Degradation of methylene blue using ZnSe–graphene nanocomposites under visible-light irradiation

Degradation of methylene blue using ZnSe–graphene nanocomposites under visible-light irradiation

Available online at www.sciencedirect.com CERAMICS INTERNATIONAL Ceramics International 41 (2015) 13759–13766 www.elsevier.com/locate/ceramint Degr...

1MB Sizes 0 Downloads 25 Views

Available online at www.sciencedirect.com

CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 13759–13766 www.elsevier.com/locate/ceramint

Degradation of methylene blue using ZnSe–graphene nanocomposites under visible-light irradiation S.H. Hsieha, W.J. Chenb,n, T.H. Yehb b

a Department of Materials Science and Engineering, National Formosa University, 64, Wunhua Road, Huwei, Yunlin 632, Taiwan Graduate School of Materials Science, National Yunlin University of Science and Technology, 123 University Road, Section 3, Douliou, Yunlin 64002, Taiwan

Received 3 April 2015; received in revised form 20 July 2015; accepted 10 August 2015 Available online 17 August 2015

Abstract In this study, graphene oxide (GO) sheets were synthesized by using a modified Hummers' method and Offeman's method. The GO was mixed with ZnSe prepared using various amounts of N2H4 at a ratio of 1:1 to form precursors. The ZnSe/graphene nanocomposites were synthesized under hydrothermal conditions (180 1C; 12 h) from the previous precursor. The obtained ZnSe/graphene photocatalysts were characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy, X-ray diffraction analysis (XRD), Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Photocatalytic activity under visible-light irradiation was evaluated in a methylene blue (MB) dye degradation reaction in the aqueous phase. The results show that the ZnSe–N2H4 reacted with the GO, possibly because the N2H4 contained in the ZnSe reduced the GO to graphene and formed a ZnSe/graphene nanocomposite. The GN-5-mL ZnSe nanocomposite prepared in this study could degrade more than 90% of the MB in 2 h and almost completely degrade the MB in 4 h under visible-light irradiation. The degradation efficiency of the ZnSe/graphene nanocomposite was 99.6% after 6 h of visible light irradiation; the pseudo-first-order reaction rate constant of the ZnSe/graphene nanocomposite was 1.27 h  1. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: B. Nanocomposites; ZnSe; Graphene; Methylene blue

1. Introduction The high-efficiency photocatalytic process shows significant potential in solving many environmental pollution problems. Therefore, the development of semiconductor photocatalysts with high-efficiency photocatalytic characteristics has become one of the most important objectives of materials science. In addition, the application of semiconductor photocatalysts in areas such as air purification, water disinfection and the removal of toxic chemicals from wastewater represent a relatively advanced technology. Ideal photocatalytic materials should exhibit a combination of high activity and high solar energy conversion efficiency in addition to other characteristics such as non-toxicity; biological and chemical inertness; stability; the ability to be used at any time and ease of processing. However, to date, no material or system has been able n

Corresponding author. E-mail address: [email protected] (W.J. Chen).

http://dx.doi.org/10.1016/j.ceramint.2015.08.052 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

to meet all of these requirements. Among semiconductor photocatalysts, titanium dioxide (TiO2) has been widely used in photocatalytic and photochemical processes due to its stability, low cost and non-toxicity [1,2]. For instance, TiO2 has been used to solve environmental problems. However, the photocatalytic activity, microstructure and physical properties of TiO2 powder are determined by the preparation conditions and methods employed. Due to its energy gap (3.2 eV), anatase TiO2 can only absorb approximately 5% of solar radiation, which is its primary disadvantage. Therefore, the development of photocatalysts that only use visible light for photocatalysis has already drawn great attention. The light absorption range of zinc selenide (ZnSe) falls in the visible light region. Compared with TiO2, ZnSe can use a wider range of the solar spectrum for catalytic reactions. Hence, ZnSe is an indispensable photocatalytic material. In addition, the nanominiaturization of photocatalytic materials can result in a relatively high specific surface area and a relatively high oxidation– reduction potential, which in turn results in an increase in the band

