Transition Metal-Based Thiometallates as Surface Ligands for Functionalization of All-Inorganic Nanocrystals

Article abstract of DOI:10.1021/acs.chemmater.7b04387

We report a new family of inorganic ligands, namely, transition metal-based thiometallates, for the surface functionalization of colloidal nanocrystals (NCs). We synthesized Pt-, Fe-, Co-, and Ni-based thiometallates, in which transition metal ions were complexed with polysulfides. These inorganic anions easily exchanged the surface organic ligands of various nanocrystals of metal, semiconductor, and oxide materials, without affecting the NCs' primary structural and optical characteristics. Furthermore, upon heating, these complexes were decomposed and transformed into crystalline phases of metal sulfides or pure metals, accompanied by the evaporation of S. Based on this effect, we selectively synthesized homogeneously distributed atomic Pt clusters or Pt nanoparticles on Fe₃O₄ nanomaterials by heating thioplatinate-capped Fe₃O₄ NCs. As a model application, we tested the prepared Pt-functionalized Fe₃O₄ nanomaterial as a heterogeneous catalyst for CO oxidation reaction and Pt-Fe₃O₄ catalysts exhibited the high turnover frequency due to the homogeneous distribution of atomic Pt over Fe₃O₄ and the corresponding strong metal-support interaction. This approach opens up a new avenue to functionalize nanocrystals for catalytic applications.

Full text of DOI:10.1021/acs.chemmater.7b04387

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Article
Transition Metal-Based Thiometallates as Surface Ligands
for Functionalization of All-Inorganic Nanocrystals
Hyewon Jeong, Sinmyung Yoon, Jung Hwa Kim, Do-Hyun Kwak, Da Hwi
Gu, Seung Hwae Heo, Hyunhong Kim, Sangmin Park, Hyeong Woo Ban,
Jongnam Park, Zonghoon Lee, Jong-Soo Lee, Kwangjin An, and Jae Sung Son
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04387 • Publication Date (Web): 22 Nov 2017
Downloaded from http://pubs.acs.org on November 25, 2017
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Page 1 of 9
Chemistry of Materials
Transition Metal-Based Thiometallates as Surface Ligands for Func-
tionalization of All-Inorganic Nanocrystals
Hyewon Jeong,† Sinmyung Yoon,‡ Jung Hwa Kim,† Do-Hyun Kwak,§ Da Hwi Gu,† Seung Hwae Heo,†
Hyunhong Kim,‡ Sangmin Park,† Hyeong Woo Ban,† Jongnam Park,‡ Zonghoon Lee,† Jong-Soo Lee,§
Kwangjin An,*,‡ and Jae Sung Son*,†
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†School of Materials Science and Engineering and ‡School of Energy and Chemical Engineering, Ulsan National Institute of
Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea.
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§Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu,
42988, Republic of Korea.
ABSTRACT: We report a new family of inorganic ligands, namely, transition metal-based thiometallates, for the surface function-
alization of colloidal nanocrystals (NCs). We synthesized Pt-, Fe-, Co-, and Ni-based thiometallates, in which transition metal ions
were complexed with polysulfides. These inorganic anions easily exchanged the surface organic ligands of various nanocrystals of
metal, semiconductor, and oxide materials, without affecting the NCs’ primary structural and optical characteristics. Furthermore,
upon heating, these complexes were decomposed and transformed into crystalline phases of metal sulfides or pure metals, accompa-
nied by the evaporation of S. Based on this effect, we selectively synthesized homogeneously distributed atomic Pt clusters or Pt
nanoparticles on Fe3O4 nanomaterials by heating thioplatinate-capped Fe3O4 NCs. As a model application, we tested the prepared Pt-
functionalized Fe3O4 nanomaterial as a heterogeneous catalyst for CO oxidation reaction and Pt-Fe3O4 catalysts exhibited the high
turnover frequency due to the homogeneous distribution of atomic Pt over Fe3O4 and the corresponding strong metal-support inter-
action. This approach opens up a new avenue to functionalize nanocrystals for catalytic applications.
