1. Art and Diseño

Electronic Supplementary Material (ESI) for Energy & Environmental Science.
This journal is © The Royal Society of Chemistry 2015
Supporting Information
Nonmetal P-doped Hematite Photoanode with Enhanced
Electron Mobility and High Water Oxidation Activity
Yuchao Zhang, a Shiqi Jiang, b Wenjing song, a Peng Zhou, a Hongwei Ji, a Wanhong Ma, a Weichang Hao, b
Chuncheng Chen,* a Jincai Zhao a
a
Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b
Center of Materials Physics and Chemistry, Beihang University, Beijing 100191, China
EXPERIMENTAL SECTION
Photoanode preparation. Hematite nanowires were prepared on a fluorine-doped tin oxide (FTO, TCO-15,
Nippon Sheet Glass, Japan, 14 ohm/sq) glass substrate by a modified procedure reported by Yat Li and
coworkers.1 Briefly, 100 ml aqueous solution containing 2.43 g of ferric chloride (FeCl3.6H2O, Alfa Aesar, 98%)
and 0.85 g of sodium nitrate (NaNO3, J&K, 99%) at pH = 1.4 (adjusted by HCl) were prepared in a Teflon-lined
stainless steel autoclave. Several FTO glass slides (2  4 cm), washed with acetone, ethanol, and then deionized
water, were placed in the autoclave and heated at 95 °C for 4 hours. After the hydrothermal treatment, a uniform
layer of iron oxyhydroxides (FeOOH) was coated on the FTO glass, which was washed completely with deionized
water to remove any residual salts. The obtained film was sintered in air at 550°C for 2 hours to convert the
FeOOH nanowires into hematite nanowires. The hematite nanowires were further annealed at 650°C for 20 min, or
at 750°C for 15 min.
P-doped hematite nanowires were prepared by the same procedure as hematite nanowires, except that FeOOH
films were soaked in 0.05M of Na2HPO4 solution with different pH for 1min and then immediately underwent the
sintering and annealing procedure without other modifications. The pH of the soaking solution was adjusted by
HCl and NaOH solutions and measured by a pH-meter (Thermo Scientific, 3-Star).
“Co-Pi” modification was fabricated according to the photo-assisted electrodeposition methods.2 The hematite
film was submerged in a solution of 0.5 mM cobalt nitrate in 0.1 M potassium phosphate (KPi) buffer (pH 7). A
150 W Xenon lamp coupled to a filter (AM 1.5G) was used as the white light source. The light power density of
100 mW/cm2 was measured with a radiometer (CEAULIGHT, CEL-NP2000). “Co-Pi” was electrodeposited at
+0.1 V vs. Ag/AgCl for 15 min.
Photoelectrochemical characterization PEC performances of the films were measured in 1M NaOH electrolyte
solution (pH 13.6) in a three-electrode electrochemical cell. The electrolyte solution was deaerated by purging
argon for 30 min before J-V scan. The measured potentials vs Ag/AgCl were converted to the reversible hydrogen
electrode (RHE) scale according to the Nernst equation: ERHE = EAg/AgCl + 0.059pH + 0.1976. A 150 W Xenon
lamp coupled to a filter (AM 1.5G) was used as the white light source. The light power density of 100 mW/cm2
was measured with a radiometer (CEAULIGHT, CEL-NP2000). The exposed area for all the photoanodes were
controlled by a Teflon mold with a standard 1 cm2 hole.
Electrochemical impedance spectra were measured under visible light illumination (λ>420 nm, I0=60 mW/cm2),
recorded by an electrochemical workstation (PGSTAT302N autolab, Metrohm). A sinusoidal voltage pulse of 10
mV amplitude was applied on a bias voltage, with a frequency that ranged from 10 kHz to 1Hz. Mott-Schottky
plots were collected with bias voltage scanned from -0.4 to 0.1 V vs. Ag/AgCl with a 50 mV interval, in the dark
at a frequency of 1000Hz. The raw data were fitted using Nova 1.8 software from Metrohm Inc.
Oxygen detection Oxygen was detected by a Thermo Scientific Orion 3-Star dissolved oxygen meter. The
photoelectrochemical cell was a two-compartment cell divided by a porous ceramics for cutting off the gas flow
but keeping the conduction of the circuit. The hematite photoanode together with Ag/AgCl reference electrode and
the sensor of the dissolved oxygen meter were placed in one cell, and the Pt counter electrode was placed into the
other one. Both cells were filled with 1M NaOH electrolyte solution (pH 13.6). The electrolyte in the working
compartment was 30 mL. A 150 W Xenon lamp coupled to a filter (AM 1.5G) was used as the white light source.
