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Perovskite Nanoparticle-Sensitized Ga2O3 Nanorod Arrays for CO Detection at High Temperature

TL;DR: Detailed electron microscopy and X-ray photoelectron spectroscopy studies suggested the LSFO nanoparticle sensitization effect is attributed to a spillover-like effect associated with the gas-LSFO-Ga2O3 triple-interfaces that spread the negatively charged surface oxygen ions from LS FO nanoparticles surfaces over to β-Ga 2O3 nanorod surfaces with faster surface CO oxidation reactions.
Abstract: Noble metal nanoparticles are extensively used for sensitizing metal oxide chemical sensors through the catalytic spillover mechanism. However, due to earth-scarcity and high cost of noble metals, finding replacements presents a great economic benefit. Besides, high temperature and harsh environment sensor applications demand material stability under conditions approaching thermal and chemical stability limits of noble metals. In this study, we employed thermally stable perovskite-type La0.8Sr0.2FeO3 (LSFO) nanoparticle surface decoration on Ga2O3 nanorod array gas sensors and discovered an order of magnitude enhanced sensitivity to carbon monoxide at 500 °C. The LSFO nanoparticle catalysts was of comparable performance to that achieved by Pt nanoparticles, with a much lower weight loading than Pt. Detailed electron microscopy and X-ray photoelectron spectroscopy studies suggested the LSFO nanoparticle sensitization effect is attributed to a spillover-like effect associated with the gas-LSFO-Ga2O3 triple-...

Summary (2 min read)

■ INTRODUCTION

  • Therefore, there is an urgent need to develop new sensor materials meeting such performance criteria in sensitivity and robustness, which are preferentially combined with low cost.
  • 13 However, due to the earth-scarcity, the concern over high cost of noble metals is an ongoing issue, and therefore, reduction or complete elimination of noble metal usage in the catalysts and related catalytic sensors would promise benefits not only to the relevant industries but also for addressing overarching concerns over global energy and environmental issues.
  • 20−22 FeO 3 shows good thermal stability at high temperature.
  • 24−29 Herein, the authors report a new discovery in which trace amounts of alternative perovskite oxide nanoparticles dramatically sensitize metal oxide nanorod gas sensors at high temperature.

■ EXPERIMENTAL METHODS

  • To remove surface grease and organic deposits, the Si/SiO 2 substrates were immersed in acetone solution and sonicated for 5 min.
  • The solution was then heated at 160 °C for 3 h, with a N 2 flow passing through the reaction system to take away water and organic byproducts, finally yielding a transparent dark-brown homogeneous colloidal solution of the Pt metal nanocluster without any precipitates.
  • The structural characteristics of intermediate GaOOH and final β-Ga 2 O 3 nanorods with either Pt-or LSFO-nanoparticle surface decoration were studied by X-ray diffractometry (XRD, Bruker D8 Advance), scanning electron microscopy (SEM, JEOL JSM-6335F), and transmission electron microscopy (TEM, FEI Tecnai T12, acceleration voltage 120 keV).
  • Charge neutralization was employed to minimize the effects of sample charging.

■ RESULTS AND DISCUSSIONS

  • X-ray diffraction (XRD) analysis successfully resolved the crystal phases of intermediate GaOOH and final β-Ga 2 O 3 nanorods before and after Pt or LSFO nanoparticle surface decoration .
  • The 45°-tilted view of as-grown GaOOH nanorod arrays shows a well aligned vertical structure, while the energy dispersive X-ray spectra (EDXS) confirm the presence of Ga and O from the nanorods and Si from the underlying Si substrate .
  • 4−6 Specifically, these chemisorbed oxygen molecules trap mobile electrons from the conduction band of β-Ga 2 O 3 , creating charge carrier depletion at the surface.
  • It is worth pointing out that the residual gas analysis confirmed that ambient oxygen molecules existed in both ultrahigh purity N 2 and CO/N 2 flows used in the experiment .
  • The authors note that there are many potential material properties that can also affect the observed sensitization, such as surface states, oxygen vacancies, and interfaces between LSFO nanoparticles and the Ga 2 O 3 nanorod, which can act as adsorption sites for the CO gas molecules.

