Indium(II) Chloride as a Precursor in the Synthesis
of Ternary (Ag–In–S) and Quaternary (Ag–In–Zn–S)
Nanocrystals
A new indium precursor, namely, indium(II) chloride, was tested as a precursor in the synthesis of ternary Ag–In–S and quaternary Ag–In–Zn–S nanocrystals. This new precursor, being in fact a dimer of Cl2In–InCl2 chemical structure, is significantly more reactive than InCl3, typically used in the preparation of these types of nanocrystals. This was evidenced by carrying out comparative syntheses under the same reaction conditions using these two indium precursors in combination with the same silver (AgNO3) and zinc (zinc stearate) precursors. In particular, the use of indium(II) chloride in combination with low concentrations of the zinc precursor yielded spherical-shaped (D = 3.7–6.2 nm) Ag–In–Zn–S nanocrystals, whereas for higher concentrations of this precursor, rodlike nanoparticles (L = 9–10 nm) were obtained. In all cases, the resulting nanocrystals were enriched in indium (In/Ag = 1.5–10.3). Enhanced indium precursor conversion and formation of anisotropic, longitudinal nanoparticles were closely related to the presence of thiocarboxylic acid type of ligands in the reaction mixture. These ligands were generated in situ and subsequently bound to surfacial In(III) cations in the growing nanocrystals. The use of the new precursor of enhanced reactivity facilitated precise tuning of the photoluminescence color of the resulting nanocrystals in the spectral range from ca. 730 to 530 nm with photoluminescence quantum yield (PLQY) varying from 20 to 40%. The fabricated Ag–In–S and Ag–In–Zn–S nanocrystals exhibited the longest, reported to date, photoluminescence lifetimes of ∼9.4 and ∼1.4 μs, respectively. It was also demonstrated for the first time that ternary (Ag–In–S) and quaternary (Ag–In–Zn–S) nanocrystals could be applied as efficient photocatalysts, active under visible light (green) illumination, in the reaction of aldehydes reduction to alcohols.
Introduction<!>Synthesis and Characterization
of the Nanocrystals<!><!>Synthesis and Characterization
of the Nanocrystals<!><!>Synthesis and Characterization
of the Nanocrystals<!><!>Synthesis and Characterization
of the Nanocrystals<!>Optical Properties<!><!>Optical Properties<!><!>Optical Properties<!>Surface Characterization<!><!>Surface Characterization<!><!>Surface Characterization<!><!>Surface Characterization<!>Photocatalytic Activity<!><!>Photocatalytic Activity<!>Conclusions<!>Materials<!>Preparation of the S/OLA
Precursor<!>Preparation of Ag–In–S and
Alloyed Ag–In–Zn–S
Nanocrystals<!>Ligand Recovery<!>Procedures for the Reduction of Aldehyde<!><!>Author Contributions<!>
<p>The preparation of
highly luminescent semiconductor nanocrystals,
which do not contain toxic metals, is of great importance, especially
in view of their application in medicine and biomedical sciences.1−3 This necessity of eliminating toxic elements resulted in quick development
of new preparation methods focused on cadmium- and lead-free nanocrystals
of various binary, ternary and quaternary semiconductors.4,5 Luminescent binary such as InP6 and ternary
AgInS2 and CuInS27−10 nanocrystals are especially interesting
in this respect. Colloidal ternary nanocrystals of core/shell structure
AgInS2/ZnS as well as alloyed quaternary ones (AgInS2–ZnS) are highly luminescent, yielding emission tunable
in a wide spectral range. For this reason, they are tested as components
of quantum dot light-emitting diodes (LEDs)11−14 as well as in various types of
biomedical applications.15−19 Moreover, in recent years, increasing number of reports can be found
on their photovoltaic20−22 and photocatalytic23,24 applications.</p><p>In the synthesis of these indium-containing nanocrystals, indium(III)
chloride and indium(III) acetate are more frequently used as indium
precursors.25−27 There also exist reports on the application of indium(III)
nitrate, indium(III) acetylacetonate, indium(III) mercaptoacetate,
and more recently, InBr3 and InI3 as precursors.28,29 The reason for testing this rather large number of different simple
precursors has its origin in their varying reactivities, inherently
associated, among others, with their different solubilities in the
reaction medium. The second factor of significant importance is the
capability of transforming the initial precursor into its active form
through binding ligands to cations originating from this precursor.
The decomposition of this active form, strongly dependent on the energy
of particular bonds breaking and forming new ones, results in the
nucleation of nanocrystals.28,30 A combination of the
above-mentioned factors may lead to products of different chemical
compositions.28,31</p><p>Simple indium precursors
are usually used in association with higher
fatty acids ligands in varying ratios.32 Alternatively salts of indium(III) with higher carboxylic acids
can be used, playing a dual role of a metal precursor and a source
of ligands.33 According to the theory of
hard and soft acids and bases, In3+ being a hard acid readily
binds to hard bases like higher carboxylic acids or amines containing
long alkyl substituents.34 It should, however,
be noted that In3+ ions present in the reaction mixture
at different stages of nucleation or crystal growth are significantly
harder acids than surfacial In3+ ions.35 Different hard acid character of surfacial In3+ ions is clearly manifested by strong effects of ligands on the luminescent
properties of indium-containing nanocrystals.36,37</p><p>There exist several compounds in which the formal oxidation
state
of indium is lower than +3.38 Among them,
indium(II) chloride seems especially interesting as a candidate for
indium precursor in the synthesis of nanocrystals since it is stable
in air and commercially available. In reality, this compound is present
in a form of a dimer In2Cl4. In–In bond
in this compound is weak compared to In–Cl one. This leads
to its disproportionation with simultaneous formation of In+[InCl4]− (see Scheme 1).39 This disproportionation
reaction can be strictly controlled40,41 and applied
for in situ generation of In3+ ions in
the reaction mixture used in the preparation of nanocrystals. The
interest in new precursors containing indium in the oxidation state
lower than +3 can be exemplified by a recent report of the use of
complexes generated from In(I)Cl in the synthesis of colloidal InAs
nanocrystals.42</p><p>In this paper, we demonstrate for the first time that indium(II)
chloride can be used as an efficient precursor in the preparation
of highly luminescent inorganic semiconductor nanocrystals varying
in composition, shape, and size. In particular, using this precursor
it is possible to prepare ternary AgInS2 and alloyed quaternary
AgInS2–ZnS nanocrystals exhibiting strong photoluminescence
covering the whole visible range of the spectrum. The new precursor
turned out to be more reactive than In(III) precursors investigated
to date. Its enhanced reactivity had a pronounced effect on the chemical
constitution and morphology of the resulting nanocrystals and on surfacial
ligand transformations, previously never observed for this type of
nanocrystals.</p><!><p>Similarly to indium(III) chloride, indium(II)
chloride is stable
under ambient conditions; thus, all operations involving this reagent
can be performed in air. A comparison of the reactivities of both
types of precursors was made possible by replacing indium(III) chloride
with indium(II) chloride in the reaction mixture, while keeping all
other reaction conditions unchanged.43−45 Ternary Ag–In–S
nanocrystals were obtained from a reaction mixture consisting of AgNO3 and InCl2 precursors, 1-dodecanethiol (DDT), and
1-octadecene (ODE) as a solvent (batch AIS). In the case
of the synthesis of quaternary Ag–In–Zn–S nanocrystals,
zinc stearate was additionally added (batches A-(1–4)). An increasing number of A-X samples corresponds to
increasing zinc content. Before describing the procedure of nanocrystals
preparation, it is instructive to discuss the color changes of the
reaction mixtures containing InCl2 upon their heating from
room temperature to 150 °C. At 90 °C, their color changed
to yellow, upon further increase of the temperature to 150 °C
this color again changed to dark brown. The observed changes were
inherently associated with the transformations of InCl2 precursor since heating of the precursor mixtures containing InCl3 did not cause any visual color change but only gradual dissolution
of precursors which yielded colorless, clear liquids. Figure S1 in the Supporting Information presents
photographs of both types of reaction mixtures taken at different
temperatures. Moreover, in Figure S2 in
the Supporting Information, UV–vis spectra, registered for
the reaction mixture at different stages of its heating before and
after the injection of the sulfur precursor, are compared. This is
completed by a short comment explaining these changes.</p><p>The reactions
leading to nanocrystals formation were initiated by injection of sulfur
dissolved in oleylamine (S/OLA) to these mixtures at 150 °C.
