DOI:10.30919/es8d689

Received: 03 Nov 2018
Accepted: 29 Jan 2019
Published online: 01 Feb 2019

In Situ Preparation of WO3/g-C3N4 Composite and Its Enhanced Photocatalytic Ability, a Comparative Study on the Preparation Methods of Chemical Composite and Mechanical Mixing

Zengying Zhao1*, Hua Ma1, Mingchao Feng1, Zhaohui Li2,3, Dapeng Cao4,5 and Zhanhu Guo6

1School of Science, China University of Geosciences, 29 Xueyuan Rd, Beijing 100083, China

2Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China

3Department of Geosciences, University of Wisconsin - Parkside, Kenosha, WI 53141-2000, USA

4 State Key Lab of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

5 International Research Center of Soft Matter, Beijing University of Chemical Technology, Beijing 100029, China

6Integrated Composites Laboratory (ICL), Chemical and Biomolecular Engineering Department, University of Tennessee, Knoxville, TN 37996, USA

*E-mail: [email protected]


Abstract

A facile and efficient WO3/g-C3N4 composite photocatalyst was developed by an in situ raw material decomposition method. The photocatalyst was synthesized by direct heating low-cost ammonium tungstate and melamine together at the same time. The characterization and photocatalytic performance of the WO3/g-C3N4 chemical composite samples were compared with those from mechanical mixing. The characterization includes X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectra (FT-IR), etc. The composite formed by the chemical method exhibited obviously better photocatalytic activity than the mechanical mixing sample under visible light irradiation. The possible mechanism for the enhanced catalytic efficiency may be due to the synergistic effect between the WO3 and g-C3N4 tighter interface and the improved optical absorption in visible region.


Table of Content

WO3/g-C3N4 photocatalyst was developed by an new in situ raw material decomposition process and it was comparatively studied with mechanical mixing samples

 

Keywords 

WO3     g-C3N4    Photocatalysis     Comparative study     Chemical composite


1. Introduction

With the fast development of industry, environmental problems have become a severe threat to human beings. As photocatalyst can be applied to wastewater treatment, environmental cleaning, and producing hydrogen from water splitting, etc., it has been attracting much attention in recent years.1-3 TiO2 is the most widely used photocatalyst because of its excellent oxidation ability, availability, and stability.4 However, TiO2 has low quantum efficiency and it is inactive under visible-light irradiation. Many scientists have tried their best to improve its visible-light response, such as doping with impurity,5,6 co-catalyst loading,7 and shape control.8-9 Scientists have also found other photocatalysts that show high catalytic activity under visible light irradiation. 10 Unfortunately, these types of catalysts, such as BiVO4 and CdS,11-14 are always toxic for human health and harmful to the environment. As a consequence, it is very important to find new and nontoxic photocatalysts with excellent catalytic activity under visible light.

The stable metal-free photocatalyst, polymeric graphite-like carbon nitride (g-C3N4), has attracted great interests recently. It has applications in water splitting and decomposition of organic pollutants under visible light.15-16 However, the photocatalytic efficiency of the pure g-C3N4 is limited by the high recombination rate of its photogenerated electron-hole pairs.17 One of the techniques for increasing the separation efficiency of photo-generated electron-hole pairs is to form a composite photocatalyst using two kinds of semiconductors.18 As for g-C3N4, various semiconductors composited with g-C3N4 have been reported. These semiconductors include TiO2 19, ZnO20, Ag3PO4 21, and Bi2WO6 22, etc. Tungsten oxide (WO3) is an n-type semiconductor. It has attracted much interest for the uses in electrochromic devices23, gas sensors24, and in supercapacitors 25 due to its relatively narrow band gap (i.e. 2.36-2.8 eV) and stable chemical and physical properties. Moreover, WO3 has also gained much attention as a promising visible light driven photocatalyst in recent years.30-34 However, pure WO3 is not an efficient photocatalyst because of its low conduction band (CB) level, which limits its photocatalytic ability to react with electron acceptors such as oxygen.35

Although there have been reports of WO3/g-C3N4 photocatalysts, the preparation methods are mechanical mixing of WO3 and g-C3N4,36-41 and none of these has been focused on the comparative study of samples prepared by in situ raw material decomposition and mechanical mixing. In this study, the WO3/g-C3N4 chemical composite photocatalyst has been prepared by an in situ decomposition reaction of ammonium tungstate and melamine. Its characterization, the photocatalytic activity and reaction mechanism in the degradation of methylene blue (MB), have been comparatively discussed with the WO3/g-C3N4 samples created by mechanical mixing.

