A Tunable library of substituted thiourea precursors to metal sulfide nanocrystals

Article abstract of DOI:10.1126/science.aaa2951

Controlling the size of colloidal nanocrystals is essential to optimizing their performance in optoelectronic devices, catalysis, and imaging applications. Traditional synthetic methods control size by terminating the growth, an approach that limits the reaction yield and causes batch-to-batch variability. Herein we report a library of thioureas whose substitution pattern tunes their conversion reactivity over more than five orders of magnitude and demonstrate that faster thiourea conversion kinetics increases the extent of crystal nucleation. Tunable kinetics thereby allows the nanocrystal concentration to be adjusted and a desired crystal size to be prepared at full conversion. Controlled precursor reactivity and quantitative conversion improve the batch-tobatch consistency of the final nanocrystal size at industrially relevant reaction scales.

Full text of DOI:10.1126/science.aaa2951

RESEARCH
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mostly aseismic in this experiment (Fig. 3B).
Micro-earthquakes are triggered only when the
estimated size of the slip zone exceeds the pres-
surized zone (Fig. 3C), suggesting that they occur
off the pressurized zone. However, they might
occur within the sliding zone or be triggered off the
sliding area by static stress increase. Aftershocks
might relate to afterslip in a similar way (23).
The ratio of the shear stress to the effective
normal stress increases from about 0.4 to about
0.8 and indicates a friction between 0.6 and 0.8
once slip becomes notable (fig. S4). Qualitatively,
this evolution correlates with slip rate better than
with slip. The data suggest a logarithmically vary-
ing, rate-dependent friction, as frequently observed
in laboratory measurements of rock friction (24)
or derived from studies of afterslip following large
earthquakes (23, 25). We tested such a law based
on a simple one-dimensional model (16). The
model, which involves only two adjustable pa-
rameters, qualitatively fits the observations well
(Fig. 3D). We determine the friction m0 = 0.67
induced injections and shallow aseismic slip.
Mechanism observed in this experiment could
also be of relevance to explain natural processes
at greater depth, such as deep afterslip, slow-slip
events, and tremors.
20. F. Cappa, Y. Guglielmi, J. Virieux, Geophys. Res. Lett. 34,
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ACKNOWLEDGMENTS
The experimental work was funded by the Agence Nationale de
la Recherche (ANR) Captage de CO2 through the HPPP-CO2 project
and by PACA through the PETRO-PRO project. The rate-and-
state fault models for this study were supported by the French
Academy of Sciences, the California Institute of Technology
Tectonics Observatory, and the ANR HYDROSEIS under contract
ANR-13-JS06-0004-01. We thank the SITES S.A.S. engineers
H. Caron, C. Micollier, R. Blin, and H. Lançon, and the Petrometalic
S.A. engineer J. B. Janovczyk, who jointly developed and operated
the probe that enabled this experiment. We also thank the
Laboratoire Souterrain à Bas Bruit (LSBB) engineers’ team for
their technical support during the installation of the experiment
in one of the LSBB boreholes (www.lsbb.eu). Experimental data are
available in the supplementary materials.
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(+/−0.05) at a reference velocity of v0 = 0.1 mm/s,
∂m
∂lnv
with a rate dependency of a ¼
¼ 0:0447
ðT0:005Þ. We also tested rate-and-state friction
laws (16) but found that the improvement was
irrelevant in view of the uncertainties of the
measurements. The friction coefficient increases
to about 0.7 for slip rates of 1 to 20 mm/s. Similar
values have been measured in the laboratory on
faults formed in limestone and at comparable
sliding rates (26–28). These laboratory experi-
ments also show a rate-strengthening behavior
at temperatures less than 100°C but a rate de-
pendency at steady-state typically one order of
magnitude lower than the value we obtained.
