Thin-walled NiO tubes functionalized with catalytic Pt for highly selective C2H5OH sensors using electrospun fibers as a sacrificial template

Article abstract of DOI:10.1039/c1cc13876f

Hollow thin walled NiO tubes functionalized by catalytic Pt were synthesized via nanofiber templating and multilayered sputter-coating of Pt and NiO thin overlayers followed by heat-treatment at 600°C. Sandwich Pt-NiO-Pt tube networks exhibited superior C₂H₅OH sensing response and remarkable selectivity against CO and H₂ gases.

Full text of DOI:10.1039/c1cc13876f

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 11300–11302
www.rsc.org/chemcomm
COMMUNICATION
Thin-walled NiO tubes functionalized with catalytic Pt for highly
selective C2H5OH sensors using electrospun fibers as a sacrificial
templatew
Nam Gyu Cho,a Hyung-Sik Woo,b Jong-Heun Lee*b and Il-Doo Kim*a
Received 29th June 2011, Accepted 7th August 2011
DOI: 10.1039/c1cc13876f
Hollow thin walled NiO tubes functionalized by catalytic Pt
were synthesized via nanofiber templating and multilayered
sputter-coating of Pt and NiO thin overlayers followed by
heat-treatment at 600 8C. Sandwich Pt–NiO–Pt tube networks
exhibited superior C2H5OH sensing response and remarkable
selectivity against CO and H2 gases.
literature include e-beam evaporation,4 in situ electrospinning
(e-spin),8a,9 sputtering coating of catalyst on a metal oxide
layer,10 and embedding of nanoparticles in fibers.11 Among
various synthetic routes of catalyst-decorated 1D metal oxide
nanostructures, the e-spin templating route, which utilizes
matrix polymeric nanofibers as a sacrificial layer for subsequent
coating of metal oxide and catalyst layers and high temperature
heat-treatment for producing hollow oxide tubes, would be
one of the most versatile techniques for creating novel catalysts-
loaded metal-oxide tube networks. The main strengths of
e-spin templating are the fabrication of well-defined poly
crystalline hollow fibers and selective functionalization of catalyst
layers on inner, outer, or both surfaces of tubular structures.
To the best of our knowledge, there have been no reports in
the literature on the fabrication of NiO tubes functionalized
with catalytic Pt layers and on the morphological/configura-
tional design of catalyst layers on inner, outer, or both
surfaces of NiO tube structures. Here, we report a facile route
to prepare thin-walled NiO tube networks whose inner and/or
outer layers are coated with nanocrystalline Pt islands and that
the functionalization of Pt is very effective to achieve ultra-
selective C2H5OH sensing characteristics against H2 and CO.
Outer and/or inner surfaces of hollow NiO tubes with a
total wall thickness of 50 nm were decorated by very thin and
discrete configuration of catalytic Pt overlayers of 7 nm via
reactive co-sputtering of NiO and Pt combined with subsequent
calcination at 600 1C. A co-sputtering approach was conducted
to prevent over-coating of continuous Pt layers. The pristine
NiO, Pt–NiO (Pt decoration on the outer surface of NiO
tubes), NiO–Pt (Pt decoration on the inner surface of NiO
tubes), Pt–NiO–Pt (Pt decoration on both of outer and inner
surfaces of NiO tubes) have been fabricated and those gas
sensing characteristics were measured. The roles of Pt in the
gas sensing reaction were investigated from the correlation
between the location/configuration of Pt catalysts and gas
sensing characteristics such as selectivity and sensitivity.
