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This article can be cited before page numbers have been issued, to do this please use: J. Wang, Y. li, J. Ge, B. Zhang and W. Wan, Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP02352A.
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Physical Chemistry Chemical Physics View Article Online
DOI: 10.1039/C5CP02352A
Improving Photocatalytic Performance of ZnO via Synergistic Effects of Ag
Jun Wang, Yan Li∗, Juan Ge, Bo-Ping Zhang, Wan Wan School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing
Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
100083, China
Abstract Here, we reported a simple and “green” method for preparing the ternary photocatalyst Ag-graphene quantum dots (GQDs)-ZnO. In this method, an aqueous solution of GQDs not only acted as a substituent for the organic solvent for preparing the ZnO precursor but was also used as a reducing agent for the in-situ synthesis of Ag nanoparticles (NPs). X-ray diffraction analysis and scanning electron microscopy were employed to confirm the effects of the GQDs solution as solvent on the ZnO structure. Transmission electron microscopy confirmed the synthesis of Ag NPs in the GQDs solution as well as the formation of close interconnections between them. Furthermore, photocatalytic tests involving the degradation of Rhodamine B showed that the synthesized ternary photocatalyst displayed excellent visible-light photocatalytic activity, which was much higher than those of pure ZnO and binary photocatalysts such as Ag-ZnO and GQDs-ZnO. We believe this method will lead to the “green” synthesis of hybrid metal/carbon/semiconductor photocatalysts with higher photocatalytic activities.
Keywords: graphene quantum dots, silver nanoparticles, surface plasmon resonance, photocatalyst
∗
Corresponding author: Tel.: +86 10 62333140; E-mail:[email protected] (Yan Li)
Physical Chemistry Chemical Physics Accepted Manuscript
Nanoparticles and Graphene Quantum Dots
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Introduction
scientific attention for their use in photocatalysis filed.1-3 In particular, ZnO represents a new class of semiconductors that have been investigated extensively, because of their relatively high photosensitivity,
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nontoxic nature, photochemical stability and low cost. 4, 5However, ZnO exhibits low photocatalytic activity due to its distinct characteristics, including its low utilization of visible light, given its wide bandgap (3.37eV), 6 and because it allows for the rapid recombination of the photoexcited electron-hole pairs; this process occurs on the nanosecond timescale and has faster kinetics than those of surface redox reactions.7 Therefore, extending the photoresponse of ZnO to the visible-light region and retarding the recombination of the photogenerated electron-hole pairs are the two challenges for increasing the use of ZnO as a photocatalyst. Various approaches have been developed to overcome these challenges, such as controlling the morphology and defects of the ZnO crystals,
3, 8
doping9 or incorporating other narrow-bandgap
semiconductors into ZnO, 10 and modifying it with carbon materials to form binary ZnO composites.11, 12 Of these approaches, hybridizing ZnO with carbon materials has been shown to be a simple yet effective method for increasing the photocatalytic activity of ZnO. On the one hand, carbon materials, such as carbon nanotubes and graphene, because of their excellent electrical conductivity, can serve as reservoirs for the photogenerated electrons and allow them to be shuttled away from ZnO, resulting in an increase in the electron lifetime. 11, 13, 14 On the other hand, if the carbon material used has the ability to absorb light in the ultraviolet (UV) or visible region, it can itself act as a photosensitizer for the generated electrons, thus increasing the number of photoelectrons available for participating in the photoreactions.15 Moreover, carbon materials can also increase the number of organic molecules adsorbed on the surface of the photocatalyst, owing to the similarity in the sp2-bonded structure of the carbon in the carbon materials and that in the organic molecules and because of the high specific surface area of the carbon materials.16,
17
Thus,
ZnO-carbon hybrids have attracted a lot of attention for use in the photodegradation of organic pollutants. Graphene quantum dots (GQDs), which are a zero-dimensional graphene material, have the intrinsic layered structure of graphene along their lateral plane and oxygen-containing functional groups on their surfaces and at their edges. These structural characteristics endow them with many outstanding properties, including high thermal and chemical stabilities, high mechanical strength, a large specific surface area, good electron conductivity and water solubility.18 Several ZnO-GQDs and TiO2-GQDs composites with high photocatalytic activities have been synthesized and used for the photodegradation of organic pollutants.19, 20 However, little attention had been paid to using GQDs-containing solutions as metal cation solvents, even
Physical Chemistry Chemical Physics Accepted Manuscript
In recent years, semiconductor metal oxides with suitable bandgaps have attracted considerable
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though GQDs exhibit high electronegativity resulting from their surface oxygen groups, which can also act as anchoring sites for cations.
exhibit enhanced photocatalytic performance, as electron-hole recombination is delayed in these materials, they show low photoabsorption efficiencies in the visible-light region, which dramatically limits their use in Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
practical applications.21 Hence, many efficient strategies have been proposed for extending their photoresponse to the visible-light region. These include modification with noble metal NPs to form ternary photocatalysts, because when the frequency of the incident photons matches the overall vibrational frequency of the conduction electrons of the noble metal NPs, the NPs can absorb incident photons and generate plasmon-induced photoexcited electrons. 22 If the photoexcited electrons can be made to migrate into the conduction band of ZnO and subsequently participate in the photodegradation process, it increases the visible-light photocatalytic efficiency of the ternary photocatalyst. In particular, Ag-NP-doped ZnO semiconductor nanocomposites exhibit high photocatalytic activity, given by the frequency dependence of the real and imaginary parts of the dielectric function of Ag. 23 Beside this, given that the price of Ag is much lower than that of other noble metals (i.e., Au and Pt) it has been used widely to improve the photocatalytic properties of ZnO-carbon materials in recent years. For instance, Gao24 synthesized sulfonated graphene oxide-ZnO-Ag composites via a nanocrystal-seed-directed hydrothermal method combined with a polyol-reduction process. The results showed that these ternary catalysts exhibited improved visible-light photodegradation efficiency, because of the surface-plasmon-resonance effect of the Ag NPs and the decrease in the recombination rate of the photogenerated electron-hole pairs. Ahmad and coworkers
25
fabricated a graphene-Ag-ZnO nanocomposite and demonstrated that this ternary ZnO-based nanocomposite exhibited much higher photodecomposition efficiency than those of binary ZnO nanocomposites. This was attributable to the synergetic effects of the Ag NPs and graphene. Therefore, it is clear that ternary ZnO-carbon hybrid catalysts would exhibit better photocatalytic ability than those of their binary counterparts, owing to the formation of two heterojunctions between the various components in the former case. However, most of the methods for synthesizing ternary photocatalysts employ organic solvents as the solvent or reducing agents for the metal NPs or alkali solutions (such as ethanol, ethylene glycol, 24, 25 and NaOH
26
) and involve complex procedures. The use of organic solvents and alkali solutions inevitably
causes pollution-related problems and can result in personal harm to those handling them and thus is unsuited for industrial use. Herein, in order to enhance the photoabsorption efficiency of ZnO-carbon composite materials and to avoid having to use organic solvents for the synthesis of ternary photocatalysts, as well as to exploit the properties of aqueous solution of GQDs, we report a “green” method for preparing an Ag-GQDs-ZnO
Physical Chemistry Chemical Physics Accepted Manuscript
At the same time, although ZnO-carbon hybrid photocatalysts (including ZnO-GQDs photocatalysts)
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ternary composite. This composite can be employed as a heterostructure photocatalyst, which is synthesized by using the GQDs solution as both a reducing agent and a cation solvent. The obtained ternary
be used to synthesize other metal-carbon hybrid nanomaterials as well as semiconductor ternary
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photocatalysts.
