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. 2020 Mar 3;5(10):5407–5411. doi: 10.1021/acsomega.9b04465

Copper-Ion-Assisted Precipitation Etching Method for the Luminescent Enhanced Assembling of Sulfur Quantum Dots

Qi-Le Li †,§,∥, Lin-Xing Shi †,‡,*, Ke Du †, Yong Qin †, Shu-Jie Qu †, De-Qian Xia †, Zhen Zhou †, Zeng-Guang Huang †, Shou-Nian Ding ∥,*
PMCID: PMC7081439  PMID: 32201831

Abstract

graphic file with name ao9b04465_0005.jpg

In this study, we report a metal-ion-assisted precipitation etching strategy that can be used to manipulate the optical properties associated with the assembling of sulfur quantum dots (S dots) using copper ions. Transmission electron microscopy confirmed that the S dots were mainly distributed within 50–80 nm and that they exhibited an ambiguous boundary. After the post-synthetic Cu2+-assisted modification was completed, the assisted precipitation-etching S dots (APE-S dots) were observed to exhibit a relatively clear boundary with a high fluorescence (FL) quantum yield (QY) of 32.8%. Simultaneously, the Fourier transform infrared radiation, X-ray photoelectron spectra, and time-resolved FL decay spectra were used to illustrate the improvement in the FL QY of the APE-S dots.

Introduction

Quantum dots (QDs) are promising materials exhibiting unique photophysical and photochemical properties owing to their unique three-dimensional quantum confinement. Therefore, they can be applied in various fields, including biological imaging, lasers, solar cells, and light-emitting diodes.1 However, majority of the high-performance QDs, including CdS, CdSe, CdTe, HgTe, PbS, and PbSe, and their hybrid structures are limited by the toxicity of the metal elements, including Cd, Hg, and Pb.2−4 The intrinsic toxicity of the conventional heavy-metal-based QDs has hindered their application prospects. To solve the problem of toxicity, many metal-free elemental QDs have been developed to satisfy the novel requirements required to obtain the next-generation green nanomaterials. High-quality pure element QDs that exhibit low toxicity, excellent solubility, and stable photoluminescence are of particular interest. Typical examples of metal-free elemental QDs include carbon QDs,5 graphene QDs,6 black phosphorus QDs,7 and silicon QDs,8 which have been extensively pursued and studied. In majority of the semiconductor QDs, the fluorescence (FL) is commonly quenched by aggregation.9 On the contrary, the aggregation of carbon QDs does not result in any FL quenching. Several studies have proved that cross-linked carbon dots or polymer carbon nanomaterials, including carbon nanoribbons, carbon nanosphere, and nanobranch, are attractive materials owing to their photoluminescence.10,11 Until now, other aggregated pure elements of QDs exhibiting this phenomenon have not yet been reported.

Sulfur materials are synthesized in different sizes and forms and are applied in various fields.12−14 In particular, sulfur nanomaterials have attracted considerable attention owing to their inherent optoelectronic properties. During the previous decade, many researchers have enabled the facile synthesis of sulfur nanomaterials in various fields, including sodium–sulfur batteries,15 lithium–sulfur batteries,16 potassium–sulfur batteries,17 and supercapacitors,18 as well as in some optical applications.19 However, sulfur has been rarely investigated as a luminescent material. As novel heavy-metal-free QDs, sulfur QDs present a potential alternative to fluorescent semiconductor nanocrystals for several promising applications in bioimaging, biosensing, and light emission owing to their excellent aqueous dispersibility, high biocompatibility, photobleaching resistance, and low toxicity. However, until now, only few groups have reported the synthesis and the luminescent property of the sulfur QDs (S QDs).20,21 Initially, Li et al. converted the CdS QDs into a low-FL quantum yield (QY) of S QDs.22 Recently, a satisfactory QY of up to 3.8% has been obtained for the S QDs synthesized using polyethylene glycol-400 (PEG-400).23 The further increase in the high FL QY of S QDs was reported by Wang et al. They proposed using hydrogen peroxide to assist the synthesis of S QDs by etching the surface polysulfide species, resulting in a high FL QY of 23%.24 Another method was reported to improve the QY of S QDs by the oxidation of the divalent polysulfide (Sx2–) ions to zero-valent sulfur (S[0]) under a pure O2 atmosphere with a FL QY of 21.5%.25 It is reasonable to believe that other methods can obtain high-FL efficient S QDs.