This was followed by heating to 180 °C. At this temperature,
the reaction mixtures were kept for an additional 1 h. Mixtures with
InCl3 precursors yielded black, insoluble precipitates
at the end of the reaction, whereas in the case of the use of InCl2 precursors no precipitates could be observed.45 Nanocrystals of AIS and A-(1–4) were separated from the reaction mixture by precipitation with acetone,
followed by centrifugation. They were then redispersed in typical
nonpolar or weakly polar solvents such as toluene, chloroform, and
methylene chloride.</p><p>AIS nanocrystals were prepared
using the metal precursors
ratio InCl2/AgNO3 of 3.0. The application of
a significant excess of the precursor of indium was based on experimentally
established findings indicating that the presence of defects in nonstoichiometric
ternary nanocrystals (In/Ag > 1.0) is beneficial for the resulting
photoluminescence quantum yield (PLQY), in accordance with donor–acceptor
mechanism of radiative recombination.45−47 As expected AIS nanocrystals were strongly nonstoichiometric and enriched in indium: Ag1.0In1.4S2.5(S2.6), where the value given in parentheses
denotes theoretical content of sulfide anions corresponding to the
sum of silver and indium cations. Compositions of nanocrystals of
all prepared batches (AIS and A-(1–4)) are collected in Table 1, whereas their energy-dispersive spectra (EDS) are presented
in Figures S3 and S4 in the Supporting
Information.</p><p>The powder diffractogram of AIS is shown in Figure 1a. The following
peaks can be distinguished which can be ascribed to (120), (002),
(121), (320), (123), and (322) of orthorhombic AgInS2 (JCPDS
00-025-1328). Enhanced intensity of (002) reflection has its origin
in the strong overlap of this peak with (121) reflection of chalcopyrite
AgInS2 (JCPDS 00-25-1330). Orthorhombic-chalcopyrite polytypism
was reported for Ag–In–S nanocrystals48,49 similarly to wurtzite-chalcopyrite polytypism in the case of Cu–In–S
nanocrystals.50 The aspect ratio of AIS nanocrystals is rather low (1.2 ± 0.20); thus, they
can be considered as almost spherical of 9.8 ± 2.3 nm diameter
(n = 300). Their high-resolution transmission electron
microscopy (HR-TEM) and transmission electron microscopy (TEM) images
are presented in Figure 1b,c, indicating an interplanar distance of 0.359 nm, which corresponds
to the (120) reflection of orthorhombic AgInS2.51−53 A close inspection of these HR-TEM images reveals the presence of
nanocrystals of different orientations of planes, thus corroborating
polytypism of the obtained nanoparticles.50</p><!><p>(a)
X-ray diffraction (XRD) patterns of Ag1.0In1.4S2.5(S2.6) (AIS) nanocrystals; (b) HR-TEM image
and selected area electron diffraction (SAED) patterns of Ag1.0In1.4S2.5(S2.6) (AIS); and (c) TEM image
and the corresponding histogram of Ag1.0In1.4S2.5(S2.6) (AIS) nanocrystals.</p><!><p>In the preparation of quaternary Ag–In–Zn–S
nanocrystals (batches A-(1–4)), the same composition
of the reaction mixture was maintained with a fixed molar ratio of
indium to silver precursors (InCl2/AgNO3 = 3).
The only parameter being changed was the content of zinc precursor
(zinc stearate) which increased from A-1 to A-4. The most striking difference in chemical compositions of AIS nanocrystals and A-(1–4) ones was
the ratio of inorganic core elements to organic ones. In the case
of AIS nanocrystals, the elements of the inorganic core
predominated with the content of carbon originating from organic ligands
being as low as 7 wt %. EDS analysis of A-(1–4) nanocrystals yielded carbon contents in the range of 46–63
wt %, clearly showing higher ligand content. This was expected, taking
into account a significantly higher surface-to-volume ratio originating
from the higher surface-to-volume ratio of A-(1–4) nanocrystals compared to the AIS ones. For A-(1–3) nanocrystals, introduction of the precursor of zinc had no or little
effect on the conversion of indium precursor since the determined
indium-to-silver ratios (In/Ag = 1.5) were very close to the value
found for AIS nanocrystals (In/Ag = 1.4). Only for the
highest zinc precursor content in the reaction mixture (batch A-4), the indium precursor conversion abruptly increased,
yielding In/Ag = 10.3 in the resulting nanocrystals.</p><p>In Figure 2, Zn/Ag
molar ratios in A-(1–4) nanocrystals are plotted
against the corresponding molar ratios in the reaction mixture. Nanocrystals
of A-(1–3) batches were enriched in Ag, i.e., their Zn/Ag ratios were lower than the molar ratios
of their precursors. On the contrary, A-4 nanocrystals
were enriched in Zn since precursors in the molar ratio Zn/Ag = 9.0
yielded nanocrystals of Zn/Ag = 12.4. Additional points in the plot B-1 (Ag1.0In2.8Zn1.3S4.0(S6.0)) and B-2 (Ag1.0In1.5Zn7.8S17.0(S10.5)) represent nanocrystals obtained with InCl3 as
a precursor, which were reported in ref (54). Elemental analyses of nanocrystals obtained
with two different indium precursors (InCl2vs InCl3) indicated distinct differences in their reactivities
in the reaction medium. In the case of InCl2, with increasing
concentration of the precursor of zinc, the conversion of indium precursor
remained at a similar level A-(1–3). It abruptly
increased for a high degree of zinc precursor conversion (A-4). On the contrary, conversion of InCl3 dropped with increasing
conversion of the precursor of zinc.</p><!><p>Molar ratio of zinc stearate to AgNO3 in the reaction
mixture vs Zn/Ag ratio in the resulting AgInS2–ZnS nanocrystals (molar ratio of indium precursor
to AgNO3 = 3.0). Circles: this research (InCl2) A-(1–4), squares: experimental data (InCl3) B-1 and B-2 from ref (54). The insets show photographs
of UV-illuminated (365 nm) nanocrystals dispersed in toluene solutions.</p><!><p>In Figure 3a, powder
diffractograms of A-(1–4) nanocrystals are presented.
Three broadened peaks can be distinguished in each case, whose positions
are intermediate between the reflections characteristic of orthorhombic
AgInS2 (JCPDS 00-025-1328) and hexagonal ZnS (JCPDS 00-036-1450).
The effect of zinc content on the position of Bragg reflections could
be conveniently followed by analyzing the shift of (002) reflection,
which is located at 2θ = 26.6 and 28.5° for orthorhombic
AgInS2 and hexagonal ZnS, respectively. With increasing
zinc content in the studied quaternary nanocrystal (002) reflection
was being increasingly shifted toward higher 2θ values: 26.6°
(A-1), 26.8° (A-2), 27.2° (A-3), and 28.0° (A-4). The introduction
of zinc to yield quaternary nanocrystals resulted in a significant
decrease of their size from 9.8 ± 2.3 nm in ternary Ag1.0In1.4S2.5(S2.6) (AIS), to 6.2 ±
1.1 nm in quaternary Ag1.0In1.5Zn0.3S3.3(S3.0) (A-1). Both AIS and A-1 nanocrystals could be considered
as close to spherical but somehow irregular in shape (aspect ratio
= 1.2) (see Figures 1 and 3). A-2 (Ag1.0In1.5Zn1.9S3.6(S4.6)) nanocrystals containing more zinc were even smaller (D = 3.7 ± 0.7 nm) and were characterized by an aspect ratio reduced
to 1.1. Further increase in the content of zinc resulted in a radical
change in the shape of nanocrystals from spherical to longitudinal.