The aseismic behavior triggered by the fluid
injection in this experiment is apparently due to
an intrinsically rate-strengthening behavior,
rather than to conditional stable creep of a rate-
weakening fault. However, the seismicity, which
was probably triggered outside the pressurized
zone, requires some areas to allow earthquake
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/348/6240/1224/suppl/DC1
Materials and Methods
Figs. S1 to S6
References (29–32)
Database S1
3 March 2015; accepted 9 May 2015
10.1126/science.aab0476
NANOMATERIALS
A tunable library of substituted
nucleation, hence weakening during deformation. thiourea precursors to metal
This observation is an indication that the fric-
tional properties are likely heterogeneous, as sup-
sulfide nanocrystals
ported by the observations of the main slip surface
that displays gouge-rich zones and slickensided
Mark P. Hendricks, Michael P. Campos, Gregory T. Cleveland,
Ilan Jen-La Plante, Jonathan S. Owen*
areas cutting solid rock (Fig. 1C). During the in-
jection, the effective behavior is rate-strengthening.
This behavior is possibly due to the fault zone be-
ing rate-strengthening on average, with the gouge-
rich zones being possibly more rate-strengthening
than the zones where the fault cuts solid rock
(27) or because the rate-weakening asperities are
brought to conditional stability by the increase in
pore pressure. This interpretation could explain
why seismicity is observed only when the crack
Controlling the size of colloidal nanocrystals is essential to optimizing their performance
in optoelectronic devices, catalysis, and imaging applications. Traditional synthetic methods
control size by terminating the growth, an approach that limits the reaction yield and causes
batch-to-batch variability. Herein we report a library of thioureas whose substitution pattern tunes
their conversion reactivity over more than five orders of magnitude and demonstrate that faster
thiourea conversion kinetics increases the extent of crystal nucleation. Tunable kinetics thereby
allows the nanocrystal concentration to be adjusted and a desired crystal size to be prepared at
full conversion. Controlled precursor reactivity and quantitative conversion improve the batch-to-
radius has become larger than the radius of the
pressurized zone. Our results prove unambigu-
batch consistency of the final nanocrystal size at industrially relevant reaction scales.
ously that fluid injection can trigger primarily
aseismic slip, with seismicity induced as a sec-
ondary effect. This is observed in the context of
our experiment, which is characterized by a par-
ticularly low effective normal stress. Thus, our
results may be of particular relevance to seismic
activity triggered at shallow depth by human-
he tunable electronic properties of nano-
meter scale crystals have inspired many
synthetic methods that control crystal size
and shape with extraordinary fidelity. Mod-
ern metal chalcogenide quantum dots, in
varies by less than a layer of surface atoms across
the distribution. This precise control is afforded
by the homogeneous nucleation and growth mech-
anism first described by LaMer and Dinegar to
follow a three-phase sequence shown in Fig. 1A
(1). In this mechanism, nucleation only occurs
T
particular, can be synthesized with a size that
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during a brief period of time when crystal mono-
mers ([ME]i) reach a so-called “critical concentra-
tion.” Modern syntheses achieve these conditions
by using nanocrystal precursors that slowly con-
vert to crystal monomers at a rate that limits the
crystallization steps (Fig. 1B) (2–4). The precursor
conversion reaction kinetics, therefore, plays a
central role in the nucleation and growth pro-
cess. In particular, the rate of monomer supply to
the crystallization medium during nucleation
determines the number of nanocrystals produced
(3, 5–8). Moreover, if conversion kinetics can be
precisely tuned, the number of nanocrystals can
be used to control the size at full conversion.
The most widely used sulfide precursors include
bis(trimethylsilyl) sulfide [(TMS)2S], phosphine
sulfides (R3P=S, where R indicates an alkyl or
aryl group), and hydrogen sulfide produced by
heating elemental sulfur in alkane or amine sol-
vents (9). Depending on the conditions of the
crystallization, a precursor is selected that pro-
vides the necessary rate of monomer supply.
For example, (TMS)2S typically reacts rapidly
with metal salts, allowing it to be used near room
temperature; however, the rapid reactivity can
lead to mixing limitations during the injection
step that hinder the reaction scale. R3P=S deriv-
atives, on the other hand, typically react sluggishly
until ~300°C and produce low reaction yields.
