Enhanced photocatalytic degradation and adsorption of methylene blue via TiO2 nanocrystals supported on graphene-like bamboo charcoal

Enhanced photocatalytic degradation and adsorption of methylene blue via TiO2 nanocrystals supported on graphene-like bamboo charcoal

Accepted Manuscript Title: Enhanced photocatalytic degradation and adsorption of methylene blue via TiO2 nanocrystals supported on graphene-like bambo...

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Accepted Manuscript Title: Enhanced photocatalytic degradation and adsorption of methylene blue via TiO2 nanocrystals supported on graphene-like bamboo charcoal Author: Fangjun Wu Wei Liu jielong Qiu Jinzhen Li Wuyi Zhou Yueping Fang Shuting Zhang Xin Li PII: DOI: Reference:

S0169-4332(15)01974-1 http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.161 APSUSC 31108

To appear in:

APSUSC

Received date: Revised date: Accepted date:

30-7-2015 17-8-2015 19-8-2015

Please cite this article as: F. Wu, W. Liu,Qiu, J. Li, W. Zhou, Y. Fang, S. Zhang, X. Li, Enhanced photocatalytic degradation and adsorption of methylene blue via TiO2 nanocrystals supported on graphene-like bamboo charcoal, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.08.161 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced photocatalytic degradation and adsorption of methylene blue via TiO2 nanocrystals supported on graphene-like bamboo charcoal

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Fangjun Wu, Wei Liu,* jielong Qiu, Jinzhen Li, Wuyi Zhou, Yueping Fang, Shuting Zhang, * Xin Li*

College of Materials and Energy, South China Agricultural University, Guangzhou

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d

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an

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510642, P. R. China

*

Corresponding author at: College of Materials and Energy, South China Agricultural University, Guangzhou 510642, P. R. China. Tel.: +86 20 85280325; fax: +86 20 85282366. E-mail address: [email protected] (W. Liu), [email protected] (S. Zhang),[email protected] (X. Li) 1

Page 1 of 39

Abstract : In this study, a novel efficient photocatalytic nanomaterial, TiO2 nanocrystals supported on graphene-like bamboo charcoal, has been successfully synthesized via a facile multi-step process. The structural and optical properties of the as-prepared samples were characterized by different techniques, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis absorption

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spectroscopy, photoluminescence spectra (PL), Raman spectra and nitrogen adsorption–desorption isotherms. The photocatalytic activities under sunlight were

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evaluated by the degradation of methylene blue (MB). The results indicated that the

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ternary hybrid photocatalysts exhibited much higher photocatalytic activities toward the degradation of MB than the pure TiO2 under UV light irradiation. Moreover, the

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optimum weight content of graphene-like bamboo charcoal in composite photocatalysts was 6 wt % for achieving the maximum photocatalytic degradation rate. The apparent rate constant of the best sample (0.0509 min−1) was about 3 times

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greater than that of the commercial P25 (0.0170 min-1). The adsorption and degradation kinetics of MB can be described by the pseudo-first-order model and

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apparent first-order kinetics model, respectively. The highly enhanced photocatalytic

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performance was attributed to the synergetic effect of graphene-like carbon and bamboo charcoal, which lead to the promoted charge separation and reduction reaction of oxygen, and enhanced adsorption capacities of MB, respectively. The composite photocatalysts displayed a high photochemical stability under repeated irradiation. This work may provide new insights and understanding on the graphene-like bamboo charcoal as an excellent support for photocatalyst nanoparticles to enhance their visible-light photocatalytic activity.

Keywords: TiO2; Bamboo charcoal; photocatalysis; degradation; graphene-like carbon

2

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1. Introduction Over the past decades, heterogeneous semiconductor photocatalysis has attracted enormous attention, because of its significant and valid applications in environmental remediation,[1, 2] solar-fuel production,[3-7] and organic photosynthesis[8, 9]. To

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date, numerous types of active photocatalysts have been reported.[2-4] Among them, TiO2 is still one of the most suitable photocatalysts for widespread environmental

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applications due to its outstanding stability, good photostability, relatively high oxidation ability, nontoxicity and low price. To date, pure TiO2 have been widely

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applied in photocatalytic mineralization of gaseous/liquid pollutants,[10-12] CO2 reduction,[13-16] NOx oxidation,[17, 18] reduction of Cr(VI),[19-22] inactivation of

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bacteria,[23-25] and oxidation of ammonia[26] under UV light illumination. Unfortunately, TiO2 suffers from the limited absorption of solar light as a result of its

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large band gap (~3.2 eV) and low quantum yield because of the fast recombination of photo-generated charges.[4] Furthermore, the agglomeration of TiO2 nanoparticles

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could significantly decrease their photocatalytic activity. In addition, it is also difficult

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to separate and recover the conventional nanosized TiO2 photocatalysts from the aqueous suspensions after the photocatalytic reaction due to their small particle sizes, thus leading to the increased cost of industrial applications and the re-pollution of the treated water.[27] To solve these abovementioned problems, a variety of strategies have been employed to develop highly photoactive and effectively recyclable TiO2-based photocatalysts, such as introducing localized electronic states into TiO2 by impurity doping,[28-31] immobilizing TiO2 nanoparticles onto an inert and porous supporting

materials,[32]

incorporating

magnetic

components

into

TiO2

nanoparticle,[27, 33, 34] altering the exposed crystal facets ratio,[35-37] constructing nanostructures[38, 39] and fabricating heterojunctions[40-43]. In particular, the hybridization of TiO2 nanoparticles with various nanocarbon materials, including graphene, carbon nanotubes, fullerenes, graphite oxide, carbon fiber and activated carbon has been proven to be the most promising method for increasing the photocatalytic activity.[44-49] 3