Experimental Preparation of Ag-GQDs hybrid compound Films of the Ag-GQDs-ZnO ternary composite were prepared via UV irradiation, spin coating, and a subsequent annealing process. All the reagents used were analytical grade and were employed without further purification. First, an aqueous solution of the GQDs (0.1mg/ml) was prepared using a previously reported electrochemical method.
27
Next, 100 µl of an AgNO3 solution with a molar concentration of
0.1mol/l was added to 2 ml of the GQDs solution. The resulting mixture was stirred for 30 min and subjected to UV irradiation for 3 h; the UV radiation wavelength was 365 nm and power was 0.08 W/cm2. After the completion of the UV irradiation process, the colorless solution turned amber, indicating the formation of Ag-NPs. Fabrication of Ag-GQDs-ZnO ternary composite films After the preparation of the Ag-GQDs hybrid compound, 233 µl of a Zn(NO3)2 solution with a molar concentration of 0.1 mol/l was added to the amber mixture to prepare the Ag-GQDs-Zn(NO3)2 precursor solution. The Ag-GQDs-ZnO composite films were obtained by spin coating the precursor solution on glass substrates which had dimensions of 2 cm×2 cm and had been ultrasonically cleaned in acetone and dried in an oven. The spinning speed was 3000 rpm, and the spinning time was 1 min. Then, the films were annealed at 500°C in an atmosphere of Ar (95%) and H2 (5%) for 10 min to remove the water, yielding the Ag-GQDs-ZnO ternary composite films. This sequence of coating was repeated 4 times for the samples used for UV-visible (UV-vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS) measurements, and 80 times for those used for the X-ray diffraction (XRD) analysis. Fabrication of comparative samples Five comparative samples were fabricated simultaneously to investigate the effects of the Ag NPs and GQDs on the properties of the ternary photocatalyst as well as those of UV irradiation on the formation of Ag NPs in the GQDs solution. The only difference between these samples and the experimentally used ones was in the processes used for producing the precursor solutions. Details of these processes are listed in Table S1.
Physical Chemistry Chemical Physics Accepted Manuscript
photocatalyst exhibits improved photodegradation efficiency under visible light. We hope this method will
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Characterization Transmission electron microscopy (TEM) images were obtained using JEM-2010 and H-7650B
2500H X-ray diffractometer using a Cu Kα radiation (λ=0.15406 nm) source at a scanning rate of 4°/min with voltage 40KV and current 200mA. The morphologies of the samples were investigated using SEM Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
images obtained with a 7401F SEM system. The XPS data were obtained using an ESCALAB 250xi electron spectrometer with an Al monochromatic Kα radiation (hυ = 1486.6 eV) source. The energies of all the spectra were calibrated with respect to the C 1s peak at 284.5 eV, which gave binding energy values with an accuracy of ±0.1 eV. The C 1s, O 1s, and Ag 3d spectra were fitted using the XPS data, which were obtained by the deconvolution of the spectra using the software program XPS Peak 4.0. The UV-vis absorption spectra of the samples were measured with a UNIC UV-2800 spectrophotometer. The electrochemical measurements of the samples were carried out on a CHI660E electrochemical station and the electrochemical impedance spectroscopy (EIS) was recorded by applying an AC voltage of 5mV amplitude in the frequency range of 0.01Hz to100000Hz with the initial potential referring to the open circuit voltage in 0.1 M KCl solution containing 1 mM [Fe (CN)6]3-/4- using a three-electrode cell, where the ITO glass substrates coated with Ag-GQDs-ZnO ternary film as the working electrode, a platinum wire as the counter electrode and the Ag/AgCl as the reference electrode. Photocatalytic measurements The photocatalytic activities of the samples were evaluated through the degradation of Rhodamine B (RhB) (supplied by Beijing Beihua Fine Chemical Co., Ltd., China) under visible-light irradiation using a 500 W daylight lamp at a distance of 10 cm (Beijing institute of Electrical Light Sources, China). A layer of RhB was spin coated onto the surfaces of the composite films at a rate of 3000 rpm for 1 min. Before the start of the degradation process, the equilibrium between the adsorption and desorption of RhB on the composite films was conducted by placing the samples in the dark for 2 h. All the samples were analyzed by recording the changes in the absorption band of RhB (550-580 nm) for the same illumination time.
Results and discussion First, we investigated the reducing ability of the GQDs under UV irradiation. According to previous report, the presence of hydroxyl groups in some organic compounds allows them to reduce metal ions to metal NPs under UV irradiation.
28
Singha et al. successfully synthesized a DNA-Ag nanocomposite in the
presence of DNA and UV light and confirmed that the hydroxyl groups on the surfaces of the DNA molecules played an important role in reduction of the Ag ions. 29 Based on the above reports and our previous work of the electrochemically synthesized GQDs consisting hydroxyl, carbonyl, and carboxylic
Physical Chemistry Chemical Physics Accepted Manuscript
electron microscopes. The X-ray diffraction (XRD) patterns of the samples were obtained by a D/MAX
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acid groups on their surfaces
27
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, it is believed that GQDs synthesized by the electrochemical method have
the ability of reducing Ag ions to Ag NPs under UV irradiation. When we added the AgNO3 solution into
colorless to amber as shown in Fig.1. This color change of solution along with the appearance of large NPs shown in the TEM images confirmed the formation of Ag NPs. When Zn (NO3)2 was subsequently added to Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
this amber solution, the solution color turned deeper within 5 s; this phenomenon may have been caused by the formation of large compounds originating from the electrostatic adsorption of the GQDs (zeta potential = -5.58 mV) and Zn cations. TEM images of the precursor solution clearly reflected the formation of Ag NPs after UV irradiation. Figures 2 (a) and (b) showed large particles with diameters of 13-33 nm (average size ~28nm), which were identified as Ag NPs, while the surrounding smaller particles, which had diameters of 2-5 nm, were regarded as the GQDs in the solution. The high-resolution TEM image shown in Fig.2 (c) further confirmed this deduction. The lattice spacing, d, of 0.24 nm corresponded to the (111) planes of the metallic Ag, which had a face-centered cubic structure, while the lattice spacing, d, of 0.33 was attributed to the (002) facet of graphite. Further observation showed that the distance between adjacent NPs was very small, indicating the Ag NPs were formed near the surfaces of the GQDs. On the other hand, so close interconnections between the GQDs and the Ag NPs were believed to favor for the transfer and separation of the photogenerated electrons from the Ag NPs to the GQDs, which, in turn, affected the photocatalytic activity. In addition to the formation of the Ag NPs around the GQDs, the fact that the unchanged colors of sample (3) (using water as solvent), and (6) (not irradiated with UV light) (Figure S1) demonstrated that the GQDs solution and UV irradiation played important roles in the reduction of the Ag cations. Figure 3 showed the XRD patterns of the six samples after annealing treatment. Both samples (1) and (2) exhibited diffraction peaks characteristic of wurtzite-structured ZnO, indicating that the present of GQDs had not influence the formation of ZnO during annealing treatment. A weak, broad diffraction peak at around 24° in the cases of sample (2) (4) (5) and (6), could be attributed to the small-sized GQDs.30Apart from them, those typical of face-centered cubic Ag (PDF#87-0597) were also observed in the case of samples (3) to (6), indicating that metallic Ag was also formed in these photocatalysts. Xu et al., 31 who prepared ZnO in water and organic solvents, found that the preferred orientation of ZnO crystals was [0001] in water because of its high polarities and that almost equiaxed ZnO crystals were obtained in the organic solvents, since low-polarity solvents afforded weaker interactions leading to poor anisotropic growth rates. Similar to that work, in our cases of sample (1) and (3), which were prepared using water as solvents, the ZnO crystals exhibited a preferred [0001] orientation; but for samples (2), (4), (5) and (6), which were prepared using GQDs solution, they showed an isotropic growth. This growing feature demonstrated that the effects of the
Physical Chemistry Chemical Physics Accepted Manuscript
GQDs solution and after the continuous UV irradiation for 3 h, the color of GQDs solution turned from
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GQDs solution as solvent on the growth process of ZnO crystals were similar to those of organic solvents, which may be due to the electronegativity of GQDs that lowered the polarity of solution. While, this result
