Highly Luminescent Ag–In–Zn–S Quaternary Nanocrystals: Growth

Jan 4, 2019 - Institute of Chemistry, Faculty of Mathematics, Physics and Chemistry, University of Silesia , Szkolna 9, 40-007 Katowice , Poland. # Ac...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Highly Luminescent Ag−In−Zn−S Quaternary Nanocrystals: Growth Mechanism and Surface Chemistry Elucidation Piotr Bujak,*,† Zbigniew Wrob́ el,‡ Mateusz Penkala,§ Kamil Kotwica,† Angelika Kmita,# Marta Gajewska,# Andrzej Ostrowski,† Patrycja Kowalik,†,∥ and Adam Pron† †

Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland § Institute of Chemistry, Faculty of Mathematics, Physics and Chemistry, University of Silesia, Szkolna 9, 40-007 Katowice, Poland # Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland ∥ Faculty of Chemistry, University of Warsaw, Pasteura 1 Str., PL-02-093 Warsaw, Poland

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/04/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: The presented research is focused on the synthesis of alloyed Ag−In−Zn−S colloidal nanocrystals from a mixture of simple metal precursors such as AgNO3, InCl3, zinc stearate combined with 1-dodecanethiol (DDT), 1-octadecene (ODE), and sulfur dissolved in oleylamine (OLA). In particular, the focus is on the effect of the solvent (ODE vs 1,2dichlorobenzene (DCB)) and the type of sulfur precursor (S/ OLA vs S/n-octylamine (OCA)) on the metal precursors reactivates and on the chemical composition, crystal structure, and luminescent properties of the resulting nanocrystals. The replacement of ODE by DCB as a solvent lowers the reactivity of metal precursors and results in a 3-fold decrease of the photoluminescence quantum yields (Q.Y.) values (from 67% to 21%). This negative effect can be fully compensated by the use of S/OCA as a source of sulfur instead of S/OLA (Q.Y. increases from 21% to 64%). NMR studies of the isolated organic phase indicate that the S/OLA precursor generates two types of ligands being products of (Z)-1-amino-9-octadecene (OLA) hydrogenation. These are “surface bound” 1-aminooctadecane (C18H37NH2) and crystal bound, i.e., alkyl chain covalently bound to the nanocrystal surface via surfacial sulfur (C18H37-NH-S crystal). Highly luminescent Ag−In−Zn−S nanocrystals exhibit a cation-enriched (predominantly indium) surface and are stabilized by a 1-aminooctadecane ligand, which shows more flexibility than OLA. These investigations were completed by hydrophilization of nanocrystals obtained via exchange of the primary ligands for 11-mercaptoundecanoic acid, (MUA) with only a 2-fold decrease of photoluminescence Q.Y. in the most successful case (from 67% to 31%). Finally, through ligand exchange, an electroactive inorganic/organic hybrid was obtained, namely, Ag−In−Zn−S/7-octyloxyphenazine-2-thiol, in which its organic part fully retained its electrochemical activity.

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INTRODUCTION Preparation of core/shell or alloyed nanocrystals of binary cadmium chalcogenides (CdS, CdSe, CdTe) usually combined with zinc chalcogenides (ZnS, ZnSe, ZnTe) has been a popular approach aimed at controlling their spectroscopic and other physical properties.1,2 Although spectacular results were obtained for these nanocrystals, a different trend in this domain of nanomaterials chemistry has emerged in the past decade, stimulated by the necessity to fabricate semiconductor nanocrystals which do not contain toxic elements. Thus, significant research efforts were put into the preparation of ternary (CuInS2, CuInSe2, AgInS2, AgInSe2, CuFeS2, CuFeSe2) and quaternary (Cu2ZnSnS4, Cu2ZnSnSe4) semiconductor nanocrystals.3−6 Similarly as in the case of cadmium−zinc chalcogenide systems, core−shell (CuInS2/ZnS, AgInS2/ZnS) © XXXX American Chemical Society

and alloyed ((CuInS 2 ) x (ZnS) 1−x , (AgInS 2 ) x (ZnS) 1−x , Cu2ZnSn(S1−xSex)4) nanocrystals were fabricated. In these multicomponent nanocrystals, their chemical composition is a key parameter determining their properties at a greater extent than the crystal structure, size, or shape.7 Among a plethora of these multicomponent semiconducting nanocrystals, two groups, namely, CuInS2−ZnS and AgInS2−ZnS, deserve special attention because of their excellent, compositiondependent luminescent properties8 which facilitate their application in electronics9−13 as well as in biology and medicine.14−37 These quaternary (Cu−In−Zn-S and Ag−In− Zn-S) nanocrystals greatly differ in composition and, by Received: October 14, 2018

A

DOI: 10.1021/acs.inorgchem.8b02916 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Preparation of Ag−In−Zn−S Nanocrystals. All operations were carried out under constant dry argon flow. Silver nitrate (0.03 g, 0.17 mmol), indium(III) chloride (0.13 g, 0.59 mmol), zinc stearate (0.40 g, 0.63 mmol), and DDT (0.20 g, 1.0 mmol) were mixed with ODE or DCB (15 mL) in a three-neck flask. The mixture was heated to 150 °C until a homogeneous solution was formed. Then, 1 mL of S/OLA or S/OCA precursor was quickly injected into the reaction solution. The temperature was increased to 180 °C, and the mixture was kept at this temperature for 60 min. After the mixture was cooled to room temperature, toluene (20 mL) was added, and the reaction mixture was centrifuged and the isolated black precipitate was separated. For sample (A), the supernatant was treated with 30 mL of acetone leading to the precipitation of the desired fraction of nanocrystals. The nanocrystals were separated by centrifugation (7000 rpm, 5 min) and then redispersed in toluene (or hexane, chloroform, dichloromethane). For obtaining nanocrystals of samples (B) and (C), a special size sorting procedure was developed. Addition of acetone to the dispersion of as prepared nanocrystals in small aliquots resulted in the precipitation of batch 1 of nanocrystals. Nanocrystals of this fraction were separated by centrifugation (7000 rpm, 5 min) and then redispersed in toluene (or hexane, chloroform, dichloromethane). The supernatant was subsequently treated with small aliquots of acetone until a second batch of smaller size nanocrystals precipitated. This fraction was separated and redispersed in an identical manner as the first one. Thus, for samples (B) and (C), two nanocrystals’ batches (batches 1 and 2) of different sizes were isolated by this procedure. Ligand Recovery. A colloidal solution of nanocrystals (in 10 mL of chloroform) and 10 mL of concentrated HCl were placed in a screw-capped ampule, which was vigorously shaken for about 60 min. Next, 20 mL of water was added, and the as-obtained mixture was centrifuged (15 000 rpm, 5 min) to achieve phase separation. Solids were discarded. The organic phase was collected, and the aqueous phase was extracted with 15 mL of chloroform. The combined organic extracts were washed two times with water, evaporated, and dried under reduced pressure. Ligand Exchange for 11-Mercaptoundecanoic Acid. A total of 0.5 g (2.29 mmol) of 11-mercaptoundecanoic acid and 0.1 g (2.50 mmol) of NaOH were dissolved in 10 mL of water and transferred to a three-neck flask. Separately, a hexane solution of nanocrystals capped with initial ligands was evaporated, and solid residue was dissolved in 5 mL of toluene and then injected into the first solution. The as-obtained two-phase mixture was heated at 80 °C for 8 h under an argon atmosphere. At this point the organic layer became colorless. After cooling, the reaction mixture was centrifuged to obtain complete phase separation, and solids and the organic phase were discarded. Water solution was then mixed with 20 mL of acetone, which led to the precipitation of nanocrystals. After centrifugation, the nanocrystals were redispersed in 10 mL of water. Ligand Exchange for 7-Octyloxyphenazine-2-thiol. In a screw-capped ampule under an argon atmosphere, a mixture of colloidal solution of nanocrystals capped with initial ligands (in 10 mL of toluene) and a ligand: (7-octyloxyphenazine-2-thiol) (200 mg; 0.59 mmol) was stirred at room temperature for 8 h. Nanocrystals were precipitated with acetone, centrifuged, and redispersed in toluene (or hexane, chloroform, dichloromethane). Characterization Methods. Elemental analysis was carried out with a multichannel Quantax 400 energy-dispersive X-ray spectroscopy (EDS) system with a 125 eV xFlash detector 5010 (Bruker) using a 15 kV electron beam energy. X-ray powder diffractograms were recorded at room temperature on a Bruker D8 Advance diffractometer equipped with a LYNXEYE position-sensitive detector using Cu Kα radiation (λ = 0.15418 nm). The data were collected in the Bragg−Brentano (θ/2θ) horizontal geometry (flat reflection mode) between 10° and 70° (2θ) in a continuous scan, using 0.04° steps at 960 s/step. The incident-beam path in the diffractometer was equipped with a 2.5° Soller slit and a 1.14° fixed divergence slit, whereas the path of the diffracted beam was equipped with a programmable antiscatter slit (fixed at 2.20°), a Ni β-filter, and a 2.5° Soller slit. The sample holder was rotated at an angular speed of 15