The applied potential was 1.23 VRHE. Before every experiment, the whole system was deaerated by purging
high-purity argon for one hour and then all of the connectors were sealed by silicone rubber. This procedure was
repeated for several times until the reading of the dissolved oxygen meter kept below 0.1 mg/L for 30 min. The
quantity of electric charge passing through the circuit was recorded by an electrochemical workstation (CHI 760D).
The charge passed during the experiment was then converted to moles by dividing by 4F (F = 96485 C/mol).
Structural characterization X-ray diffraction (XRD) measurements were performed on a Regaku D/Max-2500
diffractometer with the Cu KR radiation (1.5406 Å). Diffraction patterns were recorded from 20 to 80° 2θ with a
step size of 0.04° at 4°/min. X-ray photoelectron spectroscopy (XPS) data were obtained with an
ESCALab220i-XL electron spectrometer from VG Scientific using 300W Al Kα radiation. The base pressure was
about 3×10-9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. Ar
ion etching was conducted at an Ar partial pressure of 2 × 10-7 mbar at 3 kV and 10 mA resulting in 0.07 nm/sec
etching rate. SEM data were obtained with a Hitachi S4800 scanning electron microscope (HITACHI Ltd) with the
acceleration voltage of 15 KV. TEM data were performed on a Tecnai G2 F30 S-TWIN 300kV field-emission
microscope. XAFS experiments were carried out at the XAFS station of the Beijing Synchrotron Radiation Facility
(BSRF). The beamline provided a focused X-ray beam from 5 to 20 keV with a photon flux on the order of 1011
phs/s (@ 10 keV) and a beam size of 0.9（H）× 0.3（V）mm2. The energy was calibrated by a standard Fe metal foil
before experiments started. Fe K-edge X-ray absorption fine structure (XAFS) spectra of the samples were
collected by a Lyttle fluorescence detector at room temperature. The X-ray absorption fine structure (XAFS) was
collected from -20 to +60 eV relative to the Fe K-edge. P K-edge XAFS spectra were collected at beamline 4B7A
of BSRF, with energy range from 2.05keV to 6.0keV, and energy bandpass about 0.3eV at P K-edge. The sample
was measured in PFY model with a SDD detector. The data analysis of XANES was carried out using the Athena
and Artemis interfaces to the IFEFFIT program package.
DFT calculations The first-principle calculations were carried out using the Vienna ab initio simulation package
(VASP) based on DFT+U with projector-augmented wave (PAW) pseudo-potential method. The exchange and
correlation energy was treated via the generalized gradient approximation (GGA) using the Perdew-Burke-Emzerh
of formulation. A conjugate-gradient algorithm was used to relax the ions into their ground states, and the energies
and the forces on each ion were converged within 1.0 × 10-4 eV/atom and 0.01 eV/Å, respectively. The Koln-Sham
orbitals were expanded by a plane wave basis set and an energy cutoff of 400 eV was used throughout. O 2p
orbitals were regarded as valence band orbitals while Fe 3d and 4s were regarded as conduction band orbitals. The
Brillouin-zone integration is per-formed by using the Gamma-centered Monkhorst-Pack scheme with 5×5×1
k-points, together with a Gaussian smearing broadening of 0.1 eV.
The electron effective mass can be calculated as:
1.
Y. Ling, G. Wang, D. A. Wheeler, J. Z. Zhang and Y. Li, Nano. Lett., 2011, 11, 2119-2125.
2.
D. K. Zhong, M. Cornuz, K. Sivula, M. Graetzel and D. R. Gamelin, Energy. Environ. Sci., 2011, 4,
1759-1764.
2
J (mA/cm )
2.5
2.0
1.5
1.0
A
0.5
0.0
0.6
0.8
1.0
1.2
1.4
Potential (V vs. RHE)
1.6
1.8
1.0
1.8
1.6
B
2-
-
HPO4
H2PO4
0.8
1.4
1.2
0.6
1.0
0.8
0.4
0.6
3-
PO4
H3PO4
0.4
0.2
0.2
0.0
2
4
6
8
10
Mole Fraction
3
5
6
7
8
9
10
11
12
13
Photocurent density (mA/cm-2)
3.0
0.0
14
12
pH
Figure S1. (A) J-V scans collected for P-doped hematite synthesized by phosphate solutions with different pH and
annealed at 650C. (B) Photocurrent densities obtained at 1.23 VRHE.
