8
246 Inorganic Chemistry, Vol. 48, No. 17, 2009
Lepleux et al.
fabrication of electrochromic devices. In this work, we report
an improved hydrothermal route adapted from the work of
18
Xi and co-workers to prepare in situ self-organized nano-
porous NiO films on the surface of FTO coated glass
substrates. We show that this technique leads to reproducible
thick and crack free films which are suitable to set up
p-DSSCs with good photovoltaic performances.
2. Results and Discussion
2.1. In Situ Preparation of the NiO Films on SnO:F
Substrates. The synthetic route used to prepare nanos-
tructured NiO films is summarized in Figure 1a. This
protocol was inspired from the work of Xi and co-work-
1
8
ers who prepared NiO films by a two steps procedure,
that is a reaction of nickel nitrate hexahydrate with
hexamethylenetetramine in aqueous solution at 90 °C,
followed by heating at 300 °C. In our modified protocol,
[
Ni(NO ) 6H O] is replaced by [Ni(OCOCH ) 4H O],
3 2 2 3 2 2
3
3
and reaction takes place at 100 °C under hydrothermal
conditions (i.e., the precursors were placed in a sealed
autoclave and reacted under an autogenous pressure).
This leads to the formation of a pseudo amorphous
intermediate species, labeled hereafter NiX, which de-
poses as a green film on both sides of the FTO coated
substrate (Figure 1b and 1c). The green intermediate
species NiX was then converted into NiO upon heating
in air at 450 °C for 30 min, which led to a colorless or dark
film depending on the film thickness (Figure 1d) (vide
infra). At this stage, it is worthwhile to mention that (i) the
sintering temperature was set at 450 °C to enable a
straightforward comparison of the physical characteris-
tics of our photovoltaic cells with those reported in the
Figure 1. (a) Sketchy chemical route leading to the formation of NiO
films. (b) Representation of the FTO substrate covered by a green film in
the Teflon bomb after hydrothermal treatment at 100 °C. (c) 5 μm thick
Ni(OCOCH
before annealing. (d) NiO films prepared after hydrothermal treatment at
00 °C for 100, 150, 200, and 300 min and heating at 450 °C. The thickness
increases from 0.06, to 1.0, 1.6, and 3.5 μm from left to right. (e) TGA/
DSC curves of the green Ni(OCOCH O precursor labeled NiX in
the text.
3 2 2
) 4H O film after hydrothermal treatment for 300 min and
3
1
)
3 2
3 4H
2
parameter to consider, because it governs the total absor-
bance of the electrode and therefore the amount of photo-
generated charges. Usually, the preparation of nanoporous
NiO electrodes for DSSCs requested several steps, that is the
3
,6,8,21
literature,
even though this temperature is probably
not optimal (vide infra) and (ii) the deposition of the green
film on the FTO-electrode is strongly dependent on the
sample slant in the Teflon bomb. An angle of about 45°
appears optimal to give homogeneous films, while sub-
strates layed flat remained colorless and uncovered. In
our hands, this modified protocol yields thick, very
adhesive, and reproducible films on FTO substrates. It
is also worth mentionning that green films deposited on
the surface of FTO at 90 °C by simple coprecipitation and
sintered at 450 °C led to NiO films much less homoge-
neous, more fragile and less regular in thickness.
synthesis of the precursor (e.g., Ni(OH) ) as nanoparticles,
2
their intimate mixing in a “black box” paste in a mortar, the
deposition via the doctor blade technique onto the conduct-
5,7,8,12-14
ing substrate, and a reactive sintering at 450 °C.
Nevertheless, in our hands, this technique turned out to be far
from fully satisfactory. First, because it is very difficult to
obtain thick and crack free NiO films, and second its
reproducibility is relatively low. Several alternative routes
to prepare NiO films are reported in the literature, such as
1
5
9,16
chemical vapor deposition, electrodeposition,
thermal synthesis,
hydro-
2.2. Characterization of the NiO Films and Their
Intermediate Species. The specific surface area of NiX
powder determined by the Brunauer-Emmett-Teller
6,17,18
19
20
sputtering, and sol-gel. How-
ever, so far, these methods were essentially used for the
(
BET) and the Barrett-Joiner-Halenda (BJH) methods
2
is estimated at 22 m /g. Slow evaporation at room
temperature of the mother liquor remaining in the reactor
after hydrothermal treatment yields monocrystals, whose
structure could be solved by X-ray diffraction. This study
revealed the Ni O C N H compound built upon
(
(
12) Boschloo, G.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 3039.
13) Qin, P.; Zhu, H.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A.; Sun, L.
J. Am. Chem. Soc. 2008, 130, 8570.
14) Sumikura, S.; Mori, S.; Shimizu, S.; Usami, H.; Suzuki, E.
(
J. Photochem. Photobiol., A 2008, 199, 1.
4
32 34 12 84
(
(
15) Velevska, J.; Ristova, M. Sol. Energy Mater. Sol. Cells 2002, 73, 131.
16) Uplane, M. M.; Mujawar, S. H.; Inamdar, A. I.; Shinde, P. S.;
[Ni(H O) (OCOCH ) ] and [Ni(H O) (OCOCH ) ] poly-
2 2 3 2 2 3 3 2
Sonavane, A. C.; Patil, P. S. Appl. Surf. Sci. 2007, 253, 9365.
17) Xia, X. H.; Tu, J. P.; Zhang, J.; Wang, X. L.; Zhang, W. K.; Huang,
hedra connected via [C H N ] molecules. Nevertheless,
6 12 4
(
based on the examination of the powder X-ray diffrac-
tion (XRD) analysis and TGA curve of the green NiX
film (vide infra), this material cannot be retained as the
precursor of NiO in the aforementioned hydrothermal
reaction. Formally, the powder pattern of the green NiX
H. Sol. Energy Mater. Sol. Cells 2008, 92, 628. Pejova, B.; Kocareva, T.;
Najdoski, M.; Grozdanov, I. Appl. Surf. Sci. 2000, 165, 271. Kuang, D.-B.; Lei,
B.-X.; Pan, Y.-P.; Yu, X.-Y.; Su, C.-Y. J. Phys. Chem. C 2009, 113, 5508.
(
18) Xi, Y. Y.; Li, D.; Djurisic, A. B.; Xie, M. H.; Man, K. Y. K.; Chan,
W. K. Electrochem. Solid-State Lett. 2008, 11, D56.
19) Urbano, A.; Ferreira, F. F.; deCastro, S. C.; Landers, R.; Fantini,
M. C. A.; Gorenstein, A. Electrochim. Acta 2001, 46, 2269.
20) Park, J.-Y.; Ahn, K.-S.; Nah, Y.-C.; Shim, H.-S.; Sung, Y.-E.
J. Sol-Gel Sci. Technol. 2004, 31, 323.
(
::
(
(21) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarstrom, L.
J. Phys. Chem. C 2008, 112, 9530.