consequence, exhibit different photoluminescent properties. Despite many similarities between both groups of nanocrystals, mainly related to the donor−acceptor mechanism of radiative recombination,38−40 major differences exist in their preparation methods elaborated to date. In the case of alloyed CuInS2− ZnS nanocrystals, practically only one strategy involving the use of a mixture of simple precursors predominates.11,41−43 For quaternary alloyed nanocrystals of AgInS2 (Eg = 1.87 eV) and ZnS (Eg = 3.61 eV), two synthetic approaches were developed in parallel. The first one involves the synthesis of a single, multicomponent precursor, namely (AgIn)xZn2(1‑x)(S2CN(C2H5)2)4, and its subsequent, controlled decomposition to yield the desired nanocrystals.44−47 This type of approach works very well in the synthesis of colloidal (AgInS2)x(ZnS)1−x alloyed nanocrystals. The second, single-step method involves direct use of a mixture of simple precursors and allows for the preparation of colloidal Ag−In−Zn-S nanocrystals covering a larger composition range.12,48−54 Finally, an important trend that emerged in this domain of research is the preparation of hydrophilic Ag−In−S, Ag−In−S/ZnS, and Ag−In−Zn-S nanocrystals as evidenced by numerous publications.21,55−57 The research presented here is a continuation of our studies on the preparation of alloyed Ag−In−Zn-S nanocrystals. In our first paper devoted to this domain of research we demonstrated that the use of a mixture of simple precursors yielded Ag−In−Zn−S nanocrystals emitting light in the spectral range from 553 to 696 nm.51 Optimization of the preparation procedure was the subject of out subsequent paper54 in which we reported on green (543 nm) and red (730 nm) light emitting Ag−In−Zn−S nanocrystals, exhibiting the photoluminescence quantum yield (PLQY) values of 48% and 59%, respectively. In this article we discuss two important issues closely associated with the preparation of these nanocrystals. First we compare the role of two different noncoordinating solvents, i.e., 1-octadecene (ODE) and 1,2-dichlorobenzene (DCB), elucidating their influence on the nanocrystals formation. Second, we explore the possibility of replacing popular primary amine oleylamine ((Z)-1-amino-9-octadecene, OLA) with noctylamine (OCA) as a sulfur dissolving agent (sulfur precursor) and investigate the effect of this change on nanocrystal formation. The conclusions drawn from the results of this research can constitute an inspiration to similar studies focused on other types of multicomponent nanocrystals prepared with the use of ODE as a solvent and S/OLA as a precursor of sulfur. Moreover, our elucidation of the nanocrystal surface chemistry enabled us to elaborate effective ligand exchange procedures yielding either colloidally stable hydrophilic nanocrystals or organic/inorganic hybrids consisting of inorganic nanocrystalline core capped with electroactive phenazine-type ligands.

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EXPERIMENTAL SECTION

Materials. Silver nitrate (99%), indium(III) chloride (98%), zinc stearate (technical grade), 1-dodecanethiol (DDT, 98%), sulfur (99%), 1-octadecene (ODE, 90%), 1,2-dichlorobenzene (DCB, 99%), oleylamine (OLA, 70%), n-octylamine (OCA, 99%), and 11mercaptoundecanoic acid (MUA, 95%) were supplied by SigmaAldrich. Preparation of S/OLA and S/OCA Precursors. Fifteen milligrams (0.47 mmol) of sulfur powder and 1.0 mL of OLA (or OCA) were loaded into a glass vial, which was then immersed in an ultrasonic bath. The mixture was sonicated at room temperature (for about 10 min) until a clear red solution was formed. B

DOI: 10.1021/acs.inorgchem.8b02916 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry rpm. The data were collected under standard conditions (temperature and relative humidity). Transmission electron microscopy (TEM) analysis was performed on a Zeiss Libra 120 electron microscope operating at 120 kV. High-resolution images were acquired by a Tecnai TF20 X-TWIN (FEI) microscope operated at 200 kV. 1H NMR spectra were recorded on a Bruker (400 MHz) spectrometer and referenced with respect to tetramethylsilane (TMS) and solvents. UV−vis-NIR spectra were registered using a Cary 5000 (Varian) spectrometer. Steady-state fluorescence was recorded using an Edinburgh FS 900 CDT fluorometer (Edinburgh Analytical Instruments). Emission quantum yields were measured using quinine sulfate in 0.05 mol dm−3 H2SO4 (ϕfl = 0.51) as a standard.58 Cyclic voltammetry investigations of the nanocrystals with the ligand (7octyloxyphenazine-2-thiol) (nanocrystals after the ligands exchange were redispersed in methylene chloride) and free ligand (c = 1 × 10−4 M) were carried out in 0.1 M Bu4NBF4 solution in methylene chloride with a platinum working electrode of the surface area of 3 mm2, a platinum wire counter electrode, and an Ag/0.1 M AgNO3/ CH3CN reference electrode.

reactivity of the indium precursor as compared to the zinc one. Second, in all studied samples, the Ag/In ratio varied in the range from 0.19 to 0.06, wherein the highest conversion of the silver precursor (0.17−0.19) was observed for the highest concentrations of the indium and zinc precursors in the reaction mixture. In this case, the composition of the obtained nanocrystals was the closest to the composition of the reaction mixture. The lowest conversion of the silver precursor (0.06) was obtained for low concentrations of the indium and zinc precursors. For the same samples, the biggest difference between the reaction mixture composition and the metals molar ratios in the resulting nanocrystals was observed. Third, changing the nature of some components of the reaction mixture (solvent, ligand), while keeping the same concentration of the metal and sulfur precursors, led to nanocrystals of different compositions. This was demonstrated in two experiments. In the first one, DDT was used as a solvent, and in the second one the reaction was performed in DDT-free conditions (indium precursor/zinc precursor = 0.60). This change in the reaction medium led to nanocrystals of different compositions: Ag/In = 0.83 and In/Zn = 0.66 in the former case and Ag/In = 0.62 and In/Zn = 0.26 in the latter one. In both cases the change in the reaction medium led to an increase of the silver precursor conversion with a concomitant decrease of the indium precursor conversion. Fourth, an increase of the silver precursor conversion (Ag/In = 0.37) and a decrease of the indium precursor conversion (In/Zn = 0.77) were observed with decreasing sulfur precursor (S/OLA) concentration. In these samples, the composition of the resulting nanocrystals deviated the least from the reaction mixture composition.51,54 This set of experimental results constituted a starting point for our further investigations aimed at the elucidation of the Ag−In−Zn−S nanocrystals growth mechanism. For this purpose, we selected a reference sample (A) which was synthesized in the optimized reaction conditions, i.e., starting from the metal precursors, DDT and S/OLA molar ratios: Ag/ In/Zn/SDDT/SS = 1.00/3.40/3.60/5.60/2.65 (with ODE as a solvent). These conditions ensured the highest value of the luminescence quantum yields (Q.Y.) exceeding 60%.54 The second Ag−In−Zn−S nanocrystals sample (B) was obtained using the same molar ratio of the reagents but replacing ODE with a different noncoordinating solvent, namely, 1,2dichlorobenzene (DCB). The reaction was carried out under the same conditions, i.e., by injection of S/OLA at 150 °C and subsequent heating of the reaction mixture for 1 h at 180 °C (boiling point of the used solvent). The third sample (C) was prepared in the same conditions, using DCB as a solvent and a different sulfur precursor (sulfur dissolved in n-octylamine, S/ OCA). It is worth mentioning that the choice of DCB as a solvent was not accidental. In the chemistry of colloidal semiconducting nanocrystals, some reagents are often used in pairs: 1-octadecene, ODE, CH3(CH2)15CHCH2, (Z)-1amino-9-octadecene, oleylamine, OLA, CH3(CH2)7CH = CH(CH2)8NH2, (9Z)-octadec-9-enoic acid, oleic acid, OA, CH3(CH2)7CHCH(CH2)7COOH. They all are derivatives of the common hydrocarbon octadecane, C18H38. This makes the studies on the mechanism of nanocrystals’ growth and stabilization more difficult because of the possibility of mutual transformations of these reagents as well as possible occurrence of processes leading to the same products. Moreover, even if the reagents differ in their skeletons, the individual products’ identification must involve the determination of the chain

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RESULTS AND DISCUSSION Figure 1 summarizes the effect of the reaction mixture composition (indium chloride/zinc stearate molar ratio) on

Figure 1. Molar ratio of InCl3 to zinc stearate in the reaction mixture versus the Ag/In (blue) and In/Zn (red) ratio in the resulting Ag− In−Zn-S (molar ratio of AgNO3 to InCl3 = 0.29). Squares: this research, circles and triangles: experimental data from refs 51 and 54 respectively.

the resulting Ag−In−Zn−S nanocrystals composition (In/Zn and Ag/In ratios). In each experiment, the AgNO3/InCl3 ratio was kept constant and equal to 0.29. Red and blue circles and triangles represent the results reported in refs 51 and 54, whereas red and blue squares (samples A, B, and C) correspond to the present research. All studied nanocrystals were prepared from a mixture of simple precursors such as AgNO3, InCl3, and zinc stearate dissolved in ODE with DDT capping ligands. As seen from the data presented in Figure 1, the developed method allows for effective tuning of the nanocrystals’ composition through simple modifications of the metal precursors molar ratios. In addition, the data collected from refs 51 and 54 seem to indicate that four principal trends in the preparation of these nanocrystals can be noticed. First, for all reaction mixtures with a molecular ratio of InCl3 to zinc stearate ranging from 0.94 to 0.25, the resulting nanocrystals were In-enriched since their EDS analyses revealed the In/Zn varying from 3.00 to 0.32 respectively. This finding clearly indicated much higher C

DOI: 10.1021/acs.inorgchem.8b02916 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Reactions Parameters (Solvent and Sulfur Source) and Characteristics (Composition, Size, Maximum of the Photoluminescence Band, and Photoluminescence Quantum Yield) of the Synthesized Ag−In−Zn−S Nanocrystals sample

solvent

sulfur source

Ag + In + Zn + S (wt %)

compositiona

Ag/In

In/Zn

size (nm)

PL (nm)

Q.Y. [%]

A B C

ODE DCB DCB

S/OLA S/OLA S/OCA

15.0 17.0 32.0

Ag1.0In3.1Zn1.0S4.0(S6.1) Ag1.0In2.1Zn0.5S4.2(S4.2) Ag1.0In4.5Zn0.6S3.8(S7.8)

0.32 0.47 0.22

3.10 3.88 7.50

7.2 ± 1.6 5.9 ± 0.9 5.7 ± 1.2

720 665 625

67 21 64

a

The theoretical content of sulfur per specific content of cations is given in parentheses.

