Supporting Information
Integration of Semiconducting Sulfides for Full-Spectrum Solar Energy
Absorption and Efficient Charge Separation
Tao-Tao Zhuang+, Yan Liu+, Yi Li+, Yuan Zhao, Liang Wu, Jun Jiang,* and Shu-Hong Yu*
anie_201601865_sm_miscellaneous_information.pdf
Supporting Information
Materials
Sodium diethyldithiocarbamate (Na(S2CNEt2), 99%), AgNO3 (99.9%), Zn(NO3)2
(99%), tetrakis(acetonitrile)copper(I)hexafluorophosphate ([MeCN]4CuPF6, 98%),
Cd(NO3)2 (99%), hexane (97%), toluene (99.5%), methanol (99.5%), ethanol
(99.7%), 1-dodecanethiol (DDT, 97%), oleic acid (OA, 85%), tri-n-butylphosphine
(TBP, 99%), and tetrachloromethane (CCl4) were purchased from the Shanghai
Reagent Company (P. R. China). All chemicals were used as received without further
purification.
Methods
Synthesis of metal complexes precursor: Ag(S2CNEt2) (Ag(dedc)) and
Zn(S2CNEt2)2 (Zn(dedc)2)
The metal complexes were synthesized by a simple solution reaction at room
temperature.[1] First, AgNO3 (2 mmol) and Na(S2CNEt2) (2 mmol) were dissolved in
deionized water (100 ml), respectively. Then, the two solutions were mixed together
by dropwise addition of AgNO3 aqueous solution. And the resulting Ag(dedc) were
washed three times with deionized water and ethanol followed by drying. Zn(NO3)2 (1
mmol) was used instead of AgNO3 (2 mmol) to prepare Zn(dedc)2.
Synthesis of single ZnS nanorods with UV absorption
Through our previously reported catalyst-assisted method, the ultrathin ZnS nanorods
were prepared.[1] First, Ag(dedc) (15 mg) and Zn(dedc)2 (235.5 mg) were dissolved
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by 10 mL 1-dodecanethiol (DDT) with magnetic stirring and heated to 210 oC. When
the solution reacted for 5 min at 210 oC, 10 mL oleic acid (OA) was injected. After
keeping the reaction for 60 min, the resulting ZnS nanorods were collected and
centrifuged. Then, the products were washed three times with hexane and ethanol for
next use.
Synthesis of binary ZnS-CdS heteronanorods with UV and vis absorption
The multi-node sheath ZnS-CdS heteronanorods were prepared by previously
reported method.[2] Colloidal binary ZnS-CdS heteronanorods with segmented CdS
node sheaths were prepared using chemical transformation at room temperature,
containing two steps: (I) synthesis of ZnS-Ag2S heteronanorods through partial Zn/Ag
exchange from ultrathin ZnS nanorods and (II) transformation to ZnS-CdS by
complete Ag/Cd exchange. A solution of 30 mg AgNO3 dissolved in 10 mL methanol
was injected to a solution of 20 mg ZnS nanorods in 20 mL toluene. The ZnS-Ag2S
product washed with methanol and dispersed in toluene. The ion exchange stock
solution was prepared by mixing 2.0 g Cd(NO3)2 in 10 mL methanol and a solution of
0.5 mL TBP in 1 mL toluene. After the ion exchange, the stock solution was injected
to a solution of 20 mg ZnS-Ag2S heteronanorods in 20 mL toluene, the sample shook
for 10 min and a slower color change was observed. The multi-node sheath ZnS-CdS
heteronanorods were washed with methanol and dispersed in toluene for next use.
Synthesis of ternary multi-node sheath ZnS-CdS-Cu2-xS heteronanorods with UV,
vis and NIR absorption
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The reaction was performed at room temperature through partial Cd2+/Cu+ exchange
in ZnS-CdS heteronanorods. A solution of 22.4 mg [MeCN]4CuPF6 dissolved in 10
mL methanol was injected to a solution of 20 mg ZnS-CdS heteronanorods in 20 mL
toluene. The sample slightly shook for 15 min, and then a slower color change from
yellow to brown occurred. The product washed with methanol and dispersed in
toluene.