■ CONCLUSIONS

  • The structural and CO-sensing characteristics of the pristine, Pt-nanoparticle-, and LSFOnanoparticle-modified β-Ga 2 O 3 nanorod arrays show that 10 nm LSFO nanoparticle decoration greatly enhances the hightemperature (500 °C) CO sensitivity of pristine β-Ga 2 O 3 nanorod arrays by an order of magnitude, rivaling the sensitizing performance of Pt noble metal nanoparticle catalysts.
  • Pt oxidation state changes and surface oxygen ion populating sites of LSFO may help to enable the faster CO sensing response for LSFO decorated nanorod sensors than that for Pt decorated sensors.
  • The demonstrated perovskite LSFO-nanoparticle-modified Ga 2 O 3 nanorod represents a promising candidate for highperformance sensor material for high-temperature gas detection.

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BNL-112718-2016-JA
Perovskite Nanoparticle-Sensitized Ga
2
O
3
Nanorod
Arrays for CO Detection at High Temperature
Hui-Jan Lin, John P. Baltrus, Haiyong Gao, Yong Ding,
Chang-Yong Nam, Paul Ohodnicki, and Pu-Xian Gao
Submitted to ACS Applied Materials & Interfaces
April 2016
Center for Functional Nanomaterials
Brookhaven National Laboratory
U.S. Department of Energy
USDOE Office of Science (SC),
Basic Energy Sciences (SC-22)
Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under
Contract No. DE- SC0012704 with the U.S. Department of Energy. The publisher by accepting the
manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up,
irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others
to do so, for United States Government purposes.

DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the
United States Government. Neither the United States Government nor any
agency thereof, nor any of their employees, nor any of their contractors,
subcontractors, or their employees, makes any warranty, express or implied, or
assumes any legal liability or responsibility for the accuracy, completeness, or any
third party’s use or the results of such use of any information, apparatus, product,
or process disclosed, or represents that its use would not infringe privately owned
rights. Reference herein to any specific commercial product, process, or service
by trade name, trademark, manufacturer, or otherwise, does not necessarily
constitute or imply its endorsement, recommendation, or favoring by the United
States Government or any agency thereof or its contractors or subcontractors.
The views and opinions of authors expressed herein do not necessarily state or
reflect those of the United States Government or any agency thereof.