length which may significantly complicate the analysis of the ligands and organic products of the reaction. Therefore, in our studies instead of using the ODE + S/OCA, indirect reaction system, we used direct combination of DCB + S/OCA to simplify the analysis. Highly luminescent Ag−In−Zn−S nanocrystals of sample (A) were isolated from the postreaction mixture, after separating the insoluble residue, as reported in our previous paper.51 In the case of the post-reaction mixtures of (B) and (C), after separating the insoluble residues and conducting fractionated precipitation, we successfully isolated in each case two batches of the nanocrystals: the first batch, just a few milligrams of nanoparticles showing no luminescence, and the second one of significantly larger mass showing intense luminescence. TEM images of batch 1 nanocrystals of samples (B) and (C) are shown in Figure S1 of Supporting Information. They reveal nanoparticles of the size exceeding 10 nm. EDS spectra of these nanocrystals are presented in Figure S2 of Supporting Information. In both cases, ternary Ag−In−S nanocrystals are formed of the following compositions: Ag1.0In2.5S4.2(S4.2) (B) and Ag1.0In3.1S4.1(S5.1) (C). On the other hand, sample (A) and batch 2 of samples (B) and (C) showed intense luminescence. They were subjected to detailed analyses involving their size and shape (from TEM images), composition (from EDS spectra presented in Figure S3 of Supporting Information), and photoluminescence Q.Y. The results of these analyses are presented in Table 1. In the obtained nanocrystals ((A) and batch 2 of (B)), the inorganic core content (Ag + In + Zn + S), determined on the basis of EDS data, is similar and equal to 15% and 17% respectively, whereas in the case of batch 2 of sample (C) this content is higher and reaches 32%. Comparing the composition of Ag−In−Zn−S nanocrystals from sample (A) with the composition of nanocrystals prepared in DCB (sample (B)), it can be noted that the conversion of indium and zinc precursors is much lower in nanocrystals (B), which results from lower solubility of the precursors in DCB as compared to ODE. On the contrary, in the case of (C) nanocrystals, despite the fact that the reaction was also carried out in DCB, the conversion of the indium precursor is much higher. This may indicate greater reactivity of the S/OCA sulfur precursor compared to the S/OLA one. At this point, a large difference between the analytically determined sulfur content and the theoretical one, calculated per specific content of the cations, should be noted in the case of samples (A) and (C). This deficit of sulfur indicates a clear dominance of cations on the surface of nanocrystals, requiring stabilization by appropriate surface bound ligands. In nanocrystals of sample (B) the sulfur content is stoichiometric, which indicates the balanced surface. To verify these findings, we have performed XPS investigations of sample (A) (see Figure S4 and S5 in Supporting Information). Its survey spectrum yields a Ag/S ratio equal to 1.0/3.6, a value that is close to that determined from the EDS analysis (Ag/S = 1.0/4.0). The obtained ratios

additionally corroborate the deficit of sulfur since for the balanced surface this ratio should be equal to 1.0/6.1. In the HR-XPS (Ag 3d, In 3d, Zn 2p, and S 2p) spectra of this sample lines characteristic of alloyed Ag−In−Zn−S nanocrystals can be distinguished.54 The unequivocal determination of the type of surface in Cu−In−Zn−S nanocrystals, even by combined EDS and XPS investigations, can sometimes be difficult. This is caused by the fact that sulfur originating from organic ligands can interfere, as evidenced by recent investigations of CuInS2 nanocrystals.59 For this reason, detailed NMR studies of the collected ligands were made, showing no DDT presence. These studies will be discussed in detail in the subsequent part of the paper. In Figure 2 powder X-ray diffractograms of samples (A) to (C) are compared. The broad nature of the observed Bragg reflections is consistent with the small sizes of the prepared nanocrystals. In the diffractogram of sample (A) nanocrystals, two overlapping reflections at 2θ in the range from 24° to 34°

Figure 2. X-ray powder diffractograms of Ag1.0In3.1Zn1.0S4.0(S6.1) (A), Ag1.0In2.1Zn0.5S4.2(S4.2) (B), and Ag1.0In4.5Zn0.6S3.8(S7.8) (C) nanocrystals. D

DOI: 10.1021/acs.inorgchem.8b02916 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. TEM (a−c) and HR-TEM (d−f) images of Ag1.0In3.1Zn1.0S4.0(S6.1) (A), Ag1.0In2.1Zn0.5S4.2(S4.2) (B), and Ag1.0In4.5Zn0.6S3.8(S7.8) (C) nanocrystals and their corresponding histograms.

diameter of the nanocrystals was about 1 nm larger as compared to samples (B) and (C) which showed similar diameters. The interplanar distance of the nanocrystals structure was determined from the HRTEM images. For all samples, the obtained values ranged from 0.338 to 0.365 nm, which confirmed that alloyed nanocrystals of orthorhombic AgInS2 (JCPDS 00-025-1328) and hexagonal ZnS were obtained. Lower values of these interplanar distances could be assigned to the (002) plane, typical of the AgInS2 orthorhombic and ZnS hexagonal structures for which the calculated values of the distance, based on the model diffractograms, are 0.335 and 0.331 nm, respectively.60−62 However, higher values of this interplanar distance were also previously observed for a number of alloyed Ag−In−Zn−S nanocrystals exhibiting this type of structure.53 It should also be mentioned that in the case of alloyed Ag−In−Zn−S of tetragonal AgInS2 and regular ZnS usually significantly lower values of the interplanar distance by 0.1−0.2 nm are measured.48,60,63 In conclusion, regardless of the reaction conditions and the resulting composition of the obtained alloyed Ag−In−Zn−S nanocrystals, careful analysis of the recorded powder diffractograms and HR-TEM images clearly confirms a practically identical structure of the nanocrystalline cores, being a combination of orthorhombic (AgInS2) and hexagonal (ZnS) structures. Taking into account increased contents of indium in all studied samples, it can be assumed that in the conditions of the performed reactions, regardless of the used solvent (ODE or DCB) and injected sulfur precursor (S/OLA or S/OCA), nonstoichiometric ternary Ag−In−S nanocrystals with increased indium content are obtained at the nucleation stage. This is followed by an exchange of indium ions for zinc ones. Clear evidence in favor of this mechanism comes from

can be distinguished, together with three reflections which peaked at 45°, 49°, and 54°. The maxima of these reflections do not correspond to the reflections of either bulk AgInS2 or bulk ZnS. They are positioned in-between the appropriate reflections of orthorhombic AgInS2 (JCPDS 00-025-1328) and hexagonal ZnS (JCPDS 00-036-1450). This indicates the alloyed nature of the obtained nanocrystals. The alloyed nature of these nanocrystals is best reflected by the position of the (002) reflection which is located ca. at 27° (2θ), i.e., almost ideally in between the corresponding reflections of pure AgInS2 and ZnS phases at about 26° and 28°, respectively. The same applies to the remaining reflections present in the diffractogram which are located respectively in-between the corresponding pairs of reflections of pure AgInS2 and ZnS phases: (120) and (100), (121) and (101), (320) and (110), (123) and (103) as well as (322) and (112). Similar relationships concerning (002) and other reflections can also be found in the diffractograms of (B) and (C) nanocrystals. In these cases, however, the peak broadening is more pronounced due to the smaller size of the nanoparticles. Finally, the number of the observed reflections and their positions exclude for all samples the presence of alloyed nanocrystals of tetragonal AgInS2 (JCPDS 00-025-1330) and regular ZnS (JCPDS 00-0050566). This strong indication of the formation of alloyed nanocrystals of orthorhombic AgInS2 and hexagonal ZnS is in line with previous findings since nanocrystals of this type were obtained either from a single precursor60 or from a mixture of precursors.53 TEM and HRTEM images of samples (A−C) nanocrystals are collected in Figure 3. On the basis of the statistical analysis of the recorded images, the diameters of the obtained nanoparticles were determined: 7.2 ± 1.6 nm (A), 5.9 ± 0.9 nm (B), and 5.7 ± 1.2 nm (C). In the case of sample (A), the E

DOI: 10.1021/acs.inorgchem.8b02916 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry combined EDS and gravimetric studies which show that the ternary phase can be isolated at any reaction stage but its share in the total mass of nanocrystals decreases with increasing reaction time at the expense of the quaternary phase. Furthermore, the proposed modifications of the reaction mixture composition did not have a significant impact on the shape and size of the obtained nanocrystals. Complete elucidation of the nanocrystals structural and chemical constitution requires unequivocal identification of the primary ligands in samples (A−C). However, in our previous research we have demonstrated that relying on 1H NMR spectra recorded for the Cu−In−Zn−S nanocrystals dispersions leads to incomplete information concerning the ligands’ identity, particularly those which are strongly bonded to the nanocrystal surface. This is caused by significant broadening of the registered lines, resulting in some cases in complete disappearance of the diagnostic signals.64 For primary surfacial ligands identification in Ag−In−Zn−S nanocrystals, described here, we have used a modification of the method, previously elaborated by us, which consists of inorganic cores’ dissolution and isolation of the organic residue, whose chemical constitution is then determined by 1 H NMR spectroscopy. Hydrochloric acid, a typical nonoxidizing acid, is used to dissolve the nanocrystals’ inorganic cores, thus preventing subsequent reactions which might lead to chemical transformations of the ligands and make their identification difficult.65 In Figure 4 1H NMR spectra of the organic residues separated after dissolving inorganic cores of the Ag−In−Zn-S nanocrystals (A−C) are presented. The spectra of free molecules of potential ligands are shown for comparison. In all 1H NMR spectra of the collected organic residue, overlapping multiplets in the range from 0.87 to 0.90 ppm and 1.20 to 1.30 ppm can be distinguished. They originate, respectively, from protons of methyl groups and numerous methylene groups constituting long chains of the initial ligands. Their individual identification is practically impossible because of very small differences in the NMR lines positions. In the spectral range from ca. 1.50 ppm to ca. 3.50 ppm additional signals originating from methylene groups protons can be seen. Their differentiation results from the presence of either functional groups or double bonds in the ligand molecule. It should be remembered that also in this case unequivocal identification of the ligand on this basis is difficult because of small differences between chemical shifts of these signals and their low intensity as compared to intense multiplets of other numerous methylene groups constituting long ligand chains. For these reasons, it is worth comparing these spectra with 1H NMR spectra of potential ligands molecules. In the case of sample (A) a multiplet in the spectral range from 2.02 to 2.07 ppm can be observed, corresponding to protons from the methylene group located in the immediate vicinity of the double bond of the OLA vinyl segment (−CHCH−) or the allyl segment (−CHCH2). Moreover, in this spectral range a triplet at 2.35 ppm, characteristic of −CH2COOH group is clearly visible, which indicates the presence of stearic acid. Another triplet at 2.68 ppm characteristic of the methylene group adjacent to the amine group (−CH2NH2) can also be distinguished. For sample (B) a broadened signal at ca. 3 ppm is observed. It corresponds to the methylene group located next to the amine group in its hydrochloride form (−CH2NH2 × HCl). In the spectrum of sample (C) in addition to triplets at 2.35 ppm (CH2COOH) and 2.68 ppm (CH2NH2), a