many fields. In addition, the half-peak widths of ZnO diffraction peaks corresponding to samples (2), (4), (5) and (6) were broader than those of samples (1) and (3); this implied that smaller sized ZnO nanocrystals Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
were grown in the aqueous GQDs solutions than in water. It is believed that because of the electrostatic interactions between the Zn cations and the electronegative GQDs (zeta potential = -5.58 mV), GQDs in the solutions acted as anchors for the formation of ZnO core in the annealing process, the more the ZnO core, the smaller the size. The effect of GQDs on ZnO size was also reflected by the SEM images (shown as Figure S2). From it, it was clear that using GQDs solution as the solvent ZnO flower-like structures with small size were obtained. Especially, the sample (5) showed the smallest ZnO flower-like structures and had a more uniform distribution. However, for the smaller-sized of ZnO flower-like structures of sample (5), except for the effects of the GQDs, we believed that the advanced formation of Ag NPs before the annealing process also played an important role, because the existence of them inhibited the further growth of ZnO. A few researchers have suggested that ZnO nanocrystals and Ag NPs with a small size and a uniform distribution are favorable for increasing the number of active surface sites for the surface adsorption of organic molecules and for decreasing the diffusion time of the photogenerated electron-hole pairs, given that the surface-to-volume ratio in these cases will be high.32 This, in turn, increases the photocatalytic ability of the Ag-GQDs-ZnO photocatalyst. Figure 4 showed the UV-vis absorption spectra of all the samples. As can be seen from the spectra, the thin film of pure ZnO (i.e., sample (1)) exhibited a sharp characteristic peak at ca. 375 nm, which corresponded to the band-band transition of the electrons (3.37eV), as had also been reported by others. 6 In contrast, the film of the GQDs-ZnO binary photocatalyst exhibited an additional broad absorption peak centered at 270-300 nm which originated from the electronic transition from the n orbital to π* orbital of the C=O bond in the GQDs.
33
In the case of the Ag-ZnO binary film, in addition to the peak characteristic of
ZnO, a strong absorption band located at ~400 nm and arising from the surface plasmon resonance (SPR) effect of the metallic Ag NPs34 was also observed. As for the ternary composites (i.e., those of samples (4)-(6)), they exhibited absorption peaks related to ZnO, GQDs, and Ag NPs, with the absorbance edge expanding to the visible-light region. Compared to the SPR peak of the Ag NPs synthesized by annealing (sample 3, 453 nm), the SPR peaks of the ternary composite films exhibited a blue shift (sample 4: 433 nm; sample 5: 404 nm; sample 6: 408 nm). This suggested that the size of the Ag NPs had been reduced in the latter case, which can be ascribed to the introduction of the GQDs and explained as follows. GQDs had a high electronegativity (zeta potential = -5.58 mV), when Ag+ ions were added to an aqueous GQDs solution,
Physical Chemistry Chemical Physics Accepted Manuscript
promised that GQDs solution as a substituting solvent for organic solvents had a potential application in
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GQDs can be used as anchoring sites to form a number of Ag+-GQDs compounds. Subsequently, under continuous UV irradiation for 3 h, the Ag+ ions were gradually reduced to Ag NPs on the surfaces of the
number of anchoring sites for the formation of Ag seeds. Thus, the resulting Ag NPs were smaller. XPS analysis, which is a widely used method for studying the surface components and chemical states Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
of materials, was also performed. As shown in Fig. 5 (a), the XPS survey spectra of the composite films showed Zn 2p, Zn 3s, Zn LMM, O 1s, Ag 3d, and Si 2p photoelectron peaks. Apart from a Si-related peak, which stemmed from the Si substrate, no other peaks were observed. Therefore, it could be concluded that the Ag-GQDs-ZnO ternary photocatalyst was composed of Zn, O, Ag, and C, in keeping with the XRD results. Figures 5 (b)-(d) showed the XPS (b) C-1s, (c) O-1s, and (d) Ag-3d spectra of the core level regions of sample (5). The C1s spectrum, shown in Fig. 5 (b), containing a broad asymmetric band ranging from 283 to 290 eV, could be deconvoluted into three individual peaks, assigning to C-C (284.5 eV), C-OH (286.0eV), and the carbonyl group C=O (287.6eV), respectively.
35
The broad and asymmetric peak in the O 1s
spectrum, shown in Fig. 5 (c), could also be divided into three symmetrical peaks. The ones located at 530.1 eV and 532.6 eV could be attributed to the lattice oxygen of Zn-O36 and C=O37, while the other one centered at 531.3 eV was assigned to the chemisorbed oxygen because of the surface hydroxyl group.36 Figure 5 (d) showed the high-resolution Ag3d spectrum, the peaks at 367.65 and 373.65 eV corresponded to the binding energies of Ag3d5/2 and Ag3d3/2, respectively. The spin energy separation was 6.0 eV, indicating that the silver in Ag-GQDs-ZnO had a metallic nature. Further observations showed that the binding energies of the Ag 3d5/2 and Ag 3d3/2 states of the Ag-GQDs-ZnO ternary photocatalyst were lower than those for bulk Ag (368.2 and 374.2 eV, respectively)38 , which suggested that the electron density of Ag in the Ag-GQDs-ZnO was lower. To further explore the charge-transfer process at the electrode-electrolyte interface in the case of the as-prepared samples, EIS was performed using a 0.1 M KCl solution containing 1 mM [Fe (CN)6]3-/4-; the results were shown in Fig. 6. Usually, the Nyquist plots for electrode-electrolyte interfaces contain two semicircles and a straight line. The semicircle in the high-frequency range is related to the resistance of the electrolyte, while the semicircle in the mid-frequency region corresponds to the limited charge-transfer process. In addition, the straight line, which is observed at low frequencies, is caused by a diffusion process; 39, 40
this line is not shown in the Nyquist plots for the investigated samples. Previous report showed that the
smaller the radius of the impedance-related arc in the mid-frequency region, the faster is charge transfer 41
between the electrode and the electrolyte (i.e., [Fe (CN) 6]3-/4- ). As seen from Fig. 6, the radius of the arc
was the highest for pure ZnO, indicating that it had the largest charge-transfer resistance (Rct). For the binary films of GQDs-ZnO and Ag-ZnO, Rct was lower, implying that there was an increase in the charge-transfer
Physical Chemistry Chemical Physics Accepted Manuscript
GQDs. In contrast to the Ag NPs synthesized by annealing, the surfaces of the GQDs allowed for a greater
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rate at the electrode-electrolyte interface. Moreover, the Rct values of the ternary composite films were further lower. In particular, the Ag-GQDs-ZnO ternary photocatalyst (sample (5)) exhibited the lowest Rct
The degrees of photodegradation of RhB by the six samples under visible-light irradiation were determined; the results were shown in Fig. 7. It can be seen that all the samples exhibited photocatalytic Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
ability with respect to RhB. After 8 h, the removal rates of RhB for the six samples were 23.43%, 32.30%, 48.59%, 50.05%, 57.49%, and 34.27%. The improved RhB degradation rate of Ag-GQDs-ZnO, especially for the simple 5, implied that a synergistic effect existed between the metallic Ag NPs and the GQDs, which prevented the direct recombination of the electron-hole pairs and enhanced photon absorption in visible region. To help explain the photocatalytic mechanism of the Ag-GQDs-ZnO ternary photocatalyst, a diagram for the photocatalytic reaction is shown in Scheme 1. We propose that an electron-transfer-enhancing synergetic photocatalytic mechanism is responsible for the high photocatalytic activity. Because the Fermi energy level of ZnO is lower than that of Ag, the electrons on the Ag NPs are transferred to the surfaces of the ZnO nanocrystals, where they form a new Fermi energy level before irradiation; this is labeled as Ef in Scheme 1. When the Ag-GQDs-ZnO catalyst is exposed to visible light, the electrons below the Fermi level (Ef) of the Ag NPs will be excited to the surface plasmon states, leaving behind positive charges (h+) below the Ef level. These energetic electrons will subsequently be transferred to the conduction band of ZnO at the interface of the Ag NPs and the ZnO, resulting from the energy difference between them.42 These transferred electrons then get surrounded by O2, resulting in the formation of superoxide anion radicals (•O2−), •HO2 radicals, and •OH radicals, which are strong oxidants and readily photodegrade organic pollutants. At the same time, the holes that remain on the Ag NPs combine with H2O or OH- to also produce •OH radicals, which further oxide and degrade the pollutants. According to the above TEM images, it was clear that the Ag NPs were surrounded by GQDs. Here, GQDs acted as good electron trappers would favor for electron transfer between Ag NPs and GQDs, thus an improved separation rate of the photogenerated electrons and holes in the Ag NPs could be expected.