13760

S.H. Hsieh et al. / Ceramics International 41 (2015) 13759–13766

gap energy; therefore, nanominiaturizations techniques have been considered for increasing the degradation efficiencies of photocatalytic materials [3]. Recently, Qian et al. discovered that the photocatalytic activity of ZnSe nanoribbons is higher than that of TiO2 particles when used for the degradation of magenta acid dye under ultraviolet (UV) irradiation, indicating that ZnSe is an effective photocatalyst for the photocatalytic degradation of organic pollutants [4]. Due to its application in areas such as electronics, optics and photocatalysis, graphene has become popular in materials science research. Graphene exhibits excellent electron mobility, high transparency and stability at room temperature [5]. When used to modify other materials, graphene can retard the recombination of electron–hole pairs that are photochemically or electrochemically generated; thus, graphene can enhance the transfer of photo-generated charge carrier and further increases catalytic activity [6]. Therefore, it is expected that a ZnSe/ graphene composite prepared by combining ZnSe with graphene can improve the degradation of organic pollutants under visible-light irradiation. Chen et al. and Liu et al. proposed the use of a ZnSe/nitrogen–graphene composite to degrade organic pollutants under visible-light irradiation, which, to the best of our knowledge, is the only report on this subject to date [7,8]. Therefore, the hydrothermal method was used in this study to prepare a ZnSe/graphene nanocomposite; in addition, the effect of conditions used to prepare the ZnSe/graphene composite on the degradation of methyl blue (MB) dye was thoroughly investigated. 2. Experimental 2.1. Synthesis of the graphene oxide In this study, graphene oxide (GO) sheets were synthesized using a modified Hummers and Offemans' method. 9 g of potassium permanganate (KMnO4) was gradually added to a mixture of concentrated sulfuric acid (H2SO4, 70 mL), sodium nitrate (1.5 g), and natural graphite powder (1.5 g) in an ice– water bath. Next, 120 mL of deionized (DI) water was slowly added. The solution was maintained at 35 1C for 1 h. Then, H2O2 (9 mL) and deionized (DI) water (300 mL) were slowly added. The solution became bright yellow after being oxidized. During this process, the natural graphite powder particles were exfoliated by a strong oxidizing agent (KMnO4 þ H2O2). The precipitates were washed with methanol and DI water via centrifugation to remove any residual negative or positive ions. Graphene oxide powder was obtained after drying for 2 h. The GO powder (45 mg) was dispersed in 100 mL of ultrapure water via 2 h of ultrasonic treatment. Then, 800 mg of NaBH4 was slowly added to the mixture, under stirring for 1 h at 80 1C. Graphene was obtained after filtration and washing. 2.2. Synthesis of ZnSe/graphene nanocomposites To deposit the ZnSe on the surface of the graphene, first, 5 mL of hydrazine (N2H4), 14 mL of diethylenetriamine (C4H13N3, DETA) and 16 mL of H2O (volume ratio: 5:14:16) were mixed to

form a solution. Zinc sulfate heptahydrate (ZnSO4  7H2O) and sodium selenite (Na2SeO3) were then added to the solution (35 mL), and the solution was stirred. The uniformly mixed solution was transferred to a 50-mL Teflon stainless steel autoclave to undergo a hydrothermal reaction for 12 h at 180 1C. After the hydrothermal reaction, white flocculates were obtained. The white flocculates were then collected via centrifugation and rinsed with ethanol. The precipitates were collected and vacuum-dried in an oven at 80 1C for 6 h. The dried product consisted of ZnSe–N2H4 nanopowder particles. ZnSe–N2H4 containing 5 mL of N2H4 is denoted ZnSe(5 mL)–N2H4. In addition, ZnSe–N2H4 was prepared with different amounts of N2H4 (4 mL, 5 mL, 6 mL, 7 mL and 8 mL). ZnSe–N2H4 prepared from X mL of N2H4, 14 mL of DETA and 16 mL of H2O is denoted ZnSe (X mL)–N2H4 ( i.e., ZnSe–N2H4 prepared from 6 mL of N2H4 is denoted ZnSe (6 mL)–N2H4). When mixed solution of hydrazine and diethylenetriamine was added into the zinc sulfate heptahydrate (ZnSO4  7H2O) solution, the complex Zn(EDTA)20.5þ was formed because of the strong coordination effect of the diethylenetriamine due to its a bidentate ligand. The sodium selenite (Na2SeO3) would react with Zn(EDTA)20.5þ to form ZnSe(EDTA)20.5þ . Then ZnSe (EDTA)20.5þ reacts with hydrazine (N2H4) to form ZnSe–N2H4 finally. The formation of the precursor and the product ZnSe– N2H4 can be formulated as follows: Zn2 þ þ 0.5 EDTA-Zn(EDTA)20.5þ Zn(EDTA)2 þ þ Se2  -ZnSe(EDTA)20.5þ ZnSe(EDTA)20.5þ þ N2H4-ZnSe–N2H4 ZnSe/graphene samples were also synthesized using the hydrothermal method. First, 16 mg of dried graphene oxide (GO) was added to 16 mL water. Second, 16 mg of dried ZnSe (X mL)–N2H4 powder was collected and placed in 16 mL of water for ultrasonic agitation (i.e., the ZnSe solution). The ZnSe solution was then slowly added to the GO solution, which was magnetically stirred at 25 1C for 5 min. The uniformly mixed solution was then transferred to a 50-mL Teflon stainless steel autoclave to undergo a hydrothermal reaction for 12 h at 180 1C. The product was then collected via centrifugation and rinsed multiple times with water and ethanol. The precipitates were collected and dried in an oven at 70 1C until completely dry. The dried product was a ZnSe/graphene nanocomposite (i.e., the weight ratios of GO to ZnSe (X mL)–N2H4 was 1:1.), which is denoted GN-X mL ZnSe. 2.3. Characterization methods The ZnSe(X mL)–N2H4 and GN-X mL ZnSe were characterized using a transmission electron microscope (TEM, JEOL JEM-2010) operated at 200 kV. TEM samples were prepared by depositing a drop of sample dispersion onto Cu grids coated with a carbon layer. Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advance diffractometer with Cu Kα radiation. A Bruker Hyperion Fourier transform infrared (FT-IR) spectrometer was used to scan the samples.