INTRODUCTION
chalcogenide ligands can open up new opportunities for all-in-
organic NCs, since transition metals possess catalytic activity
by having multiple oxidation states.20-22 For example, atomic-
to nano-scale transition metals on the surface of oxides can
work well as supported heterogeneous catalysts,9,10 because of
their high surface-to-volume ratio as most catalytic reactions
occur on the surfaces. Recently, single-site atomic Pt on metal
oxide support has gained significant attention in the field of ca-
talysis, owing to the maximized use efficiency.23-27
Colloidally synthesized nanocrystals (NCs) have attracted tre-
mendous attention in various disciplines, due to their monodis-
persity and tunable sizes and shapes, which determine their in-
trinsic electronic, optical, and magnetic properties.1-3 Generally,
the surfaces of colloidal NCs are sterically stabilized by organic
capping ligands with long hydrocarbon chains (e.g., oleic acid,
oleylamine, trioctylphosphine oxide), which play a key role
during the formation of NCs in the synthesis stage and and in
ensuring easy dispersion of the prepared NCs in nonpolar or-
ganic media.4 However, these sterically bulky ligands some-
times block the access of foreign molecules. Therefore, post-
treatments such as calcination are required to expose the sur-
faces of NCs for catalytic applications.5-7 Furthermore, the elec-
trically insulating characteristics of organic ligands restrict the
application of colloidal NCs in solid-state electronic devices.4,8
Hence, intensive research efforts are being devoted to designing
surface ligands to provide new functionality to colloidal NCs
and expand their application areas.4,9
In recent years, surface ligands consisting of soluble anionic
inorganic compounds have been developed for colloidal NCs.10
These inorganic ligands easily exchange the organic ligands on
the surface of NCs to generate all-inorganic NCs, which exhibit
dramatically enhanced device performance in arrays due to the
strong electronic coupling. So far, various classes of inorganic
ligands have been suggested: chalcogenidometallates,11 metal-
free anions12-14 (e.g. SCN-, S2-, Cl-), and oxoanions.15 Among
these, chalcogenidometallates have been intensively studied us-
ing hydrazine-based chemistry due to their semiconducting
functionality, by dissolving metal chalcogenide powders in hy-
drazine in the presence of excess chalcogens.16-19 While many
classes of such materials could be prepared using this approach,
however, the majority of the reported soluble chalcogenido-
metallates are based on post-transition metals including In, Ga,
Sn, and Sb as the metal center, which restrict the application of
all-inorganic NCs in electronic devices. Transition metal-based
Figure 1. (a) Molecular structure of synthesized Na2[FeS6]2. (b)
Photographs showing synthesized thiometallates. (c) UV-Vis ab-
sorption spectra of transition metal-based thiometallates and
(NH4)2S solutions. (d) Schematic illustration of the ligand ex-
change process with thiometallates.
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Table 1. Nanocrystals, thiometallate ligands, and dispersible solvents for NCs capped with these ligands.
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NCs
Thiometallates
Solvents
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(NH4)2PtS15, (NH4)2CoS9, (NH4)2NiS6
Na2Fe2S12, (NH4)2Fe2S12
NMF, DMF, DMSO, FA
NMF, DMF
PbS, CdSe, CdS, FePt, Au, Bi,
Fe3O4
(NH4)2PtS15, Na2Fe2S12, (NH4)2Fe2S12, (NH4)2CoS9, (NH4)2NiS6
DMF
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In this work, we synthesized transition-metal based thiometal-
lates containing Pt, Fe, Co, and Ni and systematically studied
the general use of these anions as capping ligands for various
colloidal NCs. To introduce transition metals for the metal cen-
ters, we revisited the old coordination chemistry28-30 where tran-
sition metal ions are directly reacted with polysulfide, instead
of reducing bulk metal chalcogenide powders. The synthesized
compounds were thiometallates, in which each transition metal
ion is chelated with several sulfur atoms in polysulfides (Figure
1a and b). We also studied the heating-induced molecules-to-
crystal transition in these thiometallates upon heating under var-
ious environments, which could introduce transition metal
phases on oxide NC surfaces.
Next, we illustrate potential applications of this inorganic lig-
and chemistry for nano-catalysts. The combination of NC oxide
support and transition metal-based ligands provides a great op-
portunity to design heterogeneous catalysts with transition
metal active sites. For example, selective formation of atomic
Pt clusters or Pt nanoparticles was achieved on Fe3O4 NCs after
thermal treatment of thioplatinate-capped Fe3O4 NCs in air, and
the products can be applied to various catalytic reactions such
as oxygen-reduction reaction in fuel cells and water splitting.
Accordingly, we applied our prepared materials for catalytic
CO oxidation as a model reaction, and a high turnover fre-
quency (TOF) was achieved.
chloride (99.99%, Aldrich), sulfur (99.998%, Aldrich), octyla-
mine (99%, Aldrich), lead(II) chloride (PbCl2, reagent grade
99%, Alfa Aesar) were used as received.
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Synthesis of thiometallate.
Thioplatinate complex was prepared according to the proce-
dure reported by Wickenden et al.28 with modifications in reac-
tion time and purifying solvents. 5 mL of (NH4)2S 45-48 wt%
aqueous solution was diluted with 5 mL of distilled water while
stirring. 3.0 g of sulfur was added to the mixture and stirred until
it became clear red solution. 1.25 mL of chloroplatinic acid so-
lution was then added dropwise to the polysulfide solution. It
was then stirred until it the solution became turbid. The mixture
was precipitated by centrifugation and the precipitates were pu-
rified with a small amount of methatnol (MeOH) and excess
toluene for three times. The final product was re-dispersed in
10 mL of N-Methylformamide (NMF) or N,N-Dimethylforma-
mide (DMF) and filtered with a 0.20 μL PTFE filter.
