- Email: [email protected]
A study of photocatalytic graphene–TiO2 synthesis via peroxo titanic acid refluxed sol
Materials Research Bulletin 48 (2013) 2809–2816
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
A study of photocatalytic graphene–TiO2 synthesis via peroxo titanic acid refluxed sol Wasu Low a, Virote Boonamnuayvitaya b,* a b
The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand Department of Chemical Engineering, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 May 2012 Received in revised form 26 March 2013 Accepted 2 April 2013 Available online 17 April 2013
In the present work, graphene–TiO2 (GR–TiO2) photocatalyst with various weight ratios of graphene was synthesized using peroxo titanic acid solution (PTA) as a precursor for TiO2. Graphene oxide prepared by Hummer’s method was converted to graphene under ultraviolet (UV) irradiation in ethanol–water solvent for 48 h. The as-prepared GR–TiO2 composites were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, UV–vis spectrophotometry, and transmission electron microscopy (TEM). The automated potentiostat was applied to measure the photocurrent generations of prepared catalysts. The photocatalytic activities of GR–TiO2 (PTA) catalysts were determined by measuring the percentage methylene blue (MB) degradation. The results showed that TiO2 nanoparticles were successfully loaded onto graphene sheet and the surface area of catalysts increased with increasing weight ratio of graphene. In addition, GR–TiO2 (PTA, 1:50) exhibited the highest photocatalytic activity among the catalysts under UV and visible light irradiation. The adsorption edge of GR–TiO2 was shifted to a longer wavelength of 400 nm in comparison with that of pure TiO2 (PTA). The increase in the photocatalytic performance of GR–TiO2 (PTA) catalyst may be attributed to the increase in surface area, the extension of light absorption in the visible light region, and prevention of charge recombination. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Composites B. Chemical synthesis C. X-ray diffraction D. Catalytic properties
1. Introduction Recently, graphene has attracted interest and has been used in many applications such as nanoelectronic devices and catalysis, owing to its two-dimensional sp2 bonding network of carbon atoms, which gives it outstanding electrical, thermal, mechanical and optical properties [1,2]. The enhanced photocatalytic activity of semiconductor photocatalysts by adding graphene has received much attention in many applications such as hydrogen production [3], degradation of organic pollutants [4] including selective redox reaction [5]. Graphene can be synthesized using natural graphite as a starting material. The Hummer’s method is widely used to synthesize graphene oxide. In this method, natural graphite is oxidized using dimanganeseheptoxide (Mn2O7) as a high oxidizing agent produced from the reaction between potassium permanganate (KMnO4) and sulfuric acid (H2SO4) [6]. Oxygen-containing functional groups on the surface of graphene oxide such as epoxy, alkoxy and carboxyl act as a good support material to produce GR–
* Corresponding author. Tel.: +66 2 470 9221 30; fax: +66 2 428 3534. E-mail addresses: [email protected] (W. Low), [email protected] (V. Boonamnuayvitaya). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.04.020
TiO2 [7,8]. Hydrothermal treatment is a popular technique to prepare GR–TiO2 composites, the homogeneous GR–TiO2 suspension is transferred to autoclave and maintain at 120–180 8C for 2– 24 h [9–12]. The UV-assisted photocatalytic reduction of graphene oxide, which has been reported as a new reduction technique for preparation GR–TiO2 photocatalyst, exhibits well-separated GR– TiO2 composite sheet [13]. The photocatalytic activities of pure TiO2 and GR–TiO2 at different weight ratio of graphene have been examined. The results illustrate that GR–TiO2 shows higher photocatalytic activity than that of pure TiO2 under UV and visible light irradiation. The increase in photocatalytic activity of GR–TiO2 is attributed to the improvement of adsorbability, charge transfer rate and photoresponse in the visible light region [8,9]. Graphene has a high surface area of 2600 m2 g1 [10,14]. Therefore, GR–TiO2 composite exhibits an effective adsorption material. The electronic conductivity of graphene by its p–p conjugation structure enhances photocatalytic activity by suppression charge recombination of electron–hole pairs. In addition, the longer lifetime of electron– hole pairs of TiO2 has been considered as significant effect of graphene on TiO2 photocatalyst to enhance the photocatalytic activity of TiO2 [15,16]. It has been reported that the extendable photoexcitation under visible light irradiation resulting in the formation of Ti–O–C bonds [17,18]. Recently, considerable
2810
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
information of GR–ZnS photocatalytic experiments showed that the photoactivity of GR–ZnS occurs under visible light irradiation. The wide band gap of ZnS can be narrowed from 3.60 to 3.44– 3.55 eV by adding graphene but these reduced band gap value of GR–ZnS is still high for photoexcitation process under visible light illumination. Therefore, it implied that graphene acts as a macromolecular photosensitizer [19]. It was noted that excessive graphene content in TiO2 leads to decrease in photodegradation efficiency because of light obstruction [8]. Thus, the optimal graphene content in TiO2 needs to be investigated and controlled to achieve an adequate photocatalytic activity of GR–TiO2. In earlier reports, the traditional sol–gel method has been widely used for TiO2 synthesis. This method requires an acid or base catalyst in order to drive hydrolysis and poly-condensation processes. TiO2 nanoparticles in acid or base solution are then obtained after calcination at high temperature. Therefore, the acid or base nature of TiO2 sol and high temperature calcination limit its application, including the choice of substrates [20–22]. Use of peroxo titanic acid (PTA) is an alternative method for synthesis of pure anatase TiO2 under neutral pH and low temperature conditions. This method was also reported as an environmental friendly method for synthesis of TiO2 because peroxotitanium complex molecules can be converted to TiO2 nanoparticles at relatively low temperature (100 8C) [23]. It was already observed that the photocatalytic activity of TiO2 depends on its crystalline structure, crystal size, and surface area. TiO2 anatase phase photocatalyst thin films were prepared from peroxo titanic acid refluxed sols using titanyl sulfate (TiOSO4) as a titanium source [24]. Titanium tetrachloride (TiCl4) was applied to synthesize peroxotitanium solution and it was noted that TiO2 nanopowder with a high surface area can be obtained after heating at 80 8C [25]. Previously, our group has reported the synthesis of aminefunctionalized SiO2/TiO2 photocatalytic films prepared from refluxed PTA solution at 100 8C for 10 h. The results show that the adsorption capability of photocatalyst plays an important role in VOC degradation [26]. In this work, refluxed PTA solution at 100 8C for 10 h was applied as a starting material for TiO2 nanoparticles to synthesize GR–TiO2 composite. Graphene oxide– TiO2 was reduced to graphene–TiO2 using UV-assisted photocatalytic reduction without heating. The photocatalytic activity of prepared GR–TiO2 (PTA) catalysts was examined by evaluating the degradation of MB aqueous solution under UV and visible light irradiation. 2. Experimental 2.1. Materials Titanyl sulfate (TiOSO4, Sigma–Aldrich), hydrogen peroxide (H2O2, 30%, Merck), ammonia solution (NH4OH, 30%, Merck), sodium nitrate (NaNO3, Rankem), potassium permanganate (KMnO4, Qrec), sulfuric acid (H2SO4, 97%, Merck), natural graphite (fine powder extra pure, Merck), Sodium sulfate (Na2SO4, Sigma– Aldrich) and ethanol (C2H5OH, Merck). All chemicals were high purity reagents and were used without further purification. 2.2. Preparation of graphene oxide powder Graphene oxide was synthesized using Hummer’s method. The concentrated H2SO4 (32 mL) and NaNO3 (1.0 g) were mixed and stirred in an ice bath. After NaNO3 was completely dissolved, natural graphite (1.0 g) was added to the mixture. KMnO4 (4.5 g) was slowly added to the above mixture under continual stirring at 30 8C for 1 h. The mixture solution was diluted by adding 250 mL of distilled water and further heated at 98 8C for 1 h. The remaining KMnO4 in the mixture was terminated by adding 10 mL of 30%
H2O2. The dark brown precipitate was separated by centrifugation and washed with distilled water for purification until a neutral pH of 7 was reached. The graphene oxide power was obtained after drying at 35 8C [1]. 2.3. Preparation of peroxo titanic acid (PTA) solution and TiO2 (PTA) nanoparticles Titanyl sulfate (TiOSO4, 5.0 g) was dissolved in distilled water. The white precipitate (Ti(OH)4) was observed by adding NH4OH (1 M). After filtering and rinsing with de-ionized water several times, H2O2 (0.98 M) solution was added to the Ti(OH)4 at a 1:5 molar ratio of H2O2 to titanium under continual stirring until the yellow transparent PTA solution was obtained. The PTA solution was diluted with de-ionized water in a 500 mL volumetric flask. TiO2 nanoparticles can be obtained by refluxing PTA solution at 100 8C for 2 and 10 h. In order to elucidate the effect of the reflux process on the functional groups of PTA molecules, PTA powder was prepared by drying PTA solution at 35 8C [24,27]. 2.4. Preparation of GR–TiO2 (PTA) photocatalysts Graphene oxide was added at weight ratios of graphene oxide to TiO2 of 1:100, 1:50, 1:20, and 1:10 to the TiO2 (PTA refluxed at 100 8C for 10 h) colloidal suspension (50 mg/L) in ethanol (70%). Next, graphene oxide–TiO2 suspensions were irradiated with two 8 W UVA lamps for 48 h. GR–TiO2 powders were obtained after drying in a hot air oven at 50 8C for 6 h [13]. 2.5. Characterization The crystalline structure of the TiO2 photocatalyst was determined by X-ray diffractometer (Bruker, D8-Discover). Nickel-filtered CuKa radiation (l = 0.15418 nm) was used with a generator voltage of 40 kV and a current of 40 mA, scanned at a speed of 0.18/s and step size of 0.028 at an angular range of 20–808. The crystalline sizes of anatase and rutile were calculated via Scherrer’s equation:
D¼
kl Bcos u B
where D is the crystallite particle size, l is the wavelength of CuKa irradiation (0.15406 nm), k is a constant of 0.9, B is the full width at half maximum, and uB is Bragg’s angle of the anatase (1 0 1) plane [20]. The functional groups of catalysts were determined by Fourier transform infrared (FTIR) spectroscopy (Perkin-Elmer, Spectrum One). The nitrogen adsorption/desorption isotherms were measured at a liquid nitrogen temperature of 77 K using a high precision surface area and pore size analyzer (Belsorp, Belsorpmini). The specific surface area and total pore volume were calculated by the Brunauer–Emmett–Teller (BET) method. The morphology and selected area electron diffraction pattern (SAED) of catalysts was observed using a transmission electron microscope (TEM, JEOL, JEM-2100). The absorption edge wavelength of catalysts was recorded by UV–Vis spectrometer (UV1900, UV/VIS, Hitachi). Photocurrent tests were measured using automated potentiostat (ACM Instrument, Gill AC). The conventional threeelectrode cell configuration with a standard Ag/AgCl as a reference electrode, inert platinum (Pt) sheet as a counter electrode and a working electrode was immersed in the 0.1 M Na2SO4 solution. A black light blue (BLB, l = 365 nm, Philips) or fluorescent lamp (l > 420 nm, Osram) was used as the source of the UV and visible light, respectively.
