V2O5 composites for visible light-driven photocatalytic activity

Accepted Manuscript Title: One-pot synthesis of g-C3 N4 /V2 O5 composites for visible light-driven photocatalytic activity Author: Qinqin Liu Chunya Fan Hua Tang Xiujuan Sun Juan Yang Xiaonong Cheng PII: DOI: Reference:

S0169-4332(15)02101-7 http://dx.doi.org/doi:10.1016/j.apsusc.2015.09.010 APSUSC 31224

To appear in:

APSUSC

Received date: Revised date: Accepted date:

16-7-2015 30-8-2015 1-9-2015

Please cite this article as: Q. Liu, C. Fan, H. Tang, X. Sun, J. Yang, X. Cheng, One-pot synthesis of g-C3 N4 /V2 O5 composites for visible light-driven photocatalytic activity, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.09.010 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.

One-pot synthesis of g-C3N4/V2O5 composites for visible light-driven photocatalytic activity Qinqin Liu∗, Chunya Fan, Hua Tang, Xiujuan Sun, Juan Yang, Xiaonong Cheng

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School of Materials Science and Engineering, Jiangsu University, 301 Xuefu Road,

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Zhenjiang Jiangsu 212013, PR China

Abstract: The g-C3N4/V2O5 composites with visible light photocatalytic performance

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have been prepared by one-pot method. The g-C3N4/V2O5 composites were

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characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), thermogravimetric analysis (TG), diffuse

reflectance

spectroscopy

(DRS),

photoluminescence

(PL)

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UV–vis

spectroscopy, Brunauer–Emmett–Teller (BET) and X-ray photoelectron spectroscopy

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(XPS). The results indicated that, within the g-C3N4/V2O5 composites, V2O5

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nanoparticles were highly crystallized and intertwined with the lamellas of sheet-like

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g-C3N4 materials, resulting in the generation of well-defined heterostructures. The photocatalytic activity of the g-C3N4/V2O5 composites was evaluated using Rhodamine B as a target organic molecule. Under visible light illumination, as-prepared

g-C3N4/V2O5

hybrid

materials

demonstrated

highly

improved

photocatalytic activity than g-C3N4 and V2O5 materials. The enhancement in visible-light-driven photocatalytic activity can be ascribed to the formation of heterojunctions between V2O5 and g-C3N4, which promoted faster electron–hole separation and favored more efficient charge transfer. Keymords:g-C3N4; V2O5; heterojunctions; photocatalytic activity

∗Corresponding author. Tel.: +86511 88780195; fax: +86 51188791947. E-mail address:[email protected] (QQ Liu). 1

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1 Introduction Photocatalysts have drawn immense attention due to their potential applications in the photodegradation of organic and inorganic pollutants, fuel and hydrogen production

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[1-3]. However, a number of traditional photocatalysts ( such as TiO2 [4] and ZnO

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[5] ) are only active under ultraviolet ( UV ) light due to their wide band gaps, which

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restricts their practical application. Therefore, efforts have been made worldwide to development of new visible-light-responsive photocatalysts.

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A metal-free semiconductor, graphitic carbon nitride ( g-C3N4 ), has been proved to be a novel and efficient photocatalyst due to its characteristics of high thermal,

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chemical, photochemical stability, suitable band position and low-cost [6-9]. However, photocatalytic performance of g-C3N4 remains limited by deficient sunlight

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absorption and high recombination rate of photo induced electrons and holes. Therefore, many attempts have been made to improve the photocatalytic performance

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of g-C3N4, for example, ions doping [10,11], porous structure fabrication [12],

heterojunction construction [13] and so on. Among these methods, integrating g-C3N4 with other semiconductors to form heterojunctions is an effective and feasible strategy. Recently, a large number of g-C3N4-based composite photocatalysts ( g-C3N4/MoS2 [14], g-C3N4/Ag3VO4 [15], g-C3N4/Bi2MoO6 [16], g-C3N4/BaTiO3 [17], g-C3N4/AgX

