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Investigation of the appropriate content of graphene in AgTiO2-graphene ternary nanocomposites applied as photocatalysts
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
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Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts Fu-Jye Sheu, Chun-Pei Cho* Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou County 54561, Taiwan, ROC
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abstract
Article history:
AgeTiO2-graphene ternary nanocomposites with varying graphene contents were fabri-
Received 30 September 2016
cated by photocatalytic reduction. SEM and TEM imaging of the nanocomposites showed
Received in revised form
that TiO2 nanoparticles decorated with Ag nanoparticles covered graphene nanosheets. A
18 May 2017
higher content of graphene was shown to be favorable for dye photodegradation. The re-
Accepted 4 June 2017
sults of electrochemical analysis revealed that a higher graphene content contributed to
Available online xxx
increased conductivity and reduced interfacial impedance, which led to more efficient electron transport and thus higher photocatalytic activity. The highest efficiency in dye
Keywords:
photodegradation and hydrogen production from water splitting was achieved when the
Graphene
ratio of TiO2 to graphene in the nanocomposite was 5: 1. The corresponding mass-
Ternary nanocomposite
normalized hydrogen evolution rate and quantum efficiency were 129.5 mmol g1 h1
Photodegradation
and 4.8%, respectively. A mechanism for photocatalysis was proposed and discussed. This
Hydrogen evolution
study demonstrates that the AgeTiO2-graphene ternary nanocomposite could be a prom-
Hydrogen production
ising photocatalyst. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Titanium dioxide (TiO2) is the most common semiconductor material applied in photocatalysts and dye-sensitized solar cells (DSSCs) due to its non-toxicity, thermal and chemical stability, low cost, high availability and lack of photocorrosion. However, the fast recombination of photogenerated electron-hole pairs and absence of optical response to visible light have limited its photocatalytic activity [1]. Over the past decade, many efforts have been made to extend the optical absorption of TiO2. Despite the development of photocatalysts with highly efficient visible-light response,
potential applications in photodegradation and hydrogen production from water splitting still remains a challenge [2e4]. TiO2 can be mixed with noble metallic nanoparticles (NPs) or carbonaceous materials to obtain binary nanocomposites for the aforementioned purposes. C. Liu et al. reported that doping TiO2 with silver (Ag) NPs increased visiblelight absorption. The greatly enhanced photocatalytic activity was favorable to the degradation of organic pollutants [1]. Y. Zhao et al. fabricated Ag-decorated TiO2 nanofelts which could achieve self-cleaning under ultraviolet (UV) irradiation. These obtained low-cost nanocomposites could be utilized repeatedly [5]. M. Q. Yang et al. utilized fullerene, carbon nanotubes and graphene to fabricate binary nanocomposites.
* Corresponding author. E-mail address: [email protected] (C.-P. Cho). http://dx.doi.org/10.1016/j.ijhydene.2017.06.024 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
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They found that, for every nanocomposite, there would be an optimum mixing ratio between TiO2 and a carbonaceous material [6]. Z. Mou et al. used nitrogen (N) doping to reduce defects in graphene. The improved interfacial contact and charge separation resulted in better photocatalytic activity and durability of the functionalized N-doped TiO2-graphene (TG) nanocomposite when it was applied to hydrogen production [7]. Y. Yan et al. tuned the ratio of anatase and rutile to form anatase-graphene-rutile heterostructures. The highest hydrogen production efficiency was achieved when the ratio was 7:3 [8]. From the above-mentioned studies, it is perceived that the combination of TiO2 and graphene can enhance light absorption and photocatalytic activity. Graphene is a twodimensional single-layer carbonaceous material with chemical stability, transparency, high specific surface area, and superior charge transport properties [9]. It plays different roles, such as being an adsorbent, electron acceptor, and the platform for charge transport when applied to the fabrication of composites, biochemical detection, nanoelectronic devices and solar cells. While there have been quite a few studies concerning binary nanocomposites used as photocatalysts, less attention has been paid to ternary nanocomposites containing graphene. Y. Yang et al. used hydrothermal and microwave-assisted methods to prepare AgeTiO2-graphene (ATG) ternary nanocomposites showing higher efficiency for the photodegradation of dye and hydrogen production from water splitting than that of TG binary nanocomposites [10]. K. C. Hsu et al. fabricated reusable AgeTiO2-reduced graphene oxide (rGO) substrates with high sensitivity and self-cleaning surfaces. The higher intensity of Raman signals was ascribed to surface enhanced Raman scattering due to the presence of the Ag NPs [11]. X. Yang et al. prepared stable P25e Ag3PO4-GO ternary nanocomposites with good photocatalytic characteristics. The use of Ag3PO4 increased the absorption of visible light, facilitated charge separation and reduced the recombination probability. By varying the amount of GO and the molar ratio of Ag3PO4, the capabilities of photodegradation and bacterial inactivation could be adjusted [12]. It has been indicated by these reports that ternary nanocomposites applicable as photocatalysts are the latest trend. Thus, in this study, we are interested in whether photocatalytic activity can be further enhanced by ternary nanocomposites. It has been demonstrated that Ag NPs assembled on TiO2 nanotube arrays were favorable to hydrogen evolution owing to the surface plasmon resonance (SPR) effect [13e16]. When noble metallic NPs are excited by light, a strong oscillation of the conduction electrons occurs if the frequency of the incident photon is resonant with the collective oscillation of electrons. By altering the dielectric environment, the size and shape of the NPs, their absorption and scattering characteristics can be flexibly tuned. Localized SPR of noble metallic NPs usually results in strong and broad absorption bands in the visible light region. Therefore, this feature can be exploited to develop visible-light activated photocatalysts. In this study, ATG ternary nanocomposites were obtained by photocatalytic reduction, an approach with more simplified procedures requiring only 2 h and completed in one pot. Compared to other methods for obtaining ternary nanocomposites [10e12], the total duration of fabrication was
considerably shortened. The SPR effect caused by Ag NPs was favorable to extend the optical response to a wider wavelength. Due to the excellent electrical properties, the presence of graphene not only accelerated the separation of photogenerated electron-hole pairs but also enhanced the adsorption. The ratio of graphene in the nanocomposites was adjusted to investigate its impacts on dye photodegradation, electrochemical characteristics and hydrogen evolution. Efforts have been aimed to investigate the most appropriate ratio of TiO2 to graphene in an ATG nanocomposite fabricated with a fixed amount of Ag and to achieve higher photocatalytic performance. It has been demonstrated that a higher content of graphene would be conducive to photocatalytic activity.
Experimental Preparation of GO GO was synthesized from graphite powder by the modified Hummers' method [17]. A previous graphite oxidation procedure was carried out before the synthesis of GO [18,19]. 4 g of graphite powder was added into a solution composed of 2 g of potassium persulfate (K₂S₂O₈), 2 g of phosphorus pentoxide (P₂O5) and 30 mL of conc. sulfuric acid (H2SO4). The mixture solution was heated to 80 C under continuous stirring for 6 h. When it was cooled to room temperature, rinsing with deionized (DI) water was performed repeatedly by centrifugation until a neutral pH level was achieved. Afterwards, 4 g of pre-oxidized graphite powder was added to 100 mL of conc. H2SO4 solution in an ice bath. 12 g of potassium permanganate (KMnO4) was then slowly added at 35 C. Stirring was continued for 2 h until the color of the mixture turned to dark brown. Subsequently, 200 mL of DI water and 40 mL of hydrogen peroxide (H₂O₂) solution (30 wt % in water) were added slowly while a violent chemical reaction occurred. The color turned to yellow brown when the reaction was completed. The obtained product was put in a dilute aqueous solution of hydrochloric acid (HCl) to remove metal ions. After ultrasonication for 1 h, rinsing with DI water was repeatedly performed by centrifugation until a neutral pH level was achieved to obtain GO powder.
Fabrication of ATG ternary nanocomposites 50 mg of GO was added to 50 mL of absolute ethanol. Ultrasonication for 1 h was performed to better disperse GO in the solution. 85 mg of silver nitrate (AgNO3) dissolved in 100 mL of DI water was stirred for 1 h. Afterwards, P25 was added into the mixture of GO and AgNO3 solutions in a custom-built reactor surrounded by a water cooling system. After stirring for 10 min, a 350 W xenon lamp 15 cm above the reactor was turned on to initiate the photocatalytic reduction. The irradiation was continued for 2 h. When the reaction was completed, rinsing with DI water was repeated several times by centrifugation to collect the ATG ternary nanocomposites, which were then dried at 80 C for 12 h. The nanocomposites were named as ATG (x:1), in which x:1 represents the weight ratio of TiO2 and GO during the preparation processes (x was
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
for TiO2). There were five ratios (x ¼ 5, 10, 20, 100 and 1000) for comparison.