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
2811
2.6. Photocatalytic activity Photocatalytic activities of TiO2 (PTA) and GR–TiO2 (PTA) series samples were evaluated by degradation of MB aqueous solution. An irradiation box with dimensions of 410 mm 350 mm 123 mm was equipped with UV or visible light lamps: two 8 W black light blue (BLB, l = 365 nm, Philips) or 8 W fluorescent lamps (l > 420 nm, Osram), respectively. The light sources were placed parallel to one another at an equal distance of 20 mm from the photoreaction vessel. The experiment was conducted in two steps. In the first step, the catalyst suspension comprising catalyst powder (1 mg) dispersed in 40 mL of 4.5 ppm MB solution was kept in the dark for 3 h in order to ascertain the adsorption–desorption equilibrium. In the second step, under ambient conditions and stirring, the beakers were exposed to UV or visible light for 15, 30, 45, 60, 90, and 150 min, respectively. Then, 3.0 mL of sample was taken out after a determined period of time and centrifuged at 5250 rpm for 10 min. The concentration of MB in the sample was determined by UV–Vis spectrophotometer (U1900 UV/VIS, Hitachi) at a visible wavelength line of 659 nm. The photocatalytic activities of TiO2 (PTA) and GR–TiO2 (PTA) series were measured in terms of the degradation efficiency (%) of MB by the following equation:
Fig. 2. Degradation efficiency of TiO2 (PTA) at various refluxed time.
Fig. 1 shows the X-ray diffraction (XRD) patterns of TiO2 (PTA refluxed at 100 8C for 2 h), TiO2 (PTA refluxed at 100 8C for 10 h) and GR–TiO2 (PTA) composites, there are five distinctive TiO2 peaks at 25.38, 37.98, 48.08, 54.68, and 62.88 which correspond to anatase phase (JCPDS 21-1272). It is clear from Fig. 1 that TiO2 synthesized via PTA as a precursor demonstrates exclusively anatase phase. The crystal sizes of TiO2 (PTA refluxed at 100 8C for 2 h) and TiO2 (PTA refluxed at 100 8C for 10 h) calculated from Scherrer’s equation are 12.9 and 14.8 nm, respectively. The intensity of the anatase diffraction peak increases with increasing reflux time and the
crystal size of anatase phase tends to increase in size because of the enhanced crystallinity. The XRD diffraction patterns of GR–TiO2 (PTA) composites are similar to that of pure TiO2. This implies that the crystal structure of TiO2 (PTA) do not change during the graphene oxide reduction process. However, the increase in crystal size of GR–TiO2 (PTA) about 15.2 nm may be attributed to the enhanced crystallinity during drying of GR–TiO2 (PTA) catalyst in a hot air oven at 50 8C for 6 h. The photocatalytic activities of TiO2 at various refluxed time are investigated as shown in Fig. 2. The degradation efficiency of MB in the presence of TiO2 (PTA) increases with increasing refluxed time because of the enhanced crystallinity (see Fig. 1(a) and (b)). It was noted that TiO2 (PTA refluxed at 100 8C for 10 h) with 40.3% photodegradation seems to be an optimal refluxed time for preparation TiO2 nanoparticles. The functional groups of GR–TiO2 (PTA) composite, TiO2 (PTA refluxed at 100 8C for 10 h), graphene oxide powder and PTA (nonrefluxed) are presented in Fig. 3. A broad absorption band at 3000– 3600 cm1 and a strong peak at 1665 cm1 can be assigned to the vibration of OH groups of adsorbed water and Ti–OH group. The adsorption peak at around 1400 cm1 is due to the stretching vibration from the N–H bond of the residual NH4+. The peak at 900 cm1 can be assigned to the stretching mode of the peroxo group. The intensity of this peak decreased during refluxing because of the decomposition of the peroxo group [21,24]. The
Fig. 1. XRD diffraction patterns of (a) TiO2 (PTA refluxed at 100 8C for 2 h), (b) TiO2 (PTA refluxed at 100 8C for 10 h), (c) GR–TiO2 (PTA, 1:100), (d) GR–TiO2 (PTA, 1:50), (e) GR–TiO2 (PTA, 1:20) and (f) GR–TiO2 (PTA, 1:10).
Fig. 3. FTIR spectra of (a) GR–TiO2 (PTA) composite, (b) TiO2 (PTA refluxed at 100 8C for 10 h), (c) graphene oxide powder and (d) PTA (non-refluxed).