Perovskite Nanoparticle-Sensitized Ga
2
O
3
Nanorod Arrays for CO
Detection at High Temperature
Hui-Jan Lin,
John P. Baltrus,
Haiyong Gao,
Yong Ding,
§
Chang-Yong Nam,
Paul Ohodnicki,
,
and Pu-Xian Gao*
,
Department of Materials Science and Engineering & Institute of Materials Science, University of Connecticut, 97 North Eagleville
Road, Storrs, Connecticut 06269-3136, United States
National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236, United States
§
School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, Georgia 30332, United
States
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15216, United States
*
S
Supporting Information
ABSTRACT: N
oble metal nanoparticles are extensively used for sensitizing
metal oxide chemical sensors through the catalytic spillover mechanism.
However, due to earth-scarcity and high cost of noble metals, nding
replacements presents a great economic benet. Besides, high temperature and
harsh environment sensor applications demand material stability under
conditions approaching thermal and chemical stability limits of noble metals.
In this study, we employed thermally stable perovskite-type La
0.8
Sr
0.2
FeO
3
(LSFO) nanoparticle surface decoration on Ga
2
O
3
nanorod array gas sensors
and discovered an order of magnitude enhanced sensitivity to carbon
monoxide at 500 °C. The LSFO nanoparticle catalysts was of comparable
performance to that achieved by Pt nanoparticles, with a much lower weight
loading than Pt. Detailed electron microscopy and X-ray photoelectron
spectroscopy studies suggested the LSFO nanoparticle sensitization eect is
attributed to a spillover-like eect associated with the gas-LSFO-Ga
2
O
3
triple-
interfaces that spread the negatively charged surface oxygen ions from LSFO nanoparticles surfaces over to β-Ga
2
O
3
nanorod
surfaces with faster surface CO oxidation reactions.
KEYWORDS:
semiconductor, nanowire, gas sensor, harsh environment, catalytic eect
INTRODUCTION
According to the U.S. Department of Energy, harsh environ-
ment sensors are predicted to save 0.25 quadrillion BTU/year
of energy across all energy-consuming industries if successfully
employed.
1,2
In automotive and stationary energy industries,
monitoring and controlling combustion-related emissions are
top priorities for enhancing energy and environmental
sustainability. However, commercially available sensor tech-
nologies for harsh environments are extremely limited due to
the stringent requirements for sensor materials high sensitivity,
selectivity as well as stability in structure and performance
under harsh operating conditions. Therefore, there is an urgent
need to develop new sensor materials meeting such perform-
ance criteria in sensitivity and robustness, which are
preferentially combined with low cost.
Traditionally, metal oxides have been used as basic sensor
materials, and, in particular, a wide band gap β-Ga
2
O
3
(4.9
eV)
3
is promising for high temperature gas sensing, owing to its
high thermal and chemical stabilities.
46
Ga
2
O
3
thin-lm-based
gas sensors have been proven as promising oxygen sensors at
high temperature of 6001000 °C. It also can detect reducing
gases such as H
2
, CO, CH
4
, etc., at elevated temperature.
7
Meanwhile, with decreased size and increased surface-to-
volume ratio, metal oxide nanorods have shown good potential
for chemical sensors.
8, 9
To further improve the sensor
performance, various strategies can be used to directly control
and enhance the fundamental material properties aecting
sensing characteristics, such as doping,
10,11
surface functional-
ization,
1214
and heterojunction design.
15,16
Kim et al.
demonstrated that the response of multiple-networked Ga
2
O
3
nanowire sensors was enhanced 17-fold by surface decoration
of Pt nanoparticles.
17
Park et al. synthesized Ga
2
O
3
-core/GaN-
shell nanowires by directly nitriding the surface of Ga
2
O
3
nanowire and the results showed the CO sensing performance
can be enhanced at 150 °C by the created heteojunction.
18

