A novel route to graphite-like carbon supporting SnO2 with high electron transfer and photocatalytic activity

Accepted Manuscript Title: A novel route to graphite-like carbon supporting SnO2 with high electron transfer and photocatalytic activity Author: Xianjie Chen Fenglin Liu Bing Liu Lihong Tian Wei Hu Qinghua Xia PII: DOI: Reference:

S0304-3894(15)00039-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.01.037 HAZMAT 16538

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

20-10-2014 31-12-2014 13-1-2015

Please cite this article as: Xianjie Chen, Fenglin Liu, Bing Liu, Lihong Tian, Wei Hu, Qinghua Xia, A novel route to graphite-like carbon supporting SnO2 with high electron transfer and photocatalytic activity, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.01.037 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.

A novel route to graphite-like carbon supporting SnO2 with high electron transfer and photocatalytic activity Xianjie Chen, Fenglin Liu, Bing Liu, Lihong Tian*, Wei Hu, Qinghua XiaHighlights ►

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Highlights ► Mesoporous nanocomposites that graphite-like carbon supporting SnO2 are prepared by solvothermal method combined with a post-calcination. ► The polyvinylpyrrolidone not only promotes the nucleation and crystallization but also provides the carbon source in the process. ► The graphite-like carbon hinders the recombination of photogenerated electron and holes efficiently. ► The mesoporous carbon-SnO2 nanocomposite shows high photocatalytic activity on the degradation of Rhodamine B and glyphosate under simulated sunlight.

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Abstract

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Mesoporous graphite-like carbon supporting SnO2 (carbon-SnO2) nanocomposites were

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prepared by a modified solvothermal method combined with a post-calcination at 500℃ under a

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nitrogen atmosphere. The polyvinylpyrrolidone not only promotes the nucleation and

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crystallization, but also provides the carbon source in the process. The results of scanning

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electron microscopy and transmission electron microscopy show a uniform distribution of SnO2

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nanoparticles on the graphite-like carbon surface. Raman and X-ray photoelectron spectra

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indicate the presence of strong C-Sn interaction between SnO2 and graphite-like carbon.

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Photoelectrochemical measurements confirm that the effective separation of electron-hole pairs

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on the carbon-SnO2 nanocomposite leads to a high photocatalytic activity on the degradation of Rhodamine B and glyphosate under simulated sunlight irradiation. The nanocomposite materials

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show a potential application in dealing with the environmental and industrial contaminants under sunlight irradiation. *

Hubei Collaborative Innovation Center for Advanced Organochemical Materials; Ministry-of-Education Key

Laboratory for the Synthesis and Applications of Organic Functional Molecules, Hubei University, Wuhan 430062, P. R. China. Corresponding author: [email protected] (L. Tian)

Key words: SnO2 nanoparticles; graphite-like carbon; nanocomposite; glyphosate; photocatalysis 1. Introduction Semiconductor photocatalysis has attracted an increasing attention on degrading environmental pollution in past decades, driven by its utility of solar energy. Glyphosate, an organophosphorus herbicide, has been extensively used in order to increase the agriculture

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production. However, the residue has posed a threat to ecosystem, so the study on photocatalytic

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degradation of organophosphorus compounds is essential and practical. Up to now, most reported

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significant achievements have mainly focused on the studies of photocatalytic degradation of

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organophosphorus compounds and the mechanism on titanium dioxide catalyst because of its

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high photo-oxidation ability and chemical stability [1-3]. Compared to TiO2, SnO2 has more

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positive valence band and stronger oxidation ability [4]. Nevertheless, the application of SnO2

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semiconductor is restricted in the field of photocatalysis under the solar light due to low

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efficiency and narrow light response. At present, to improve its photocatalytic performance,

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many approaches have been tested by tuning its bandgap or inhibiting the recombination of

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photogenerated electron-hole pairs such as the formation of heterostructures with narrow

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bandgap semiconductors [5, 6], employing noble metal nanoparticles for SnO2 [7, 8] and

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non-metal doping SnO2 [9]. Carbon materials are widely studied for their conjugated structure, large surface area, and

unique electrical and optical properties. Carbon supported semiconductor nanoparticles like graphene, carbon nanotube and activated carbon have been made to enhance the photocatalytic performance of semiconductor photocatalysts. It is found that these conjugated carbon can inhibit

the recombination of photocarries efficiently, leading to the amazing photocatalytic performance of semiconductor [10-12]. Usually, the synthesis of carbon-semiconductor nanocomposites is complicated. The pre-treatment by strong oxidant or modification of carbon materials is essential to create active sites and groups. In addition, it is also a challenge to disperse metal oxide particles on the surface of carbon materials uniformly [13, 14]. Therefore, the development of novel and facile approaches for carbon-metal oxide nanocompsites with high activity is

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significant and meaningful.

