Communication pubs.acs.org/cm
Size-Tunable Synthesis of Nearly Monodisperse Ag2S Nanoparticles and Size-Dependent Fate of the Crystal Structures upon Cation Exchange to AgInS2 Nanoparticles Hyunmi Doh, Sekyu Hwang, and Sungjee Kim* Department of Chemistry, Pohang University of Science & Technology, San 31, Hyojadong, Namgu, Pohang 790-784, South Korea S Supporting Information *
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ilver sulfide (Ag2S) nanoparticles (NPs) have attracted great interest because they do not contain heavy metal ions and can have a narrow band gap, thereby emitting at nearinfrared (NIR) wavelengths. Wang et al. reported NIR emitting Ag2S NPs prepared by thermolysis at 200 °C.1 They controlled the NP size by reaction temperature to tune the emission wavelengths.2 Pang et al. synthesized Ag2S NPs using a hot injection method and tuned the size with a subsequent growth step.3 The size of Ag2S NPs synthesized in an organic phase is typically controlled by the reaction temperature and time.4 NIR-emitting Ag2S NPs can also be synthesized in aqueous media; however, their size and size distribution (SD) are not as flexibly controlled.5−8 Herein, we report a simple size-tunable synthetic route for Ag2S NPs under ambient room temperature (RT) conditions that produces nearly monodisperse NPs. A silver precursor solution was prepared by dissolving silver nitrate in toluene with excess octylamine (see Supporting Information (SI)). A sulfur precursor solution was separately prepared by dissolving sulfur powder in toluene. Simple mixing of the two precursor solutions at RT resulted in a rapid color change, and stirring for 1 h yielded nearly monodisperse Ag2S NPs. Fully dissolving the two precursors separately for 1 h in toluene before mixing was a crucial step. The excess amines in the silver precursor solution form Ag+−amine complexes that are ready to react. Upon mixing, the excess amines remaining in the silver precursor solution rapidly react with sulfur and generate in situ active sulfur species.9 The burst of active sulfur species is considered to result in controlled nucleation and growth, yielding nearly monodisperse NPs. The size of Ag2S NPs is flexibly controlled by changing the silver to sulfur (Ag/ S) precursor ratio. Varying the Ag/S ratio from 100 to 1/100 tuned the Ag2S NP size from 2.8 to 14.4 nm (Figure 1a). The seeds for Ag2S NPs are considered to be rich in silver,10 and a higher Ag/S ratio results in a larger number of nuclei that yield smaller NP size. The size span ranged the photoluminescence (PL) wavelengths from 1000 to over 1200 nm that fit well in the NIR-II window, important for biological imaging (Figure 1b).11,12 The SD showed a relative standard deviation as small as 4%. Ag2S NPs larger than 5 nm shared a similar PL spectrum profile, suggesting bulk emissions considering the Bohr size of 4.4 nm.2 The absorption spectra were featureless for all prepared sizes (Figure 1c), presumably because of convoluted inhomogeneity from silver vacancy oriented composition distribution and size deviation. Our Ag2S NP synthesis allows simple size control, and the reaction is rapid and mild, fitting well for potential mass production. © 2016 American Chemical Society
Figure 1. (a) TEM images of a Ag2S NP size series prepared by controlling the Ag/S ratio, and their PL (b) and absorption (c) spectra.
Cation exchange (CE) has emerged as a powerful tool to tailor the composition and crystal structure of NPs in a postsynthetic way.13−17 Ag2S nanostructures have been cationexchanged to CdS counterparts typically by adding cadmium ions with phosphines.18−21 Recently, Donega et al. reported partial CE reactions of spherical Cu2S NPs and Cu2Se/Cu2S dot-in-rods into the CuInS2 counterparts.22,23 Ternary semiconductor NPs, including CuInS2 and AgInS2, are of great interest for potential applications such as optoelectronics and for biological applications.24,25 Partial CE reactions of Ag2S into AgInS2 NPs have not yet been reported. Unlike the cases of Cu2S NP CEs, Ag2S NP CE could not be performed using indium ions with phosphines. With indium ions and phosphines, Ag2S NPs became too fragile and often showed dissolution. As an alternative, we added dodecanethiol (DT) in conjunction with methanolic In(NO3)3 to a Ag2S NP toluene solution (see SI for details). The addition of DT was critical as no CE reaction proceeded in a control experiment that omitted the addition of DT (Figure S1). DT is considered to act as an Ag+ extracting agent. It is a softer base than amines and may effectively bind to Ag+ ions while preserving the NP integrity.26 Received: September 21, 2016 Revised: October 22, 2016 Published: October 24, 2016 8123
DOI: 10.1021/acs.chemmater.6b04011 Chem. Mater. 2016, 28, 8123−8127
Communication
Chemistry of Materials
Figure 2. (a) Schematic representation of a CE reaction of Ag2S NPs into AgInS2 NPs. CE reaction of 4.1 nm NPs: absorption spectra (b), PL spectra (c), XRD patterns (d), TEM images (e), and size histogram (f) of samples before (Ag2S: black) and after (AgInS2: red) CE. CE reaction of 8.8 nm NPs: absorption spectra (g), PL spectra (h), XRD patterns (i), TEM images (j), and size histogram (k) of samples before (Ag2S: black) and after (AgInS2: red) CE reaction and the intermediate (Ag2S/AgInS2: blue). PL spectra (l) and XRD patterns (m) of a size series of Ag2S NPs before and after CE.