S.H. Hsieh et al. / Ceramics International 41 (2015) 13759–13766

Fourier transform infrared spectroscopy was performed over the wavelength range of 4000–400 cm  1. A Hitachi F-7000 fluorescence spectrophotometer was used to record the photoluminescence (PL) spectra. Photoluminescence emission spectra were obtained using a 340-nm excitation source at room temperature, and the emission spectra were collected between 400 and 700 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5000 VersaProbe system. 2.4. Photocatalytic activity test The volume of the methylene blue solution was 60 mL, with a concentration of 35 mg/L at a pH level of 4, and 10 mg of the ZnSe–N2H4 (or GN–ZnSe) photocatalyst was used for each test. The test solution was first stirred by a magnetic stirrer for 0.5 h in the dark. For the photocatalytic degradation experiments, the photocatalytic reaction was initiated by exposing solution under visible light; then, 4-mL aliquots of the solution were removed after 1, 2, 3, 4, 5 and 6 h of reaction. The visible light was generated by a solar simulator under the 1-sun global solar spectrum of air mass (AM) 1.5 (i.e., under 100 mW cm  2). Each sample solution was analyzed by a UV–vis spectrophotometer to monitor the concentration of MB left in the aqueous system by detecting the maximum absorption wavelength of MB at 663 nm. The intensity of the primary absorption peak (663 nm) of the MB dye was considered a measure of the residual MB dye concentration, C, and the initial concentration of dye was referred to as Co.

13761

Powder Diffraction Standards (JCPDS) cards; the presence of the spectrum could only be confirmed by some peaks corresponding to the peaks of ZnSe–N2H4. However, the XRD spectrum of ZnSe (5 mL)–N2H4 is identical to that of the ZnSe precursor reported by Ni et al. [12]. The compound in question should be an organic–inorganic hybrid semiconductor [12]. Fig. 2(b) shows the pattern of the hexagonal ZnSe phase, which exhibits a wurtzite structure (PDF 15-0105). The diffraction peak angles of the hexagonal ZnSe phase are 25.61, 27.41, 29.21, 37.91, 45.31, 49.51, 53.81, 60.61, 69.41, 72.11, and 78.11, which correspond to the (100), (002), (101), (102), (110), (103), (112), (202), (203), (210) and (105) planes, respectively; no other phases were identified, indicating that ZnSe–N2H4 reacted with GO, possibly because the N2H4 contained in ZnSe–N2H4 reduced GO to graphene and formed a ZnSe/graphene composite. This reduction reaction is the N2H4 reduction method that has been widely used to prepare graphene [13]. During the reduction reaction process, the organic or inorganic compounds originally present in the ZnSe–N2H4 phase likely participated in the reaction and formed

3. Results and discussion 3.1. Structural characterization Fig. 1(a), (b) and (c) shows the X-ray diffraction (XRD) spectra of graphite, graphene oxide (GO) and graphene, respectively. As shown in Fig. 1(a), the characteristic peak of graphite (002) is located at a 2θ angle of 26.51, and the spacing between the layers is 0.337 nm. After oxidation, GO was formed. As shown in Fig. 1(b), the characteristic peak of GO shifted to 10.41. GO differs from graphite primarily in terms of its diffraction intensity and peak broadening. The spacing between the layers of GO increased to 0.85 nm. The expansion of the spacing between the layers of GO is due to the existence of epoxy groups, carboxyl groups and water molecules between the layers of GO; as a result, the surface of GO contains oxygen-containing functional groups and is hydrophilic [9]. Fig. 1(c) shows the XRD spectrum of graphene, which was reduced from GO by sodium borohydride (NaBH4). Due to the relatively small number of graphite layers in the graphene film, the diffraction intensity of the graphene film was relatively weak, and could not be effectively detected by XRD, making it difficult to resolve the diffraction peak of graphene at 23.31 [9–11]. Fig. 2(a) and (b) shows the XRD spectra of the ZnSe (5 mL)– N2H4 and the GN-5 mL ZnSe nanocomposites. It was difficult to locate the spectrum of the compound that corresponds to the XRD spectrum of the ZnSe–N2H4 in the Joint Committee on

Fig. 1. XRD patterns of (a) graphite, (b) graphene oxide and (c) graphene.

Fig. 2. XRD patterns of (a) ZnSe(5 mL)–N2H4 and (b) GN-5 mL ZnSe nanocomposites.