( X = Br, I) [18], g-C3N4/Bi2O3 [19], g-C3N4/BiOBr [20], g-C3N4/Ni(dmgH)2 [21], g-C3N4/Bi2O2CO3 [22] and C3N4/Ag3PO4 [23,24] ) have been prepared and used for photodegradation of organic dyes. Vanadium pentoxide ( V2O5 ), with layered structure, has attracted considerable 2

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interest over past few years owing to its unique physical and chemical properties [25]. It has been well studied for various applications, such as catalysts, solar cells, gas sensors, electrochromic devices, a cathode material in rechargeable lithium batteries,

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and electrooptic switches [26]. As a photocatalyst, V2O5 has been studied extensively

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owing to its chemical inertness, strong oxidizing power and long-term stability against

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photo and chemical decay [27]. However, the practical application of V2O5 is limited and the construction of well-defined V2O5/g-C3N4 heterostructures [28] has been

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proved to be advantageous for the separation and transportation of photogenerated carriers, and thus resulting in the enhancement of visible-light-driven photocatalytic

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activity.

However, nowadays the most commonly used method for the generation of

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g-C3N4-based heterostructures is solid state reaction. The problem for solid state method is that it is difficult to mix raw materials at the atomic level which may cause

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composition segregation and uneven distribution. To our best knowledge, there is rare report on synthesis of V2O5 nanoparticle modified g-C3N4 photocatalysts by a facile one-pot method for improved visible-light-induced photocatalysis. In the present work, we successfully prepared a series of V2O5 modified g-C3N4

composites via a one-pot method. The as-prepared g-C3N4/V2O5 composites displayed

a wider absorption in visible light region than bare g-C3N4 and exhibited enhanced photodegradation activity of rhodamine B (RhB) under visible light irradiation. Subsequently the possible photocatalytic mechanism is proposed on the basis of results and discussion. 3

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2 Experimental 2.1 Preparation All raw materials in the present study were of analytical grade and used as

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received without further purification. Typically, 3.0 g melamine and 0.1 M NH4VO3

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with different volume aqueous solution were added into 100 mL distilled water. The obtained solution was heated at 100 oC for complete water evaporation. The resulting

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mixture was put into an alumina crucible with a cover, and heated to 520o C with

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ramping rate of 20 oC/min for 4 h. Pure g-C3N4 and pure V2O5 were prepared by the same method without adding NH4VO3 or melamine using the same heat treatment

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process. According to thermogravimetric analysis (TG) results, the weight of g-C3N4

2.2 Characterization

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respectively.

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in g-C3N4/V2O5 composites was estimated to be 5.1 %, 7.9 %, 60.3 %, and 91.4 %,

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The phases structure of the products were collected on a Bruker D8 Advance

X-ray diffractometer (Cu Kα radiation, λ = 0.15406 nm) in a 2θ range from 10° to 60° at room temperature. The morphologies of the as-synthesized products were examined by transmission electron microscopy (TEM, H-600-II, and Hitachi). UV-vis diffuse reflectance spectra (UV-vis DRS) were achieved on a UV-visible spectrometer (Shimadzu UV-2450) using BaSO4 as the reference. The absorbency of solution was

tested on a Shimadzu UV-2550 spectrometer at room temperature. TG analysis was performed on STA-449C Jupiter (NETZSCH Corporation, Germany). The experiment temperature ranged from 25 oC to 900 oC at a constant heating rate of 10 oC min-1 in 4

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air. BET surface area and porosity measurements were carried out by N2 adsorption at 77 K using a Micromeritics 2010 instrument. The surface electronic states were analyzed using X-ray photoelectron spectroscopy (XPS, Perkin–Elmer PHI 5000C).