Figure 4. 1H NMR spectra of the organic residue from Ag−In−Zn−S nanocrystals (A−C), 1-octadecene (ODE), stearic acid (SA), oleylamine (OLA), oleylamine hydrochloride (OLA-HCl), n-octylamine (OCA), and 1-dodecanethiol (DDT) recorded in CDCl3.

number of multiplets can be distinguished. Among them, that in the spectral range 2.49−2.54 ppm can be assigned to the methylene group (CH2SH) in primary thiols such as DDT. Signals in the range from 3.50 to 6.00 ppm can also be seen in the recorded spectra of the collected organic residues. They are characteristic of the presence of CC double bonds. For all samples in the ranges of 4.91−5.02 ppm and 5.77−5.85 ppm multiplets corresponding to protons (CH2) and proton (−CH) are observed. They are characteristic of the terminal double bond in the allyl group (−CH2−CHCH2). Moreover, in the case of sample (B) an intense multiplet can be seen in the spectral range from 5.28 to 5.41 ppm, which is characteristic of vinyl protons of no terminal groups containing double bonds (−CHCH−). The performed analysis of the 1H NMR spectra that unambiguously confirms the presence of terminal alkene in the mixture of ligands collected after the core dissolution, independently of the Ag−In−Zn−S nanocrystals preparation procedure used (ODE or DCB as solvents). Terminal alkenes are not capable of forming strong bonds with the nanocrystal F

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Inorganic Chemistry surface and, by consequence, they cannot act as surfacial ligands ensuring nanocrystal colloidal stability. Their presence in the reaction mixture must be an effect of chemical reactions accompanying the process of the inorganic core dissolution. In the most plausible explanation, it can be assumed that some ligands are present in a form of alkyl chains directly attached to surfacial sulfur atoms via -S-CH2- covalent bonds. A similar type of ligand bonding has already been observed for Cu2S nanocrystals. In this case two types of DDT bonding were distinguished: (i) covalent (“crystal-bound”) where a sulfur atom from the ligand is built into the nanocrystal structure and (ii) coordination (“surface-bound”) where a sulfur atom from the ligand is attached to the nanocrystal surface.66 In our earlier studies on kesterite Cu2ZnSnS467 and chalcopyrite CuFeS268 nanocrystals we have also proved the presence of initial ligands in the form of alkyl chains covalently bonded to the nanocrystals surface. It is worth noting that in both analyzed cases, in the preparation of nanocrystals, we used S/ OLA as a precursor of sulfur. Also in both cases this type of ligand strongly dominated, with other types of ligands appearing in negligible quantities. In the case of alloyed Ag− In−Zn-S nanocrystals, studied in this research, this picture is much more complicated because in addition to covalently bonded alkyl chain ligands other ligands typically coordinatively bonded appear. The recorded 1H NMR spectra clearly show the presence of above all primary amines (originating from the sulfur precursor) as well as stearic acid (coming from the applied zinc precursor). A comparison of the chemical compositions of samples (A) [Ag1.0In3.1Zn1.0S4.0(S6.1)] and (B) [Ag1.0In2.1Zn0.5S4.2(S4.2)] clearly indicates a decrease of the indium and zinc precursors’ conversion, in the latter which can be associated with their lower solubility in DCB. In both cases, the same sulfur precursor was used, i.e., a highly reactive ionic entity (C18H35NH3+)(C18H35NH-S8−) formed by dissolution of sulfur in OLA, as demonstrated in our earlier studies.68 In the 1H NMR spectrum of the organic residue of sample (A) a triplet at 2.68 ppm can be distinguished which indicates the presence of −CH2NH2, while a multiplet, typical of the vinylene group (−CHCH−) in OLA, expected in the spectral range from 5.283 to 5.415 ppm is not observed. In the spectrum of the organic part of sample (B), besides the triplet at 2.68 ppm, clear signals confirming the presence of OLA hydrochloride can be found. This indicates that increased conversion of metal precursors is accompanied by hydrogenation of OLA to a primary amine, combined with oxidation of DDT to didodecyl disulfide which does not bond to the nanocrystal’s surface. Hydrogenation of (Z)-1-amino-9-octadecene to 1-aminooctadecane is also thermodynamically advantageous due to transition from a rigidly bent chain to a simple hydrocarbon chain which can more easily bind to the surface (Scheme 1). At lower conversion of the precursors (sample (B)) hydrogenation of OLA is not complete. In this case, only part of OLA undergoes hydrogenation and is typically covalently bonded to the nanocrystal surface while the remaining part acts as a typical coordination-type surfacial ligand. Comparing samples (A) and (B) with sample (C) [Ag1.0In4.5Zn0.6S3.8(S7.8)], an increase in the indium precursor conversion can be observed with the zinc precursor conversion remaining essentially unchanged. This finding indicates the strong effect of the applied amine (OCA) on the indium precursor reactivity. The observed effect may result from the linear shape of the applied amine, which makes binding with

Scheme 1. Proposed Reaction Pathways: Conversion of Oleylamine to 1-Aminooctadecane and Oxidation of 1Dodecanethiol (DDT) to Didodecyl Disulfide

the nanocrystal surface much easier than in the case of the stiffened structure of OLA. The approach presented here is somehow similar to the strategy of the preparation of colloidal InP/ZnS nanocrystals where a mixture of hexadecylamine and stearic acid ligands was used to coordinate indium and zinc, respectively, with the goal to increase Q.Y.69 In the case of sample (C) the use of noctylamine (OCA) for dissolving sulfur significantly increases the reactivity of the whole system. In 1H NMR spectrum a number of multiplets can be observed corresponding to a mixture of organic products. Above all, signals indicating the formation of terminal alkene from the used amine can also be identified. However, only in the spectrum of sample (C) signals indicating the presence of DDT can be found. Its role as a ligand binding to the nanocrystal surface indirectly confirms our earlier conclusions regarding the participation of DDT in the hydrogenation of OLA. The conclusions presented above are additionally confirmed by the sulfur content analysis in the obtained nanocrystals. For samples (A) and (C) with increased indium content, we clearly observe a decreased content of sulfur, which proves the dominance of cations on the surface (mainly indium ones), stabilized by the amine ligands. However, a decrease of the indium content for sample (B) leads to the stoichiometric composition of the surface and does not require strong involvement of typically coordinatively bonded ligands. The results of this research combined with previous findings67,68 enables us to propose the mechanism of the precursors and ligands transformations occurring in the synthesis of alloyed Ag−In−Zn−S nanocrystals. The reactive sulfur precursor (C18H35NH3+)(C18H35NH-S8−) dissolved in ODE and introduced to the mixture of metal precursors (AgNO3, InCl3, zinc stearate), and DDT becomes, in the presence of indium, a source of two different types of ligands. The intermediate step of both transformations is hydrogenation of (Z)-1-amino-9-octadecene to 1-aminooctadecane combined with oxidation of DDT to didodecyl disulfide. Next, as a result of these concomitant reactions, two processes occur. The first one involves attachment of alkyl chains to the surface of nanocrystals through formation of a “crystal-bound”-type covalent bond (C18H37-NH-S-crystal). Inorganic core dissolution leads in this case to 1-octadecene. Covalently bonded alkyl chains provide stability of the nanocrystals dispersion. The formation of these direct bonds with sulfur does not involve any coordination of surfacial cations. When this process dominates, an increase of the sulfur precursor G

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Figure 5. Comparison of samples (A), (B (batch 1 and 2)), (C (batch 1 and 2)) nanocrystals’ absorption (a−c) and photoluminescence (d−f) spectra for dispersion in toluene (solid lines) and water (dashed lines).

corresponding emission spectra are characterized a large Stokes shift and a high full width at half-maximum (fwhm) value. These spectral features are characteristic of the donor− acceptor mechanism of radiative recombination.38−40 In the case of sample (A) (Ag1.0In3.1Zn1.0S4.0(S6.1)), the absorption spectra of toluene and water dispersions are very similar; only a small bathochromic shift of the emission peak maximum (from 720 to 728 nm) is observed upon the nanocrystals transfer from toluene to water. A 2-fold decrease of the Q.Y. value from 67% to 31% upon the transfer to water must be noted. Two batches (1 and 2) of nanocrystals (B) and (C) were studied. TEM images of batches 1 are presented in Figure S1 in Supporting Information. Comparing the absorption spectra recorded for batch 1 with those of batch 2 of samples (B) and (C) a clear bathochromic shift of absorption thresholds can be noticed. A similar relationship is observed in the emission spectra where the broad emission peak maxima of batch 1 nanocrystals are clearly shifted toward longer wavelengths. The determined Q.Y. for toluene dispersions of batch 1 nanocrystals did not exceed 1% for both examined samples. The EDS spectra of batch 1 nanocrystals (Figure S2 in Supporting Information) showed no zinc and led to the following compositions Ag1.0In2.5S4.2(S4.2) and Ag1.0In3.1S4.1(S5.1) for samples (B) and (C), respectively. The obtained results unambiguously show, that batch 1 nanoparticles isolated from samples (B) and (C) are in fact Ag−In−S nanocrystals. The presence of Ag−In−S is a result of unfavorable influence of DCB solvent inducing radical decrease of the used precursors solubility. The selection of DCB clearly differentiates the indium precursor (InCl3) solubility and that of the zinc precursor. As a result, in the first step nucleation of Ag−In−S nanocrystals occurs, while in the second step a cation exchange process occurs, leading to alloyed Ag−In− Zn−S nanocrystals. In the case of sample (B) batch 2 (Ag1.0In2.1Zn0.5S4.2(S4.2)) a superposition of the absorption thresholds can be noticed in the absorption spectra of toluene and water dispersions. However, the maximum of its emission spectrum shifts from