13762

S.H. Hsieh et al. / Ceramics International 41 (2015) 13759–13766

products that were soluble in the liquid phase; hence, a nearly pure ZnSe/graphene nanocomposite was obtained. Fig. 3 shows the Fourier transform infrared spectroscopy (FTIR) spectra of ZnSe (5 mL)–N2H4, GO and GN-5 mL ZnSe. As shown in the spectrum in (a), the absorption peaks exhibited by ZnSe–N2H4 correspond to various types of functional groups. The vibration of the –OH group of water is located at 3300–3400 cm  1; the symmetrical and asymmetrical vibrations of the NH2 or (N–H) group of N2H4 are located at 3231 and 3125 cm  1, respectively; and the shear vibration of the NH2 group of N2H4 is located at 1596 cm  1. The absorption bands at 1102 and 1063 cm  1 can be assigned to C–N vibrations. The peaks located at 1446 and 1350 cm  1 are caused by the bending vibrations of the CH2 group; the symmetrical and asymmetrical stretching vibrations of the CH2 group are located at 2800 and 3000 cm  1, respectively [14,15]. Fig. 3(a) shows that certain organic compounds were also present, in agreement with the XRD results. As shown in the spectrum in (b), the absorption peaks exhibited by GO correspond to various types of oxygen-containing functional groups. The stretching vibration of the –OH group of water is located at 3410 cm  1; the vibration of the CQO group of the carboxylates or the carbonyl ketones occurs at 1734 cm  1; and the peak observed at 1620 cm  1 is a resonance peak that can be assigned to C–C stretching and absorbed hydroxyl groups in the GO. The stretching vibration of tertiary C–OH groups is located at 1420 cm  1. The bands at 1227 and 1055 cm  1 can be attributed to the vibration of C–O–C and C–O groups, respectively [16,17]. This finding shows that many oxygencontaining functional groups exist on the surface of the GO nanosheets. The spectrum in (c) shows a small number of absorption peaks on the ZnSe/graphene curve; these peaks have a low intensity, indicating that ZnSe–N2H4 reacted with GO and formed ZnSe/graphene nanocomposites because the NH2 and N–H functional groups of ZnSe–N2H4 facilitated the reduction of GO to graphene. It can be inferred from Fig. 3(c) that nearly all oxygen-containing functional groups on GO participated in the reaction and thus disappeared, leaving only CQC and C–O–C bonds. Therefore, the spectra in Figs. 2 and

Fig. 3. Fourier transform infrared spectroscopy (FTIR) spectra of ZnSe–N2H4, GO and GN-5 mL ZnSe.

3 confirm that ZnSe/graphene nanocomposites were successfully prepared in this study. 3.2. Photocatalytic activity of the ZnSe–N2H4 and GN-5 mL ZnSe samples Fig. 4 shows the variation in the concentration of the MB dye with time; this concentration is denoted C/Co, where C represents the concentration of the pollutant (i.e., the MB dye) and Co represents the initial concentration of the pollutant during the photodegradation process of ZnSe–N2H4 and/or GN-5 mL ZnSe. As shown in curve (a), after 6 h of visible light irradiation, only 35.4% of MB was removed when using ZnSe–N2H4, indicating a 35.4% degradation efficiency. Therefore, the effect of the photocatalytic degradation was unsatisfactory when using ZnSe–N2H4 alone. As shown in curve (b), after 1 h of visible-light irradiation, the degradation efficiency reached 85% when using GN-5 mL ZnSe, and after 4 h of visible-light irradiation, the degradation efficiency reached 99.5%. Thus, the ZnSe/graphene nanocomposite positively affected the degradation of the MB dye. In addition, Fig. 4 indicates that the degradation efficiency of ZnSe/graphene was far superior to that of ZnSe. Fig. 5 presents the photo-stimulated luminescence (PL) spectra of ZnSe–N2H4 and GN-5 mL ZnSe. As shown in curve (a), numerous photo-excited charge carriers were easily generated after ZnSe was irradiated by a laser; it was easy for these photo-excited charge carriers to emit due to the recombination of electron–electron hole pairs at the illumination center (i.e., the defect location); the wavelength of this defect peak ranges from 450 to 600 nm. As shown in curve (b), we know that the intensity of the defect peak in the ZnSe/graphene nanocomposite decreased, indicating that the recombination rate of the electron–hole pairs decreased. This phenomenon occurred because the photo-excited carriers were rapidly transferred to graphene, resulting in the effective separation of charges, which in turn reduced the probability of electron– hole recombination [18]. Therefore, the degradation effect of ZnSe/graphene was far superior to that of ZnSe. To understand the effect of N2H4 on the preparation of ZnSe/graphene nanocomposites, we will discuss the effect of various amounts

Fig. 4. Variations in the concentration (C/Co) of MB dye over time during the photodegradation of ZnSe–N2H4 and GN-5 mL ZnSe.