In the case of A-3 (Ag1.0In1.5Zn4.4S6.8(S7.1)) nanocrystals,
their width (3.9 ± 0.9 nm) was comparable to the diameter of A-2 nanocrystals, but their length was 9.0 ± 1.7 nm,
yielding an aspect ratio of 2.4. An even higher aspect ratio (3.3)
was obtained for A-4 (Ag1.0In10.3Zn12.4S11.8(S28.3)) nanocrystals
characterized by smaller width (3.1 ± 0.7 nm) and higher length
(9.9 ± 2.2 nm).</p><!><p>X-ray powder diffractograms of Ag1.0In1.5Zn0.3S3.3(S3.0) (A-1), Ag1.0In1.5Zn1.9S3.6(S4.6) (A-2), Ag1.0In1.5Zn4.4S6.8(S7.1) (A-3), and Ag1.0In10.3Zn12.4S11.8(S28.3) (A-4) nanocrystals (a),
HR-TEM (b, d), and TEM (c, e) images of Ag1.0In1.5Zn0.3S3.3(S3.0) (A-1) and Ag1.0In10.3Zn12.4S11.8(S28.3) (A-4) alloyed nanocrystals
and their corresponding histograms.</p><!><p>It is instructive to compare HR-TEM images of A-1 and A-4 nanocrystals significantly differing in their size, shape,
and zinc content (see Figure 3b,d). The interplanar distances derived from these images
were in the range of 0.310–0.339 nm and could be ascribed to
(002) interplanar distances of orthorhombic AgInS2 and
hexagonal ZnS, corroborating alloyed-type structure of the studied
nanocrystals.23,45,55,56 Possible, alternative alloyed-type structures
of tetragonal AgInS2 (JCPDF 00-25-1330) and cubic ZnS (JCPDF
00-50-0566) can be excluded since in this case significantly shorter
interplanar distances, in the range 0.1–0.2 nm should have
been observed.57−59 Moreover, HR-TEM studies excluded the possibility
of the formation of hexagonal InAgZn2S4-type
structures (JCPDF 00-025-0383) since they are characterized by interplanar
distances of 0.25–0.29 nm.60−62 TEM images of A-1 and A-4 nanocrystals are shown in Figure 3c,e. The corresponding
images of A-2 and A-3 nanocrystals can be
found in the Supporting Information.</p><p>Nanocrystals A-1 and A-4 were compared
with the corresponding nanocrystals obtained in the same conditions
but using a different indium precursor, i.e., InCl3. TEM images of these nanocrystals denoted as B-1 and B-2 are shown in Figure S7 in the Supporting Information. For small concentrations of the precursor
of zinc, no significant differences could be noticed between InCl3 and InCl2 precursors. In both cases, relatively
large, spherical nanocrystals were obtained of very similar size: A-1 (D = 6.2 ± 1.1 nm) and B-1 (D = 6.2 ± 0.9 nm). Large concentrations of
zinc precursor in the reaction mixtures resulted in pronounced differentiation
in shape and size of nanocrystals obtained using different indium
precursors. As already mentioned, in the case of InCl2 precursor,
longitudinal nanoparticles were formed (A-4, L = 9.9 ± 2.2 nm; D = 3.3 ± 0.9
nm). The use of InCl3 precursor resulted in the formation
of spherical nanocrystals of smaller size than A-1 nanoparticles
(B-2, D = 4.2 ± 0.6 nm).</p><p>These morphological and composition differences clearly seen in
quaternary Ag–In–Zn–S nanocrystals obtained with
InCl2 and InCl3 have their origin in differences
in these precursors reactivities and are consistent with our previous
findings concerning the reactivity of silver precursors.63 If highly reactive silver precursor (AgNO3) in combination with InCl2 and zinc precursor
of low concentration were used, AgInS2 phase was formed
at the nucleation stage, which was then converted into quaternary
Ag–In–Zn–S phase at subsequent stages of the
crystal growth. In the case of reaction mixtures of high zinc precursor
concentrations, with the highly reactive indium precursor (InCl2) concentration unchanged (case of A-4 nanocrystals),
nucleation of ZnIn2S4 phase took place, which
was then followed by the formation of quaternary Ag–In–Zn–S
nanocrystals of longitudinal shape. This change in the nanocrystals
morphology could be ascribed to the high reactivity of InCl2 combined with the high concentration of zinc precursor, both factors
favoring anisotropic growth of nanoparticles.64 If InCl3, exhibiting significantly lower reactivity compared
to InCl2, was present in the reaction mixture, it preferentially
reacted with AgNO3 to nucleate the AgInS2 phase.
This process occurred even in reaction mixtures of high zinc precursor
concentrations (case of B-2 nanocrystals). Nucleation
of this type resulted in a quick drop of the precursor concentration
and was followed by subsequent growth, converting ternary germs into
quaternary Ag–In–Zn–S nanocrystals of spherical
shape and relatively low diameter. It should be, however, noted that
nucleation via ZnIn2S4 phase
formation was previously observed by us even in the presence of the
InCl3 precursor, but only in the case when a highly reactive
silver precursor was not present in the reaction mixture at the stage
of nucleation. This was demonstrated by performing a two-step injection
of precursors in which the sulfur precursor was injected in the first
step followed by the addition of the silver precursor in the second
step.65</p><!><p>Tetragonal AgInS2 of
chalcopyrite structure is thermodynamically stable at temperatures
inferior to 620 °C. Its band gap is equal to 1.87 eV49,66 and its exciton Bohr radius is relatively small (5.5 nm).67 Orthorhombic AgInS2 of pseudowurtzite
structure is thermodynamically stable at relatively higher temperatures,
its band gap is larger—1.98 eV.49,66 The quantum
confinement effect is observed for its nanocrystals of diameter inferior
to 5 nm,68 indicating a similar value of
the exciton Bohr radius as in the case of chalcopyrite-type AgInS2. The observed increase of the orthorhombic AgInS2 band gap can originate from its nonstoichiometry and, in particular,
an increased ratio of In/Ag > 1.0. It is postulated that in this
case,
the upper edge of the valence band corresponds to hybridized S 3p
and Ag 4d orbitals, whereas the lower edge of the conduction band
is attributed to hybridized S 3p and Ag 4d orbitals.69</p><p>UV–vis–near-infrared (NIR) of ternary
(AIS) and quaternary (A-(1–4)) nanocrystals
dispersed in toluene are presented in Figure 4. On the basis of these spectra, optical
band gaps were determined following the relationship (AhÎ½)2vshÎ½
(see Figure S8 in the Supporting Information). Eg(opt) ∼ 2.0 eV was determined for AIS (Ag1.0In1.4S2.5(S2.6)), nanocrystals,
a value very close to that of bulk orthorhombic AgInS2.
This was not unexpected since no quantum confinement was expected
for these relatively large nanoparticles (9.8 ± 2.3 nm). Essentially
the same Eg(opt) ∼ 2.0 eV was calculated
for A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)) nanocrystals characterized
by small content of zinc and relatively large size (6.2 ± 1.1
nm). Eg(opt) steadily enlarged with increasing
content of zinc: 2.4 eV (A-2), 2.7 eV (A-3) and 3.2 eV (A-4). These values consistently fell in
the range determined by the bands of orthorhombic AgInS2 (1.98 eV)66 and hexagonal ZnS (3.68 eV).70</p><!><p>Absorbance, photoluminescence excitation, and emission
spectra
of toluene dispersion of Ag1.0In1.4S2.5(S2.6) (AIS) (a) and alloyed Ag1.0In1.5Zn0.3S3.3(S3.0) (A-1) (b), Ag1.0In1.5Zn1.9S3.6(S4.6) (A-2) (c), Ag1.0In1.5Zn4.4S6.8(S7.1) (A-3) (d), and Ag1.0In10.3Zn12.4S11.8(S28.3) (A-4) (e) nanocrystals.