Conclusion In conclusion, ternary photocatalysts consisting of wurtzite-structured ZnO, face-centered cubic Ag, and graphite-structured GQDs were fabricated successfully through UV irradiation, spin coating, and subsequent annealing with an aqueous solution of GQDs as the solvent and reducing agent. The excellent visible-light-induced photocatalytic performances of these Ag-GQDs-ZnO ternary photocatalysts can be ascribed to the synergistic effects of the Ag NPs and the GQDs. The roles of the Ag NPs in these composites can be described as follows. (1) The phenomenon of localized SPR of the Ag NPs increases their absorbance of visible light, producing energetic electrons. (2) The presence of Ag NPs in the precursor solution prevents the growth of the ZnO flower-like nanostructures, resulting in small-sized ZnO nanostructures with a high
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value, showing that it had the highest electron-transfer ability.
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specific surface area for the photocatalytic reaction. In addition, the roles of the GQDs can be described as follows. (1) They act as anchoring sites for the in-situ formation of Ag NPs and hinder the agglomeration of
dispersibility, owing to their low polarity with respect to water. Finally, the energetic electrons of Ag are transferred to the GQDs, resulting in the hindering of the electron-hole recombination process. Given the
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low-cost, simple, and “green” nature of the described fabrication, it is reasonable to believe that thus-fabricated Ag-GQDs-ZnO ternary nanocomposite will find wide application as an industrial catalyst.
Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 51202011), and Beijing Organization department outstanding talented person project (2013D009006000001) and the Fundamental Research Funds for the Central Universities (FRF-TP-14-010A2)
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the ZnO nanoparticles. (2) They control the growth orientation of the ZnO nanocrystals and improve their
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19. Y. Li, B. P. Zhang, J. X. Zhao, Z. H. Ge, S. K. Zhao and L. Zou, Appl. Surf. Sci., 2013, 279, 367-373. 20. D. Qu, M. Zheng, P. Du, Y. Zhou, L. Zhang, D. Li, H. Tan, Z. Zhao, Z. Xie and Z. Sun, Nanoscale,
21. C. Han, M. Q. Yang, B. Weng and Y. J. Xu, Phys. Chem. Chem. Phys., 2014,16, 16891-16903. 22. J. S. Zhong, Q. Y. Wang and Y. F. Yu, J. Alloys Compd., 2015, 620, 168-171. Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
23. X. Hou, Mater. Lett. , 2015, 139, 201-204. 24. P. Gao, K. Ng and D. D. Sun, J. Hazard. Mater. , 2013,262, 826-835. 25. M. Ahmad, E. Ahmad, Z. L. Hong, N. R. Khalid, W. Ahmed and A. Elhissi, J. Alloys Compd., 2013, 577, 717-727. 26. F. Xu, Y. Yuan, D. Wu, M. Zhao, Z. Gao and K. Jiang, Mater. Res. Bull. , 2013, 48, 2066-2070. 27. Y. Li, Y. Zhao, G. Shi, L. Deng, Y. Hou and L. Qu, Adv. Mater., 2011, 23, 776-780. 28. K. Esumi, A. Suzuki, N. Aihara, K. Usui and K. Torigoe, Langmuir, 1998, 14, 3157-3159. 29. D. Majumdar, A. Singha, P. K. Mondal and S. Kundu, ACS Appl. Mater. Interfaces, 2013, 5, 7798-7807. 30. Z. W. Liang, X. Cai, S. Z. Tan, P. H. Yang, L Zhang, X. Yu, K. Q. Chen, H. M. Zhu, P. Y. Liu and W. J. Mai, Phys. Chem. Chem. Phys., 2012, 14,16111-16114. 31. L. Xu, Y. L. Hu, C. Pelligra, C. H. Chen, L. Jin, H. Huang, S. Sithambaram, M. Aindow, R. Joesten and S. L. Suib, Chem. Mater., 2009, 21, 2875-2885. 32. H. Zhai, L. Wang, D. Sun, D. Han, B. Qi, X. Li and L. Chang, J. Phys. Chem. Solids, 2015, 78, 35-40. 33. P. Yu, X. Wen, Y. R. Toh and J. Tang, J. Phys. Chem. C, 2012, 116, 25552-25557. 34. A. Singha, S. Dasgupta, A. Roy, Biophys. Chem., 2006, 120, 215-224. 35. L. Bao, Z. L. Zhang, Z. Q. Tian, L. Zhang, C. Liu, Y. Lin, B. Qi and D. W. Pang, Adv. Mater. , 2011, 23, 5801-5806. 36. V. Kumar, V. Kumar, S. Som, L. P. Purohit, O. M. Ntwaeaborwa and H.C. Swart, J. Alloys Compd., 2014,594, 32-38. 37. T. Shao, G. Wang, X. An, S. Zhou, Y. Xia and C. Zhu, RSC Adv., 2014, 4, 47977-47981. 38. Q. Deng, H. Tang, G. Liu, X. Song, G. Xu, Q. Li, D. H. L. Ng and G. Wang, Appl. Surf. Sci., 2015, 331, 50-57. 39. W. H. Leng, Z. Zhang, J.Q. Zhang and C. N. Cao, J. Phys. Chem. B, 2005, 109, 15008-15023. 40. X. H. Tan, P. F. Qiang, D. D. Zahng, X. C.ai, S. Z. Tan, P. Y. Liu and W. J. Mai, CrystEngComm, 2014, 16, 1020-1025. 41. H. Gu, Y. Yang, J. Tian and G. Shi, ACS Appl. Mater. Interfaces, 2013, 5, 6762-6768. 42. K. H. Leong, B. L. Gan, S. Ibrahim, P. Saravanan, Appl. Surf. Sci., 2014,319, 128-135.