Among these approaches described above, the decoration of
catalytically active noble metal on sensor material surfaces or
interfaces has been one of the most eective and widely used
techniques in practice that resulted in substantial improvements
in the sensor performance through the catalytic spillover
mechanism.
13
However, due to the earth-scarcity, the concern
over high cost of noble metals is an ongoing issue, and
therefore, reduction or complete elimination of noble metal
usage in the catalysts and related catalytic sensors would
promise benets not only to the relevant industries but also for
addressing overarching c oncerns over global energy and
environmental issues.
19
Therefore, nding replacements of
noble metals presents a great economic benetwitha
signicant opportunity for enhancing material and manufactur-
ing sustainability.
On the other hand, the signicantly decreased melting points
of noble metal nanoparticles (for example, the melting point of
Pt nanoparticles could be reduced to 600 °C) due to a size
eect coupled with inherent chemical instabilities also hinder
their usage at elevated temperatures as sensitizers for harsh
environment chemical sensors.
2022
Tietz et al. reported
perovskite material La
0.8
Sr
0.2
FeO
3
shows good thermal stability
at high temperature. After sintering at 9001300 °C for 6 h, the
crystalline phases essentially remain the same.
23
In addition,
rare-earth-based perovskite oxides have shown their potentials
for catalytic and functional applications as in our recent
demonstrations of the improved performance of various metal
oxide nanowire a rrays via the application of thin lm
perovskites.
2429
Herein, we report a new discovery in which trace amounts of
alternative perovskite oxide nanoparticles dramatically sensitize
metal oxide nanorod gas sensors at high temperature. In the
present work, we conducted a comparative study on the sensing
properties of pristine, Pt-nanoparticle-, and
La
0.8
Sr
0.2
FeO
3
(LSFO)-nanoparticle-sensitize Ga
2
O
3
nanorod
arrays and clearly show that the perovskite-nanop article
decoration can enhance the gas sensitivity by an order of
magnitude at high temperature with excellent dynamic gas
sensing r esponse characteris tics, which overall rivals the
performance of Pt sensitizing eects.
EXPERIMENTAL METHODS
First we grow β-Ga
2
O
3
nanorod arrays by combining a hydrothermal
method and high temperature calcination. A Si (100) wafer with 1 μm
SiO
2
insulator layer is used as a substrate. To remove surface grease
and organic deposits, the Si/SiO
2
substrates were immersed in acetone
solution and sonicated for 5 min. A 50 nm thick tin dioxide (SnO
2
)
lm was sputter-coated as a seed layer followed by postdeposition
ambient-annealing at 900 °C for 2 h in order to make it crystalline. A
Ga(NO
3
)
3
solution was prepared by dissolving 0.6 g Ga(NO
3
)
3
·9H
2
O
in 40 mL of deionized (DI) water, with the pH of solution controlled
at 2. The SnO
2
-coated substrates were then incubated in Ga(NO
3
)
3
solution at 150 °C for 12 h for the hydrothermal growth of the
intermediate products, vertically aligned gallium hydroxide (GaOOH)
nanorod arrays. After the growth, GaOOH nanorod arrays were
washed by DI water, dried overnight in air at 80 °C, and nally
subjected to annealing at 1000 °C for 4 h to be converted into pure β-
Ga
2
O
3
nanorod arrays. To prepare Pt nanoparticles, a glycol solution
of NaOH (50 mL, 0.5M) was added into a glycol solution of H
2
PtCl
6
·
6H
2
O (1.0 g, 1.93 mmol in 50 mL) via stirring to obtain a transparent
yellow platinum hydroxide or oxide colloidal solution. The solution
was then heated at 160 °C for 3 h, with a N
2
ow passing through the
reaction system to take away water and organic byproducts, nally
yielding a transparent dark-brown homogeneous colloidal solution of
the Pt metal nanocluster without any precipitates. Pt nanoparticles
were dip-coated with a control of 3.0 wt % loading on the β-Ga
2
O
3
nanorod arrays, followed by a post heat-treatment at 450 °C for 2 h in
order to remove the surface residual glycol ligands. The decoration of
LSFO nanoparticles on Ga
2
O
3
nanorod arrays was achieved by