For comparison, the emission spectra (dot lines) of toluene dispersions
of Ag1.0In2.8Zn1.3S4.0(S6.0) (B-1) and Ag1.0In1.5Zn7.8S17.0(S10.5) (B-2) alloyed nanocrystals are presented.</p><!><p>Photoluminescence of binary nanocrystals such as CdSe is governed
by a simple mechanism of radiative recombination (1S(e) → 1S3/2(h)) between the states located in the
vicinity of the band edge. This gives rise to a very narrow emission
peak (full width at half-maximum, fwhm, of 80–150 meV at room
temperature), small Stokes shift (21 ± 4 meV), and room-temperature
radiative decay times of the order of 18–40 ns. Spectral parameters
are also dependent on the size of nanocrystals (quantum confinement)
and the presence of surfacial defects, which may quench the photoluminescence.71−73 In ternary nanocrystals such as AgInS2 as well as in
core/shell AgInS2/ZnS, alloyed AgInS2–ZnS
and nonstoichiometric Ag–In–Zn–S (In/Ag ≠
1.0) ones, the emission is governed by the donor–acceptor pair
recombination mechanism.74 The donor states
are located in the band gap, below the lower edge of the conduction
band and are associated with the presence of S vacancies and interstitial
Ag atoms. The acceptor states located above the upper edge of the
valence band originate from the presence of Ag vacancies and interstitial
S atoms.</p><p>Surfacial defects in the forms of vacancies, dangling
bonds, and
oxygen adatoms may also contribute to this mechanism.68,75 Radiative recombination mechanism in ternary AgInS2 nanocrystals
and related alloyed and nonstoichiometric nanoparticles gives rise
to broad emission peaks (fwhm ∼ 0.4 eV) and large Stokes shifts.74 These characteristic spectral features are rationalized
by strong electron–phonon interactions of trapped carriers.68 Deeper elucidation of these phenomena requires
the use of time-resolved emission spectroscopy techniques. Decay curves
registered for ternary AgInS2 and core/shell AgInS2/ZnS ones are of complex character and cannot be fitted with
a single-exponential function and usually require the application
of two23,68,74,76,77 or three-exponential
models.17,24,49,75,78,79 The two-exponential model yields two decay times: the longer one
(τ2) is usually ascribed to the photoluminescence
originating from the bulk of the nanocrystal according to the above-described
donor–acceptor pair mechanism. The shorter decay time (τ1) is related to the photoluminescence involving surfacial
defects.68 In the case of photoluminescence
of ternary AgInS2 and CuInS2 nanocrystals which
had to be fitted using three-exponential model, different interpretations
could be found in the literature explaining the presence of the third
exponent.75,80,81 For example,
detailed analysis of the photoluminescence decay in alloyed AgInS2–ZnS nanocrystals clearly demonstrated the necessity
of fitting the obtained data using three-exponential functions. The
shortest decay time (τ1) was attributed to the presence
of surficial defects, the intermediate (τ2) and the
longest (τ3) decay times could be tentatively ascribed
to the recombination between the donor and acceptor states at the
surface and in the bulk of a nanocrystal, respectively.75 These briefly outlined photoluminescence mechanisms
are schematically illustrated in Figure 5b. It should, however, be noted that such
characteristic Stokes-shifted and long-lived luminescence was also
observed for stoichiometric AgInS2 and CuInS2 nanocrystals. This was rationalized by theoretical and experimental
investigations as effects originating from the detailed structure
of the valence band featuring two sublevels with different parity.82,83</p><!><p>Photoluminescence
decay curves of Ag1.0In1.4S2.5(S2.6) (AIS) (a) and alloyed Ag1.0In1.5Zn0.3S3.3(S3.0) (A-1) (a, c) and Ag1.0In10.3Zn12.4S11.8(S28.3) (A-4) (c) nanocrystals
and the corresponding bi (AIS and A4) and
three (A-1)-exponential fitting curves. Schematic of
the relaxation dynamics proposed for AIS, A-1, and A-4 nanocrystals (b).</p><!><p>UV–vis–NIR absorption spectra as well as excitation
and stationary photoluminescence spectra of AIS and A-(1–4) nanocrystals are compared in Figure 4. Photoluminescence decay curves
registered for AIS, A-1, and A-4 nanocrystals are presented in Figure 5a,c. The corresponding curves obtained for A-2 and A-3 nanocrystals are shown in Figure S9 in the Supporting Information. The decay profiles
were fitted using two (AIS and A-4) and
three (A-1, A-2, and A-3) exponential
models yielding high accuracy, as indicated by the narrow distribution
of fitting residuals (see eqs 1 and 2)8412where I(t) is the photoluminescence intensity (au); τ1, τ2, and τ3 are the photoluminescence
lifetime
components (ns); A0 is the background
noise level; and A1, A2, and A3 represent the relative
weight of the decay components (%) at t = 0.</p><p>For all fitting functions, the obtained χ2 values
were in the range of 0.98–1.1, proving adequacy of the applied
fitting method. Average photoluminescence lifetimes were calculated
according to eq 33where τav denotes the average
photoluminescence decay time (ns) and Ai and τi are the
normalized amplitude (%) and the lifetime (ns) of the (i) component, respectively.</p><p>All optical parameters determined
for ternary (AIS) and quaternary (A-(1–4)) nanocrystals
are collected in Table 2.</p><p>The excitation spectrum
of AIS (Ag1.0In1.4S2.5(S2.6)) nanocrystals
consisted of a relatively narrow
peak with a clear maximum at 625 nm located close to the absorption
threshold in the UV–vis–NIR spectrum and roughly corresponding
to the optical band gap (∼2.0 eV). Its emission peak was broad
showing a maximum at ∼755 nm. The introduction of even small
amounts of zinc to yield quaternary A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)) nanocrystals resulted in profound spectroscopic changes. In particular,
the excitation peak broadened covering the spectral range of 300–600
nm. Its maximum was hypsochromically shifted to ∼450 nm. In
addition, a significant increase of PLQY was observed from 12% in
the case of AIS nanocrystals to 40% for A-1 ones. A-1 nanocrystals emitted red light (Î»max = 731 nm). With growing zinc content the emission peak
was being increasingly hypsochromically shifted to Î»max = 664 nm for A-2 (Ag1.0In1.5Zn1.9S3.6(S4.6)) nanocrystals
and Î»max = 596 nm for A-3 (Ag1.0In1.5Zn4.4S6.8(S7.1)) nanocrystals. A-4 (Ag1.0In10.3Zn12.4S11.8(S28.3)) nanocrystals
which were the richest in zinc emitted green light (Î»max = 528 nm). It should be pointed out that its excitation peak was
relatively narrow, contrary to the cases of A-1, A-2, and A-3 nanocrystals. Moreover, its maximum
at ca. 400 nm closely corresponded to the absorption
threshold in the UV–vis–NIR spectrum as well as to the
optical band gap of 3.1–3.2 eV. It should be noted that these
relatively narrow excitation spectra of AIS and A-4 are not typical. However, several examples of excitation
spectra of similar shapes were reported for a variety of nanocrystals,
for example for CuInS2–ZnS85 and AgInS2–ZnS nanocrystals69,86 The excitation spectrum of AgInS2–ZnS presented
in ref (87) was essentially
identical in shape to the spectra of AIS and A-4.</p><p>The spectroscopic peculiarity of AIS (Ag1.0In1.4S2.5) nanocrystals was
manifested
in its decay curves fitted by two-exponential functions (Figure 5a). This fitting
resulted in extremely long decay times τ1 and τ2 of ∼1.1 and ∼13.1 μs leading to an average
value (τav) of 9.4 μs—the highest ever
reported for ternary Ag–In–S nanocrystals. Typical decay
times reported for this class of nanocrystals were in the range of
3–4 μs.88 Decay curves of
quaternary nanocrystals had to be fitted with three-exponential functions.