S.H. Hsieh et al. / Ceramics International 41 (2015) 13759–13766

of N2H4 on the characteristics of the prepared ZnSe/graphene nanocomposite in the following section. 3.3. Effect of different amounts of N2H4 on the degradation characteristics of ZnSe/graphene nanocomposites First, GO was mixed with ZnSe–N2H4 prepared using various amounts of N2H4 at a ratio of 1:1 to form precursors. Second, ZnSe/graphene nanocomposites were synthesized under hydrothermal conditions (180 1C; 12 h), which are denoted GN-X mL ZnSe. Fig. 6 shows transmission electron microscopy (TEM) images of graphene (a), ZnSe/graphene containing 5 mL (b), and 8 mL of N2H4 (c); the illustration shows the selected area of the electron diffraction pattern. Fig. 6(a) shows the morphology of graphene is high surface area and ultrathin two-dimensional sheet structure with wrinkles. Fig. 6(b) and (c) shows TEM image of ZnSe/ graphene prepared with 5 mL and 8 mL of N2H4, respectively. The presence of ZnSe was confirmed by the electron diffraction pattern. The crystal structure of the ZnSe/graphene nanocomposite did not change with increasing amounts of N2H4. Fig. 6(b) and (c) also shows that ZnSe was distributed on the graphene in the forms of flakes and aggregated nanoparticles (namely, nanorods). The diameter of the ZnSe nanoparticles was approximately 20 nm. For ZnSe/graphene prepared with 5 mL of N2H4. The diameters of the ZnSe nano-rods (i.e., aggregated nanoparticles) ranged from 100 to 200 nm, and the lengths of the ZnSe nanorods ranged from 500 to 600 nm. For ZnSe/graphene prepared with 8 mL of N2H4. The diameter of the ZnSe nano-rods was approximately 100 nm, and the lengths of the ZnSe nano-rods ranged from 200 to 400 nm. Hence, the size of the ZnSe nanorods decreased with an increasing amount of N2H4; however, because the nano-rods are composed of aggregated nanoparticles, larger nano-rods contained more ZnSe nanoparticles (i.e., the number of ZnSe nanoparticles decreased with an increase in the amount of N2H4 from 5 mL to 8 mL). Fig. 7 shows a diagram of the variation of the MB dye concentration during the photodegradation process over time for different ZnSe/graphene compositions prepared with various amounts of N2H4 as a catalyst. Fig. 7 demonstrates that when the amount of N2H4 was increased from 4 mL to 5 mL, the degradation efficiency of the ZnSe/graphene nanocomposites showed an increasing trend after 1–6 h of visible light irradiation; however, when the amount of N2H4 was increased from 5 mL to 8 mL, the degradation efficiency of the ZnSe/ graphene nanocomposites exhibited a decreasing trend after 1– 6 h of visible light irradiation. These results indicate that an initial increase in the amount of N2H4 will increase the degradation efficiency, whereas a continuous increase in the amounts of N2H4 will decrease the degradation efficiency. When the amount of N2H4 was 4 mL, the N2H4 contained in ZnSe–N2H4 could not reduce all of the GO to graphene; therefore, the degradation efficiency could not reach its maximum value. When the amount of N2H4 was increased from 4 mL to 5 mL, the N2H4 contained in ZnSe–N2H4 could reduce all of the GO to graphene. In addition, ZnSe–N2H4 was transformed into ZnSe, which in turn formed ZnSe/graphene

13763

nanocomposites, upon which the amount of ZnSe distributed on graphene was the maximized (Fig. 6(b)); the degradation efficiency was thus maximized under this condition. When the amount of N2H4 was further increased to over 6 mL, even though the N2H4 contained in ZnSe–N2H4 could reduce all of the GO to graphene, there was a surplus of ZnSe–N2H4, which reduced the amount of ZnSe distributed on the graphene in the formed ZnSe/graphene nanocomposites (Fig. 6(c)); therefore, the degradation efficiency decreased as the amount of N2H4 was increased from 5 mL to 8 mL. These results demonstrate that the optimum degradation efficiency of the ZnSe/graphene nanocomposites prepared with 5 mL of N2H4 was 99.6% after 6 h of visible light irradiation. With respect to the MB aqueous solution, the photocatalytic reaction of ZnSe/graphene is consistent with the Langmuir– Hinshelwood (L–H) kinetic model: in which ln(Co/C) varies linearly with time (t). If ln(Co/C) is plotted against time (t), a straight line with a slope of k is obtained, and the value of the regression coefficient of this straight line can be obtained via linear regression. The slope, k, is the pseudo-first-order reaction rate constant, and Co and C are the initial and reaction concentrations of aqueous MB, respectively. Fig. 8 shows the kinetic fitting curves of the photo-degradation of MB under visible-light irradiation using ZnSe/graphene prepared with various amounts of N2H4 as a catalyst. The figure demonstrates that the photocatalytic reactions primarily agree with a firstorder kinetic model; in addition, different samples exhibited the same trend in which high values of k gave rise to improved photocatalytic activity. Table 1 describes the simplified L–H kinetic model. The value of the regression coefficient of the straight line (R2) ranges from 0.94185 to 0.99882. The value of k of GN-5 mL ZnSe was the highest (e.g., 1.27158), indicating that the photocatalytic activity of GN-5 mL ZnSe was excellent. The GN-5 mL ZnSe nanocomposites prepared in this study could degrade more than 90% of MB in 2 h compared with the 4 h reported by Liu et al., and could almost completely degrade MB at 4 h. Fig. 9(a) and (b) shows the X-ray photoelectron spectroscopy (XPS) spectra of ZnSe(5 mL)–N2H4 and GN-5 mL ZnSe. XPS is a powerful tool for determining the chemical

Fig. 5. Photo-stimulated luminescence (PL) spectra of ZnSe–N2H4 and GN5 mL ZnSe.