conversion is expected leading to balanced stoichiometric composition of the surface and the whole nanocrystal, as is observed in the case of sample (B). In the second process, typical coordination bonding of the resulting ligand, C18H37NH2,with surfacial indium ions occurs (“surface-bound” ligands). This process balances the excess positive charge on the surface. According to the hard and soft acids and bases (HSAB) theory, in the group of cations present in the mixture (Ag+, In3+, Zn2+) indium ions can be classified as hard acids forming stable complexes with hard bases - primary amines.6 In this case, nanocrystals of nonstoichiometric composition are obtained showing an excess of indium cations on the nanocrystal surface (samples (A) and (C)). In previously studied cases, i.e., Cu2ZnSnS467 and CuFeS268 nanocrystals, because of the absence of indium, we were unable to identify the presence of 1-aminooctadecane (C18H37-NH2), an intermediate in the transition from OLA to ODE, the latter obtained each time after dissolution of the analyzed nanocrystals. Nanocrystals’ composition together with the presence of specific primary ligands bound to their surface significantly affects the spectroscopic properties of the resulting colloidal dispersions, with the luminescent ones being the most sensitive. Moreover, emission properties of these colloids can be strongly affected by the exchange of hydrophobic ligands for hydrophilic ones. For all studied samples transfer of the nanocrystals to water was carried out following one step procedure described in ref 54, which involves the exchange of primary ligands for 11mercaptoundecanoic acid. In all cases, stable water dispersions of nanocrystals were obtained. Figure 5 shows a comparison of the absorption and emission spectra recorded for toluene and water dispersions of the isolated Ag−In−Zn−S nanocrystals samples (A−C). The recorded absorption spectra are typical of ternary (Ag− In−S) and quaternary (Ag−In−Zn−S) alloyed nanocrystals showing no or a poorly defined exciton peak and an absorption tail increasing toward the UV part of the spectrum. The H

DOI: 10.1021/acs.inorgchem.8b02916 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Synthetic route for 7-octyloxyphenazine-2-thiol.

Further studies on Ag−In−Zn−S nanocrystals surface engineering involved the preparation of an electroactive inorganic/organic hybrid through the exchange of primary ligands for 7-octyloxyphenazine-2-thiol. Such hybrids are of interest in view of their diversified applications involving electronics, analytics, and other domains.71 The electroactive ligand was designed on purpose. It contains a phenazine core functionalized with a thiol group which ensures its binding to the nanocrystal surface. The choice of the core was not accidental since the phenazine structure occurs in many azaacenes, promising organic semiconductors72 as well as constituting structural units of several groups of biologically active compounds.73 To improve solution processing of this electroactive ligand, we additionally functionalized the phenazine core and octyloxy group. The presence of this electron donating alkoxyl group in combination with the phenazine core should also render this molecule ambipolar in nature through donor−acceptor interactions. Ambipolar molecules and macromolecules are still scarce and of great interest.74 In the preparation of this ligand, we have applied a strategy based on the synthesis of properly functionalized phenazine ring (see Figure 6) using modifications of procedures described in refs 75 and 76. Detailed description of the performed syntheses can be found in Supporting Information. The studied hybrid was prepared using Ag−In−Zn−S nanocrystals of sample (A) which showed the highest value of Q.Y. = 67%. The exchange of primary ligands for 7octyloxyphenazine-2-thiol was performed using a simple procedure of mixing the nanocrystals dispersion with excessive amounts of ligands. The reaction was carried out at room temperature, for about 8 h, under intensive stirring. Then, the Ag−In−Zn−S/Ligand hybrids were precipitated with acetone in order to remove the excess ligands, centrifuged and redispersed in toluene. In Figure 7 absorption and emission spectra of a dispersion of Ag−In−Zn−S nanocrystals capped with primary ligands, dispersion of Ag−In−Zn−S/7-(octyloxy)phenazine-2-thiol and a solution of 7-octyloxyphenazine-2-thiol in toluene are compared. The absorption spectrum of the hybrid can be treated as a superposition of the corresponding spectra of the free ligand and the nanocrystals capped with primary ligands. The absorption threshold does not change upon the ligand exchange; however, three new bands appear with clear maxima at 359, 414, and 430 nm, confirming binding of 7octyloxyphenazine-2-thiol to the nanocrystal surface. Binding of 7-octyloxyphenazine-2-thiol to the nanocrystal surface results in essentially complete quenching of the inorganic core photoluminescence (see Figure 7b). The only present weak emission bands originate from the photoluminescence of the ligand as evidenced by the comparison of the free ligand and Ag−In−Zn−S/7-octyloxyphenazine-2thiol spectra. This quenching can be considered as an indirect proof of strong binding of the conjugated ligand to the nanocrystal surface since nanocrystals photoluminescence quenching by conjugated ligands is a common phenomenon

665 to 695 nm upon the nanocrystals transfer to water. Significantly lower Q.Y. values are measured for (B) sample nanocrystals as compared to the corresponding values determined for nanocrystals of sample (A). Q.Y. drop to 21% for the toluene dispersions and to 8% for the aqueous ones. This radical loss of Q.Y. is caused by a decrease of indium content in the nanocrystals which is closely related to the mechanism of the ligands’ transformation described above. In the case of sample (A), increased conversion of the indium precursor was accompanied by hydrogenation of oleylamine to 1-aminooctadecane (C18H37-NH2), a ligand which forms bonds with indium ions, providing better stability of the resulting colloidal dispersions and higher Q.Y. values. Nanocrystals of sample (B), exhibiting decreased indium content, are stabilized with oleylamine ligands. These molecules because of their chain rigidity associated with the presence of a double bond do not provide enough stability of the system and yield nanoparticles of low Q.Y. values. The above conclusions are confirmed by the analysis of the emission spectrum of sample (C) batch 2 (Ag1.0In4.5Zn0.6S3.8(S7.8)). The spectrum of its toluene dispersion shows an emission peak at 625 nm with the Q.Y. value of 64%, i.e., practically the same as in the case of sample (A). The use of OCA leads to an increase of the indium precursor conversion and ensures binding of surface indium cations with ligand molecules providing stability of the system and high Q.Y. It is worth noting that the conclusions presented are in line with previous works on nonstoichiometric Ag−In−S nanocrystals70 and alloyed (AgInS2)x(ZnS)1−x nanocrystals45 obtained, in contrast to our research, using sophisticated precursors for which studies on the influence of ligands inter alia OLA and OCA on Q.Y. were conducted. In the case of sample (C) batch 2 it is interesting to compare the absorption and emission spectra of sample (C) batch 2 nanocrystals, before and after their transfer from toluene to water. In the absorption spectra a bathochromic shift of the absorption threshold is clearly visible upon water transfer, while in the photoluminescence spectra the emission peak maximum is also bathochromically shifted from 625 to 690 nm. This is in contrast to the case of sample (A) nanocrystals where only minimal shift was observed. These large shifts are accompanied by a decrease of Q.Y. from 64% to 7%. This 9fold drop of Q.Y. is much more marked than the 2-fold Q.Y. decrease caused by the transfer of sample (A) nanocrystals to water. It can be postulated that the observed differences result from differences in the composition of the obtained samples. For sample (C) (Ag1.0In4.5Zn0.6S3.8(S7.8)) a much larger deficit of sulfur is determined in relation to the cations. In this case the surface of alloyed nanocrystals contains an increased number of indium and zinc cations, coordinatively stabilized by surface ligands. In the case of sample (A) the exchange of stearic acid molecules bonded to zinc ions for MUA dominates the process of rendering nanocrystals hydrophilic. It affects to a lesser extent the Q.Y. values than the exchange of amine molecules bonded to indium ions for MUA in the case of sample (C). I

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Figure 8. Cyclic voltammograms of free ligand (7-octyloxyphenazine2-thiol) (black line) and Ag−In−Zn−S nanocrystals capped with this ligand (blue line). Electrolyte: 0.1 M Bu4NBF4/CH2Cl2, reference electrode Ag/0.1 M AgNO3 in acetonitrile, scan rate = 50 mV/s.

Figure 7. Absorption (a) and photoluminescence (b) spectra recorded in toluene solution of Ag−In−Zn−S/7-octyloxyphenazine2-thiol hybrid, Ag−In−Zn−S nanocrystals capped with initial ligands and “free” 7-octyloxyphenazine-2-thiol.

potentials by 90 mV (from −1.51 V to −1.60 V vs Ag/Ag+) with respect to the reduction peak maximum recorded for the free ligand. However, the electrochemical band gap,78,79 determined from the onset of the reduction and oxidation peaks, is almost identical for the free and nanocrystal-bound ligands - ca. 2.2 eV. This value is typical of donor−acceptor type small molecules and corroborates the expected ambipolarity of this ligand.80 As clearly seen from this study, the electrochemical activity of the ligand is fully retained upon its binding to the nanocrystal surface, which makes the obtained hybrid a suitable candidate for the use in inorganic/ organic electronic devices.