The results of these fittings showed that τav values
determined for quaternary nanocrystals (A-(1–3)) were 1 order of magnitude smaller than that calculated for ternary
(AIS) ones. Moreover, they were dependent on zinc content
decreasing from 1.5 to 0.5 μs when going from A-1 to A-3. τ1 values were also strongly
reduced to 2–140 ns. Finally, τ2 and τ3, i.e., decay times reflecting donor–acceptor
radiative processes associated with the nanocrystal core and surface,
respectively, were of the order of 0.3 and 1.0 Î¼s. It should
also be noted that for quaternary nanocrystals, the contribution of
surficial processes characterized by τ1 was significantly
smaller than in the case of AIS nanocrystals, dropping
to 1% for A-2 nanocrystals, where the donor–acceptor
radiative recombination mechanism predominated.</p><p>The richest
in zinc A-4 (Ag1.0In10.3Zn12.4S11.8(S28.3))
nanocrystals behaved distinctly different. Its decay curve could be
fitted with two-exponential functions (see Figure 5b,c) yielding τav ∼
0.8 μs. The calculated τ1 and τ2 were 251 and 1039 ns, respectively. Compared to A-(1–3) nanocrystals, the contribution of surficial mechanisms characterized
by τ1 was significantly more pronounced (>60%),
whereas
the contribution of donor–acceptor recombination occurring
in the core diminished to ca. 36% (see Table 2). These distinct differences
between decay curves of A-(1–3) and A-4 and the different nature of its excitation spectrum are perfectly
in line with our already expressed conclusion suggesting different
nucleation mechanisms of these zinc-rich nanocrystals which involves
the formation of ZnIn2S4 rather than AgInS2 as in the case of A-(1–3) nanocrystals.</p><p>Comparison of nanocrystals prepared using the InCl2 precursor
with those synthesized applying InCl3 in identical conditions, i.e., B-1 in the same conditions as A-1 and B-2 in the same conditions as A-4,
clearly showed that the dissimilarity of their spectroscopic properties
originated from significant differences in their chemical compositions
caused by the use of indium precursors of different reactivities. B-1 (Ag1.0In2.8Zn1.3S4.0(S6.0)) and B-2 (Ag1.0In1.5Zn7.8S17.0(S10.5)) emitted red (720 nm) and green (543 nm) light, respectively,
consistent with their different Zn/Ag ratios.54 It should be noted that A-4 and B-2 exhibit
similar emission spectra with maxima at 528 and 542 nm, respectively,
despite different shapes and sizes. In the case of ternary or quaternary
nanocrystals of sizes inferior to their exciton Bohr radii, the luminescence
band position is a complex product of their composition and the quantum
confinement phenomenon. The hypsochromic shift of the photoluminescence
band of A-4 to the green region of the visible spectrum
is most probably caused by the high content of Zn in these quaternary
nanocrystals, although the quantum confinement effect can also interfere,
taking into account their rather small diameter (L = 9.9 ± 2.2 nm, D = 3.3 ± 0.9 nm). The
same two factors influence the position of the photoluminescence band
of spherical B-2 nanocrystals, the effect of quantum
confinement being probably more pronounced (D = 3.3
± 0.9 nm). It is difficult to separate these two factors but
their combination may lead to similar spectra of nanocrystals differing
in composition, shape, and size. The application of more reactive
indium(II) chloride significantly extended the range of nanocrystals
compositions possible to obtain. For example, Zn/Ag ratios could be
varied from 0.3 to 12.4, allowing for precise tuning of the photoluminescence
in the spectral range from 755 to 528 nm.</p><p>To summarize this
part of the research, the use of InCl2 as a new precursor
of indium in the preparation of semiconductors
nanocrystals resulted in the preparation of ternary Ag–In–S
and quaternary Ag–In–Zn–S nanoparticles which
distinctly differed in their morphological and spectroscopic properties
from nanocrystals synthesized using different precursors of indium
and stabilized either with hydrophobic or hydrophilic ligands. Detailed
comparison of spectroscopic properties of nanocrystals prepared in
this research and other ternary and quaternary nanocrystals of this
family, reported in the literature, is presented in Table S1 in the Supporting Information.</p><p>In this respect,
special attention should be paid to extremely
long decay times of τav ∼ 9.4 and ∼1.5
Î¼s, determined for AIS (Ag1.0In1.4S2.5(S2.6)) and A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)) nanocrystals,
respectively. Further investigations of these nanocrystals should
be carried out in view of their promising applications in bioimaging,
photocatalysis, and photovoltaics.68,88</p><!><p>InCl2 precursor
as well as the nanocrystals prepared with its use (AIS, A-1, and A-4) were additionally studied
by X-ray photoelectron spectroscopy (XPS). The resulting survey spectra
are presented in Figures S10 and S11 in
the Supporting Information. In Figure 6a,b, high-resolution XPS (HR-XPS) spectra of indium(II)
chloride are presented. In the XPS In 3d spectrum, only one doublet
is present at 446.2 eV (In 3d5/2) and 453.7 eV (In 3d3/2). The measured binding energy Eb, is distinctly different than that of In(I) (444.9–445.1
eV),89,90 but it falls in the range reported for InCl3 (445.9–446.9 eV)89,90 and In2O3 (444.3–446.7 eV).91,92 Similarly, Eb derived from the XPS Cl
2p spectrum (199.3 eV for Cl 2p3/2) is close to that reported
for InCl3 (199.1 eV) but is clearly different from the
value reported for InCl (198.5 eV).90 The
presence of only one type of indium and chlorine seems to favor the
dimeric form of the precursor (Cl2In–InCl2), characterized by strong contribution of the In–In covalent
bond, rather than its disproportionated form of In(I)In(III)Cl4 where two oxidation states of indium are expected. As seen
from the survey spectrum, the content of carbon and oxygen were of
the order 0.05 wt %. These impurities were identified as water and
organic origin contaminants by recording HR-XPS C 1s and O 1s (see Figure S12 in the Supporting Information).</p><!><p>In 3d (a) and
Cl 2p (b) high-resolution XPS spectra in the surface
region of indium(II) chloride and Ag1.0In1.4S2.5(S2.6) (AIS) nanocrystals. Binding energy of the reference
substances: InCl3, InCl, and In2O3 are also shown.</p><!><p>In Figure 6a, XPS
In 3d spectrum of AIS (Ag1.0In1.4S2.5(S2.6)) nanocrystals is shown. Similar to that in the case of InCl2, only one doublet can be observed, excluding nonequivalence
of indium surfacial atoms. It should be noted that Eb values 444.7 eV (In 3d5/2) and 452.2 eV (In
3d3/2) are lower than the corresponding values in InCl2, indicating oxidation of In(II) to In(III). Indeed, Eb measured for In 3d5/2 is in the
range reported of ternary Ag–In–S and quaternary Ag–In–Zn–S
of different compositions and prepared using In(III)-based precursors
(443.5–445.0 eV).62,93−95 Similar shift by ca. 1 eV toward lower Eb values is observed for residual chlorine (ca. 1 wt %) in AIS (Ag1.0In1.4S2.5(S2.6)) nanocrystals (Figure 6b). The determined values are: 198.3 eV (Cl 2p3/2) and 199.9 eV (Cl 2p1/2). Lower Eb values of In 3d5/2 and Cl 2p3/2 measured for these nanocrystals compared to InCl3 and
InCl seem to indicate the effect of stoichiometric differentiation
of the surface, originating predominantly from the presence of ligands,
which significantly lower the hard acid character of In(III). A similar
phenomenon was reported for InP nanocrystals.35</p><p>In Figure S13 in the Supporting
Information,
Ag 3d, In 3d, and Zn 2p HR-XPS registered for ternary AIS (Ag1.0In1.4S2.5(S2.6)) as well as quaternary A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)) and A-4 (Ag1.0In10.3Zn12.4S11.8(S28.3)) nanocrystals. No significant differences can be seen in
the measured Eb values: Ag 3d5/2 ∼367 eV, In 3d5/2 ∼444.6 eV, and Zn 2p3/2 ∼1021 eV. They all fall in the range reported for
ternary Ag–In–S and quaternary Ag–In–Zn–S
nanocrystals.62,93−95</p><p>Figure 7 shows the
valence band region of the XPS spectra of AIS, A-1, and A-4 nanocrystals. In all three samples,
signals at ∼18.1 and ∼5.4 eV can be seen, originating
from In 4d + C 2s and Ag 4d orbitals. For nanocrystals containing
zinc (A-1 and A-4), an additional peak can
be noticed at 10.2 eV, ascribed to the Zn 3d orbital.69,96,97 It should be noted that the onset