13764

S.H. Hsieh et al. / Ceramics International 41 (2015) 13759–13766

Fig. 6. Transmission electron microscopy (TEM) images of graphene (a), ZnSe/graphene containing 5 mL (b) and 8 mL of N2H4 (c).

6

GN-4ml ZnSe GN-5ml ZnSe GN-6ml ZnSe GN-7ml ZnSe GN-8ml ZnSe

5

ln(C0/C)

4

3

2

1

0 0

1

2

3

4

5

6

time (h)

Fig. 7. Diagram of the variation in the concentration of MB dye during the photodegradation process over time for different ZnSe/graphene nanocomposites prepared with various amounts of N2H4 as a catalyst.

states of elements in materials. Fig. 9 shows that the peak of ZnSe (5 mL)–N2H4 is located at 285.1 eV in the C1s region, corresponding to the C–OH bond; the peak of GN-5 mL ZnSe is located at 284.7 eV in the C1s region, corresponding to the CQC bond. The peak of ZnSe (5 mL)–N2H4 is located at 397.6 eV in the N1s region, and the peak of GN-5 mL ZnSe is located at 399.8 eV in the N1s region. Fig. 9(b) confirms that the nitrogen in GN-5 mL ZnSe was doped with graphene and formed nitrogen-doped graphene, which is consistent with the research results reported by Liu et al. and Chen et al. [7,8]. The mechanism of photodegradation for ZnSe/graphene is as follows. Under visible-light irradiation, electrons (e  ) are excited from the valence band to the conduction band of ZnSe, creating a hole (h þ ) in the valence band. In the absence of graphene, most of electrons and holes quickly recombine without doing any chemistry. Typically, only a small number of these charges are trapped and participate in photocatalytic reactions, resulting in low reactivity. When ZnSe nanoparticles are in intimate contact with graphene, the excited-electrons are transferred to the graphene from the surface of ZnSe, allowing charge separation, stabilization, and hindered recombination.

Fig. 8. Kinetic fitting curves of the photo-degradation of MB under visiblelight irradiation using ZnSe/graphene prepared with various amounts of N2H4 as a catalyst.

Table 1 L–H kinetic model for different ZnSe/graphene materials prepared with various amounts of N2H4 as the catalyst. Sample label

L–H kinetic model

GN-4 mL GN-5 mL GN-6 mL GN-7 mL GN-8 mL

ln ln ln ln ln

ZnSe ZnSe ZnSe ZnSe ZnSe

(Co/C) ¼0.04896þ0.70031t (Co/C) ¼0.31709þ1.27158t (Co/C) ¼0.52642þ0.76549t (Co/C) ¼0.29263þ0.77763t (Co/C) ¼0.33948þ0.83446t

R2

k

0.99882 0.97871 0.94185 0.98332 0.96388

0.70031 1.27158 0.76549 0.77763 0.83446

The photoexcited holes in the valence band can form hydroxyl radical (∙OH). The MB molecules can be subsequently destroyed into CO2 and H2O by the ∙OH, owing to its high activities. The excited electrons can be shuttled freely along the conducting network of graphene. Since O2 may be adsorbed on the surfaces of graphene, the e  in the graphene reacts with O2 and forms ∙O2 . The routes of ∙OH formation and the photodegradation of MB can be described as follows:

S.H. Hsieh et al. / Ceramics International 41 (2015) 13759–13766

3

Zn2p

O1s

25000

N1s

C1s

15000

10000

Se3d

Intensity (a.u.)

20000

5000

Table 2 Comparison of the performance of various photocatalysts. Catalyst

Degradation efficiency (%)

Reference

F-doped TiO2 g-C3N4–CdS carbon modified TiO2 nanotube NiO/graphene oxide TiO2–graphene BiVO4/TiO2 G/TiO2

91 90.5 92.7 97.5 98.8 84 94.1

19 20 21 22 23 24 25

5ml ZnSe-N 2H4

0 0

200

400

600

800

1000

Binding Energy (eV)

30000

3

Zn2p 25000

N1s

C1s

20000

15000

10000

Se3d

Intensity (a.u.)