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Since Kamat and co-workers pioneeringly fabricated the TiO2 –reduced graphene oxide (RGO) nanocomposites through in situ UV-induced photocatalytic reduction of GO in 2008,[50] graphene as a new type of two-dimensional carbon nanomaterials has been extensively used to construct highly effective photocatalysts for the diversified photocatalytic applications.[51-54] Clearly, graphene not only has a

capacity,

but

also

exhibits a

high electrical conductivity

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very large specific surface area, strong chemical stability and good adsorption and

a

perfect

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two-dimensional (2D) structure. These properties make it an excellent support for developing hybrid photocatalysts due to promoted charge transfer and separation, as

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compared to other nanocarbon materials or 2D materials such as carbon nanotubes, fullerene, 2D TiO2 and g–C3N4 nanosheets.[53] Recently, Zhu’s group reported that a

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monomolecular layer of conjugative π structure material such as such as C60,[55, 56] polyaniline,[57-59] g–C3N4,[60, 61] graphene-like[62-64] and graphite-like carbon[65,

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66] can obviously enhance the photocatalytic activity of semiconductor photocatalysts through the in-situ surface hybridization, due to the efficient electronic coupling of π

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states of conjugated materials and conduction band states of semiconductors. For

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example, it was found that the enhanced photocatalytic activity of hybridized graphene-like carbon/TiO2 photocatalysts for degradation of methylene blue (MB) could be achieved through via a facile in-situ graphitization of melamine because of the improved photo-induced charge separation.[62] Moreover, it has been also widely reported that the direct coupling of graphene and TiO2 could also lead to the significantly improved photocatalytic activity for the degradation of polluants.[44-46, 49, 67-69] Although the partly enhanced photocatalytic activity of TiO2 could be achieved through the surface hybridization with graphene or graphene-like carbon materials, some disadvantages of the hybridized TiO2/graphene photocatalysts in practical applications such as the obvious agglomeration and difficulty in the recovery are still needed to be solved. Furthermore, the instinct structure difference between inorganic TiO2 surface and organic pollution molecule consisting of aromatic rings, is unfavorable for the adsorption of organic molecules on TiO2 due to the weak interactions between them, 4

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thus significantly limiting the rate of photocatalytic degradation.[45] Thereby, from an engineering point of view, it is generally accepted that the introduction of porous carbon supporters such as activated carbon, activated carbon fibers and bamboo charcoal could not only efficiently increase intimate interface relationship between between TiO2 photocatalyst and organic molecule, but also overcome the limitations

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of agglomerate and separation of TiO2 photocatalyst, because of their high surface

area, high adsorption capacity, large pores (macroporosity) and particle sizes.[70-74]

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In general, the hierarchical and three-dimensional pores in carbon supports would provide the perfect channels for the mass diffusion, transport and adsorption of

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pollutants molecules, thus leading to the enhanced accumulation of carriers and improved efficiency of photocatalytic degradation.[74] Particularly, bamboo charcoal

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(BC), as a potential support for TiO2 photocatalyst, has drawn increasing attention in the advanced oxidation degradation processes of polluants, due to improved degree of

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dispersion and inhibited TiO2 aggregation.[70, 75-77] For example, Wu et.al suggested that TiO2 nanoparticles were mostly deposited on the bamboo charcoal

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surface rather than in the mouth of pore channels.[75] Furthermore, the linearity

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relationship between the overall photocatalytic efficiency and the TiO2 loading indicated that each dispersed TiO2 particulate could act as an identical location for photocatalytic reactions, thus leading to the significantly enhanced photocatalytic activity. It was also revealed that the significant synergistic effects of adsorptive and photocatalytic performance of the photocatalysts also play very important roles in enhancing the degradation efficiency.[70, 72] Thus, it is naturally expected the graphene is in-situ formed on the surface or inside the pores of bamboo carbon. As a result, the hierarchically porous structure of bamboo carbon and the unique electronic conductive property of graphene will be synergistically utilized for design and synthesis of highly effective photocatalysts. Furthermore, inspired by the in-situ formation of 2D or 3D graphene by brief pyrolysis of the organic carbon sources such as food, insects, waste, thin polymer films or a metal ion-exchange resin at low temperatures (<1000 ℃) in inert gas atmosphere,[78-80] the phenolic resin as raw materials could be expected to be in-situ converted into graphene-like carbon inside 5

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the pores of bamboo carbon. However, to our knowledge, the in-situ fabrication of TiO2 nanocrystals supported on graphene-like bamboo charcoal and their application in the photocatalytic oxidation of MB have not been reported. Thus, it is highly expected that the introduction of bamboo carbon can not only increase the surface area, adsorption and dispersity, and decrease the aggregation of graphene-like

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carbon/TiO2 composites, but also facilitate their recovery.

Herein, we describe the in-situ fabrication of a new TiO2 nanocrystals supported

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on graphene-like bamboo charcoal via a two-step strategy. That is, graphene is firstly

in-situ synthesized in the porous bamboo carbon, and then TiO2 nanoparticles were

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deposited uniformly on the surface of graphene. The formation process of the nanocomposite is illustrated in Scheme 1. Remarkably, the TiO2/graphene/bamboo

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carbon composites exhibit excellent photocatalytic activities for degradation of MB under simulated solar irradiation. The design strategy would greatly improve the poor

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performance of the traditional photocatalyst TiO2. This unique structure could not only avoid the weak adsorption of MB on TiO2 nanocrystals and agglomeration of

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TiO2 and graphene, but also enable easier charge transport across the TiO2-GC

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interfaces and the recovery of photocatalysts. All these factors are beneficial for the enhancements of charge separation and photodegradation activity.

