Study on the separation mechanisms of photogenerated electrons and holes for composite photocatalysts g-C3N4-WO3

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Highlights

  • g-C3N4-WO3 photocatalysts with different main parts of C3N4 or WO3 were prepared.

  • The band–band transfer for g-C3N4/WO3 and the Z-scheme for WO3/g-C3N4 were proposed.

  • The activity of g-C3N4/WO3 is not obviously increased compared with pure WO3.

  • The activity of WO3/g-C3N4 is increased greatly compared with pure g-C3N4.

Abstract

The separation mechanisms of photogenerated electrons and holes for composite photocatalysts have been a research focus. In this paper, the composite g-C3N4-WO3 photocatalysts with different main parts of C3N4 or WO3 were prepared by ball milling and heat treatment methods. The photocatalytic performance was evaluated by degradation of methylene blue (MB) and fuchsin (BF) under visible light illumination. The photocatalyst was characterized by X-ray powder diffraction (XRD), UV–vis diffuse reflection spectroscopy (DRS), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) methods. The separation mechanisms of photogenerated electrons and holes of the g-C3N4-WO3 photocatalysts were investigated by electron spin resonance technology (ESR), photoluminescence technique (PL), and determination of reactive species in the photocatalytic reactions. When the main part of the g-C3N4-WO3 photocatalyst is WO3 (namely g-C3N4/WO3), the transport process of the photogenerated electrons and holes adopts the generic band–band transfer. Meanwhile, g-C3N4 is covered by WO3 powder, and the role of g-C3N4 can not be played fully. The photocatalytic activity of the photocatalyst is not obviously increased. However, when the primary part of the WO3-g-C3N4 photocatalyst is g-C3N4 (namely WO3/g-C3N4), the migration of photogenerated electrons and holes exhibits a typical characteristic of Z-scheme photocatalyst, and the photocatalytic activity of the photocatalyst is increased greatly.

Introduction

As photocatalysis can be applied to waste water treatment, environmental cleaning, and producing hydrogen from water splitting, it has been attracting much attention in recent years [1], [2], [3]. In order to achieve the above goals, two problems must be resolved, i.e., increasing the separation efficiency of photoexcited electron-hole pairs and extending the excitation wavelength range of photocatalysts [4], [5]. To increase the absorption wavelength range, the semiconductor materials, such as oxides, sulfides, nitrides, and solid solutions etc., which can be excited by visible light, have been investigated extensively [6], [7], [8], [9]. However, a single-phase photocatalyst exhibits significant limitations in the process of photocatalytic reactions due to the quick combination of photogenerated electrons and holes. To enhance the separation efficiency of photoexcited electron-hole pairs, the heterojunction composite photocatalysts have been fabricated extensively [10], [11], [12], [13]. In general, when two semiconductors with the matching of band gap were coupled into a heterojunction photocatalyst, the photoexcited carriers are transferred into valence band (VB) and conduction band (CB) of opposite semiconductor respectively due to their potential difference of VB and CB [14], [15], [16]. However, the oxidation and reduction ability of the transferred photoexcited carriers are lower than that of original photoexcited carriers because of the difference of band positions. So exploitation of semiconductor photocatalysts with simultaneous high photooxidation and photoreduction performance is always a hot topic. Recently, the Z-scheme principle of photocatalyst has become a focus of research because of its stronger oxidation and reduction capability and higher photocatalytic performance than the single component. For example, a plasmonic Z-scheme visible-light photocatalyst H2WO4·H2O/Ag/AgCl exhibits a much higher photocatalytic activity than the one-component or two-component photocatalysts [17]. ZnO/CdS Z-scheme photocatalyst prolongs the lifetime of photoexcited carriers [18], and increases the photocatalyst activity.

Recently, a polymer photocatalyst named graphitic carbon nitride (g-C3N4) has attracted intensive attention for hydrogen and oxygen evolution via water splitting, photocatalytic degradation of organic pollutants, and photosynthesis under visible light illumination [19], [20], [21]. It is known that the band gap of g-C3N4 is about 2.7 eV, which can absorb visible light up to 460 nm. Furthermore, the CB minimum (−1.12 eV vs. NHE) of g-C3N4 is extremely negative, so photogenerated electrons should have high reduction ability. However, the photocatalytic efficiency of single g-C3N4 is limited due to the high recombination probability of photoexcited electron-hole pairs. In order to improve photocatalytic activity, many strategies such as doping and coupling g-C3N4 with other semiconductor materials or metal and nonmetal have been used to modify g-C3N4 [22], [23], [24], [25], [26].