Although reactions of elemental sulfur with al-
kanes and amines are more versatile and can be
used at intermediate temperatures, their con-
version reactions follow ill-defined radical path-
ways that are difficult to control and sensitive
to the presence of impurities. Further, sulfur-
containing by-products contribute to batch-to-batch
variability and have detrimental effects on nano-
crystal properties. Recently, several phosphine
chalcogenides and dichalcogenides have been
investigated whose conversion kinetics depends
on the organic substituents (10, 11). These com-
pounds show promise for tuning nanocrystal size
and shape, but few are commercially available
and others require involved, multistep syntheses.
We report a library of inexpensive and air-
stable substituted thioureas whose conversion
reactivity can be finely tuned over many orders
of magnitude by adjusting the organic substitu-
ents (Fig. 2). The widely tunable reactivity allows
an optimum chalcogen precursor to be matched
with the reactivity of a metal complex at a de-
sired reaction temperature. By controlling the
monomer supply kinetics, we adjusted the extent
of nucleation in syntheses of lead sulfide, cad-
mium sulfide, zinc sulfide, and copper sulfide
nanocrystals and reliably prepared a desired size
with a narrow size distribution and in quantita-
tive yield.
primary or secondary amines (Fig. 2A). Based
on the number of commercially available start-
ing materials, we estimate that 103 to 104 structures
can be accessed in a single step. In most cases, the
electrophilicity of isothiocyanates makes the reac-
tion with amines rapid at room temperature,
allowing N,N′-diaryl; N,N′-dialkyl; mixed N-alkyl-
N′-aryl; and N,N,N′-trialkylthiourea structures to
be prepared (1 to 11).
disubstituted thioureas (1 to 8) react with lead
oleate at temperatures at or below 150°C, pro-
viding access to colloidal PbS nanocrystals with
linewidths of the 1Se – 1Sh absorption that are
among the narrowest reported, including at sizes
relevant for PbS solar cells (13, 14). Concentrated
conditions also proved accessible, allowing up to
6 g of nanocrystals to be prepared in 150 ml of
solvent with a high degree of reproducibility.
Figure 2E shows the results of nine syntheses
conducted using N,N′-diphenylthiourea (2) and
six syntheses conducted using N-dodecyl-N′-
phenylthiourea (3d) that produce between 0.2
and 6 g of nanocrystals. Within one set of condi-
tions, a final size and size distribution are reliably
obtained (Fig. 2E and fig. S13). Moreover, little
change in the final size and size distribution is ob-
served as the scale, concentration, and stoichiom-
etry are varied (10 to 150 ml, 25 to 100 mM, 1.2:1
to 3:1 Pb:S) (Fig. 2E and fig. S13). The reproduci-
bility and monodispersity are unusual given the
high nanocrystal concentrations (up to 0.12 mM),
illustrating the reliability and homogeneity pro-
vided by well-defined lead and thiourea precursors.
To determine the relationship between con-
version reactivity and the final size, we moni-
tored the kinetics of lead sulfide formation in situ
by tracking the absorbance at l = 400 nm. Lead
sulfide formation approaches 100% yield within a
few minutes and can be approximated by a single
exponential process (fig. S15), from which rate
constants [kobs(1) to kobs(8) (s−1)] are extracted. By
normalizing kobs(1) through kobs(8) to the rate
constant of the slowest precursor [kobs(8)], we
determined relative first-order rate constants
The thiourea conversion reactivity depends
on the number of substituents and their electron-
ic and steric properties. The rate decreases as the
number of substituents increases: Tetrasubsti-
tuted thioureas convert most slowly, followed by
trisubstituted and then disubstituted deriva-
tives. Thus, the substitution pattern can be used
to optimize monomer supply kinetics at the de-
sired crystallization temperature. For example,
monodisperse lead sulfide nanocrystals can be
synthesized from lead oleate and reactive disub-
stituted thioureas at temperatures from 90° to
150°C (Fig. 2D). However, monodisperse CdS
nanocrystals require higher temperature con-
ditions (150° to 250°C), where disubstituted
thioureas convert at a rate that is limited by mix-
ing during the injection. Instead, less-reactive
N,N,N′- trialkylthioureas (9 to 11) or N,N,N′,N′-
tetramethylthiourea (12) were found to have the
appropriate conversion reactivity (fig. S1). Mono-
disperse zinc sulfide nanocrystals could also be
obtained, but only at high temperatures (≥240°C)
(fig. S2). However, the lower reactivity of zinc
oleate compared with cadmium oleate allows
more-reactive disubstituted thioureas to be used.