2. Experimental

2.1. Preparation methods

2.2.1 Pretreatment of the bamboo charcoal All of the reagents employed in this study were of analytical grade. The bamboo charcoal (denoted as BC) was synthesized according to the previously reported procedure. A certain amount of BC was put into deionized water, using the microwave to heat it for 30 minutes. After being washed and dried, the cleaning bamboo charcoal was heated and stirred in 6 mol/L HNO3 for 12 hours. Then cleaned BC were again washed with deionized water to pH = 7, and dried under 60

for 24

hours for further use. 6

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2.2.2 Preparation of phenolic resins The phenolic resins were synthesized following a previously reported procedure. An aqueous solution containing 600 mg of phenol, 2 mL of formaldehyde, and 15 mL of NaOH (0.1 mol/L) was firstly mixed and stirred at 70 °C for 0.5 h to obtain low-molecular-weight phenolic resins. After that, 0.96 g of triblock copolymer

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Pluronic F127 (Mw =12 600, PEO106PPO70PEO106, Sigma.) dissolved in 15 mL H2O

was added into the above solution and stirred for another 2 h. Then, 50 mL of

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Ni(NO3)2 (0.005 mol/L) was added into the same solution and further stirred for over

9 h at 70 °C until the deposition was observed. During this process, the aqueous

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solution changed from colorless transparent to green and finally to yellow. The

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solution was kept at room temperature for further use.[81] 2.2.3 Synthesis of graphene-like BC

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Firstly, 5 mL of the as-prepared solution was transferred into an autoclave (50 mL volume), and a certain amount of the BC was put into the solution. The mixture was

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kept at room temperature for 2h to achieve the sufficient contact between the BC and solution. After that, 30 mL of H2O was added to the solution, and the mixture was

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stirred for 2 h to ensure that all reagents were dissolved. Hydrothermal treatment was then carried out in an oven at 130 °C for 20 h. After cooling down to room temperature, the sediment was washed with water several times. It was then dried in an oven at 40 °C for 2 days, followed by carbonization at 500 °C for 2 h in argon to obtain the mesoporous carbon nanosheets in BC. The mesoporous carbon nanosheets were further carbonized at 900 °C for 3h in argon to obtain mesoporous graphene-like carbon nanosheets in BC, which is the so-called graphene-like BC in this paper (denoted as GC).

2.2.4 Preparation of TiO2-GC The anatase TiO2 sol (RS)[82, 83] was prepared in the following way: 7.996 g titanyl sulfate was dissolved in 100 mL the distilled water at room temperature, then precipitated by adding ammonia solution (NH4OH, 3mol/L), thus resulting in the formation of white precipitate [Ti(OH)4]-. The white precipitate was filtered and 7

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sufficiently washed with distilled water to remove the NH4+ and SO42− formed in the reaction. Then, 450 ml distilled water was added to disperse the precipitate homogeneously. The precipitate was peptized in 50 ml aqueous hydrogen peroxide (30%).Here, continual magnetic stirring was required to avoid the immediate dense gel formation during dissolution and to keep the reactant mixed uniformly. The

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obtained transparent sol adding different content of GC was kept under reflux condition (around 130℃) for 13h. Finally, the GC with anatase TiO2 crystals was

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obtained and dried in air oven. The obtained ternary composites with percentages of

GC at 3 wt%, 6 wt%, 9 wt% and 12 wt% were denoted as TiO2-3%GC, TiO2-6%GC,

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TiO2-9%GC and TiO2-12%GC, respectively.

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2.3. Characterization of Samples

The powder X-ray diffraction (XRD) patterns of the prepared samples were

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collected using an X-ray diffractometer with Cu target (36 kV, 25 mA). Scanning electron microscope (SEM) and transmission electron microscope (TEM) analyses

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were conducted with a Hitachi S4800 scanning electron microscope operated at 5 kV

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and a JEM-2010HR microscope (JEOL) transmission electron microscope at 200 kV, respectively. Nitrogen adsorption–desorption isotherms were obtained at 77 K on a Gemini VII 2390 surface area analyzer (MQL). UV–vis diffuse reflectance data were collected with a spectrophotometer (UV-2550, Shimadzu) equipped with an integrated sphere, and the Raman spectra were carried out by a Raman spectroscopy (Renishaw in via). Photoluminescence (PL) spectra were performed by a RF-5301PC (Shimadzu) using excitation wavelength of 300 nm. All the samples were degassed at 417.16K before the nitrogen adsorption measurements.

2.4. Adsorption kinetics and photocatalytic activity The adsorption kinetics of MB was also performed in this study. In a typical experiment, 0.05 g of photocatalyst powder was added to a 200 mL MB aqueous solution (30 mg L−1) in the dark. The suspension was then placed in a shaker at a rate 8

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of 150 rpm at 25 ℃. During the adsorption, exactly 5 mL of suspension were taken from the reactor at given time intervals, and centrifuged to analyze the dye concentration by recording variations in the absorption of MB using a UV-vis spectrophotometer (UV725-P) at 660 nm.

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The photocatalytic activity of the TiO2-GC was evaluated by the degradation of MB (30 mg/L). Firstly, 0.05 g of TiO2 or TiO2-GC was dispersed into a 200 mL MB

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aqueous solution in a quartz vessel, and then the dispersion were kept in the dark for 60 min at room temperature to establish the absorption equilibrium. The light

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irradiation to the solution was performed by a Xenon lamp (300 W) without using a UV cutoff filter. During the irradiation, 5 mL of suspension were taken from the

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reactor at given time intervals, filtered, and then analyzed by recording variations in the absorption of MB using a UV-vis spectrophotometer (UV725-P) at 660 nm.

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Controlled experiments using different radical scavengers (ammonium oxalate (AO) scavenger for photo-generated holes, benzoquinone (BQ) scavenger for superoxide

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radical species (O2−•), and tert-butyl alcohol (TBA) scavenger for hydroxyl radical species (•OH)) were performed similar to the above photocatalytic experiments

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except that the radical scavengers (1 mmol/L) were added to the reaction system. A recycled photoactivity on the used catalyst was carried out as following. Typically, after the reaction of 1st run under UV light irradiation, the suspensions were centrifuged to obtain the photocatalysts, which was washed with anhydrous ethanol and deionized water carefully and then dried in an oven. The fresh MB solution was mixed with the used photocatalyst to perform the 2nd run photoactivity testing. Similarly, the recycled 3rd and 4th tests were also performed.

