One pot and room temperature photochemical synthesis of high quantum yield NIR emissive Ag2S@Ag(In, Zn)S2 core-shells at the presence of air in water

https://doi.org/10.1016/j.jphotochem.2019.111854Get rights and content

Highlights

  • AgInS2 was synthesized using a one pot method.

  • The synthesis is at room temperature and in air atmosphere.

  • quantum yield (QY) of AgInS2 is above 25% in aqueous media.

  • the emission is located in NIR region > 750 nm.

  • their emission is stable for a long time in water.

Abstract

Here a new pathway for obtaining high Quantum Yield emission AgInS2 nanoparticles in water using defect engineering at air atmosphere is proposed. Ag2S@AgInS2 nanoparticles with zinc as both doping and alloying agent were synthesized systematically using a room temperature photochemical approach in air. 3-mercaptipropionic acid (MPA) was introduced as efficient capping agent in water. The high nucleation rate of Ag-S compare to In-S was the innovative key in the formation of core-shells using a one pot method. Zn insertion was found to have synergistic effect on controlling the growth rate and emissive defect states creation in the final product. TEM and XRD results revealed the formation of Ag2S@AgInS2 core-shell structure. PL spectroscopies revealed a broad NIR emission of Ag2S@AgInS2 core-shells with FWHM of 0.4 eV, large stokes shift of 1.1 eV and QY of 27% in water media.

Introduction

Ag2S [[1], [2], [3]] and AgInS2 [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]] nanoparticles are the novel class of direct band gap semiconductors with near IR (NIR) emission properties which have been investigated due to their unique NIR emission and absorption, low toxicity and biocompatibility. AgInS2 is a direct band gap semiconductor with two main phases of orthorhombic and tetragonal with close band gaps of about ˜1.8−2.6 eV. The narrow band gap of AgInS2 has made it a very interesting candidate for different biological, electronic and photonic applications [[4], [5], [6], [7], [8], [9]12,16]. The tetragonal phase is the most stable phase at room temperature and its synthesis with various optical properties especially its emission has been investigated extensively [4]. One of the major problems in exciton emission properties of I-III-VI QDs are the misalignment of Ag and In in this structure which leads to considerable density of defects that results in intrinsic and surface defects. Thus one of the major problems in the synthesis of AgInS2 nanoparticles are the control of defects for emission properties. Another major problem is their synthesis in air ambient and in aqueous media. Different methods have been employed for the synthesis of highly emissive AgInS2 QDs and nanoparticles [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]]. Park et al. have synthesized AgInS2 in an organic solvent using Silver nitrate (AgNO3), indium(III) acetylacetonate (In(acac)3), dodecanethiol (DDT), oleic acid (OA), oleylamine (OLA), and 1-octadecene (ODE) with purged nitrogen gas at elevated temperatures (120–180 °C). The trap state emission was obtained at 688 nm (1.8 eV) [4]. Chen and co-workers also reported Ag-In-S/ZnS core-shells for brain tumor cell targeting. They used Silver acetate, indium(III) acetate, Sulfur, trioctylphospine (TOP), 1-dodecanethiol (DDT) to synthesis AgInS2 QDs under Argon atmosphere at elevated temperatures (130 °C, 170 °C) in multistep procedure and then they transferred the synthesized QDs to hexane to control the growth of QDs. According to their report, the QDs (4.5 nm in size) showed an emission at 594 nm with QY 13%. They also reported an increase of QY after the growth of ZnS shell up to 41% but the emission peak was blue shifted to 545 nm (visible range) [5]. Ogawa et al. [6] and Hamanaka et al. [7,9,12,13] reported also the synthesis of AgInS2 QDs with sizes below 4 nm with a broad emission band about 820 nm (1.5 eV) with FWHM of 0.4 eV and QY of >40% and stokes shift of 0.8–1 eV. They used a thermochemical decomposition approach in Ar-flushing atmosphere at elevated temperatures of 130–200 °C [6]. The synthesis was performed in three steps procedure and overall time of 6 h in Ar neutral atmosphere with complex and expensive chemicals and the florescence measurements were obtained from QDs in Hexane media [6]. Some reports containing the growth of ZnS shell to passivate surface defects and increase the QY of AgInS2@ZnS nanoparticles [5,8,9,15,[18], [19], [20]]. Suzuki et al reports are based on the synthesis of AgInS2 in ionic solutions such as 1-ethyl-3-metylimidazolium ethylsulfate (EMI-es) and tris (2-hydroxyethyl) methylammonium Methylsulfate (HEMA-mes) with the help of sodium 2-mercaptoethanesulfonic acid (MES) instead of oleylamine (OLA) as capping agent to make them water-soluble which needs expensive and complex chemistry [8]. The report of Suzuki et al. for example shows that the emission of nanoparticles is not stable more than several hours in water [8]. Mao et al. reported the NIR emissive (about 800 nm) AgInS2/ZnS NCs well dispersed in aqueous solution. Their method is multistep at elevated temperatures of typical 75–180 °C using In(acac)3, zinc stearate, oleic acid, dodecanethiol, and ODE under Ar flushing. They claimed that it is not still clear Zn or ZnS has a leading effect on the NIR emission of AgInS2 NCs but they found that Zn plays an inevitable role in the control of growth rate of AgInS2 [15].