Photocatalysis The photodegradation of methylene orange (MO) was carried out to evaluate the photocatalytic activity of the ATG photocatalysts. 200 mg of an ATG nanocomposite was dispersed in a 200 mL of MO solution with a concentration of 20 ppm. A 350 W xenon lamp 15 cm above the reactor was used as a solar simulator light source. Prior to irradiation, the mixture was kept in the dark stirring for 20 min to establish an adsorptiondesorption equilibrium. Under irradiation, 8 mL aliquots from each sample were taken out at the desired time intervals, followed by centrifugation and filtration to remove the photocatalyst. The supernatant was used to determine the concentration of residual dye in solution analyzed by recording the characteristic optical absorbance using a UVeVis spectrophotometer. Another reactor with a volume of 65 mL was used for hydrogen production from water splitting. A 300 W xenon lamp was employed as the solar simulator light source. The light luminous intensity was 450.1 mW/cm2, and the irradiated area was 15.9 cm2. The fixed distance between the reactor and lamp was 14 cm. At first, 20 mg of a ternary nanocomposite was dispersed in methanol (20 vol % in water) in the reactor. Prior to irradiation, the solution was degassed for 20 min by high nitrogen gas. Strong stirring was maintained for 6 h during irradiation. The gas generated in the reactor was collected once per hour. It went through a thin stainless steel pipeline and was injected into a gas chromatograph. Then, the amount of hydrogen produced from water splitting could be recorded.
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(Ag/AgCl) electrode and a platinum (Pt) wire served as the reference and auxiliary electrodes, respectively. The scanning rate for CV was 0.05 V s1. The frequency range explored for EIS was 102 Hz to 105 Hz, and the AC amplitude was set as 10 mV between the two electrodes.
Results and discussion XRD patterns are often used to identify the crystal structure of a solid material. Fig. 1a and b shows XRD patterns of GO and the five ATG ternary nanocomposites, respectively. In Fig. 1a, the diffraction peak at approximately 10.5 is a typical characteristic of GO, demonstrating the successful preparation of GO. In Fig. 1b, the peaks at 25.3 , 37.8 , 48.0 , 53.9 , 55.1 , 62.7 and 75.0 can be ascribed to the (101), (004), (200), (105), (211), (204) and (215) planes, respectively, of anatase TiO2. A smaller peak at 27.4 can be ascribed to the (110) plane of rutile TiO2 [20]. Another peak at 44.3 can be indexed to the cubic structure of Ag, corresponding to diffraction from the (200) plane of the Ag NPs. This indicates that Ag NPs were anchored to TG by reducing Agþ ions to Ag successfully and formed ATG nanocomposites. Due to the low content of Ag in the nanocomposites, the intensity of its peak on the XRD patterns is relatively lower compared to other diffraction peaks [9]. The characteristic diffraction peak of graphene supposed to appear at approximately 24.5 , but is not found in Fig. 1b. It is probably shielded by the strong peak of anatase at 25.3 , since
Characterizations The surface morphologies of the nanocomposites were examined by a field emission gun scanning electron microscope (SEM). The microstructure and lattice fringes were observed by a high-resolution transmission electron microscope (HRTEM). The X-ray diffraction (XRD) patterns ranging from 20 to 80 (2q) were collected by an X-ray powder A. The diffractometer with copper Ka radiation of l ¼ 1.5406 angle step was 0.02 , and the scan rate was 2 min1. The absorption spectra ranging from 250 nm to 800 nm were recorded by the UVeVis spectrophotometer. The chemical compositions of the nanocomposites were examined by the Xray photoelectron spectrometer (XPS). According to the binding energies of photoelectrons emitted from the surfaces of the nanocomposites, the chemical states of the elements could be ascertained. The vibrational modes of the molecules could be identified and the chemical structures of the nanocomposites could be thereby determined by Raman spectroscopy. A three-electrode configuration was used for electrochemical measurements, including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) by a potentiostat/galvanostat analyzer using a 0.5 M sodium sulfate (Na2SO4) solution as the electrolyte. The nanocomposite powder was mixed with anhydrous ethanol to form a paste which was coated on the FTO conductive glass by a doctor blade method to form the working electrode. A silver chloride
Fig. 1 e XRD patterns of (a) GO and (b) ATG ternary nanocomposites.
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
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the content of graphene is much lower than TiO2 and the two peaks are very close to each other. The disappearance of the characteristic peak of GO on the XRD patterns of the nanocomposites has also proven that GO was successfully reduced to graphene by photocatalytic reduction. Raman spectroscopy is a widely employed technique to examine the structure and electronic properties of graphene. Fig. 2a and b shows the Raman spectra of ATG ternary nanocomposites ranging from 1000 cm1 to 2000 cm1 and 100 cm1 to 850 cm1, respectively. The two peaks in Fig. 2a are D and G bands. The D band at 1354 cm1 is due to the presence of disorder in sp2-hybridized carbon systems. It can be used to estimate the defect level and the content of impurities in graphene sheets. The G band at 1594 cm1 is derived from the stretching of the sp2-hybridized carboncarbon bonds and is highly sensitive to strain effects in the sp2 system within graphene sheets. Furthermore, the intensity ratio of D and G bands, ID/IG, can be considered as a measure of the relative concentration of local defects or interferences, i.e., it can be used to estimate the changes of sp3 graphite oxide converting to sp2 graphene [21]. Therefore, an
Fig. 2 e Raman spectra of GO and ATG nanocomposites ranging from (a) 1000 cm¡1 to 2000 cm¡1 and (b) 100 cm¡1 to 850 cm¡1.
increment of ID/IG value implies an increase in the number of defects. As displayed in Fig. 2a, the ID/IG value of GO is 1.757. It becomes 1.395, 1.197, 1.430 and 1.340 after the ATG ternary nanocomposites are obtained. The reduced ID/IG values
Fig. 3 e XPS spectra of ATG nanocomposites: (a) Ti 2p, (b) C 1s, and (c) Ag 3d.
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
indicate that the hydroxyl and epoxy groups in GO have been successfully removed to form graphene [21]. However, the ID/ IG value of ATG (1000:1) cannot be calculated because the content of graphene in it is too low and the intensity of the D and G bands are too low. There are five peaks at 147 cm1, 199 cm1, 399 cm1, 515 cm1 and 639 cm1 in Fig. 2b. The corresponding vibrational modes are Eg, Eg, B1g, B1g and Eg of anatase TiO2, respectively [5]. This demonstrates that the TiO2 in the five nanocomposites is mainly anatase. Fig. 3 displays XPS spectra of ATG ternary nanocomposites. The composition of a nanocomposite and the chemical states of the elements can be investigated by XPS. Fig. 3a shows the Ti 2p spectra. The two energy peaks centered at approximately 458.8 eV and 464.4 eV can be ascribed to the Ti 2p3/2 and Ti 2p1/2 spin-orbit splitting states, respectively. They are the corresponding feature of TieO bonding (Ti4þ), which is typical for TiO2 [19,22]. Fig. 3b displays the C 1s spectra. According to the literature, the energy peak centered at approximately 284.5 eV is usually assigned to sp2-hybridized carbon (C]C) atoms, whereas another two weaker peaks at higher binding energies (286 eVe289 eV) can be assigned to oxygenated carbon atoms, such as hydroxyl (CeOH) and carboxyl (HOeC]O) species on the surface [12,23]. Compared to the other four nanocomposites, ATG (5: 1) shows higher
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intensities of the two peaks since more GO was added during its preparation. A longer irradiation time might be required when the photocatalytic reduction approach was used to fabricate graphene with reduced oxygenated carbon atoms. The absence of the peak at approximately 281 eV suggests that no carbon doping in the lattice of TiO2 occurred and no TieC bonds formed between graphene and TiO2 [24]. Fig. 3c shows the Ag 3d spectra. The two energy peaks centered at approximately 367.7 eV and 373.7 eV can be ascribed to the Ag 3d5/2 and Ag 3d3/2 spin-orbit splitting states, respectively. They are typical for zero-valent (neutral) Ag and also demonstrate that all the AgNO3 was reduced to Ag [25]. GO and ATG ternary nanocomposites were dispersed and coated on copper grids with lacey carbon films for SEM observation. Fig. 4 shows the surface morphologies. The typical sheet-like structure of GO can be observed, as shown in Fig. 4a. A large number of TiO2 and Ag NPs were found on the graphene nanosheets, as shown in Fig. 4bef. It was difficult to observe sheet structures by SEM when the ratio of TiO2 in a nanocomposite was larger. The formation of ATG nanocomposites can be confirmed by TEM analysis. Fig. 5 displays the TEM micrographs of ATG (5:1). The ultra-thin transparent membranous structures are observed, as shown in Fig. 5aec, demonstrating the presence of graphene with a crumpled
Fig. 4 e SEM micrographs of (a) GO and ATG (1: x): (b) x ¼ 5, (c) x ¼ 10, (d) x ¼ 20, (e) x ¼ 100, (f) x ¼ 1000. Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
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Fig. 5 e (a), (b), (c) TEM micrographs and (d) HRTEM microstructure of ATG (5:1). layered structure. The spherical and oval TiO2 NPs with diameters varying from several to tens of nm were randomly distributed over the surface of graphene nanosheets. Because the content of Ag is relatively lower and many TiO2 NPs are present in the nanocomposite, it is not easy to distinguish the Ag NPs in Fig. 5a and b. Fortunately, the presence of Ag NPs was indicated by the microstructure shown in Fig. 5d. The smaller black dots decorated on the surface of TiO2 in Fig. 5c are Ag NPs. One lattice fringe spacing value is 0.35 nm, corresponding to the d-spacing of the (101) plane of anatase. Another two lattice spacing values are 0.34 nm and 0.24 nm, corresponding to graphene and the (111) plane of the Ag NPs, respectively. The results have proven the successful preparation of the ATG nanocomposite. Fig. 6 displays the UVeVis absorption spectra of the five ATG nanocomposites. A higher absorption intensity in the visible light region is found as a nanocomposite contains more graphene, which would give rise to a smaller energy bandgap (Eg) than anatase TiO2. This is similar to the bandgap values of other carbonaceous TiO2 materials [26,27]. A wider light absorption range is also beneficial to improve photocatalytic activity. As shown in Fig. 6, ATG (5:1) shows the largest absorbance and a wider absorption range, so it is expected to exhibit the optimum photocatalytic characteristics among the five nanocomposites. The decolorization of dye is often used to evaluate the photocatalytic performance of a photocatalyst. Fig. 7a displays the photodegradation curves of MO by ATG nanocomposites, in which C0 and C represent the MO
concentrations before and after xenon lamp irradiation, respectively. The results revealed that a larger content of graphene contributed to higher photodegradation efficiency. Due to the presence of graphene, both the intensity and range of light absorption are increased. The enhanced light harvesting generates more photogenerated carriers. Electrons are injected to the CB of TiO2 and then transferred through graphene sheets to the Ag NPs, leading to higher electron
Fig. 6 e UVeVis absorption spectra of ATG ternary nanocomposites.