In each case, the conversion reactivity could be
optimized to induce nucleation shortly after in-
jection at the temperature needed to obtain nar-
row size distributions. In this manner, a wide
variety of metal sulfides could be synthesized—
including photoluminescent core-shell nanocrystals
(CdSe/CdS/ZnS), plasmonic nanocrystals (Cu2–xS),
catalyst materials (NiS), complex compound semi-
conductors (CuZnSnS4), and one-dimensional (1D)
and 2D anisotropic nanostructures (CdS nanorods,
SnS nanosheets)—across a diverse set of growth
media, including amine, carboxylate, and phos-
phonate surfactant mixtures (figs. S3 to S10).
In addition to matching the demands of the
crystallization and co-reactants, tunable mono-
mer supply can control the number of nanocrys-
tals and thereby define a desired size after 100%
yield is reached. We demonstrate this principle
by optimizing an inexpensive synthesis of lead
sulfide nanocrystals with diameters from 2.5 to
7.2 nm [wavelength lmax(1Se – 1Sh) = 850 to
1800 nm, full width at half maximum (FWHM) =
30 to 160 meV] on multigram scales and in quan-
titative yield [as determined by the final absorbance
at l = 400 nm, where the extinction is proportional
to the concentration of lead sulfide formula units
within nanocrystals (12)] (Fig. 2D and fig. S11).
To ensure highly reproducible reactivity, 100-g
batches of pure, hydroxide-free lead oleate were
prepared by dissolving lead oxide in trifluoro-
acetic anhydride solution and neutralizing the
lead trifluoroacetate product with oleic acid
and triethylamine (Fig. 2B and fig. S12). N,N′-
Fig. 1. The mechanism of precursor-limited
homogeneous nucleation and growth of colloi-
dal metal chalcogenide nanocrystals. (A) The
time evolution of the monomer concentration as
described by LaMer and Dinegar, where monomer
supersaturation (I) precedes crystal nucleation
(II) and is followed by growth (III). (B) Metal and
chalcogen precursors (MX2, ER2) react and supply
monomers ([ME]i) to the growth medium at a
rate that limits the crystallization.
Multigram quantities of air-stable N,N′-
disubstituted and N,N,N′-trisubstituted thioureas
are obtained in quantitative yields via a one-step
“click” reaction between inexpensive, commer-
cially available substituted isothiocyanates and
Department of Chemistry, Columbia University, New York, NY
10027, USA.
*Corresponding author. E-mail: jso2115@columbia.edu
SCIENCE sciencemag.org
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Fig. 2. A library of
thioureas can be
accessed in a single
step and used to pre-
pare PbS nanocrys-
tals with highly
reproducible control
over the final size.
(A) Synthesis of
substituted thiourea
precursors. (B) Syn-
thesis of lead oleate
from lead oxide and
synthesis of PbS
nanocrystals from
substituted thioureas
and lead oleate. Et,
ethyl group. (C) Table
of substituted thiourea
precursor structures.
Me, methyl; Ph, phenyl.
(D) Ultraviolet–visible–
near-infrared absorp-
tion spectra of PbS nanocrystals synthesized using substituted thioureas: 1 and 2 (95°C); 3b, c, d, and f (120°C); 6 and 8 (150°C). a.u., arbitrary units.
(E) Results of a reproducibility study showing the nanocrystal diameter and relative size distribution of three reactions for two different precursors (2 and 3d).
The varying shades of dot color represent differing reaction scale, concentration, or amount of excess lead oleate used in the reaction. See fig. S14 for detailed
reaction conditions.