in inorganic/organic hybrids of this type.71,77 Moreover, as evidenced by our studies (vide supra), quenching of luminescence of Ag−In−Zn−S nanocrystals, induced by the ligands exchange, is caused by replacing primary ligand molecules, i.e., long-chain amines attached to indium ions, by thiol-type ligands. Thus, similar effects are responsible for the 2-fold decrease of Q.Y. (to 30%) upon transfer of the alloyed Ag−In−Zn−S nanocrystals to water and the total photoluminescence quenching following the exchange of initial ligands for thiol-functionalized phenazine derivative. Ambipolar molecules are characterized by relatively low ionization potential (IP) and high electron affinity (EA), leading to low band gap.74 These parameters can be determined from the potential of the first oxidation and first reduction peaks using cyclic voltammetry.78,79 We have performed the cyclic voltammetry investigations of the free ligand and the hybrid with the goal to verify whether 7octyloxyphenazine-2-thiol shows features characteristic of an ambipolar molecule and how they are affected by this ligand binding to the nanocrystal surface. In Figure 8 cyclic voltammograms of “free” 7-octyloxyphenazine-2-thiol and Ag−In−Zn−S/7-(octyloxy)phenazine-2-thiol are compared. Both “free” and nanocrystal surface-bound ligands undergo one irreversible oxidation and one irreversible reduction. The bound ligand is slightly more difficult to oxidize and to reduce. The maximum of its oxidation peaks is shifted to higher potentials by 50 mV (from 1.30 to 1.35 V vs Ag/Ag+) as compared to the maximum of the corresponding peak in the free ligand voltammogram. The reduction peak maximum of the surface bound ligand is, in turn, shifted toward lower

CONCLUSIONS Quaternary Ag−In−Zn−S colloidal nanocrystals are interesting nanomaterials since they exhibit excellent and tunable optical properties and contain no toxic elements, contrary to popular cadmium or lead chalcogenides. The presented research was aimed at the elucidation of the Ag−In−Zn−S nanocrystals’ growth mechanism with the goal to obtain nanocrystals exhibiting high Q.Y. values. We have demonstrated that starting from the reaction mixture consisting of popular metal precursors, AgNO3, InCl3, zinc stearate, and DDT in ODE and S/OLA as sulfur precursor it is possible to tune the reactivity of the mixture by replacing ODE with a different noncoordinating solvent, namely, DCB, and by modifying the sulfur precursor by the use of a primary amine (OCA) instead of OLA. Through these changes it is possible to tune the indium precursor conversion, consequently leading to controlled changes in the composition of the nanocrystals and their luminescence properties. More precisely, the main factors influencing the final nanocrystal compositions and luminescent properties are the indium precursor and the affinity of indium ions to primary amines which are present on the nanocrystals’ surface. Our spectroscopic analysis of the initial ligands showed that in the reaction mixture consisting of metal precursors, sulfur precursor, i.e., sulfur dissolved in OLA which forms reactive (C18H35NH3+)(C18H35NH-S8−), chemical reactions occur involving hydrogenation of OLA to 1-aminooctadecane (C18H37-NH2) and oxidation of DDT to didodecyl disulfide. As a result, two types of ligands are formed. The first type involves “crystal-bound” ligands (C18H37-NH-S-crystal) which

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form a typical covalent bond with nanocrystals’ surface through sulfur. They provide colloidal stability of the dispersion, while not directly coordinating the surface cations. These ligands are dominant in the case of nanocrystals of stoichiometric composition with properly balanced charges of surfacial cations and anions. “Surface-bound” 1-aminooctadecane (C18H37NH2) molecules constitute the second type of ligands in Ag−In− Zn−S nanocrystals. They are coordinatively bonded to surfacial indium ions and are dominant in the case of nanocrystals of nonstoichiometric composition with their surface enriched in cations. To obtain alloyed Ag−In−Zn−S nanocrystals of high photoluminescence Q.Y. it is beneficial to increase the indium level in the nanocrystals with simultaneous decreasing of the sulfur content. Nanocrystals of typically cationic-rich surface are then obtained, stabilized by longchained amines. The structure of the amine ligand influences the nanocrystal stability and has an effect on the photoluminescence Q.Y. In this respect, OLA is unfavorable since the presence of a double bond induces some chain rigidity. The product of OLA hydrogenation, namely, 1-aminooctadecane (C18H37NH2) - a primary amine better more suitably binds to the nanocrystal surface as proven by the ligand analysis. Stearic acid, containing a long saturated alkyl chain, is the second ligand in these nanocrystals coordinating surfacial zinc ions. The influence of typically coordinating ligands on the luminescent properties of alloyed Ag−In−Zn−S nanocrystals is also confirmed by the studies on the exchange of initial ligands for 11-mercaptoundecanoic acid and for the conjugated, electrochemically active ligand containing phenazine core. Our results concerning the mechanism of the growth of alloyed Ag−In−Zn−S nanocrystals must be considered as an important supplement to our previous studies on the mechanism of Cu2ZnSnS4 and CuFeS2 nanocrystals’ growth. Some generalizations can be made. First, in all three systems the same transformations take place, leading to the sulfur precursor (C18H35NH3+)(C18H35NH-S8−), through hydrogenation of OLA to C18H37NH2 and the creation of typically covalent connection (“crystal-bound”) with the surface of the nanocrystals (C18H37−NH−S crystal). In all these cases, the formation of 1-octadecene, as a result of dissolving the nanocrystals inorganic cores, can unequivocally be explained by these transformations.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Center of Poland, Grant No. 2015/17/B/ST4/03837. P.K. additionally acknowledges financial support from the Project TRI-BIOCHEM which is implemented under the Operational Program Knowledge Education Development 2014-2020 cofinanced by the European Social Fund. M.P. wish to acknowledge financial support from National Science Centre, Poland, Grant No. 2018/02/X/ST5/01519.

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(1) Reiss, P.; Protière, M.; Li, L. Core/Shell Semiconductor Nanocrystals. Small 2009, 5, 154−168. (2) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Prospect of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389−458. (3) Aldakov, D.; Lefrançois, A.; Reiss, P. Ternary and Quaternary Metal Chalcogenide Nanocrystals: Synthesis, Properties and Applications. J. Mater. Chem. C 2013, 1, 3756−3776. (4) Reiss, P.; Carrière, M.; Lincheneau, C.; Vaure, L.; Tamang, S. Synthesis of Semiconductor Nanocrystals, Focusing on Nontoxic and Earth-Abundant Materials. Chem. Rev. 2016, 116, 10731−10819. (5) Bujak, P. Core and Surface Engineering in Binary, Ternary and Quaternary Semiconductor Nanocrystals - A Critical Review. Synth. Met. 2016, 222, 93−114. (6) Coughlan, C.; Ibáñez, M.; Dobrozhan, O.; Singh, A.; Cabot, A.; Ryan, K. M. Compound Copper Chalcogenide Nanocrystals. Chem. Rev. 2017, 117, 5865−6109. (7) Yarema, O.; Yarema, M.; Wood, V. Tuning the Composition of Multicomponent Semiconductor Nanocrystals: The Case of I-III-VI Materials. Chem. Mater. 2018, 30, 1446−1461. (8) Zhong, H.; Bai, Z.; Zou, B. Tuning the Luminescence Properties of Colloidal I-III-VI Semiconductor Nanocrystals for Optoelectronics and Biotechnology Applications. J. Phys. Chem. Lett. 2012, 3, 3167− 3175. (9) Chen, B.; Zhong, H.; Zhang, W.; Tan, Z.; Li, Y.; Yu, C.; Zhai, T.; Bando, Y.; Yang, S.; Zou, B. Highly Emissive and Color-Tunable CuInS2-Based Colloidal Semiconductor Nanocrystals: Off-Stoichiometry Effects and Improved Electroluminescence Performance. Adv. Funct. Mater. 2012, 22, 2081−2088. (10) Song, W.-S.; Yang, H. Efficient White-Light-Emitting Diodes Fabricated from Highly Fluorescent Copper Indium Sulfide Core/ Shell Quantum Dots. Chem. Mater. 2012, 24, 1961−1967. (11) Zhang, W.; Lou, Q.; Ji, W.; Zhao, J.; Zhong, X. Color-Tunable Highly Bright Photoluminescence of Cadmium-Free Cu-Doped ZnIn-S Nanocrystals and Electroluminescence. Chem. Mater. 2014, 26, 1204−1212. (12) Yoon, H. C.; Oh, J. H.; Ko, M.; Yoo, H.; Do, Y. R. Synthesis and Characterization of Green Zn-Ag-In-S and Red Zn-Cu-In-S Quantum Dots for Ultrahigh Color Quality of Down-Converted White LEDs. ACS Appl. Mater. Interfaces 2015, 7, 7342−7350. (13) Ji, C.; Lu, M.; Wu, H.; Zhang, X.; Shen, X.; Wang, X.; Zhang, Y.; Wang, Y.; Yu, W. W. 1,2-Ethanedithiol Treatment for AgIn5S8/ ZnS Quantum Dot Light-Emitting Diodes with High Brightness. ACS Appl. Mater. Interfaces 2017, 9, 8187−8193. (14) Li, L.; Daou, T. J.; Texier, I.; Chi, T. T. K.; Liem, N. Q.; Reiss, P. Highly Luminescent CuInS2/ZnS Core/Shell Nanocrystals: Cadmium-Free Quantum Dots for In Vivo Imaging. Chem. Mater. 2009, 21, 2422−2429. (15) Yong, K.-T.; Roy, I.; Hu, R.; Ding, H.; Cai, H.; Zhu, J.; Zhang, X.; Bergey, E. J.; Prasad, P. N. Synthesis of Ternary CuInS2/ZnS Quantum Dot Bioconjugates and Their Applications for Targeted Cancer Bioimaging. Integr. Biol. 2010, 2, 121−129.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02916.

