Graphene facilitated visible light photodegradation of methylene blue over titanium dioxide photocatalysts

Chemical Engineering Journal 214 (2013) 298–303

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Graphene facilitated visible light photodegradation of methylene blue over titanium dioxide photocatalysts Shizhen Liu, Hongqi Sun, Shaomin Liu, Shaobin Wang ⇑ Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" Graphene–titania composites

(G–TiO2) were synthesized by a sol–gel method. " G–TiO2 catalysts showed high efficiency in degradation of methylene blue under visible light. " In situ prepared G–TiO2 presented higher activity than that of G–TiO2(P25). " The mechanism of graphene in the enhanced visible light photocatalytic activity was proposed.

a r t i c l e

i n f o

Article history: Received 19 July 2012 Received in revised form 29 October 2012 Accepted 30 October 2012 Available online 7 November 2012 Keywords: Graphene TiO2 Methylene blue Photocatalysis Visible light

a b s t r a c t Several graphene–titania composites (G–TiO2) were synthesized by a sol–gel method using titanium isopropoxide (or P25) as Ti-precursors and reduced graphene oxide (RGO). The structural, morphological, and physicochemical properties of the samples were thoroughly investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), UV–vis diffuse reflectance (UV–vis DRS), and thermogravimetric-differential thermal analysis (TG-DTA). A significant increase in light absorption to visible light was observed by G–TiO2 compared with that of naked TiO2. The photocatalytic activity of G–TiO2 in methylene blue bleaching under visible light (>430 nm) is much enhanced. G–TiO2 synthesized from titanium isopropoxide hydrolysis presented higher activity than that of G–TiO2(P25). Contribution of graphene on the enhancement of visible-light photocatalytic activity of the composite was discussed. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Photocatalytic decomposition of various organic compounds in aqueous solutions has been widely studied and many nanomaterials have been developed as photocatalysts for this technology [1–4]. TiO2 has been intensively investigated as a photocatalyst for environmental clean-up and solar energy conversion. However, TiO2 can only decompose aromatic organics into CO2 and H2O under UV-illumination and suffers from a barrier in responding ⇑ Corresponding author. Tel.: +61 8 93663776; fax: +61 8 92662681. E-mail address: [email protected] (S. Wang). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.10.058

to visible light at wavelengths higher than 387 nm due to a large band gap of 3.2 eV. As a result, only 3–5% of the solar energy that reaches onto the earth surface can be utilized. The common strategies for extending the absorption threshold of TiO2 to visible light region include doping, coupling or anchoring with other organic or inorganic elements such as nitrogen, carbon, halogen, and metals into the titania lattice [5–11]. Combination of different types of carbon with TiO2 has been suggested as a promising method for an enhanced photocatalytic performance [12,13]. In the past a few years, graphene as a novel carbonaceous nanomaterial has attracted more and more interests due to its unique and excellent performance in chemical, structural

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and electrical properties. Graphene is considered as a single sheet of graphite which has impressive theoretical surface area of 2600 m2/g [12]. Because it has a hexagonal honeycomb network structure, the sp2-bonded carbon lattice can have numerous outstanding electronic and spacious delocalized p-bonds which enhance its structure stability and conductivity capacity [14]. Several attempts in using graphene oxide (GO) or reduced graphene oxide (RGO) for modification of TiO2 for photocatalytic degradation of organics have been reported [15,16]. Zhang et al. [17] used a commercial TiO2(P25) and GO to obtain a TiO2–graphene nanocomposite. Nguyen-Phan et al. [18] prepared a TiO2(P25)–GO composite using a simple colloidal blending method. Liang et al. [19] reported a graphene/TiO2 nanocrystal hybrid fabricated by directly growing TiO2 nanocrystals onto GO sheets. The reported graphene/TiO2 nanocrystal hybrid has a superior photocatalytic activity over other TiO2 materials in the degradation of a dye of rhodamine B. Chen et al. [20] also investigated GO/TiO2 composites via a self-assembly method on GO using TiCl3 as a Ti-precursor. However, using P25 to deposit on GO usually resulted in aggregation of TiO2. In addition, exfoliated GO exhibits poor electronic conductivity because of the interruption of the p system by substitution with oxygen functional groups. Therefore, a form of graphene/TiO2 composite with a high interfacial contact of TiO2 with graphene surface without aggregation will be highly in demand for an improved photocatalytic performance. The unique structure will facilitate the charge separation and electron transfer from TiO2 to graphene upon irradiation [21]. In this paper, we report a preparation of G–TiO2 composite using a reduced graphene oxide and a titanium precursor by a sol–gel method. For a comparison, a G–TiO2(P25) was also synthesized. These photocatalysts were tested in photocatalytic degradation of a dye, methylene blue, under simulated sunlights and visible lights.