Tungsten oxide (WO3) is regarded as a promising material because of its special photocatalytic and electrochromic properties [27], [28], [29], [30]. Compared with TiO2, WO3 has a smaller optical band gap (2.7 eV), and the VB position of WO3 is extremely close to that of TiO2. Therefore, the hole generated on the VB of WO3 has a similar oxidative capability to that of TiO2. However, the CB level of WO3 is more positive than that of TiO2, which results in the electron generated on the CB of WO3 with a limited reductive ability than of TiO2.

When WO3 is combined with g-C3N4, a g-C3N4/WO3 heterojunction photocatalyst may be formed between WO3 and g-C3N4. The transfer of the photoexcited carriers of g-C3N4 and WO3 will happen because of the position differences of VB and CB. There are two ways for the transfer of the photoexcited carriers. One is band–band transfer, and the other is Z-scheme principle transfer. It is known that the VB position of g-C3N4 is about 1.57 eV, and the CB position of WO3 is about 0.74 eV [21], [29]. Because of the short distance between the VB of g-C3N4 and the CB of WO3, a Z-scheme system photocatalyst may be formed. If so, the holes generated on the VB of g-C3N4 are easily combined with the electrons generated on the CB of WO3. Consequently, the photogenerated electrons on the CB of g-C3N4 exhibit strong reduction ability, and the photogenerated holes on the VB of WO3 show excellent oxidation ability. However, to the best of our knowledge, there has been no report on the investigation of g-C3N4-WO3 photocatalyst. Especially, the separation mechanisms of photoexcited carriers for the heterojunction photocatalysts have not been investigated extensively.

In this paper, different ratios of g-C3N4-WO3 photocatalysts were prepared with ball milling and heat treatment methods. The photocatalytic activity was evaluated by degradation of methylene blue and fuchsin under visible light illumination. The g-C3N4-WO3 photocatalysts were characterized in detail. The separation mechanisms of photogenerated electrons and holes of the g-C3N4-WO3 photocatalysts were investigated by electron spin resonance technology, photoluminescence technique, and determination of reactive species in the photocatalytic reactions. Some interesting results were obtained. The separation mechanisms of photoexcited carriers for the composite photocatalysts were proposed.

Section snippets

Materials

Melamine powder (Pur. >99.0%) used in the experiments was supplied by Aladdin Chemistry Co. Ltd. Ammonium tungstate hydrate was supplied by Sinopharm Chemical Reagent Co. Ltd. MB, BF, and other chemicals used in the experiments were purchased from Shanghai and other China chemical reagent Ltd. They are of analytically pure grade and used without further purification. Deionized water was used throughout this study.

Preparation of samples

g-C3N4 powder was prepared via heating melamine in a tube furnace. A certain

XRD analysis

Fig. 1a shows the XRD patterns of pure g-C3N4 and WO3/g-C3N4, and Fig. 1b shows the XRD patterns of WO3 and g-C3N4/WO3. From Fig. 1a, it is clear that g-C3N4 shows two basic diffraction peaks at around 27.8° and 13.3°, which can be indexed as (0 0 2) and (1 0 0) diffraction planes (JCPDS 87-1526). The former, which corresponds to the interlayer distance of 0.325 nm, is attributed to the long-range interplanar stacking of aromatic systems; the latter with a much weaker intensity, which corresponds to

Conclusions

The composite photocatalyst WO3(wt.%)-g-C3N4 was fabricated via ball milling and heat treatment methods. The coupling between g-C3N4 and WO3 may happen on the g-C3N4-{0 0 2} facets. When g-C3N4 is the major part of composite photocatalyst (namely WO3/g-C3N4), the transfer of the photoexcited electrons and holes is according to the Z-scheme mechanism. In the system, the photoexcited holes in the VB of g-C3N4 and electrons in the CB of WO3 are quickly combined. The accumulated electrons in the CB

Acknowledgement

This study was supported by the Natural Science Foundation of China (NSFC, grant No. 20973071, 51172086, 51272081 and 21103060).

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