Results and Discussion 3.1 XRD analysis

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XRD measurements were firstly used to investigate the crystalline structure and phase composition of the as-synthesized photocatalysts. The XRD patterns of the as-prepared TiO2-GC composites are shown in Figure 1. It is obvious that the TiO2-GC composites with different weight addition ratios of GC exhibit similar XRD patterns. The presence of characteristic peaks at about 25.3o, 37.9o, 48.0o, 54.3o, 55.2o

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and 62.7o are observed for all diffraction patterns, which can be readily indexed to the (101), (004), (200), (105), (211), (204), (116) planes of anatase TiO2 with tetragonal

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lattice parameters of a=3.785 Å and c=9.514 Å (JCPDS # 21–1272), respectively.[11,

36, 84, 85] No rutile diffraction peaks were detected in all samples, suggesting that

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GC and bamboo charcoal have no obvious effects on the transformation of anatase TiO2 to rutile phase. Based on the Debye–Scherrer equation,[40] the calculated

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crystalline grain size were summarized in Table 1. As observed from Table 1, it is clear that the lattice size decreased with increasing the content of the carbon

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composite, indicating that the introduction of GC and bamboo charcoal can efficiently inhibit the aggregation of TiO2 nanocrystals. Furthermore, for the three reference

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samples of GC, BC and BC(950 °C), a large broaden band from the 2θ of less than

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20–55◦ can be observed from Figure 1, in good agreement with the literature values.[72, 75, 86] In addition, it is also noticed that the graphite diffraction intensity of GC at 26.4° is obviously stronger those of BC and BC(950 °C), partially indicating the formation of graphene-like carbon on the surface of bamboo charcoal. All above results indicated anatase TiO2 has been loaded onto GC or BC successfully.

3.2. Microscopic studies

The morphologies of the samples were characterized via SEM, TEM, and HRTEM. The TEM images of the TiO2-GC with the GC content of 9% are shown in Figure 2a. It is clearly observed from Figure 2a that the TiO2 nanoparticles with the size of 8–15 nm are well deposited in the GC. And the selected area electron diffraction (SAED) pattern (inset in Figure 2b) confirms the formation of anatase TiO2. The structure of composites was further investigated by HRTEM and shown in Figure 2b. The clear 10

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lattice fringes in HRTEM image further confirm the high crystallinity of graphene-like carbon and TiO2 in GC. The lattice spacings of 0.345 and 0.204 nm correspond to the (101) crystal plane of TiO2 and (101) lattice plane of graphene-like carbon material, respectively.[87] A typical nanotubular structure in GC can be obtained through the ultrasonic process, which was shown in Figure 2c. It is clear

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observed from the inset of Figure 2c that some irregularly distributed graphene-like

carbon nanosheets with the thickness of about 2-4 nm can be formed on the wall of

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nanotubular structure in the natural bamboo by carbonizing phenolic resin. Figures 2d, 2e and 2f show that lots of TiO2 nanocrystals have been uniformly dispersed on GC.

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All these results indicated that the ternary hybrids have been successfully fabricated.

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3.3. Raman spectra

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To further confirm the simultaneous presence of TiO2, GC, and BC in the composite, the sample was studied using Raman spectroscopy. It is known that Raman spectroscopy is a powerful tool for characterizing the electronic structure of

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the graphitized carbon materials. Figure 3 shows the Raman spectra of TiO2-GC and

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pure TiO2. For TiO2, four characteristic Raman bands can be observed at around 144, 393, 513 and 635 cm-1 corresponding to E1g, B1g, A1g+B1g and E2g modes, respectively,[88] which are similar to the reported values for anatase TiO2. In the spectra of GC, there are two typical bands, the D band at 1354 cm-1 and the G band at 1598 cm-1.[89] The D band is a common feature for sp3 defects form disordered/amorphous carbon, which is attributed to the A1g vibration mode of nanocrystalline graphite, while the G band provides information on in plane vibrations of sp2 bonded carbons, which can be assigned to the E2g vibration mode. The graphitization degree can be identified by the ratio of the integrated area of the D band and the G band, R (ID/IG). A lower R value generally represents a higher graphitization degree of the carbon. For TiO2-GC, the four peaks of anatase TiO2 still exist. Compared to TiO2-3% GC, the D-band of TiO2-12% GC shifts 6 cm−1 to high wave numbers, and the G-band of shifts 4 cm−1 to high wave numbers, respectively, 11

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indicating the increased number of layers in graphene. A marked decrease in the intensity ratio of D and G (ID/IG) is also observed. The ratios for TiO2-3%GC, TiO2-6%GC, TiO2-9%GC and TiO2-12%GC are 0.895, 0.833, 0.823 and 0.753, respectively. The result shows that more numerous sp2 domains have been formed in TiO2-GC. This shows that the proportion of sp2-hybridization carbon atoms in the

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sample increases with increasing the concentration of graphene-like carbon in TiO2-GC. In addition, the sp2 hybridization of carbon atoms has the lone pair

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electrons that do not participate in the hybridization, which can form large the

as the rapid conduction electrons in a material.[90]

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3.4. Textural properties

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conjugative π system with other sp2 hybridization of carbon atoms. Thus, they can act

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The N2 adsorption-desorption isotherms of some typical nanocomposites are performed to obtain further information about their porous structures. For comparison, the results of pure TiO2 and GC are also included. Figure 4 shows the isotherms and

d

pore size distribution of different samples. It can be seen that the nitrogen

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adsorption–desorption isotherms of all composites and GC show the classical type IV curve with a H4 adsorption hysteresis loops according to the International Union of Pure and Applied Chemistry classification, suggesting the presence of mesopores.[91] The corresponding pore size distribution curves of the different samples were further deduced using the BJH method (the inset in Fig 4b), further confirming the existence of mesopores. The structure parameters of the photocatalysts are summarized in Table 2. As shown in Table 2, the specific surface area of the TiO2-9%GC sample reached 277.2 m2 g−1, which was almost three times that of the pure TiO2 (97.26 m2 g−1). Clearly, the Brunauer–Emmett–Teller (BET) surface area of TiO2-GC is gradually enhanced with increasing the contents of GC, while the pore diameter is decreased. The results indicate that the introduction of GC could greatly increase the BET surface areas of composite materials, which generally increase with decreasing

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the pore sizes.[92, 93] The data further confirms that these composite photocatalysts are mesoporous materials.