Physical Chemistry Chemical Physics Accepted Manuscript
2013,5, 12272-12277.
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Figure captions
and (c) after the addition of the Zn (NO3)2 solutions. Figure 2 TEM images of the precursor solution for sample (5).
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Figure 3 XRD patterns of the six samples. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.) Figure 4 UV-vis spectra of the six films. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.) Figure 5 (a) XPS survey spectra of the Ag-GQDs-ZnO ternary composite film (sample (5)), (b)-(d) show the C1s, O1s, and Ag3d spectra, respectively. Figure 6 Nyquist plots of the six samples in a 0.1 M KCl solution containing 1 mM [Fe (CN) 6]3-/4-.((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.) Figure 7 Rates of photodegradation of RhB for the six samples. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.) Scheme 1 Working mechanism of the plasmonic Ag-GQDs-ZnO ternary composite film.
Physical Chemistry Chemical Physics Accepted Manuscript
Figure 1 Changes in the color of the precursor solution for sample (5) (a) before and (b) after UV irradiation
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Fig.1. Changes in the color of the precursor solution for sample (5) (a) before and (b) after UV irradiation and (c) after the addition of the Zn (NO3)2 solution.
Fig.2. TEM images of the precursor solution for sample (5).
Fig.3. XRD patterns of the six samples. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.)
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DOI: 10.1039/C5CP02352A
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Fig.4. UV-vis spectra of the six films. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.)
Fig.5. (a) XPS survey spectra of the Ag-GQDs-ZnO ternary composite film (sample (5)), (b)-(d) show the C1s, O1s, and Ag3d spectra, respectively.
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Fig.6. Nyquist plots of the six samples in a 0.1 M KCl solution containing 1 mM [Fe (CN) 6]3-/4-.((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.)
Fig.7. Rates of photodegradation of RhB for the six samples. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.)
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DOI: 10.1039/C5CP02352A
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Scheme 1 Working mechanism of the Ag-GQDs-ZnO ternary composite film.
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DOI: 10.1039/C5CP02352A
PCCP Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. Wang, Y. li, J. Ge, B. Zhang and W. Wan, Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP02352A.
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DOI: 10.1039/C5CP02352A
Improving Photocatalytic Performance of ZnO via Synergistic Effects of Ag
Jun Wang, Yan Li∗, Juan Ge, Bo-Ping Zhang, Wan Wan School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing
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100083, China
Abstract Here, we reported a simple and “green” method for preparing the ternary photocatalyst Ag-graphene quantum dots (GQDs)-ZnO. In this method, an aqueous solution of GQDs not only acted as a substituent for the organic solvent for preparing the ZnO precursor but was also used as a reducing agent for the in-situ synthesis of Ag nanoparticles (NPs). X-ray diffraction analysis and scanning electron microscopy were employed to confirm the effects of the GQDs solution as solvent on the ZnO structure. Transmission electron microscopy confirmed the synthesis of Ag NPs in the GQDs solution as well as the formation of close interconnections between them. Furthermore, photocatalytic tests involving the degradation of Rhodamine B showed that the synthesized ternary photocatalyst displayed excellent visible-light photocatalytic activity, which was much higher than those of pure ZnO and binary photocatalysts such as Ag-ZnO and GQDs-ZnO. We believe this method will lead to the “green” synthesis of hybrid metal/carbon/semiconductor photocatalysts with higher photocatalytic activities.
Keywords: graphene quantum dots, silver nanoparticles, surface plasmon resonance, photocatalyst
∗
Corresponding author: Tel.: +86 10 62333140; E-mail:[email protected] (Yan Li)
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Nanoparticles and Graphene Quantum Dots
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Introduction
scientific attention for their use in photocatalysis filed.1-3 In particular, ZnO represents a new class of semiconductors that have been investigated extensively, because of their relatively high photosensitivity,
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nontoxic nature, photochemical stability and low cost. 4, 5However, ZnO exhibits low photocatalytic activity due to its distinct characteristics, including its low utilization of visible light, given its wide bandgap (3.37eV), 6 and because it allows for the rapid recombination of the photoexcited electron-hole pairs; this process occurs on the nanosecond timescale and has faster kinetics than those of surface redox reactions.7 Therefore, extending the photoresponse of ZnO to the visible-light region and retarding the recombination of the photogenerated electron-hole pairs are the two challenges for increasing the use of ZnO as a photocatalyst. Various approaches have been developed to overcome these challenges, such as controlling the morphology and defects of the ZnO crystals,
3, 8
doping9 or incorporating other narrow-bandgap
semiconductors into ZnO, 10 and modifying it with carbon materials to form binary ZnO composites.11, 12 Of these approaches, hybridizing ZnO with carbon materials has been shown to be a simple yet effective method for increasing the photocatalytic activity of ZnO. On the one hand, carbon materials, such as carbon nanotubes and graphene, because of their excellent electrical conductivity, can serve as reservoirs for the photogenerated electrons and allow them to be shuttled away from ZnO, resulting in an increase in the electron lifetime. 11, 13, 14 On the other hand, if the carbon material used has the ability to absorb light in the ultraviolet (UV) or visible region, it can itself act as a photosensitizer for the generated electrons, thus increasing the number of photoelectrons available for participating in the photoreactions.15 Moreover, carbon materials can also increase the number of organic molecules adsorbed on the surface of the photocatalyst, owing to the similarity in the sp2-bonded structure of the carbon in the carbon materials and that in the organic molecules and because of the high specific surface area of the carbon materials.16,
17
Thus,
ZnO-carbon hybrids have attracted a lot of attention for use in the photodegradation of organic pollutants. Graphene quantum dots (GQDs), which are a zero-dimensional graphene material, have the intrinsic layered structure of graphene along their lateral plane and oxygen-containing functional groups on their surfaces and at their edges. These structural characteristics endow them with many outstanding properties, including high thermal and chemical stabilities, high mechanical strength, a large specific surface area, good electron conductivity and water solubility.18 Several ZnO-GQDs and TiO2-GQDs composites with high photocatalytic activities have been synthesized and used for the photodegradation of organic pollutants.19, 20 However, little attention had been paid to using GQDs-containing solutions as metal cation solvents, even
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In recent years, semiconductor metal oxides with suitable bandgaps have attracted considerable
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though GQDs exhibit high electronegativity resulting from their surface oxygen groups, which can also act as anchoring sites for cations.