Simulation of LSPR of Cu2-xS nanocrystals
The 3D numerical simulations were performed using the finite element method
(FEM) with an implement of the commercial software package (Comsol 4.3a).[3]
LSPR effect is generally believed to be manifested in local electrical field
enhancement. The 2D contour of the electron field intensity (XOY-plane, z = 50 nm)
by FEM simulation with illumination source of 900-1800 nm are simulated by
locating the Cu2-xS nanocrystal in the center of a simulation box (Figure S5b).
Clearly, the electrical field intensities for 1300 nm and 1400 nm irradiations are much
stronger than other incident wavelengths, indicating the LSPR peak of around
1300-1400 nm. For imitating the dielectric circumstance in the experiment, the
background dielectric constant was fixed at 1.46 (Carbon tetrachloride). The dielectric
function of Cu2-xS is described using the Drude model.
ε(ω) = 1 −
𝜔𝑝2
𝜔 2 + 𝑖𝛾𝜔 2
Here ωp is the bulk plasma oscillation frequency associated with the free carriers and
γ is their bulk collision frequency. ωp and γ were deduced by the experimental
absorption spectra as 3.1 × 1015 rad/s and 7.6 ×1014 rad/s, respectively. The simulation
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area was 100 nm × 100 nm. In the vertical direction, the perfectly-matched-layer
(PML) was set to achieve absorbing boundary conditions[4]. The diameter of Cu2-xS
nanosphere was 4 nm. The mesh maximum element size was set as 0.2 nm in the
domains representing Cu2-xS and 3 nm for CCl4 subdomains.
Density functional theory simulation methods
We used the Vienna ab initio Simulation (VASP) package to simulate the electronic
structures of various interfaces.[5] Calculations were performed at the spin-polarized
density functional theory (DFT) level using frozen-core all-electron projector
augmented wave (PAW) model with the generalized gradient approximation (GGA)
and Perdew-Burke-Ernzerhof (PBE) functions.
An energy cutoff of 400 eV was
used for the plane-wave expansion of the electronic wave function. The force and
energy convergence criterion were set to 0.01 eV/Å and 10-5 eV, respectively. The
interfaces of CdS/ Cu2S was modeled with the system of 5 layers of CdS slab (2 × 1)
and 8 layers of metal slab (2 × 2), with 2 bottom layers and 4 top layers were fixed to
the bulk positions during the relaxation. The 8 × 8 × 1 k-points test was performed for
the first Brillouin zone using the gamma center scheme.
Photocurrent measurement
Photocurrent measurements were performed on a CHI 670D electrochemistry
potentiostat in a standard three-electrode configuration with the photocatalyst-coated
ITO as the working electrode, the Ag/AgCl as the reference electrode, and the Pt foil
as the counter electrode. A full-spectrum illumination was provided by a 300 W Xe
lamp with the light intensity of 100 mW/cm2. 0.5 M NaOH solution purged with N2
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for 30 min before measurement was used as electrolyte. The working electrodes were
prepared by spin coating a toluene suspension (100 L) made of ZnS, ZnS-CdS and
ZnS-CdS-Cu2-xS (with the concentration of 10 mg/mL) onto a pre-cleaned 1.5×1.5
cm2 ITO-coated glass at 1500 r.p.m. for 1 min. The working electrodes were dried at
300 oC in Ar atmosphere. The linear sweep voltammetry was conducted from -0.2 to
0.65 V vs. Ag/AgCl with a scan rate of 10 mV/s. The photoelectrochemical response
curves were obtained at 0.6 V vs. Ag/AgCl under chopped illumination.
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Figure S1. Energy band alignment of CdS-Cu2-xS. CdS is an n-type semiconductor
with a band-gap of ca. 2.4 eV, while Cu2-xS features p-type, representing a bulk band
gap of ca. 1.2 eV. The band gap of Cu2-xS embeds within that constructing CdS. CdS
contacts with Cu2-xS to form pn junction, and consequently, the Fermi level difference
would drive electrons to flow from CdS to Cu2-xS while holes to flow from Cu2-xS to
CdS, leading to the shifting of the two Fermi levels until they reach equilibrium. The
formation of pn junction between CdS and Cu2-xS is able to achieve type-II
heterojunction, facilitating the electron-hole separation. Ec = energy of conduction
band; Ef = energy of Fermi level; Ev = energy of valence band.