In order to fabricate NiO and Pt-decorated NiO tubes,
poly(vinyl pyrrolidone) (PVP, Mw: 1 300 000 g molÀ1) nanofibers,
used as a sacrificial polymeric template, were directly electro-
spun onto sensor substrates with Au electrodes from a solution
of N,N-dimethylformamide (DMF, Fluka, 98%). Following
the electrospinning of the PVP fiber templates, an overlayer of
NiO (50 nm), Pt (7 nm)–NiO (43 nm), NiO (43 nm)–Pt (7 nm),
One dimensional (1D) metal oxide sensing architectures, such
as wires, tubes, fibers and hollow fibers, have been considered
as ideal building blocks for high sensitivity gas sensors due to
high surface-to-volume ratio and a readily gas accessible
networked structure.1,2 Thus far, various single and poly-
crystalline 1D metal oxides such as SnO2,3 ZnO,4 TiO2,5
7
WO3,6 and Zn2SnO4 have been successfully assembled into
gas sensing layers. When single crystalline nanowires are used
as gas sensors, facile modulation of surface depletion depth
along the longitudinal direction of wires and formation of
additional potential barriers at their junctions lead to remarkably
enhanced gas response.1a Compared to single crystalline nano-
wire counterparts, polycrystalline nanofibers with a number of
grain-boundary contacts between the primary particles within
a nanofiber are promising sensing platforms to enhance gas
response.1b,2 Gas response can be enhanced further by employing
tubular or quasi-tubular structures because both of inner and outer
surfaces of nanotubes participate in the gas sensing reaction.
Moreover, under the tubular configuration, the ultimate design
of selectivity and sensitivity becomes possible by the functionalization
of a catalytic layer on the inner and/or outer surfaces of nanotubes.
To date, various catalysts such as Pt, Pd, Ag, CuO, and NiO
have been loaded on the surface of 1D nanostructures to
enhance gas sensing performances.8 Loading contents, distri-
bution, and size of metallic and/or metal oxide catalysts are
key parameters for improving gas selectivity as well as sensi-
tivity. Physico-chemical methods to load catalysts in the
a Department of Materials Science and Engineering, Korea Advanced
Institute of Science and Technology, Daejeon 305-701, Republic of
Korea. E-mail: idkim@kaist.ac.kr; Fax: +82-42-359-3310;
Tel: +82-42-350-3329
b Department of Materials Science and Engineering, Korea University,
Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea.
E-mail: jongheun@korea.ac.kr; Fax: +82-42-359-3310;
Tel: +82-42-350-3329
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc13876f
c
11300 Chem. Commun., 2011, 47, 11300–11302
This journal is The Royal Society of Chemistry 2011

View Article Online
images (Fig. 1i) revealed that catalytic Pt–NiO–Pt tubes had much
smaller grain sizes (5–10 nm) due to the grain growth inhibition
during film growth and high temperature calcination.8a,13 Inter-
planar distances of 2.41 A and 2.26 A correspond to the (111) of
NiO and (111) of Pt. TEM-EDS analysis showed the atomic ratio
(6.67 : 1) of Ni (47.4%) and Pt (7.1%) for Pt–NiO–Pt tubes.
XRD pattern also showed the existence of Pt phases, i.e.,
(200), (220), and (222) diffraction planes, in Pt–NiO–Pt tube
strucutres. (see ESIw, Fig. S1) These nanocrytalline Pt particles
can serve as effective catalysts in the gas sensing reaction. In
order to understand the role of catalytic Pt on the outer and/or
inner surface of NiO tubes and demonstrate the potential use
of Pt-decorated NiO tubes in highly sensitive and selective gas
sensing, we measured the gas responses (CO, H2, and
C2H5OH) of sensor prototypes comprising 4 different net-
works of NiO, Pt–NiO, NiO–Pt, and Pt–NiO–Pt tubes,
respectively. The resistance in air (Ra) of NiO, Pt–NiO,
NiO–Pt, and Pt–NiO–Pt tubes at 400 1C were 2.9, 14.2, 14.7,
and 42.6 kO, respectively (see ESIw, Fig. S2).
Pristine NiO tubes showed the lowest Ra value (2.9 kO)
while Pt–NiO–Pt tubes had the highest Ra of 42.6 kO. Considering
the reactive sputtering in oxidizing atmosphere and high
temperature heat treatment (600 1C), the electronic interaction
between Pt-related species and NiO, caused by partial oxidation
of Pt,14 can be considered as a plausible reason for the Pt-induced
increase of Ra although further studies are necessary. And the
grain growth inhibition by Pt doping can also play the role in
resistance variation. Interestingly Pt–NiO and NiO–Pt tubes
exhibited similar base resistance levels (see ESIw, Fig. S2).