exhibit enhanced photocatalytic performance, as electron-hole recombination is delayed in these materials, they show low photoabsorption efficiencies in the visible-light region, which dramatically limits their use in Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
practical applications.21 Hence, many efficient strategies have been proposed for extending their photoresponse to the visible-light region. These include modification with noble metal NPs to form ternary photocatalysts, because when the frequency of the incident photons matches the overall vibrational frequency of the conduction electrons of the noble metal NPs, the NPs can absorb incident photons and generate plasmon-induced photoexcited electrons. 22 If the photoexcited electrons can be made to migrate into the conduction band of ZnO and subsequently participate in the photodegradation process, it increases the visible-light photocatalytic efficiency of the ternary photocatalyst. In particular, Ag-NP-doped ZnO semiconductor nanocomposites exhibit high photocatalytic activity, given by the frequency dependence of the real and imaginary parts of the dielectric function of Ag. 23 Beside this, given that the price of Ag is much lower than that of other noble metals (i.e., Au and Pt) it has been used widely to improve the photocatalytic properties of ZnO-carbon materials in recent years. For instance, Gao24 synthesized sulfonated graphene oxide-ZnO-Ag composites via a nanocrystal-seed-directed hydrothermal method combined with a polyol-reduction process. The results showed that these ternary catalysts exhibited improved visible-light photodegradation efficiency, because of the surface-plasmon-resonance effect of the Ag NPs and the decrease in the recombination rate of the photogenerated electron-hole pairs. Ahmad and coworkers
25
fabricated a graphene-Ag-ZnO nanocomposite and demonstrated that this ternary ZnO-based nanocomposite exhibited much higher photodecomposition efficiency than those of binary ZnO nanocomposites. This was attributable to the synergetic effects of the Ag NPs and graphene. Therefore, it is clear that ternary ZnO-carbon hybrid catalysts would exhibit better photocatalytic ability than those of their binary counterparts, owing to the formation of two heterojunctions between the various components in the former case. However, most of the methods for synthesizing ternary photocatalysts employ organic solvents as the solvent or reducing agents for the metal NPs or alkali solutions (such as ethanol, ethylene glycol, 24, 25 and NaOH
26
) and involve complex procedures. The use of organic solvents and alkali solutions inevitably
causes pollution-related problems and can result in personal harm to those handling them and thus is unsuited for industrial use. Herein, in order to enhance the photoabsorption efficiency of ZnO-carbon composite materials and to avoid having to use organic solvents for the synthesis of ternary photocatalysts, as well as to exploit the properties of aqueous solution of GQDs, we report a “green” method for preparing an Ag-GQDs-ZnO
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At the same time, although ZnO-carbon hybrid photocatalysts (including ZnO-GQDs photocatalysts)
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ternary composite. This composite can be employed as a heterostructure photocatalyst, which is synthesized by using the GQDs solution as both a reducing agent and a cation solvent. The obtained ternary
be used to synthesize other metal-carbon hybrid nanomaterials as well as semiconductor ternary
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photocatalysts.
Experimental Preparation of Ag-GQDs hybrid compound Films of the Ag-GQDs-ZnO ternary composite were prepared via UV irradiation, spin coating, and a subsequent annealing process. All the reagents used were analytical grade and were employed without further purification. First, an aqueous solution of the GQDs (0.1mg/ml) was prepared using a previously reported electrochemical method.
27
Next, 100 µl of an AgNO3 solution with a molar concentration of
0.1mol/l was added to 2 ml of the GQDs solution. The resulting mixture was stirred for 30 min and subjected to UV irradiation for 3 h; the UV radiation wavelength was 365 nm and power was 0.08 W/cm2. After the completion of the UV irradiation process, the colorless solution turned amber, indicating the formation of Ag-NPs. Fabrication of Ag-GQDs-ZnO ternary composite films After the preparation of the Ag-GQDs hybrid compound, 233 µl of a Zn(NO3)2 solution with a molar concentration of 0.1 mol/l was added to the amber mixture to prepare the Ag-GQDs-Zn(NO3)2 precursor solution. The Ag-GQDs-ZnO composite films were obtained by spin coating the precursor solution on glass substrates which had dimensions of 2 cm×2 cm and had been ultrasonically cleaned in acetone and dried in an oven. The spinning speed was 3000 rpm, and the spinning time was 1 min. Then, the films were annealed at 500°C in an atmosphere of Ar (95%) and H2 (5%) for 10 min to remove the water, yielding the Ag-GQDs-ZnO ternary composite films. This sequence of coating was repeated 4 times for the samples used for UV-visible (UV-vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS) measurements, and 80 times for those used for the X-ray diffraction (XRD) analysis. Fabrication of comparative samples Five comparative samples were fabricated simultaneously to investigate the effects of the Ag NPs and GQDs on the properties of the ternary photocatalyst as well as those of UV irradiation on the formation of Ag NPs in the GQDs solution. The only difference between these samples and the experimentally used ones was in the processes used for producing the precursor solutions. Details of these processes are listed in Table S1.
Physical Chemistry Chemical Physics Accepted Manuscript
photocatalyst exhibits improved photodegradation efficiency under visible light. We hope this method will
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Characterization Transmission electron microscopy (TEM) images were obtained using JEM-2010 and H-7650B
2500H X-ray diffractometer using a Cu Kα radiation (λ=0.15406 nm) source at a scanning rate of 4°/min with voltage 40KV and current 200mA. The morphologies of the samples were investigated using SEM Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
images obtained with a 7401F SEM system. The XPS data were obtained using an ESCALAB 250xi electron spectrometer with an Al monochromatic Kα radiation (hυ = 1486.6 eV) source. The energies of all the spectra were calibrated with respect to the C 1s peak at 284.5 eV, which gave binding energy values with an accuracy of ±0.1 eV. The C 1s, O 1s, and Ag 3d spectra were fitted using the XPS data, which were obtained by the deconvolution of the spectra using the software program XPS Peak 4.0. The UV-vis absorption spectra of the samples were measured with a UNIC UV-2800 spectrophotometer. The electrochemical measurements of the samples were carried out on a CHI660E electrochemical station and the electrochemical impedance spectroscopy (EIS) was recorded by applying an AC voltage of 5mV amplitude in the frequency range of 0.01Hz to100000Hz with the initial potential referring to the open circuit voltage in 0.1 M KCl solution containing 1 mM [Fe (CN)6]3-/4- using a three-electrode cell, where the ITO glass substrates coated with Ag-GQDs-ZnO ternary film as the working electrode, a platinum wire as the counter electrode and the Ag/AgCl as the reference electrode. Photocatalytic measurements The photocatalytic activities of the samples were evaluated through the degradation of Rhodamine B (RhB) (supplied by Beijing Beihua Fine Chemical Co., Ltd., China) under visible-light irradiation using a 500 W daylight lamp at a distance of 10 cm (Beijing institute of Electrical Light Sources, China). A layer of RhB was spin coated onto the surfaces of the composite films at a rate of 3000 rpm for 1 min. Before the start of the degradation process, the equilibrium between the adsorption and desorption of RhB on the composite films was conducted by placing the samples in the dark for 2 h. All the samples were analyzed by recording the changes in the absorption band of RhB (550-580 nm) for the same illumination time.
Results and discussion First, we investigated the reducing ability of the GQDs under UV irradiation. According to previous report, the presence of hydroxyl groups in some organic compounds allows them to reduce metal ions to metal NPs under UV irradiation.