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In this work, a modified solvothermal reaction involving the controlled and induced

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nucleation of SnO2 nanoparticles with the aid of polyvinylpyrrolidone (PVP) was developed to

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synthesize the PVP-SnO2 xerogel. During the solvothermal reaction, Sn-O bond formed via the

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ether elimination of tin alkoxides and the Sn2+ cation was oxidized to SnO2, while C=O groups of

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PVP were reduced. Then the mesoporous graphite-like carbon supporting SnO2 nanocomposite

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was obtained by carbonization of PVP-SnO2 xerogel in nitrogen gas at 500℃. The formation

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process of the nanocomposite is illustrated in Scheme 1. The photocatalytic activity of

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carbon-SnO2 nanocomposites on the degradation of Rhodamine B (RhB) and glyphosate under

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the simulated sunlight, and the photogenerated electron transfer between SnO2 and graphite-like

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carbon were investigated.

Scheme 1. The formation of the graphite-like carbon supporting SnO2 nanocomposite.

2. Experimental section

2.1 Synthesis of graphite-like carbon supporting SnO2 All chemical reagents (A. R. grade) were purchased from Sinopharm Chemical Regent Co., Ltd and used without further purification. Firstly, the PVP-SnO2 was synthesized by a solvothermal

method.

Typically,

0.5g

SnCl2 ⋅

2H2O

and

a

desired

amount

of

polyvinylpyrrolidone (PVP) were dissolved in 20 mL ethanol, respectively. Then above two solutions were mixed with a vigorous stir, and the resulting solution was transferred into a 50 mL

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Teflon-lined stainless autoclave and maintained at 120 °C for 24h. The xerogel was obtained by

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centrifugation separation, washed several times with distilled water and dried at 70°C for 24 h.

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The carbon-SnO2 nanocomposites were acquired by sintering the xerogel at 500 °C for 4 h under

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N2 atmosphere. The as-prepared carbon-SnO2 nanocomposites with different amounts of PVP

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(0.05, 0.1, 0.15 or 0.2g) were named S-1, S-2, S-3 and S-4, respectively. As a comparison, the

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sample that xerogel obtained with adding 0.15g PVP and calcined in O2 atmosphere at 500 °C is

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denoted as S-0.

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2.2 Characterization

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The crystal structure of the carbon-SnO2 nanocomposites was characterized by employing a

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Rigaku X-Ray Diffractometer (XRD) with a CuKα (λ=0.15418nm) radiation source. The UV-vis

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diffuse reflection spectra were obtained for the dry-pressed disk samples on a UV–Vis spectrophotometer (JASCO) by BaSO4 as the reflectance sample. The morphologies and particle

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sizes of the samples were observed by transmission electron microscopy (TEM, FEI Tecnai G20 and HRTEM, JEM2100) and scanning electron microscopy (SEM, JSM7100F). Simultaneous thermogravimetric and differential thermal analyses (TGA/DTA) of the samples were performed on a thermal analyzer (SDT Q600) in air with a heating rate of 10 °C·min-1. X-ray photoelectron

spectroscopy (XPS) measurements of the materials were carried out using a VG Multilab 2000 spectrometer with an AlKα X-ray source (Thermo Electron Corporation), and all the spectra were calibrated to the C 1s peak at 284.6 eV. Raman spectra were recorded on a laser micro-Raman spectrometer (Invia, Renishaw) with the exciting wavelength at 532 nm. BET surface area and pore structure of samples were performed on a Micromeritics ASAP2020 surface area and porosity analyzer. Prior to the BET analysis, the powder was degassed at 120 °C for 5 h to

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remove the adsorbed H2O from the surface.