3.5 UV–vis spectroscopy The optical properties of the as-fabricated composites were further investigated by

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UV-vis diffuse reflectance spectroscopy, as shown in Figure 5. It can be seen that the

pure TiO2 exhibits typical and intense absorption in the UV region shorter than 390

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nm, attributing to the electron transitions from the valence band (VB) to conduction

band (CB) of TiO2. Compared with the pure TiO2, the presence of different GC

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content significantly affects the light absorption properties of the composites. Although the absorption edge of the TiO2-GC composites shows no red-shift to higher

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wavelength, the absorption intensity significantly increases in the visible light region (400-800 nm), indicating the absence of C-doped TiO2.[94] The enhanced

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visible-light absorption should be attributed to the introduction of BC and GC.

d

3.6 Adsorption kinetics

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It is known that the adsorption capacity of a given semiconductors has an

important impact on its photocatalytic performance. To demonstrate diversified engineering application of the ternary hybrid photocatalysts, we firstly used MB as a probe molecule to investigate the adsorption kinetics. For all samples, the adsorption tests were conducted for 60 min. The corresponding adsorption kinetics of MB on different photocatalysts was shown in Figure 6. Clearly, the introduction of GC or BC can greatly increase the adsorption rate of MB on the composite photocatalysts, as compared to the pure TiO2 and P25.

To further compare their adsorption rate, two widely used models (the pseudo-first-order and pseudo-second-order models) were employed to fit the date. The differential and integral forms of the pseudo-first-order model can be described as:[95, 96] dqt/dt= k1(qe-qt)

(1)

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ln(qe-qt)= lnqe-k1t

(2)

The pseudo-second-order model can be expressed as:[95-97] dqt/dt= k2(qe-qt)2

(3)

t/qt=1/(k2qe2)+t/qe

(4)

where k1 and k2 are the first-order and second-order rate constants for the

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adsorption process, respectively; qt is the amount of MB adsorbed (mg/g) at various time t, qe is the maximum adsorption capacity and t is the adsorption time (min).

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The parameters were calculated and presented in Table 3. According to the results, the pseudo-first-order model demonstrated better fitting for all activated carbons than

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the pseudo-second -order model. The results are different from those in literatures, where the pseudo-second-order model fitted well for bamboo-based activated

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carbon.[95, 96] The possible reason may be that the bamboo charcoal deposited by the graphene-like carbon layers exhibits a decrease in the adsorption capacity of MB,

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due to due to altered surface performances. It is also noted that the maximum adsorption capacities were significantly influenced by the surface areas, while the k1

d

values become larger with increasing the pore diameters. Nevertheless, it is clear that

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all composite photocatalysts exhibited significantly improved adsorption kinetics, further confirming that the introduction of bamboo charcoal or graphene-like carbon could efficiently improve the adsorption performances of composites. In addition, it should be noted that the P25 and TiO2 possess the slowest adsorption rates and the lowest adsorption capacities, further indicating the weak interactions between the inorganic TiO2 surface and MB molecules. In a word, all results clearly indicate that the enhanced adsorption kinetics of MB over different composite photocatalysts is strongly dependent on the increased surface areas due to the introduction of bamboo charcoal and graphene-like carbon, which is also benefical for the improvements of their photocatalytic activities.

3.6 Photocatalytic activity and stability

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Page 14 of 39

The photocatalytic degradation kinetics of MB on different photocatalysts was also shown in Figure 7a. As shown in Figure 7a, the MB is unstable and can be easily decomposed under UV light irradiation without photocatalysts. Furthermore, it can be also seen that photocatalytic activities of TiO2-6%GC and TiO2-3%GC are obviously better than the pure TiO2 and TiO2-6%BC, especially better than P25. More than 90%

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of MB has been degraded over them within 2 h. However, only 70% of MB could be

decomposed over P25 after 2.5 h irradiation. Obviously, the content of GC plays an

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important role in improving their photocatalytic activities. The optimal content of GC is 6%. Continuing to increase the content of GC, the photodegradation efficiency of

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MB gradually decreased, which may be due to the mask effects of excess GC.[53]

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It is known that the kinetic linear curves of photodegradation of MB follow a Langmuir–Hinshelwood apparent first-order kinetics model, described by:[98]

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r =dC/dt=kKC/(1+KC)

(5)

where r is the degradation rate of the reactant (mg L-1min-1), C the concentration

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d

of the reactant (mg L-1 ), t the UV light irradiation time, k the reaction rate constant (mg L-1min-1), and K the adsorption coefficient of reactant (L mg-1). When the initial concentration (C0 ) was very low (C0=30 mg L-1 for MB), eqn (1) can be simplified to an apparent first-order model: [99]

ln(C0/C) = kKt=kat

(6)

where C0 is the initial concentration of dye, ka is the apparent first-order rate constant, C is the concentration of the dye and t is the illumination time. For comparison purposes, the rate constants can be obtained from the slope of linear fitting. The apparent first order linear transforms are given in Figure 7b. The fitting parameters were presented in Table 4. It is clear that all reaction kinetics for the photodegradation of MB over different photocatalysts followed the apparent first-order equation, with higher R2 values. The obtained apparent rate constants for the degradation of MB over MB, P25, TiO2, TiO2-3%GC, TiO2-6%GC, TiO2-9%GC, 15