Degradation efficiency ð%Þ ¼
C0 Ci 100 C0
where C0 is the initial concentration of MB and Ci is the concentration of MB solution after UV or visible light irradiation. 3. Results and discussion 3.1. Characterization of synthesized materials
2812
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
Fig. 4. Photographs of (a) graphene oxide powder, (b) TiO2 (PTA refluxed at 100 8C for 10 h) and (c) GR–TiO2 (PTA). (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
absorption peak between 400 and 690 cm1 was corresponded to the signal of Ti–O–Ti bond. In the case of graphene, the peaks at 1723, 1600 and 1228 cm1 are assigned as the carbonyl groups (C5 5O), hydroxyl groups (OH) and epoxy groups (C–O), respectively. The FTIR data of GR–TiO2 (PTA, Fig. 3(a)) shows different FTIR spectral patterns in comparison with that of graphene oxide (Fig. 3(c)). After the graphene oxide reduction process, the peaks of the carbonyl and epoxy groups disappeared. This implies that the functional groups on graphene oxide were completely reduced to graphene using the UV-assisted photocatalytic reduction method. In addition, it was also observed that the signal of Ti–O–Ti shifted to a higher wavenumber around 790 cm1 because of the combined signal of Ti–O–Ti and Ti–O–C vibration [17,18]. The interaction between the functional groups of graphene oxide and the OH group on the surface of TiO2 nanoparticles is a basic mechanism to obtain GR–TiO2 composites [13]. Fig. 4 shows the digital photographs of graphene oxide powder, TiO2 (PTA refluxed at 100 8C for 10 h), and GR–TiO2 (PTA). Graphene oxide has a brown color and the TiO2 (PTA refluxed at 100 8C for 10 h) powder is light-yellow in color because of the remaining peroxo group. After graphene oxide reduction under UV irradiation for 48 h, the color of GR–TiO2 (PTA) changed to light gray. The wrinkled two-dimensional structure of graphene oxide can be clearly observed in Fig. 5(a). In the case of TiO2 (PTA refluxed at 100 8C for 10 h), the needle-like or rhombus anatase crystals have an average length of 40–80 nm and diameter of 10–20 nm as shown in Fig. 5(b). Fig. 5(c) shows the TEM image of GR–TiO2 (PTA) and Fig. 5(d) is a high resolution TEM (HRTEM) image of GR–TiO2 (PTA) sample. The crystal spacing of 0.235 nm is observed and corresponding to the (0 0 1) plane of anatase. Fig. 5(e) shows the selected area electron diffraction (SAED) pattern of GR–TiO2 (PTA). The diffraction rings of GR–TiO2 (PTA) indexed to (1 0 1), (0 0 4), (2 0 0) and (1 0 5) crystal planes of anatase TiO2, are in accordance with the XRD analysis. The results demonstrate that TiO2 nanoparticles are successfully loaded onto graphene sheet. However, the crystal size of TiO2 (PTA) calculated using Scherrer’s equation (14.8 nm) was different from that observed in the TEM
image (40–80 nm in length and 10–20 in diameter) because of the effect of diffraction line width and instrumental broadening [24]. The formation of TiO2 anatase phase using PTA has been reported. The white precipitate of Ti(OH)4 is dissolved in H2O2 solution to form a homogenous yellow aqueous PTA solution. The complex anion of titanium is basically considered as binuclear species, Ti2O5(OH)x(x2) (x > 2). When increased the temperature, binuclear complex species condenses to octahedral TiO6 units in hydrated form and further crystalize to anatase TiO2 via facesharing condensation [21,23,28–30]. The UV–vis absorption spectra of TiO2 (PTA) and GR–TiO2 (PTA) catalysts are presented in Fig. 6. The spectra of GR–TiO2 (PTA) catalysts show a red shift in the band gap transition with increases in the amount of graphene dopant. The absorption below 400 nm results from the excitation of electrons from the valence band (VB) to the conduction band (CB) of TiO2 [18]. The band gap energy of catalysts can be estimated by the Kubelka–Munk function [10,12,31]. 1=2
ðahnÞ
¼ Ai ðhn Eg Þ
where a is the absorption coefficient (cm1). hn is the photon energy (eV) and Ai is a constant value. In Fig. 7, the estimated band gaps of GR–TiO2 (PTA) are 3.24, 3.22, 3.20, 3.09, and 2.90, corresponding to TiO2 (PTA), GR–TiO2 (PTA, 1:100), GR–TiO2 (PTA, 1:50), GR–TiO2 (PTA, 1:20) and GR– TiO2 (PTA, 1:10), respectively. Analogous results for band gap narrowing of the GR–P25 series were also found [10,12]; the decrease in the band gap in the GR–TiO2 composite was due to the interaction between TiO2 (PTA) and graphene, which can be clearly observed from the FTIR broad absorption of Ti–O–Ti and Ti–O–C vibration around 790 cm1. Fig. 8 shows the photocurrent behavior of TiO2 (PTA) and GR– TiO2 (PTA) photocatalysts under UV and visible light irradiation. It was observed that photocurrent of all samples show a uniform photoresponse. The photocurrent generations from GR–TiO2 (PTA) composites are higher than that of pure TiO2 (PTA). The GR–TiO2
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
2813
Fig. 5. TEM image of (a) graphene oxide, (b) TiO2 (PTA refluxed at 100 8C for 10 h), (c) GR–TiO2 (PTA), (d) HRTEM image of GR–TiO2 (PTA) and (e) SAED pattern of GR–TiO2 (PTA).
3.2. Photocatalytic activities
(PTA, 1:50) photocatalyst, which demonstrated the highest photocurrent generation, yielded the current density of 4.33 and 2.98 mA/cm2 under UV and visible light illumination, respectively. The increasing in photocurrent can be attributed to the twodimensional p–p conjugation structure of carbon atoms of graphene resulting in an excellent electrical conduction. However, the decreasing in photocurrent can be observed on excessive graphene content in GR–TiO2 (PTA, 1:20 and 1:10) composites. The higher graphene content in GR–TiO2 (PTA) composite may hinder or scatter the light transmission. Therefore, the photoexcitation of TiO2 could be decreased because of light obstruction of graphene [8,11].
Fig. 9 shows the concentration of MB solution after reaching the adsorption–desorption equilibrium in dark conditions. The equilibrium concentrations of MB in the presence of GR–TiO2 (PTA) series are lower than that of pure TiO2 (PTA). This confirms that MB was adsorbed more on the surface of GR–TiO2 (PTA) photocatalysts. In addition, GR–TiO2 (PTA) demonstrates the increased adsorption capacity with increasing weight fraction of graphene. The photocatalytic activities of P25, TiO2 (PTA) and GR–TiO2 (PTA) photocatalysts under UV and visible light irradiation are presented in Figs. 10 and 11, respectively. The order of photo-
Fig. 6. UV–vis absorption spectra of TiO2 (PTA) and GR–TiO2 (PTA) catalysts with different weight ratio of graphene.
Fig. 7. A plot of (ahn)1/2 versus photon energy of TiO2 (PTA) and GR–TiO2 (PTA) catalysts.
2814
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
Fig. 8. Photocurrent generation from catalysts under UV and visible light irradiation (a) TiO2 (PTA), (b) GR–TiO2 (PTA, 1:10), (c) GR–TiO2 (PTA, 1:20), (d) GR–TiO2 (PTA, 1:50) and (e) GR–TiO2 (PTA, 1:100).
degradation of MB under UV irradiation is as follows: GR–TiO2 (PTA, 1:50) > GR–TiO2 (PTA, 1:20) > GR–TiO2 (PTA, 1:10) > GR– TiO2 (PTA, 1:100) TiO2 (PTA) > P25. Under visible light irradiation, the degradation efficiency of MB follows the order as below: GR–TiO2 (PTA, 1:50) > GR–TiO2 (PTA, 1:20) > GR–TiO2 (PTA, 1:100) > GR–TiO2 (PTA, 1:10) > TiO2 (PTA) > P25. The optimal amount of graphene seems to be within the range of 1:10–1:100 because the photocatalytic activities of GR–TiO2 (PTA) at 1:10 and 1:100 become close to that of pure TiO2 (PTA). The appropriate amount of graphene to TiO2 for preparation GR–TiO2 (PTA) in this experiment is about 1:50. The decrease in MB degradation
efficiencies of GR–TiO2 (PTA, 1:10 and 1:20) under UV and visible light compared to that of GR–TiO2 (PTA, 1:50) was ascribed to light obstruction. In addition, the change of photocatalytic activities under UV and visible light irradiation are observed between GR– TiO2 (PTA, 1:10) and GR–TiO2 (PTA, 1:100) photocatalysts. Under visible light irradiation, the photocatalytic activity of GR–TiO2 (PTA, 1:10) is lower in comparison with that of GR–TiO2 (PTA, 1:100). As the results from UV–vis absorption spectra (Fig. 6) and photocurrent generation (Fig. 8) of photocatalysts, it was noted that GR–TiO2 (PTA, 1:10 and 1:20) yields higher absorption over all visible light wavelengths but the photocurrent generations of that
Fig. 9. Concentration at equilibrium of MB solution in the presence of TiO2 (PTA) at different weight ratio of graphene.
Fig. 10. Photocatalytic degradation of MB solution over TiO2 (PTA), GR–TiO2 (PTA), P25 catalysts and GR under UV irradiation.