As shown in Fig. 2a and b, NiO, Pt–NiO, NiO–Pt, Pt–NiO–Pt
tube-based sensors showed typical p-type gas sensing behaviors,
i.e., a resistance increase by reducing gases (100 ppm CO, H2, and
C2H5OH). The gas sensing transients of Pt-decorated NiO tube
networks to 1–100 ppm C2H5OH exhibited stable response and
recovery characteristics (Fig. 2b). The gas response of
Pt–NiO–Pt tubes to 100 ppm C2H5OH (Rg/Ra: 11, Rg: resistance
in gas, Ra: resistance in air) was 5.85-fold higher than that of pure
NiO tubes (Rg/Ra: 1.88) (Fig. 2c). In particular, the cross-
responses to 100 ppm CO and H2 were negligible (Fig. 2d).
Based on the similar tube morphologies and thickness of pristine
NiO and Pt-functionalized NiO tubes in the present study, the
enhanced C2H5OH sensing characteristics of Pt-decorated NiO
tubes are mainly attributed to catalytic functions of Pt on the
surface of thin-walled NiO tubes.
Fig. 1 Morphologies and crystal structures of NiO, Pt-coated NiO,
Pt–NiO–Pt tubes: (a) SEM image of the NiO tube network; (b) TEM
image of a single NiO tube; (c) magnified TEM image of (b); (d) selected
area electron diffraction pattern of NiO tubes; (e) SEM image of Pt–NiO
tubes; (f) SEM image of NiO–Pt tubes; (g) SEM image of sandwich
Pt–NiO–Pt tubes; (h) TEM image of Pt–NiO–Pt tubes; (i) magnified
TEM image of (h), inset shows lattice image of Pt–NiO–Pt tubes.
Pt (7 nm)–NiO (36 nm)–Pt (7 nm) was sputtered onto PVP
fiber mats. The NiO and Pt layers were co-sputtered from two
inch Pt and Ni metallic targets using DC (5 W, Ar/O2 12 : 8)
and RF (70 W) sputtering in a single chamber. NiO films were
reactively coated in oxygen atmospheres. After the room
temperature deposition of the Pt and NiO layers,
a
post-annealing process was performed using a box furnace at
600 1C for 2 hours in air to remove the as-spun PVP fiber
templates, leading to the formation of hollow NiO, Pt–NiO,
NiO–Pt, and Pt–NiO–Pt tube structures, respectively.
Fig. 1a presents an FE-SEM image of NiO tube networks. The
PVP templates were decomposed during the calcination step,
resulting in randomly oriented networks of NiO and/or
Pt-functionalized NiO hollow tubes. The diameters of the
individual tube were commensurate with that of the original
PVP fiber templates of 200–400 nm. Fig. 1b and c show TEM
images of a polycrystalline NiO tube with average grain size of
15–20 nm. The characteristic ring diffraction patterns in Fig. 1d
reveal that the NiO tube has a polycrystalline structure showing
the (111), (200), and (220) diffraction patterns indexed to the NiO
rock salt crystalline structure. Fig. 1e and f exhibit SEM images
of Pt–NiO and NiO–Pt tube networks, respectively. No signfi-
cant difference was observed in both morphologies. Fig. 1g
shows an SEM image of a thin-walled sandwich Pt–NiO–Pt tube
structure. Interestingly, as indicated by the arrows in Fig. 1g, open
tubular networks with uncoated parts are frequently observed
probably due to non-complete NiO and Pt coating on the surface
of round-shaped fibers by sputtering. Asymmetric coating nature
was proven in the previous report.12 Fig. 1h shows clearly a
hollow nature of the Pt–NiO–Pt tube (orange arrow). The wall
thickness was 15–20 nm. Careful examination of HR-TEM
A nearly full monolayer of oxygen ions (OÀ) is known to be
adsorbed on the surface of NiO by the facile oxidation of Ni2+
into Ni3+ 15
Thus, the electron generation by electrochemical
.