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
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Fig. 7 e (a) Photodegradation curves and (b) corresponding reaction rate constants of ATG ternary nanocomposites.
transport and photodegradation efficiency. According to firstorder kinetics, the corresponding reaction rate constants (k') can be calculated, as displayed in Fig. 7b. ATG (5: 1) exhibits the best photocatalytic activity and largest k'. It can be therefore demonstrated that the most appropriate ratio of TiO2 to graphene for ATG ternary nanocomposites is 5: 1. The oxidation and reduction capabilities of a material can be evaluated by the magnitude of oxidation and reduction potentials obtained by CV tests. Fig. 8a shows the CV curves of the electrodes covered with ATG ternary nanocomposites, where the voltammetric features of the oxidation of Ag on the anodic scan and the reduction of the oxide on the cathodic scan can be observed. Larger oxidation and reduction currents are obtained when the content of graphene is higher. Owing to the high conductivity, the increased current can be attributed to more graphene sheets acting as the platform for electron transport. The electrode with ATG (5:1) has the largest oxidation potential, implying that it acquires electrons most readily (the strongest oxidizer). It exhibits the largest reduction potential as well, indicating that it is least likely to lose electrons (the weakest reductant). Recently, the EIS method has been widely used to analyze the nature of charge transfer at the interface of a material. A semicircle in a Nyquist plot is related to charge transfer impedance at the interface.
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Fig. 8 e (a) CV curves and (b) Nyquist plots of the electrodes covered with ATG ternary nanocomposites.
According to the radius of the semicircle, the charge transfer rate on each interface can be compared. Fig. 8b shows the Nyquist plots of the electrodes covered with various ATG ternary nanocomposites obtained by EIS analysis. The semicircle of the plot for ATG (5:1) exhibits the smallest radius, implying that the smallest interfacial impedance contributed to more efficient electron transport. It is also found that, with a reduced content of graphene in an ATG nanocomposite, the radius becomes larger and the interfacial impedance increases. As electron transport becomes faster, charge recombination is restrained and higher photocatalytic activity is achieved. The CV and EIS results demonstrate that more graphene is indeed beneficial to enhance the photocatalytic activity of an ATG nanocomposite. Under the irradiation of a 300 W xenon lamp, the hydrogen evolution curves of the ATG ternary nanocomposites are plotted, as displayed in Fig. 9a. A methanol solution was used as the sacrificial agent. As predicted, a larger hydrogen evolution rate can be obtained when more graphene is contained in a nanocomposite. ATG (5:1) shows the largest amount of hydrogen production of 778 mmol g1 after 6 h of photocatalytic water splitting. Compared to ATG (1000:1), there is an over four-fold difference in the amount of hydrogen production. A
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
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Fig. 9 e (a) Hydrogen evolution curves, (b) mass-normalized evolution rates and quantum efficiencies of ATG ternary nanocomposites.
higher graphene content contributes to not only the photodegradation of dye but also to hydrogen production. To have better comprehension of these effects, the corresponding mass-normalized evolution rates and apparent quantum efficiencies (QE) are calculated and shown in Fig. 9b. According to the literature [28,29], QE can be calculated after obtaining the mass-normalized evolution rate: QEð%Þ ¼ ¼
Benzoquinone (BQ), ammonium oxalate (AO) and tert-butyl alcohol (TBA) were used for capturing superoxide radicals (O2), holes (hþ) and hydroxyl radicals (OH), respectively. After 2 h of irradiation, the inactivation of ATG (5:1) with the degradation efficiency of only 6.57% was remarkably caused by BQ. On the other hand, the degradation efficiency was 26.9% when AO was used. Even if there was not much reduction in degradation efficiency, TBA is still considered to be able to somewhat deactivate ATG (5:1). The results have revealed that the irradiated O2 and photo-induced hþ contribute most to the ATG nanocomposite photocatalytic system, and OH is considered to be of lesser importance. Based on the aforementioned results and by referring to the literature [16], a possible photocatalytic mechanism of ATG ternary nanocomposites can be proposed, as depicted in Fig. 11. A Schottky junction is formed between TiO2 and Ag. Since the Fermi energy level (EF) of TiO2 is higher than that of Ag, the Schottky barrier would hinder electrons at the boundary of Ag from transferring to TiO2. However, thanks to the SPR effect, which has been observed in binary composite systems such as AgeTiO2 [30,31], electrons can actually be transferred from Ag NPs to TiO2. When the SPR effect induces the collective oscillation of electrons and thus interband excitation, electrons gain sufficient energy to move to the junction and overcome the Schottky barrier. Once electrons are transferred to the CB of TiO2 due to the SPR effect, they capture oxygen (O2) molecules adsorbed on the surface of TiO2, and O2 radicals are produced. They can also be transferred from the CB of TiO2 to graphene sheets, and charge separation occurs as O2 molecules adsorbed on graphene accept the electrons. If GO is not completely reduced to graphene, the defects obstruct charge separation, and the nanocomposite will show inferior photocatalytic activity. The hþ in the Ag NPs can also accept electrons from water molecules (H2O) adsorbed on the TiO2 surface, hydroxide ions or dye molecules in solution. OH radicals are thereby produced, and organics can be decomposed to H2O and carbon dioxide (CO2). The mechanism has illustrated that hþ, O2 and OH are all active species involved in the photodegradation process.
number of reacted electrons 100 number of incident photons number of evolved H molecules 2 100 number of incident photons
The QE and mass-normalized hydrogen evolution rate for ATG (5:1) are 4.8% and 129.5 mmol g1 h1, respectively. Nevertheless, those for ATG (1000:1) are 1.0% and 27.1 mmol g1 h1, respectively. Apparently, a higher ratio of graphene contributes to higher QE. More graphene contained in a nanocomposite increases the adsorption area, improves charge transport, lowers recombination probability and enhances hydrogen production efficiency. Hole and radical trapping experiments were carried out to further elucidate the photocatalytic reaction mechanism. The degradation of MO was observed to investigate the main reactive species involved in the photocatalytic process, as displayed in Fig. 10. Three scavengers with a concentration of 20 ppm were selected for the trapping experiments [6,12].
Fig. 10 e Hole and radical trapping experiments. Photocatalytic degradation of MO in the presence of different scavengers.