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
2815
Table 1 Texture characterization of catalysts. Sample
BET surface (m2 g1)
Total pore volume (cm3 g1)
Average pore diameter (nm)
P25 TiO2 (PTA) GR–TiO2 (PTA, GR–TiO2 (PTA, GR–TiO2 (PTA, GR–TiO2 (PTA,
50.75 88.00 93.76 94.13 102.00 105.70
0.47 0.36 0.33 0.38 0.20 0.31
37.24 16.20 14.07 16.21 8.02 11.56
1:100) 1:50) 1:20) 1:10)
photocatalyst under UV and visible light irradiation are lower than that of GR–TiO2 (PTA, 1:50). This phenomenon is attributed to the high photon absorption of excessive graphene. These evidences imply that the overdose graphene content in GR–TiO2 reduces the light absorption efficiency of TiO2 [19,20]. In dark condition, MB molecules adsorbed onto catalysts and reached equilibrium within 3 h. The remaining concentration of MB in the presence of GR–TiO2 composites is lower than that of TiO2 as shown in Fig. 9. This result is corresponding with the BET specific area of TiO2 (PTA), GR–TiO2 (1:100), GR–TiO2 (1:50), GR– TiO2 (1:20) and GR–TiO2 (1:10) as shown in Table 1. It was obvious that the BET specific area of GR–TiO2 (PTA) increases with increasing weight ratio of graphene resulting in an increase in adsorbability. In this study, the photodegradation of MB in the presence of P25 photocatalyst under UV and visible light irradiation is not as high as that of TiO2 (PTA). The average crystal size of P25 calculated using Scherrer’s equation is about 21.1 nm and it was also observed that P25 has lower BET specific area than that of TiO2 (PTA). Therefore, the decrease in photocatalytic activity of P25 is due to its lower specific surface area. The possible mechanism for enhanced adsorbability on GR–TiO2 (PTA) should be a noncovalent formation which is formed on the surface of graphene via the interaction between the p–p stacking of MB molecules and aromatic regions of graphene [32]. It has been reported that 17b-estradiol molecules were trapped on graphene pores and further degraded by Fe doped TiO2 [33]. Thus, the MB molecules, also adsorbed, trapped and then degraded by TiO2 photocatalyst. The photocatalytic activity of GR–TiO2 (PTA) under visible light irradiation was influenced by narrowing of the band gap from 3.24 to 2.90 eV, which resulted in the formation of Ti–O– C bonds. The GR–TiO2 (PTA, 1:50) catalyst showed higher photocatalytic activity than any other catalyst. However, in the case of GR–TiO2 (PTA, 1:20) and GR–TiO2 (PTA, 1:10) the BET surface areas are 102.00 and 105.70 m2 g1, respectively; both catalysts show larger surfaces than that of GR–TiO2 (PTA, 1:50) but
their photocatalytic activities are not as high as that of GR–TiO2 (PTA, 1:50). Therefore, the addition of higher graphene content in a GR–TiO2 (PTA) photocatalyst not only leads a decrease of photocatalytic activity because of light obstruction but also shields the active sites on TiO2 photocatalyst [12]. Theoretically, it is well known that the electrons in the valence band (VB) can be excited to the conduction band (CB) of TiO2 by photo-irradiation. This reaction leads to positive holes in the VB and electrons in the CB that can further react with adsorbed water and oxygen molecules to produce hydroxyl radicals (OH) and superoxide radical anions (O2) which can oxidize volatile organic compounds [34]. The formation of OH and O2 in the presence of TiO2 photocatalyst is well known and written as shown in Eqs. (1)– (5) [27,35]. TiO2 þ hn ! hþ þ e
(1)
O2 þ e ! O2
(2)
2O2 þ 2H2 O ! 2H2 O2 þ O2
(3)
H2 O2 þ e ! OH þ OH
(4)
hþ þ TiOH ! TiOH
(5)
The photocatalytic activity of GR–TiO2 (PTA) composite is improved by separation of photo-generated electrons and holes. Graphene acts as electron acceptor and transporter due to its p–p conjugation structure [14,36,37]. The TiO2 electrons from the VB can be excited to the CB, and then rapidly transfer to graphene, leaving more electron–hole pairs to produce highly reactive species (OH and O2), resulting in higher photocatalytic activity. 4. Conclusion The GR–TiO2 photocatalysts were synthesized via PTA as a precursor with different weight ratios of graphene. GR–TiO2 (PTA) contains anatase phase crystal with an average length of 40–80 nm and diameter of 10–20 nm. The surface area of GR– TiO2 (PTA) photocatalyst increased with increasing weight ratio of graphene. Under UV and visible light irradiation, GR–TiO2 (PTA, 1:50) photocatalyst has the optimal amount of added graphene to achieve the highest photocatalytic activity, while a higher weight ratio of graphene content seems to decrease the photocatalytic activity. The enhancement of photocatalytic activity can be attributed to the influence of the increase in adsorption capacity and the reduction of the band gap by the formation of Ti–O–C bonds together with the separation of the electron–hole pairs. Acknowledgements
Fig. 11. Photocatalytic degradation of MB solution over TiO2 (PTA), GR–TiO2 (PTA), P25 catalysts and GR under visible light irradiation.
This work was supported by the Royal Golden Jubilee Ph.D. Program, the Thai Research Fund (TRF), and the Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi.
2816
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
References [1] G. Li, T. Wang, Y. Zhu, S. Zhang, C. Mao, J. Wu, B. Jin, Y. Tian, Appl. Surf. Sci. 257 (2011) 6568–6572. [2] Z. Qianqian, B. Tang, H. Guoxin, J. Hazard. Mater. 198 (2011) 78–86. [3] X. Zhang, Y. Sun, X. Cui, Z. Jiang, Int. J. Hydrogen Energy 37 (2012) 811–815. [4] D. Zhao, G. Sheng, C. Chen, X. Wang, Appl. Catal. B: Environ. 111/112 (2012) 303–308. [5] N. Zhang, Y. Zhang, X. Pan, M.Q. Yang, Y.J. Xu, J. Phys. Chem. C 116 (2012) 18023– 18031. [6] S.P. Daniel, R. Dreyer, W. Christopher, Bielawski, S. Rodney, Ruoff, Chem. Soc. Rev. 39 (2010) 228–240. [7] G. Jiang, Z. Lin, C. Chen, L. Zhu, Q. Chang, N. Wang, W. Wei, H. Tang, Carbon 49 (2011) 2693–2701. [8] F. Wang, K. Zhang, J. Mol. Catal. A: Chem. 345 (2011) 101–107. [9] H. Zhang, X. Lv, Y. Li, Y. Wang, J. Li, ACS Nano 4 (2010) 380–386. [10] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 4 (2010) 7303–7314. [11] P. Wang, Y. Ao, C. Wang, J. Hou, J. Qian, J. Hazard. Mater. 223/224 (2012) 79–83. [12] P. Cheng, Z. Yang, H. Wang, W. Cheng, M. Chen, W. Shangguan, G. Ding, Int. J. Hydrogen Energy 37 (2012) 2224–2230. [13] G. Williams, B. Seger, P.V. Kamt, ACS Nano 2 (2008) 1487–1491. [14] N.R. Khalid, Z. Hong, E. Ahmed, Y. Zhang, H. Chan, M. Ahmad, Appl. Surf. Sci. 258 (2012) 5827–5834. [15] N. Zhang, Y. Zhang, X. Pan, X. Fu, S. Liu, Y.J. Xu, J. Phys. Chem. C 115 (2011) 23501– 23511. [16] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 5 (2011) 7426–7435. [17] T.D. Nguyen Phan, V.H. Pham, E.W. Shin, H.D. Pham, S. Kim, J.S. Chung, E.J. Kim, S.H. Hur, Chem. Eng. J. 170 (2011) 226–232. [18] Y. Min, K. Zhang, W. Zhao, F. Zheng, Y. Chen, Y. Zhang, Chem. Eng. J. 193/194 (2012) 203–210.
[19] Y. Zhang, N. Zhang, Z.R. Tang, Y.J. Xu, ACS Nano 6 (2012) 9777–9789. [20] Y.J. Liu, M. Aizawa, Z.M. Wang, H. Hatori, N. Uekawa, H. Kanoh, J. Colloid Interface Sci. 322 (2008) 497–504. [21] N. Sasirekha, B. Rajesh, Y.W. Chen, Thin Solid Films 518 (2009) 43–48. [22] T.X. Liu, F.B. Li, X.Z. Li, J. Hazard. Mater. 152 (2008) 347–355. [23] S.I. Seok, M. Vithal, J.A. Chang, J. Colloid Interface Sci. 346 (2010) 66–71. [24] L. Ge, M.X. Xu, M. Sun, Mater. Lett. 60 (2006) 287–290. [25] J. Lu, L.P. Bauermann, P. Gerstel, U. Heinrichs, P. Kopold, J. Bill, F. Aldinger, Mater. Chem. Phys. 115 (2009) 142–146. [26] V. Boonamnunyvitaya, S. Photong, Water Air Soil Pollut. 210 (2009) 453–461. [27] C. Kaewtip, P. Chadpunyanun, V. Boonamnuayvitaya, Water Air Soil Pollut. 223 (2012) 1455–1465. [28] N. Murakami, Y. Kurihara, T. Tsubota, T. Ohno, J. Phys. Chem. C 113 (2009) 3062– 3069. [29] Y. Gao, Y. Masuda, Z. Peng, T. Yonezawa, K. Koumoto, J. Mater. Chem. 13 (2002) 608–613. [30] H. Ichinose, M. Terasaki, H. Katsuki, J. Sol–Gel Sci. Technol. 22 (2001) 33–40. [31] L. Ge, X. Mingxia, F. Haibo, M. Sun, Appl. Surf. Sci. 253 (2006) 720–725. [32] Y. Zhang, C. Pan, J. Mater. Sci. 46 (2011) 2622–2626. [33] F. Nasrin, C.R. Roy, M.G. Yaocihuatl, M.B. Ray, P.A. Charpentier, Appl. Catal. B: Environ. 110 (2011) 25–32. [34] T.K. Tseng, Y.S. Lin, Y.J. Chen, H. Chu, J. Mol. Sci. 11 (2010) 2336–2361. [35] A. Houas, H. lachhed, M. Ksibi, E. Elaloui, C. Guillard, J.M. Herrmann, Appl. Catal. B: Environ. 31 (2001) 145–157. [36] N. Farhangi, R.R. Chowdhury, Y. Medina Gonzalez, M.B. Ray, P.A. Charpentier, Appl. Catal. B: Environ. 110 (2011) 25–32. [37] C. Hou, Q. Zhang, Y. Li, H. Wang, J. Hazard. Mater. 205/206 (2012) 229–235.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
A study of photocatalytic graphene–TiO2 synthesis via peroxo titanic acid refluxed sol Wasu Low a, Virote Boonamnuayvitaya b,* a b
The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand Department of Chemical Engineering, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 May 2012 Received in revised form 26 March 2013 Accepted 2 April 2013 Available online 17 April 2013
In the present work, graphene–TiO2 (GR–TiO2) photocatalyst with various weight ratios of graphene was synthesized using peroxo titanic acid solution (PTA) as a precursor for TiO2. Graphene oxide prepared by Hummer’s method was converted to graphene under ultraviolet (UV) irradiation in ethanol–water solvent for 48 h. The as-prepared GR–TiO2 composites were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, UV–vis spectrophotometry, and transmission electron microscopy (TEM). The automated potentiostat was applied to measure the photocurrent generations of prepared catalysts. The photocatalytic activities of GR–TiO2 (PTA) catalysts were determined by measuring the percentage methylene blue (MB) degradation. The results showed that TiO2 nanoparticles were successfully loaded onto graphene sheet and the surface area of catalysts increased with increasing weight ratio of graphene. In addition, GR–TiO2 (PTA, 1:50) exhibited the highest photocatalytic activity among the catalysts under UV and visible light irradiation. The adsorption edge of GR–TiO2 was shifted to a longer wavelength of 400 nm in comparison with that of pure TiO2 (PTA). The increase in the photocatalytic performance of GR–TiO2 (PTA) catalyst may be attributed to the increase in surface area, the extension of light absorption in the visible light region, and prevention of charge recombination. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Composites B. Chemical synthesis C. X-ray diffraction D. Catalytic properties