28
Singha et al. successfully synthesized a DNA-Ag nanocomposite in the
presence of DNA and UV light and confirmed that the hydroxyl groups on the surfaces of the DNA molecules played an important role in reduction of the Ag ions. 29 Based on the above reports and our previous work of the electrochemically synthesized GQDs consisting hydroxyl, carbonyl, and carboxylic
Physical Chemistry Chemical Physics Accepted Manuscript
electron microscopes. The X-ray diffraction (XRD) patterns of the samples were obtained by a D/MAX
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acid groups on their surfaces
27
DOI: 10.1039/C5CP02352A
, it is believed that GQDs synthesized by the electrochemical method have
the ability of reducing Ag ions to Ag NPs under UV irradiation. When we added the AgNO3 solution into
colorless to amber as shown in Fig.1. This color change of solution along with the appearance of large NPs shown in the TEM images confirmed the formation of Ag NPs. When Zn (NO3)2 was subsequently added to Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
this amber solution, the solution color turned deeper within 5 s; this phenomenon may have been caused by the formation of large compounds originating from the electrostatic adsorption of the GQDs (zeta potential = -5.58 mV) and Zn cations. TEM images of the precursor solution clearly reflected the formation of Ag NPs after UV irradiation. Figures 2 (a) and (b) showed large particles with diameters of 13-33 nm (average size ~28nm), which were identified as Ag NPs, while the surrounding smaller particles, which had diameters of 2-5 nm, were regarded as the GQDs in the solution. The high-resolution TEM image shown in Fig.2 (c) further confirmed this deduction. The lattice spacing, d, of 0.24 nm corresponded to the (111) planes of the metallic Ag, which had a face-centered cubic structure, while the lattice spacing, d, of 0.33 was attributed to the (002) facet of graphite. Further observation showed that the distance between adjacent NPs was very small, indicating the Ag NPs were formed near the surfaces of the GQDs. On the other hand, so close interconnections between the GQDs and the Ag NPs were believed to favor for the transfer and separation of the photogenerated electrons from the Ag NPs to the GQDs, which, in turn, affected the photocatalytic activity. In addition to the formation of the Ag NPs around the GQDs, the fact that the unchanged colors of sample (3) (using water as solvent), and (6) (not irradiated with UV light) (Figure S1) demonstrated that the GQDs solution and UV irradiation played important roles in the reduction of the Ag cations. Figure 3 showed the XRD patterns of the six samples after annealing treatment. Both samples (1) and (2) exhibited diffraction peaks characteristic of wurtzite-structured ZnO, indicating that the present of GQDs had not influence the formation of ZnO during annealing treatment. A weak, broad diffraction peak at around 24° in the cases of sample (2) (4) (5) and (6), could be attributed to the small-sized GQDs.30Apart from them, those typical of face-centered cubic Ag (PDF#87-0597) were also observed in the case of samples (3) to (6), indicating that metallic Ag was also formed in these photocatalysts. Xu et al., 31 who prepared ZnO in water and organic solvents, found that the preferred orientation of ZnO crystals was [0001] in water because of its high polarities and that almost equiaxed ZnO crystals were obtained in the organic solvents, since low-polarity solvents afforded weaker interactions leading to poor anisotropic growth rates. Similar to that work, in our cases of sample (1) and (3), which were prepared using water as solvents, the ZnO crystals exhibited a preferred [0001] orientation; but for samples (2), (4), (5) and (6), which were prepared using GQDs solution, they showed an isotropic growth. This growing feature demonstrated that the effects of the
Physical Chemistry Chemical Physics Accepted Manuscript
GQDs solution and after the continuous UV irradiation for 3 h, the color of GQDs solution turned from
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GQDs solution as solvent on the growth process of ZnO crystals were similar to those of organic solvents, which may be due to the electronegativity of GQDs that lowered the polarity of solution. While, this result
many fields. In addition, the half-peak widths of ZnO diffraction peaks corresponding to samples (2), (4), (5) and (6) were broader than those of samples (1) and (3); this implied that smaller sized ZnO nanocrystals Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
were grown in the aqueous GQDs solutions than in water. It is believed that because of the electrostatic interactions between the Zn cations and the electronegative GQDs (zeta potential = -5.58 mV), GQDs in the solutions acted as anchors for the formation of ZnO core in the annealing process, the more the ZnO core, the smaller the size. The effect of GQDs on ZnO size was also reflected by the SEM images (shown as Figure S2). From it, it was clear that using GQDs solution as the solvent ZnO flower-like structures with small size were obtained. Especially, the sample (5) showed the smallest ZnO flower-like structures and had a more uniform distribution. However, for the smaller-sized of ZnO flower-like structures of sample (5), except for the effects of the GQDs, we believed that the advanced formation of Ag NPs before the annealing process also played an important role, because the existence of them inhibited the further growth of ZnO. A few researchers have suggested that ZnO nanocrystals and Ag NPs with a small size and a uniform distribution are favorable for increasing the number of active surface sites for the surface adsorption of organic molecules and for decreasing the diffusion time of the photogenerated electron-hole pairs, given that the surface-to-volume ratio in these cases will be high.32 This, in turn, increases the photocatalytic ability of the Ag-GQDs-ZnO photocatalyst. Figure 4 showed the UV-vis absorption spectra of all the samples. As can be seen from the spectra, the thin film of pure ZnO (i.e., sample (1)) exhibited a sharp characteristic peak at ca. 375 nm, which corresponded to the band-band transition of the electrons (3.37eV), as had also been reported by others. 6 In contrast, the film of the GQDs-ZnO binary photocatalyst exhibited an additional broad absorption peak centered at 270-300 nm which originated from the electronic transition from the n orbital to π* orbital of the C=O bond in the GQDs.
33
In the case of the Ag-ZnO binary film, in addition to the peak characteristic of
ZnO, a strong absorption band located at ~400 nm and arising from the surface plasmon resonance (SPR) effect of the metallic Ag NPs34 was also observed. As for the ternary composites (i.e., those of samples (4)-(6)), they exhibited absorption peaks related to ZnO, GQDs, and Ag NPs, with the absorbance edge expanding to the visible-light region. Compared to the SPR peak of the Ag NPs synthesized by annealing (sample 3, 453 nm), the SPR peaks of the ternary composite films exhibited a blue shift (sample 4: 433 nm; sample 5: 404 nm; sample 6: 408 nm). This suggested that the size of the Ag NPs had been reduced in the latter case, which can be ascribed to the introduction of the GQDs and explained as follows. GQDs had a high electronegativity (zeta potential = -5.58 mV), when Ag+ ions were added to an aqueous GQDs solution,
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promised that GQDs solution as a substituting solvent for organic solvents had a potential application in
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GQDs can be used as anchoring sites to form a number of Ag+-GQDs compounds. Subsequently, under continuous UV irradiation for 3 h, the Ag+ ions were gradually reduced to Ag NPs on the surfaces of the
number of anchoring sites for the formation of Ag seeds. Thus, the resulting Ag NPs were smaller. XPS analysis, which is a widely used method for studying the surface components and chemical states Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
of materials, was also performed. As shown in Fig. 5 (a), the XPS survey spectra of the composite films showed Zn 2p, Zn 3s, Zn LMM, O 1s, Ag 3d, and Si 2p photoelectron peaks. Apart from a Si-related peak, which stemmed from the Si substrate, no other peaks were observed. Therefore, it could be concluded that the Ag-GQDs-ZnO ternary photocatalyst was composed of Zn, O, Ag, and C, in keeping with the XRD results. Figures 5 (b)-(d) showed the XPS (b) C-1s, (c) O-1s, and (d) Ag-3d spectra of the core level regions of sample (5). The C1s spectrum, shown in Fig. 5 (b), containing a broad asymmetric band ranging from 283 to 290 eV, could be deconvoluted into three individual peaks, assigning to C-C (284.5 eV), C-OH (286.0eV), and the carbonyl group C=O (287.6eV), respectively.