0.20
pH3
pH8
pH12
2
J (mA/cm )
0.15
0.10
0.05
0.00
0.6
0.8
1.0
1.2
1.4
1.6
Potential (V vs. RHE)
Figure S2. J-V scans collected for undoped hematite photoanodes synthesized by solutions with different pH and
annealed at 650C.
80
NW650
PNW650
NW750
PNW750
8
6
70
A
Faradaic efficiency
O2 produced (mol)
10
4
2
0
0
5
10
15
20
Time (min)
25
30
B
60
50
40
30
20
PNW650
NW750
PNW750
10
0
5
10
15
20
Time (min)
25
30
Figure S3. (A) O2 evolution and (B) the corresponding faradic efficiency detected by a dissolved oxygen meter
during the PEC water oxidation at 1.23 VRHE for the pristine and P-doped hematite photoanodes annealed at 650C
and 750C. The faradaic efficiencies for all the PEC oxygen evolution were around 70%. Since no other species
except H2O can be oxidized in our PEC systems, the deviation of the faradaic efficiencies from unity should be
caused by some systematic errors.
3.4
3.0
B
A
3.3
2.9
3.1
2
J (mA/cm )
2
J (mA/cm )
3.2
3.0
2.9
2.8
2.7
2.6
2.7
2.6
0.0
2.8
0.2
0.4
0.6
0.8
2.5
1.0
0
5
10
15
Time (h)
20
25
30
35
40
Time (h)
Figure S4. Long-term PEC experiments for (A) PNW750+CoPi and (B) PNW750 in 1 M NaOH at 1.23 VRHE.
NW650
PNW650
A
Absorbance (a.u.)
Absorbance (a.u.)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
400
500
600
700
Wavelength (nm)
800
IPCE (%)
40
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
B
400
NW750
PNW750
500
600
700
Wavelength (nm)
800
PNW750
NW750
PNW650
NW650
C
30
20
10
0
350
400
450
500
550
Wavelength (nm)
600
650
Figure S5. UV-vis data collected for P-doping hematite photoanodes annealed at (A) 650C and (B) 750C. (C)
IPCE values collected for undoped and P-doped hematite photoanodes.
-2
10
-2
4
C ( 10 F cm )
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
NW650
PNW650
0.60 0.65 0.70 0.75 0.80 0.85 0.90
Potential (V vs. RHE)
Figure S6. Mott-Schottky plots collected in the dark for undoped and P-doped hematite photoanodes annealed at
650C.
12
NW650-exp
PNW650-exp
NW650-fit
PNW650-fit
8
2
-Z'' (10 )
10
6
Cbulk
Rs
4
CPE
Rct,trap
Rtrapping
2
Ctrap
CPE
0
0
2
4
6
2
8
10
12
14
Z' / (10 )
Figure S7. Nyquit plot collected for NW650 and PNW650 at 1.2VRHE. Inset is the equivalent circuit used for the fit
and simulation. In this model, Rs stands for all the series resistances in the electrochemical cell, Cbulk represents the
capacitance of the space charge depletion region at the surface of the electrode, Rtrapping represents the resistance in
the process of trapping holes by surface states, Ctrap represents the amount of active sites in surface states, and
Rct,trap represents the resistance in the process of charge transfer across the interface.
4
3
30
0
2
-30
1
-60
-90
pH3
pH8
pH12
0
B
120
2
60
150
Rs (cm )
A
Photocurrent (mA)
3
2
Rct,trap (  cm )
90
90
60
30
0
pH3
pH8
pH12
2.5
-2
Cbulk(10 F cm )
C
2.0
-5
1.5
1.0
0.5
0.0
pH3
pH8
pH12
Figure S8. Fitted values of (A) Rct,trap, (B) Rs, (C) Cbulk from EIS spectra collected for P-doped hematite anodes
impregnated by phosphate solutions with different pH and annealed at 650C.
Fe2O3
20
-1
-1
DOS (states.eV .atom )
40
0
-20
2.0 eV
-40
-20
-15
-10
-5
Energy (eV)
Figure S9. DOS of pure hematite.
0
5
10
Figure S10. DOS of P-doped hematite structures.