13765

5000

GN-5ml ZnSe

0 0

200

400

600

800

In this study, due to the presence of graphene, the capacity of the GN–ZnSe composite for absorbing pollutant molecules was enhanced. In addition, the band gap energy of ZnSe is within the visible-light range. Thus, the absorption range of ZnSe under visible-light irradiation was expanded due to its capacity to absorb visible light and the incorporation of graphene. Nitrogen atom doping can widen the band gap of graphene and transform it into a semiconductor. Therefore, the prepared GN–ZnSe composite was a heterogeneous system composed of two semiconductors, and a p–n junction was also formed; this system could promote the collection and separation of charge carriers at the interface and could thus improve the degradation capacity of GN–ZnSe [7,8]. Therefore, GN5 mL ZnSe showed the optimum degradation performance.

1000

Binding Energy (eV)

Fig. 9. X-ray photoelectron spectroscopy (XPS) spectra of (a) ZnSe(5 mL)– N2H4 and (b) GN-5 mLZnSe.

 þ ZnSe þ hν-ZnSe(eCB þ hVB )  eCB (ZnSe)-etr (graphene) þ hVB (ZnSe) þ H2O-H þ þ ∙OH

etr (graphene) þ O2(ads) -∙O2 2etr (graphene) þ 2H þ þ O2 -H2O2 H2O2 þ etr (graphene)-HO  þ ∙OH MB þ ∙OH-CO2 þ H2O In order to demonstrate the degradation capacity of the catalyst, a comparison of degradation of methylene blue under visible light is listed in Table 2. The highest degradation efficiency of ZnSe/ graphene nanocomposites was estimated to be 99.6% for the sample prepared with 5 mL of N2H4. The degradation efficiency of this photocatalyst is much higher than that previously reported, for example, on F-doped TiO2 from Yu (91%) [19], g-C3N4–CdS (90.5%) [20], carbon modified TiO2 nanotube (92.7%) [21], NiO/ graphene oxide (97.5%) [22], TiO2–graphene (98.8%) [23], BiVO4/TiO2 (84%) [24], and reduced graphene oxide modified TiO2 (94.1%) [25].

4. Conclusions In this study, ZnSe/graphene nanocomposites were successfully prepared from ZnSe–N2H4/GO precursors using a hydrothermal method. XRD analysis demonstrated that the ZnSe was composed of many inorganic and organic compounds. After adding GO, the N2H4 contained in the ZnSe reacted with the oxygen-containing functional groups on GO and formeda ZnSe/graphene nanocomposite, which had the structure of hexagonal wurtzite ZnSe. The crystal structure of the ZnSe/ graphene nanocomposite did not change with the increase in the amount of N2H4. Photoluminescence analysis demonstrated that the addition of graphene to ZnSe resulted in a decrease in the intensity of the defect peak, indicating that the recombination rate of electron–hole pairs decreased because the photo-excited charge carriers were rapidly transported onto the graphene; this process resulted in the effective separation of charges, which in turn decreased the probability of the electron–hole recombination. The degradation efficiency of ZnSe–N2H4 alone toward MB was only 35.4% after 6 h of visible-light irradiation. The degradation efficiency of the ZnSe/graphene nanocomposite prepared from the ZnSe precursor, which was prepared from GO and 5 mL of N2H4 (weight ratio: 1:1), under hydrothermal conditions (180 1C; 12 h) was optimum (99.6%) after 6 h of visible-light irradiation; moreover, the pseudo-first-order reaction rate constant of the ZnSe/graphene nanocomposite was optimum (1.27158 h  1).

13766

S.H. Hsieh et al. / Ceramics International 41 (2015) 13759–13766

Acknowledgments The authors are grateful for the financial support of the Ministry of Science and Technology of the Republic of China under contract MOST 103-2221-E-224-015.

[13]

[14]

References [15] [1] D. Wodka, E. Bielańska, R.P. Socha, M. Elżbieciak-Wodka, J. Gurgul, P. Nowak, P. Warszyński, I. Kumakiri, Photocatalytic activity of titanium dioxide modified by silver nanoparticles, ACS Appl. Mater. Interfaces 2 (7) (2010) 1945–1953. [2] K. Kądzioła, I. Piwoński, A. Kisielewska, D. Szczukocki, B. Krawczyk, J. Sielskic, The photoactivity of titanium dioxide coatings with silver nanoparticles prepared by sol–gel and reactive magnetron sputtering methods – comparative studies, Appl. Surf. Sci. 288 (2014) 503–512. [3] Z. Wang, C.J. Medforth, J.A. Shelnutt, Self-metallization of photocatalytic porphyrin nanotubes, J. Am. Chem. Soc. 126 (2004) 16720–16721. [4] S. Xiong, B. Xi, C. Wang, G. Xi, X. Liu, Y. Qian, Solution-phase synthesis and high photocatalytic activity of wurtzite ZnSe ultrathin nanobelts: a general route to 1D semiconductor nanostructured material, Chem. Eur. J. 13 (2007) 7926–7932. [5] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [6] N.R. Khalid, E. Ahmed, Z. Hong, L. Sana., M. Ahmed, Enhanced photocatalytic activity of graphene–TiO2 composite under visible light irradiation, Curr. Appl. Phys. 13 (2013) 659–663. [7] P. Chen, T.Y. Xiao, H.H. Li, J.J. Yang, Z. Wang, H.B. Yao, S.H. Yu, Nitrogen-doped graphene/ZnSe nanocomposites: hydrothermal synthesis and their enhanced electrochemical and photocatalytic activities, ACS Nano 6 (2012) 712–719. [8] B. Liu, L. Tian, Y. Wang, One-pot solvothermal synthesis of ZnSe  xN2H4/GS and ZnSe/N–GS and enhanced visible-light photocatalysis, ACS Appl. Mater. Interfaces 5 (2013) 8414–8422. [9] G.X. Wang, J. Yang, J.S. Park, X.L. Gou, B. Wang, H. Liu, J. Yao, Facile synthesis and characterization of graphene nanosheets, J. Phys. Chem. C 112 (2008) 8192–8195. [10] H.K. Jeong, M.H. Jin, K.H. An, Y.H. Lee, Structural stability and variable dielectric constant in poly sodium 4-styrensulfonate intercalated graphite oxide, J. Phys. Chem. C 113 (2009) 13060–13064. [11] M.A. Rafiee, J. Rafiee, Z. Wang, H. Song, Z.-Z. Yu, N. Koratkar, Enhanced mechanical properties of nanocomposites at low graphene content, ACS Nano 3 (2009) 3884–3890. [12] Y.H. Ni, L. Zhang, L. Zhang, X. Wei, Synthesis, conversion and comparison of the photocatalytic and electrochemical properties of

