RGO/Ag2S/TiO2 ternary heterojunctions with highly enhanced UV-NIR photocatalytic activity and stability

https://doi.org/10.1016/j.apcatb.2016.12.011Get rights and content

Highlights

  • Novel RGO/Ag2S/TiO2 UV-NIR response heterostructure photocatalysts have been synthesized successfully.

  • Strong absorption of UV-NIR light improves the sunlight utilization effectively and the degradation rate of organic pollutants.

  • The synergetic effects of RGO, Ag2S and TiO2 enhance the photocatalytic activity and stability.

Abstract

A series of novel RGO/Ag2S/TiO2 heterostructure photocatalysts has been synthesized successfully by a facial hydrothermal and simple ionexchange method. During the preparation, the reduction of graphene oxide (GO) and growth of TiO2 and Ag2S are accomplished smoothly, and the functions of each part are developed well together. Compared with pure TiO2, Ag2S and Ag2S/TiO2 nanocomposite, the obtained 3 wt.% RGO/Ag2S/TiO2 nanocomposite exhibits a significant enhancement of ultraviolet (UV) and near-infrared (NIR) light photocatalytic activities. Owing to its extended photoresponsive range, great adsorptive property and efficient electron-hole separation capacity. Furthermore, benefited by the introduction of reduced graphite oxide (RGO), the deterioration of Ag2S is inhibited well and the stability of RGO/Ag2S/TiO2 nanocomposite has been enhanced greatly.

Introduction

Utilization of photocatalytic technology to solve the energy crisis and environmental contaminations has been thriving, owning to its clean, non-polluting and high-efficiency properties [1], [2], [3]. Up to now, UV and visible (Vis) light respond photocatalysts have been studied vastly and some excellent photocatalysts have been developed well. Titanium dioxide (TiO2) has been widely employed as a photocatalyst because of its low cost, high photoinduced charge carries utilization efficiency and outstanding photocatalytic activity, but the narrow absorption band in UV region (which accounts for only 5% of the solar spectrum) astricts its practical application [4], [5], [6]. In order to utilize solar light more efficiently, modifying TiO2 by surface modification [7], [8], doping of metal or nonmetal elements [9], [10], [11], and combination with narrow band gap semiconductors to enhance the visible light absorption and photocatalytic activity has been a main research direction in recent years [12], [13]. Nevertheless, the NIR light of large fraction (∼50%) in sunlight remains untapped since its low photon energy cannot directly give rise to the response of most semiconductors during the solar energy conversion process [14]. In other words, the relative low-usage of sunlight has been restraining the photocatalytic efficiency in the environmental remediation, and the development of the NIR light excited photocatalysts to utilize solar light efficiently is an urgent task [6], [15].

Lately, Jiang et al. have prepared Ag2S by a simple ion-exchange method at room temperature and used it directly as a NIR light induced photocatalyst, but the narrow band gap about 0.9 eV leads to the fast recombination of photo carries and low photodegradation rate under the irradiation of different light [16], [17]. Depending on the broad and strong absorption property of Ag2S in the whole solar energy spectra, especially in NIR light region, the fabrication of heterojunction with TiO2 is a good modification method to improve the charge carries transfer and separation capacity, and then enhance the photocatalytic activity [6], [18]. In previous reports, there has been some positive results of Ag2S-coupled TiO2 composite in photocatalysis area. Zhu et al. have reported the photocatalytic activity of Ag2S sensitized TiO2 catalysts towards methylene blue photodegradation, and the nanocomposite exhibits much higher photocatalytic activity than that of pure Ag2S and TiO2 under visible light irradiation [19]. Furthermore, Gholami et al. have fabricated Ag2S-TiO2 nanotube arrays to split water and it exhibits about 15-fold photo-enhancement in current density as compared with the pure TiO2 nanotubes under similar conditions [20]. Regardless of these few reports on the visible-light-induced photocatalytic properties of Ag2S modified TiO2 composite, the photocatalytic activity of Ag2S-TiO2 system under the irradiation of NIR light has not been completely studied yet. Therefore, it is highly interesting and important to explore the potential of NIR photocatalytic activity of Ag2S-TiO2 composite, so as to utilize solar light more efficiently by responding in the extended solar energy spectra region.