Only some general effective methods have been used for surface etching although surface modification based on a ligand exchange reaction has been actively explored to enhance the FL QY in various nanocrystalline systems. Herein, a new and facile method has been used for the post-synthesis of S dots (the assembled S QDs were defined as “S dots”), where Cu2+ was used to modify the highly luminescent S dots via a simple surface modification approach in which Cu2+ was the precipitator. When compared with the H2O2-assisted top-down approach (23%), the Cu2+-assisted precipitation etching S dots (APE-S dots) exhibit a higher FL QY of 32.8% (Figure S1). In this method, no additional treatment is required and results in the production of a unique fluorescent nanomaterial for further applications.

Results and Discussion

Figure 1 schematically presents the synthesis of the APE-S dots through a Cu2+-assisted precipitation etching approach. First, the uniform and well-dispersed assembling of S dots was conducted using the “assemble-fission” approach of the sublimated sulfur powders with alkali using PEG-400 as the passivation agent. Briefly, the sublimated S powder, PEG-400, NaOH (4.0 g), and 50 mL of ultrapure water were heated to 70 °C under continuous stirring, similar to the synthesis method reported by Shen et al.23 After 72 h, S dots were observed to be the as-obtained product assembling particles. The as-synthesized S dots are deep yellow in natural light; however, they emit poor FL under the radiation of a UV lamp at 365 nm (inset of Figure 1). However, the black precipitation produced by the addition of Cu2+ did not exhibit any FL. Interestingly, FL disappeared from the raw liquid; however, after filtration heating at 70 °C, stronger FL could be observed under continuous stirring for 30–120 min. The obtained APE-S dots were light yellow in natural light but emitted a remarkable blue FL under the radiation of the UV lamp at 365 nm. This may be attributed to surface etching on the pristine S dots. Thus, the usage of Cu2+ after synthesis is essential to improve the FL activity of the S dots, which is when compared with the value of 23% reported by Wang et al.24

Figure 1.

Figure 1

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Schematic of the synthesis and etching of S dots through the Cu2+-assisted precipitation approach and photographic changes of the S dot FL spectra before and after etching (inset is the photograph and fluorescent photograph obtained under 365 nm UV light).

Transmission electron microscopy (TEM) was used to characterize the morphology of the as-synthesized assembling S dots (A) and APE-S dots (B). The S dots aggregate to form large assembled particles. The assembled S dots were mainly distributed in 50–80 nm with an average diameter of 65 nm (Figure 2A) and an ambiguous boundary. Unlike the H2O2-assisted top-down method, size-controlled optical properties can be obtained within the S QDs by etching the large S dots to smaller sizes, obtaining a size-dependent emission color.22 The difference is that the etched APE-S dots show a relatively clear boundary when compared with the ambiguous boundary of the S dots (Figure 2B). Based on the results of the experimental data, we can conclude that during etching with Cu2+, the edge of the active sulfur atoms of the APE-S dots react with the Cu2+ ion to etch the surface-active components of the APE-S, enhancing the FL of APE-S owing to the passivation of the surface-active components of the S dots. The enlarged high-resolution TEM images (Figure S2) indicate the high crystallinity of the S and APE-S dots and a lattice spacing of 0.34 nm. Mutual intersection lattice fringes of 0.344/0.321 nm were observed in the sulfur nanocrystals. The intersecting angle of the lattice fringes was approximately 120°, corresponding to a (026)/(040) plane and reflecting the well-defined crystalline structure of sulfur.

Figure 2.

Figure 2

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TEM of the S dots (A) and APE-S dots (B).

UV–vis spectroscopy was used to characterize the optical properties of the obtained S dots and APE-S dots (Figure 3A). The UV–visible spectra of the S dots and the APE-S dots in 200–400 nm showed a clear peak at approximately 220 nm, which may be attributed to the n → σ* transitions, indicating the formation of nanodots.21Figure 3B is the FL spectra of pristine-assembled particles S dots (black) and the APE-S dots (red). The inset of Figure 3B is the enlarged FL spectrum of the S dots. Our results indicate that the FL intensity of the APE-S dots was considerably dependent on the usage of the injected Cu2+. Figure 3C shows the FL spectra (excited at 365 nm) of the APE-S dots in an aqueous solution synthesized by subsequent heating for 2 h after precipitation with various amounts of Cu2+ (0.1–1.0 mM). With the increasing amount of Cu2+, the FL intensity of the APE-S dots increased from 0.1 to 0.6 mM and then decreased gradually. This could be attributed to the more effective etching of the surface polysulfide species, which reduced the nonradiative recombination rate through the surface state and facilitated the radiation recombination channel. The decrease in FL intensity from 0.6 to 1.0 mM may be attributed to the over-etching of the S dots.