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furnace at 500 °C from ambient temperature, maintained for 5 min and then naturally cooled down to room temperature. In G–TiO2 samples, graphene loading was kept at 1, 3, 5, and 7 wt.% by adjusting RGO amount at 0.02, 0.06, 0.1 and 0.14 g, respectively. Graphene assisted commercial titanium oxide (TiO2–P25) was also prepared by the similar method as described above using 1.94 g P25 powders mixing with 0.06 g RGO and 0.5 g CTAB in 30 mL ethanol. The dried compound was annealed under 500 °C for 5 min. 2.3. Characterization of materials The crystalline structure of samples was analyzed by powder X-ray diffraction (XRD) using a Bruker D8-Advance X-ray diffractometer with Cu Ka radiation (k = 1.5418 Å) operated at 40 kV and 30 mA, respectively. FTIR analysis was performed on a Perkin–Elmer Model FTIR-100 with a MIR detector. UV–vis diffuse reflectance spectra (DRS) of samples were recorded on a JASCO V670 spectrophotometer with an Ø60 mm integrating sphere, and BaSO4 as a reference material. Field emission scanning electron microscopy (FE-SEM), performed on a Zeiss Neon 40EsB, was used to evaluate the morphology, size and texture information of the samples. Thermogravimetric-differential thermal analysis (TG-DTA) was carried out on a TGA/DSC 1 instrument of Mettler-Toledo under air flow at a heating rate of 10 °C/min. 2.4. Photocatalytic tests

Natural graphite powders (AF99, 325 mesh, 99.995% carbon content) were used for GO synthesis. All other reagents, sulfuric acid (95–98%, Shcarlau), KMnO4 (Fluka), H2O2 (30%, Biolab), were used as received. Titanium (IV) isopropoxide (TTIP) was used as a Ti-precursor and supplied by Aldrich Chemicals with a purity of 97%. A reference material of TiO2–P25 was received from Degussa, Germany.

Photocatalytic performances of various catalysts were evaluated by the photodegradation of methylene blue (MB) under either artificial solar light or visible light. In a typical process, aqueous solution of MB (10 mg/L, 200 mL) and the photocatalysts (100 mg) were put into a 1 L double-jacket cylindrical reactor with cycled cooling water (25 °C) under constant stirring. The photoreaction vessel was positioned 30 cm away from the radiation source with a cut-off filter. Two light sources were employed. One is UV– vis light with intensities at 2.31 lW/cm2 (220–280 nm), 6.94 mW/ cm2 (315–400 nm), 129.3 mW/cm2 (400–1050 nm). The other is visible light with an intensity of 84 mW/cm2 at 400–1050 nm. The reaction solution was firstly stirred for 30 min in dark to achieve adsorption equilibrium. The photocatalytic reaction was started by turning on a halogen lamp. At given time intervals, the suspension solution was centrifuged and the MB solution was analyzed by a JASCO UV–vis spectrophotometer at a wavelength of 664 nm.