[krel(1) to krel(8)] across a range of temperatures
(90° to 150°C), allowing the reactivity to be quan-
titatively compared over more than three orders of
magnitude (Fig. 3C). These measurements show a
well-defined dependence of the conversion reac-
tivity on the thiourea structure that is defined by
the substituents.
Within the disubstituted thiourea deriva-
tives, the conversion rate constants decrease
over three orders of magnitude upon replacing
electron-withdrawing aryl substituents with alkyl
substituents. Thus, N,N′-diarylthioureas such as
N-(3,5,-bis-trifluoromethylphenyl)-N′-phenylthiourea
(1) convert most rapidly, whereas N,N′-di-n-
alkylthioureas (8) react 4000-fold more slowly.
Similar reactivity trends were found with cad-
mium oleate (fig. S16). Mixed N-alkyl-N′-aryl
variants showed intermediate reactivity toward
lead oleate that can be finely adjusted by append-
ing electron withdrawing or donating substituents
on the aromatic ring. The conversion rate con-
stants of para-substituted N-p-X-phenyl-N′-n-
dodecylthioureas (3a to 3f, where X = CN, CF3,
Cl, H, Me, MeO) increase by a factor of 20 as the
para-substituent becomes increasingly electron
withdrawing. The logarithms of the observed first-
order rate constants are plotted versus the Ham-
mett sigma parameter of the para-substituent in
Fig. 3D (15). A linear relationship is observed,
demonstrating the well-behaved dependence of
conversion kinetics on the thiourea acidity.
A large positive slope (r = 1.3) indicates a
buildup of negative charge during the conver-
sion reaction. These results can be explained
by rate-limiting deprotonation of the thiourea
or nucleophilic attack on the thiocarbonyl car-
bon. Conversion of 3d is faster in the presence
of tri-n-butylamine and slower when oleic acid
Fig. 3. The reaction of disubstituted thioureas with lead oleate in hexadecane and diphenyl
ether follows single exponential kinetics, allowing the conversion rate constants to be quanti-
tatively compared. (A) Schematic of PbS formation reaction used to measure conversion kinetics at 90°
to 150°C. (B) Kinetics of lead sulfide formation as measured in situ by the absorbance at l = 400 nm.
(C) Effect of thiourea substitution pattern on the relative thiourea conversion rate constants [krel(1) to
krel(8), e.g., krel(7) = k(7)/k(8)]. The wide range of reactivity requires that kinetics are measured at
multiple temperatures. To account for the temperature dependence of the conversion rate constant,
3b and 3f were measured at two temperatures, and the change in rate constant was used to normalize
the relative rate constants of the respective temperatures {e.g., krel(3b) = [k(3b)120°C/k(8)150°C] ×
[k(3f)150°C/k(3f)120°C]}. (D) Hammett plot illustrating the well-defined relationship between the electronic
structure of the thiourea and the rate of lead sulfide formation.
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Fig. 4. Increasing thiourea conversion reactivity produces a higher nanocrystal concentration and a smaller final nanocrystal diameter. (A)
Nanocrystal concentration and final diameter plotted versus the rate constant of lead sulfide formation (kobs) for 3a to 3f ([Nanocrystal]t º [PbS]t/(rt3),
where t is time and r is the nanocrystal radius at time t). (B to D) Transmission electron micrographs of PbS nanocrystals synthesized under reaction
conditions used for kinetics experiments (3a, 3d, and 3f).
is present, both of which suggest that depro-
tonation of the thiourea precedes the formation of
lead sulfide. Deprotonation of thiourea in water is
known to speed its hydrolysis to cyanamide
(16). Increasing steric bulk of the thiourea sub-
stituents also speeds the rate of conversion (5 to
8); the increased bulk may accelerate elimina-
tion of lead sulfide from an intermediate lead
thioureate complex formed by deprotonation
of a lead-bound thiourea. Although detailed work
is required to determine the precise conversion
mechanism, these observations highlight the
importance of the microscopic steps leading to
the rate-determining precursor conversion step,
which vary depending on the surfactants used
as well as the nature of the metal co-reactant.