Thioferrite complex was synthesized following the procedure
reported by Strasdeit et al.29 with modifications as follows. In a
N2-filled glove box, FeCl2 (1 mmol), sulfur (6 mmol), and so-
dium (3 mmol) in 5 mL of DMF were stirred at 70 oC for 6 hours
and cooled to RT. 3 mL of methanol (MeOH) was then added
to the mixture and was precipitated by centrifugation (at 4000
rpm for 3 min). The supernatant was stirred with excess MeOH
(up to 40-50 mL) overnight. It was then precipitated by centrif-
ugation at 7000 rpm for 10 min. The precipitates were purified
with 10 mL of MeOH followed by centrifugation for two times.
The final product was re-dispersed in NMF and was further
used after filtering with a 0.20 μL PTFE filter.
EXPERIMENTAL SECTION
Materials. Ammonium sulfide solution ((NH4)2S, 40-48 wt%
in H2O, Aldrich), ammonium chloride (NH4Cl, 99,0%, Sam-
chun Chemical), amberlyst® 15 hydrogen form (Aldrich), cad-
mium oxide (CdO, ≥99.99%, Aldrich), borane t-butylamine
complex (97%, Aldrich), chloroplatinic acid solution (H2PtCl6,
8 wt% in H2O, Aldrich), trioctylphosphine oxide (TOPO, 99%,
Aldrich), tetradecylphosphonic acid (TDPA, 97%, Aldrich), oc-
tadecylphosphonic acid (ODPA, 97%, Aldrich), selenium
(powder, 99.99%, Aldrich), bismuth neodecanoate (technical
grade, Aldrich), 1-octadecene (ODE, 90%, Aldrich), 1-dodec-
anethiol (DDT, 98%, Aldrich), trioctylphosphine (TOP, 90%,
Aldrich), gold(III) chloride trihydrate (HAuCl4∙3H2O, 99.9%,
Aldrich), 1,2,3,4-tetrahydronaphthalene (tetralin, 97%, Alfa
Aesar), iron chloride (FeCl2, 98%, Aldrich), Iron(III) chloride
hexahydrate (ACS reagent, 97%), sodium oleate (≥82%, Al-
drich), nickel(II) acetate tetrahydrate (98%, Aldrich), nitro-
sonium tetrafluoroborate (NOBF4, 97%, Acros), cobalt(II) ace-
tate tetrahydrate (reagent grade, Aldrich), sodium (ACS reagent,
dry, Aldrich), platinum(II) acetylacetonate (Pt(acac)2, 99.99%,
Aldrich), iron(III) acetylacetonate (Fe(acac)2, 99.95%, Aldrich),
octyl ether (99%, Aldrich), 1,2-Hexanedecanediol (90%, Al-
drich), oleylamine (OAm, approximate C18-content 80-90%,
Acros organics), oleic acid (OA, 90%, Aldrich), tetrae-
thylammonium bromide (TEAB, 99%, Aldrich), cadmium
Thionickelate complex was synthesized according to the pro-
cedure reported by Müller et al.30 with modifications in polysul-
fide solution. 5 mL of (NH4)2S solution (45-48 wt%) was di-
luted with H2O to a 50 % solution and 1g of sulfur was added
to the solution. 0.8 g of TEAB was dissolved in 10 mL of the
mixture. 0.4 g of nickel(II) acetate tetrahydrate was then added
step-wise and the mixture was heated to 45 oC while stirring. As
it reached the temperature, the mixture was quickly precipitated
by centrifugation at 4000 rpm for 3 min. The supernatant was
kept in a closed conical tube for an hour. No crystals were ob-
served. The mixture was then purified using 2-propanol (IPA)
and methanol (MeOH) for three times and was finally dispersed
in 10 mL of NMF or DMF.
Thiocobaltate complex was synthesized by using a slightly
modified procedure from the synthesis of thionickelate. The
ammonium sulfide aqueous solution was diluted to half with
MeOH since cobalt precursor possessed higher solubility in
MeOH than water. 0.38 g of cobalt (II) acetate tetrahydrate was
o
added step-wise and the solution was stirred at 50 C. As it
reached the temperature, the mixture was quickly precipitated
by centrifugation at 4000 rpm for 3 min. The supernatant was
kept in a closed conical tube for an hour. No crystals were ob-
served. The mixture was then purified using excess 2-propanol
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Chemistry of Materials
(IPA). The final solution was dispersed in 10 mL of either NMF
toluene (DMF:Toluene = 1:2) for two times and was finally dis-
persed in DMF. Meanwhile, the thioplatinate ligand solution
was dispersed in DMF and followed by filtration. Each solution
was then mixed together and sonicated for the interaction be-
tween slightly positive surface charge over Fe3O4 and the ani-
onic thioplatinate ligand.
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or DMF.
