Supporting information
Hydrothermal growth of highly oriented single crystalline Ta2O5 nanorod arrays
and their conversion to Ta3N5 for efficient solar driven water splitting
Zixue Su,[a] Lei Wang,[a] Sabina Grigorescu,[a] Kiyoung Lee,[a] Patrik Schmuki [a,b] *
[a] Department of Materials Science and Engineering, WW4-LKO, University of Erlangen-Nuremburg,
Martensstr. 7, D-91058 Erlangen, Germany.
* Corresponding Author: [email protected] (P. Schmuki)
[b] Department of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia.
Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2014
Experimental Section
Two different sizes of Ta foils with a thickness of 0.10 mm (99.9% purity, Advent,
England) were used including the standard size of 1.25 cm x 1.25 cm, and the small size of
0.6 cm x 0.6 cm. In a typical synthesis of Ta2O5 nanorod arrays, a piece of Ta foil was first
degreased by sonication successively in ethanol, acetone, and deionized water. After drying in
N2, the pretreated Ta foil was put into a 250 ml Teflon-lined autoclave filled with an aqueous
solution of 0.1 M HF with or without H2O2. The filling factor of the solution (volume of
liquid to total autoclave volume) varied from 10% to 40%. The autoclave was then heated to
the target temperature of 240 °C for 6 ~ 48 h to enable the growth of Ta2O5 nanorod arrays on
the surface of Ta foil. To obtain Ta3N5 nanorod arrays, the hydrothermally grown Ta2O5
nanorod arrays on the Ta foils were then heated in a quartz tube furnace in a gaseous
atmosphere of NH3 with a flow rate of about 200 mL min-1 at 1000 °C for 2 h.
Before the water splitting measurement, Co(OH)x or Co-Pi were deposited on the Ta3N5
nanorod surface as oxygen evolution catalysts. (resulting in both cases in approx. 0.3 at.% Co
content (EDX in Figure S4)) In a typical Co(OH)x treatment, the nanorod photoanodes were
immersed in a mixed solution of 0.1 M CoSO4 and 0.1 M NaOH with a ratio of 1:1 for 25 min,
then washed and dried in N2. The Co-Pi catalyst was loaded by electrodeposition in a solution
of 0.5 mM Co(NO3)2 in 0.1 M potassium phosphate buffer at pH=7 at 1 V versus Ag/AgCl for
8 min. The photoelectrochemical water splitting was carried out in a standard three-electrode
system, where the Ta3N5 nanorod arrays, Ag/AgCl electrode and Pt electrode acted as
working electrode, reference electrode and counter electrode, respectively. The electrolyte
used was aqueous KOH solution with a pH value of 13.7. The working electrode was
illuminated under AM 1.5G irradiation (100 mW cm-2). According to the Nernst equation
(ERHE = EAg/AgCl+0.059pH+0.196), potentials vs. Ag/AgCl can be converted to potentials vs.
the RHE.
Morphological characterizations of the grown Ta2O5 and Ta3N5 nanorod arrays were
carried out on a field emission scanning electron microscopy (Hitachi FE-SEM S4800, Japan).
X-ray diffraction (X’pert Philips MPD with a Panalytical X’celerator detector, Germany) was
carried out using graphite monochromized Cu Kα radiation (Wavelength 0.154056 nm).
Transmission electron microscopy (TEM) was performed by using a Philips CM300
UltraTWIN, equipped with a LaB6 filament and operated at 300 kV. TEM images and
diffraction patterns were recorded with a fast scan (type F214) charge-coupled device camera
from TVIPS (Tietz Video and Image Processing Systems), with an image size of 2048 x 2048
pixels. The SAED patterns were evaluated by using the software JEMS1 and incorporating
crystal data information from the inorganic crystal structure data base (ICSD). For TEM
investigations the samples were first mechanically scratched from the Ta substrate and the
resulting sample powder was prepared on commonly used copper TEM grids coated with
lacey carbon film.
1. A. Stadelmann, Jems Electron Microscopy Software (1999–2012), java version 3.7624U2012,
CIME-EPFL, Switzerland.
Figure S1. TEM image showing the micropores present in the base oxide layer underneath
the nanorod arrays.