1. Introduction Recently, graphene has attracted interest and has been used in many applications such as nanoelectronic devices and catalysis, owing to its two-dimensional sp2 bonding network of carbon atoms, which gives it outstanding electrical, thermal, mechanical and optical properties [1,2]. The enhanced photocatalytic activity of semiconductor photocatalysts by adding graphene has received much attention in many applications such as hydrogen production [3], degradation of organic pollutants [4] including selective redox reaction [5]. Graphene can be synthesized using natural graphite as a starting material. The Hummer’s method is widely used to synthesize graphene oxide. In this method, natural graphite is oxidized using dimanganeseheptoxide (Mn2O7) as a high oxidizing agent produced from the reaction between potassium permanganate (KMnO4) and sulfuric acid (H2SO4) [6]. Oxygen-containing functional groups on the surface of graphene oxide such as epoxy, alkoxy and carboxyl act as a good support material to produce GR–
* Corresponding author. Tel.: +66 2 470 9221 30; fax: +66 2 428 3534. E-mail addresses: [email protected] (W. Low), [email protected] (V. Boonamnuayvitaya). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.04.020
TiO2 [7,8]. Hydrothermal treatment is a popular technique to prepare GR–TiO2 composites, the homogeneous GR–TiO2 suspension is transferred to autoclave and maintain at 120–180 8C for 2– 24 h [9–12]. The UV-assisted photocatalytic reduction of graphene oxide, which has been reported as a new reduction technique for preparation GR–TiO2 photocatalyst, exhibits well-separated GR– TiO2 composite sheet [13]. The photocatalytic activities of pure TiO2 and GR–TiO2 at different weight ratio of graphene have been examined. The results illustrate that GR–TiO2 shows higher photocatalytic activity than that of pure TiO2 under UV and visible light irradiation. The increase in photocatalytic activity of GR–TiO2 is attributed to the improvement of adsorbability, charge transfer rate and photoresponse in the visible light region [8,9]. Graphene has a high surface area of 2600 m2 g1 [10,14]. Therefore, GR–TiO2 composite exhibits an effective adsorption material. The electronic conductivity of graphene by its p–p conjugation structure enhances photocatalytic activity by suppression charge recombination of electron–hole pairs. In addition, the longer lifetime of electron– hole pairs of TiO2 has been considered as significant effect of graphene on TiO2 photocatalyst to enhance the photocatalytic activity of TiO2 [15,16]. It has been reported that the extendable photoexcitation under visible light irradiation resulting in the formation of Ti–O–C bonds [17,18]. Recently, considerable
2810
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
information of GR–ZnS photocatalytic experiments showed that the photoactivity of GR–ZnS occurs under visible light irradiation. The wide band gap of ZnS can be narrowed from 3.60 to 3.44– 3.55 eV by adding graphene but these reduced band gap value of GR–ZnS is still high for photoexcitation process under visible light illumination. Therefore, it implied that graphene acts as a macromolecular photosensitizer [19]. It was noted that excessive graphene content in TiO2 leads to decrease in photodegradation efficiency because of light obstruction [8]. Thus, the optimal graphene content in TiO2 needs to be investigated and controlled to achieve an adequate photocatalytic activity of GR–TiO2. In earlier reports, the traditional sol–gel method has been widely used for TiO2 synthesis. This method requires an acid or base catalyst in order to drive hydrolysis and poly-condensation processes. TiO2 nanoparticles in acid or base solution are then obtained after calcination at high temperature. Therefore, the acid or base nature of TiO2 sol and high temperature calcination limit its application, including the choice of substrates [20–22]. Use of peroxo titanic acid (PTA) is an alternative method for synthesis of pure anatase TiO2 under neutral pH and low temperature conditions. This method was also reported as an environmental friendly method for synthesis of TiO2 because peroxotitanium complex molecules can be converted to TiO2 nanoparticles at relatively low temperature (100 8C) [23]. It was already observed that the photocatalytic activity of TiO2 depends on its crystalline structure, crystal size, and surface area. TiO2 anatase phase photocatalyst thin films were prepared from peroxo titanic acid refluxed sols using titanyl sulfate (TiOSO4) as a titanium source [24]. Titanium tetrachloride (TiCl4) was applied to synthesize peroxotitanium solution and it was noted that TiO2 nanopowder with a high surface area can be obtained after heating at 80 8C [25]. Previously, our group has reported the synthesis of aminefunctionalized SiO2/TiO2 photocatalytic films prepared from refluxed PTA solution at 100 8C for 10 h. The results show that the adsorption capability of photocatalyst plays an important role in VOC degradation [26]. In this work, refluxed PTA solution at 100 8C for 10 h was applied as a starting material for TiO2 nanoparticles to synthesize GR–TiO2 composite. Graphene oxide– TiO2 was reduced to graphene–TiO2 using UV-assisted photocatalytic reduction without heating. The photocatalytic activity of prepared GR–TiO2 (PTA) catalysts was examined by evaluating the degradation of MB aqueous solution under UV and visible light irradiation. 2. Experimental 2.1. Materials Titanyl sulfate (TiOSO4, Sigma–Aldrich), hydrogen peroxide (H2O2, 30%, Merck), ammonia solution (NH4OH, 30%, Merck), sodium nitrate (NaNO3, Rankem), potassium permanganate (KMnO4, Qrec), sulfuric acid (H2SO4, 97%, Merck), natural graphite (fine powder extra pure, Merck), Sodium sulfate (Na2SO4, Sigma– Aldrich) and ethanol (C2H5OH, Merck). All chemicals were high purity reagents and were used without further purification. 2.2. Preparation of graphene oxide powder Graphene oxide was synthesized using Hummer’s method. The concentrated H2SO4 (32 mL) and NaNO3 (1.0 g) were mixed and stirred in an ice bath. After NaNO3 was completely dissolved, natural graphite (1.0 g) was added to the mixture. KMnO4 (4.5 g) was slowly added to the above mixture under continual stirring at 30 8C for 1 h. The mixture solution was diluted by adding 250 mL of distilled water and further heated at 98 8C for 1 h. The remaining KMnO4 in the mixture was terminated by adding 10 mL of 30%
H2O2. The dark brown precipitate was separated by centrifugation and washed with distilled water for purification until a neutral pH of 7 was reached. The graphene oxide power was obtained after drying at 35 8C [1]. 2.3. Preparation of peroxo titanic acid (PTA) solution and TiO2 (PTA) nanoparticles Titanyl sulfate (TiOSO4, 5.0 g) was dissolved in distilled water. The white precipitate (Ti(OH)4) was observed by adding NH4OH (1 M). After filtering and rinsing with de-ionized water several times, H2O2 (0.98 M) solution was added to the Ti(OH)4 at a 1:5 molar ratio of H2O2 to titanium under continual stirring until the yellow transparent PTA solution was obtained. The PTA solution was diluted with de-ionized water in a 500 mL volumetric flask. TiO2 nanoparticles can be obtained by refluxing PTA solution at 100 8C for 2 and 10 h. In order to elucidate the effect of the reflux process on the functional groups of PTA molecules, PTA powder was prepared by drying PTA solution at 35 8C [24,27]. 2.4. Preparation of GR–TiO2 (PTA) photocatalysts Graphene oxide was added at weight ratios of graphene oxide to TiO2 of 1:100, 1:50, 1:20, and 1:10 to the TiO2 (PTA refluxed at 100 8C for 10 h) colloidal suspension (50 mg/L) in ethanol (70%). Next, graphene oxide–TiO2 suspensions were irradiated with two 8 W UVA lamps for 48 h. GR–TiO2 powders were obtained after drying in a hot air oven at 50 8C for 6 h [13]. 2.5. Characterization The crystalline structure of the TiO2 photocatalyst was determined by X-ray diffractometer (Bruker, D8-Discover). Nickel-filtered CuKa radiation (l = 0.15418 nm) was used with a generator voltage of 40 kV and a current of 40 mA, scanned at a speed of 0.18/s and step size of 0.028 at an angular range of 20–808. The crystalline sizes of anatase and rutile were calculated via Scherrer’s equation:
D¼
kl Bcos u B
where D is the crystallite particle size, l is the wavelength of CuKa irradiation (0.15406 nm), k is a constant of 0.9, B is the full width at half maximum, and uB is Bragg’s angle of the anatase (1 0 1) plane [20]. The functional groups of catalysts were determined by Fourier transform infrared (FTIR) spectroscopy (Perkin-Elmer, Spectrum One). The nitrogen adsorption/desorption isotherms were measured at a liquid nitrogen temperature of 77 K using a high precision surface area and pore size analyzer (Belsorp, Belsorpmini). The specific surface area and total pore volume were calculated by the Brunauer–Emmett–Teller (BET) method. The morphology and selected area electron diffraction pattern (SAED) of catalysts was observed using a transmission electron microscope (TEM, JEOL, JEM-2100). The absorption edge wavelength of catalysts was recorded by UV–Vis spectrometer (UV1900, UV/VIS, Hitachi). Photocurrent tests were measured using automated potentiostat (ACM Instrument, Gill AC). The conventional threeelectrode cell configuration with a standard Ag/AgCl as a reference electrode, inert platinum (Pt) sheet as a counter electrode and a working electrode was immersed in the 0.1 M Na2SO4 solution. A black light blue (BLB, l = 365 nm, Philips) or fluorescent lamp (l > 420 nm, Osram) was used as the source of the UV and visible light, respectively.