The nanocrystal concentrations obtained from
3a to 3f are plotted versus kobs in Fig. 4A, where
an eightfold increase in the conversion rate leads
to a fourfold increase in the nanocrystal concen-
tration. The finely tuned monomer supply kinetics
controls the extent of nucleation, because the rate
of Ostwald ripening is negligibly slow under these
conditions (Fig. 4A and fig. S17). A similar depen-
dence is observed in studies of zinc sulfide, copper
sulfide, nickel sulfide, and cadmium sulfide nano-
crystals (figs. S2, S4, S5, and S16). Tuning the
precursor reactivity can also be used to finely con-
trol the aspect ratio of CdS nanorods; thioureas of
decreasing reactivity (3f, 9, and 12) lead to a
systematic increase in the volume and rod aspect
ratio, a trend that results from the lower nano-
crystal concentration produced by slower con-
version kinetics (figs. S7 to S9) (17).
leation rate on monomer supersaturation, like the
one predicted by classical nucleation theory, is
expected to produce a linear relationship between
the nanocrystal concentration and the precursor
conversion rate during nucleation (5, 6). How-
ever, a subcritical dependence can be observed
in Fig. 4A that will require more accurate models
of nucleation to explain. Nonetheless, the smooth
correlation between precursor conversion rate and
nanocrystal concentration can be used to pre-
dictably obtain a desired nanocrystal size.
Controlling the final size with the precursor
reaction rate rather than modifying the crystal-
lization medium (e.g., reaction temperature, sol-
vent, and surfactant concentration), or limiting
the conversion, greatly simplifies the nanocrystal
composition because both the starting materials
as well as the reaction by-products can bind the
nanocrystals (19). Although the thiourea conver-
sion by-products were not found to bind the
nanocrystals in this case (fig. S18), cadmium
and lead oleate precursors are known to rever-
sibly bind and passivate nanocrystal surfaces,
and their coverage may depend on the concen-
tration of these complexes remaining after con-
version (20).
Tuning size with precursor reactivity and run-
ning reactions to full conversion allows the final
ratio of lead sulfide product and unconverted
lead oleate to be determined by the amounts of
reactants used. This advance enables a standard
purification procedure to be optimized to repro-
ducibly control the final oleate ligand shell and
nanocrystal stoichiometry. Moreover, by using
preformed lead oleate, syntheses run at or below
120°C can be conducted in low-boiling solvents
like 1-octene that are conveniently distilled under
vacuum after completion of the synthesis. This
reduces the volume of solvent used during the
isolation and facilitates large-scale reactions. For
example, nanocrystals with a 3.4-nm diameter
were synthesized on a 3-g scale from a 1.5:1 lead
oleate–to–2 mixture and isolated with 5.7 oleates
per square nanometer of surface area, from which
we estimate a Pb:S ratio of 1.26. Larger nano-
crystals (6.5 nm) with a lower surface area–to–
volume ratio were synthesized on a 6-g scale
with a lower lead oleate–to–3d ratio (1.2:1) and
isolated with 2.9 oleates nm−2, from which we
estimate a Pb:S ratio of 1.07 (fig. S18). In both
cases, the purification is greatly simplified com-
pared with methods where the conversion is
limited. Rather than removing a large excess
of unreacted metal precursor—a process that
is complicated by the polymeric structures and
low solubilities of zinc, cadmium, and lead
carboxylates, phosphonates, and halides—a desired
amount of remaining metal precursor can be
chosen by the starting metal-to-sulfur ratio. Thus,
obtaining a desired size at complete conversion is
an important step toward reproducibly defining
the surface structure and optoelectronic proper-
ties at large reaction scales.
Tailoring precursor reactivity will greatly
advance our ability to rationally design high-
performance materials and to relate atomic struc-
ture to nanocrystal function. In particular, highly
reproducible syntheses of PbS nanocrystals with
narrow size distributions and well-defined surface
chemistry can advance the systematic develop-
ment of solution-processed nanocrystal photo-
voltaics. Tunable sulfide precursors will also
advance the synthesis of complex metal chal-
cogenide alloys and heterostructures used in
solid-state lighting and luminescent displays.