Figure 3.

Figure 3

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UV (A) and FL (B) spectra of the pristine assembling particles of the S dots (black) and APE-S dots(red); inset is the enlarged FL spectra of the S dots. (C) FL spectra (excited at 365 nm) of the APE-S dots, which were produced using different Cu2+ concentrations (0.1–1.0 mM); inset denotes the histogram of FL intensity vs different Cu2+ concentrations. (D) FL spectra of the APE-S dots under different excitation wavelengths from 320 to 450 nm.

The full width at half-maximum of the emission spectrum with respect to the APE-S dots etched with various amounts of Cu2+ (0.1–1.0 mM) remained almost unchanged at approximately 85 nm, indicating that the APE-S dots have a uniform and stable size distribution. Furthermore, the APE-S dots showed an excitation-dependent emission, with the peak shifting toward longer wavelength (from 425 to 525 nm) when the excitation wavelength was varied from 320 to 450 nm in Figure 3D, consistent with a previously conducted study.21,22 Along with the change in FL intensity of the APE-S dots with various amounts of Cu2+, a surface etching mechanism is proposed to explain the formation of the APE-S dots.

The Fourier transform infrared (FT-IR) radiation measurements were conducted on pure PEG 400, the S dots, and the APE-S dots to study any possible surface interactions between the S dots and the APE-S dots. The bands at 2880 and 1452 cm–1 correspond to the stretching vibrations of C–H and C–H bending, respectively. The band at 938 cm–1 can be related with the stretching vibration of O–H. A decrease in intensity can be observed at 1290 and 1243 cm–1 in the S dots and the APE-S dots, which are the characteristic peaks of PEG 400. The peaks at 1138 and 1452 cm–1 in PEG 400 overlap to obtain a broader band at 1395 cm–1 in the S dots and the APE-S dots, which is in accordance with a previously conducted research. No other additional new peaks could be observed in either the S dots or the APE-S dots, suggesting that there were no significant changes.

The time-resolved FL decays were measured and presented in Figure 4B to obtain insights with respect to the reasons behind the improved FL QY. Although the FL decay of the S dots and the APE-S dots could be fitted by two exponential functions, the lifetime of the APE-S dots (2.58 ns) is observed to be shorter than that of the S dots (6.35 ns) (Table 1). The average FL lifetime of the S dots is longer than that of the APE-S dots, indicating a high amount of surface states in the pristine S dots. After etching with Cu2+, the long-lived FL lifetime τ1 considerably decreased from 62.4 to 2.55 ns, whereas τ2 decreased slightly from 4.06 to 2.53 ns (Table 1). On the contrary, no significant change could be observed with respect to the percentage of long- and short-lived FL lifetimes according to the nonradiative ratio between the S dots and the APE-S dots. Furthermore, a reduction in nonradiative pathways could be observed after etching with Cu2+. The results were highly consistent with those presented via TEM, which further indicated that the FL enhancement can be attributed to the surface etching of the pristine S dots.

Figure 4.

Figure 4

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(A) FT-IR spectra of the S dots and APE-S dots, (B) FL decay of the S dots and APE-S dots, and the XPS spectra of the S dots (C) and APE-S dots (D).

Table 1. Spectroscopic Parameters of the S Dots and APE-S Dots.

sample τave (ns) τ1 (ns) τ2 (ns)
S dots 6.35 62.4 (32.61%) 4.06 (67.39%)
APE-S dots 2.58 2.55 (36.34%) 2.53 (63.66%)
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X-ray photoelectron spectra (XPS) measurements were performed for conducting the surface elemental analysis of the S dots and the APE-S dots. The S 2p spectrum could be deconvoluted into two peaks at 161.68 and 162.87 eV in case of the S dots (Figure 4C), whereas the four sub-peaks at 161.87, 162.48, 163.10, and 163.90 eV in case of the APE-S dots could be attributed to atomic sulfur (Figure 4C). Meanwhile, the binding energies at 166.11, 167.79, 168.70, and 169.16 eV were attributed to SO2–(2p3/2), SO2–(2p1/2), SO3–(2p3/2), and SO3–(2p1/2), respectively. This phenomenon was also observed for the APE-S dots at 166.52, 167.93, 168.24, and 169.10 eV, verifying the presence of similar S bonding characteristics for these two nanodots. The amount of SO2–(2p1/2) and SO3–(2p1/2) with respect to the S dots are 36.99 and 13.78%, respectively. The corresponding amounts in case of the APE-S dots are 24.25 and 23.80%, respectively. When compared with the S dots, the amount of SO2–(2p1/2) and SO3–(2p1/2) in the APE-S dots decreased and increased, respectively. The XPS results clearly indicate a significant change in the chemical nature of the S dots after etching and oxidation using Cu2+.