Page 15 of 39

TiO2-12%GC and TiO2-6%BC are 0.0118, 0.0170, 0.0198, 0.0379, 0.0509, 0.0218, 0.0089 and 0.0273 min−1, respectively. Obviously, the apparent rate constant of TiO2-6%GC (0.0509 min−1) was about 3 times greater than that of the commercial P25 (0.0170 min-1). The possible reason was that the graphene-like structure in GC could accept the photo-excited electrons from TiO2 particles quickly and effectively

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and suppressed the electron-hole recombination. The prolong lifetime of charge

carriers in TiO2-GC composites leads to more active oxidizing agents generation,

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which could improve the photodegradation of MB significantly in comparison to the

pure TiO2. In addition, it should be also noted that the apparent rate constant for the

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degradation of MB over TiO2 is slight better than that over P25, which may be due to that the surface area of TiO2 prepared by our group (97.26 m2·g-1) is about two times

an

higher than that of P25 (52.7 ± 3.6 m2·g-1).[100] More importantly, the enhanced adsorption kinetics may play very important roles in improving the photocatalytic

M

degradation activities of TiO2-GC photocatalysts due to the introduction of graphene-like carbon. The hierarchical pores of BC in the GC would provide the best

Ac ce pt e

and graphene, respectively.

d

channel for MB to flow, transmit and absorb, and prevent the agglomeration of TiO2

An ideal photocatalyst should maintain photochemical durability under repeated irradiation. To further evaluate the stability and reusability of the TiO2-GC hybrid photocatalysts, the recycled photoactivities were therefore measured by executing repeated photodegradation reaction of MB over TiO2-6%GC for four times under UV light irradiation. The results were shown in Figure 8. As observed from Figure 8, no distinct activity decay is observed after four recycling runs. These results indicate that the composite photocatalyst are relatively stable and cannot be easily photocorroded during the photocatalytic process.

3.7 Effect of operating parameters 16

Page 16 of 39

In general, the solution pH has a vital influence on the photocatalytic degradation of MB. Thus, the effect of pH on the photo-degradation of MB was carefully optimized in the pH range 1−9, and the corresponding results are shown in Figure 9a. As shown in Figure 9a, it can be seen that the weak acid or alkaline conditions are favorable for the photocatalytic degradation of MB. When the pH value of the

ip t

solution is 8, the degradation rate is the fastest. After 60 min, the absorbance of solution reaches about 0. The surface of the photocatalyst in alkaline condition is

cr

negatively charged, which is benefical for the adsorption of MB with positively charges. Furthermore, under UV light irradiation, the photo-generated holes can react

us

with the adsorpted OH- and facilitate the formation of hydroxyl radicals (•OH), whose strong oxidation ability can significantly enhance the photodegradation efficiency of

an

MB. However, on the other hand, with increasing the pH value (more than PH8), the adsorpted OH- can cover the negatively charged surface of TiO2 particles, which is

M

not conducive and not favorable for the transfer of electrons on the surface of particles,

in the photocatalytic activity.

d

thus leading to the fast recombination of photo-generated electrons and the decrease

Ac ce pt e

The effect of the photocatalyst dosage on the degradation of MB has been carried

out through adjusting the concentrations of photocatalyst in solution. The corresponding results are shown in Figure 9b. For our system, the catalyst amount of 50mg in 200 mL of MB solution, the photodegradation efficiency is the highest. At higher concentration of the catalyst (above 50mg in 200 mL of MB solution), the decreased photodegradation efficiency This is due to the reduced utilization rate of light, which is caused by diffuse reflection and scattering effects.[101] Thus, the optimum photocatalyst dosage on the degradation of MB (50mg in 200 mL of MB solution) can be identified in our system.

3.8 Photocatalysis mechanism It is known that the photoluminescence (PL) spectra is often employed to reveal the separation performances of electron–hole pairs in semiconductors.[6, 7, 102] 17

Page 17 of 39

Therefore, to further confirm the promoted separation of photo-generated electronic-hole pairs through the introduction of GC, the PL spectra of the composite photocatalysts were also investigated. Figure 10 presents the PL spectra of several representative composite photocatalysts at an excitation wavelength of 350 nm. As shown in Figure 10, the photoluminescence (PL) spectroscopy of all samples exhibit a

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strong emission peak centering at the light wavelength of about 368 nm, which can be attributed to the band-band PL phenomenon and is approximately equal to the

cr

band-gap energy(3.1eV) of TiO2. The PL intensities of the composite photocatalysts

are lower than that of the pure TiO2 and gradually decrease with the increasing of the

us

GC contents (for TiO2-3%GC and TiO2-6%GC composites with the GC contents of less than 6%). However, the PL intensities of composite photocatalysts gradually

an

increase with increasing the GC contents (for TiO2-9%GC and TiO2-12%GC composites with the GC contents of more than 6%). The results further indicated that

M

the excess GC is not favorable for the enhancement in the photocatalytic degradation

GC.

d

activity of composite photocatalysts toward MB, due to the mask effects of the excess

Ac ce pt e

Generally, it is well accepted that a series of reactive oxygen species, such as photo-generated holes (h+), hydroxyl radicals (•OH), or superoxide radicals (O2−•), play important key roles in the photocatalytic redox process.[103-105] To further determine the role of these primary active species for photodegradation, a series of controlled experiments with adding different scavengers have been performed. Figure 11 shows the photocatalytic activities of TiO2-6%GC toward the degradation of MB in presence of different scavengers under UV light irradiation, i.e., ammonium oxalate (AO) scavenger for holes, benzoquinone (BQ) scavenger for O2−•, and tert-butyl alcohol (TBA) scavenger for • OH, respectively.[106] It can be seen that the photocatalytic activity of TiO2-6%GC toward the degradation of MB could be significantly inhibited by the addition of TBA, BQ and AO in the solution due to the inhibited charge transfer to MB molecules by the scavengers. This results clearly suggest that three kinds of reactive oxygen species (h+, •OH, and O2−•) are all the active species for photo-degradation of MB. Comparatively speaking, the strong and 18