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2.3 Photocatalytic test

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Before the test, the standard curve of concentration and absorbance of the

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Rhodamine B ( RhB ) is plotted and given in Fig.S1. It can be seen that concentration is linear to absorbance. The photocatalytic activity was examined by the degradation

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of RhB using simulated sunlight irradiation from a 200 W xenon lamp at room temperature. The initial concentration of RhB solution was 10 mg/L. 50 mg of the

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prepared photocatalyst was added into 50 mL RhB solution and the reaction mixture was continuously aerated by a pump to provide oxygen and aid in the mixing of the

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reaction solution. After regular intervals, the samples were removed and centrifuged (6000 rpm) to separate the photocatalyst for analysis. The concentration of RhB was

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determined by measuring the absorption intensity at its maximum absorbance wavelength of RhB. The degradation percentage of the dyes was defined as: Degradation(%) =

Ct At (Eq. (1)) = C0 A0

where C0 is initial dye concentration, Ct is dye concentration at certain reaction time t

(min), A0 is UV–vis absorption of the original solution and At is UV–vis absorption of degraded solution at certain minutes. 3 Results and discussion In order to quantify g-C3N4 amounts in final products, thermogravimetric analysis was performed. The results are shown in Fig.1. It can be seen that pure V2O5 5

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shows negligible weight loss, while other samples exhibit obvious mass loss. For pure g-C3N4, the beginning temperature of the weight loss is located at approximately 600oC. This weight loss is mainly attributed to the sublimation or decomposition of

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g-C3N4 [29]. However, the weight loss of the g-C3N4/V2O5 composites is lower than

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that of g-C3N4. Similar phenomena were reported in other g-C3N4 based composites

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[30-32]. It was reported that most of the semiconductors are metal oxides which own the capability of activating oxygen. Therefore, g-C3N4 is possible to be catalytic

o

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oxidized by the coupled metal oxide even if the heating temperature is lower than 600 C [30-32]. V2O5 in this work is speculated to be a catalyst that can absorb and

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activate the oxygen in the air, and then oxidize the g-C3N4 at a relatively low temperature. The contents of g-C3N4 in the composites are calculated according to the

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weight loss of g-C3N4 after heating the samples over 700 oC. The mass ratio of g-C3N4 in composites is determined to be 5.1 %, 7.9 %, 60.3 % and 91.4 %, respectively.

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The XRD patterns of pure g-C3N4, V2O5 and g-C3N4/V2O5 composites are shown

in Fig.2. The XRD pattern of the pure g-C3N4 reveals two distinct diffraction peaks at

13.01 º and 27.66 º, which can be indexed as the (110) and (002) planes for graphitic materials, corresponding to the in-plane structural packing motif and interlayer stacking of aromatic segments, respectively [23,32]. For the V2O5, all peaks can be indexed to orthorhombic V2O5 (JCPDS 65-0131) [33]. For composites materials, the

XRD patterns are similar, all representative peaks of g-C3N4 and V2O5 are clearly observed, indicating that the as-prepared g-C3N4/V2O5 composites are indeed composed of g-C3N4 and V2O5. Besides that, the XRD peaks of V2O5 strengthen with 6

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the increasing concentration of V2O5 at the expense of those of g-C3N4. To further reveal the structural information of g-C3N4/V2O5 hybrid materials, FT-IR spectra of the synthesized V2O5, g-C3N4 and g-C3N4/V2O5 are shown in Fig. 3.

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For g-C3N4, the band at 809 cm−1 is attributable to the s-triazine ring mode [34]. The

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strong bands in the 1200–1700 cm−1 region are found, with the bands at 1236, 1320,

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1405, 1547 and 1634 cm−1, which corresponds to the stretching vibration modes of C=N and aromatic C–N heterocycles [22,34]. For V2O5, two characteristic absorption

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bands at 829 cm−1 and 1018 cm−1 are observed. The band at 829 cm−1 is assigned to the asymmetric stretching modes of the V–O–V bond and the other band at 1018 cm−1 is

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attributed to the stretching vibration of the V=O bond [28]. The FT-IR spectra of g-C3N4/V2O5 composites depict the overlap of the FT-IR spectra of g-C3N4 and V2O5.

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All the FT-IR spectra of the composites exhibit the characteristic bands of both g-C3N4 and V2O5, and no impurity band is detected.