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
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Fig. 11 e Schematic photocatalytic mechanism of ATG ternary nanocomposites.
Conclusions ATG ternary nanocomposites with various contents of graphene were fabricated and used as photocatalysts. The TiO2 in the nanocomposites was mainly anatase, and graphene was successfully obtained from GO by a photocatalytic reduction method. TiO2 NPs decorated with smaller Ag NPs were observed to be distributed over the surface of graphene nanosheets. The formation of ATG nanocomposites was confirmed by TEM analysis. As nanocomposites contained a large amount of graphene, a wide range and high intensity of light absorption that were beneficial to improving photocatalytic activity were found. Among the five ternary nanocomposites, ATG (5: 1) exhibited the highest efficiency of photodegradation. Therefore, the most appropriate ratio of TiO2 to graphene in an ATG nanocomposite was deduced to be 5: 1. The CV and EIS results demonstrated that a higher content of graphene was beneficial to increased conductivity and reduced interfacial impedance. As electron transport was more efficient, charge recombination was restrained and a superior photocatalytic performance was achieved. ATG (5:1) also exhibited the largest amount of hydrogen production from water splitting. Its QE and mass-normalized hydrogen evolution rate were 4.8% and 129.5 mmol g1$h1, respectively. This study demonstrated that an appropriate content of graphene contributed not only to dye photodegradation but also to hydrogen evolution. From the results of hole and radical trapping experiments, the photocatalytic mechanism of an ATG ternary nanocomposite was proposed. It has been verified that hþ, O2 and OH are all active species involved in the photodegradation process.
Acknowledgements We are grateful for the support from the Ministry of Science and Technology Taiwan and National Chi Nan University.
references
[1] Liu C, Yang D, Jiao Y, Tian Y, Wang Y, Jiang Z. Biomimetic synthesis of TiO2-SiO2-Ag nanocomposites with enhanced visible-light photocatalytic activity. ACS Appl Mater Interfaces 2013;5:3824e32. [2] Xiang QJ, Yu JG, Jaroniec M. Graphene-based semiconductor photocatalysts. Chem Soc Rev 2012;41:782e96. [3] Tong H, Ouyang S, Bi Y, Umezawa N, Oshikiri M, Ye J. Nanophotocatalytic materials: possibilities and challenges. Adv Mater 2012;24:229e51. [4] Kubacka A, Fernandez-Garcia M, Colon G. Advanced nanoarchitectures for solar photocatalytic applications. Chem Rev 2012;112:1555e614. [5] Zhao Y, Sun L, Xi M, Feng Q, Jiang C, Fong H. Electrospun TiO2 nanofelt surface-decorated with Ag nanoparticles as sensitive and UV-cleanable substrate for surface enhanced Raman scattering. ACS Appl Mater Interfaces 2014;6:5759e67. [6] Yang MQ, Zhang N, Xu YJ. Synthesis of fullerene-, carbon nanotube, and graphene-TiO2 nanocomposite photocatalysts for selective oxidation: a comparative study. ACS Appl Mater Interfaces 2013;5:1156e64. [7] Mou Z, Wu Y, Sun J, Yang P, Du Y, Lu C. TiO2 nanoparticlesfunctionalized N-doped graphene with superior interfacial contact and enhanced charge separation for photocatalytic hydrogen generation. ACS Appl Mater Interfaces 2014;6:13798e806. [8] Yan Y, Wang C, Yan X, Xiao L, He J, Gu W, et al. Graphene acting as surface phase junction in anatase-graphene-rutile heterojunction photocatalysts for H2 production from water splitting. J Phys Chem C 2014;118:23519e26. [9] Xu T, Zhang L, Cheng H, Zhu Y. UV-assisted photocatalytic synthesis of TiO2-reduced graphene oxide with enhanced photocatalytic activity in decomposition of sarin in gas phase. Appl Catal B Environ 2011;101:382e7. [10] Yang Y, Liu E, Dai H, Kang L, Wu H, Fan J, et al. Photocatalytic activity of Ag-TiO2-graphene ternary nanocomposites and application in hydrogen evolution by water splitting. Int J Hydrogen Energy 2014;39:7664e71. [11] Hsu KC, Chen DH. Highly sensitive, uniform, and reusable surface-enhanced Raman scattering substrate with TiO2
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
10
[12]
[13]
[14]
[15]
[16]
[17] [18]
[19]
[20]
[21]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
interlayer between Ag nanoparticles and reduced graphene oxide. ACS Appl Mater Interfaces 2015;49:27571e9. Yang X, Qin J, Jiang Y, Chen K, Yan X, Zhang D, et al. Fabrication of P25/Ag3PO4/graphene oxide heterostructures for enhanced solar photocatalytic degradation of organic pollutants and bacteria. Appl Catal B Environ 2015;166e167:231e40. Wu F, Fan J, Hu X, Liu E, Sun T, Kang L, et al. Photocatalytic activity of Ag/TiO2 nanotube arrays enhanced by surface plasmon resonance and application in hydrogen evolution by water splitting. Plasmonics 2012;8:501e8. ar B, Sturm Logar M, Janc S, Suvorov D. Weak polyion multilayer-assisted in situ synthesis as a route toward a plasmonic Ag/TiO2 photocatalyst. Langmuir 2010;26:12215e24. Awazu K, Fujimaki M, Rockstuhl C, Tominaga J, Murakami H, Ohki Y, et al. A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide. J Am Chem Soc 2008;130:1676e80. Christopher P, Ingram DB, Linic S. Enhancing photochemical activity of semiconductor nanoparticles with optically active Ag nanostructures: photochemistry mediated by Ag surface plasmons. J Phys Chem C 2010;114:9173e7. Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80. 1339e9. Kovtyukhova NI, Ollivier PJ, Martin BR, Mallouk TE, Chizhik SA, Buzaneva EV, et al. Layer-by-layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations. Chem Mater 1999;11:771e8. Wen Y, Ding H, Shan Y. Preparation and visible light photocatalytic activity of Ag/TiO2/graphene nanocomposite. Nanoscale 2011;3:4411e7. Xu YJ, Zhuang Y, Fu X. New insight for enhanced photocatalytic activity of TiO2 by doping carbon nanotubes: a case study on degradation of benzene and methyl orange. J Phys Chem C 2010;114:2669e76. Zhang Y, Zhang N, Tang ZR, Xu YJ. Improving the photocatalytic performance of graphene-TiO2 nanocomposites via a combined strategy of decreasing
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
defects of graphene and increasing interfacial contact. Phys Chem Chem Phys 2012;14:9167e75. McCafferty E, Wightman JP. Determination of the concentration of surface hydroxyl groups on metal oxide films by a quantitative XPS method. Surf Interface Anal 1998;26:549e64. Yumitori S. Correlation of C1s chemical state intensities with the O1s intensity in the XPS analysis of anodically oxidized glass-like carbon samples. J Mater Sci 2000;35:139e46. Irie H, Watanabe Y, Hashimoto K. Carbon-doped anatase TiO2 powders as a visible-light sensitive photocatalyst. Chem Lett 2003;32:772e3. Sher Shah MS, Zhang K, Park AR, Kim KS, Park NG, Park JH, et al. Single-step solvothermal synthesis of mesoporous AgTiO2-reduced graphene oxide ternary composites with enhanced photocatalytic activity. Nanoscale 2013;5:5093e101. Ren W, Ai Z, Jia F, Zhang L, Fan X, Zou Z. Low temperature preparation and visible light photocatalytic activity of mesoporous carbon-doped crystalline TiO2. Appl Catal B 2007;69:138e44. Pan X, Yang MQ, Tang ZR, Xu YJ. Non-covalently functionalized graphene-directed synthesis of ultralarge graphene-based TiO2 nanosheet composites: tunable morphology and photocatalytic applications. J Phys Chem C 2014;118:27325e35. Phan TDN, Luo S, Liu Z, Gamalski AD, Tao J, Xu W, et al. Striving toward noble-metal-free photocatalytic water splitting: the hydrogenated-graphene-TiO2 prototype. Chem Mater 2015;27:6282e96. Braslavsky SE, Braun AM, Cassano AE, Emeline AV, Litter MI, Palmisano L, et al. Glossary of terms used in photocatalysis and radiation catalysis. Pure Appl Chem 2011;83:931e1014. Furube A, Du L, Hara K, Katoh R, Tachiya M. Ultrafast plasmon-induced electron transfer from gold nanodots into TiO2 nanoparticles. J Am Chem Soc 2007;129:14852e3. Tian Y, Tatsuma T. Mechanisms and applications of plasmon-induced charge separation at TiO2 films loaded with gold nanoparticles. J Am Chem Soc 2005;127:7632e7.
