Enhanced photoelectrochemical and photocatalytic activityin
Visible-light-driven Ag/BiVO4 inverse opals
Liang Fang1, a), Feng Nan1, Ying Yang2 and Dawei Cao2, a)
1
College of Physics, Optoelectronics and Energy and Jiangsu Key Laboratory of Thin Films, Soochow University,
Suzhou, 215006, People’s Republic of China
2
a)
Institute of Physics & IMN MacroNano (ZIK), Ilmenau University of Technology, Ilmenau 98693, Germany
Author to whom correspondence should be addressed. e-mail: [email protected];
[email protected]
Summary: There are 7 pages including 4 figures.
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1. Preparation of Ag/io-BiVO4 photoelectrode
The preparation of Ag/BiVO4 inverse opals is illustrated in Figure S1.
Fluorine-doped SnO2 (FTO) substrates were cleaned by acetone, ethanol and
deionized water for 0.5 h each with sonication. A polystyrene (PS) sphere (SuZhou
Nanomicro Tech.) opal template was prepared by self-assembled method on the FTO
substrates. Then the self-assembled PS opal template was immersed vertically in
BiVO4 precursor to form io-BiVO4 by dip-coating sol-gel technique. The BiVO4
precursor is obtained by addition of 0.01 mol of Bi(NO3)3, 0.01 mol of NH4VO3 and
0.02 mol of citric acid into 30 mL of 23.3% HNO3 aqueous solution. Then 2.4 g of
polyvinyl alcohol and 7.5 mL of acetic acid were added into the solution with
intensely continuous stirring to promote the dissolution and reaction until a blue
transparent solution emerged. The template was dipped into the precursor sol for 5
min and then dried for 1 h in an oven (at 80°C) to transform the sol into a gel. To
ensure that the voids in the template were completely filled with the gel, the above
procedures were repeated at least 3 times. Finally, the sample was annealed at 350 °C
in air for 2 h to crystallize monoclinic BiVO4.
Ag nanoparticles were fabricated onto io-BiVO4 (Ag/io-BiVO4) in the dark from
a solution of 0.01M AgNO3 solution and 0.1M NaNO3 (in deionized water) by using a
standard three-electrode configuration. The io-BiVO4 photoelectrode acted as the
working electrode, Pt wire as the counter electrode, and Ag/AgCl as the reference
electrode. Then, a pulsed current of 8 mA·cm–2 with an alternating 0.1 s on time and
0.3 s off time was applied to the working electrode for Ag deposition using a
specifically designed software module. The electrodeposition time was controlled at
15, 35, 55 seconds, respectively. The obtained samples were labeled as
15s-Ag/io-BiVO4, 35s-Ag/io-BiVO4, and 55s-Ag/io-BiVO4, respectively. The sample
was cleaned with deionized water after one period of electrodeposition. For
comparison, 35s-Ag/d-BiVO4 photoelectrode was also prepared by the same method.
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Figure S1 Schematic diagram of the fabrication procedures for Ag/io-BiVO4
photoelectrode.
2. Characterization
The morphologies of the samples were characterized by using a JEOL
JSM-6700F field-emission scanning electron microscope (FESEM). Transmission
electron microscopy (TEM) images were obtained by a Tecnai G2F20 electron
microscope at an acceleration voltage of 200 kV. The X-ray diffraction (XRD)
patterns of the samples were measured by utilizing a Shimadzu thin film
diffractometer equipped with Cu Kα radiation (λ=1.540598Å). The ultraviolet-visible
(UV-vis) absorption spectra of the samples were obtained by a Perkin Elmer Lambda
750 UV/Vis/NIR Spectrometer. Photoluminescence spectra were measured by Jobin
Yvon Time-correlated Single Photon Counting System (Horibfm-2015).
3. Photoelectrochemical measurements
Photocurrent density and electrochemical impedance spectroscopy (EIS)
measurements were obtained by using electrochemical analyzer (CHI-660D, Shanghai
Chenhua Instrument Co. Ltd.) in a standard three-electrode configuration. The prepared
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BiVO4 photoelectrodes acted as the working electrode, Pt wire as the counter electrode,
and Ag/AgCl as the reference electrode. The Na2SO4 aqueous solution (0.1 M, pH=7)
acted as the electrolyte. Photocurrents were measured under applied potentials of 0V
vs. Ag/AgCl with 300W Xe-lamp illumination (λ > 420 nm). The intensity of the light
was measured to 100 mW·cm-2, calibrated with a Newport 1918-C photometer. EIS
spectrum was obtained by applying an open-circuit voltage (about 0.65 V) in the
frequency range of 0.01 Hz to 10 MHz with oscillation amplitude of 10 mV under no
illumination. Before performing EIS measurement, the measurement cell was
discharged to a designated potential, then kept under open-circuit condition for 1 h to
ensure the equilibrium of cell.