Figure S2. Current-potential curves of PEC water splitting cell with a photoanode of
Co(OH)x treated Ta3N5 nanorod arrays obtained from 2 h nitridation at 1000 °C of 12
h, 24 h, and 48 h grown Ta2O5 nanorod arrays. All curves were measured in aqueous
KOH solution (pH=13.7) under chopped AM 1.5G simulated sunlight, with a scan rate
of 10 mV s-1 from negative to positive.
Figure S3. TEM analysis of the Ta3N5 sample: a) and b) are BF TEM images showing
agglomerates of nanoparticles. The BF TEM image b) shows a magnified region from a). The
SAED pattern in c) was recorded at the position marked with the dashed circle in a). In d-f)
the same experimental diffraction pattern (which is shown in c) is shown with the
corresponding simulations obtained by software JEMS.
Figure S3 shows the TEM analysis of the sample Ta3N5. The bright-field (BF) TEM images
in a) and b) show a representative agglomerate of particles found on the TEM grid after the
preparation procedure. The black dashed circle in a) marks the area where the selected area
electron diffraction (SAED) pattern, which is shown in c), is acquired. The SAED patern in c)
exhibits dotted circles, which indicates the presence of nanoparticles, being in good
agreement with observations from BF TEM imaging. The evaluation of the diffraction
patterns was performed by simulating diffraction patterns with the software JEMS and
comparison with the experimental SAED pattern. Representative results are shown in Figure
S3 d-f), which confirm the presence of Ta3N5 and TaN, having orthorhombic and hexagonal
crystal structure, respectively. For a better overview, in d) the simulated and experimental
diffraction pattern of Ta3N5 (ICSD 66533) is shown, while e) shows the simulated and
experimental pattern of TaN (ICSD 25659). In f) the experimental and simulated patterns of
both, Ta3N5 and TaN, are shown.
Figure S4. EDS analyses of (a) Co(OH)x and (b) Co-Pi treated Ta3N5 nanorods.
Element at.%
Ta 51.7
N 47.95
Co 0.35
Element at.%
Ta 55.56
N 44.12
Co 0.32
(a)
(b)
Figure S5. Typical SEM images of (a) Ta2O5 films hydrothermally grown on pure Ta foils in
0.1 M HF at 240 °C for 24 h with a filling factor of 40%, (b) Ta2O5 films grown on Ta foils
by 40 min oxidation at 500 °C in air, and Ta2O5 films grown on the pre-oxidized Ta foil under
the same condition of (a) for 6 h (c) and 24 (h) respectively.
Figure S6. SEM images of Ta2O5 films grown on a Ta foil by 48 h hydrothermal reaction in
0.1 M HF with 40% filling factor of solution under 240 °C. The Ta foils used in (a, c) are of
standard size, while Ta foils used in (b, d) are of small size.
Figure S7. SEM images of Ta2O5 films grown on a Ta foil by 24 h hydrothermal reaction in
0.1 M HF with 10% filling factor of solution under 240 °C.
Influence of various experimental parameters such as pre-oxidation of the Ta, Ta foil size,
and solution filling factor on the morphologies of Ta2O5 film were studied. Figure S5 shows
that a Ta2O5 film with a thickness of about 1.6 µm was produced after 40 min oxidation at
500 °C in air. Due to the dissolution of the pre-formed Ta2O5 films, the H2TaF7 concentration
in the solution would be higher compared to that when standard pure Ta foils were used after
the same reaction time. This led to an earlier growth of the nanorod arrays (Figure S5c) for 6
h and thicker base oxide films for 24 h reaction (Figure S5c). Since Ta foils of small size
have smaller contact areas with Ta foils of standard size, the H2TaF7 concentration in the
solution will also be lower after the same condition. Figure S6 shows that after 48 h
hydrothermal reaction, the thickness of the base oxide layer is much thinner when a smaller
Ta foil was used. The concentration of H2TaF7 could also be increased by decreasing the
solution filling factor after the same time reaction due to smaller amount of water in the
solution. Figure S7 shows that when the solution filling factor decreased to 10%, 24 h
hydrothermal reaction in 0.1 M under 240 °C led to the growth of a thin nanorod layer and a
thick base oxide layer. Moreover, a thick layer of dispersed flower-like Ta2O5 nanorods was
also found to be deposited on top of the nanorod layer, which probably formed directly in the
bulk solution due to very high concentration of H2TaF7.