depositing LSFO (nominal thickness of 510 nm, monitored by a
quartz microbalance) on β-Ga
2
O
3
nanorod arrays by radio frequency
(RF) magnetron sputtering followed by postannealing to improve the
crystallinity of LSFO nanoparticles.
The structural characteristics of intermediate GaOOH and nal β-
Ga
2
O
3
nanorods with either Pt- or LSFO-nanoparticle surface
decoration were studied by X-ray diractometry (XRD, Bruker D8
Advance), scanning electron microscopy (SEM, JEOL JSM-6335F),
and transm ission electron microscopy (TEM, FEI Tecnai T12,
acceleration voltage 120 keV). The selected area electron diraction
(SAED) in TEM was used to further conrm the crystal structures of
the grown intermediate and nal nanorods while scanning TEM
(STEM, FEI Tecnai G2 F30, acceleration voltage: 300 keV) was used
to examine the detailed morphology and composition distributions of
β-Ga
2
O
3
based nanorods. The Pt and perovskite loadings on Ga
2
O
3
nanowire array sensors were measured using an inductively coupled
plasma (ICP) optical emission spectrometer (PerkinElmer Optima
7300DV).
The high-temperature gas sensing properties of β-Ga
2
O
3
nanorod
arrays were tested by monitoring the potentiostatic current response of
the β-Ga
2
O
3
nanorod array device to carbon monoxide (CO) exposure
in a high-temperature tube furnace equipped with an alumina tube,
electrical feedthroughs (Ni/Cr lead wires), and a gas injection system.
The resistor-type β-Ga
2
O
3
-based nanorod arrays were installed on an
Al
2
O
3
ceramic holder, shown in Figure S1. Two Pt wires (10 μmin
diameter) were used to connect the nanorod gas sensor placed in the
center position of the tube to Ni/Cr lead wires, which were externally
connected to an electrochemical workstation (CHI 601C). The
nanorod sensor was subjected to a xed 1 V direct current (DC) bias
while being heated from room temperature to 500 °C in air with a
ramp rate of 20 °C/min. Gas sensing tests were then performed at 500
°C under varying concentration of CO (N
2
balance; from 20, 50, 80,
to 100 ppm; total chamber pressure 1 atm). The responses of the
nanorod sensor to CO were evaluated by measuring the magnitude of
current change upon the exposure to various concentrations of CO
under a dynamic gas ow condition with a constant ow rate of 1.5 L/
min (with high purity N
2
as carrier gas), which was regulated by a
computer-controlled gas mixing system (S-4000, Environics Inc.,
USA). The desired CO concentrations can be achieved by a 2% CO
cylinder connected to one port of the system and diluted by a pure N
2
cylinder connected in another port of the gas mixing system. In detail,
the nanorod sensor device was rst exposed to CO/N
2
mixture for 16
min, followed by high purity N
2
purge for 24 min (i.e., one gas
exposure cycle) to recover the sensor, and then multiple exposure
cycles were repeated. We dene the device gas sensitivity as (R
0
/R
CO
)
1, where R
CO
is the resistance under CO/N
2
mixture and R
0
is the
resistance under high purity N
2
. The response time is dened as the
time duration required for gas sensitivity to reach 90% upon exposure
of a sensor to CO, and recovery is dened as the time duration
required for the gas sensitivity to decrease to 10% of the sensitivity
upon the termination of CO injection.
To investigate the chemical characteristics of nanoparticle-decorated
β-Ga
2
O
3
nanorods, we conducted X-ray photoelectron spectroscopy
(XPS) analysis (PHI 5600ci with monochromatic Al Kα characteristic
X-ray) with the pass energy of the analyzer at 58.7 eV. Measured
binding energies were referenced to the Ga 2p3/2 peak, which was
assigned a binding energy of 1117.9 eV for Pt/Ga
2
O
3
and 1117.5 eV
for LSFO/Ga
2
O
3
, based on its position relative to the C 1s peak at
284.6 eV, which originates from adventitious carbon on the samples.
Charge neutralization was employed to minimize the eects of sample
charging. Treatments with O
2
,N
2
, and CO (10% in N
2
) gases were
performed at atmospheric pressure for 20 min at 500 °C in a reaction
chamber directly attached to the XPS instrument, which permitted
sample transfer between the reaction and analysis chambers without
exposure to air.

