Insitu synthesis of highly efficient visible light driven stannic oxidegraphitic carbon nitride heterostructured photocatalysts

Yan, In-situ synthesis of highly efficient visible light driven stannic oxide/ graphitic carbon nitride heterostructured photocatalysts, Journal of Colloid and Interface Science 2016, do

Accepted Manuscript

In-situ synthesis of highly efficient visible light driven stannic oxide/graphitic

carbon nitride heterostructured photocatalysts

Binglin Tao, Zifeng Yan

DOI: http://dx.doi.org/10.1016/j.jcis.2016.07.009

Reference: YJCIS 21395

To appear in: Journal of Colloid and Interface Science

Received Date: 26 April 2016

Revised Date: 2 July 2016

Accepted Date: 6 July 2016

Please cite this article as: B Tao, Z Yan, In-situ synthesis of highly efficient visible light driven stannic oxide/ graphitic carbon nitride heterostructured photocatalysts, Journal of Colloid and Interface Science (2016), doi: http:// dx.doi.org/10.1016/j.jcis.2016.07.009

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In-situ synthesis of highly efficient visible light driven stannic oxide/graphitic carbon nitride heterostructured photocatalysts

Binglin Tao and Zifeng Yan*

State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China University of Petroleum, Qingdao 266580, China

*Corresponding Author

Phone: +86 532 86981296; Fax: +86 532 86981295;

Email: zfyancat@upc.edu.cn

Abstract

Novel and efficient visible-light-driven stannic oxide/graphitic carbon nitride heterostructured photocatalysts are prepared via a simple in-situ solvothermal method Characterization results demonstrate that there exist strong interactions between SnO2 nanoparticles and g-C3N4 matrix, which indicates the formation of SnO2/g-C3N4 heterojunction The as-synthesized SnO2/g-C3N4composite exhibits improved efficiency for photodegradation of rhodamine B in aqueous solutions, with an apparent rate constant 6.5 times higher than that of commercial TiO2 (Degussa P25) The enhanced photocatalytic activity is attributed to synergistic effect between SnO2 and g-

C3N4, resulting in effective interfacial charge transfer and prolonged charge-hole separation time Moreover, SnO2/g-C3N4 composite photocatalysts possess excellent durability and stability after

6 recycling runs, and a possible photocatalytic mechanism is also proposed This research highlights the promising applications of two dimensional g-C3N4 based composite photocatalysts

in the field of waste water disposal and environmental remediation

Keywords: composite, solvothermal, catalytic property, heterojunction

1 Introduction

Photocatalysis is of great interest because it provides a simple technique to solve selected environmental pollutions (e.g., gas and water purification [1]) and energy issues (e.g., water splitting [2] and carbon dioxide conversion [3]) facing the modern society During the last 40 years, titanium oxide (TiO2) has attracted much attention for its high efficiency, low cost and availability in the fields of water splitting for hydrogen production [4], water detoxification [5],

commercial photocatalyst, Degussa P25, composing of about 80% anatase and 20% rutile, is well-known to possess an insurmountable photocatalytic activity Despite these remarkable advantages and considerable achievements, TiO2 (P25) has also been much criticized for its two intrinsic shortcomings: the wide band gap and high recombination rate of photoinduced electron-hole pairs Besides, the toxicity and safety for TiO2 need to be further studied in detail [9] Thus, numerous novel photocatalytic materials, for example, metal oxides [10, 11], metal sulfides [12], metallates [13, 14], metal halide [15], and even some metal-free semiconductors [16] have been proposed as substitutes for conventional TiO2 photocatalysts

Since the first report about hydrogen production by graphitic carbon nitride (g-C3N4) in 2009 [17], this kind of metal-free polymer has attracted much attention in the field of photocatalysis

recombination rate, indicating its short lifetime and low separation efficiency of photogenerated electron-hole pairs, which leads to low quantum efficiency Many approaches have been proposed to enhance the photocatalytic activity of g-C3N4, including exfoliation of bulk g-C3N4into nanosheets [18], preparation of mesoporous g-C3N4 with large specific surface area [19],

prepare heterostructured photocatalysts is supposed to be the most effective way to fully address

Ag2O/g-C3N4 [21], TiO2/g-C3N4 [23], Bi2WO6/g-C3N4 [24], AgVO4/g-C3N4 [25] and In2O3/g-C3N4 [26] were prepared, which exhibited improved separation efficiency of photogenerated charge carriers and enhanced photocatalytic activities