In addition, the poor stable property of most Ag-containing compounds hinders the practical application of Ag2S as a recyclable and highly efficient photocatalyst, because the Ag (I) could be reduced to metallic Ag by the photogenerated electrons under light irradiation [3], [21], [22], [23]. Hence, it is important to accelerate the transfer of photogenerated electrons for higher photocatalytic stability. Reduced graphite oxide (RGO) has recently attracted intense scientific interest owing to its large specific surface area, high chemical stability and excellent electrons capture and transport properties [24], [25], [26]. Benefiting from these characteristics, some RGO-based photocatalysts have been carried out (e.g., RGO/Ag3PO4, Ag/AgCl/RGO and Cu2O–RGO composite), and the stability has been promoted after the introduction of RGO owing to the enhancement of the transfer and separation of photogenerated carriers [24], [27], [28]. Meanwhile, under the inhibiting effort of the recombination of electron-hole pairs, the photocatalytic activity has enhanced distinctly in these composites. Therefore, it is favorable to fabricate RGO with unstable Ag2S to improve its photocatalytic stability and enhance the activity simultaneously.

Inspired by the above discoveries, RGO-Ag2S-TiO2 ternary heterostructured composites are designed and prepared via a hydrothermal method combined with a facile ionexchange method, and applied to the photocatalytic degradation of a methyl orange (MO) solution under UV and NIR-light irradiation. It is found that the UV and NIR photocatalytic activities are improved significantly with Ag2S assemble onto the TiO2 nanoparticles. Furthermore, the content of RGO obviously affects the photocatalytic activity of RGO-Ag2S-TiO2 nanocomposite and the addition of RGO could enhance the photocatalytic stability of Ag2S effectively. Subsequently, the possible mechanism for the enhanced photocatalytic activity and stability of RGO-Ag2S-TiO2 are thoroughly proposed based on the obtained experimental results.

Section snippets

Preparation of RGO-TiO2 nanocomposite

A certain amount of graphene oxide (GO) was redispersed via the assist of ultrasonication in 30 mL deionized water to obtain the homogeneous suspension. Then, 7.5 mL absolute ethyl alcohol with 1.71 mL tetrabutyl titanate (TBOT) were added dropwise to the suspensions under vigorous stirring for 30 min. Finally, the above suspensions were transferred into 50 mL Teflon-lined autoclaves and maintained at 150 °C for 10 h. After this hydrothermal reaction, the suspensions were centrifuged at 17000 rpm

Results and discussion

Fig. 1 presents the XRD patterns of the pure TiO2, Ag2S, AT and RAT nanocomposites with different RGO ratios. The reflection in Fig. 1a matches best with the single anatase TiO2 phase (JCPDS 21–1272). Moreover, XRD pattern from the AT nanostructures (Fig. 1c) obviously consists of two sets of diffraction peaks. Compared to the peaks of the anatase TiO2, there are three sharp diffraction peaks at 2θ values of 34.4°, 36.8° and 37.7°, which could be indexed to the (1¯21), (121) and (1¯03) crystal

Conclusion

In summary, we have proposed and proven a new strategy by introducing the Ag2S with wide spectrum absorption property and RGO sheets with excellent ionic conductivity into high-efficiency TiO2 to extend the photoresponsive range and enhance the photocatalytic activity. Specifically, a series of RGO-Ag2S-TiO2 heterojunctions with good photocatalytic performance has been realized via a facial hydrothermal and simple ionexchange method. The enhanced UV-NIR photocatalytic activity could be

Acknowledgements

This research is supported by the Gansu Province Development and Reform Commission (NDRC, No. 2013-1336), the National Natural Science Funds of China (Grant No. 51372105), the International Sci. & Tech. Cooperation Foundation of Gansu Provincial, China (Grant Nos 1504WKCA088), the Fundamental Research Funds for the Central Universities (No. lzujbky-2016-131), the JSPS KAKENHI Grant Number JP16H06439 (Grant-in-Aid for Scientific Research on Innovative Areas), the Dynamic Alliance for Open

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