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Fig.4a shows the survey XPS spectrum of g-C3N4 (60.3%)/V2O5 composite. It

could be seen that C, N, V, O elements are detected in the composite. Fig. 4b–e shows the high-resolution spectra of C 1s, N 1s, V 2p and O 1s, respectively. The C 1s peak at 284.9 eV is attributed to sp2-bonded carbon in the form of C-C [35]. The other C 1s peak at 288.5 eV is assigned to a C-N-C coordination mode [24,35]. The N 1s peak of g-C3N4 at 399.2 eV is typically attributed to N atoms sp2-bonded to two carbon atoms (C=N-C), suggesting the presence of sp2-bonded graphitic carbon nitride [36]. For high-resolution XPS spectrum of V 2p, the peaks centered at 518.2 eV and 525.8 eV correspond to V5+ 2p3/2 and V5+ 2p1/2 doublet states respectively, with a typical spin 7

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orbit splitting of 7.6 eV, matching with previous reported values [33]. The O 1s peak centered at 530.8 eV is associated with the O2− in the orthorhombic V2O5 [33].

verify the presence of both V2O5 and g-C3N4 in the hybrid materials.

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Therefore, in combination with XRD, FT-IR and XPS measurements, the results

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The morphological features of the obtained g-C3N4 and g-C3N4/V2O5 composites

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were characterized by TEM, as shown in Fig.5. The morphology of pure g-C3N4 (Fig. 5a) appears to be the aggregated morphology with a lamellar structure. Fig.5b–d

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shows typical TEM images of the g-C3N4( 91.4 % )/V2O5, g-C3N4( 60.3 % )/V2O5 and g-C3N4( 7.9 % )/V2O5. It is obvious that a few of small V2O5 nanoparticles are

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deposited on the lamellas or wrapped inside of g-C3N4 nanosheets. The V2O5 nanoparticles have spherical shapes with an average diameter of 5nm. As the content

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d

of V2O5 in the composites increased, more and more V2O5 nanoparticles intertwine with g-C3N4 nanosheets. The interaction between V2O5 nanoparticles and g-C3N4

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layered materials is so strong that ultra-sonication during the sample preparation procedure for TEM analysis could not peel off these nanoparticles. The formation of this intimate junction structure is favorable for the charge transfer between V2O5 and

g-C3N4 and may promote the separation of photo-excited electron–hole pairs, and

therefore enhanced photocatalytic activity is expected to be obtained [37]. The BET surface area of samples was investigated by N2 adsorption–desorption isotherm. The BET surface area of g-C3N4 and V2O5 is 16.1 and 5.1 m2/g, respectively. It is noted that the BET surface area of g-C3N4(60.3%)/V2O5 composite is 27.9 m2/g, which is higher than that of g-C3N4 or V2O5. The increase in surface area can provide 8

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more active sites for reaction with dye molecules. This phenomenon favors for the enhancement in photocatalytic activity. Barret–Joyner–Halenda (BJH) proposes a calculation method for the distribution curves of the pore volume as a function of pore

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diameter from nitrogen adsorption–desorption data [38]. The pore-size distribution

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curves calculated from BJH method is shown in Fig.6. Average pore sizes are also

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estimated. The mean pore size of g-C3N4 observed at 3.4 nm is significantly increased towards larger pore size when V2O5 is incorporated, indicating that the surface

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features of composite are notably affected [39].

Fig.7(A) exhibits UV–vis diffuse reflectance spectra of g-C3N4, V2O5 and

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g-C3N4/V2O5 composites. It can be seen that g-C3N4 holds an absorption edge of around 450 nm while the absorption edge of V2O5 is at 570 nm. After combining two

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semiconductors, the absorption edge of g-C3N4/V2O5 composites is shifted to longer wavelength in comparison with g-C3N4. This may be attributed to the hybrid structure

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between g-C3N4 and V2O5 interfaces. No obvious difference is observed among the composites, indicating that the content of V2O5 does not influence the light-harvesting

property of prepared composites. It is notable that the g-C3N4/V2O5 composites also exhibit a broad background absorption in the visible light region, which could be attributed to the presence of V2O5 nanoparticles. The results also indicate that all g-C3N4/V2O5 composites are able to function under visible light illumination. The band gap energy ( Eg ) of a semiconductor can be calculated using the formula of ahv=A(hv -Eg)2, where a, h, v, Eg and A are absorption coefficient, Planck constant, light frequency, band gap energy, and a constant, respectively. The Eg value 9

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can be estimated by extrapolating the straight portion of (ahv)2−(ahv) plot to the a = 0 point. As shown in 7(B), the band gaps of g-C3N4 and V2O5 are estimated to be 2.76 and 2.23 eV, respectively. This result is in agreement with previous reports [28].