The cation exchange from Na+ to NH4+ or vice versa was con-
ducted following the procedure described in the literature re-
ported by Zhang et al.14 Acid beads (Amberlyst® 15 hydrogen
form, Aldrich) was immersed in 30 mL of NH4Cl (0.2 M~0.3
M, pH: ~4-5) and sonicated for over 10 min. Resin-beads were
collected after centrifugation. The pH value of the supernatant
almost reached ~2. The beads were dried under vacuum after
washing with D.I water. The NCs capped with thioferrite ligand
was dispersed in NMF. This mixture was vigorously vortexed
Characterization. Transmission electron microscopy (TEM)
images of nanocrystals (NCs) were collected using a JEOL-
2100 instrument operated at 200 kV. The samples dispersed in
non-polar solvents were prepared by drop-casting on carbon-
coated TEM grids and dried under air while the samples dis-
persed in polar solvents were dried under vacuum. STEM-EDS
and HAADF-STEM characterization was conducted in a Cs
probe aberration-corrected FEI Titan Cube TEM with an elec-
tron monochromator operated at 80 kV. X-ray diffraction (XRD)
patterns were obtained using a Rigaku D/Max2500V diffrac-
tometer equipped with a Cu-rotating anode X-ray source. Elec-
tronic states of Pt species were obtained using a ThermoFisher
K-alpha X-ray photoelectron spectrometer. The absorption
spectra of colloidal NCs and ligands were acquired using a Cary
5000 UV-Vis-NIR spectrophotometer. Photoluminescence (PL)
spectra of colloidal solutions in the wavelength range of 500-
700 nm were obtained using a Cary Eclipse fluorescence. Fou-
rier transform infrared (FT-IR) spectra were obtained in ATR
mode using a 670/620 varian FT-IR spectrometer with averag-
ing over 32 scans. ζ-potential data were collected using a Mal-
vern Zetasizer Nano-ZS90. Elemental analysis by inductively
coupled plasma optical emission spectrometers (ICP-OES) was
carried out using a varian 700-ES. Thermogravimetric analysis
(TGA) was conducted using a TA-Q500 thermal analyzer with
a heating rate of 10 oC/min under air or under nitrogen.
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+
with NH4 -resin for 15-20 min. It was then precipitated by cen-
trifugation and washed with toluene. The final product was dis-
persed in 10 mL of NMF or DMF.
Synthesis of organics-capped NCs.
16-20 nm-sized Fe3O4 NCs were synthesized by the method
reported by Park et al.31 In a typical synthesis, iron oleate (10
mmol), and oleic acid (7 mmol) were dissolved in 30 g of 1-
octadecene. The mixture was heated to 320 C for 30 min with
a heating rate of 3.3 C/min and was cooled to RT under inert
o
o
atmosphere. The product was collected using ethanol by cen-
trifugation and was dispersed in hexane.
6.7 nm-sized PbS NCs were synthesized according to the re-
ported literature by Weidman et al.32 with slight modifications.
The sulfur precursor was prepared by using a heating mantle
with a heating rate of 5 oC/min for 20 min. As it reached 120 oC,
the orange and amber-like mixture was cooled to RT.
6.0 nm-sized Au NCs were synthesized following the reported
literature by Zhu et al.33 with a few modifications. Here, the 0.2
g of HAuCl4 was dissolved in 10 mL of tetralin and 10 mL of
oleylamine at room temperature under N2 flow.
11 nm-sized Bi NCs were synthesized according to the method
developed by Son et al.34 with slight alterations. Acetone was
only used as anti-solvent instead of tetrahydrofuran (THF).
RESULTS AND DISCUSSION
The synthesis of Pt-, Co-, and Ni-based thiometallates was
conducted using a modified version of the reported synthetic
route for thioplatinate.28 Typically, aqueous solutions of metal
ions were reacted with ammonium polysulfide at near room
temperature. Thioferrite was synthesized by an alternative route
in which FeCl2 precursor was reacted with elemental S in the
presence of Na as a reducing agent.29 The produced thiometal-
lates had (NH4)+ or Na+ as counter ions, which could be ex-
changed by a cationic exchange reaction. The metal-to-sulfur
ratio spans a large range depending on the metal center. For ex-
ample, this ratio in thioferrite and thioplatinate analyzed by in-
ductively coupled plasma optical emission spectroscopy (ICP-
OES) is 1:6 and 1:15, respectively, corresponding to literature
values.28,29 On the other hand, thiocobaltate and thionickelate
synthesized by the modified route for thioplatinate showed the
respective ratios of 1:9 and 1:6. The synthesized compounds
were characterized by UV-Vis absorption spectroscopy. As
shown in Figure 1c, thioplatinate and thioferrite exhibited ab-
sorption peaks at 391 and 351 nm, respectively. Thiocobaltate
and thionickelate also displayed broad absorption bands
throughout the visible region with peaks at 360 nm. These peaks
are completely different from that observed in the spectrum of
(NH4)2S (a single peak at 372 nm), demonstrating the formation
of thiometallates.