of the least energetic peak in the spectra of AIS and A-1 is located near ∼0.5 eV whereas in the case of A-4 it is shifted to ∼1.1 eV. This finding is consistent
with the differences in their optical band gaps derived from UV–vis–NIR
investigation (see Figure S8 in the Supporting
Information). AIS and A-1 show similar band
gaps (Eg ∼ 2.0 eV), whereas the
band gap of A-4 is significantly larger (Eg ∼ 3.2 eV).</p><!><p>Comparison of the valence bands XPS spectra
of Ag1.0In1.4S2.5(S2.6) (AIS), Ag1.0In1.5Zn0.3S3.3(S3.0) (A-1), and Ag1.0In10.3Zn12.4S11.8(S28.3) (A-4) nanocrystals.</p><!><p>S 2p, C 1s, and O 1s HR-XPS studies are especially useful in analyzing
binding of ligands to the nanocrystal surface. HR-XPS S 2p spectra
of AIS (Ag1.0In1.4S2.5(S2.6)), A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)) and A-4 (Ag1.0In10.3Zn12.4S11.8(S28.3)) nanocrystals are presented in Figure S14a–c, Supporting Information. Spectra of AIS and A-1 can be deconvoluted to two and three doublets,
respectively, indicating certain nonequivalence of surficial sulfur
atoms. The dominant doublet at ∼161 eV (S 2p3/2)
can be ascribed to the structural (S2–) form of
sulfur. The second doublet of weak intensity at 162.9 eV (S 2p3/2) and 162.6 eV (S 2p3/2) in the spectra of AIS and A-1, respectively, should be ascribed
to surficial sulfur forming a covalent bond with carbon atoms of the
ligand (C–S). This type of covalent bonding may be formed during
chemical processes of redox nature occurring in the reaction mixture
and accompanying the nanocrystal growth, as demonstrated previously.44,98 In the case of A-1 nanocrystals, an additional doublet
can be noticed 166.9 eV (S 2p3/2) corresponding to oxidized
forms of surficial sulfur (SOx).44,99</p><p>HR-XPS C 1s spectra (see Figure S14d–f) provide information concerning surfacial ligands. The studied nanocrystals
of different compositions (AIS, A-1, and A-4) exhibit similar spectral features. The dominant signal
at 284.6 eV corresponds to aliphatic carbons of long-chain ligands,
whereas signals in the range of 285.2–285.6 eV can be ascribed
to these carbon atoms of the ligand, which are located in close vicinity
of a functional group. Thus, they can also be attributed to carbons
covalently bound to surfacial sulfur atom (C–S) which is consistent
with the HR-XPS S 2p spectra (vide supra). In the
case of AIS and A-4 nanocrystals, additional
signals can be seen at ca. 286 and 288 eV, which
originate from the presence of ether (C–O) and carbonyl (C=O)
types of carbon.100 O 1s HR-XPS spectra
are presented in Figure S15 in the Supporting
Information. In the case of AIS nanocrystals, only one
signal corresponding to oxygen in organic ligands can be distinguished
in the spectral range of 531.7–531.9 eV. Deconvolution of the
spectra of A-1 and A-4 gives rise to an
additional peak of small intensity at ca. 530 eV
which can be ascribed to In2O3.100</p><p>NMR techniques applied directly to the
identification of organic
ligands in colloidal dispersions of the nanocrystals provide very
limited information due to significant differences in the relaxation
times of different groups of nonequivalent protons. This especially
applies to protons of anchor groups, directly bound to the nanocrystal
surface, for which partial or total quenching of the proton signals
may occur.45,101 Thus, to gain more information
on surfacial ligands, in addition to spectra recorded for dispersions
of AIS, A-1, and A-4 nanocrystals
we registered spectra of organic residue recovered after selective
dissolution of the inorganic core. This approach was previously developed
in our group and applied to the analysis of nanocrystals of different
nature.101,102</p><p>In Figure S16 in the Supporting Information, 1H NMR spectra recorded
for colloidal dispersions of AIS, A-1, and A-4 nanocrystals in
C6D6 are compared with the corresponding spectra
of their organic residue collected after the inorganic core dissolution.
In the case of AIS (Ag1.0In1.4S2.5(S2.6)) nanocrystals, the 1H NMR spectrum of the colloidal
dispersion is very similar to that registered for the organic residue.
In both spectra, only signals characteristic of a long-chain alkene
can be found, namely, multiplets in the following spectral ranges:
1.98–2.04 ppm (−CH2–CH=), 4.91–5.02 ppm (=CH2), and 5.77–5.85 ppm (−CH=). They undoubtedly
originate from the presence of 1-octadecene. Although this compound
was used as a solvent, its presence is not associated with incomplete
solvent removal. It is formed as a product of the decomposition of
nanocrystals stabilizing ligands, according to Scheme 2. This conclusion is supported by the results
of our previous studies confirming the presence of ODE in NMR spectra
of various nanocrystals such as Ag–In–Zn–S45 or Cu2ZnSnS4102 despite the fact that this compound was not
present in the initial reaction mixture. 1H NMR spectra
of colloidal dispersions of A-1 and A-4 nanocrystals
and their organic residues are distinctly different. In the spectra
of the colloidal dispersions, the lines are broadened whereas in the
spectra of organic residues clear multiplets can be distinguished,
allowing for unequivocal identification of the investigated chemical
species. In Figure 8a, 1H NMR spectra of recovered organic residues of AIS, A-1, and A-4 nanocrystals are
presented. In the spectrum of the organic part of A-4, in addition to a multiplet at 1.98–2.04 ppm (−CH2–CH=CH2) originating from the presence of ODE, a triplet appears
2.08 ppm (−CH2COOH), which can be ascribed to stearic
acid (SA). In the spectral range of 2.16–2.21 ppm, a multiplet
is present which can be correlated with the signal at 5.56 ppm (see 1H–1H correlated spectroscopy (COSY) spectrum
in Figure 8b), which
strictly excludes the presence of thiol. To a first approximation,
coupling between these signals might originate from the following
segment −CH2–CH=CH–CH2– characteristic of OLA. However, in view of the absence of
a signal at 2.52 ppm, which is diagnostic of −CH2– adjacent to −NH2, this possibility has
to be excluded. This can be further corroborated by a comparison of
the spectrum of colloidal A-4 nanocrystals with that
of its organic residue (see Figure S16e,f in the Supporting Information).</p><!><p>(a) 1H NMR spectra of the organic
residue from Ag1.0In1.4S2.5(S2.6) (AIS), Ag1.0In1.5Zn0.3S3.3(S3.0) (A-1), and Ag1.0In10.3Zn12.4S11.8(S28.3) (A-4) nanocrystals, 1-octadecene (ODE), stearic
acid (SA), 1-dodecanethiol
(DDT), and oleylamine (OLA) recorded in C6D6, (b) 1H–1H COSY NMR spectrum of the
organic residue (in C6D6) from Ag1.0In10.3Zn12.4S11.8(S28.3) (A-4) nanocrystals.</p><!><p>In the spectrum of
the colloidal dispersion, a broadened signal
at ∼2.2 ppm can be noticed together with another broadened
signal at ∼5.62 ppm, the latter being shifted with respect
to the corresponding signal in the solution spectrum of the organic
residue. This shift can be considered as a spectroscopic manifestation
of binding of the ligand to the nanocrystal surface. In the case of
OLA bound to the nanocrystal surface, this shift is not observed as
the functional group is distant from the binding site. The above short
discussion clearly shows that in the course of the nanocrystal synthesis,
chemical transformation of ligands may occur, leading to new ligands
of different chemical identity. Below, this problem is discussed in
detail.</p><p>It is known that the dissolution of elemental sulfur
in OLA results
in a formation of an active precursor of the following chemical formula
(C18H35NH3+)(C18H35NH-S8–), which can consecutively
be transformed to carboxylic acids and thioacids.103,104 It can therefore be postulated that in the reaction mixture, a thiocarboxylic
acid is formed of the formula R-CH2(C=O)SH, which
gives rise to a multiplet at 2.21 ppm corresponding to the methylene
group (−CH2−) adjacent to the functional
group, whereas the broadened signal appearing at 5.56 ppm has to be
ascribed to −SH bound to the nanocrystal surface.105 A detailed analysis of the 1H–13C heteronuclear multiple bond correlation (HMBC) NMR spectrum
registered for the organic residue of A-4 is presented
in Figure S17 in the Supporting Information
in view of the unequivocal confirmation of the chemical structure
of this thioacid-type ligand. In 1H NMR spectra of all
nanocrystals dispersions (AIS, A-1, and A-4), this broadened signal, originating from R-CH2(C=O)SH, is present in the spectral range of 5.00–5.75
ppm (see Figure S16 in the Supporting Information).