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Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts Fu-Jye Sheu, Chun-Pei Cho* Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou County 54561, Taiwan, ROC
article info
abstract
Article history:
AgeTiO2-graphene ternary nanocomposites with varying graphene contents were fabri-
Received 30 September 2016
cated by photocatalytic reduction. SEM and TEM imaging of the nanocomposites showed
Received in revised form
that TiO2 nanoparticles decorated with Ag nanoparticles covered graphene nanosheets. A
18 May 2017
higher content of graphene was shown to be favorable for dye photodegradation. The re-
Accepted 4 June 2017
sults of electrochemical analysis revealed that a higher graphene content contributed to
Available online xxx
increased conductivity and reduced interfacial impedance, which led to more efficient electron transport and thus higher photocatalytic activity. The highest efficiency in dye
Keywords:
photodegradation and hydrogen production from water splitting was achieved when the
Graphene
ratio of TiO2 to graphene in the nanocomposite was 5: 1. The corresponding mass-
Ternary nanocomposite
normalized hydrogen evolution rate and quantum efficiency were 129.5 mmol g1 h1
Photodegradation
and 4.8%, respectively. A mechanism for photocatalysis was proposed and discussed. This
Hydrogen evolution
study demonstrates that the AgeTiO2-graphene ternary nanocomposite could be a prom-
Hydrogen production
ising photocatalyst. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Titanium dioxide (TiO2) is the most common semiconductor material applied in photocatalysts and dye-sensitized solar cells (DSSCs) due to its non-toxicity, thermal and chemical stability, low cost, high availability and lack of photocorrosion. However, the fast recombination of photogenerated electron-hole pairs and absence of optical response to visible light have limited its photocatalytic activity [1]. Over the past decade, many efforts have been made to extend the optical absorption of TiO2. Despite the development of photocatalysts with highly efficient visible-light response,
potential applications in photodegradation and hydrogen production from water splitting still remains a challenge [2e4]. TiO2 can be mixed with noble metallic nanoparticles (NPs) or carbonaceous materials to obtain binary nanocomposites for the aforementioned purposes. C. Liu et al. reported that doping TiO2 with silver (Ag) NPs increased visiblelight absorption. The greatly enhanced photocatalytic activity was favorable to the degradation of organic pollutants [1]. Y. Zhao et al. fabricated Ag-decorated TiO2 nanofelts which could achieve self-cleaning under ultraviolet (UV) irradiation. These obtained low-cost nanocomposites could be utilized repeatedly [5]. M. Q. Yang et al. utilized fullerene, carbon nanotubes and graphene to fabricate binary nanocomposites.
* Corresponding author. E-mail address: [email protected] (C.-P. Cho). http://dx.doi.org/10.1016/j.ijhydene.2017.06.024 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
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They found that, for every nanocomposite, there would be an optimum mixing ratio between TiO2 and a carbonaceous material [6]. Z. Mou et al. used nitrogen (N) doping to reduce defects in graphene. The improved interfacial contact and charge separation resulted in better photocatalytic activity and durability of the functionalized N-doped TiO2-graphene (TG) nanocomposite when it was applied to hydrogen production [7]. Y. Yan et al. tuned the ratio of anatase and rutile to form anatase-graphene-rutile heterostructures. The highest hydrogen production efficiency was achieved when the ratio was 7:3 [8]. From the above-mentioned studies, it is perceived that the combination of TiO2 and graphene can enhance light absorption and photocatalytic activity. Graphene is a twodimensional single-layer carbonaceous material with chemical stability, transparency, high specific surface area, and superior charge transport properties [9]. It plays different roles, such as being an adsorbent, electron acceptor, and the platform for charge transport when applied to the fabrication of composites, biochemical detection, nanoelectronic devices and solar cells. While there have been quite a few studies concerning binary nanocomposites used as photocatalysts, less attention has been paid to ternary nanocomposites containing graphene. Y. Yang et al. used hydrothermal and microwave-assisted methods to prepare AgeTiO2-graphene (ATG) ternary nanocomposites showing higher efficiency for the photodegradation of dye and hydrogen production from water splitting than that of TG binary nanocomposites [10]. K. C. Hsu et al. fabricated reusable AgeTiO2-reduced graphene oxide (rGO) substrates with high sensitivity and self-cleaning surfaces. The higher intensity of Raman signals was ascribed to surface enhanced Raman scattering due to the presence of the Ag NPs [11]. X. Yang et al. prepared stable P25e Ag3PO4-GO ternary nanocomposites with good photocatalytic characteristics. The use of Ag3PO4 increased the absorption of visible light, facilitated charge separation and reduced the recombination probability. By varying the amount of GO and the molar ratio of Ag3PO4, the capabilities of photodegradation and bacterial inactivation could be adjusted [12]. It has been indicated by these reports that ternary nanocomposites applicable as photocatalysts are the latest trend. Thus, in this study, we are interested in whether photocatalytic activity can be further enhanced by ternary nanocomposites. It has been demonstrated that Ag NPs assembled on TiO2 nanotube arrays were favorable to hydrogen evolution owing to the surface plasmon resonance (SPR) effect [13e16]. When noble metallic NPs are excited by light, a strong oscillation of the conduction electrons occurs if the frequency of the incident photon is resonant with the collective oscillation of electrons. By altering the dielectric environment, the size and shape of the NPs, their absorption and scattering characteristics can be flexibly tuned. Localized SPR of noble metallic NPs usually results in strong and broad absorption bands in the visible light region. Therefore, this feature can be exploited to develop visible-light activated photocatalysts. In this study, ATG ternary nanocomposites were obtained by photocatalytic reduction, an approach with more simplified procedures requiring only 2 h and completed in one pot. Compared to other methods for obtaining ternary nanocomposites [10e12], the total duration of fabrication was
considerably shortened. The SPR effect caused by Ag NPs was favorable to extend the optical response to a wider wavelength. Due to the excellent electrical properties, the presence of graphene not only accelerated the separation of photogenerated electron-hole pairs but also enhanced the adsorption. The ratio of graphene in the nanocomposites was adjusted to investigate its impacts on dye photodegradation, electrochemical characteristics and hydrogen evolution. Efforts have been aimed to investigate the most appropriate ratio of TiO2 to graphene in an ATG nanocomposite fabricated with a fixed amount of Ag and to achieve higher photocatalytic performance. It has been demonstrated that a higher content of graphene would be conducive to photocatalytic activity.
Experimental Preparation of GO GO was synthesized from graphite powder by the modified Hummers' method [17]. A previous graphite oxidation procedure was carried out before the synthesis of GO [18,19]. 4 g of graphite powder was added into a solution composed of 2 g of potassium persulfate (K₂S₂O₈), 2 g of phosphorus pentoxide (P₂O5) and 30 mL of conc. sulfuric acid (H2SO4). The mixture solution was heated to 80 C under continuous stirring for 6 h. When it was cooled to room temperature, rinsing with deionized (DI) water was performed repeatedly by centrifugation until a neutral pH level was achieved. Afterwards, 4 g of pre-oxidized graphite powder was added to 100 mL of conc. H2SO4 solution in an ice bath. 12 g of potassium permanganate (KMnO4) was then slowly added at 35 C. Stirring was continued for 2 h until the color of the mixture turned to dark brown. Subsequently, 200 mL of DI water and 40 mL of hydrogen peroxide (H₂O₂) solution (30 wt % in water) were added slowly while a violent chemical reaction occurred. The color turned to yellow brown when the reaction was completed. The obtained product was put in a dilute aqueous solution of hydrochloric acid (HCl) to remove metal ions. After ultrasonication for 1 h, rinsing with DI water was repeatedly performed by centrifugation until a neutral pH level was achieved to obtain GO powder.
Fabrication of ATG ternary nanocomposites 50 mg of GO was added to 50 mL of absolute ethanol. Ultrasonication for 1 h was performed to better disperse GO in the solution. 85 mg of silver nitrate (AgNO3) dissolved in 100 mL of DI water was stirred for 1 h. Afterwards, P25 was added into the mixture of GO and AgNO3 solutions in a custom-built reactor surrounded by a water cooling system. After stirring for 10 min, a 350 W xenon lamp 15 cm above the reactor was turned on to initiate the photocatalytic reduction. The irradiation was continued for 2 h. When the reaction was completed, rinsing with DI water was repeated several times by centrifugation to collect the ATG ternary nanocomposites, which were then dried at 80 C for 12 h. The nanocomposites were named as ATG (x:1), in which x:1 represents the weight ratio of TiO2 and GO during the preparation processes (x was
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
for TiO2). There were five ratios (x ¼ 5, 10, 20, 100 and 1000) for comparison.