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
2811
2.6. Photocatalytic activity Photocatalytic activities of TiO2 (PTA) and GR–TiO2 (PTA) series samples were evaluated by degradation of MB aqueous solution. An irradiation box with dimensions of 410 mm 350 mm 123 mm was equipped with UV or visible light lamps: two 8 W black light blue (BLB, l = 365 nm, Philips) or 8 W fluorescent lamps (l > 420 nm, Osram), respectively. The light sources were placed parallel to one another at an equal distance of 20 mm from the photoreaction vessel. The experiment was conducted in two steps. In the first step, the catalyst suspension comprising catalyst powder (1 mg) dispersed in 40 mL of 4.5 ppm MB solution was kept in the dark for 3 h in order to ascertain the adsorption–desorption equilibrium. In the second step, under ambient conditions and stirring, the beakers were exposed to UV or visible light for 15, 30, 45, 60, 90, and 150 min, respectively. Then, 3.0 mL of sample was taken out after a determined period of time and centrifuged at 5250 rpm for 10 min. The concentration of MB in the sample was determined by UV–Vis spectrophotometer (U1900 UV/VIS, Hitachi) at a visible wavelength line of 659 nm. The photocatalytic activities of TiO2 (PTA) and GR–TiO2 (PTA) series were measured in terms of the degradation efficiency (%) of MB by the following equation:
Fig. 2. Degradation efficiency of TiO2 (PTA) at various refluxed time.
Fig. 1 shows the X-ray diffraction (XRD) patterns of TiO2 (PTA refluxed at 100 8C for 2 h), TiO2 (PTA refluxed at 100 8C for 10 h) and GR–TiO2 (PTA) composites, there are five distinctive TiO2 peaks at 25.38, 37.98, 48.08, 54.68, and 62.88 which correspond to anatase phase (JCPDS 21-1272). It is clear from Fig. 1 that TiO2 synthesized via PTA as a precursor demonstrates exclusively anatase phase. The crystal sizes of TiO2 (PTA refluxed at 100 8C for 2 h) and TiO2 (PTA refluxed at 100 8C for 10 h) calculated from Scherrer’s equation are 12.9 and 14.8 nm, respectively. The intensity of the anatase diffraction peak increases with increasing reflux time and the
crystal size of anatase phase tends to increase in size because of the enhanced crystallinity. The XRD diffraction patterns of GR–TiO2 (PTA) composites are similar to that of pure TiO2. This implies that the crystal structure of TiO2 (PTA) do not change during the graphene oxide reduction process. However, the increase in crystal size of GR–TiO2 (PTA) about 15.2 nm may be attributed to the enhanced crystallinity during drying of GR–TiO2 (PTA) catalyst in a hot air oven at 50 8C for 6 h. The photocatalytic activities of TiO2 at various refluxed time are investigated as shown in Fig. 2. The degradation efficiency of MB in the presence of TiO2 (PTA) increases with increasing refluxed time because of the enhanced crystallinity (see Fig. 1(a) and (b)). It was noted that TiO2 (PTA refluxed at 100 8C for 10 h) with 40.3% photodegradation seems to be an optimal refluxed time for preparation TiO2 nanoparticles. The functional groups of GR–TiO2 (PTA) composite, TiO2 (PTA refluxed at 100 8C for 10 h), graphene oxide powder and PTA (nonrefluxed) are presented in Fig. 3. A broad absorption band at 3000– 3600 cm1 and a strong peak at 1665 cm1 can be assigned to the vibration of OH groups of adsorbed water and Ti–OH group. The adsorption peak at around 1400 cm1 is due to the stretching vibration from the N–H bond of the residual NH4+. The peak at 900 cm1 can be assigned to the stretching mode of the peroxo group. The intensity of this peak decreased during refluxing because of the decomposition of the peroxo group [21,24]. The
Fig. 1. XRD diffraction patterns of (a) TiO2 (PTA refluxed at 100 8C for 2 h), (b) TiO2 (PTA refluxed at 100 8C for 10 h), (c) GR–TiO2 (PTA, 1:100), (d) GR–TiO2 (PTA, 1:50), (e) GR–TiO2 (PTA, 1:20) and (f) GR–TiO2 (PTA, 1:10).
Fig. 3. FTIR spectra of (a) GR–TiO2 (PTA) composite, (b) TiO2 (PTA refluxed at 100 8C for 10 h), (c) graphene oxide powder and (d) PTA (non-refluxed).