Stannic oxide (SnO2), as an n-type semiconductor with a band gap of ~3.60 eV, was shown to

be a less active photocatalyst for water detoxification [27] However, SnO2 has been more widely employed as an additive to improve catalytic behaviors of the main photocatalysts For example,

much higher photocatalytic activity than the pure ZnO and SnO2 nanofibers for the degradation

heterojunctions which could facilitate the separation of electron-hole pairs [28] Theoretically, the combination of SnO2 and g-C3N4 could lead to the formation of a new kind of semiconductor heterojunction (SnO2/g-C3N4), which possesses a well matched band structure and reduces the electron-hole recombination rate

photocatalysts Most recently, SnO2/g-C3N4 composite was synthesized by a traditional solvent

interfacial charge transfer thus prolonged electron-hole pairs’ lifetime However, the large crystal size (20 nm), small specific surface area (38.5 m2/g-1), and bulk g-C3N4 sheets render SnO2/g-C3N4composite possesses a limited photocatalytic activity Besides, the synthesis procedure was still tedious

A novel in-situ method for the fabrication of SnO2/g-C3N4 heterostructured photocatalysts is described here These new composites were prepared via a solvothermal method in which urea-

derived g-C3N4 acted as matrix and stannic acetate served as SnO2 precursor They exhibited considerable activity for rhodamine B degradation in aqueous solutions, which was directly related to their unique structures

2 Experiment section

2.1 Materials Urea and dimethylsulfoxide were obtained from Sinopharm Chemical Reagent

rutile and 80% anatase) was provided by Degussa (China) Co., Ltd All the reagents were analytical pure and used as received without further purification Ultrapure water (resistivity•18

2.2 Synthesis of g-C 3 N 4 The metal-free semiconductor g-C3N4 powders were synthesized by simply heating urea in a muffle furnace Typically, 20.0 g of urea was placed in a 100 mL semi-closed crucible with a cover Then temperature was raised to 550 oC at an accelerating rate of 15

o

C min-1 and the condensation process was maintained at 550 oC for 4 h [30] After the crucible was cooled down to room temperature naturally, the yellow product was collected and ground into fine powders For 20.0 g of urea, the g-C3N4 yield was 0.87-0.89 g

2.3 Fabrication of SnO 2 /g-C 3 N 4 composite photocatalysts SnO2/g-C3N4 composite photocatalysts were synthesized via an ordinary solvothermal method In a typical synthesis, 0.4

dimethylsulfoxide contained in a 250 mL beaker After sonication for 60 min, the above solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, and then heated at 180 o

C for 20 h Subsequently, the precipitate was collected by filtration, washed with water and ethanol for several times, and dried at ambient conditions Finally the composite photocatalysts were

annealed at 400 C for 3 h According to this method, a series of SnO2/g-C3N4 composites with

stannic acetate in dimethylsulfoxide directly

2.4 Characterization Crystal structure of the as-prepared products were characterized by a

PANalytical PRO X-ray Diffractometer equipped with Cu K radiation ( =1.5406nm) Fourier radiation ( =1.5406nm) Fourier =1.5406nm) Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 spectrometer using KBr disc technique Nitrogen adsorption-desorption isotherms were obtained on a Micromerities Tristar

3000 equipment at -196 o

C Prior to measurement, all the samples were degassed at 300 o

C for 3 hours Surface property analysis was performed on an Escalab 250 X-ray photoelectron spectrometer The C1s binding energy of 284.6 eV of adventitious carbon was used as the reference Morphologies of the as-prepared composites were obtained from a Hitachi S-4800 scanning electron microscope at an accelerating voltage of 5 kV The lattice structure and selected area electron diffraction were observed on a JEM-2100UHR transmission electron microscope Diffuse reflectance spectra were recorded by a Jena SPECORD 210 PLUS UV-Vis

used as reference The transient photocurrent measurement was performed on an electrochemical workstation (CHI 660E, Chenhua Instrument Corporation, Shanghai, PR China) with a standard three-electrode system The preparation of working electrodes and testing method were according to the literature[31]

2.5 Evaluation of photocatalytic activity The photocatalytic activities of the samples were

evaluated by using the visible-light-driven degradation of rhodamine B in aqueous solution as a probe reaction A double layered Pyrex glassware was used as the reactor Typically, 10 mg of

suspension After magnetically stirred in the dark for 30 min, the Pyrex reactor was irradiated under a 350 W xenon lamp equipped with a 420 nm cut off filter, which was placed 20 cm above the solution At regular time intervals of 3 min, 2 mL of aliquots were sampled and centrifuged for further analysis The concentration of rhodamine B was analyzed by recording the maximum absorption peak of rhodamine B solution (initially value = 554 nm, very slightly blue shift could

be observed during the reaction process) on a Jena SPECORD 210 PLUS UV-Vis spectrophotometer During the photoreactions, no oxygen was bubbled into the system and the temperature was maintained at 20 oC by using a circulating water bath