2.2. Preparation of TiO2 and G–TiO2 samples

3. Results and discussion

Graphene oxide (GO) was prepared by a modified Hummers method [22,23] and the reduction of exfoliated GO was obtained by hydrothermal reaction using hydrazine hydrate. Typically, GO (100 mg) was loaded in a 250 mL round bottom flask with 100 mL deionized water and subjected to ultrasonic treatment for 2 h, yielding a homogeneous yellow–brown dispersion. Hydrazine hydrate (1.00 mL) was then added in and the solution was heated at 100 °C for 24 h. The reduced GO (RGO) was gradually precipitated as a black solid. This product was separated by filtration and washed with ethanol and water several times and then dried at 80 °C. For a typical synthesis of G–TiO2, RGO powder, cetyl trimethylammonium bromide (CTAB, 0.5 g) and 30 mL ethanol were placed in a 100 mL beaker with stirring. After 30 min, titanium isopropoxide (11 mL) was dropwised into the reactor. Then 20 mL deionized water was added into the mixed solution. The suspension was stirred for 8 h and dried at 80 °C. The solid was annealed in a muffle

Fig. 1 shows XRD patterns of prepared and commercial TiO2 and a variety of G–TiO2 photocatalysts. G–TiO2 and G–P25 showed different patterns owing to their varying crystalline structures of G–TiO2 (anatase) and G–P25 (30% rutile and 70% anatase). For synthesized TiO2 and G–TiO2, X-ray diffraction at 25.4°, 37.8°, 48.0°, 54.3°, and 62.7° were found, corresponding to the crystal planes (1 0 1), (0 0 4), (2 0 0), (2 1 1), (2 0 4) of anatase, respectively [24], suggesting a pure anatase phase of prepared and modified TiO2. The Scherrer equation [25] was used to estimate the size of crystallites of G–TiO2 samples and the particle diameter was obtained at about 8.6–9.1 nm. Meanwhile, P25 and G–P25 showed mixed crystalline phases of anatase and rutile. A comparison of P25 and G–P25 suggested that addition of graphene would not change the crystalline structure of TiO2. For all G–TiO2, no graphene peak is observed from XRD patterns, the same as other investigations [17,26,27]. This is probably due to low graphene content in the catalysts, the disruption and well exfoliation of reduced graphene oxide in the

2. Experimental 2.1. Materials and reagents

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(1) P25 (2) G-P25-3% (3) G-TiO2-7%

(101)A

Intensity/(a.u.)

(4) G-TiO2-5% (110)R

(5) G-TiO2-3% (200)A (6) G-TiO -1% 2

(004)A (101)R (111)R

(1)

(105)A(211)A(204)A

(3) (4) (5) (6)

20

30

40 50 2θ/degree

60

70

Fig. 1. XRD patterns of P25, G–TiO2 and G–P25. (A: anatase and R: rutile).

Weight loss (TGA,%)

(2)

100 80 60 40 20 0 100 80 60 40 20 0 100

GO

RGO

RAW

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DTG,%

300

80 70

composite or its low diffraction intensity (below the detection limit of the instrument) [18]. Li et al. [28] prepared mesoporous anatase TiO2 nanospheres/graphene composites by template-free self-assembly and detected a overlapping shoulder peak of graphene (0 0 2). TGA analysis showed that the graphene loading is high at 6.5 wt.%. FTIR spectra of GO, graphene, G–TiO2–3% before and after calcination were presented in Fig. 2. For GO, O–H stretching at 3000 cm 1, C–O stretching at 1030 cm 1, and C–OH stretching at 1165 cm 1 were clearly observed, suggesting the presence of hydroxyl, carboxyl and oxygenation functional groups. For RGO, the bands associated with the oxygen functional groups were entirely eliminated. The OH band also disappeared due to thermal treatment and hydrophobic surface of graphene [29,30]. However a new band at 1500 cm 1 was identified, which was attributed to the skeletal vibration of the graphene sheets [31]. For G–TiO2–3% skeletal ring vibrations of domains were observed around 1616 cm 1, which indicates the presence of Ti–O–C stretching [18,32,33]. Fig. 3 shows TG-DTA curves of GO, RGO, G–TiO2–3% before and after calcination. Weight loss below 110 °C was observed on all samples, contributing to the desorption of surface water. For GO, TG curve showed two clear steps of mass loss. The first one was due to the removal of oxygen-containing groups accompanied by the liberation of COx and H2O species from about 200–300 °C [34], and the second was owing to carbon combustion with a sharp exothermic peak at 580 °C in DTA curve. The RGO decomposition temperature is much higher than GO. The poor stability of GO

Transmittance (a.u.)