4. Photocatalytic degradation of organic pollutant
The decolorization of methyl blue (MB) was chosen to evaluated the
photocatalytic degradation efficiency of the samples in water at room temperature.
The experiment was conducted as the following steps: the samples were placed in a
rectangular beaker 20 ml of MB aqueous solution (2.5×10−5 M). In order to achieve
the adsorption-desorption equilibrium among the photocatalyst, MB, and water, the
solution was magnetically stirred in dark for 2 h before the photodegradation reaction.
The MB concentration after equilibration was determined by measuring the
absorbance at 663 nm, and taken as the initial concentration (C0). The illumination
source was a 300 W Hg lamp, which was placed 10 cm above the beaker. Visible light
irradiation was performed with a cut off filter (λ>420 nm). After one period of
reaction, the concentration of MB (C) was measured.
5. FDTD Simulation:
A commercial FDTD simulation package (Solutions Lumerical Solutions Inc.) is
utilized for calculating the local electrical field distributions for the ordered and
disordered BiVO4 inverse opal with a 25 nm Ag NP at λ = 450 nm. The diameter of
the PS spheres is 155 nm in ordered structure, while 155 nm and 200 nm PS spheres
are applied in disordered one in the simulation. The Ag NP is located on the surface
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of the BiVO4 inverse opal, but in 200 nm opal for disordered structure. The refractive
index of BiVO4 and water are set to 2.45 and 1.33, respectively. In both Figure 4(a)
and (b), the illumination was by plane wave source. The materials file of Ag used for
the simulations was from Johnson and Christy.
6. The morphology and composition of the different samples
Figure S2(a) shows the typical scanning electron microscopy (SEM) image of
io-BiVO4 sample. Black holes connecting the neighboring pores are three
dimensionally ordered in a hexagonal close-packed array, and this continuous porous
structure enables the subsequent introduction of Ag NPs as well as allowing electrolyte
unhindered or organic pollutant diffusion throughout the entire photocatalyst. The inset
of Figure S2(a) shows a cross-sectional SEM image of the inverse opal, which is
approximately 4.7 μm. As the reference sample, a disordered porous BiVO4 structure
(d-BiVO4, Figure S2(b)) was also fabricated with the same thickness under the same
conditions as the inverse opals. But it was prepared from a randomly packed template
formed with a mixture of 255 and 360 nm PS spheres. Figure S2(c) shows the SEM
image of the 35s-Ag/io-BiVO4 inverse opal, which has a similar morphology as the
pure BiVO4 inverse opal sample, indicating Ag NPs deposition process does not
damage the ordered structure of BiVO4. Closer observation reveals that Ag NPs with
size 25~30 nm are dispersed on both outside and inside of the BiVO4 inverse opals, as
indicated by the arrows. The elemental composition of the 35s-Ag/io-BiVO4 sample
was analyzed by energy dispersive spectrometry (EDS), confirming the presence of Ag,
Bi, V and O (Figure S2(d)). In order to investigate the dispersion of Ag NPs more
deeply, TEM was used to observe the 35s-Ag/io-BiVO4 sample. Figure S2(e) shows the
TEM image of the broken fractal of the samples obtained after ultrasonication, showing
that the sample has the hexagonal close-packed array structure. The formation of the
hetero-junctions between Ag and BiVO4 is also demonstrated by the High-resolution
TEM results, as shown in Figure S2(f). The observed lattice spacing of 0.309 nm
corresponds to the (112) crystallographic plane of monoclinic phase BiVO4, while that
of 0.236 nm corresponds to the (111) plane of the cubic face of Ag.
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Figure S2 SEM images of top view for (a) the io-BiVO4 sample (the inset is the cross
section view for the io-BiVO4 sample); (b) the d-BiVO4 sample; (c) the
35s-Ag/io-BiVO4 sample; (d) EDX of the 35s-Ag/io-BiVO4 sample; (e) TEM image
of the 35s-Ag/io-BiVO4 sample; (f) High resolution image of the interface region.
7. The XRD patterns of the different samples
The XRD patterns (Figure S3) of all the samples match the Joint Committee on
Powder Diffraction Standards File (JCPDS No. 75-2481) for BiVO4, which confirms
the monoclinic phase of these BiVO4 microstructures.
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Figure S3 XRD patterns of the different samples.
8. The photoelectrochemical and photocatalytic properties of io-BiVO4 with
different Ag NPs electrodeposition times
Figure S4 (a) Chronoamperometry measurements of the Ag/io-BiVO4 samples with
different Ag NPs electrodeposition times; (b) Photocatalytic degradation efficiencies
of MB using Ag/io-BiVO4 samples with different Ag NPs electrodeposition times.
The inset is first order rate constants of different samples.
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