Photocatalysis The photodegradation of methylene orange (MO) was carried out to evaluate the photocatalytic activity of the ATG photocatalysts. 200 mg of an ATG nanocomposite was dispersed in a 200 mL of MO solution with a concentration of 20 ppm. A 350 W xenon lamp 15 cm above the reactor was used as a solar simulator light source. Prior to irradiation, the mixture was kept in the dark stirring for 20 min to establish an adsorptiondesorption equilibrium. Under irradiation, 8 mL aliquots from each sample were taken out at the desired time intervals, followed by centrifugation and filtration to remove the photocatalyst. The supernatant was used to determine the concentration of residual dye in solution analyzed by recording the characteristic optical absorbance using a UVeVis spectrophotometer. Another reactor with a volume of 65 mL was used for hydrogen production from water splitting. A 300 W xenon lamp was employed as the solar simulator light source. The light luminous intensity was 450.1 mW/cm2, and the irradiated area was 15.9 cm2. The fixed distance between the reactor and lamp was 14 cm. At first, 20 mg of a ternary nanocomposite was dispersed in methanol (20 vol % in water) in the reactor. Prior to irradiation, the solution was degassed for 20 min by high nitrogen gas. Strong stirring was maintained for 6 h during irradiation. The gas generated in the reactor was collected once per hour. It went through a thin stainless steel pipeline and was injected into a gas chromatograph. Then, the amount of hydrogen produced from water splitting could be recorded.
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(Ag/AgCl) electrode and a platinum (Pt) wire served as the reference and auxiliary electrodes, respectively. The scanning rate for CV was 0.05 V s1. The frequency range explored for EIS was 102 Hz to 105 Hz, and the AC amplitude was set as 10 mV between the two electrodes.
Results and discussion XRD patterns are often used to identify the crystal structure of a solid material. Fig. 1a and b shows XRD patterns of GO and the five ATG ternary nanocomposites, respectively. In Fig. 1a, the diffraction peak at approximately 10.5 is a typical characteristic of GO, demonstrating the successful preparation of GO. In Fig. 1b, the peaks at 25.3 , 37.8 , 48.0 , 53.9 , 55.1 , 62.7 and 75.0 can be ascribed to the (101), (004), (200), (105), (211), (204) and (215) planes, respectively, of anatase TiO2. A smaller peak at 27.4 can be ascribed to the (110) plane of rutile TiO2 [20]. Another peak at 44.3 can be indexed to the cubic structure of Ag, corresponding to diffraction from the (200) plane of the Ag NPs. This indicates that Ag NPs were anchored to TG by reducing Agþ ions to Ag successfully and formed ATG nanocomposites. Due to the low content of Ag in the nanocomposites, the intensity of its peak on the XRD patterns is relatively lower compared to other diffraction peaks [9]. The characteristic diffraction peak of graphene supposed to appear at approximately 24.5 , but is not found in Fig. 1b. It is probably shielded by the strong peak of anatase at 25.3 , since
Characterizations The surface morphologies of the nanocomposites were examined by a field emission gun scanning electron microscope (SEM). The microstructure and lattice fringes were observed by a high-resolution transmission electron microscope (HRTEM). The X-ray diffraction (XRD) patterns ranging from 20 to 80 (2q) were collected by an X-ray powder A. The diffractometer with copper Ka radiation of l ¼ 1.5406 angle step was 0.02 , and the scan rate was 2 min1. The absorption spectra ranging from 250 nm to 800 nm were recorded by the UVeVis spectrophotometer. The chemical compositions of the nanocomposites were examined by the Xray photoelectron spectrometer (XPS). According to the binding energies of photoelectrons emitted from the surfaces of the nanocomposites, the chemical states of the elements could be ascertained. The vibrational modes of the molecules could be identified and the chemical structures of the nanocomposites could be thereby determined by Raman spectroscopy. A three-electrode configuration was used for electrochemical measurements, including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) by a potentiostat/galvanostat analyzer using a 0.5 M sodium sulfate (Na2SO4) solution as the electrolyte. The nanocomposite powder was mixed with anhydrous ethanol to form a paste which was coated on the FTO conductive glass by a doctor blade method to form the working electrode. A silver chloride
Fig. 1 e XRD patterns of (a) GO and (b) ATG ternary nanocomposites.
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
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the content of graphene is much lower than TiO2 and the two peaks are very close to each other. The disappearance of the characteristic peak of GO on the XRD patterns of the nanocomposites has also proven that GO was successfully reduced to graphene by photocatalytic reduction. Raman spectroscopy is a widely employed technique to examine the structure and electronic properties of graphene. Fig. 2a and b shows the Raman spectra of ATG ternary nanocomposites ranging from 1000 cm1 to 2000 cm1 and 100 cm1 to 850 cm1, respectively. The two peaks in Fig. 2a are D and G bands. The D band at 1354 cm1 is due to the presence of disorder in sp2-hybridized carbon systems. It can be used to estimate the defect level and the content of impurities in graphene sheets. The G band at 1594 cm1 is derived from the stretching of the sp2-hybridized carboncarbon bonds and is highly sensitive to strain effects in the sp2 system within graphene sheets. Furthermore, the intensity ratio of D and G bands, ID/IG, can be considered as a measure of the relative concentration of local defects or interferences, i.e., it can be used to estimate the changes of sp3 graphite oxide converting to sp2 graphene [21]. Therefore, an
Fig. 2 e Raman spectra of GO and ATG nanocomposites ranging from (a) 1000 cm¡1 to 2000 cm¡1 and (b) 100 cm¡1 to 850 cm¡1.
increment of ID/IG value implies an increase in the number of defects. As displayed in Fig. 2a, the ID/IG value of GO is 1.757. It becomes 1.395, 1.197, 1.430 and 1.340 after the ATG ternary nanocomposites are obtained. The reduced ID/IG values
Fig. 3 e XPS spectra of ATG nanocomposites: (a) Ti 2p, (b) C 1s, and (c) Ag 3d.
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
indicate that the hydroxyl and epoxy groups in GO have been successfully removed to form graphene [21]. However, the ID/ IG value of ATG (1000:1) cannot be calculated because the content of graphene in it is too low and the intensity of the D and G bands are too low. There are five peaks at 147 cm1, 199 cm1, 399 cm1, 515 cm1 and 639 cm1 in Fig. 2b. The corresponding vibrational modes are Eg, Eg, B1g, B1g and Eg of anatase TiO2, respectively [5]. This demonstrates that the TiO2 in the five nanocomposites is mainly anatase. Fig. 3 displays XPS spectra of ATG ternary nanocomposites. The composition of a nanocomposite and the chemical states of the elements can be investigated by XPS. Fig. 3a shows the Ti 2p spectra. The two energy peaks centered at approximately 458.8 eV and 464.4 eV can be ascribed to the Ti 2p3/2 and Ti 2p1/2 spin-orbit splitting states, respectively. They are the corresponding feature of TieO bonding (Ti4þ), which is typical for TiO2 [19,22]. Fig. 3b displays the C 1s spectra. According to the literature, the energy peak centered at approximately 284.5 eV is usually assigned to sp2-hybridized carbon (C]C) atoms, whereas another two weaker peaks at higher binding energies (286 eVe289 eV) can be assigned to oxygenated carbon atoms, such as hydroxyl (CeOH) and carboxyl (HOeC]O) species on the surface [12,23]. Compared to the other four nanocomposites, ATG (5: 1) shows higher
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intensities of the two peaks since more GO was added during its preparation. A longer irradiation time might be required when the photocatalytic reduction approach was used to fabricate graphene with reduced oxygenated carbon atoms. The absence of the peak at approximately 281 eV suggests that no carbon doping in the lattice of TiO2 occurred and no TieC bonds formed between graphene and TiO2 [24]. Fig. 3c shows the Ag 3d spectra. The two energy peaks centered at approximately 367.7 eV and 373.7 eV can be ascribed to the Ag 3d5/2 and Ag 3d3/2 spin-orbit splitting states, respectively. They are typical for zero-valent (neutral) Ag and also demonstrate that all the AgNO3 was reduced to Ag [25]. GO and ATG ternary nanocomposites were dispersed and coated on copper grids with lacey carbon films for SEM observation. Fig. 4 shows the surface morphologies. The typical sheet-like structure of GO can be observed, as shown in Fig. 4a. A large number of TiO2 and Ag NPs were found on the graphene nanosheets, as shown in Fig. 4bef. It was difficult to observe sheet structures by SEM when the ratio of TiO2 in a nanocomposite was larger. The formation of ATG nanocomposites can be confirmed by TEM analysis. Fig. 5 displays the TEM micrographs of ATG (5:1). The ultra-thin transparent membranous structures are observed, as shown in Fig. 5aec, demonstrating the presence of graphene with a crumpled
Fig. 4 e SEM micrographs of (a) GO and ATG (1: x): (b) x ¼ 5, (c) x ¼ 10, (d) x ¼ 20, (e) x ¼ 100, (f) x ¼ 1000. Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
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Fig. 5 e (a), (b), (c) TEM micrographs and (d) HRTEM microstructure of ATG (5:1). layered structure. The spherical and oval TiO2 NPs with diameters varying from several to tens of nm were randomly distributed over the surface of graphene nanosheets. Because the content of Ag is relatively lower and many TiO2 NPs are present in the nanocomposite, it is not easy to distinguish the Ag NPs in Fig. 5a and b. Fortunately, the presence of Ag NPs was indicated by the microstructure shown in Fig. 5d. The smaller black dots decorated on the surface of TiO2 in Fig. 5c are Ag NPs. One lattice fringe spacing value is 0.35 nm, corresponding to the d-spacing of the (101) plane of anatase. Another two lattice spacing values are 0.34 nm and 0.24 nm, corresponding to graphene and the (111) plane of the Ag NPs, respectively. The results have proven the successful preparation of the ATG nanocomposite. Fig. 6 displays the UVeVis absorption spectra of the five ATG nanocomposites. A higher absorption intensity in the visible light region is found as a nanocomposite contains more graphene, which would give rise to a smaller energy bandgap (Eg) than anatase TiO2. This is similar to the bandgap values of other carbonaceous TiO2 materials [26,27]. A wider light absorption range is also beneficial to improve photocatalytic activity. As shown in Fig. 6, ATG (5:1) shows the largest absorbance and a wider absorption range, so it is expected to exhibit the optimum photocatalytic characteristics among the five nanocomposites. The decolorization of dye is often used to evaluate the photocatalytic performance of a photocatalyst. Fig. 7a displays the photodegradation curves of MO by ATG nanocomposites, in which C0 and C represent the MO
concentrations before and after xenon lamp irradiation, respectively. The results revealed that a larger content of graphene contributed to higher photodegradation efficiency. Due to the presence of graphene, both the intensity and range of light absorption are increased. The enhanced light harvesting generates more photogenerated carriers. Electrons are injected to the CB of TiO2 and then transferred through graphene sheets to the Ag NPs, leading to higher electron
Fig. 6 e UVeVis absorption spectra of ATG ternary nanocomposites.