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REFERENCES

TEM images of nanocrystals (samples B and C, batch 1), EDS spectra of Ag−In−S and Ag−In−Zn−S nanocrystals (samples A, B, and C), survey and highresolution XPS spectra of Ag−In−Zn−S nanocrystals (sample A), experimental procedures and characterization for ligand (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Piotr Bujak: 0000-0003-2162-961X K

DOI: 10.1021/acs.inorgchem.8b02916 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (16) Pons, T.; Pic, E.; Lequeux, N.; Cassette, E.; Bezdetnaya, L.; Guillemin, F.; Marchal, F.; Dubertret, B. Cadmium-Free CuInS2/ZnS Quantum Dots for Sentinel Lymph Node Imaging with Reduced Toxicity. ACS Nano 2010, 4, 2531−2538. (17) Lee, S. J.; Kim, Y.; Jung, J.; Kim, M. A.; Kim, N.; Lee, S. J.; Kim, S. K.; Kim, Y.-R.; Park, J. K. Rapid and Facile Synthesis of a (ZnxAgyInz)S2 Nanocrystal Library via Sono-combichem Method and its Characterization Including Single Nanocrystal Analysis. J. Mater. Chem. 2012, 22, 11957−11963. (18) Subramaniam, P.; Lee, S. J.; Shah, S.; Patel, S.; Starovoytov, V.; Lee, K.-B. Generation of a Library of Non-Toxic Quantum Dots for Cellular Imaging and siRNA Delivery. Adv. Mater. 2012, 24, 4014− 4019. (19) Liu, L.; Hu, R.; Law, W.-C.; Roy, I.; Zhu, J.; Ye, L.; Hu, S.; Zhang, X.; Yong, K.-T. Optimizing the Synthesis of Red- and NearInfrared CuInS2 and AgInS2 Semiconductor Nanocrystals for Bioimaging. Analyst 2013, 138, 6144−6153. (20) Fahmi, M. Z.; Chang, J.-Y. Forming Double LayerEncapsulated Quantum Dots for Bio-Imaging and Cell Targeting. Nanoscale 2013, 5, 1517−1528. (21) Regulacio, M. D.; Win, K. Y.; Lo, S. L.; Zhang, S.-Y.; Zhang, X.; Wang, S.; Han, M.-Y.; Zheng, Y. Aqueous Synthesis of Highly Luminescent AgInS2-ZnS Quantum Dots and their Biological Applications. Nanoscale 2013, 5, 2322−2327. (22) Peng, E.; Choo, E. S. G.; Tan, C. S. H.; Tang, X.; Sheng, Y.; Xue, J. Multifunctional PEGylated Nanoclusters for Biomedical Applications. Nanoscale 2013, 5, 5994−6005. (23) Liu, L.; Hu, R.; Roy, I.; Lin, G.; Reynolds, J. L.; Liu, J.; Liu, J.; Schwartz, S. A.; Zhang, X.; Yong, K.-T.; Ye, L. Synthesis of Luminescent Near-Infrared AgInS2 Nanocrystals as Optical Probes for In Vivo Applications. Theranostics 2013, 3, 109−115. (24) Xiong, W.-W.; Yang, G.-H.; Wu, X.-C.; Zhu, J.-J. MicrowaveAssisted Synthesis of Highly Luminescent AgInS2/ZnS Nanocrystals for Dynamic Intracellular Cu(II) Detection. J. Mater. Chem. B 2013, 1, 4160−4165. (25) Deng, D.; Cao, J.; Qu, L.; Achilefu, S.; Gu, Y. Highly Luminescent Water-Soluble Quaternary Zn-Ag-In-S Quantum Dots for Tumor Cell-Targeted Imaging. Phys. Chem. Chem. Phys. 2013, 15, 5078−5083. (26) Fahmi, M. Z.; Ou, K.-L.; Chen, J.-K.; Ho, M.-H.; Tzing, S.-H.; Chang, J.-Y. Development of Bovine Serum Albumin-Modified Hybrid Nanoclusters for Magnetofluorescence, Imaging and Drug Delivery. RSC Adv. 2014, 4, 32762−32772. (27) Shinchi, H.; Wakao, M.; Nagata, N.; Sakamoto, M.; Mochizuki, E.; Uematsu, T.; Kuwabata, S.; Suda, Y. Cadmium-Free Sugar-ChainImmobilized Fluorescent Nanoparticles Containing Low-Toxicity ZnS-AgInS2 Cores for Probing Lectin and Cells. Bioconjugate Chem. 2014, 25, 286−295. (28) Zhao, P.; Zhang, J.; Zhu, Y.; Yang, X.; Jiang, X.; Yuan, Y.; Liu, C.; Li, C. A Novel Strategy for the Aqueous Synthesis of Down-/UpConversion Nanocomposites for Dual-Modal Cell Imaging and Drug Delivery. J. Mater. Chem. B 2014, 2, 8372−8377. (29) Wu, P.-J.; Ou, K.-L.; Chen, J.-K.; Fang, H.-P.; Tzing, S.-H.; Lin, W.-X.; Chang, J.-Y. Methotrexate-Conjugated AgInS2/ZnS Quantum Dots for Optical Imaging and Drug Delivery. Mater. Lett. 2014, 128, 412−416. (30) Song, J.; Jiang, T.; Guo, T.; Liu, L.; Wang, H.; Xia, T.; Zhang, W.; Ye, X.; Yang, M.; Zhu, L.; Xia, R.; Xu, X. Facile Synthesis of Water-Soluble Zn-Doped AgIn5S8/ZnS Core/Shell Fluorescent Nanocrystals and Their Biological Application. Inorg. Chem. 2015, 54, 1627−1633. (31) Shamirian, A.; Appelbe, O.; Zhang, Q.; Ganesh, B.; Kron, S. J.; Snee, P. T. A Toolkit for Bioimaging Using Near-Infrared AgInS2/ ZnS Quantum Dots. J. Mater. Chem. B 2015, 3, 8188−8196. (32) Chen, S.; Ahmadiantehrani, M.; Publicover, N. G.; Hunter, K. W., Jr.; Zhu, X. Thermal Decomposition Based Synthesis of Ag-In-S/ ZnS Quantum Dots and their Chlorotoxin Modified Micelles for Brain Tumor Cell Targeting. RSC Adv. 2015, 5, 60612−60620.

(33) Song, J.; Ma, C.; Zhang, W.; Li, X.; Zhang, W.; Wu, R.; Cheng, X.; Ali, A.; Yang, M.; Zhu, L.; Xia, R.; Xu, X. Bandgap and Structure Engineering via Cation Exchange: From Binary Ag2S to Ternary AgInS2Quaternary AgInZnS Alloy and AgInZnS/ZnS Core/Shell Fluorescent Nanocrystals for Bioimaging. ACS Appl. Mater. Interfaces 2016, 8, 24826−24836. (34) Chen, S.; Demillo, V.; Lu, M.; Zhu, X. Preparation of Photoluminescence Tunable Cu-Doped AgInS2 and AgInS2/ZnS Nanocrystals and their Application as Cellular Imaging Probes. RSC Adv. 2016, 6, 51161−51170. (35) Xuan, T.-T.; Liu, J.-Q.; Yu, C.-Y.; Xie, R.-J.; Li, H.-L. Facile Synthesis of Cadmium-Free Zn-In-S:Ag/ZnS Nanocrystals for BioImaging. Sci. Rep. 2016, 6, 24459. (36) Ogihara, Y.; Yukawa, H.; Kameyama, T.; Nishi, H.; Onoshima, D.; Ishikawa, T.; Torimoto, T.; Baba, Y. Labeling and In Vivo Visualization of Transplanted Adipose Tissue-Derived Stem Cells with Safe Cadmium-Free Aqueous ZnS Coating of ZnS-AgInS2 Nanoparticles. Sci. Rep. 2017, 7, 40047. (37) Matysiak-Brynda, E.; Bujak, P.; Augustin, E.; Kowalczyk, A.; Mazerska, Z.; Pron, A.; Nowicka, A. M. Stable Nanoconjugates of Transferrin with Alloyed Quaternary Nanocrystals Ag-In-Zn-S as a Biological Entity for Tumor Recognition. Nanoscale 2018, 10, 1286− 1296. (38) Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. Synthesis and Characterization of Colloidal CuInS2 Nanoparticles from a Molecular Single-Source Precursor. J. Phys. Chem. B 2004, 108, 12429−12435. (39) Ogawa, T.; Kuzuya, T.; Hamanaka, Y.; Sumiyama, K. Synthesis of Ag-In Binary Sulfide Nanoparticles-Structural Tuning and their Photoluminescence Properties. J. Mater. Chem. 2010, 20, 2226−2231. (40) Nam, D.-E.; Song, W.-S.; Yang, H. Noninjection, One-Pot Synthesis of Cu-Deficient CuInS2/ZnS Core/shell Quantum Dots and their Fluorescent Properties. J. Colloid Interface Sci. 2011, 361, 491−496. (41) Pan, D.; Weng, D.; Wang, X.; Xiao, Q.; Chen, W.; Xu, C.; Yang, Z.; Lu, Y. Alloyed Semiconductor Nanocrystals with Broad Tunable Band Gaps. Chem. Commun. 2009, 4221−4223. (42) Zhang, J.; Xie, R.; Yang, W. A Simple Route for Highly Luminescent Quaternary Cu-Zn-In-S Nanocrystal Emitters. Chem. Mater. 2011, 23, 3357−3361. (43) De Trizio, L.; Prato, M.; Genovese, A.; Casu, A.; Povia, M.; Simonutti, R.; Alcocer, M. J. P.; D’Andrea, C.; Tassone, F.; Manna, L. Strongly Fluorescent Quaternary Cu-In-Zn-S Nanocrystals Prepared from Cu1‑xInS2 Nanocrystals by Partial Cation Exchange. Chem. Mater. 2012, 24, 2400−2406. (44) Torimoto, T.; Adachi, T.; Okazaki, K.-i.; Sakuraoka, M.; Shibayama, T.; Ohtani, B.; Kudo, A.; Kuwabata, S. Facile Synthesis of ZnS-AgInS2 Solid Solution Nanoparticles for a Color-Adjustable Luminophore. J. Am. Chem. Soc. 2007, 129, 12388−12389. (45) Chevallier, T.; Le Blevennec, G.; Chandezon, F. Photoluminescence Properties of AgInS2-ZnS Nanocrystals: the Critical Role of the Surface. Nanoscale 2016, 8, 7612−7620. (46) Chevallier, T.; Benayad, A.; Le Blevennec, G.; Chandezon, F. Method to Determine Radiative and Non-Radiative Defects Applied to AgInS2-ZnS Luminescent Nanocrystals. Phys. Chem. Chem. Phys. 2017, 19, 2359−2363. (47) Sharma, D. K.; Hirata, S.; Bujak, L.; Biju, V.; Kameyama, T.; Kishi, M.; Torimoto, T.; Vacha, M. Influence of Zn on the Photoluminescence of Colloidal (AgIn)xZn2(1‑x)S2 Nanocrystals. Phys. Chem. Chem. Phys. 2017, 19, 3963−3969. (48) Tang, X.; Yu, K.; Xu, Q.; Choo, E. S. G.; Goh, G. K. L.; Xue, J. Synthesis and Characterization of AgInS2-ZnS Heterodimers with Tunable Photoluminescence. J. Mater. Chem. 2011, 21, 11239− 11243. (49) Tang, X.; Ho, W. B. A.; Xue, J. M. Synthesis of Zn-Doped AgInS2 Nanocrystals and Their Fluorescence Properties. J. Phys. Chem. C 2012, 116, 9769−9773. L