2.6 Determination of reactive species In order to detect the active species generated in the

reaction system, various representative scavengers, including silver nitrate (AgNO3, 6 mmol L-1, scavenger for e-

,

(NaHCO3, 6 mmol L-1, scavenger for h+ and •OH) and 1, 4-benzoquinone (PBQ, 6 mmol L-1, scavenger for O2•-) were introduced into the solution before illumination The next experimental steps were the same as above mentioned photocatalytic activity test

3 Result and discussion

3.1 Structure and composition X-ray diffraction patterns (XRD) of SnO2/g-C3N4 composites

are shown in Fig 1 Spectra of pure SnO2 and g-C3N4 are also provided for comparison purpose For pure SnO2 sample, all diffraction peaks could be clearly indexed as (110), (101), (211), and (301) planes of rutile stannic oxide (JCPDS 72-1147) In the pattern of pure g-C3N4 sample, there are two diffraction peaks at 13.2o and 27.3o, which are stemmed from the lattice planes parallel to the c-axis and the graphite-like layer stacking of the conjugated aromatic units, respectively

These two characteristic peaks are in agreement with graphitic carbon nitride reported in previous literatures [17] In the spectrum of SnO2/g-C3N4 composite with the mass ratio of 1:4, all diffraction peaks of SnO2 and g-C3N4 could be detected, and the overlap of diffraction patterns leads to broadening of (110) peak These results indicate the coexistence of SnO2 and g-C3N4 in SnO2/g-C3N4 composites Also, there are no detectable impurities, such as, SnO, SnS or Sn2S3 in the resulting composites SnO2 crystalline sizes in different samples are estimated according to

(101) peak by Scherrer equation, and the results are shown in Table S1

Fig 2 shows the FT-IR spectra of pure SnO2, g-C3N4, and a series of SnO2/g-C3N4 composites with selected mass ratios The board peak appears at wave number of 656 nm-1 can be attributed

to Sn-O bond vibration [32, 33] The characteristic peak at 808 cm-1 corresponds to the breathing mode of triazine units [34] A series of small peaks in the range of 1200-1700 cm-1

are directly related to skeletal stretching modes of s-triazine or tri-s-triazine units [35] It is obvious that the intensity of the peak at 808 cm-1 weakens with the decrease of g-C3N4 content, which is consistent with the (002) peak variations in XRD analysis

The composition and valence status of the SnO2/g-C3N4 composites were also characterized by

X-ray photoelectron spectroscopy (XPS) technique Fig S1 depicts the survey spectra of pure

SnO2, g-C3N4 and SnO2/g-C3N4 composite with a mass ratio of 1:4 (the best performing one in photodegradation) These results reveal that elemental Sn, O, C, N coexist in the SnO2/g-C3N4composites The high resolution XPS spectra of Sn 3d, O 1s, C 1s, and N 1s in SnO2/g-C3N4-1:4

binding energies of 487.2 eV and 495.7 eV are ascribed to Sn 3d5/2 and Sn 3d3/2, which are close

to the literature values [36] Besides, the ratio of their integral areas conforms to 3:2 While in the SnO2/g-C3N4-1:4 heterostructured photocatalyst, the binding energies for Sn 3d5/2 and Sn 3d3/2

have shifted to 486.8eV and 495.7 eV, respectively This phenomenon is probably due to the intimate interactions between Sn4+ and g-C3N4 matrix Similarly, the binding energy of O 1s has

an obvious opposite direction shift, from 531.2 eV to 532.0 eV, as shown in Fig 3B In Fig 3C,

the signal of C 1s peak in SnO2/g-C3N4-1:4 composite is deconvoluted to 4 individual peaks, with binding energies at 284.5 eV, 286.0 eV, 287.8 eV and 289.0 eV It is believed that the strong peak centered at binding energy of 284.5 eV corresponds to the C-C coordination of sp2 hybrid carbon atoms [37], and the peak centered at binding energy of 286.0 eV is attributed to C-N bond

in N-containing groups [29] Besides, the peak signals at 287.8 eV and 289.0 eV are ascribed to

