, bare CdSxSe1-x and CdSxSe1-x-graphene composites were recorded in Fig.3. It can be seen from Fig.3a that GO exhibits a relatively strong absorption peak at 227nm due to the π-π* transition of C=C and a shoulder around 300nm due to n-π* transitions of C=O. In the absorption spectrum of graphene (Fig.3b), the peak at 270nm appears and the shoulder around 300nm disappears due to the removal of oxygen-containing groups from GO. As shown in Fig.3c, the absorption peak of bare CdSxSe1-x is present at 480 nm, while that of CdSxSe1-x-graphene is increased with a red shift in the presence of graphene. The introduction of graphene into ternary CdSxSe1-x is able to promote the light response of the CdSxSe1-x-graphene composites due to electronic interactions between graphene and CdSxSe1-x, which is beneficial to the photoelectric performance in the visible region.

3.4 Transient photocurrent tests Fig.4a-4b shows a comparison of the photocurrent-time (I-t) curves for CdSxSe1-x-graphene and bare CdSxSe1-x. Since CdSxSe1-x can absorb light and generate photogenerated electron-hole pairs under irradiation, the photocurrent density in Fig.4a achieves the maximum value once the light turns on and drops rapidly to zero once the light turns off. An obvious decreased photocurrent response of bare CdSxSe1-x can be observed at the initial time of irradiation due to the charge carrier recombination process. Graphene can act as a center to provide a conductive path for the photogenerated electron instead of the recombination of electron–hole pair. As shown in Fig.4b, an obvious increase in the photocurrent density can be seen for CdSxSe1-x-graphene. By comparing the I-t curves in Fig.4a and Fig.4b, the photocurrent transient response of CdSxSe1-x-graphene is about seven times larger than that of bare CdSxSe1-x, which is attributed to electron capture and transfer ability of graphene resulting in high photocurrent performance of CdSxSe1-x-graphene as compared with that of bare CdSxSe1-x. Since the potential of graphene lies below the conduction band (CB) of CdSxSe1-x, the graphene sheet promotes the effective charge separation due to its huge π−π network with a high electron mobility that allows photogenerated electrons from CdSxSe1-x CB to be captured and spontaneously delivered to the FTO substrate. The incorporation of graphene with uniform distribution of CdSxSe1-x particles leads to higher mobility of the photoinduced electrons across the network and consequently boosts the photocurrent response.

3.5 Cyclic voltammetry tests Fig.5 compares the CV curves of CdSxSe1-x and CdSxSe1-x-graphene in the aqueous solution of 0.1M H2SO4 at a scanning rate of 50 mV

·s

−1

. It can be seen that the symmetrical CV curve is

close to a rectangle after the CV curves went to be stable in the process of ten successive scanning cycles, indicative of excellent electrochemical stability and charge/discharge properties. Compared with bare CdSxSe1-x, CdSxSe1-x-graphene presents the remarkable improvement in both the electrochemical area and the current density, which may be ascribed to the improved electron

transfer aided by graphene. The introduction of graphene is beneficial to facilitate the electron

transfer process and electrical conductivity due to its high electron mobility and acceptance as well as the supporting matrix for uniformly dispersed CdSxSe1-x particles, resulting in the enhanced current of CdSxSe1-x-graphene as compared with bare CdSxSe1-x. This is in accordance with the results of transient photocurrent response.

4

Conclusions In conclusion, CdSxSe1-x-graphene composites were successfully synthesized by the

solvothermal method. The diffraction peaks of CdSxSe1-x-graphene composites are assigned to hexagonal CdSxSe1-x, indicating that the formation of the alloyed CdSxSe1-x instead of independent CdS and CdSe. Compared with bare CdSxSe1-x, the absorption peak of CdSxSe1-x-graphene is increase with a red shift. CdSxSe1-x-graphene composites display the remarkable improvement in both the photocurrent density and the electrochemical area in the presence of graphene.

Acknowledgments The work was supported by National Natural Science Foundation of China No. 51204129.

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Figures

a

Intensity / a.u.

b

c

10

20

30

40

50

60

70

2θ / degree

Fig.1 XRD patterns of GO (a), graphene (b) and CdSxSe1-x-graphene composites(c)

Fig.2 SEM images of grapheme (a) and CdSxSe1-x-graphene composites (b)

Absorbance

a

c

d b 250 300 350 400 450 500 550 600 650 700 750 800

Wavelength / nm

Fig.3 UV-vis absorption spectra of GO (a), Graphene (b),CdSxSe1-x (c) and CdSxSe1-x-graphene composites (d)

b

-5 Photocurrent / 10 A/cm2

1.0

0.8

0.6

0.4

a

0.2

0.0 0

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200

Time / s

Fig.4 Transient photocurrent responses of the CdSxSe1-x (a) and CdSxSe1-x-graphene composites (b)

1.5

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Current /10 A

1.0 0.5 0.0

a

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b

-1.0 -1.5 -2.0 -0.6

-0.4

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0.0

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0.4

0.6

Potential / V

Fig.5 CV curves of the CdSxSe1-x (a) and CdSxSe1-x-graphene composites (b) at a scanning rate of 50mV s-1