A novel high-efficiency visible-light sensitive Ag2CO3 photocatalyst with universal photodegradation performances: Simple synthesis, reaction mechanism and first-principles study

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Abstract

A novel high-efficiency visible-light sensitive Ag2CO3 semiconductor photocatalyst was prepared by a simple ion-exchange method based on a strategy incorporating of p-block C element into a narrow bandgap Ag2O. This photocatalyst exhibits universal high-efficient degradation ability for typically several RhB, MO and MB dyes. Getting insight into degradation patterns of dyes over Ag2CO3 identifies they are self-oxidation behavior of semiconductor rather than the effect of photosensitization. The reaction mechanism investigated by a series of radical trapping experiments, not only ascertains the major photoreaction approaches of dyes on the surface of Ag2CO3, but also reveals the unique universality advantage that arises from selective using one of many activated species to decompose many kinds of dyes such as RhB, MO and MB. The theoretical calculation based on first-principles provides inherently essential evidences for high-efficient oxidation performance of Ag2CO3 photocatalyst.

Highlights

► A novel visible-light sensitive Ag2CO3 synthesized by a simple method. ► Universal and high-efficiency photodegradation performance. ► Evidence of photooxidation ability are provided by theory calculation. ► Mechanism and approaches of dyes are studied in detail.

Introduction

Environmental pollution and energy shortage have become two arduous challenges to the sustainable development of modern human society. Over the past decades, the “Green-life” concept is inspiring enthusiasm to exploit the novel, high-efficient and eco-friendly photocatalysts for air purification and wastewater cleaning [1]. The photocatalytic approach has been broadly investigated and consistently considered as a promising candidate to complement advanced oxidation technologies such as chemical, physical and biological methods [1], [2]. Semiconductor photocatalysts have attracted extensive attention because of their potential applications to energy and environmental problems, e.g. water splitting [3], [4], carbon dioxide reduction [5], organic contaminant degradation [4], [6] and so on. As is well known, the photodegradation of organic pollutants using semiconductor involves photogenerated electrons and holes separating, migrating to the surface of semiconductor, and serving as redox sources that then react with adsorbed organic molecules by a series of complicated redox process, in consequence, giving rise to decomposing organic pollutants. We emphasize the significance of developing novel visible-light (about 48% of sunlight) sensitive photocatalysts with excellent photodegradation ability and universality for decomposing various organic pollutants. Therefore, the development of the visible-light sensitive photocatalysts for more efficient utilization of the abundant sunlight and indoor light has become one of the desired directions and most important topics in the photocatalytic fields [2], [3], [4]. Up to now, more and more studies were carried out for the development of visible-light sensitive photocatalysts, the attempts of which were concentrated on oxides/composite oxides, such as TiO2 [6], N–TiO2 [7], Ag–TiO2 [8], [9], MnO2 [10], Nb2O5 [11], Fe3O4/Fe2O3 [12], and sulfides/composite sulfides, such as CdS [13], ZnS [14], MoS2/MoS3 [15], Cu2ZnSnS4 [16]. In recent years, some for Ag-based compounds are believed to be promising high-efficient photocatalysts such as reported silver halides AgX (X = Cl [17], [18], [19], [20], Br [21], I [22]), AgInW2O8 [23], [24], Ag2Mo3O11 [25], AgNb(Ta)O3 [26], Ag3VO4 [27] and Ag2V4O11 [28]. Especially, Ag-based transition metal complex oxide semiconductors, the top of the valence band (VB) of them consist of the unique hybridized Ag 4d and O 2p orbitals, which can lift the top position of the VB and narrow down the bandgap. The bottom of the conduction band (CB) that consists of the relatively delocalized s and/or p orbitals displays significantly dispersity so that it possesses high photogenetated electrons and holes mobility resulting to enhancement of photocatalytic activity of photocatalysts [29]. Moreover, it is also a feasible and effective designed strategy incorporating of p-block elements into a simple narrow bandgap oxide to develop new semiconductor photocatalysts [30]. The reported Ag2O belongs to a narrow bandgap semiconductor with bandgap of 1.3 eV [31]. Therefore, the incorporation of p-block elements into Ag2O may broaden the bandgap, enhance oxidative ability, and improve stability of the obtained photocatalyst compared to Ag2O. At present, a serious of successful research works were achieved by this strategy to prepare many new photocatalysts, for instance, the reported AgSbO3 [29], AgMO2 (M = Al, Ga, In) [32], [33], [34], [35] and Ag3PO4 [30], [36], [37], [38], [39], [40] exhibit high photocatalytic activity under visible-light irradiation.

Herein, in this study, silver carbonate was selected as a novel candidate of visible-light sensitive photocatalyst that prepared by a simple ion-exchange method. The photodegradation patterns and reaction approaches of the methyl orange (MO), methylene blue (MB), and rhodamine B (RhB) dyes over Ag2CO3 under visible-light with λ  400 nm irradiation were investigated in detail. In addition, the theoretical calculation of the energy band dispersion and density of states (DOS) of Ag2CO3 based on plane-wave-based density functional theory (DFT) was applied to revealing the intrinsically essential evidences of high-efficient photooxidation ability for Ag2CO3.

Section snippets

Preparation and characterization of the Ag2CO3 photocatalyst

The Ag2CO3 samples were prepared by a typically simple ion-exchange method. NaHCO3 aqueous solutions (0.05 mol L−1) were added to the 100 ml AgNO3 aqueous solutions (0.05 mol L−1) drop by drop on ice-water bath conditions until the plenty of yellow green precipitations were generated. The precipitations were washed in turn with secondary distilled water and absolute ethanol to dissolve any unreacted raw materials. Then, the as-prepared Ag2CO3 products were blow-dried using blower under atmosphere at

Results and discussion

Fig. 1a presents XRD pattern of the as-prepared Ag2CO3 sample. All the diffraction peaks can be indexed to Ag2CO3 crystal (JCPDS card No. 70-2184.) with a monoclinic structure, and no diffraction peaks from the impurities are detected. Compared with standard diffraction spectrum, the main diffraction peak at 2θ = 33.6° corresponding to lattice plane (1 3 0) exhibits the drastic enhancement relative to the others, which indicates that (1 3 0) may be the main exposed and activated lattice plane. Fig. 1

Conclusions

We successfully prepared a new visible-light sensitive Ag2CO3 semiconductor photocatalyst by a simple ion-exchange method, which exhibits universal high-efficient photodegradation performance for RhB, MB and MO dyes. The theory calculation based on the plane-wave-based DFT indicates the incorporation of p-block C element into narrow bandgap Ag2O semiconductor broadens the bandgap width and enhances photodegradation ability. The weakening of Agsingle bondO bonds and the occurrence of Ag s–Ag s states

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (21071036 and 21271055) and Province Natural Science Foundation of Heilongjiang Province (ZD201011). We acknowledge the State Key Laboratory of Urban Water Resource and Environment for the help in characterization.

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