Page 18 of 39

non-selective hydroxyl radicals are not the most active species for the photocatalytic degradation of MB under UV light irradiation in our system. On the contrary, the addition of BQ could drastically inhibit the photocatalytic degradation of MB in these photocatalytic redox processes, indicating that O2−• played a decisive role in determining the photodegradation of MB. The results provided direct evidences to

ip t

support the argument that the superoxide radicals (O2−•) are the main oxygen active species the photodegradation of MB under UV light irradiation. Analogous

cr

phenomena has also been observed in other systems.[107] It was also further

confirmed that the single-electron reduction reaction of O2 is the rate-determining step

us

in solar photocatalytic mineralization of MB.[103, 105, 108]

On the basis of the above results, the photocatalytic mechanism for photocatalytic

an

degradation of MB over ternary hybrids of TiO2, graphene-like carbon and bamboo charcoal has been proposed and presented in Scheme 2. As shown in Scheme 2,

M

TiO2 nanocrystals supported on graphene-like bamboo charcoal can be only excited to generate the photo-generated electrons in the conduction band (CB) and the holes in

d

the valence band (VB) of TiO2 nanocrystals, respectively, by UV light irradiation.

Ac ce pt e

Then, the excited electrons in CB of TiO2 could be transferred to the adjacent graphene-like carbon due to its high electrical conductivity and suitable Fermi level position.[53] Subsequently, the O2 molecules adsorbed on the surface of TiO2 nanocrystals or graphene-like carbon, can react with photo-excited e− to generate superoxide radical anions (O2−•). Interestingly, it has been demonstrated that the graphene-like carbon itself as cocatalyst can also promote the reduction reaction of oxygen.[109] Furthermore, the photo-excited h+ in CB of TiO2 can react with OH-/H2O to produce active hydroxyl radicals (·OH). It has been demonstrated by controlled experiments that these three kinds of reactive radical species (·O2−, ·OH and h+) are powerful oxidizing agents, all of which can attack MB molecules adsorpted on the bamboo charcoal and finally degrade them into H2O and CO2.[110] Thus, from what has been discussed above, it is clear that the synergistic effect of graphene-like carbon and bamboo charcoal leads to the greatly enhanced photocatalytic activity. On the one hand, the in-situ formed graphene-like carbon can 19

Page 19 of 39

accelerate the charge separation and the reduction reaction of oxygen; on the other hand, the bamboo charcoal can enhance the adsorption capacities of MB over composite photocatalysts. Therefore, the research may provide a new strategy to synergistically improve the adsorption and photocatalytic degradation activities of MB over TiO2 through the introduction of both graphene-like carbon and bamboo

ip t

charcoal.

cr

4 Conclusions

In summary, the graphene-like bamboo charcoal could be prepared through the

us

high-temperature treatment of phenolic resin in the bamboo charcoal. Then, TiO2 nanocrystals supported on graphene-like bamboo charcoal was further fabricated by

an

liquid-precipitation method. The results demonstrated that the TiO2 nanocrystals were uniformly loaded on the surface and in the pores of the graphene-like bamboo

M

charcoal. It was found that that the ternary hybrids exhibit better photodegradation degradation efficiency and adsorption capacities toward MB, as compared with the

d

pure TiO2 and P25. The optimum photocatalytic efficiency for MB degradation could

Ac ce pt e

be obtained under the suitable conditions (50mg of photocatalyst in 200 mL of MB solution (30 mg/L) at pH=8). The highly enhanced photocatalytic performance was attributed to the synergetic effect of graphene-like carbon and bamboo charcoal, which lead to the promoted charge separation and reduction reaction of oxygen, and enhanced adsorption capacities of MB, respectively. The graphene-like bamboo charcoal may provide an excellent support to improve the photocatalytic degradation activities toward the degradation of pollutant molecules over various semiconductor nanocrastals. Accordingly, the composite materials also show a potential application in dealing with the environmental contaminants because of their high photocatalytic activities under sunlight.

20

Page 20 of 39

Acknowledgements The work was supported by the National Natural Science Foundation of China (20906034, 21173088, 21207041 and 31101854), the National Science and Technology Support Project of China (2015BAD15B03), and the Dongguan Innovative Research Team Program (2014607101005). This work also partly

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supported by the State Key Laboratory of Advanced Technology for Material

cr

Synthesis and Processing (Wuhan University Of Technology) (2015-KF-7).

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d

nanocomposites.

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Scheme 1. The formation of the TiO2/graphene/bamboo carbon

Scheme 2 Proposed mechanism scheme for photocatalytic degradation of MB under UV light irradiation.

26

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(200)

(204)

(004)

(105) (211)

(101)

(116)

TiO2-6% GC TiO2-9% GC

ip t

TiO2-12% GC TiO2- 6% BC TiO2

cr

Intensity(a.u.)

TiO2-3% GC

GC 002

(100) (101) (002)

BC(950℃)

20

30

40

50

60

70

80

an

10

us

BC

2Theta(degree)

Ac ce pt e

d

of GC.

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Figure 1 XRD patterns of TiO2-6%BC,TiO2,GC and TiO2-GC with different contents

27

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Figure 2. TEM (a, d) ,HRTEM (b and c) and SEM(e) images of TiO2-GC, and SEM(f) images of BC

28

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Raman signal(a.u.)