35
The broad and asymmetric peak in the O 1s
spectrum, shown in Fig. 5 (c), could also be divided into three symmetrical peaks. The ones located at 530.1 eV and 532.6 eV could be attributed to the lattice oxygen of Zn-O36 and C=O37, while the other one centered at 531.3 eV was assigned to the chemisorbed oxygen because of the surface hydroxyl group.36 Figure 5 (d) showed the high-resolution Ag3d spectrum, the peaks at 367.65 and 373.65 eV corresponded to the binding energies of Ag3d5/2 and Ag3d3/2, respectively. The spin energy separation was 6.0 eV, indicating that the silver in Ag-GQDs-ZnO had a metallic nature. Further observations showed that the binding energies of the Ag 3d5/2 and Ag 3d3/2 states of the Ag-GQDs-ZnO ternary photocatalyst were lower than those for bulk Ag (368.2 and 374.2 eV, respectively)38 , which suggested that the electron density of Ag in the Ag-GQDs-ZnO was lower. To further explore the charge-transfer process at the electrode-electrolyte interface in the case of the as-prepared samples, EIS was performed using a 0.1 M KCl solution containing 1 mM [Fe (CN)6]3-/4-; the results were shown in Fig. 6. Usually, the Nyquist plots for electrode-electrolyte interfaces contain two semicircles and a straight line. The semicircle in the high-frequency range is related to the resistance of the electrolyte, while the semicircle in the mid-frequency region corresponds to the limited charge-transfer process. In addition, the straight line, which is observed at low frequencies, is caused by a diffusion process; 39, 40
this line is not shown in the Nyquist plots for the investigated samples. Previous report showed that the
smaller the radius of the impedance-related arc in the mid-frequency region, the faster is charge transfer 41
between the electrode and the electrolyte (i.e., [Fe (CN) 6]3-/4- ). As seen from Fig. 6, the radius of the arc
was the highest for pure ZnO, indicating that it had the largest charge-transfer resistance (Rct). For the binary films of GQDs-ZnO and Ag-ZnO, Rct was lower, implying that there was an increase in the charge-transfer
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GQDs. In contrast to the Ag NPs synthesized by annealing, the surfaces of the GQDs allowed for a greater
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rate at the electrode-electrolyte interface. Moreover, the Rct values of the ternary composite films were further lower. In particular, the Ag-GQDs-ZnO ternary photocatalyst (sample (5)) exhibited the lowest Rct
The degrees of photodegradation of RhB by the six samples under visible-light irradiation were determined; the results were shown in Fig. 7. It can be seen that all the samples exhibited photocatalytic Published on 15 June 2015. Downloaded by North Dakota State University on 18/06/2015 12:04:23.
ability with respect to RhB. After 8 h, the removal rates of RhB for the six samples were 23.43%, 32.30%, 48.59%, 50.05%, 57.49%, and 34.27%. The improved RhB degradation rate of Ag-GQDs-ZnO, especially for the simple 5, implied that a synergistic effect existed between the metallic Ag NPs and the GQDs, which prevented the direct recombination of the electron-hole pairs and enhanced photon absorption in visible region. To help explain the photocatalytic mechanism of the Ag-GQDs-ZnO ternary photocatalyst, a diagram for the photocatalytic reaction is shown in Scheme 1. We propose that an electron-transfer-enhancing synergetic photocatalytic mechanism is responsible for the high photocatalytic activity. Because the Fermi energy level of ZnO is lower than that of Ag, the electrons on the Ag NPs are transferred to the surfaces of the ZnO nanocrystals, where they form a new Fermi energy level before irradiation; this is labeled as Ef in Scheme 1. When the Ag-GQDs-ZnO catalyst is exposed to visible light, the electrons below the Fermi level (Ef) of the Ag NPs will be excited to the surface plasmon states, leaving behind positive charges (h+) below the Ef level. These energetic electrons will subsequently be transferred to the conduction band of ZnO at the interface of the Ag NPs and the ZnO, resulting from the energy difference between them.42 These transferred electrons then get surrounded by O2, resulting in the formation of superoxide anion radicals (•O2−), •HO2 radicals, and •OH radicals, which are strong oxidants and readily photodegrade organic pollutants. At the same time, the holes that remain on the Ag NPs combine with H2O or OH- to also produce •OH radicals, which further oxide and degrade the pollutants. According to the above TEM images, it was clear that the Ag NPs were surrounded by GQDs. Here, GQDs acted as good electron trappers would favor for electron transfer between Ag NPs and GQDs, thus an improved separation rate of the photogenerated electrons and holes in the Ag NPs could be expected.
Conclusion In conclusion, ternary photocatalysts consisting of wurtzite-structured ZnO, face-centered cubic Ag, and graphite-structured GQDs were fabricated successfully through UV irradiation, spin coating, and subsequent annealing with an aqueous solution of GQDs as the solvent and reducing agent. The excellent visible-light-induced photocatalytic performances of these Ag-GQDs-ZnO ternary photocatalysts can be ascribed to the synergistic effects of the Ag NPs and the GQDs. The roles of the Ag NPs in these composites can be described as follows. (1) The phenomenon of localized SPR of the Ag NPs increases their absorbance of visible light, producing energetic electrons. (2) The presence of Ag NPs in the precursor solution prevents the growth of the ZnO flower-like nanostructures, resulting in small-sized ZnO nanostructures with a high
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value, showing that it had the highest electron-transfer ability.
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specific surface area for the photocatalytic reaction. In addition, the roles of the GQDs can be described as follows. (1) They act as anchoring sites for the in-situ formation of Ag NPs and hinder the agglomeration of
dispersibility, owing to their low polarity with respect to water. Finally, the energetic electrons of Ag are transferred to the GQDs, resulting in the hindering of the electron-hole recombination process. Given the
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low-cost, simple, and “green” nature of the described fabrication, it is reasonable to believe that thus-fabricated Ag-GQDs-ZnO ternary nanocomposite will find wide application as an industrial catalyst.
Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 51202011), and Beijing Organization department outstanding talented person project (2013D009006000001) and the Fundamental Research Funds for the Central Universities (FRF-TP-14-010A2)
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Figure captions
and (c) after the addition of the Zn (NO3)2 solutions. Figure 2 TEM images of the precursor solution for sample (5).
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Figure 3 XRD patterns of the six samples. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.) Figure 4 UV-vis spectra of the six films. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.) Figure 5 (a) XPS survey spectra of the Ag-GQDs-ZnO ternary composite film (sample (5)), (b)-(d) show the C1s, O1s, and Ag3d spectra, respectively. Figure 6 Nyquist plots of the six samples in a 0.1 M KCl solution containing 1 mM [Fe (CN) 6]3-/4-.((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.) Figure 7 Rates of photodegradation of RhB for the six samples. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.) Scheme 1 Working mechanism of the plasmonic Ag-GQDs-ZnO ternary composite film.
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Figure 1 Changes in the color of the precursor solution for sample (5) (a) before and (b) after UV irradiation
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Fig.1. Changes in the color of the precursor solution for sample (5) (a) before and (b) after UV irradiation and (c) after the addition of the Zn (NO3)2 solution.
Fig.2. TEM images of the precursor solution for sample (5).
Fig.3. XRD patterns of the six samples. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.)
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DOI: 10.1039/C5CP02352A
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Fig.4. UV-vis spectra of the six films. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.)
Fig.5. (a) XPS survey spectra of the Ag-GQDs-ZnO ternary composite film (sample (5)), (b)-(d) show the C1s, O1s, and Ag3d spectra, respectively.
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Fig.6. Nyquist plots of the six samples in a 0.1 M KCl solution containing 1 mM [Fe (CN) 6]3-/4-.((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.)
Fig.7. Rates of photodegradation of RhB for the six samples. ((1) Pure ZnO film, (2) GQDs-ZnO binary film, (3) Ag-ZnO binary film,(4) Ag-GQDs-ZnO ternary films with Zn (NO3)2 and AgNO3 were added to the GQDs solution simultaneously (5) Ag-GQDs-ZnO ternary films with advanced formation of Ag-GQDs (6) Ag-GQDs-ZnO ternary films without UV irradiation.)
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DOI: 10.1039/C5CP02352A
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Scheme 1 Working mechanism of the Ag-GQDs-ZnO ternary composite film.
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DOI: 10.1039/C5CP02352A