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2.3 Photoelectrochemical measurement

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Photocurrents were measured on an electrochemical analyzer (CHI660C) in a standard

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two-electrode system with the as-prepared samples as the working electrodes and a Pt plate as

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the counter electrode. A 300 W Xenon lamp was used as the light source and the electrolyte was

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0.1mol ⋅ L-1 Na2SO4 aqueous solution. For the working electrode, 10 mg of sample (S-0 or S-3)

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was dispersed in 4 mL ethanol to obtain a suspension. Then, the suspension was coated onto a

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glassy carbon electrode and dried in an oven.

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2.4 Photocatalytic experiments

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The photocatalytic activities of the carbon-SnO2 nanocomposites were evaluated on the

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degradation of Rhodamine B and glyphosate under simulated sunlight. In a typical run, 10 mg of as-prepared sample was dispersed into 50 mL of 10 mg. L-1 RhB aqueous solution with the aid of

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ultrasound for 20 min. The experiment of adsorption / desorption equilibrium of RhB on catalyst (S-3) was conducted and displayed in Fig. S1. The result showed that the adsorption of RhB on catalyst mainly took place before 1 h. Therefore, before the exposure to illumination, the suspension was stirred in the dark for 1 h is to ensure the establishment of adsorption / desorption

equilibrium of substrate (RhB) on the sample surface. Subsequently, the solution was placed in a quartz reactor and stirred with a magnetic stirrer. A high-pressure xenon lamp (CEL-HXF300) (including UV and visible light region) was used as the light source to simulate the sunlight. At given time intervals, aliquot 4 mL solution was taken out. After the suspended catalysts were removed by centrifugation, the absorbance of the supernatant was measured by a UV-Vis spectrophotometer (Shimadzu 722N). The similar method was applied for the degradation of

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1×10-4 M glyphosate solution and the conversion rate of glyphosate to PO43- was determined by

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molybdenum blue method [15, 16].

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3. Results and discussion

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3.1 XRD patterns

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Fig.1 shows the XRD patterns of the different as-prepared samples. Clearly, all the diffractive

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peaks correspond to (110), (101), (200), (211), (220), (002), (310), and (301) planes of the rutile

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phase of SnO2 (JCPDS No.41-1445) and no impurity can be observed. The average particle size

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of each sample was calculated by the Scherrer equation D = Kλ/(βcosθ) according to the (110)

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plane diffraction peak and listed in Table 1. The crystallite sizes of carbon-SnO2 nanocomposites

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decrease with the increase of the mass ratio of PVP, indicative of the inhibiting effect of PVP

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from the growth of SnO2 nanocrystals. However, no obvious peaks related to carbon are observed from the XRD patterns of carbon-SnO2 nanocomposites, due to the fact that the

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characteristic peak of graphite carbon at 26.5° might be shielded by the main peak of rutile SnO2 at 26.45°. 3.2 TG-DTA and BET-BJH analyses

The carbon content of nanocomposites was determined by TG-DTA analysis and displayed in Table 1. Fig. 2a shows the TG curve of S-3 from room temperature to 800 °C during the calcination in air. The first weight loss of 7.2% appears between room temperature and 410 °C, attributed to the removal of absorbed water and organic compound. Then a steep decrease of about 16.8% in mass emerges in the range of 410-700 °C due to the combustion of carbon [17, 18]. To evaluate the pore size and surface area of the graphite-like carbon supporting SnO2

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nanocomposites, the N2 adsorption-desorption isotherms of all the nanocomposites are

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investigated. Fig. 2b shows the adsorption-desorption isotherm and pore size distribution of the

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S-3. It can be seen that the adsorption-desorption isotherm of S-3 corresponds to the classical

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type IV curve with a H4 hysteresis loop, indicating the presence of mesoporous framework in

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S-3 nanocomposite [19]. Moreover, the adsorption at high relative pressure (> 0.9) indicates the

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presence of layered structure, i.e. graphite-like carbon. The mesoporous S-3 exhibits a narrow

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pore size distribution with the average porous diameters of about 6.495 nm. It is found that the

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formation of graphite-like carbon in nanocomposites increases the surface areas and pore size of

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the nanocomposites in comparison of S-0 in Table 1. The sample S-3 with 16.8 % carbon content

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possesses the largest surface area (56.67 m2 / g) and average pore size among all the carbon-SnO2

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nanocomposites.