This can be considered as a proof that the ligand is not a side product
of the inorganic core dissolution process but it was formed in situ in the reaction mixture. In the case of AIS and A-1 nanocrystals, the content of this thiocarboxylic
acid ligand is rather small, in contrast to A-4 nanocrystals
where additional broadened and signals are observed at 3.27 and 7.80
ppm. Strong coupling between these signals suggests the presence of
imine-type groupings (−CH2–N=CH−).106 Their formation can be considered as a proof
of hydrogenation of DDT and OLA, yielding 1-aminooctadecane, which
binds to the nanocrystal surface (C18H17NH–S-crystal).
In the inorganic core dissolution process, this ligand is eliminated
in a form of ODE. Chemical transformations occurring in the reaction
during the nanocrystals growth and stabilization are depicted in Scheme 2. The total absence
of signals attributable to DDT and OLA can be considered as an indirect
proof of the validity of the proposed transformation mechanism.</p><p>XPS and NMR results presented here were almost perfectly complementary.
XPS analysis of AIS (Ag1.0In1.4S2.5(S2.0)) clearly indicated the presence of carbonyl groups, despite
the fact that compounds containing this group were present in the
reaction mixture. Through complementary NMR analysis, it was unequivocally
demonstrated that thiocarboxylic acids, i.e., compounds
comprising carbonyl groups were formed in the reaction medium through
transformation of the precursor of sulfur into these chemical entities.
Moreover, these analyses indicated a clear correlation between the
nanocrystal's chemical composition (in particular the content
of sulfur) and the presence of a given type of ligands. AIS (Ag1.0In1.4S2.5(S2.0)) and A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)) nanocrystals, enriched in sulfur as seen from the elemental
EDS analysis, could be characterized by an equilibrated surface and
were stabilized by crystal-bound C18H17NH–S-crystal
ligands. Upon dissolution of the inorganic core these ligands were
eliminated in a form of ODE. NMR spectra of organic residues of these
nanocrystals revealed only minute amounts of typical coordinating
ligands containing thiocarboxylic anchor groups. In the case of A-1, signals ascribed to sulfur of higher oxidation states
could also be observed.</p><p>Elemental analysis of A-4 (Ag1.0In10.3Zn12.4S11.8(S28.3))
nanocrystals clearly indicated that they were strongly cation-enriched,
representing unequilibrated surface. NMR spectra of their organic
residues revealed the presence of thiocarboxylates and stearates as
ligands bound to indium and zinc, respectively. Although there exist
several reports on coordination compounds of indium(III) and thiocarboxylate
ligands,107 no indium-containing inorganic
nanocrystals capped with thiocarboxylates have been reported to date.</p><p>In the case of B-1 (Ag1.0In2.8Zn1.3S4.0(S6.0)) and B-2 (Ag1.0In1.5Zn7.8S17.0(S10.5)), i.e., nanocrystals prepared
under the same experimental conditions as A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)) and A-4 (Ag1.0In10.3Zn12.4S11.8(S28.3)) nanocrystals,
respectively, but using less reactive InCl3 precursor instead
of the InCl2 one, the NMR analysis revealed two different
types of stabilizing ligands. These were stearate anions originating
from the precursor of zinc and 1-aminooctadecane ligands being the
product of OLA hydrogenation and acting as "crystal-bound"
or "surface-bound" ligands.45</p><p>To summarize this part of the paper, application of a more
reactive
indium precursor, namely, InCl2 instead of InCl3, results in the formation of nanocrystals capped with thiocarboxylate
anions as ligands. It should be noted here that carboxylate ligands
have not been detected in nanocrystals obtained from the InCl3 precursor under the same experimental conditions. Thiocarboxylates
are less hard bases than stearate anions which stabilize nanocrystals
obtained with InCl3. Surfacial In(III) ions in the resulting
Ag–In–Zn–S quaternary nanocrystals constitute
an example of less hard acids compared to bulk (macrocrystalline)
indium ions. Moreover, the use of this more reactive InCl2 precursor allows for significant extension of the obtainable nanocrystals
compositions and, by consequence, facilitates precise tuning of the
nanocrystals photoluminescence spectra.</p><!><p>In the last few years, several
papers appeared devoted to the application of inorganic semiconductors
nanocrystals as photocatalyst of many important reactions, mainly
the reduction ones108,109 and C–C coupling reactions.110 In the majority of cases, cadmium chalcogenide
nanocrystals, including core/shell ones, were used for this purpose.111 The most important drawback of the above-mentioned
photocatalysts is their toxicity. However, a few reports of the use
of nontoxic nanocrystals, such as core–shell binary InP/ZnS,
appeared in the literature.112 It is therefore
tempting to verify whether alloyed or nonstoichiometric ternary (Ag–In–S)
and quaternary (Ag–In–Zn–S) can be used as photocatalysts.
Apart from nontoxicity, the great advantage of this family of nanocrystals
is the possibility of tuning their optical properties in a wide spectral
range.</p><p>In the research described here, AIS (Ag1.0In1.4S2.5(S2.0)) and A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)) were tested as photocatalysts in aldehyde reduction reactions,
following the procedure elaborated previously for CdSe/CdS core/shell
nanocrystalline photocatalysts.109 Thiols
are frequently used as reducing agents, which readily bind to the
nanocrystal surface. Two aldehydes were selected for test reactions,
namely, 4-chlorobenzaldehyde and 4-methoxybenzaldehyde. The same reaction
set was used in all four cases: CH3COOCs/p-toluenethiol/nanocrystals (AIS or A-1)
in benzene-d6. This solvent was purposely
used with the goal of monitoring the changes in the reaction mixture
by 1H NMR. The catalytic tests were carried out at room
temperature using an LED monochromatic source (Î» = 528 nm) of
10 W power (see Figure 9a,b).</p><!><p>(a) Setup for investigation of photocatalytic reactions under 10
W green LED (Î» = 528 nm) irradiation, (b) photocatalytic reduction
of aldehydes to alcohols, (c) 1H NMR spectra of the photocatalytic
reduction reaction mixture used for photocatalytic reduction of 4-chlorobenzaldehyde
or 4-methoxybenzaldehyde with AIS (Ag1.0In1.4S2.5(S2.0)) or A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)) nanocrystals
as photocatalysts (in C6D6). All four processes
were carried out under the same conditions.</p><!><p>In Figure 9c, 1H NMR spectra of reaction mixtures, exposed to green light
irradiation, for 5 h, are compared. For 4-chlorobenzaldehyde and 4-methoxybenzaldehyde,
the diagnostic −CHO singlets can be distinguished at 9.55 and
9.79 ppm. Lines at 4.28 and 4.49 ppm correspond, in turn, to methylene
protons in the −CH2OH units of the reaction products.
The ratio of the integrated intensities of these lines allows for
direct determination of the conversion degree. AIS (Ag1.0In1.4S2.5(S2.0)) is a rather inefficient photocatalyst,
and the conversion of aldehyde to alcohol is ∼1% for 4-chlorobenzaldehyde.