oxidation of reducing gases will increase the resistance of p-type
NiO. The Pt catalyst is widely known to promote a gas sensing
reaction by the spill over effect.16 C2H5OH is known to dissociate
into small molecules such as CH3CHO + H2 on the surface of
basic oxide such as NiO.17 Catalytically dissociated small gases
can either participate in the gas sensing reaction further or diffuse
away to the ambient atmosphere. Accordingly, the retention of
CH3CHO and H2 near the NiO sensing surface can play the key
role. Considering nearly closed configuration of tubes, these
gases will stay longer near the inner surfaces of NiO–Pt tubes
(Rg/Ra to 100 ppm C2H5OH: 6.7) rather than near the outer
surfaces of Pt–NiO tubes (Rg/Ra to 100 ppm C2H5OH: 2.4),
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11300–11302 11301

View Article Online
again. Pt–NiO–Pt tube networks showed a much higher gas
response (Rg/Ra to 100 ppm C2H5OH: 11.7) compared to
planar NiO thin films (Rg/Ra to 100 ppm C2H5OH: 1.69)
(see ESIw, Fig. S3d).
In summary, catalytic Pt-decorated NiO tubes were synthesized
via a nanofiber templating method. Multi-layered coating of Pt
onto the inner and/or outer surfaces of NiO and subsequent
heat-treatment lead to the formation of highly porus NiO tubes
functionalized by Pt. Thickness and loading contents of Pt
overlayers were carefully controlled via the film growth condition.
We applied NiO, Pt–NiO, NiO–Pt, and Pt–NiO–Pt tube networks
to semiconducting gas sensors and compared their gas sensing
characteristics. Remarkably high selectivity for C2H5OH against
CO and H2 gases was observed in sandwich Pt–NiO–Pt tube
sensors. The NiO tubes whose outer and inner surfaces were
decorated by Pt exhibited approximately 5.85-fold higher
C2H5OH sensitivity compared to the pristince NiO tube sensor,
leading to a versatile approach for application in volatile organic
compound sensors.
This work was supported by a grant from the Ministry of
Research, Korea and the Ministry of Science & Technology,
Israel. The work of J.-H. Lee was supported by the KOSEF
NRL Program (R0A-2008-000-20032-0).
Notes and references
1 (a) V. V. Sysoev, J. Goschnick, T. Schneider, E. Strelcov and
A. Kolmakov, Nano Lett., 2007, 7, 3182; (b) I. D. Kim,
A. Roschild, B. H. Lee, D. Y. Kim, S. M. Jo and H. L. Tuller, Nano
Lett., 2006, 6, 2009; (c) J. H. Lee, Sens. Actuators, B, 2009, 140, 319.
2 I. D. Kim and A. Rothschild, Polym. Adv. Technol., 2011, 22, 318.
3 J. M. Paik, M. Zielke, M. H. Kim, K. L. Turner, A. M. Wodtke
and M. Moskovits, ACS Nano, 2010, 4, 3117.
4 C. W. Na, H. S. Woo, I. D. Kim and J. H. Lee, Chem. Commun.,
2011, 47, 5148.
5 O. Landau, A. Rothschild and E. Zussman, Chem. Mater., 2009,
21, 9.
6 (a) B. B. Cao, J. J. Chen, X. J. Tang and W. L. Zhou, J. Mater.
Chem., 2009, 19, 2323; (b) G. Wang, Y. Ji, X. Huang, X. Yang,
P. I. Gouma and M. Dudley, J. Phys. Chem. B, 2006, 110, 23777.
7 S. H. Choi, I. S. Hwang, J. H. Lee, S. G. Oh and I. D. Kim, Chem.
Commun., 2011, 47, 9315.