A scheme for CE reaction from a Ag2S NP to a AgInS2 NP is shown in Figure 2a. The reaction requires exchange of three Ag+ ions with one In3+ ion, which should be accompanied by a reduction in the NP size. The narrow SD of our Ag2S NPs enabled TEM measurements, confirming the expected size reduction. Figure 2b shows the absorption spectra of a 4.1 nm Ag2S NP sample and the sample after partial CE to AgInS2 NPs. In contrast to the featureless absorption profile of the Ag2S NP sample, the AgInS2 NPs exhibited a distinct absorption peak around 500 nm. The inset shows a color change from blackish brown to light-red brown after the CE reaction. Figure 2c shows the PL spectra before and after the CE. The PL peak shifts from 1230 to 730 nm as the CE reaction completes from Ag2S to AgInS2 NPs. As the CE reaction proceeded, the PL peak blue-shifted (Figure S1). The blue shift PL is attributed to an increase in the band gap as the NP composition approached AgInS2. A control sample with DT only and no In3+ showed no PL shift (Figure S1). XRD measurements showed the crystal structure transformation from monoclinic acanthite Ag2S to tetragonal AgInS2 (Figure 2d). TEM images before and after the CE are shown in Figure 2e (see Figure S2 for enlarged TEM images). EDX elemental analysis on the AgInS2 NPs returned an atomic number ratio of 58.5:41.5 for Ag:In, indicating slightly incomplete CE to AgInS2. Size histograms are shown in Figure 2f; these histograms show the average size decreased from 4.1 ± 0.2 nm to 3.9 ± 0.2 nm. The 14% size reduction is comparable to the expected 16.9% volume decrease for complete bulk transformation from acanthite Ag2S to tetragonal AgInS2 via CE of three Ag+ ions by one In3+ ion. In the presence of sulfur precursors, another CE route that replaces one Ag+ ion with a combination of one In3+ ion and one S2− ion might be possible; however, attempts at such CE reactions were unsuccessful and typically resulted in uncontrollable NP growth. Partial cation exchange has been reported during the growth of Ag2S NPs into AgInS2/Ag2S heterostructure NPs.27
We exploited our size-tunable synthesis of Ag2S NPs to study the size-dependent behaviors for the CE reaction. The CE experiment was repeated using an 8.8 nm Ag2S NP sample. The 8.8 nm sample has a larger average volume by a factor of 10 compared to 4.1 nm NPs. For the 4.1 nm sample, the CE reaction was completed in a few hours at RT; however, the CE rate was noticeably slower for the 8.8 nm sample, taking 4 days, allowing capture of intermediates. Initiation of CE for Ag2S NPs was heterogeneous from NP to NP. Some NPs immediately initiated the CE, while others needed some induction time. Once initiated by addition of the first foreign atoms, the following atoms should be easier to be incorporated within the lattice.28,29 Such cooperativity of CE reactions may have contributed to the heterogeneity. For the 8.8 nm NP CE, an intermediate exhibiting both Ag2S and AgInS2 crystal structures can be obtained as early as 40 min. The sample at 5.5 h showed only slight additional CE progress compared to progress after 40 min. The 40 min sample was selected as a representative intermediate because it bears the optical and crystalline properties of both Ag2S and AgInS2 NPs in the most distinctive fashion. Intermediate aliquots at other time points were also taken, and the PL, TEM, and XRD were characterized (Figure S3). Figure 2g shows the absorption spectra of the 8.8 nm sample before and after CE and the intermediate. The AgInS2 NPs after CE did not show an absorption peak as distinctive as that of the 4.1 nm sample; however, it showed a resolvable peak around 600 nm. The PL peak shifted from 1200 to 1030 nm as the CE reached completion (Figure 2h). The final 8.4 nm AgInS2 NPs showed an emission wavelength over 1000 nm, typically nonaccessible through direct synthesis. For example, 10 nm AgInS2 NPs were reported to emit at 820 nm.30 We speculate that our AgInS2 NPs have a PL at particularly long wavelengths because of deep PL bands created during the CE. CuInS2 NPs prepared by CE from Cu2−xS NPs also showed a PL at much longer wavelengths compared to conventional NPs of similar size.22 8124
DOI: 10.1021/acs.chemmater.6b04011 Chem. Mater. 2016, 28, 8123−8127
Communication
Chemistry of Materials
determines the fate of crystal structures upon CE. For example, a distinctive orthorhombic (122) XRD peak that appears at 2θ 37° was absent for the two small samples, whereas it stood out for the four large samples. The narrow SDs of our samples allowed clear-cut elucidation of size-dependent CE. Both 4.1 and 4.5 nm samples had SDs of 0.2 nm (4.9% and 4.4% relative SDs, respectively). Tetragonal AgInS2 is the preferred crystal structure at RT. The 4.5 nm and larger samples have a thermodynamically less favored crystal structure, suggesting another factor such as size-dependent rigidity of the anionic crystal framework. Figure 3 illustrates anion sublattices of acanthite Ag2S [200], orthorhombic AgInS2 [001], and tetragonal AgInS2 [102], with
The intermediate sample showed a broad PL, which is a combination of the two PL peaks before and after the CE samples. The PL time evolution showed complicated changes. At first (