(1) (2)

C=O C-OH C-O

Graphene

(3) (4)

C=C; C-H -OH

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber (cm ) Fig. 2. FTIR spectra of GO (1), RGO (2), G–TiO2–3% before calcination (3) and G–TiO2–3% after calcination (4).

60 100

RGO@TiO2-3%

98 96 94 92 90

300

600

900

0

Temperature ( C) Fig. 3. TG-DTA spectra of GO, RGO, G–TiO2–3% before and after calcination.

was attributed to the hydroxyl and carboxyl groups that may reduce the oxidation activation energy surrounding the graphene sheets. Before calcination G–TiO2–3% dried gel did not show such clear steps of mass loss compared to GO. Two weak exothermic peaks were observed at 160 and 300 °C, respectively, due to desorption of water, ethanol, hydroxyl groups and decomposition of organic precursors from hydrolysis. The exothermic peak at about 450 °C was due to the combustion of organics. The broad peak at about 580 °C arose from decomposition of graphene. For G–TiO2–3% sample, the mass loss was much less than the associated dried gel. The weight content of reduction of GO in the composite was evaluated to be 3.8%. The chemical bonds formed via the calcination were expected to play an important role in the photo degradation [17]. Fig. 4 displays SEM images of RGO, G–P25–3% and G–TiO2–3% for evaluation of their morphologies. It was found that many layers of graphene sheets appear in RGO. For G–P25–3%, it was observed that titania nanoparticles were attached onto the surface of RGO sheets and they were intercalated between the graphene ‘‘Sandwich’’ constructed in an aggregation. P25 nanoparticles are in a diameter of 20–40 nm. No single RGO sheet was observed in G– P25–3%. For G–TiO2–3%, large aggregated TiO2 particles with diameter of several tens of nanometers were attached onto graphene sheets. Better interface between TiO2 and graphene was observed compared to G–P25–3%, owing to the in situ hydrolysis of Ti-precursor with graphene sheets. UV–vis diffuse reflectance spectra of P25, synthesized TiO2, G– P25–3% and G–TiO2–3% were shown in Fig. 5. After integration of RGO, the thresholds of G–TiO2–3% and G–P25–3% photocatalyst were extended to visible light region. In G–TiO2, graphene would work as a sensitizer and TiO2 would work as a substrate in the heterojunction [35], giving rise to the estimation of the band gap energies of the samples. The Kubelka–Munk equation, ahm = B (h/ k Eg) n (n = 0.5 for indirect transition), was used to estimate

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Fig. 4. SEM images of (A) RGO sheet, (B) G–P25–3%, (C) G–TiO2–3% in 2 lm, and (D) G–TiO2–3% in 300 nm.

0.8

(2)

0.5

(2) G-P25-3% (3) P25 (4) TiO2 Prepared

G-TiO2-7%

0.6

0.4 0.3

(3)

0.0

448.5

397.2

0.4 0.2

407.5

0.2

G-TiO2-5%

0.8

C/C0

0.6

Absorbance

G-TiO2-3%

(1) G-TiO2-3%

G-P25-3% TiO2 prepared

No radiation

0.7

0.1

G-TiO2-1%

1.0 (1)

P25 MB-UV+Visible

0.0 (4)

300

400

500

600

700

0

800

20

40

60

80

100

120

140

Irradiation Time (min)

Wavelength/nm Fig. 5. UV–vis diffuse reflectance spectra of P25, TiO2, G–TiO2–3% and G–P25–3%.

G-TiO2-1%

1.0

G-TiO2-3% G-TiO2-5%

0.8

G-TiO2-7% G-P25-3% P25 TiO2 Prepared

0.6

No radiation

C/C0

the band gap energies. The band gap of P25 is 3.04 eV, compared to the prepared anatase TiO2 of 3.12 eV. Zhang et al. [17] found that the narrowing of band gap of P25 occurred with the graphene introduction, which was attributed to the formation of Ti–O–C bond. Moreover, the band gap was significantly narrowed to 2.98 eV and 2.78 eV by the two graphene modified samples of G– P25–3% and G–TiO2–3%. Similar band gap narrowing was also observed in a recent study, in which a band gap energy of 2.80 eV was estimated for TiO2–graphene nanoparticles [36]. The greater band gap narrowing occurring on G–TiO2–3% was possibly attributed to the Ti–O–C bonds from hydrolysis of Ti-precursor which built new molecular orbital and narrowed the band gap [13,37]. Fig. 6 shows the efficiencies of various photocatalysts in MB degradation under either UV–visible or pure visible light irradiations. Under UV–vis light, MB presented gradual photolytic degradation. After 50 min irradiation, about 30% MB concentration was reduced. P25 demonstrated the highest activity in MB degradation