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The photocatalytic performances of g-C3N4, V2O5 and g-C3N4/V2O5 composites

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are evaluated by photocatalytic removal of RhB in liquid phase as shown in Fig. 8.

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The blank experiment in the absence of photocatalysts demonstrates that no RhB is photodegraded by the visible light. All the samples exhibit the photocatalytic activity

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in RhB degradation under visible light irradiation. However, the photocatalytic activity of g-C3N4 or V2O5 is low. All of the g-C3N4/V2O5 composites exhibit higher

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photocatalytic activities than that of g-C3N4 or V2O5. Furthermore, the photocatalytic activity of the composites increases as g-C3N4 concentration increases from 5.1 % to

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60.3 %, and decreases at an even higher g-C3N4 concentration. The highest photocatalytic activity is obtained from the catalyst g-C3N4(60.3 %)/V2O5. The

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increased photocatalytic activity of the g-C3N4/V2O5 composites maybe attributed to the synergic effect between g-C3N4 and V2O5, which included the light absorption,

optical property, higher BET surface and heterojunction structure. The photoluminescence (PL) measurement was used to investigate the

electron–hole separation process between V2O5 and g-C3N4. The PL spectra of the synthesized photocatalysts are shown in Fig. 9. For g-C3N4, the main emission peak is centered at about 460 nm. This emission can be attributed to the band–band PL phenomenon with the energy of light approximately equal to the band gap energy of g-C3N4 [40]. The strong PL intensity of g-C3N4 indicates that the electrons and holes 10

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recombine rapidly. However, in the case of g-C3N4/V2O5 composites, the PL peak intensity is greatly decreased, suggesting that the composites have a much lower recombination rate of photogenerated charge carriers. This is favorable for the

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enhancement of photoactivity in degradation reaction.

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Numerous researchers ascribed the promotion of the photocatalytic activity to the

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efficient charge transfer on the boundary of two the semiconductors, which could result in the effective separation of photogenerated electron–holes [22, 28, 34, 39]. A

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possible mechanism has been proposed to explain the enhanced photocatalytic activity of the as-synthesized g-C3N4/V2O5 composites for the degradation of RhB. As shown

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in Fig.10, separation of the electron–holes pairs took place in the g-C3N4 semiconductor under visible light irradiation, which generated the electrons and holes.

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The photoexcited electrons transferred from the valence band (VB) of the g-C3N4 surface to the conduction band (CB), which resulted in the formation of holes in the

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VB. The CB and VB potentials of g-C3N4 and V2O5 were calculated using the

Mulliken electronegativity theory: ECB=χ−Ec−0.5Eg [40]. Here, ECB is the conduction band edge energy, χ is the absolute electro negativity of the semiconductor, Ec is the

energy of free electrons, Eg is the band gap energy of the semiconductor. EVB could be obtained by the equation of EVB=ECB+Eg. The calculated CB and VB of g-C3N4 is

-1.15 and 1.61 eV, whereas CB and VB of V2O5 is 0.49 and 2.72 eV, respectively, which was in agreement with that reported by literatures [28]. The CB of g-C3N4 is more negative than that of V2O5. Thus, photoexcited electrons on the g-C3N4 surface could quickly move to V2O5 through the interface interaction, and the holes on the 11

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valence band of V2O5 could easily shift to g-C3N4. The holes remained on the g-C3N4 surface, which effectively prevented the recombination of the electrons and holes as well as leading to the high photo-oxidation efficiency. The generated electrons would

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subsequently transfer to the photocatalyst surface to react with water and oxygen to

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generate some active species such as hydroxyl radicals and superoxide radicals. These

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radicals are able to oxidize the pollutant due to their high oxidative capacities. As a result, the adulteration of g-C3N4 to V2O5 could effectively enhance the separation

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efficiency of photogenerated electron–hole pairs, promote the photocatalytic activity during the visible light catalytic reactions as well [41].