1.2 nm-thick CdS nanoplates were synthesized following the
same experimental procedure reported by Son et al.35
2.5 nm-sized FePt NCs were synthesized following the same
experimental procedure reported by Liu et al.36
CdSe NCs capped with ODPA were prepared according to pre-
viously reported procedures.37
Ligand exchange with thiometallate.
The ligand exchange process was conducted in a N2-filled
glovebox. Thiometallate complexes was dispersed in 7~10 mL
of NMF. For the typical ligand exchange process, 2 mL of lig-
and solution formed the bottom layer and the NCs dispersed in
hexane formed the upper layer. This two-phase mixture was
vigorously stirred for at least 30 min until the upper layer be-
came clear as shown in Figure S1. The transferred phase into
the bottom layer was purified with IPA (NMF:IPA ~ 1:3) when
the exchanging ligands were thioplatinate and thioferrite. In-
stead of IPA, acetonitrile was used as purifying solvent for the
rest of thiometallate ligands (NMF:ACN ~ 1:0.5). The mixture
was precipitated by centrifugation (13,000 rpm, 5 min) for more
than three times. The possible final dispersants are summarized
in Table 1. The ligand exchange process for Fe3O4 NCs was
performed by modified two-step method from the literature re-
ported by Dong et al.38 The organics-Fe3O4 was dispersed in
hexane and formed the upper layer and 120 mg of NOBF4 dis-
persed in 2 mL of DMF formed the bottom layer. It was then
stirred overnight. The ligand stripped Fe3O4 was washed with
The ligand exchange of organics-capped NCs with the current
thiometallates was conducted using a typical two-phase reac-
tion (Figure 1d). We studied a variety of combinations of NCs
(such as semiconductors, metals, and oxides) and thiometallate
ligands. This two-phase reaction worked very well for PbS,
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Chemistry of Materials
CdSe, CdS, Au, FePt, and Bi (Figure 3a and Figure S1 in Sup-
Page 4 of 9
the thioplatinate-capped CdS nanoplates (Figure 4b), which can
be attributed to the electronic coupling between the exciton
states of CdS nanoplates and thioplatinate, since thioplatinate
has a strong absorption band at 391 nm (Figure 1c). On the other
hand, the emission properties of CdSe NCs almost disappeared
after the ligand exchange process (Figures 4c and S6). The ob-
served PL quenching phenomenon on CdSe NCs after the lig-
and exchange reaction can be attributed to the formation of mul-
tiple surface defects on NCs, which agrees with the previous
studies on inorganic ligand-capped CdSe NCs.39 Recently, our
group reported the recovery of PL properties of all-inorganic
NCs via a photo-oxidation process, in which a robust oxidized
surface is formed on NCs to cure the surface defects.39 Likewise,
we performed the photo-oxidation treatment on PtS, FeS, CoS,
NiS thiometallates-capped CdSe NCs (Figure 4c and Figure S6).
The PL spectra showed greatly enhanced emission properties in
all cases, and these observations are in agreement with those of
previously reported MoS42- and WS42--capped CdSe NCs.39 Un-
like other NCs, the PL peak position of thioferrite-capped CdSe
NCs was shifted to a lower wavelength (Figure S6a), which in-
dicates the reduction in the NC size. This shift can be attributed
to the etching effect from residual Cl- ion in thioferrite, since
the precursor for thioferrite was FeCl2. In our previous report, a
significant blue shift was also observed in the PL spectrum of
Cl--capped CdSe NCs after the photo-oxidation process.39
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porting Information). Regardless of the NCs and thiometallate
ligands, all NCs exhibited negative zeta (ζ) potentials after lig-
and exchange. For example, the thiometallate-capped PbS NCs
showed ζ potentials that ranged from -10.0 to -31.4 mV, demon-
strating the attachment of thiometallates on the NC surface (Fig-
ure 2a). On the other hand, we found that Fe3O4 NCs dispersed
in hexane were not transferred into the polar phase. Huang, et
al. reported that ligand exchange of NCs with less nucleophilic
anions can be achieved by a two-step procedure: stripping the
organic capping ligand with tetrafluoroborate followed by the
attachment of inorganic ligands on the NC surface.15,38 We con-
ducted similar two-step reactions for the ligand exchange of
Fe3O4 NCs to obtain stable colloidal solutions of thiometallate-
capped Fe3O4 NCs in N,N-dimethylformamide (DMF) (Figure
S2). The ζ potentials of ligand-stripped Fe3O4 and PtS-Fe3O4 are
shown in Figure 2b, as +19.4 and –11.6 mV, respectively,
demonstrating the attachment of negatively charged thiometal-
lates. All studied combinations of NCs and thiometallate lig-
ands are summarized in Table 1.