3.3 Morphology and structure The morphologies and structural features of all the samples were studied by SEM and TEM. It can be seen from the SEM images that mesoporous nanocomposites are achieved by

solvothermal method with post-calcination in nitrogen atmosphere. The EDX analysis shows the existence of C, O and Sn three elements in the nanocomposites (see the supporting information Fig. S2). TEM images reveal the size of SnO2 nanoplates is about 20-25 nm for S-0, and the average crystallite size of 5-10 nm for S-1, S-2 and S-3 (Fig. 3). This indicates the presence of graphite-like carbon can prevent SnO2 from the growth of particles in the process of calcination. In addition, the morphologies of the carbon-SnO2 nanocomposites are also distinctly different

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from that of S-0. The SnO2 nanoparticles disperse on the surface of graphite-like carbon more

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and more densely with the increase of the carbon content (Fig. 3b-d), which is beneficial to the

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enhancement of the photocatalytic properties of the nanocomposites. When the carbon content

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reaches 19.8 wt. %, most SnO2 particles are embedded in carbon. The further structure of

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nanocomposites is investigated by HRTEM and shown in Fig. 3f. The clear lattice fringes in

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HRTEM image confirm the high crystallinity of graphite-like carbon and SnO2 in S-3

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nanocomposite. The lattice spacing of 0.334nm, 0.320nm and 0.264nm corresponds to the (110),

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(001) and (101) planes of SnO2, respectively. In addition, the lattice spacing of 0.204nm for the



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(101) plane of graphite carbon can be observed in Fig.3f.

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3.4 XPS and Raman spectra The XPS spectra were employed to investigate the chemical states and elements of S-0 and

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S-3 nanocomposites. Fig. 4a shows the whole spectra of S-3, revealing the presence of carbon, oxygen and tin without any hetero element, which is identical to the result of EDS. Fig. 4b displays the C 1s XPS spectrum of S-3. The peak at 284.6eV is ascribed to elemental carbon (C-C or C=C) and two peaks at 286.5 eV and 288.8 eV correspond to carbonated species [20, 21]. The

peak at 281.4 eV with low binding energy indicates the formation of C-Sn bond in S-3 [22]. Fig. 4c-d compares the Sn 3d and O1s XPS spectra of S-0 and S-3. The Sn 3d spectrum of S-0 exhibits two symmetrical peaks at 486.3 eV and 494.8 eV corresponding to Sn 3d5/2、Sn 3d3/2 of Sn 4+ ion in SnO2 [10, 23]. Compared to S-0, the two peaks of S-3 shift to the higher binding energy at 486.6 eV and 495.0 eV due to the strong interaction between SnO2 and graphite-like carbon (C-Sn). In Fig. 4d, the peak around 531.2 eV is assigned to hydroxyl groups [24] and one

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attributed to the formation of oxygen defects in SnO2 lattice [26, 27].

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at 530.1 eV to lattice oxygen in SnO2 [25]. For S-3, the up-shift of binding energy to 530.3 eV is

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The Raman spectra of S-0 and S-3 are compared in Fig. 5. In Fig. 5a, three fundamental Raman

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peaks at 475, 633 and 776 cm-1 are attributed to the Raman active modes of the rutile phase of

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SnO2 , which correspond to the symmetries of Eg, A1g and B1g of Sn-O bond respectively [28, 29].

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This further confirms the rutile SnO2 can be obtained by the solvothermal method, in consistent

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with the XRD result. For S-3 composite, two strong peaks at 1366 and 1585 cm−1 are ascribed to

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carbon-related D-bond (disordered) and G-bond (graphitic) (Fig.5b) [30, 31], indicating the

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formation of graphite-like carbon with defects. Compared to S-0, three fundamental Raman peaks

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of S-3 (465, 623 and 764 cm-1) shifts 10 cm-1 to low wave numbers respectively (see the supporting information Fig. S3), indicating the existence of C-Sn in S-3, which is identical to the

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XPS results. 3.5 Optical properties The optical properties of S-0 and all the carbon-SnO2 nanocomposites were investigated by

UV-Visible diffuse reflectance (Fig. 6). It can be clearly observed from Fig. 6a that all carbon-SnO2 nanocomposites exhibit an enhanced absorption in both ultraviolet and visible light region compared with pure SnO2 (S-0). The absorption of the carbon-SnO2 nanocomposites is improving with increasing the carbon content due to the strong absorption of graphene-like carbon in both ultraviolet and visible light region. However, the plots of the transformed Kubelka-Munk function versus the energy of exciting light of all samples confirm that the bandgap of the

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carbon-SnO2 nanocomposites has no obvious shift in comparison to that of pure SnO2 (Fig.6b),

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implying the carbon merely acts as a supporter of SnO2 and is not incorporated into the SnO2

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lattice [32].