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

ZnSeN2H4 and ZnSe microrods, Cryst. Res. Technol. 43 (2008) 1030–1035. S.G. Park, J. An, J.R. Potts, A. Velamakanni, S. Murali, R.S. Ruoff, Hydrazine-reduction of graphite- and graphene oxide, Carbon 49 (2011) 3019–3023. L.H. Zhang, H.Q. Yang, J. Yu, F.H. Shao, L. Li, F.H. Zhang, H. Zhao, Controlled synthesis and photocatalytic activity of ZnSe nanostructured assemblies with different morphologies and crystalline phases, J. Phys. Chem. C 113 (2009) 5434–5443. S. Mizushima, I. Nakangwa, D.M. Sweeny, NH2 deformation frequencies of inorganic coordination complexes, J. Chem. Phys. 25 (1956) 1006–1008. Y. Cong, M. Long, Z.W. Cui, X.K. Li, Z.J. Dong, G.M. Yuan, J. Zhang, Anchoring a uniform TiO2 layer on graphene oxide sheets as an efficient visible light photocatalyst, Appl. Surf. Sci. 282 (2013) 400–407. K. Krishnamoorthy, M. Veerapandian, K. Yun, S.-J. Kim, The chemical and structural analysis of graphene oxide with different degrees of oxidation, Carbon 53 (2013) 38–49. N.R. Khalid, Zhanglian Hong, E. Ahmed, Y.W. Zhang, H. Chan, M. Ahmad, Synergistic effects of Fe and graphene on photocatalytic activity enhancement of TiO2 under visible light, Appl. Surf. Sci. 258 (2012) 5827–5834. W. Yu, X.J. Liu, L.K. Pan, J.L. Li, J.Y. Liu, J. Zhang, P. Li, C. Chen, Z. Sun, Enhanced visible light photocatalytic degradation of methylene blue by F-doped TiO2, Appl. Surf. Sci. 319 (2014) 107–112. F. Jiang, T.T. Yan, H. Chen, A.W. Sun, C.M. Xu, X. Wang, A g-C3N4– CdS composite catalyst with high visible-light-driven catalytic activity and photostability for methylene blue degradation, Appl. Surf. Sci. 298 (2014) 164–172. Y.C. Li, Y.Q. Wang, J.H. Kong, H.X. Jia, Z.S. Wang, Synthesis and characterization of carbon modified TiO2 nanotube and photocatalytic activity on methylene blue under sunlight, Appl. Surf. Sci. 344 (2015) 176–180. X.S. Rong, F.X. Qiu, C. Zhang, L. Fu, Y.Y. Wang, D.Y. Yang, Adsorption–photodegradation synergetic removal of methylene blue from aqueous solution by NiO/graphene oxide nanocomposite, Powder Technol. 275 (2015) 322–328. X.S. Rong, F.X. Qiu, C. Zhang., L. Fu., Y.Y. Wang, D.Y. Yang, Preparation, characterization and photocatalytic application of TiO2– graphene photocatalyst under visible light irradiation, Ceram. Int. 41 (2015) 2502–2511. N. Wetchakun, S. Chainet, S. Phanichphant, K. Wetchakun, Efficient photocatalytic degradation of methylene blue over BiVO4/TiO2 nanocomposites, Ceram. Int. 41 (2015) 5999–6004. H.Q. Sun, S.H. Liu, S.M. Liu, S.B. Wang, A comparative study of reduced graphene oxide modified TiO2, ZnO and Ta2O5 in visible light photocatalytic/photochemical oxidation of methylene blue, Appl. Catal. B: Environ. 146 (2014) 162–168.