Figure S2. (a, b) TEM images of ZnS nanorods and binary heteronanorods (Zn/Cu =
0.7/0.6, in theory) by exchange reaction between ZnS (20 mg; 0.2 mmol)/toluene (20
mL) and [MeCN]4CuPF6 (44.6 mg; 0.12 mmol)/methanol (10 mL) at room
temperature. Insets show the solution color of prepared nanocrystals dispersed in
tolune, revealing the occurrence of cation exchange. (c) EDS spectrum of the binary
heteronanorods shown in b.
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Figure S3. (a) TEM image of binary multi-node sheath ZnS-CdS heteronanorods. (b)
Absorption spectrum for colloidal ZnS-CdS heteronanorods, demonstrating the
absorption range of UV and vis.
Figure S4. EDS spectrum of the ternary ZnS-CdS-Cu2-xS heteronanorods shown in
Figure 1b. The Mo and C elements are attributed to molybdenum grid and carbon film,
respectively.
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Figure S5. (a) NIR absorption band of ternary ZnS-CdS-Cu2-xS (Cu/Cd = 0.6/0.7, in
theory) obtained through experiment (blue hollow circle, extracted from the
absorption spectrum) and Gaussian function fitting (red hollow triangle). The LSPR
frequency ωsp and line width γ are 0.88 eV and 0.5 eV, respectively. (b) A finite
element method (FEM) simulation setup for ZnS-CdS-Cu2-xS. We simplified the
model as sphere Cu2-xS NPs (diameter of 4 nm) surrounded by carbon tetrachloride
(CCl4, with refractive index of 1.46).
Figure S6. Schematic illustration demonstrating the wide absorption range (UV, vis
and NIR), almost full-spectrum, resulted from the formation of ternary
ZnS-CdS-Cu2-xS heteronanorods.
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Figure S7. (a) The XPS spectra of sample ternary ZnS-CdS-Cu2-xS heterojunction
display the fine-scanned S2p, Zn2p, Cd3d and Cu2p peaks. (b) The S 2p3/2 and 2p1/2
peaks are centered at 161.7 eV and 162.4 eV; (c) The Zn 2p3/2 and 2p1/2 peaks are
centered at 1021.8 eV and 1044.1 eV; (d) The Cd 3d5/2 and 3d3/2 peaks are centered
at 404.5 eV and 411.3 eV; and (e) The Cu 2p3/2 and 2p1/2 peaks are centered at
931.1 eV and 950.6 eV.
Figure S8. TEM images of the ternary ZnS-CdS-Cu2-xS heteronanorods with different
theoretical molar ratio of Cu and Cd in the prepared products (a = 0.6/0.7; b = 1.2/0.4;
c = 1.0/0).
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Figure S9. (a) XRD patterns and (b) absorption spectra of the ternary
ZnS-CdS-Cu2-xS heteronanorods with theoretical molar ratio of Cu and Cd. The
diffraction intensity of Cu2-xS and the NIR absorption intensity became stronger with
increasing amount of copper sulfide in ZnS-CdS-Cu2-xS.
.
Figure S10. TEM images of (a, c, e) binary ZnS-CdS with different molar ratio of
ZnS and CdS in binary heteronanorods, and corresponding (b, d, f) ternary
ZnS-CdS-Cu2S heteronanorods synthesized from ZnS-CdS by adding Cu+ precursor.
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Figure S11. The atomic models for bare ZnS, CdS and Cu2S simulations. ZnS: cubic
structure, lattice parameters：a=b=c=5.448; α=β=γ=90˚. CdS: cubic structure, lattice
parameters：a=b=c=5.938; α=β=γ=90˚. Cu2S: approximating to hexagonal structure,
lattice parameters：a= 3.808, b=3.960, c=6.745; α=β=90 ˚, γ=116.260 ˚.
Figure S12. (a) The computed density of states (DOS) of bare ZnS, bare CdS and
bare Cu2S suggest bandgap values as 3.38, 2.40 and 1.10 eV, respectively. (b) Energy
band structure of bare ZnS, bare CdS and bare Cu2S aligned with the reaction
potential of water splittin.
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Scheme S1. The energy band alignment of ternary ZnS-CdS-Cu2-xS heteronanorod.
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