Conclusions

In this study, we have explored a facile post-synthetic Cu2+-assisted precipitation etching approach to prepare high-FL-QY APE-S dots. This is presumably caused by the effective etching of the APE-S dot surface states using Cu2+ and the consequent suppression of the nonradiative recombination transitions. The APE-S dots exhibit excellent FL activity, low toxicity, and excellent solubility and can be extensively applied to bioimaging. Furthermore, they are developed to satisfy the novel requirements associated with the next-generation green nanomaterials. Even though the exact mechanism remains unclear, it is reasonable to believe that the precipitation etching approach introduces a new kind of surface state in the metal-free element, offering considerable scientific insights with respect to the FL enhancement mechanism of the metal-free elemental nanomaterials.

Experimental Section

Reagents and Materials

The sublimated sulfur powder was purchased from the Wenzhou chemical plant. Polyethylene glycol 400 (PEG-400) was purchased from Sigma-Aldrich (Shanghai, China). Copper nitrate was purchased from Sinopharm Chemical Reagent Co. Ltd. Ultrapure water was obtained from the aquaplore ultrapure water system. Sodium hydroxide was obtained from Jiangyin Jianghua microelectronic-material-incorporated company. All of the chemicals were of analytical grade and were used without further purification.

Instrument

The UV-lamp photographs were obtained using Ultraviolet Analyzer (ZF-1, Qiwei, Hangzhou, China). The product morphologies were characterized by TEM (JEM-2100F) at 200 KV. The time-resolved FL decay curves were recorded on HORIBA-FM-2015. The absolute FL QY was determined on a FL3 (Edinburgh Instruments) spectrometer equipped with an integrating sphere, and the FT-IR spectra were collected using the Thermo Scientific Nicolet 6700 spectrometer.

Synthesis of the APE-S Dots Etched Using Cu2+

The sublimated sulfur powder (1.4 g), PEG-400 (3 mL), sodium hydroxide (4.0 g), and 50 mL of ultrapure water were mixed and were subjected to a reaction at 70 °C under continuous stirring, which is similar to the synthesis method reported by Shen et al.23 After 72 h, the as-obtained products were referred to as “S dots.” When Cu2+ was not used after the synthesis, the resulting nonetching S dots expressed poor luminescence with irregular spectra and weak intensity under 365 nm UV light radiation. However, intense blue emission could be observed under the UV light when Cu2+ was added into the S dot solution. S dots (1.5 mL) were mixed with 0.6 mmol of Cu2+ containing different amounts of substances (0.1–1.0 mmol) in a round bottom flask at 70 °C for 2 h under vigorous stirring. After filtration, the mixture became light yellow, which differed from the normally observed deep yellow color, and the products were referred to as the APE-S dots.

Acknowledgments

We considerably appreciate the support of the Natural Science Foundation of China (61774069), Lianyungang Haiyan Plan under Grant 2018-QD-019, the Science Foundation of Jiangsu Ocean University (KQ17015 and Z2017011), the Natural Science Fund for Colleges and Universities in Jiangsu Province, China (19KJB150023), the Jiangsu Planned Projects for Postdoctoral Research Funds (no. 2019K207), and the Open-end Funds of the Jiangsu Key Laboratory of Function Control Technology for Advanced Materials, Jiangsu Ocean University.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b04465.

  • Raw data of the APE-S dot FL QY and high-resolution TEM of the S dots and the APE-S dots (Figure S2) (PDF)

Author Contributions

⊥ Q.-L.L. and L.-X.S. contributed equally to this work.

The authors declare the following competing financial interest(s): The authors have applied for patents related to the results presented in this work.

Supplementary Material

ao9b04465_si_001.pdf (558.1KB, pdf)

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Associated Data

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Supplementary Materials

ao9b04465_si_001.pdf (558.1KB, pdf)

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