The effect of defect emissions on enhancement photocatalytic performance of ZnSe QDs and ZnSe/rGO nanocomposites

doi:10.1016/j.apsusc.2017.11.183

Applied Surface Science

Volume 435, 30 March 2018, Pages 886-893

https://doi.org/10.1016/j.apsusc.2017.11.183 Get rights and content

Highlights

•
Effect of different defect emissions of ZnSe QDs on photocatalytic performance was investigated.
•
Annealing effects of ZnSe QDs on photocatalytic performance was investigated.
•
Graphene role as an additive to enhance photocatalytic performance of ZnSe QDs was studied.

Abstract

A systematic study about the origin of defects emission of ZnSe structure was conducted by photoluminescence (PL) spectrometer at room temperature. It was observed that different intermediate energy levels in band-gap space of ZnSe structure were generated by different defects such as Se-, Zn-vacancies, Se-, Zn-interstitials, and surface states. Effects of these defects on the photocatalytic performance of ZnSe quantum dots (QDs) and ZnSe/graphene nanocomposites were investigated. The pristine ZnSe QDs and ZnSe/graphene nanocomposites were synthesized by a co-precipitation method. The PL spectra of the samples showed four emissions from four regions of the visible spectrum such as violet, green, orange, and red emissions. The violet emission was associated with the near-band-edge (NBE) of the ZnSe nanostructures, while, the other emissions were related to different defects of ZnSe structures. Annealing the samples in the H₂ atmosphere caused to increase orange emission intensity and indicated that origin of orange emission was a donor-acceptor pair (DAPs) related to singly positively charged Se-vacancies (V_Se) to singly negatively charged zinc vacancy (V_Zn⁻). Photocatalytic study of the samples to remove the methylene blue (MB) dye showed that the photocatalytic performance of the samples improved by graphene as an additive and increasing the orange emission intensity.

Graphical abstract

Introduction

Among different semiconductors used as photocatalytic materials, zinc selenide (ZnSe) from group-II-VI semiconductors is a promising photocatalytic material due to its suitable band-gap (2.67 eV) existing in the visible region of the electromagnetic spectrum. Such a suitable band gap value caused to report several works concerning the photocatalytic performance of ZnSe nanostructures under visible illumination [1], [2], [3], [4], [5]. However, there have been relatively few works regarding important factors in photocatalyst processes of ZnSe. In fact, the addition of band gap value of a semiconductor in photocatalysis process, intermediate energy levels are highly important, which were generated by defects. For example, recently Chen et al., showed how Se as an impurity generated an intermediate energy level in ZnO nanostructures and caused enhanced photocatalytic performance of ZnO nanostructures [6]. The mentioned defects in the course of photocatalysis process could be used as recombination centers for photoexcited electron-hole pairs [7]. In most cases, PL study of a semiconductor can present further information concerning the band-gap value and intermediate energy levels of a semiconductor. PL spectrum of ZnSe nanostructures typically shows two features of emission peaks, one a narrow peak centered at ∼460 nm belonging to near-band-edge (NBE) emission, and another a broader peak in the region of 510–700 nm as deep-level-emission (DLE)‎ relating to defects. These defects include vacancies, interstitials, and stacking faults as well as surface states abundant on the surface of the nanostructures [8], [9], [10], [11]. These DLEs usually show green, orange, and red emissions of the visible spectrum. Several works have been reported regarding the origin of the red emission of the ZnSe structures, but the origin of the green and orange emissions is not totally clear. Therefore, in the current work, the annealing process of ZnSe QDs in the H₂ atmosphere was used to understand origin of these emissions. Then, the effects of the deffect emissions intensity on the photocatalytic performance of the ZnSe QDs were investigated. The addition of the pristine ZnSe QDs, ZnSe/graphene nanocomposites with superior photocatalytic properties, was also studied using the same process. In fact, due to unique properties of graphene, it is one of the best additives for improvement of photocatalytic performance of semiconductors [12], [13]. Therefore, the effects of graphene on the defect emissions were also investigated.

Section snippets

Materials and synthesis

Zn(NO₃)₂·6H₂O (99.99%) and selenium (99.99%) powders (Sigma Aldrich) were utilized as zinc and selenium sources, respectively. Graphene source was high purity graphene oxide powder (GO 99.999%, US Research Nanomaterials, Inc.) with 6–10 layers. Synthesis process of the pristine ZnSe QDs and ZnSe/graphene nanocomposites was similar to that of our previous works about the synthesis of the pristine ZnS, Cu₃Se₂ nanoparticles, ZnS/rGO, and Cu₃Se₂/rGO nanocomposites[14], [15]. At the first step,

Results and discussion

Fig. 1 shows XRD patterns of the untreated GO sheets and treated GO sheets by 0.001 moles of glycine amino acid. As the figure shows, the GO sheets pattern indicates a single peak at 10.8° position, which is a characteristic peak of GO. On the other hand, the treated GO sheets pattern reveals two broad peaks at 24.90° and 43.15°, which indicate the GO sheets were changed to rGO sheets by amino acid. It is known that using amino acids as a reducing agent to reduce GO to rGO is one of the green

Conclusion

A systematic study was conducted concerning the source of the defect emissions of ZnSe QDs and ZnSe/rGO nanocomposites. It was revealed that defect emissions were from three sources by annealing the products in the H₂ atmosphere. It was observed that these sources included V_Se → V_Zn⁻ (orange emission), STS (green emission), and Zn_i → V_Zn²⁻ (red emission). Furthermore, photocatalytic performance of the products showed that ZnSe/rGO nanocomposites had superior photocatalyst properties in

Acknowledgments

R. Youseﬁ gratefully acknowledges obtaining a research grant from the Iranian National Science Foundation (INSF) for this research. In addition, R. Yousefi gratefully thanks Islamic Azad University (I.A.U), Masjed-Soleiman Branche. W. J. Basirun would like to acknowledge University of Malaya for its research grant (NO. FP039-2016).

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Full Length ArticleThe effect of defect emissions on enhancement photocatalytic performance of ZnSe QDs and ZnSe/rGO nanocomposites

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and synthesis

Results and discussion

Conclusion

Acknowledgments

Mater. Res. Bull.

J. Alloy. Compd.

J. Alloy. Compd.

Mat. Sci. Semicon. Proc.

Ceram. Int.

Mat. Sci. Semicon. Proc.

Mater. Chem. Phys.

Ceram. Int.

Solid State Sci.

J. Lumin.

Ceram. Int.

Appl. Catal. B

Appl. Surf. Sci.

J. Phys. C Solid State Phys.

Full Length Article
The effect of defect emissions on enhancement photocatalytic performance of ZnSe QDs and ZnSe/rGO nanocomposites