C=N and N-C=N coordination, respectively [38] The N 1s spectra in Fig 3D shows three

deconvoluted peaks, which are centered at 398.3 eV, 399.7 eV, and 400.7 eV According to previous reports, these peaks are derived from N-sp2C (pyridinic nitrogen, sp2 hybridization), N-(C)3 (graphitic nitrogen, sp3 hybridization), and marginal N-H structures [39] All the above analysis indicates strong interactions between SnO2 and g-C3N4, demonstrating the formation of SnO2/g-C3N4 heterojunction

3.2 Texture and optical properties Nitrogen adsorption-desorption isotherms and

Barrett-Joyner-Halenda (BJH) pore size distribution curves of pure SnO2, g-C3N4, and SnO2/g-C3N4

composites are shown in Fig S2 Obviously, adsorption-desorption isotherms of all samples are

of type • (IUPAC classification) with hysteresis loops, confirming the presence of mesopores

aggregated particles Similarly, the hysteresis loops are type H3 of all g-C3N4 based samples, which are well consistent with the slit-shaped pores formed by layer stacking of g-C3N4 sheets

m2/g, respectively It is worth mentioning that when SnO2 particles loaded on g-C3N4, BET surface areas of SnO2/g-C3N4 composites are much higher than that of both pure SnO2 and g-C3N4

This kind of “synergistic effect” has scarcely been reported before, possibly relates to the high

dispersity of SnO2 particles on g-C3N4 sheets [38] Apparently, an enlarged specific surface area

is beneficial for the improvement of photocatalytic activity All the texture properties are

summarized in Table S1

The optical properties pure SnO2, g-C3N4, and SnO2/g-C3N4 composites were characterized by

UV-Vis diffuse reflectance technique (Fig 4) As depicted in Fig S3, pure SnO2 particles show

an absorption edge at about 320 nm, which corresponds to a band gap of 3.88 eV While all the

g-C3N4 based materials, including pure g-C3N4 and SnO2/g-C3N4 composites, exhibit similar

optical absorption abilities These results indicate that the introduction of SnO2 particles on the

surface of g-C3N4 matrix would not change the absorption edges, even with an extremely high

SnO2 content at mass ratio of 2:1 Considering the uncertain optical transition types of SnO2

/g-C3N4 composites materials (direct band-gap semiconductor for pure SnO2 and indirect band-gap

empirical equation:

Eg=1240/ (1) where represents the wavelength of absorption edge Thus represents the wavelength of absorption edge Thus it can be concluded that the band gap

energies are in the range of 2.69-2.84 eV for g-C3N4 based composite materials, showing their

strong visible light absorption abilities For better comparison, the absorption edges and

corresponding calculated band gap energies of all samples are summarized in Table S1

3.3 Morphology Fig 5A and 5B present the SEM images of pure g-C3N4 sheets with lamella

structures It is apparent that the surface of g-C3N4 sheets is quite smooth, and the thickness of a

slice is in the range of 30-40 nm This estimation can be verified by Fig 5C and 5D, and the

dimensions of g-CN sheets were several micrometers After loading with SnO particles, there

are no significant morphology changes of the layer structure, as shown in SEM image of Figure

All the SnO2 particles dispersed evenly on the surface of g-C3N4 sheets, and the particle size is

about 5 nm (as depicted in Fig 5G), which can also be identified by XRD analysis The HRTEM

displayed in Fig 5G and Fig 5H, respectively These diffraction rings could be indexed as the

(110), (101), (211), and (301) planes of rutile SnO2 from inner to outside, and two sets of lattice

fringes could be recognized (Fig 5I and 5K) By measuring the lattice spacing in Fig 5J and 5L,

the interplanar distances were determined to be about 0.33 and 0.26 nm, in accordance with the (110) and (101) planes of rutile stannic oxide, respectively Moreover, the elemental mapping

images of Sn, O, C, and N (Fig S4) suggest the uniform spatial distribution, which is a direct

evidence for the high dispersity of SnO2 nanoparticles on g-C3N4 sheets

3.4 Photocatalytic activity The photocatalytic activities of different samples were evaluated

by degradation of rhodamine B solution under visible light (420-760 nm) irradiation For comparison, one of the best commercial photocatalysts, Degussa P25, was used as the reference

As displayed in Fig 6A and 6B, pure SnO2 shows the weakest ability to decompose rhodamine B, only 4.4 % of the dye was degraded in 15 min The SnO2/g-C3N4-1:4 composite photocatalyst possesses the best photocatalytic performance, and 97.5 % of the rhodamine B was decomposed under the same conditions While the reference, commercial Degussa P25 displayed a relatively

photocatalysts exhibit fairly high efficiency for the photodegradation of rhodamine B, and a

comparison between the previous literatures and this study are summarized in Table S2 In this