2. Experimental

2.1 Preparation of photocatalysts

All starting reagents (analytical grade) were purchased from Sinopharm company and were used without further purification. Distilled water was used throughout. Before the reaction, different ratios of ammonium tungstate hydrate and melamine powders were added into a motor and ground for 30 min using a pestle. The resultant mixed powder was added into a crucible with a cover in a semiclosed system. Then it was heated at a heating rate of 20 °C min-1 under air conditions. According to thermogravimetric analysis (TG) results (as shown in Fig. S1), the mass contents of WO3 in the WO3/g-C3N4 chemical composite samples were estimated to be 0.39%, 1.35%, 2.83%, 5.01%, 10.02%, respectively. Therefore, the obtained WO3/g-C3N4 chemical composites with different contents of WO3 were named as 0.3WN, 1WN, 3WN, 5WN, 10WN. The pure WO3 and pure g-C3N4 samples were also prepared using the same process with ammonium tungstate hydrate or melamine as raw materials, respectively. Besides, a mechanically mixed sample was also prepared by mixing the WO3 and g-C3N4 powders, which has the same amount of WO3 with that of 3WN, and it was named as mechanical mixing sample 3W--N, in order to compare with the chemical composite sample of 3WN.

2.2. Characterization

The crystal phase composition of the samples was obtained by X-ray diffraction (XRD, Rigaku D/max, Rigaku Corporation, Tokyo, Japan) in the 2θ range of 10-80° using Cu-Kα radiation for the X-ray radiation source. The specific surface area, pore size, and pore volume of the samples were evaluated with a surface area analyzer (AutosorbiQ Station-I, Quantachrome Instruments, Florida, US) from N2 adsorption isotherms using Brunauer-Emmett-Teller (BET) method. The mesopore size distribution and mesopore volume were calculated by desorption isotherms. The elemental composition of the samples was measured by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250xi) with photon energy of 1253.6 eV, and with Mg-Kα radiation as the exciting source. All the binding energies were referenced to the C1s peak for calibration. The crystal size and morphology of the samples were obtained by scanning electron microscopy (SEM, S-500, JEOL Ltd., Tokyo, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL Ltd., Tokyo, Japan). The crystalline structure of the samples was examined by high resolution transmission electron microscopy (HRTEM). The UV-vis absorption spectra of the samples were recorded on a UV-vis scanning spectrophotometer (Perkin-Elmer, Lambda 900, Florida) in the scan range from 200 to 800 nm equipped with an integrated sphere with Barium sulphate (BaSO4) as the reference. The EIS and PT are both tested on an electrochemical workstation (SI 1287, METEK, Benelux, UK) with electrolyte of 0.1M Na2SO4 and visible light irradiation of a 500 W Xenon lamp.

2.3. Photoreaction apparatus and procedure

Photocatalytic activities of WO3/g-C3N4 chemical composite samples were evaluated by degradation of MB (2×10-5 mol/L) under visible light irradiation of a 500 W Xenon lamp with 420 nm cutoff filters. In a typical procedure, 50 mg of photocatalyst was dispersed into 50 mL of MB solution. Before illumination, the photocatalyst powder and dye solution were stirred in dark for 60 min to achieve the adsorption-desorption equilibrium of suspensions. Afterward, about 3 mL of the reaction solution was taken at given time intervals, and separated through centrifugation. The concentration of MB was analyzed by recording the absorbance at the characteristic band of 664 nm by a U-3010 UV-vis spectrophotometer.

3. Results and discussion

3.1 XRD analysis

Fig. 1 XRD patterns of WO3, g-C3N4, WO3/g-C3N4 chemical composite samples and mechanical mixing sample.