Fe2O3
A)
B)
Energy (eV)
Energy (eV)
0
-2.5
A
H
K
M
L
F
Q
Z
Fe3
D)
Fe2
C)
2.5
Energy (eV)
Energy (eV)
0
-2.5
H
2.5
0
-2.5
Fe1
2.5
2.5
F
Q
Z
0
-2.5
F
Q
Z
E)
Fe4
Energy (eV)
2.5
0
-2.5
F
Q
Z
Figure S11. Band structures of (A) pristine hematite and (B)-(E) P-doped hematite.
A)
B)
C)
D)
E)
Figure S12. Electron density distribution plots for (A) pristine and (B)-(E) P-doped hematite (110) surface.
800000
NW650
PNW650
700000
B
9000
8500
500000
Counts
Counts
600000
9500
A
400000
300000
200000
P 2p
8000
7500
7000
6500
100000
6000
0
0
200
400
600
800
1000
Binding Energy (ev)
1200
125
130
135
140
Binding Energy (ev)
145
Figure S13. (A) The XPS survey data collected for NW650 and PNW650. (B) The P 2p detail spectrum collected
for PNW650. The XPS survey data confirm the presence of P on the surface of PNW650, while no P signal is
detected on NW650. The high resolution P 2p spectrum shows that the P 2p binding energy of 133.3 eV which is
much higher than the reported for phosphate (132.1-132.9 eV), but is nearly consistent with values reported for
FePO4 (133.4 eV).
PNW650
NW650
Figure S14. SEM images of undoped and P-doped hematite (cross-section view).
PNW750
NW750
Figure S15. SEM images for P-doped (right) and undoped (left) hematite photoanodes annealed at 750C. Similar
to the situation in 650C, the P-doped hematite nanowires annealed at 750C became thinner than the undoped
1.2
* *
*
1.0
0.8
0.6
0.4
0.2
0.0
10
20
30
40
50
60
2 Theta (degree)
70
80
*
(110)
* FTO
1.8
*
1.6
1.4
1.2
NW750
PNW750
B
*
(300)
*
1.4
2.0
(104)
1.6
NW650
PNW650
A
Normalized Intensity (a.u.)
*
* FTO
1.8
(300)
2.0
(104)
(110)
Normalized Intensity (a.u.)
ones.
**
1.0
0.8
0.6
0.4
0.2
0.0
10
20
30
40
50
60
70
80
2 Theta (degree)
Figure S16. XRD patterns of undoped and P-doped hematite for (A) annealed at 650C and (B) annealed at 750C.
pH3
pH8
pH12
0.6
pH3
pH8
pH12
8
Normolized x
Normolized x
0.8
0.4
0.2
7114 7116 7118 7120 7122 7124
6
4
2
0
2150
E (eV)
2151
2152
2153
E (eV)
Figure S17. A comparison of Fe K-edge X-Ray absorption near edge structure (XANES) spectra (left) and P
K-edge XANES spectra (right). For Fe K-edge XANES spectra, the pre-edge peak at 7114 eV exhibited a gradual
increase with the decrease of pH. This pre-edge peak is a signature of the 1s-3d transition of tetrahedrally
coordinated Fe, which is commonly found in Fe3O4 and quite weak for the α-Fe2O3. The larger pre-edge peak
4
*
3
A
*
*
* *
NW650
pH3
pH8
pH12
2
1
0
20
30
40
50
60
2 Theta (degree)
70
80
Relative Intensity of (110) Diffraction
* FTO
(300)
5
(104)
(110)
Normalized Intensity (a.u.)
suggests that the content of tetrahedrally coordinated Fe increased with the decrease of pH.
0.60
pH3
pH8
pH12
0.55
0.50
B
0.45
0.40
0.35
0.30
0.25
560
580
600
Temperature(0C)
620
640
Figure S18. (A) XRD patterns collected for undoped and P-doped hematite anodes impregnated by phosphate
solution with different pH and annealed at 650C. (B) The relative (110) diffraction from temperature programmed
XRD.
Table S1. Fitted values of the EIS data in Figure 2A
Rs
Cbulk
Rtrapping
-6
NW650
142.5 Ωcm2
PNW650
119.4 Ωcm2
5.9 × 10 F
-2
cm
2.2 × 10-5 F
cm-2
Ctrap
Rct,trap
-5
3194.2 Ωcm2
230.6 Ωcm2
4.6 × 10 F
cm-2
4.9 × 10-4 F
cm-2
25441.2 Ωcm2
81.5 Ωcm2
Table S2. The content of P from XPS experiments for P-doped hematite anodes impregnated by phosphate solution
with different pH and annealed at 650C.
P%
pH3
pH8
pH12
4.14
4.87
2.92