Mercaptopropionic acids such as TGA and MPA have shown considerable capability for the preparation of different types of QDs such as Ag2S [[1], [2], [3]], CdSe/CdS [24], ZnSe [25] with high QY of emission and long lived colloids in water. Thus, in this work with the aid of MPA as an excellent capping agent together with the synergistic role of Zn as both the growth control and alloying/doping agents in air atmosphere and using a photochemical approach at room temperature for the first time high QY Ag2S@AgInS2 core-shells with enhanced NIR emission and long lived up to 6 months were prepared in water.

Section snippets

Materials and methods

All chemicals were purchased from Merck Co. and used without further purification. 1.6 mM AgNO3, 1.6 mM InCl3.5H2O, 3.5 mM Na2S2O3, 1.6 mM Zn(NO3)2 were prepared separately in DI water. First, AgNO3 and InCl3.5H2O were mixed and stirred for 5 min. then, 4 ml of MPA 1 M was injected into the Ag-In precursor solution and NH4OH was injected drop wise to adjust pH to 8. 0.32 g Na2S2O3 was separately dissolved in 10 ml of DI water and added to it and stirred for 10 min. the solution was set under UV

Structural verification of Ag2S@Ag(In, Zn)S2 core-shells

Fig. 1 depicts the XRD data collected from the samples with various Indium (In) content. As it is increased from S1 to S4, the substitution of In in AgInS2 structure increases and the pattern diffraction changes dramatically from monoclinic Ag2S for InAg:02 with major growth plane of (112) to tetragonal AgInS2 phase with preferential growth plane of (211) as indicated by arrows [[4], [5], [6]15]. It is notable that InAg:11 sample most resembles the crystal diffraction of AgInS2 nanoparticles [15

Conclusion

Ag2S@ Ag (In, Zn)S2 core-shells were synthesized using a direct, one pot and room temperature photochemical approach in water. MPA was introduced as an efficient capping agent for the control of synthesis process. TEM images, XRD analysis and optical studies revealed the formation of Ag2S@AgInS2 core-shells. Zinc and Indium has competitive role in substitution in the core-shells. Zinc insertion was found as a crucial parameter for synergistic effect of growth control and emission properties at

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      Salavati-Niasar et al. have fabricated matchstick-like Ag2S/AgInS2 nanohybrids from a mixture of InCl3, [Ag(HSal)] and thiosemicarbazide (TSC) under microwave irradiation in presence of water/propylene glycol, followed by a annealing process under Argon atmosphere at 350 °C for 1 h [20]. Despite inspiring progress, it is well known that the usual techniques for fabricating Ag2S/AgInS2 heteronanostructures demonstrate sustainability limitations such as [8,12,15–27]: (i) necessity of complicated organic molecular precursors of Ag and In, (ii) requirement of elevated temperature over 200 °C and prolonged time to assure the thermal decomposition of Ag- and In- molecular precursors, (iii) employment of capping ligands and organic solvents such as oleylamine (OLA), octadecene (ODE), Tri-n-octylphosphine (TOP), methanol and 1-dodecanethiol that are expensive and environmentally hostile, (iV) usage of harmful and carcinogenic sulfur agents including 1,3-dibutylthiourea, thiosemicarbazide (TSC), thioactamide and dimethyl sulfoxide, which give off plenty of highly hazardous H2S, and (V) involvement of two- or multi-step reactions in a Schlenk equipment under inert-gas atmosphere, which are usually cost-, labor-, time-consuming, and what’s more, inevitably cause loose and chaotic connections between the ingredients, thus leading to the sluggish separation and transport of charge carriers and poor photocatalytic performance. In recent years, the hierarchical core/shell heteroarchitectures of two-dimensional (2D) nanosheet framework with favorable interface have stimulated ever-growing interest for heterogeneous photo/electro catalysis owing to the credits of large specific surface area, abundant exposed active centers, reduced diffusion-length from bulk-to-surface as well as multi-scattering/reflection of incident light favorable for the photocatalytic performance [28–34].

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