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Figure 2. ζ potentials measured for (a) thiometallates-capped PbS
NCs in NMF, and (b) ligand-stripped and thioplatinate-capped
Fe3O4 NCs in DMF.
Transmission electron microscopy (TEM) images were ob-
tained for the primary organics-capped spherical NCs of CdSe,
PbS, Au, FePt, Bi, and Fe3O4, after exchange with various thio-
metallate capping ligands (Figure 3 and Figure S3-S5). Figures
3b-3e, S3, and S4 reveal that the ligand exchange reactions did
not affect the size and shape of NCs, except for Fe3O4. In the
case of Fe3O4, the ligand-stripping step seemed to etch the sur-
face of NCs and reduce their size (Figure S5a,b), which can be
attributed to its oxidative properties and Lewis acidity of the
nitrosonium cation. However, the thiometallate attachment step
did not affect the size or shape of Fe3O4 NCs, as confirmed by
TEM analysis (Figure S5b,c). To test the applicability of these
ligands for NCs with different shapes, ultrathin CdS nanoplates
with a thickness of 1.2 nm were also utilized for the exchange
with PtS ligands. According to the TEM image (Figure S4d),
the CdS nanoplates retained their ultrathin characteristics after-
wards.
Optical properties of thiometallate-capped CdSe NCs and CdS
nanoplates were investigated by UV-Vis absorption and photo-
luminescence (PL) spectroscopy. The CdSe NCs showed iden-
tical spectra with either thiometallate- or primary organics-cap-
ping (Figure 4a), which demonstrates that the ligand exchange
process did not affect the structural and electronic characteris-
tics of these CdSe NCs. The CdS nanoplates also showed very
similar absorption spectra after the ligand exchange. The only
change was that the peak became broader and red-shifted with
Figure 3. (a) Photographs showing the ligand exchange process.
TEM images of NCs: (b) PtS-capped CdSe (size: 4.6 nm), (c) FeS-
capped PbS (6.7 nm), (d) CoS-capped Fe3O4 (16 nm), and (e) NiS-
capped Au (6.0 nm).
The degree of ligand exchange on PbS and CdSe NCs capped
with thiometallates was studied by Fourier transform infrared
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absorption (FT-IR) spectroscopy. As shown in Figure 4d, the
pronounced XRD peaks, while a crystalline phase correspond-
ing to the hexagonal CoS (JCPDS 03-065-8977) was observed
in the pattern after annealing at 500 °C (Figure 5c). It is note-
worthy that thioplatinate exhibits broad peaks that do not corre-
spond to any related references when heated under N2 even up
to 500 °C (Figure 5d), whereas a pure Pt crystalline phase ap-
pears upon annealing at temperatures above 300 °C under air
(Figure 6a). This transformation should be due to the sulfur ox-
idation and subsequent evaporation of SOx during the heat treat-
ment. Other thiometallates were transformed into metal sulfate
phases (Figure S8) under air. These results demonstrate the ap-
plicability of the current thiometallates as precursors for intro-
ducing transition metal sulfides or pure metals on the surfaces
of desired materials.
absorption bands of C-H stretching modes (2700–3000 cm-1)
were observed in the organic-capped NCs but not in the thio-
metallate-capped ones (Figure 4d), suggesting the complete re-
placement of the organic ligands with inorganic thiometallates.
The surface coverage of thiometallate ligands on PbS (particle
size: 6.7 nm) was estimated with ICP-OES, and all ligands were
found to have surface coverages in the range of 14–30%. These
values are in agreement with the result reported for typical in-
organic ligands.39
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Figure 4. UV-Vis absorption spectra of organics- and thiometal-
late- capped (a) CdSe NCs and (b) CdS nanoplates. (c) PL spectra
of organics-capped CdSe and PtS-capped CdSe NCs before and af-
ter photo-oxidation. (d) FT-IR spectra of organics- and thiometal-
lates-capped CdSe NCs.
Generally, chalcogenidometallate compounds synthesized by
the hydrazine- or mixed solvent-reducing route from metal
chalcogenide powders can decompose back into crystalline
semiconducting metal chalcogenides upon heat treatment.17,18
This effect has been intensively studied for solution-processed
fabrication of semiconductor thin films using chalcogenido-
metallate inks. Thus, the current thiometallates were analyzed
by thermogravimetric analysis (TGA) under N2 and air environ-
ments, and XRD analysis before and after heating. All TGA
data showed a remarkable weight loss at 150–300 °C under both
N2 and air environments, depending on the type of thiometallate
(Figure S7). Therefore, these compounds thermally decompose
into crystalline phases in a manner similar to that previously
described for typical chalcogenidometallates.16,17 At above
300 °C, a slight weight loss was observed under both N2 and air,
and tentatively attributed to S evaporation from the formed
metal sulfides.
Figure 5. XRD patterns of (a) thioferrite, (b) thionickelate, (c) thi-
ocobaltate, and (d) thioplatinate annealed at various temperature
under N2 environment. The vertical lines show the patterns of ref-
erences.
