Ce-Doped Sns Quantum Dots Sensitized SnS2 Nanocomposites for NO2 Monitoring
Authors: Su Huyin , Cai Yuchen , Li Long , Liu Huan 
Published: May 2020

Introduction
Metal sulfides have been proved to be highly sensitive to low concentration NO2 due to their unique structure and electrical properties, among which SnS2 has been widely studied to make use of its narrow band gap, large surface-to-volume ratio and large electronegativity. However, it has been proved that the low sensitivity of pure SnS2 to NO2 at the room temperature is still far from sufficient practical values. Therefore, the fabrication of nanocomposites based on SnS2 with heterojunctions have been studied as promising candidate materials for gas sensors with practical applications. For instance, Gu and his coworkers reported that the sensing properties of formed SnO2/SnS2 nanohybrids improved obviously compared with commercial pristine SnS2 nanosheets and Hao further promoted the progress by decorating the surface of SnS2 nanoflowers with SnO2 nanoparticles.
Under the inspiration from the above studies, herein cerium was later introduced into the one-step synthesis process of SnS2/SnS heterostructures to effectively modulate the sensing property. In previous studies, Ce-doped SnS2 nanoflakes were successfully synthesized and exhibited higher photocatalytic efficiency than that of un-doped SnS2 due to the porous architecture and reduced energy bandgap after Ce-doping. Ce-doped SnS2 nanoflowers were also studied as anode electrodes materials for lithium ion batteries, whose reversible capacities and cycling performance is the best among the synthesized Ce SnS2 compounds because of larger lattice space provided by the larger-radius cerium ions replacement of Sn4+ for lithium intercalation and deintercalation. Besides, it has been found that Ce dopants can effectively tune the optical gaps and produce the magnetic ground state.

Experimental

Synthesis of SnS@SnS2 nanoparticles

The pure SnS@SnS2 nanoparticles were synthesized by a hydrothermal reaction. In experiment, 0.902 g of SnCl2·2H2O and 1.805 g of thiourea were dissolved in a stoichiometric ratio of 1:3 in 32ml Milli-Q water under vigorous stirring to form a white homogeneous solution. Then, the obtained homogeneous solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and kept at 200 °C for 8 h. Then, the autoclave was allowed to cool down to room temperature naturally. The formed SnS@SnS2 nanoparticles were washed with ethanol and water by centrifugation (8 000 rpm) three times, respectively, then dried at 60 °C for several hours in air to obtain the final product named SnS@SnS2-0.

Synthesis of Ce3+ doped SnS@SnS2 nanoparticles

The Ce3+ doped SnS@SnS2 nanoparticles are synthesized in the same method. The only difference is that different amounts of Ce(OOCCH3)3·5H2O were added with SnCl2·2H2O and thiourea to obtain the homogeneous solution. The synthesized Ce3+ doped SnS@SnS2 with molar ratios of Ce3+ to tin (Ce/Sn) in the precursors of 0.005, 0.01, 0.02 and 0.04 were named SnS@SnS2-0.5, SnS@SnS2-1, SnS@SnS2-2 and SnS@SnS2-4 respectively.

Results and Conclusions

In this study, the synthesized SnS2 bulk nanocomposites are randomly arranged with a great many SnS quantum dots dispersing on the surface. At the interfaces between SnS2 and SnS phases, accumulation layers are formed and p-n heterojunctions come into existence. Besides, it has been proved that cerium is mainly found in SnS quantum dots to achieve the lowest formation energy. Therefore, the number of SnS quantum dots can be modulated by the doping concentration. Furthermore, the number of heterojunctions, the baseline resistance and the sensing properties can be effectively controlled.
In conclusion, the pristine SnS@SnS2 nanocomposites and Ce-doped SnS@SnS2 nanocomposites with different doping proportions were all synthesized by a simple one-step hydrothermal method. Due to the combined action of SnS@SnS2 p–n heterojunctions and high specific surface, 1at.% Ce-doped SnS@SnS2 sensor had obviously improved sensing sensitivity with lower baseline resistance compared to pristine SnS@SnS2 and exhibited the best gas sensitivity of 31 with response/recovery time of 19s/63s to 10 ppm NO2 at room temperature. The reason was that Ce-doping could effectively modulate the number and size of SnS quantum dots, thereby regulating the number of the SnS@SnS2 p-n heterojunction and specific surface to enhance the gas sensitivity, which was consistent with the DFT calculating results. The replacement of some Sn2+ by Ce3+ also increased the electron concentration and made contributions to the decrease of baseline resistance. By changing the proportion of Ce precursors, the sensing performance can be effectively modulated and finally achieve the ideal sensing material for NO2 gas sensing at room temperature.

References

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Figure 1