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3.6 Photocatalytic activity

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The photocatalytic activity of the SnO2 and carbon-SnO2 nanocomposites was evaluated by

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photocatalytic degradation of Rhodamine B and colorless glyphosate aqueous solution under

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simulated sunlight irradiation. Fig.7 shows the photocatalytic degradation of RhB and the

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conversion of glyphosate to inorganic PO43- over S-0 and all carbon-SnO2 nanocomposites with

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the different carbon contents. It can be seen from the Fig.7a that RhB can be drastically degraded

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over carbon-SnO2 nanocomposites. The first kinetic-order constant of RhB degradation on S-3 is 0.0107 min-1, which is 4.5 times that on S-0 (Fig.7b). In addition, the glyphosate can be

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transformed into the inorganic PO43- effectively on the nanocomposites and the conversion rate of glyphosate on S-3 reaches 91.2% (Fig. 7c). However, S-4 composite has a low photocatalytic activity on the degradation of both RhB and glyphosate, ascribable to the fact that excess graphite-like carbon shelters the incident light to SnO2 in nanocomposite [33, 34]. The results of

recycling runs for photocatalytic degradation of RhB over S-3 show that as-prepared nanocomposite of graphite-like carbon supporting SnO2 is stable and still keeps an excellent activity on the photocatalytic degradation of RhB after 5 successive recycles (Fig.7d). The high photocatalytic activity of the graphite-like carbon supporting SnO2 is likely related to the fact that

mesoporous structure and large surface area of carbon-SnO2 nanocomposites

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make a great contribution to the improvement of the absorption of RhB and glyphosate on the

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nanocomposites. More importantly, graphite-like carbon plays a vital role in hindering the

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recombination of photogenerated electron-hole pairs for carbon-SnO2 nanocomposites. To

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investigate the separation efficiency of the photo-induced electron-hole pairs, the transient

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photocurrent response measurements for S-0 and S-3 are conducted. The photocurrent resulting

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from the irradiated semiconductor was determined by the speed of excited electrons separated

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from semiconductor to electrode and the recombination of electrons and holes in the

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semiconductor interface [35]. Herein, the photocurrent response was measured in 30s on-off

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cycles. Fig. 8 shows that the photocurrent responses for each on-off behavior of the two samples.

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When the irradiation is stopped, the photocurrent is immediately reduced to the lowest. As seen

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from Fig. 8, the stable photocurrent of S-3 is about 4 times as high as that of S-0. The enhanced photocurrent on S-3 is ascribed to the presence of C-Sn bond (XPS and Raman results) and the

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excellent electron transfer property of graphite-like carbon [36]. The results suggest the more efficient separation of photo induced electron-hole pairs and the lower recombination rate take place at the S-3 nanocomposite interface compared to S-0, leading to a high photocatalytic activity on the degradation of RhB and glyphosate over graphite-like carbon supporting SnO2

(Fig.7). 4. Conclusions The

PVP-SnO2

xerogel

was

synthesized

by

a

modified

solvothermal

reaction.

Polyvinylpyrrolidone was used as the carbon source and promoted the nucleation and the crystallization of SnO2. After the xerogel was calcined in N2, the mesoporous nanocomposite of

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graphite-like supporting SnO2 was obtained. Due to the presence of C-Sn strong interaction in the

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carbon-SnO2 nanocomposite and excellent electron transfer property of graphite-like carbon, the

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photogenerated electrons could easily transfer to the graphite-like carbon and were stored. This led

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to an effective separation of electron-hole pairs on the carbon-SnO2 nanocomposites and a high

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photocatalytic activity on the degradation of RhB dye and glyphosate. Accordingly, the composite

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materials show a potential application in dealing with the environmental contaminants because of

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their high photocatalytic activities under sunlight irradiation.