Degradation efficiency ð%Þ ¼
C0 Ci 100 C0
where C0 is the initial concentration of MB and Ci is the concentration of MB solution after UV or visible light irradiation. 3. Results and discussion 3.1. Characterization of synthesized materials
2812
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
Fig. 4. Photographs of (a) graphene oxide powder, (b) TiO2 (PTA refluxed at 100 8C for 10 h) and (c) GR–TiO2 (PTA). (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
absorption peak between 400 and 690 cm1 was corresponded to the signal of Ti–O–Ti bond. In the case of graphene, the peaks at 1723, 1600 and 1228 cm1 are assigned as the carbonyl groups (C5 5O), hydroxyl groups (OH) and epoxy groups (C–O), respectively. The FTIR data of GR–TiO2 (PTA, Fig. 3(a)) shows different FTIR spectral patterns in comparison with that of graphene oxide (Fig. 3(c)). After the graphene oxide reduction process, the peaks of the carbonyl and epoxy groups disappeared. This implies that the functional groups on graphene oxide were completely reduced to graphene using the UV-assisted photocatalytic reduction method. In addition, it was also observed that the signal of Ti–O–Ti shifted to a higher wavenumber around 790 cm1 because of the combined signal of Ti–O–Ti and Ti–O–C vibration [17,18]. The interaction between the functional groups of graphene oxide and the OH group on the surface of TiO2 nanoparticles is a basic mechanism to obtain GR–TiO2 composites [13]. Fig. 4 shows the digital photographs of graphene oxide powder, TiO2 (PTA refluxed at 100 8C for 10 h), and GR–TiO2 (PTA). Graphene oxide has a brown color and the TiO2 (PTA refluxed at 100 8C for 10 h) powder is light-yellow in color because of the remaining peroxo group. After graphene oxide reduction under UV irradiation for 48 h, the color of GR–TiO2 (PTA) changed to light gray. The wrinkled two-dimensional structure of graphene oxide can be clearly observed in Fig. 5(a). In the case of TiO2 (PTA refluxed at 100 8C for 10 h), the needle-like or rhombus anatase crystals have an average length of 40–80 nm and diameter of 10–20 nm as shown in Fig. 5(b). Fig. 5(c) shows the TEM image of GR–TiO2 (PTA) and Fig. 5(d) is a high resolution TEM (HRTEM) image of GR–TiO2 (PTA) sample. The crystal spacing of 0.235 nm is observed and corresponding to the (0 0 1) plane of anatase. Fig. 5(e) shows the selected area electron diffraction (SAED) pattern of GR–TiO2 (PTA). The diffraction rings of GR–TiO2 (PTA) indexed to (1 0 1), (0 0 4), (2 0 0) and (1 0 5) crystal planes of anatase TiO2, are in accordance with the XRD analysis. The results demonstrate that TiO2 nanoparticles are successfully loaded onto graphene sheet. However, the crystal size of TiO2 (PTA) calculated using Scherrer’s equation (14.8 nm) was different from that observed in the TEM
image (40–80 nm in length and 10–20 in diameter) because of the effect of diffraction line width and instrumental broadening [24]. The formation of TiO2 anatase phase using PTA has been reported. The white precipitate of Ti(OH)4 is dissolved in H2O2 solution to form a homogenous yellow aqueous PTA solution. The complex anion of titanium is basically considered as binuclear species, Ti2O5(OH)x(x2) (x > 2). When increased the temperature, binuclear complex species condenses to octahedral TiO6 units in hydrated form and further crystalize to anatase TiO2 via facesharing condensation [21,23,28–30]. The UV–vis absorption spectra of TiO2 (PTA) and GR–TiO2 (PTA) catalysts are presented in Fig. 6. The spectra of GR–TiO2 (PTA) catalysts show a red shift in the band gap transition with increases in the amount of graphene dopant. The absorption below 400 nm results from the excitation of electrons from the valence band (VB) to the conduction band (CB) of TiO2 [18]. The band gap energy of catalysts can be estimated by the Kubelka–Munk function [10,12,31]. 1=2
ðahnÞ
¼ Ai ðhn Eg Þ
where a is the absorption coefficient (cm1). hn is the photon energy (eV) and Ai is a constant value. In Fig. 7, the estimated band gaps of GR–TiO2 (PTA) are 3.24, 3.22, 3.20, 3.09, and 2.90, corresponding to TiO2 (PTA), GR–TiO2 (PTA, 1:100), GR–TiO2 (PTA, 1:50), GR–TiO2 (PTA, 1:20) and GR– TiO2 (PTA, 1:10), respectively. Analogous results for band gap narrowing of the GR–P25 series were also found [10,12]; the decrease in the band gap in the GR–TiO2 composite was due to the interaction between TiO2 (PTA) and graphene, which can be clearly observed from the FTIR broad absorption of Ti–O–Ti and Ti–O–C vibration around 790 cm1. Fig. 8 shows the photocurrent behavior of TiO2 (PTA) and GR– TiO2 (PTA) photocatalysts under UV and visible light irradiation. It was observed that photocurrent of all samples show a uniform photoresponse. The photocurrent generations from GR–TiO2 (PTA) composites are higher than that of pure TiO2 (PTA). The GR–TiO2
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
2813
Fig. 5. TEM image of (a) graphene oxide, (b) TiO2 (PTA refluxed at 100 8C for 10 h), (c) GR–TiO2 (PTA), (d) HRTEM image of GR–TiO2 (PTA) and (e) SAED pattern of GR–TiO2 (PTA).
3.2. Photocatalytic activities
(PTA, 1:50) photocatalyst, which demonstrated the highest photocurrent generation, yielded the current density of 4.33 and 2.98 mA/cm2 under UV and visible light illumination, respectively. The increasing in photocurrent can be attributed to the twodimensional p–p conjugation structure of carbon atoms of graphene resulting in an excellent electrical conduction. However, the decreasing in photocurrent can be observed on excessive graphene content in GR–TiO2 (PTA, 1:20 and 1:10) composites. The higher graphene content in GR–TiO2 (PTA) composite may hinder or scatter the light transmission. Therefore, the photoexcitation of TiO2 could be decreased because of light obstruction of graphene [8,11].
Fig. 9 shows the concentration of MB solution after reaching the adsorption–desorption equilibrium in dark conditions. The equilibrium concentrations of MB in the presence of GR–TiO2 (PTA) series are lower than that of pure TiO2 (PTA). This confirms that MB was adsorbed more on the surface of GR–TiO2 (PTA) photocatalysts. In addition, GR–TiO2 (PTA) demonstrates the increased adsorption capacity with increasing weight fraction of graphene. The photocatalytic activities of P25, TiO2 (PTA) and GR–TiO2 (PTA) photocatalysts under UV and visible light irradiation are presented in Figs. 10 and 11, respectively. The order of photo-
Fig. 6. UV–vis absorption spectra of TiO2 (PTA) and GR–TiO2 (PTA) catalysts with different weight ratio of graphene.
Fig. 7. A plot of (ahn)1/2 versus photon energy of TiO2 (PTA) and GR–TiO2 (PTA) catalysts.
2814
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
Fig. 8. Photocurrent generation from catalysts under UV and visible light irradiation (a) TiO2 (PTA), (b) GR–TiO2 (PTA, 1:10), (c) GR–TiO2 (PTA, 1:20), (d) GR–TiO2 (PTA, 1:50) and (e) GR–TiO2 (PTA, 1:100).
degradation of MB under UV irradiation is as follows: GR–TiO2 (PTA, 1:50) > GR–TiO2 (PTA, 1:20) > GR–TiO2 (PTA, 1:10) > GR– TiO2 (PTA, 1:100) TiO2 (PTA) > P25. Under visible light irradiation, the degradation efficiency of MB follows the order as below: GR–TiO2 (PTA, 1:50) > GR–TiO2 (PTA, 1:20) > GR–TiO2 (PTA, 1:100) > GR–TiO2 (PTA, 1:10) > TiO2 (PTA) > P25. The optimal amount of graphene seems to be within the range of 1:10–1:100 because the photocatalytic activities of GR–TiO2 (PTA) at 1:10 and 1:100 become close to that of pure TiO2 (PTA). The appropriate amount of graphene to TiO2 for preparation GR–TiO2 (PTA) in this experiment is about 1:50. The decrease in MB degradation
efficiencies of GR–TiO2 (PTA, 1:10 and 1:20) under UV and visible light compared to that of GR–TiO2 (PTA, 1:50) was ascribed to light obstruction. In addition, the change of photocatalytic activities under UV and visible light irradiation are observed between GR– TiO2 (PTA, 1:10) and GR–TiO2 (PTA, 1:100) photocatalysts. Under visible light irradiation, the photocatalytic activity of GR–TiO2 (PTA, 1:10) is lower in comparison with that of GR–TiO2 (PTA, 1:100). As the results from UV–vis absorption spectra (Fig. 6) and photocurrent generation (Fig. 8) of photocatalysts, it was noted that GR–TiO2 (PTA, 1:10 and 1:20) yields higher absorption over all visible light wavelengths but the photocurrent generations of that
Fig. 9. Concentration at equilibrium of MB solution in the presence of TiO2 (PTA) at different weight ratio of graphene.
Fig. 10. Photocatalytic degradation of MB solution over TiO2 (PTA), GR–TiO2 (PTA), P25 catalysts and GR under UV irradiation.