DOI: 10.1021/acs.inorgchem.8b02916 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (50) Mao, B.; Chuang, C.-H.; Lu, F.; Sang, L.; Zhu, J.; Burda, C. Study of the Partial Ag-to-Zn Cation Exchange in AgInS2/ZnS Nanocrystals. J. Phys. Chem. C 2013, 117, 648−656. (51) Gabka, G.; Bujak, P.; Giedyk, K.; Ostrowski, A.; Malinowska, K.; Herbich, J.; Golec, B.; Wielgus, I.; Pron, A. A Simple Route to Alloyed Quaternary Nanocrystals Ag-In-Zn-S with Shape and Size Control. Inorg. Chem. 2014, 53, 5002−5012. (52) Kameyama, T.; Takahashi, T.; Machida, T.; Kamiya, Y.; Yamamoto, T.; Kuwabata, S.; Torimoto, T. Controlling the Electronic Energy Structure of ZnS-AgInS2 Solid Solution Nanocrystals for Photoluminescence and Photocatalytic Hydrogen Evaluation. J. Phys. Chem. C 2015, 119, 24740−24749. (53) Torimoto, T.; Kamiya, Y.; Kameyama, T.; Nishi, H.; Uematsu, T.; Kuwabata, S.; Shibayama, T. Controlling Shape Anisotropy of ZnS-AgInS2 Solid Solution Nanoparticles for Improving Photocatalytic Activity. ACS Appl. Mater. Interfaces 2016, 8, 27151−27161. (54) Gabka, G.; Bujak, P.; Kotwica, K.; Ostrowski, A.; Lisowski, W.; Sobczak, J. W.; Pron, A. Luminophores of Tunable Colors from Ternary Ag-In-S and Quaternary Ag-In-Zn-S Nanocrystals Covering the Visible to Near-Infrared Spectral Range. Phys. Chem. Chem. Phys. 2017, 19, 1217−1228. (55) Kang, X.; Huang, L.; Yang, Y.; Pan, D. Scaling up the Aqueous Synthesis of Visible Light Emitting Multinary AgInS2/ZnS Core/Shell Quantum Dots. J. Phys. Chem. C 2015, 119, 7933−7940. (56) Raevskaya, A.; Lesnyak, V.; Haubold, D.; Dzhagan, V.; Stroyuk, O.; Gaponik, N.; Zahn, D. R. T.; Eychmüller, A. A Fine Size Selection of Brightly Luminescent Water-Soluble Ag-In-S and Ag-In-S/ZnS Quantum Dots. J. Phys. Chem. C 2017, 121, 9032−9042. (57) Stroyuk, O.; Raevskaya, A.; Spranger, F.; Selyshchev, O.; Dzhagan, V.; Schulze, S.; Zahn, D. R. T.; Eychmüller, A. Origin and Dynamics of Highly Efficient Broadband Photoluminescence of Aqueous Glutathione-Capped Size Selected Ag-In-S Quantum Dots. J. Phys. Chem. C 2018, 122, 13648−13658. (58) Velapoldi, R. A. Proc. Conf. NBS Gaithesburg; National Bureau of Standards: Washington, D.C., 1972; Vol. 378, p 231. (59) Gromova, M.; Lefrançois, A.; Vaure, L.; Agnese, F.; Aldakov, D.; Maurice, A.; Djurado, D.; Lebrun, C.; de Geyer, A.; Schülli, T. U.; Pouget, S.; Reiss, P. Growth Mechanism and Surface State of CuInS2 Nanocrystals Synthesized with Dodecanethiol. J. Am. Chem. Soc. 2017, 139, 15748−15759. (60) Yang, X.; Tang, Y.; Tan, S. T.; Bosman, M.; Dong, Z.; Leck, K. S.; Demir, H. V.; Sun, X. W.; Ji, Y. Facile Synthesis of Luminescent AgInS2-ZnS Solid Solution Nanorods. Small 2013, 9, 2689−2695. (61) Peng, S.; Zhang, S.; Mhaisalkar, S. G.; Ramakrishna, S. Synthesis of AgInS2 Nanocrystal Ink and its Photoelectrical Application. Phys. Chem. Chem. Phys. 2012, 14, 8523−8529. (62) Deng, M.; Shen, S.; Wang, X.; Zhang, Y.; Xu, H.; Zhang, T.; Wang, Q. Controlled Synthesis of AgInS2 Nanocrystals and their Application in Organic-Inorganic Hybrid Photodetectors. CrystEngComm 2013, 15, 6443−6447. (63) Zhang, Q.-H.; Tian, Y.; Wang, C.-F.; Chen, S. Construction of Ag-Doped Zn-In-S Quantum Dots Toward White LEDs and 3D Luminescent Patterning. RSC Adv. 2016, 6, 47616−47622. (64) Gabka, G.; Bujak, P.; Gryszel, M.; Kotwica, K.; Pron, A. Anchor Groups Effect on Spectroscopic and Electrochemical Properties of Quaternary Nanocrystals Cu-In-Zn-S Capped with Arylamine Derivatives. J. Phys. Chem. C 2015, 119, 9656−9664. (65) Gabka, G.; Bujak, P.; Giedyk, K.; Kotwica, K.; Ostrowski, A.; Malinowska, K.; Lisowski, W.; Sobczak, J. W.; Pron, A. Ligand Exchange in Quaternary Alloyed Nanocrystals - a Spectroscopic Study. Phys. Chem. Chem. Phys. 2014, 16, 23082−23088. (66) Turo, M. J.; Macdonald, J. E. Crystal-Bound vs Surface-Bound Thiols on Nanocrystals. ACS Nano 2014, 8, 10205−10213. (67) Gabka, G.; Bujak, P.; Gryszel, M.; Ostrowski, A.; Malinowska, K.; Zukowska, G. Z.; Agnese, F.; Pron, A.; Reiss, P. Synthesis and Surface Chemistry of High Quality Wurtzite and Kesterite Cu2ZnSnS4 Nanocrystals Using tin(II) 2-ethylhexanoate as a New Tin Source. Chem. Commun. 2015, 51, 12985−12988.

(68) Gabka, G.; Bujak, P.; Ostrowski, A.; Tomaszewski, W.; Lisowski, W.; Sobczak, J. W.; Pron, A. Cu-Fe-S Nanocrystals Exchibiting Tunable Localized Surface Plasmon Resonance in the Visible to NIR Spectral Ranges. Inorg. Chem. 2016, 55, 6660−6669. (69) Xu, S.; Ziegler, J.; Nann, T. Rapid Synthesis of Highly Luminescent InP and InP/ZnS Nanocrystals. J. Mater. Chem. 2008, 18, 2653−2656. (70) Dai, M.; Ogawa, S.; Kameyama, T.; Okazaki, K.-i.; Kudo, A.; Kuwabata, S.; Tsuboi, Y.; Torimoto, T. Tunable Photoluminescence from the Visible to Near-Infrared Wavelength Region of NonStoichiometric AgInS2 Nanoparticles. J. Mater. Chem. 2012, 22, 12851−12858. (71) Reiss, P.; Couderc, E.; De Girolamo, J.; Pron, A. Conjugated Polymers/Semiconductor Nanocrystals Hybrid Materials-Preparation, Electrical Transport Properties and Applications. Nanoscale 2011, 3, 446−489. (72) Bunz, U. H. F.; Engelhart, J. U.; Lindner, B. D.; Schaffroth, M. Large N-Heteroacenes: New Tricks for Very Old Dogs? Angew. Chem., Int. Ed. 2013, 52, 3810−3821. (73) Laursen, J. B.; Nielsen, J. Phenazine Natural Products: Biosynthesis, Synthetic Analogues, and Biological Activity. Chem. Rev. 2004, 104, 1663−1685. (74) Bujak, P.; Kulszewicz-Bajer, I.; Zagorska, M.; Maurel, V.; Wielgus, I.; Pron, A. Polymers for Electronics and Spintronics. Chem. Soc. Rev. 2013, 42, 8895−8999. (75) Wróbel, Z.; Kwast, A. Simple Synthesis of N-Aryl-2-nitrosoanilines in the Reaction of Nitroarenes with Aniline Anion Derivatives. Synthesis 2010, 2010, 3865−3872. (76) Kwast, A.; Stachowska, K.; Trawczyński, A.; Wróbel, Z. N-Aryl2-nitrosoanilines as Intermediates in the Synthesis of Substituted Phenazines from Nitroarenes. Tetrahedron Lett. 2011, 52, 6484−688. (77) Querner, C.; Reiss, P.; Bleuse, J.; Pron, A. Chelating Ligands for Nanocrystals’ Surface Functionalization. J. Am. Chem. Soc. 2004, 126, 11574−11582. (78) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Frontier Orbital Energy Levels of Conjugated Polymers for Solar Cell Applications. Adv. Mater. 2011, 23, 2367−2371. (79) Sworakowski, J. How Accurate Are Energies of HOMO and LUMO Levels in Small-Molecule Organic Semiconductors Determined from Cyclic Voltammetry or Optical Spectroscopy? Synth. Met. 2018, 235, 125−130. (80) Pron, A.; Gawrys, P.; Zagorska, M.; Djurado, D.; Demadrille, R. Electroactive Materials for Organic Electronics: Preparation Strategies, Structural Aspects and Characterization Techniques. Chem. Soc. Rev. 2010, 39, 2577−2632.

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DOI: 10.1021/acs.inorgchem.8b02916 Inorg. Chem. XXXX, XXX, XXX−XXX