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Acknowledgements

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We gratefully acknowledge the support from National Natural Science Foundation of China (No.

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51302072), Natural Science Foundation of Hubei Provincial Department of Education (No.

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Q20131010) and Natural Science Fund for Creative Research Groups of Hubei Province of China (No. 2014CFA015).

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and their application as high-performance supercapacitors, ACS Nano 8 (2014) 5069-5078.

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[31] J. P. Liu, C. X. Guo, C. M. Li, Carbon-decorated ZnO nanowire array: A novel platform for

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direct electrochemistry of enzymes and biosensing applications, Electrochem. Commun. 11

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(2009) 202-205.

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[32] M. J. Mattle, K. R. Thampi, Photocatalytic degradation of Remazol Brilliant Blue by sol–gel derived carbon-doped TiO2, Appl. Catal. B: Environ. 140-141 (2013) 348-355.

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[33] Q. J. Xiang, J. G. Yu, M. Jaroniec, Enhanced photocatalytic H2-production activity of

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graphene-modified titania nanosheets, Nanoscale 3 (2011) 3670. [34] S. J. Yu, H. J. Yun, Y. H. Kim, J. Yi. Carbon-doped TiO2 nanoparticles wrapped with

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nanographene as a high performance photocatalyst for phenol degradation under visible light irradiation, Appl. Catal. B: Environ. 144 (2014) 893-899. [35] Q. P. Luo, X. Y. Yu, B. X. Lei, H. Y. Chen , D. B. Kuang, C. Y. Su, Reduced graphene oxide-hierarchical ZnO hollow sphere composites with enhanced photocurrent and photocatalytic activity, J. Phys. Chem. C 116 (2012) 8111-8117. [36] Y. J. Wang, R. Shi, J. Lin, Y. F. Zhu, Enhancement of photocurrent and photocatalytic

activity of ZnO hybridized with graphite-like C3N4, Energy Environ. Sci. 4 (2011) 2922-2929.

Table 1.Carbon content, crystal size, surface area and average pore size of all samples.

Sample

Carbon content (wt.%)

Crystallite sizes (nm)

SBET (m2/g)

S-0

0

25.30

32.40

--

S-1

6.9

12.08

44.23

4.942

S-2

11.4

10.54

S-3

16.8

9.92

S-4

19.8

13.23

PT

RI 52.69

5.919

56.67

6.495

SC

U N A M D TE EP CC A

Pore size(nm)

56.21

5.949

Fig.1 XRD patterns of all as-prepared samples. Fig.2 (a) TG analysis of S-3 nanocomposite catalyst in air and (b) nitrogen adsorption-desorption isotherm and pore size distribution (the inset) of S-3. Fig. 3 TEM image of S-0(a), S-1(b), S-2(c), S-3(d) and S-4(e) and HRTEM image of S-3(f). Fig. 4 XPS survey spectra, (a) whole spectrum of S-3, (b) C 1s of S-3, (c) and (d) comparisons of Sn 3d and O 1s of S-0 and S-3.

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Fig. 5 Raman spectra of S-0 (a) and S-3 (b) Fig. 6 (a) UV-Vis diffuse reflectance spectra of all samples and (b) the plots of the transformed

RI

Kubelka-Munk function versus the energy of exciting light of all the as-prepared samples.

SC

Fig.7 (a) the UV-vis spectral changes of RhB in S-3 nanocomposite aqueous dispersion as a

U

function of irradiation time, (b) the degradation of RhB on all catalysts, (c) the conversion rate of

A

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glyphosate to PO43- over different photocatalysts under simulated sunlight for 120 min, and (d) the

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cycling runs for photocatalytic degradation of RhB over S-3 sample.

A

CC

EP

TE

D

Fig. 8 Photocurrent response of S-0 and S-3.

TE

EP

CC

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.

PT

RI

SC

U

N

A

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gr1

TE

EP

CC

A D

.

PT

RI

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U

N

A

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gr2

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A D

.

PT

RI

SC

U

N

A

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gr3

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A D

.

PT

RI

SC

U

N

A

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gr4

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A D

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RI

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U

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gr5

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RI

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gr6

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gr7

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gr8