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In addition to photocatalytic efficiency, the stability of photocatalyst is also important for practical application. The cycling experiments were carried out via the

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degradation of RhB under the visible light irradiation and the results are shown in Fig.11(A). It can be seen that the photocatalytic degradation efficiency of RhB still

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reaches 97% after 4 cycles. Fig.11(B) shows the XRD patterns of the g-C3N4(60.3%)/V2O5 composite before and after the recycle experiment. There is no

obvious change of crystal structure, indicating that g-C3N4(60.3%)/V2O5 composite is a stable photocatalyst.

4 Conclusions

In short, g-C3N4/V2O5 composites were successfully synthesized via a simple one-pot method. XRD, FT-IR, TEM, XPS and UV–vis DRS analyses revealed that the g-C3N4/V2O5 composites were hybridization photocatalysts. PL analysis revealed the high separation and easy transfer of photogenerated electron–hole pairs at the 12

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heterojunction interfaces derived from the match of band positions between the g-C3N4 and V2O5. The g-C3N4/V2O5 composites showed higher photocatalytic activities than that of pure g-C3N4 and V2O5. The probable electron–hole separation

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process in the hetero-junction has also been explained schematically. This research

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work provides an easy method to fabricate g-C3N4 based photocatalyst with superior

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activity for the degradation dyes. Acknowledgements

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This work was financially supported by the National Natural Science Foundation of China ( No. 51202093, 51302111 and 51272093 ), Science Foundation of Jiangsu

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Province ( No. BK20130523 ), Jiangsu University Development Foundation for

No.1143002079

),

Jiangsu

Province

Postdoctoral

Science

Foundation

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(

d

Talents ( No. 11JDG025 ), Jiangsu University Postdoctoral Science Foundation

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( No.1101035C ), China Postdoctoral Science Foundation ( No. 20110491356 ).

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List of figures 19

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Figure1 TG curves of pure g-C3N4, V2O5 and g-C3N4/V2O5 composites Figure2 XRD patterns of pure g-C3N4, V2O5 and g-C3N4/V2O5 composites FT-IR spectura of pure g-C3N4, V2O5 and g-C3N4/V2O5 composites

Figure4

XPS of g-C3N4 (60.3%)/V2O5 composite: (a) narrow scan spectrum, (b) C1s

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Figure3

TEM images of (a) pure g-C3N4, (b) g-C3N4(91.4%)/V2O5 heterocrystals, (c)

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spectrum, (c) N 1s spectrum, (d) V 2p spectrum and (e) O 1s spectrum

g-C3N4(60.3%)/V2O5and (d) g-C3N4(7.9%)/V2O5

The pore-size distribution curves of pure V2O5, g-C3N4 and

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Figure6

g-C3N4(60.3%)/V2O5 composite

(A) UV–vis absorption spectra and (B) (ahv)2–hvcurves of cubic pure

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g-C3N4, V2O5 and g-C3N4/V2O5 heterocrystals

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Figure8 RhB degradation under visible light illumination for 80 min in the presence of pure g-C3N4, V2O5 and g-C3N4/V2O5 photocatalysts PL emission spectra for g-C3N4, V2O5 and g-C3N4/V2O5 photocatalysts

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Figure9

Figure10

Schematic illustration of the possible photocatalytic mechanism of

g-C3N4/V2O5 composites toward the degradation of RhB.

Figure11

(A) Cycling runs of g-C3N4 (60.3%)/V2O5 photocatalyst under visible light

irradiation and (B) XRD patterns of g-C3N4(60.3%)/V2O5 heterocrystals

before and after 4 recycling runs

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Highlights ► The g-C3N4/V2O5 heterocrystals are prepared by one-pot method.

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► V2O5 nanoparticles with diameter of 5 nm intertwined with g-C3N4 nanosheets.

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► The possible photocatalytic mechanism was discussed.

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► G-C3N4/V2O5 heterocrystals showed excellent photocatalytic activity.

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Graphical Abstract (for review)

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