No conversion products were detected in the case of 4-methoxybenzaldehyde.</p><p>Significantly higher photocatalytic activity was measured for A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)). In the reaction of 4-chlorobenzaldehyde
reduction, the conversion degree reached 50%, whereas in the case
of 4-methoxybenzaldehyde, it was equal to 10%. Thus, these preliminary
results confirmed the photocatalytic activity of both types (AIS and A-1) nanocrystals. Moreover, they demonstrated
the inactivity of thiols as reducing agents under these experimental
conditions. The observed differences in the photocatalytic activity
of AIS and A-1 could be rationalized by
inspection of their excitation spectra (see Figure 4). The spectrum of AIS (Ag1.0In1.4S2.5(S2.0)) presents a narrow peak with
a clear maximum at 625 nm, whereas a broad band, covering the spectral
range from 300 to 600 nm and comprising green radiation, can be noticed
in the spectrum of A-1 (Ag1.0In1.5Zn0.3S3.3(S3.0)). Finally, a clear
substituent effect on the photocatalytic activity should be noted.
The presence of an electron accepting substituent (chlorine) in the
para position promoted the photocatalytic reduction. On the contrary,
electron-donating methoxy substituent in the same position lowered
the photocatalytic activity of AIS and A-1 nanocrystals in the same experimental conditions as those reported
for CdSe/CdS photocatalysts.109 For comparative
reasons, we performed the same catalytic tests using B-1 nanocrystals as photocatalysts and carried out the experiments under
identical conditions. It should be noted that the emission spectrum
of B-1 nanocrystals was very similar to that measured
for A-1. This could imply similar photocatalytic properties.
However, no reduction products were detected in this case B-1 sample (see Figure S18, Supporting Information),
clearly demonstrating catalytic inactivity of these nanocrystals and
advantageous properties of nanocrystals prepared from the InCl2 precursor. Finally, it should be noted that the presented
results of photocatalytic studies should be treated as preliminary.
Further investigations are needed leading to the optimization of the
catalyst composition and reaction conditions.</p><!><p>A new indium precursor, namely, indium(II) chloride in its dimeric
form (In2Cl4) turned out to be significantly
more reactive in the process of ternary (Ag–In–S) and
quaternary (Ag–In–Zn–S) nanocrystals preparation
compared to traditionally used indium(III) chloride. For low and moderate
concentrations of zinc precursor (zinc stearate), the conversion of
the new In2Cl4 precursor was comparable to that
of the traditional InCl3 one. For the highest concentrations
of zinc stearate, the conversion of the new indium precursor was significantly
higher than that of InCl3, indicating different nucleation
mechanisms. In the case of low zinc stearate concentrations, AgInS2 germs were formed as a consequence of high reactivities of
silver and indium precursors, the resulting nanocrystal were spherical
in shape. Increasing concentration of zinc stearate led to the formation
of ZnIn2S4 germs and longitudinal quaternary
Ag–In–Zn–S nanocrystals in the crystal growth
step.</p><p>The observed changes in the composition, shape, and size
of nanocrystals
originated from chemical transformations of the reaction mixture leading
to ligands of different chemical nature. Ternary Ag–In–S
nanocrystals and quaternary Ag–In–Zn–S nanocrystals
of low indium and zinc contents were capped with ligands covalently
bound to structural sulfur atoms (C18H17NH–S-crystal), i.e., ligands formed in situ in the reaction
mixture as a result of its chemical transformations. Quaternary Ag–In–Zn–S
nanocrystals of high indium and zinc contents were capped with thiocarboxylate
and stearate anionic ligands bound to indium and zinc cations, respectively.
Again, thiocarboxylate ligands were formed as products of the reaction
mixture chemical transformations.</p><p>Nanocrystals obtained with
the highly reactive In2Cl4 indium precursor
exhibited interesting photoluminescent properties.
First, their photoluminescence color could be controllably tuned in
the spectra range from 730 to 530 nm, always exhibiting the PLQY values
in the range of 20–40%. Second, for ternary Ag–In–S
nanocrystals enriched in indium, the longest, ever reported for this
family of compounds photoluminescence lifetime of τav ∼ 9.4 μs, was found. A long photoluminescence lifetime
of ∼1.4 μs was also measured for quaternary Ag–In–Zn–S
nanocrystals. Finally, quaternary nanocrystals prepared from the InCl2 precursor turned out to be promising photocatalysts in the
reaction of aldehydes reduction to alcohols.</p><!><p>Silver nitrate (99%), indium(II) chloride
(99%) indium(III) chloride (98%), zinc stearate (technical grade),
1-dodecanethiol (DDT, 98%), sulfur (99%), 1-octadecene (ODE, 90%),
oleylamine (OLA, 70%), and benzene-d6 (100%,
99.6 atom % D) were supplied by Sigma-Aldrich.</p><!><p>Sulfur powder (15
mg, 0.47 mmol) and OLA (1.0 mL) 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.</p><!><p>All operations were carried out under constant
dry argon flow. Silver nitrate (0.03 g, 0.18 mmol), indium(II) chloride
(0.11 g, 0.59 mmol), DDT (0.20 g, 1.0 mmol) and zinc stearate (0.11–1.11
g, 0.18–1.8 mmol) were mixed with ODE (15 mL) in three-neck
flask. The mixture was heated to 150 °C until a homogeneous solution
was formed. Then, 1 mL of S/OLA 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 (10 mL) was added,
and the reaction mixture was centrifuged—the isolated black
precipitate consisting of organic waste and agglomerated particles
was separated (however, usually no solids were isolated in this step).
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).</p><!><p>A colloidal solution of nanocrystals
(in 10 mL of toluene) 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 10 mL chloroform. The combined organic extracts
were washed two times with water, evaporated, and dried under reduced
pressure.</p><!><p>To a solution
of 4-chlorobenzaldehyde (0.6 mmol, 84 mg) or 4-methoxybenzaldehyde
(0.6 mmol, 82 mg), CH3COOCs (0.1 mmol, 20 mg), and p-toluenethiol (3.0 mmol, 372 mg) in benzene-d6 (4 mL) were added Ag1.0In1.4S2.5(S2.0) (AIS) (∼10
mg in 1 mL of benzene-d6) or Ag1.0In1.5Zn0.3S3.3(S3.0) (A-1) (∼10 mg in 1 mL of benzene-d6) nanocrystals. The vial was sealed and purged with argon
for 15 min before illumination by a 10 W green LED (Î» = 528
nm).</p><!><p>Characterization
methods, photographs of the reaction
mixtures, EDS spectra, TEM and HR-TEM images of Ag–In–S
and Ag–In–Zn–S nanocrystals, UV–vis–NIR,
photoluminescence and decay curves of Ag–In–Zn–S
nanocrystals, XPS survey and HR-XPS spectra of indium precursor and
Ag–In–S and Ag–In–Zn–S nanocrystals, 1H NMR spectra of C6D6 dispersion of
Ag–In–S and Ag–In–Zn–S nanocrystals
capped with initial ligands and ligands recovered after dissolution
of nanocrystals, 1H–13C HMBC NMR spectrum
of the organic residue from Ag–In–Zn–S nanocrystals,
detailed comparison of spectroscopic properties of nanocrystals prepared
in this research and other ternary and quaternary nanocrystals reported
in the literature (PDF)</p><p>cm1c03800_si_001.pdf</p><!><p>P.K. and
P.B. designed the concepts. P.K. prepared the samples of nanocrystals.
P.K. carried out the catalytic tests. M.P. recorded the NMR spectra.
A.M.M. performed the PL and TCSPC experiments. A.O. made the X-ray
measurements. A.K. and M.G. made the TEM and HR-TEM measurements.
W.L. and J.W.S. measured the XPS spectra. P.K., P.B., and M.P. discussed
the results. P.K., P.B., M.P., and A.P. wrote the manuscript.</p><!><p>The authors declare no
competing financial interest.</p>