GC TiO2-3% GC

ip t

TiO2-6% GC

cr

TiO2-9% GC

us

TiO2-12% GC TiO2

500

1000

1500

2000

2500

an

100

-1

wavenumber(cm )

Ac ce pt e

d

M

Figure 3. Raman spectra of TiO2-GC and TiO2

29

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a TiO2-9 %GC

200

GC

TiO2-6 %GC

ip t

150

cr

100

TiO2 TiO2-3 %GC

50 0 0.0

0.4

0.6

0.8

1.0

an

0.2

us

3

Quantity Adsorbtion(cm /g)

250

b

1.2

3

Ac ce pt e

Pore Volume(cm /g)

1.4

d

M

Relative Pressure

1.0

TiO2

0.8

TiO2-6% GC

0.6

TiO2-9% GC TiO2-3% GC

0.4

GC

0.2 0.0

-0.2

2

10

100

Pore Size(nm)

Fig 4. N2 adsorption–desorption isotherms (a) and pore size distribution (b) of different nanocomposites. 30

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contents.

d

Figure 5. UV-vis diffuse reflectance spectra of TiO2 and TiO2-GC with different GC

31

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TiO2+6% BC

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TiO2+9% GC

20

TiO2+3% GC

TiO2+6% GC TiO2+12% GC TiO2 P25

cr

qe /mg⋅g-1

30

15

30 45 60 Time /min

an

0

75

90

M

0

us

10

Figure 6. Adsorption kinetics of MB on different photocatalysts (C0 = 30 mg/L,

Ac ce pt e

d

V=200 mL, m=50mg, T=25 °C).

32

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ip t cr us an M d Ac ce pt e Figure 7. (a) Photodegradation kinetics of TiO2, TiO2-BC, TiO2-GC and MB samples for the removal of MB under solar simulated light; (b) The absorbance in the dark after 1h (a); dye removal efficiency over photocatalysts with different GC amounts(b);the influence of different PH(c); the repetitive testing and the content of catalyst(d) on photocatalytic activity of the samples. 33

Page 33 of 39

ip t cr us an M

Ac ce pt e

light irradiation.

d

Figure 8 Stability tests for the photodegradation of MB over TiO2-6%GC under UV

34

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ip t cr us an M d Ac ce pt e Figure 9 (a) Effect of solution pH on MB degradation (200 mL of MB solution (30 mg/L), the catalyst amount of photocatalyst: 50mg, a 300 W Xenon lamp); and (b) photodegradability of MB with different amounts of photocatalysts (200 mL of MB solution (30 mg/L), PH=7, a 300 W Xenon lamp). 35

Page 35 of 39

an

us

cr

ip t

PL intensitiy(a.u.)

TiO2-12% GC TiO2 TiO2-9% GC TiO2-3% GC TiO2-6% GC

250 300 350 400 450 500 550 600 650 700

M

wavelength(nm)

Ac ce pt e

contents.

d

Figure 10 Photoluminescence (PL) spectra of TiO2-GC composites with different GC

36

Page 36 of 39

1.0

catalyst catalyst TBA added catalyst BQ added catalyst AO added

ip t

0.6 0.4

cr

C/C0

0.8

us

0.2 0.0 10

20

30

40

50

60

an

0

Time(min)

M

Figure 11. Photocatalytic activities of TiO2-6%GC toward the degradation of MB in

Ac ce pt e

d

presence of different scavengers under UV light irradiation.

Table 1. Lattice size of TiO2, TiO2-6%BC and TiO2-GC

Sample

Half peak width(rad)

X-ray diffraction(°)

lattice size(nm)

TiO2

0.42

12.76

17.7981

TiO2-6%BC

1.33

12.66

5.9354

TiO2-3%GC

1.13

12.66

7.1252

TiO2-6%GC

1.32

12.66

5.9804

TiO2-9%GC

1.45

12.66

5.4442

TiO2-12%GC

1.37

12.66

5.7621

37

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Table 2. Parameters obtained from N2 desorption isotherm measurements. Average Pore

sample

SBET /m2·g-1

Pore Volume/cm3·g-1

TiO2

97.26

0.36

15.01

TiO2-3%GC

185.48

0.38

9.23

TiO2-6%GC

193.33

0.32

6.20

TiO2-9%GC

277.2

0.25

2.17

GC

405

0.23

5.54

us

cr

ip t

diameter/nm

Table 3 Kinetic model parameters for the adsorption of MB Pseudo-first-order

Pseudo-second-order

(mg·g qe(mg

k1(min-1 R2

−1

an

qexp

qe(mg·g k2(×10-4,g·(mg

·g−1)

, ×10-2)

P25

5.35

5.317

2.649

0.968

11.40

601

0.44

TiO2

8.72

9.00

4.547

0.979

16.18

102.6

0.88

TiO2-3%GC 17.50

16.68

3.977

0.982

27.38

26.48

0.945

TiO2-6%GC 13.17

18.80

8.77

0.835

28.04

15.49

0.609

TiO2-9%GC 27.26

26.61

4.634

0.990

39.90

4.3

0.996

TiO2-12%GC 9.15

10.86

8.31

0.949

12.62

100

0.952

TiO2-6%BC 33.26

35.41

7.57

0.992

42.48

1.91

0.987

d

)

Ac ce pt e

−1

R2

M

Samples

)

·min) −1)

38

Page 38 of 39

Table 4 The rate of absorption and photogradation of MB sample

Photodegradation Rp2

MB

1.18

0.98

P25

1.70

0.98

TiO2

1.98

0.98

TiO2-3%GC

3.79

TiO2-6%GC

5.09

TiO2-9%GC

2.18

TiO2-12%GC

0.89

TiO2-6%BC

2.73

ip t

Kp(10-2)

0.99

cr

0.99

0.98 0.99

Ac ce pt e

d

M

an

us

0.99

39

Page 39 of 39