0.4 0.2 0.0

0

20

40

60

80

100

120

140

Irradiation Time (min) Fig. 6. Photodegradation of methylene blue solutions under solar irradiations (top) and visible light (bottom) .

with 100% at 50 min, meanwhile, G–P25–3% showed a higher adsorption of MB due to the addition of graphene. Several investigations

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have shown that graphene oxide and graphene are good adsorbents for MB adsorption [38,39]. However, G–P25–3% presented lower overall decolouration of MB at 100% in 80 min. The lower MB degradation rate is probably due to lower loading of P25 in solution, aggregation of G–P25, blockage of light by graphene to P25 catalyst surface and transformation to heat [40]. Zhang et al. [41] investigated MB degradation on G–P25 and found that the higher addition of graphene in G–P25 leads to a decreased photocatalytic activity. The prepared TiO2 demonstrated a high activity in MB degradation owing to the well growth of anatase crystalline. 98% MB degradation was achieved at 90 min. For G–TiO2 catalysts, G loading significantly influenced their catalytic activities. G–TiO2–5% presented the highest activity with 100% MB decomposition at 90 min. G–TiO2–3% was able to degrade MB at 100% in 110 min, G–TiO2–1% and G–TiO2–7% also showed comparable photocatalytic performance. Generally, the graphene modified TiO2 photocatalysts exhibited excellent performance in decomposition MB under visible light (wavelength longer than 420 nm). G–P25–3% composite demonstrated significant MB adsorption with about 30% reduction while other catalysts showed little MB adsorption. Under the visible light radiation, pure TiO2, either prepared sample or P25 presented little MB degradation, due to its low visible light absorption. G–P25–3% showed MB degradation with MB reduction from 70% to 40% in 90 min in photocatalytic reaction. For G–TiO2, MB decomposition rates also depended on graphene loading. G–TiO2–1% showed about 10% MB degradation in 90 min. Increased graphene loading amount would lead to a significant enhancement of the visible light photocatalytic activity of G– TiO2 samples. In 90 min, about 69% MB has been degraded for 3, 5 and 7 wt.% loading. In an extended time region, G–TiO2–3% was able to achieve 95% MB degradation in about 150 min. It was demonstrated that G–P25–3% showed a less photocatalytic activity under visible light irradiation, but it showed better activity than G–P25–3%. The lower activity of G–P25 than G-prepared-TiO2 might be due to the interface between graphene and TiO2. The results were also consistence to their optical property, in which G–TiO2–3% had strong visible light absorption (Fig. 5) and reduced band gap of 2.78 eV. The role of graphene in the visible light photocatalysis might be triple folds. (a) Increasing adsorption capacity of the photocatalyst. MB molecules would transfer from the solution to the catalysts’ surface and be adsorbed with offset face-to-face orientation via p–p conjugation between MB and aromatic regions of the graphene. Therefore, the adsorption of dyes is increased compared to bare TiO2. (b) Extending light absorption. The chemical bonds of Ti–O–C and good transparency of graphene render a red shift in the photo responding range and facilitate a more efficient utilization of light for the photocatalysis. (c) Suppressing charge recombination. Graphene can act as an acceptor of the photo-generated electrons for titanium dioxide particles and ensure a fast charge transportation in view of its high conductivity, and therefore, an effective charge separation can be achieved [15,17]. The better charge transportation would provide more photoinduced carriers for the associated photocatalytic reactions, leading to a higher photocatalytic activity.

4. Conclusion Graphene and titania composites were prepared by hydrolysis of titanium isopropoxide with reduced graphene oxide via a reduction of hydrazine hydrate. A good interface between graphene and TiO2 was created by such an in situ synthesis compared to G-

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