Full Length ArticleSnO2/graphene quantum dots composited photocatalyst for efficient nitric oxide oxidation under visible light
Graphical abstract
SnO2/GQDs has exhibited a remarkable efficiency towards photocatalytic removal of NO as a result of the improved light harvesting ability and enhanced charge separation efficiency with the introduction of GQDs.
Introduction
Nitric oxide (NO) is a typical component in air pollutant in some of the countries around the world and the removal of nitric oxide is of great importance in consideration of the air purification [1], [2]. Various techniques including physical/chemical adsorption [3], [4], selective catalytic reduction [5], [6], and thermal catalytic reduction [7], [8] have been developed for the removal of NO. However, these methods require additional conditions such as high temperature, reducing agents et al., making the removal of NO difficult at the sub-ppm level at ambient temperature. Photocatalytic removal of NO with low concentration (ppb level) is considered as one of the most promising strategies, as it could take advantage of solar light directly without introduction of additional energy source. In the past few decades, various semiconductors such as TiO2 [9], Bi2O2CO3 [10], [11], Bi2WO6 [12], C3N4 [13], [14] and Bi2O3 [15] have been explored for the photocatalytic NO oxidation and they exhibited attractive activities. However, many of these semiconductors could not oxidize NO completely into nitrate NO3−, and nitrogen dioxide NO2 was often obtained instead, which could be even more toxic in contrast to NO.
Tin dioxide (SnO2), an n-type semiconductor with the bulk band gap (∼3.60 eV), high stability, excellent electronic and optical properties, are widely used in gas sensor, photoelectrochemistry and photodetector [16], [17], [18]. Compared to TiO2, SnO2 has a more positive valence band and hence stronger oxidation ability, which could oxidize the pollutants more completely [19]. Nevertheless, the application of SnO2 in photocatalysis is quite limited as a result of its intrinsic wide bandgap and hence ineffective utilization of visible light [20], [21], [22]. This could be solved by modification of the surface state of SnO2 like doping and introduction of oxygen vacancies. [23], [24], [25], [26]. For example, our previous work showed that the iodination on the surface of SnO2 can significantly enhance its visible light response due to the formation of band tail states [27].
In addition, the enhancement of visible light response of the system could be achieved after the wide bandgap semiconductors are combined with other narrower bandgap materials. Among them, graphene quantum dots (GQDs) as a new type of zero-dimensional carbon nanomaterials with unique properties of graphene has drawn a lot of attention [28]. Specifically, owing to the size-resulted quantum confinement and strong edge effects, GQDs have exhibited remarkable visible light absorption, unique electron reservoir ability and photostability [29], [30]. GQDs could not only render the visible light response of the system, but also benefit the charge separation efficiency of the system. A series of GQDs modified semiconductor photocatalysts has been reported with enhanced photocatalytic and photoelectrochemical performances, such as TiO2/GQDs [31], CdS/GQDs [32], ZnO/GQDs [33], g-C3N4/GQDs [34], N-doped Bi2O2CO3/GQDs [35] and so on. Nevertheless, to the best of our knowledge, the influence of modification of SnO2 with GQDs on the photocatalytic behavior of SnO2 for NO removal is not yet studied.
In this work, we have successfully coupled ultrasmall SnO2 nanoparticle (∼2.4 nm) with GQDs through a facile method. The photocatalytic behavior of as-prepared SnO2/GQDs composite was evaluated by the removal of NO at ppb level. It turns out that the efficiency of SnO2 could be remarkably improved after the introduction of GQDs with a weight percentage of 1%, meanwhile the high selectivity of the oxidative products towards NO3− originated from SnO2 maintained. Though the small amount of GQDs in the composite has no obvious influence on the morphology and structure of SnO2, it does enhance the visible light response and charge separation efficiency of the system.
Section snippets
Experimental section
All the chemicals were used as received without further purification and purchased from Chengdu Kelong Co. Ltd. with analytical grade. Ultrapure deionized water (resistivity ≥ 18.25 M) was used in all experiments.
Photocatalytic performances
The photocatalytic performances of the as-prepared samples were evaluated by the degradation of air pollutant NO at an indoor ppb level (600 ppb). Before illumination, the samples were exposed to continuous NO gas flow for 30 min to reach the surface adsorption-desorption equilibrium. Under the full spectrum irradiation, all samples show good photocatalytic activity among all the investigated samples (Fig. 1a). In specific, the activity of the samples increased with a small amount of GQDs
Conclusion
In summary, we have prepared a series of SnO2/GQDs composites via a facile method. The composites could function as an efficient photocatalyst for removal of indoor NO under visible light illumination. Among them, the composite with 1% GQDs exhibits the highest photocatalytic NO degradation ratio of 57% and excellent selectivity with NO2 generated fraction as low as 5%. The introduction of GQDs in the system has little influence on the structure and morphology of SnO2, but it notably improves
Acknowledgements
We gratefully acknowledge financial support from National Natural Science Foundation of China (21403172), the Innovative Research Team of Sichuan Province (2016TD0011), Sichuan Provincial Education Department Project (15ZB0055), the Scientific Research Starting Project of SWPU (2015QHZ018) and Young Scholars Development Fund of SWPU (201499010096).
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Y. Xie and S. Yu contributed equally in this paper.