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
Fig. 7 e (a) Photodegradation curves and (b) corresponding reaction rate constants of ATG ternary nanocomposites.
transport and photodegradation efficiency. According to firstorder kinetics, the corresponding reaction rate constants (k') can be calculated, as displayed in Fig. 7b. ATG (5: 1) exhibits the best photocatalytic activity and largest k'. It can be therefore demonstrated that the most appropriate ratio of TiO2 to graphene for ATG ternary nanocomposites is 5: 1. The oxidation and reduction capabilities of a material can be evaluated by the magnitude of oxidation and reduction potentials obtained by CV tests. Fig. 8a shows the CV curves of the electrodes covered with ATG ternary nanocomposites, where the voltammetric features of the oxidation of Ag on the anodic scan and the reduction of the oxide on the cathodic scan can be observed. Larger oxidation and reduction currents are obtained when the content of graphene is higher. Owing to the high conductivity, the increased current can be attributed to more graphene sheets acting as the platform for electron transport. The electrode with ATG (5:1) has the largest oxidation potential, implying that it acquires electrons most readily (the strongest oxidizer). It exhibits the largest reduction potential as well, indicating that it is least likely to lose electrons (the weakest reductant). Recently, the EIS method has been widely used to analyze the nature of charge transfer at the interface of a material. A semicircle in a Nyquist plot is related to charge transfer impedance at the interface.
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Fig. 8 e (a) CV curves and (b) Nyquist plots of the electrodes covered with ATG ternary nanocomposites.
According to the radius of the semicircle, the charge transfer rate on each interface can be compared. Fig. 8b shows the Nyquist plots of the electrodes covered with various ATG ternary nanocomposites obtained by EIS analysis. The semicircle of the plot for ATG (5:1) exhibits the smallest radius, implying that the smallest interfacial impedance contributed to more efficient electron transport. It is also found that, with a reduced content of graphene in an ATG nanocomposite, the radius becomes larger and the interfacial impedance increases. As electron transport becomes faster, charge recombination is restrained and higher photocatalytic activity is achieved. The CV and EIS results demonstrate that more graphene is indeed beneficial to enhance the photocatalytic activity of an ATG nanocomposite. Under the irradiation of a 300 W xenon lamp, the hydrogen evolution curves of the ATG ternary nanocomposites are plotted, as displayed in Fig. 9a. A methanol solution was used as the sacrificial agent. As predicted, a larger hydrogen evolution rate can be obtained when more graphene is contained in a nanocomposite. ATG (5:1) shows the largest amount of hydrogen production of 778 mmol g1 after 6 h of photocatalytic water splitting. Compared to ATG (1000:1), there is an over four-fold difference in the amount of hydrogen production. A
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
Fig. 9 e (a) Hydrogen evolution curves, (b) mass-normalized evolution rates and quantum efficiencies of ATG ternary nanocomposites.
higher graphene content contributes to not only the photodegradation of dye but also to hydrogen production. To have better comprehension of these effects, the corresponding mass-normalized evolution rates and apparent quantum efficiencies (QE) are calculated and shown in Fig. 9b. According to the literature [28,29], QE can be calculated after obtaining the mass-normalized evolution rate: QEð%Þ ¼ ¼
Benzoquinone (BQ), ammonium oxalate (AO) and tert-butyl alcohol (TBA) were used for capturing superoxide radicals (O2), holes (hþ) and hydroxyl radicals (OH), respectively. After 2 h of irradiation, the inactivation of ATG (5:1) with the degradation efficiency of only 6.57% was remarkably caused by BQ. On the other hand, the degradation efficiency was 26.9% when AO was used. Even if there was not much reduction in degradation efficiency, TBA is still considered to be able to somewhat deactivate ATG (5:1). The results have revealed that the irradiated O2 and photo-induced hþ contribute most to the ATG nanocomposite photocatalytic system, and OH is considered to be of lesser importance. Based on the aforementioned results and by referring to the literature [16], a possible photocatalytic mechanism of ATG ternary nanocomposites can be proposed, as depicted in Fig. 11. A Schottky junction is formed between TiO2 and Ag. Since the Fermi energy level (EF) of TiO2 is higher than that of Ag, the Schottky barrier would hinder electrons at the boundary of Ag from transferring to TiO2. However, thanks to the SPR effect, which has been observed in binary composite systems such as AgeTiO2 [30,31], electrons can actually be transferred from Ag NPs to TiO2. When the SPR effect induces the collective oscillation of electrons and thus interband excitation, electrons gain sufficient energy to move to the junction and overcome the Schottky barrier. Once electrons are transferred to the CB of TiO2 due to the SPR effect, they capture oxygen (O2) molecules adsorbed on the surface of TiO2, and O2 radicals are produced. They can also be transferred from the CB of TiO2 to graphene sheets, and charge separation occurs as O2 molecules adsorbed on graphene accept the electrons. If GO is not completely reduced to graphene, the defects obstruct charge separation, and the nanocomposite will show inferior photocatalytic activity. The hþ in the Ag NPs can also accept electrons from water molecules (H2O) adsorbed on the TiO2 surface, hydroxide ions or dye molecules in solution. OH radicals are thereby produced, and organics can be decomposed to H2O and carbon dioxide (CO2). The mechanism has illustrated that hþ, O2 and OH are all active species involved in the photodegradation process.
number of reacted electrons 100 number of incident photons number of evolved H molecules 2 100 number of incident photons
The QE and mass-normalized hydrogen evolution rate for ATG (5:1) are 4.8% and 129.5 mmol g1 h1, respectively. Nevertheless, those for ATG (1000:1) are 1.0% and 27.1 mmol g1 h1, respectively. Apparently, a higher ratio of graphene contributes to higher QE. More graphene contained in a nanocomposite increases the adsorption area, improves charge transport, lowers recombination probability and enhances hydrogen production efficiency. Hole and radical trapping experiments were carried out to further elucidate the photocatalytic reaction mechanism. The degradation of MO was observed to investigate the main reactive species involved in the photocatalytic process, as displayed in Fig. 10. Three scavengers with a concentration of 20 ppm were selected for the trapping experiments [6,12].
Fig. 10 e Hole and radical trapping experiments. Photocatalytic degradation of MO in the presence of different scavengers.
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
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Fig. 11 e Schematic photocatalytic mechanism of ATG ternary nanocomposites.