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
2815
Table 1 Texture characterization of catalysts. Sample
BET surface (m2 g1)
Total pore volume (cm3 g1)
Average pore diameter (nm)
P25 TiO2 (PTA) GR–TiO2 (PTA, GR–TiO2 (PTA, GR–TiO2 (PTA, GR–TiO2 (PTA,
50.75 88.00 93.76 94.13 102.00 105.70
0.47 0.36 0.33 0.38 0.20 0.31
37.24 16.20 14.07 16.21 8.02 11.56
1:100) 1:50) 1:20) 1:10)
photocatalyst under UV and visible light irradiation are lower than that of GR–TiO2 (PTA, 1:50). This phenomenon is attributed to the high photon absorption of excessive graphene. These evidences imply that the overdose graphene content in GR–TiO2 reduces the light absorption efficiency of TiO2 [19,20]. In dark condition, MB molecules adsorbed onto catalysts and reached equilibrium within 3 h. The remaining concentration of MB in the presence of GR–TiO2 composites is lower than that of TiO2 as shown in Fig. 9. This result is corresponding with the BET specific area of TiO2 (PTA), GR–TiO2 (1:100), GR–TiO2 (1:50), GR– TiO2 (1:20) and GR–TiO2 (1:10) as shown in Table 1. It was obvious that the BET specific area of GR–TiO2 (PTA) increases with increasing weight ratio of graphene resulting in an increase in adsorbability. In this study, the photodegradation of MB in the presence of P25 photocatalyst under UV and visible light irradiation is not as high as that of TiO2 (PTA). The average crystal size of P25 calculated using Scherrer’s equation is about 21.1 nm and it was also observed that P25 has lower BET specific area than that of TiO2 (PTA). Therefore, the decrease in photocatalytic activity of P25 is due to its lower specific surface area. The possible mechanism for enhanced adsorbability on GR–TiO2 (PTA) should be a noncovalent formation which is formed on the surface of graphene via the interaction between the p–p stacking of MB molecules and aromatic regions of graphene [32]. It has been reported that 17b-estradiol molecules were trapped on graphene pores and further degraded by Fe doped TiO2 [33]. Thus, the MB molecules, also adsorbed, trapped and then degraded by TiO2 photocatalyst. The photocatalytic activity of GR–TiO2 (PTA) under visible light irradiation was influenced by narrowing of the band gap from 3.24 to 2.90 eV, which resulted in the formation of Ti–O– C bonds. The GR–TiO2 (PTA, 1:50) catalyst showed higher photocatalytic activity than any other catalyst. However, in the case of GR–TiO2 (PTA, 1:20) and GR–TiO2 (PTA, 1:10) the BET surface areas are 102.00 and 105.70 m2 g1, respectively; both catalysts show larger surfaces than that of GR–TiO2 (PTA, 1:50) but
their photocatalytic activities are not as high as that of GR–TiO2 (PTA, 1:50). Therefore, the addition of higher graphene content in a GR–TiO2 (PTA) photocatalyst not only leads a decrease of photocatalytic activity because of light obstruction but also shields the active sites on TiO2 photocatalyst [12]. Theoretically, it is well known that the electrons in the valence band (VB) can be excited to the conduction band (CB) of TiO2 by photo-irradiation. This reaction leads to positive holes in the VB and electrons in the CB that can further react with adsorbed water and oxygen molecules to produce hydroxyl radicals (OH) and superoxide radical anions (O2) which can oxidize volatile organic compounds [34]. The formation of OH and O2 in the presence of TiO2 photocatalyst is well known and written as shown in Eqs. (1)– (5) [27,35]. TiO2 þ hn ! hþ þ e
(1)
O2 þ e ! O2
(2)
2O2 þ 2H2 O ! 2H2 O2 þ O2
(3)
H2 O2 þ e ! OH þ OH
(4)
hþ þ TiOH ! TiOH
(5)
The photocatalytic activity of GR–TiO2 (PTA) composite is improved by separation of photo-generated electrons and holes. Graphene acts as electron acceptor and transporter due to its p–p conjugation structure [14,36,37]. The TiO2 electrons from the VB can be excited to the CB, and then rapidly transfer to graphene, leaving more electron–hole pairs to produce highly reactive species (OH and O2), resulting in higher photocatalytic activity. 4. Conclusion The GR–TiO2 photocatalysts were synthesized via PTA as a precursor with different weight ratios of graphene. GR–TiO2 (PTA) contains anatase phase crystal with an average length of 40–80 nm and diameter of 10–20 nm. The surface area of GR– TiO2 (PTA) photocatalyst increased with increasing weight ratio of graphene. Under UV and visible light irradiation, GR–TiO2 (PTA, 1:50) photocatalyst has the optimal amount of added graphene to achieve the highest photocatalytic activity, while a higher weight ratio of graphene content seems to decrease the photocatalytic activity. The enhancement of photocatalytic activity can be attributed to the influence of the increase in adsorption capacity and the reduction of the band gap by the formation of Ti–O–C bonds together with the separation of the electron–hole pairs. Acknowledgements
Fig. 11. Photocatalytic degradation of MB solution over TiO2 (PTA), GR–TiO2 (PTA), P25 catalysts and GR under visible light irradiation.
This work was supported by the Royal Golden Jubilee Ph.D. Program, the Thai Research Fund (TRF), and the Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi.
2816
W. Low, V. Boonamnuayvitaya / Materials Research Bulletin 48 (2013) 2809–2816
References [1] G. Li, T. Wang, Y. Zhu, S. Zhang, C. Mao, J. Wu, B. Jin, Y. Tian, Appl. Surf. Sci. 257 (2011) 6568–6572. [2] Z. Qianqian, B. Tang, H. Guoxin, J. Hazard. Mater. 198 (2011) 78–86. [3] X. Zhang, Y. Sun, X. Cui, Z. Jiang, Int. J. Hydrogen Energy 37 (2012) 811–815. [4] D. Zhao, G. Sheng, C. Chen, X. Wang, Appl. Catal. B: Environ. 111/112 (2012) 303–308. [5] N. Zhang, Y. Zhang, X. Pan, M.Q. Yang, Y.J. Xu, J. Phys. Chem. C 116 (2012) 18023– 18031. [6] S.P. Daniel, R. Dreyer, W. Christopher, Bielawski, S. Rodney, Ruoff, Chem. Soc. Rev. 39 (2010) 228–240. [7] G. Jiang, Z. Lin, C. Chen, L. Zhu, Q. Chang, N. Wang, W. Wei, H. Tang, Carbon 49 (2011) 2693–2701. [8] F. Wang, K. Zhang, J. Mol. Catal. A: Chem. 345 (2011) 101–107. [9] H. Zhang, X. Lv, Y. Li, Y. Wang, J. Li, ACS Nano 4 (2010) 380–386. [10] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 4 (2010) 7303–7314. [11] P. Wang, Y. Ao, C. Wang, J. Hou, J. Qian, J. Hazard. Mater. 223/224 (2012) 79–83. [12] P. Cheng, Z. Yang, H. Wang, W. Cheng, M. Chen, W. Shangguan, G. Ding, Int. J. Hydrogen Energy 37 (2012) 2224–2230. [13] G. Williams, B. Seger, P.V. Kamt, ACS Nano 2 (2008) 1487–1491. [14] N.R. Khalid, Z. Hong, E. Ahmed, Y. Zhang, H. Chan, M. Ahmad, Appl. Surf. Sci. 258 (2012) 5827–5834. [15] N. Zhang, Y. Zhang, X. Pan, X. Fu, S. Liu, Y.J. Xu, J. Phys. Chem. C 115 (2011) 23501– 23511. [16] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 5 (2011) 7426–7435. [17] T.D. Nguyen Phan, V.H. Pham, E.W. Shin, H.D. Pham, S. Kim, J.S. Chung, E.J. Kim, S.H. Hur, Chem. Eng. J. 170 (2011) 226–232. [18] Y. Min, K. Zhang, W. Zhao, F. Zheng, Y. Chen, Y. Zhang, Chem. Eng. J. 193/194 (2012) 203–210.
[19] Y. Zhang, N. Zhang, Z.R. Tang, Y.J. Xu, ACS Nano 6 (2012) 9777–9789. [20] Y.J. Liu, M. Aizawa, Z.M. Wang, H. Hatori, N. Uekawa, H. Kanoh, J. Colloid Interface Sci. 322 (2008) 497–504. [21] N. Sasirekha, B. Rajesh, Y.W. Chen, Thin Solid Films 518 (2009) 43–48. [22] T.X. Liu, F.B. Li, X.Z. Li, J. Hazard. Mater. 152 (2008) 347–355. [23] S.I. Seok, M. Vithal, J.A. Chang, J. Colloid Interface Sci. 346 (2010) 66–71. [24] L. Ge, M.X. Xu, M. Sun, Mater. Lett. 60 (2006) 287–290. [25] J. Lu, L.P. Bauermann, P. Gerstel, U. Heinrichs, P. Kopold, J. Bill, F. Aldinger, Mater. Chem. Phys. 115 (2009) 142–146. [26] V. Boonamnunyvitaya, S. Photong, Water Air Soil Pollut. 210 (2009) 453–461. [27] C. Kaewtip, P. Chadpunyanun, V. Boonamnuayvitaya, Water Air Soil Pollut. 223 (2012) 1455–1465. [28] N. Murakami, Y. Kurihara, T. Tsubota, T. Ohno, J. Phys. Chem. C 113 (2009) 3062– 3069. [29] Y. Gao, Y. Masuda, Z. Peng, T. Yonezawa, K. Koumoto, J. Mater. Chem. 13 (2002) 608–613. [30] H. Ichinose, M. Terasaki, H. Katsuki, J. Sol–Gel Sci. Technol. 22 (2001) 33–40. [31] L. Ge, X. Mingxia, F. Haibo, M. Sun, Appl. Surf. Sci. 253 (2006) 720–725. [32] Y. Zhang, C. Pan, J. Mater. Sci. 46 (2011) 2622–2626. [33] F. Nasrin, C.R. Roy, M.G. Yaocihuatl, M.B. Ray, P.A. Charpentier, Appl. Catal. B: Environ. 110 (2011) 25–32. [34] T.K. Tseng, Y.S. Lin, Y.J. Chen, H. Chu, J. Mol. Sci. 11 (2010) 2336–2361. [35] A. Houas, H. lachhed, M. Ksibi, E. Elaloui, C. Guillard, J.M. Herrmann, Appl. Catal. B: Environ. 31 (2001) 145–157. [36] N. Farhangi, R.R. Chowdhury, Y. Medina Gonzalez, M.B. Ray, P.A. Charpentier, Appl. Catal. B: Environ. 110 (2011) 25–32. [37] C. Hou, Q. Zhang, Y. Li, H. Wang, J. Hazard. Mater. 205/206 (2012) 229–235.