More broadly, these results highlight the value
of rationally controlling the nanocrystal syn-
thesis mechanism to obtain a desired nanocrys-
tal product in high yields and with a high degree
of batch-to-batch consistency.
Previous experimental and theoretical work
on cadmium selenide and silver halides reported
a similar correlation between the concentration
of nanocrystals and the precursor conversion rate
during nucleation (3, 5–7, 18). The correlation
results from a nucleation process that continues
until the collective consumption of monomers by
growing nuclei exceeds monomer production by
precursor conversion. At this point, the concen-
tration of nuclei is sufficiently high to cause the
supersaturation to drop and the nucleation pro-
cess to end. A “critical” dependence of the nuc-
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ACKNOWLEDGMENTS
Precursor design and kinetics studies were supported by
the Center for Re-Defining Photovoltaic Efficiency Through
Molecule Scale Control, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under award no. DE-SC0001085.
Large-scale syntheses and surface chemistry measurements
were supported by the Department of Energy under grant
no. DE-SC0006410. The authors thank A. N. Beecher
and E. Auyeung for assistance with transmission electron
microscopy, which was carried out in part at the New York
Structural Biology Center, supported by Empire State
Development’s Division of Science, Technology and Innovation and
the National Center for Research Resources, NIH, grant no. C06
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/348/6240/1226/suppl/DC1
Materials and Methods
Figs. S1 to S20
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13 November 2014; accepted 1 May 2015
10.1126/science.aaa2951
ELECTROCHEMISTRY
control over dopant incorporation of various tran-
sition metals onto the surface of dispersive and
octahedral Pt3Ni/C (termed as M‐Pt3Ni/C, where
M = V, Cr, Mn, Fe, Co, Mo, W, or Re), we have
developed ORR catalysts that exhibit both high
activity and stability. In particular, our Mo‐Pt3Ni/C
catalyst has high specific activity (10.3 mA/cm2),
high mass activity (6.98 A/mgPt), and substan-
tially improved stability for 8000 potential cycles.
We prepared highly dispersed Pt3Ni octahedra
on commercial carbon black by means of an ef-
ficient one‐pot approach without using any bulky
capping agents, which used platinum(II) acetyl-
acetonate [Pt(acac)2] and nickel(II) acetylaceton-
ate [Ni(acac)2] as metal precursors, carbon black
as support, N,N-dimethylformamide (DMF) as
solvent and reducing agent, and benzoic acid as
the structure-directing agent (fig. S1A). The sur-
face doping for the Pt3Ni/C catalyst was initiated
by the addition of dopant precursors, Mo(CO)6,
together with Pt(acac)2 and Ni(acac)2 into a sus-
pension of Pt3Ni/C in DMF, and the subsequent
reaction at 170°C for 48 hours (fig. S1B). The
transmission electron microscopy (TEM) and
high‐angle annular dark‐field scanning TEM
(HAADF‐STEM) images of the Pt3Ni/C and Mo-
Pt3Ni/C catalysts (Fig. 1, A and B, and fig. S2)
revealed highly dispersive octahedral nanocrys-
tals (NCs) in both samples, which were substan-
tially uniform in size, averaging 4.2 T 0.2 nm in
High-performance transition
metal–doped Pt3Ni octahedra for
oxygen reduction reaction
Xiaoqing Huang,1,2*† Zipeng Zhao,1,2* Liang Cao,3 Yu Chen,1,2 Enbo Zhu,1,2
Zhaoyang Lin,4 Mufan Li,4 Aiming Yan,5,6,7 Alex Zettl,5,6,7 Y. Morris Wang,8
Xiangfeng Duan,2,4 Tim Mueller,9‡ Yu Huang1,2
‡
Bimetallic platinum-nickel (Pt-Ni) nanostructures represent an emerging class of
electrocatalysts for oxygen reduction reaction (ORR) in fuel cells, but practical applications
have been limited by catalytic activity and durability. We surface-doped Pt3Ni octahedra
supported on carbon with transition metals, termed M‐Pt3Ni/C, where M is vanadium,
chromium, manganese, iron, cobalt, molybdenum (Mo), tungsten, or rhenium. The Mo‐Pt3Ni/C
showed the best ORR performance, with a specific activity of 10.3 mA/cm2 and mass activity
of 6.98 A/mgPt, which are 81- and 73‐fold enhancements compared with the commercial
Pt/C catalyst (0.127 mA/cm2 and 0.096 A/mgPt). Theoretical calculations suggest that Mo
prefers subsurface positions near the particle edges in vacuum and surface vertex/edge sites
in oxidizing conditions, where it enhances both the performance and the stability of the
Pt3Ni catalyst.