8 (a) D. J. Yang, I. Kamienchick, D. Y. Youn, A. Rothschild and
I. D. Kim, Adv. Funct. Mater., 2010, 20, 4258; (b) I. S. Hwang,
J. K. Choi, S. J. Kim, K. Y. Dong, J. H. Kwon, B. K. Ju and
J. H. Lee, Sens. Actuators, B, 2009, 142, 105; (c) Y. Shen,
T. Yamazaki, Z. Liu, D. Meng and T. Kikuta, J. Alloys Compd.,
2009, 488, L21.
Fig. 2 (a) Gas concentration variations of C2H5OH; (b) gas
responses (Rg/Ra, Rg: resistance in gas and Ra: resistance in air) of
NiO, Pt–NiO, NiO–Pt, Pt–NiO–Pt tube-based sensors to 1–100 ppm
C2H5OH at 400 1C; (c) C2H5OH response vs. C2H5OH concentration;
(d) gas responses of NiO, Pt–NiO, NiO–Pt, Pt–NiO–Pt tube-based
sensors to 100 ppm CO, H2, and C2H5OH.
which explain the superior C2H5OH response (2.79-fold higher
sensitivity) of NiO–Pt tubes. These features are in good agreement
with the enhancement of toluene sensing characteristics in
WO3 films by the decoration of discrete Pd nanoparticles
between two interdigited electrodes when the electrodes are
sandwiched by WO3 films and substrates.18
9 B. Ding, M. R. Wang, X. F. Wang, J. Y. Yu and G. Sun, Mater.
Today, 2010, 13, 16.
10 C. C. Chan, W. C. Hsu, C. C. Chang and C. S. Hsu, Sens.
Actuators, B, 2010, 145, 691.
11 G. Z. Hu, Z. P. Zhou, Y. Guo, H. Q. Hou and S. J. Shao,
Electrochem. Commun., 2010, 12, 422.
12 S. H. Choi, G. Ankonina, D. Y. Youn, S. G. Oh, J. M. Hong,
A. Rothschild and I. D. Kim, ACS Nano, 2009, 9, 2623.
13 N. G. Cho, G. C. Whitfield, D. J. Yang, H. G. Kim, H. L. Tuller
and I. D. Kim, Sens. Actuators, B, 2010, 157, J435.
14 S. L. Saenger, C. Cabral Jr., C. Lavoie and S. M. Rossnagel,
J. Appl. Phys., 1999, 86, 6084.
15 D. Kohl, J. Phys. D: Appl. Phys., 2001, 34, R125.
16 (a) D. S. Vlachos, C. A. Papadopoulos and N. A. Avaritsiotis,
Sens. Actuators, B, 1997, 44, 458; (b) V. Srivastava and K. Jain,
Sens. Actuators, B, 2008, 133, 46.
It should be noted that the responses to C2H5OH were
further enhanced 5.85 times by employing a Pt–NiO–Pt sandwich
configuration, while the responses to CO and H2 were not
improved significantly (1.55 times, and 1.56 times) by the
functionalization of outer/inner Pt compared to pristine NiO
tubes. This cleary supports that the catalytic dissociation of
C2H5OH at the inner/outer surface of NiO tubes and the
elongation of retention time using tubular morphology is a
very promising approach to accomplish ultra-selective and
sensitive detection of C2H5OH with the minimal interferences
to small-size gases.
Additionally, we compared the gas response of Pt–NiO–Pt
tubes and planar NiO thin films (see ESIw, Fig. S3a–d). For
the gas sensing test, Pt–NiO–Pt tubes were dispersed in a
solvent with ultrasonication and coated on sensor substrates
17 T. Jinkawa, G. Sakai, J. Tamaki, N. Miura and N. Yamaoze,
J.Mol. Catal. A: Chem., 2000, 155, 193.
18 K. Kanda and T. Maekawa, Sens. Actuators, B, 2005, 108, 97.
c
11302 Chem. Commun., 2011, 47, 11300–11302
This journal is The Royal Society of Chemistry 2011