To study the structural transformations of these thiometal-
lates on the NC surfaces, we investigated the annealing behav-
iors of of thiometallate-capped Fe3O4 NCs and organics-capped
Fe3O4 NCs by thermogravimetric analysis (TGA), since Fe3O4
NCs have sufficiently high thermal stability to exclude possible
side reactions between NCs and ligands. As shown in Figure S9,
all TGA spectra of thiometallate-capped Fe3O4 NCs shows a
weight loss of 6-15% at 150–300 °C, indicating the thermal de-
composition of thiometallates on NCs. Furthermore, the degree
of a weight loss in the TGA spectra is in order of thiocobaltate,
thioplatinate, thionickelate, and thioferrite, which agrees with
the TGA data of thiometallates. On the other hand, organics-
capped Fe3O4 NCs exhibit two distinct weight losses at ~250 °C
and ~380 °C, agreeing with the previously reported TGA result
of oleic acid-capped Fe3O4 NCs. Zhang, et al. claimed that these
two transitions might stem from the evaporation of oleic acid
capping on two different crystalline facets.40
XRD patterns of thioferrite and thionickelate annealed at
300 °C show sharp crystalline peaks that can be indexed as cu-
bic FeS2 (JCPDS 00-042-1340) and cubic NiS2 (JCPDS 03-065-
3325) structures (Figures 5a and 5b). At the annealing temper-
ature of 500 °C, thioferrite was transformed into the mixed
phases of FeS (JCPDS 03-065-9124(RDB)) and FeS2, indicat-
ing the evaporation of S. Also, the XRD pattern of thionickelate
at 500 °C matches those of both Ni (JCPDS 00-004-0850) and
NiS (JCPDS 03-065-5762) phases. In thiocobaltate, the sample
annealed at 300 °C might be amorphous because there were no
We further studied thioplatinate-capped Fe3O4 NCs since Pt
is an important catalytic element. The XRD pattern of the thio-
platinate-capped Fe3O4 NCs was almost identical after heat
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Chemistry of Materials
treatment, with slight peak sharpening arising from partial ag-
Page 6 of 9
8a), suggesting the formation of Pt single atoms in the oxide
lattice as a result of the partially vacant 5d orbitals.24 The XPS
spectrum of the Pt nanoparticles on Fe3O4 confirms that the na-
noparticles consisted of the fully reduced metallic phase Pt0
(Figure 8b). Similarly, in the previous report by Huang et al.,
XPS spectra of dendrimer-encapsulated Pt clusters with the Pt
size of less than 1 nm indicated that 93% of the Pt was oxidized,
while Pt nanoparticles with sizes above 2.5 nm exhibited metal-
lic Pt phase in the XPS.41 These results reveal the controllable
formation of Pt atoms and nanoclusters with different sizes and
chemical states on the surface of oxide NCs (Figure 8c).
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glomeration (Figure 6b). At the same time, we could not ob-
serve any peaks arising from the thioplatinate, meaning that no
crystalline domains of Pt formed that were detectable by XRD.
TEM analysis (Figure 6c) reveals that the size and shape of the
thioplatinate-capped Fe3O4 NCs barely changed during anneal-
ing. Elemental mapping image obtained by energy dispersive
spectroscopy (EDS) (Figure 6c) shows homogeneous distribu-
tion of Pt on the surface of Fe3O4 NCs. Also, we did not observe
any Pt nanoparticles in the TEM analysis, which was consistent
with the absence of Pt peaks in the XRD pattern. We further
characterized the configuration of the Pt on Fe3O4 NCs by high-
angle annular dark field (HAADF) scanning transmission elec-
tron microscopy (STEM). The HAADF-STEM image displays
well-distributed atomic Pt clusters showing different contrast at
the edge of Fe3O4 NCs (Figure 6d). Pt clusters with size of less
than 2 nm were clearly observed, indicating that single Pt at-
oms26,27 were incorporated into the Fe3O4 lattices homogene-
ously.
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We further heated the thioplatinate-capped Fe3O4 in the pres-
ence of excess thioplatinate under the same condition. The XRD
pattern shown in Figure 7a demonstrates the formation of a Pt
phase together with Fe3O4 NCs. The TEM image also shows
that Pt nanoparticles with an average size of 3.3 nm were
formed on the surface of Fe3O4 NCs, which were further con-
firmed by TEM-EDS mapping analyses (Figures 7c, S10, and
S11).
Figure 7. (a) XRD pattern and (b) TEM image of thioplatinate-
capped Fe3O4 NCs annealed with excess thioplatinate at 500 oC.
Figure 6. (a) XRD patterns of thioplatinate compound annealed at
300 oC and 500 oC in air. (b) XRD patterns of as-synthesized
organics-capped Fe3O4 and annealed thioplatinate-capped Fe3O4
o
NCs at 500 C. (c) TEM-EDS mapping and (d) HAADF-STEM
images of Pt-decorated Fe3O4.