Fig.1 shows the XRD patterns of the WO3, g-C3N4, chemical composite samples 0.3WN, 1WN, 3WN, and 5WN, and mechanical mixing sample 3W--N. The XRD pattern of the WO3 can be exactly indexed as the monoclinic structure (JCPDS 43-1035).18 For pure g-C3N4, the strongest XRD peak at 27.30°, corresponding to 0.326 nm, can be indexed as (002) diffraction plane (JCPDS 87-1526). It is due to the stacking of the conjugated aromatic system as in graphite. Another pronounced additional peak is found at 12.80°, corresponding to a distance of 0.675 nm. This distance is only slightly below the size of one tris-s-triazine unit (ca. 0.73 nm). The two diffraction peaks are in good agreement with the reported results of g-C3N4.41 For the mechanical mixing sample 3W--N, the XRD patterns reveal a coexistence of WO3 and g-C3N4 with the appearance of three WO3 typical peaks: (002), (020) and (200) and two g-C3N4 peaks: (100) and (002). On the contrary, in the chemical composite sample 3WN, the three peaks have combined to a broad peak. It may that the (as also shown in the following TEM images). Moreover, the intensity of the above-mentioned WO3 broad peak increases gradually with the increasing of the WO3 content in the samples; and it is also interesting to note that the peak of g-C3N4 decreased at the same time.

Besides, it is observed that in the chemical composite samples, the (002) peak of g-C3N4 shifted from 27.30° to 27.77°. Based on this result, it is proposed that the coupling between WO3 and g-C3N4 may happen on the g-C3N4 (002) facet.42 By contrast, from Fig. 1, there are no shifts of the diffraction peaks in the mixed sample 3W--N. This proved that there is no tight interaction between the WO3 and C3N4 in the mechanical mixing sample 3W--N.

3.2 BET Test

Fig. 2 N2 adsorption-desorption isotherms of the chemical composite samples and the mechanical mixing sample 3W--N.

BET N2-sorption measurements can be used to investigate the surface areas, pore volumes, and average pore diameters of the samples. The recorded N2 adsorption-desorption isotherms for the chemical composite sample and the mechanical mixing sample are shown in Fig. 2. The N2 adsorption-desorption isotherm include six types from I to VI, indicating the existence of micropore solids, non-hole or macropore structure, the interaction of gas-solid on non-hole or macropore materials, mesoporous structure, the interaction of gas-solid on micropore and mesoporous materials, and the multilayer adsorption on surface of nonhole materials, respectively. The isotherm of all the samples are type II. And both of them are with H3 hysteresis loop, indicating the existence of non-hole or macropore structure according to the IUPAC classification.43-44

Table 1 Structural properties of the mechanical mixing sample 3W--N and chemical composite sample 3WN.

Sample

SBET a

(m2 g-1)

PV b

(cm3)

PDc

(nm)

3W--N

7.323

0.059

3.785

3WN

18.984

0.032

4.396

a: the specific surface area was calculated using BET equation; b: pore volume; c: pore diameter.

According to the BET measured results, the specific surface areas, pore sizes and pore volumes of the samples were calculated, and the results are listed in Table 1. The samples show obvious difference in their surface areas, which are 7.323 and 18.984 m2g-1, for the mechanical mixing sample 3W--N and the chemical composite sample 3WN, respectively. The specific surface area of the chemical composite 3WN is much higher than that of the mechanical mixing 3W--N. This is perhaps because that the g-C3N4 has become more porous in its preparation from the raw material of ammonium tungstate hydrate and melamine. The larger specific surface area of 3WN can provide abundant reacting active sites for the reactive molecules to contact with the photocatalyst, thus improving its photocatalytic performance.

3.3 SEM, TEM and HRTEM analyses

Fig. 3 shows the SEM images of the chemical composite sample 3WN and mechanical mixing sample 3W--N. The chemical composite sample has an uneven surface (Fig. 3a), which indicates that the surface of g-C3N4 in the composite sample has been etched by the thermal treatment in the preparation procedure. Furthermore, the uneven sheets of g-C3N4 covered the WO3 particles in the 3WN sample, while the g-C3N4 and the WO3 particles are departed from each other.