Conclusions ATG ternary nanocomposites with various contents of graphene were fabricated and used as photocatalysts. The TiO2 in the nanocomposites was mainly anatase, and graphene was successfully obtained from GO by a photocatalytic reduction method. TiO2 NPs decorated with smaller Ag NPs were observed to be distributed over the surface of graphene nanosheets. The formation of ATG nanocomposites was confirmed by TEM analysis. As nanocomposites contained a large amount of graphene, a wide range and high intensity of light absorption that were beneficial to improving photocatalytic activity were found. Among the five ternary nanocomposites, ATG (5: 1) exhibited the highest efficiency of photodegradation. Therefore, the most appropriate ratio of TiO2 to graphene in an ATG nanocomposite was deduced to be 5: 1. The CV and EIS results demonstrated that a higher content of graphene was beneficial to increased conductivity and reduced interfacial impedance. As electron transport was more efficient, charge recombination was restrained and a superior photocatalytic performance was achieved. ATG (5:1) also exhibited the largest amount of hydrogen production from water splitting. Its QE and mass-normalized hydrogen evolution rate were 4.8% and 129.5 mmol g1$h1, respectively. This study demonstrated that an appropriate content of graphene contributed not only to dye photodegradation but also to hydrogen evolution. From the results of hole and radical trapping experiments, the photocatalytic mechanism of an ATG ternary nanocomposite was proposed. It has been verified that hþ, O2 and OH are all active species involved in the photodegradation process.
Acknowledgements We are grateful for the support from the Ministry of Science and Technology Taiwan and National Chi Nan University.
references
[1] Liu C, Yang D, Jiao Y, Tian Y, Wang Y, Jiang Z. Biomimetic synthesis of TiO2-SiO2-Ag nanocomposites with enhanced visible-light photocatalytic activity. ACS Appl Mater Interfaces 2013;5:3824e32. [2] Xiang QJ, Yu JG, Jaroniec M. Graphene-based semiconductor photocatalysts. Chem Soc Rev 2012;41:782e96. [3] Tong H, Ouyang S, Bi Y, Umezawa N, Oshikiri M, Ye J. Nanophotocatalytic materials: possibilities and challenges. Adv Mater 2012;24:229e51. [4] Kubacka A, Fernandez-Garcia M, Colon G. Advanced nanoarchitectures for solar photocatalytic applications. Chem Rev 2012;112:1555e614. [5] Zhao Y, Sun L, Xi M, Feng Q, Jiang C, Fong H. Electrospun TiO2 nanofelt surface-decorated with Ag nanoparticles as sensitive and UV-cleanable substrate for surface enhanced Raman scattering. ACS Appl Mater Interfaces 2014;6:5759e67. [6] Yang MQ, Zhang N, Xu YJ. Synthesis of fullerene-, carbon nanotube, and graphene-TiO2 nanocomposite photocatalysts for selective oxidation: a comparative study. ACS Appl Mater Interfaces 2013;5:1156e64. [7] Mou Z, Wu Y, Sun J, Yang P, Du Y, Lu C. TiO2 nanoparticlesfunctionalized N-doped graphene with superior interfacial contact and enhanced charge separation for photocatalytic hydrogen generation. ACS Appl Mater Interfaces 2014;6:13798e806. [8] Yan Y, Wang C, Yan X, Xiao L, He J, Gu W, et al. Graphene acting as surface phase junction in anatase-graphene-rutile heterojunction photocatalysts for H2 production from water splitting. J Phys Chem C 2014;118:23519e26. [9] Xu T, Zhang L, Cheng H, Zhu Y. UV-assisted photocatalytic synthesis of TiO2-reduced graphene oxide with enhanced photocatalytic activity in decomposition of sarin in gas phase. Appl Catal B Environ 2011;101:382e7. [10] Yang Y, Liu E, Dai H, Kang L, Wu H, Fan J, et al. Photocatalytic activity of Ag-TiO2-graphene ternary nanocomposites and application in hydrogen evolution by water splitting. Int J Hydrogen Energy 2014;39:7664e71. [11] Hsu KC, Chen DH. Highly sensitive, uniform, and reusable surface-enhanced Raman scattering substrate with TiO2
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024
10
[12]
[13]
[14]
[15]
[16]
[17] [18]
[19]
[20]
[21]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
interlayer between Ag nanoparticles and reduced graphene oxide. ACS Appl Mater Interfaces 2015;49:27571e9. Yang X, Qin J, Jiang Y, Chen K, Yan X, Zhang D, et al. Fabrication of P25/Ag3PO4/graphene oxide heterostructures for enhanced solar photocatalytic degradation of organic pollutants and bacteria. Appl Catal B Environ 2015;166e167:231e40. Wu F, Fan J, Hu X, Liu E, Sun T, Kang L, et al. Photocatalytic activity of Ag/TiO2 nanotube arrays enhanced by surface plasmon resonance and application in hydrogen evolution by water splitting. Plasmonics 2012;8:501e8. ar B, Sturm Logar M, Janc S, Suvorov D. Weak polyion multilayer-assisted in situ synthesis as a route toward a plasmonic Ag/TiO2 photocatalyst. Langmuir 2010;26:12215e24. Awazu K, Fujimaki M, Rockstuhl C, Tominaga J, Murakami H, Ohki Y, et al. A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide. J Am Chem Soc 2008;130:1676e80. Christopher P, Ingram DB, Linic S. Enhancing photochemical activity of semiconductor nanoparticles with optically active Ag nanostructures: photochemistry mediated by Ag surface plasmons. J Phys Chem C 2010;114:9173e7. Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80. 1339e9. Kovtyukhova NI, Ollivier PJ, Martin BR, Mallouk TE, Chizhik SA, Buzaneva EV, et al. Layer-by-layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations. Chem Mater 1999;11:771e8. Wen Y, Ding H, Shan Y. Preparation and visible light photocatalytic activity of Ag/TiO2/graphene nanocomposite. Nanoscale 2011;3:4411e7. Xu YJ, Zhuang Y, Fu X. New insight for enhanced photocatalytic activity of TiO2 by doping carbon nanotubes: a case study on degradation of benzene and methyl orange. J Phys Chem C 2010;114:2669e76. Zhang Y, Zhang N, Tang ZR, Xu YJ. Improving the photocatalytic performance of graphene-TiO2 nanocomposites via a combined strategy of decreasing
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
defects of graphene and increasing interfacial contact. Phys Chem Chem Phys 2012;14:9167e75. McCafferty E, Wightman JP. Determination of the concentration of surface hydroxyl groups on metal oxide films by a quantitative XPS method. Surf Interface Anal 1998;26:549e64. Yumitori S. Correlation of C1s chemical state intensities with the O1s intensity in the XPS analysis of anodically oxidized glass-like carbon samples. J Mater Sci 2000;35:139e46. Irie H, Watanabe Y, Hashimoto K. Carbon-doped anatase TiO2 powders as a visible-light sensitive photocatalyst. Chem Lett 2003;32:772e3. Sher Shah MS, Zhang K, Park AR, Kim KS, Park NG, Park JH, et al. Single-step solvothermal synthesis of mesoporous AgTiO2-reduced graphene oxide ternary composites with enhanced photocatalytic activity. Nanoscale 2013;5:5093e101. Ren W, Ai Z, Jia F, Zhang L, Fan X, Zou Z. Low temperature preparation and visible light photocatalytic activity of mesoporous carbon-doped crystalline TiO2. Appl Catal B 2007;69:138e44. Pan X, Yang MQ, Tang ZR, Xu YJ. Non-covalently functionalized graphene-directed synthesis of ultralarge graphene-based TiO2 nanosheet composites: tunable morphology and photocatalytic applications. J Phys Chem C 2014;118:27325e35. Phan TDN, Luo S, Liu Z, Gamalski AD, Tao J, Xu W, et al. Striving toward noble-metal-free photocatalytic water splitting: the hydrogenated-graphene-TiO2 prototype. Chem Mater 2015;27:6282e96. Braslavsky SE, Braun AM, Cassano AE, Emeline AV, Litter MI, Palmisano L, et al. Glossary of terms used in photocatalysis and radiation catalysis. Pure Appl Chem 2011;83:931e1014. Furube A, Du L, Hara K, Katoh R, Tachiya M. Ultrafast plasmon-induced electron transfer from gold nanodots into TiO2 nanoparticles. J Am Chem Soc 2007;129:14852e3. Tian Y, Tatsuma T. Mechanisms and applications of plasmon-induced charge separation at TiO2 films loaded with gold nanoparticles. J Am Chem Soc 2005;127:7632e7.
Please cite this article in press as: Sheu F-J, Cho C-P, Investigation of the appropriate content of graphene in AgeTiO2-graphene ternary nanocomposites applied as photocatalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.06.024