roton-exchange membrane (PEM) fuel cells
use reactions between the fuel (such as
hydrogen or alcohols) at the anode and the
oxidant (molecular oxygen) at the cathode
(1–3). Both cathode and anode reactions
so far have led to a considerable increase in ORR
activity, the champion activity as observed on
bulk Pt3Ni(111) surface has not been matched in
nanocatalyts (21–25), indicating room for further
improvement. At the same time, one noted
major limitation of Pt-Ni nanostructures is their
low durability. The Ni element in these nano-
structures leaches away gradually under detri-
mental corrosive ORR conditions, resulting in
rapid performance losses (23–27). Thus, synthesiz-
ing Pt‐based nanostructures with simultaneously
high catalytic activity and durability remains an
important open challenge (28).
Because surface and near-surface features of a
catalyst have a strong influence on its catalytic
performance, we adopted a surface engineering
strategy to further explore and enhance the per-
formance of Pt3Ni(111) nanocatalysts. We specifically
focused our efforts on Pt3Ni-based nanocatalysts
because the bulk extended Pt3Ni(111) surface has
been shown to be one of the most efficient cat-
alytic surfaces for the ORR. On the basis of the
P
1Department of Materials Science and Engineering, University
of California, Los Angeles, CA 90095, USA. 2California
NanoSystems Institute (CNSI), University of California, Los
Angeles, CA 90095, USA. 3Department of Physics and
Astronomy, Johns Hopkins University, Baltimore, MD 21218,
USA. 4Department of Chemistry and Biochemistry, University
of California, Los Angeles, CA 90095, USA. 5Department of
Physics and Center of Integrated Nanomechanical Systems,
University of California, Berkeley, CA 94720, USA. 6Materials
Sciences Division, Lawrence Berkeley National Laboratory,
Berkeley, CA 94720, USA. 7Kavli Energy NanoSciences
Institute at the University of California, Berkeley, and the
Lawrence Berkeley National Laboratory, Berkeley, CA 94720,
USA. 8Physical and Life Sciences Directorate, Lawrence
Livermore National Laboratory (LLNL), Livermore, CA 94550,
USA. 9Department of Materials Science and Engineering,
Johns Hopkins University, Baltimore, MD 21218, USA.
*These authors contributed equally to this work. †Present address:
College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, 215123 Suzhou, China. ‡Corresponding author.
E-mail: tmueller@jhu.edu (T.M.); yhuang@seas.ucla.edu (Y.H.)
need catalysts to lower their electrochemical over-
potential for high-voltage output, and so far, plat-
inum (Pt) has been the universal choice (4–6).
To fully realize the commercial viability of fuel
cells, the following challenges, which may not
be strictly independent of one another, need to
be simultaneously addressed: the high cost of
Pt, the sluggish kinetics of the oxygen reduction
reaction (ORR), and the low durability of the
catalysts (7–11).
Alloying Pt with a secondary metal reduces the
usage of scarce Pt metal while at the same time
improving performance as compared with that
of pure Pt on mass activity (12–15), which has led
to the development of active and durable Pt-
based electrocatalysts with a wide range of
compositions (16–20). However, although studies
1230 12 JUNE 2015 • VOL 348 ISSUE 6240
sciencemag.org SCIENCE

A tunable library of substituted thiourea precursors to metal sulfide
nanocrystals
Mark P. Hendricks et al.
Science 348, 1226 (2015);
DOI: 10.1126/science.aaa2951
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