Figure 8. XPS spectrum of (a) Pt atomic clusters and (b) Pt
nanoparticles on Fe3O4 NCs. (c) Scheme for the formatin of a Pt
phase on NCs. (d) Turnover frequencies and (e) Arrhenius plots of
the atomic Pt-Fe3O4 and the 3.3 nm-sized Pt-Fe3O4 catalysts for CO
oxidation in the temperature range of 180 oC-220 oC
Both Pt atomic clusters- and Pt nanoparticles-decorated
Fe3O4 samples were analyzed by X-ray photoelectron spectros-
copy (XPS). The XPS spectrum of Fe3O4 with Pt atomic clusters
shows multiple peaks assignable to Pt0, Pt2+, and Pt4+ (Figure
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Finally, as a model application, we carried out CO oxidation
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over the prepared Pt nanocatalysts supported on Fe3O4. Two Pt-
Fe3O4 catalysts with Pt atoms and 3.3-nm Pt nanoparticles were
compared in a batch reactor under 40 Torr CO, 100 Torr O2, and
620 Torr He in the temperature range of 180–220 °C. The TOFs
were determined from the rate of CO oxidation normalized by
the number of Pt active sites on the oxide NC surfaces, as de-
termined by ethylene hydrogenation.5 The loading of Pt per
Fe3O4 measured by ICP-OES was 1.18 and 14.0 % for the
atomic Pt-Fe3O4 and the 3.3 nm-Pt on Fe3O4, respectively. Fig-
ures 8d and 8e show the TOFs of Pt-catalyzed CO oxidation as
a function of the reaction temperature, and the activation ener-
gies from the Arrhenius plots. The obtained TOFs clearly con-
firm that active Pt sites were successfully created on the surface
of Fe3O4 NCs. Although the Pt nanoparticles-Fe3O4 had a higher
Pt loading, the atomic Pt-Fe3O4 exhibited CO oxidation rates
that are more than two times higher. The TOFs shown in the
present work are higher than the reported values of commercial
Pt-Al2O3, Pt nanoparticles, and Pt nanoparticles on various sup-
port oxides.6,42,43 This enhancement of the TOF in the atomic
Pt-Fe3O4 can be understood by several mechanism. First,
atomic Pt catalytic sites on Fe3O4 can be more reactive than cat-
alytic sites of Pt nanoparticles on Fe3O4. Qiao, et al. demon-
strated that the more vacant d orbitals of the charged single Pt
atoms help to reduce the CO adsorption energy as well as the
activation barriers for CO oxidation.26 However, in this work,
the activation barrier for CO oxidation was almost identical in
both atomic Pt and 3.3 nm-sized Pt on Fe3O4 (Table S1), which
distinguishes the catalytic reaction pathway in atomic Pt-Fe3O4
in this work from the previously reported one. Secondly, the
contact of Pt on Fe3O4 may affect the catalytic activity. Previous
studies reported that CO oxidation rate was increased in the
presence of oxide support due to the strong metal-support inter-
action between Pt and oxide.44 As shown in the XPS spectrum
(Figure 8a), atomic Pt was incorporated on the surface of Fe3O4
and this could be responsible for the stronger metal support in-
teraction between Pt and Fe3O4, resulting in the high TOF. On
the other hand, in the case that the size of Pt was increased to
3.3 nm, it was speculated that the metal-support interaction was
weakened with the strong metallic character of Pt, confirmed by
the XPS analysis (Figure 8b). Moreover, the homogeneous dis-
tribution of atomic Pt over Fe3O4 might further contribute to in-
crease the TOF.
ASSOCIATED CONTENT
Supporting Information. Solution photographs before and after
the ligand exchange, scheme for two-step ligand exchange, TEM
images of NCs before and after the ligand exchange, PL spectra of
NCs, TGA spectra and XRD patterns of thiometallates, size histo-
gram of Pt nanoparticles, TEM-EDS mapping image of Pt nanopar-
ticles, and TOFs and activation energy of CO oxidation.
9
AUTHOR INFORMATION
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Corresponding Author
kjan@unist.ac.kr (K.A.) & jsson@unist.ac.kr (J.S.S.)
ACKNOWLEDGMENT
This research was supported by the Nano·Material Technology
Development Program (No. 2016M3A7B4900044) (J.S.S.) and
Basic Science Research Program (No. 2015R1C1A1A01055092)
(K.A.) through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, and the 2015 Research Fund
(1.150109.01) of UNIST (K.A.).
ABBREVIATIONS
NCs, Nanocrystals; NMF, N-Methylformamide; DMF, N,N-Dime-
thylformamide, ACN, Acetonitrile.
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In summary, we have shown the general applicability of tran-
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ligands for colloidal NCs. These inorganic anions as ligands
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Chemistry of Materials
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