RESULTS AND
DISCUSSIONS
X-ray diraction (XRD) analysis successfully resolved the
crystal phases of intermediate GaOOH and nal β-Ga
2
O
3
nanorods before and after Pt or LSFO nanoparticle surface
decoration (Figure 1). As shown in Figure 1a, most of the peaks
in the GaOOH XRD spectrum are assigned to orthorhombic
GaOOH phase (JCPDS #060180), with a preferred growth
orientation perpendicular to (111) plane, instead of (110)
plane typically observed in GaOOH powder.
30,31
Strongly c-
axis-orient ed GaOOH na norods were deposited by the
hydrothermal method. Heterogeneous nucleation of GaOOH
was eciently induced on crystalline SnO
2
seed layers and
further annealing converted the GaOOH nanorods to Ga
2
O
3
nanorods without structural disintegration.
32
Minor peaks
present in the spectrum are originating from the underlying
SnO
2
seed layer. After postgrowth annealing at 1000 °C for 4 h,
the GaOOH phase is completely converted to monoclinic β-
Ga
2
O
3
(JCPDS # 41 1103) as shown in Figure 1b. These β-
Ga
2
O
3
nanorod arrays have a preferred growth orientation
perpendicular to (111) plane of the monoclinic structure. The
XRD spectrum of Pt-coated β-Ga
2
O
3
(Figure 1c) displays no
clear peaks corresponding to Pt due to its much smaller amount
than Ga
2
O
3
. Similarly, the LSFO-nanoparticle-decorated β-
Ga
2
O
3
nanorod array does not display peaks corresponding to
the LSFO perovskite, other than apparent Ga
2
O
3
peaks (Figure
1d).
From SEM, we nd that the grown vertical nanorod arrays
have diameters of 100300 nm and lengths up to 2 μm.
Figure 2a shows a representative top-view SEM micrograph of
as-synthesized GaOOH nanorods grown on the Si(100 )
substrate. The tips of the nanorods reveal diamond-shaped
cross sections with diagonal lengths of 150350 nm,
originating from its orthorhombic crystal symmetry (Figure
S2). The 45°-tilted view of as-grown GaOOH nanorod arrays
(Figure 2b) shows a well aligned vertical structure, while the
energy dispersive X-ray spectra (EDXS) conrm the presence
of Ga and O from the nanorods and Si from the underlying Si
substrate (Figure 2c). From the cross-sectional view of growth
substrate (Figure 2d), the length of vertical GaOOH nanorod
arrays was determined to be 1.8 μm. After thermal annealing
at 1000 °C for 4 h, the converted pure Ga
2
O
3
phase retained
the diamond-shaped tips, which were not aected by the
following Pt or LSFO nanoparticle decoration (Figure 2f, g). In
addition, the average diagonal length of each sample is similar,
which is shown in Figure S2. The composition analyses from
ICP (Figure S3) showed that the Pt nanoparticle loading was
3.39 wt % on β-Ga
2
O
3
nanorod arrays, while 5 nm LSFO
nanoparticle decoration led to 0.61 wt % loading on β-Ga
2
O
3
nanorod arrays, suggesting 1 0 nm LSFO nanoparticle
decoration would result in 1.22 wt % loading, only 30%
of Pt loading amount.
The low-magni cation bright-eld TEM combined with
SAED analysis provide further structural details of grown
nanorods and nanoparticle surface decoration. The as-
synthesized GaOOH nanorod, with smooth side walls, has
single-crystalline orthorhombic crystal structure (lattice con-
stants a = 4.58 Å, b = 9.8 Å, c = 2.97 Å) and growth orientation
perpendicular to the (111) plane (Figure 3a). The Ga
2
O
3
nanorod converted from GaOOH on the other hand has
seemingly roughened surfaces as the bright-eld TEM micro-
graph features local variation in contrast, which is generally
resulting from the density variation (Figure 3b). A more
detailed examination by STEM reveals that the contrast
variation was in fact caused by the porous structure of β-
Ga
2
O
3
nanorod (Figure S4). The porous β-Ga
2
O
3
nanorod is
however textured with monoclinic structure (a = 12.22 Å; b =
3.038 Å; c = 5.807 Å; α =90°, β = 103.82°, γ =90°) with a
growth orientation normal to (001) plane as conrmed by
SAED analysis (Figure 3b inset). The perovskite LSFO-
nanoparticle decoration by sputter deposition yields sparsely
distributed LSFO nanoparticles of <10 nm size on the surface
of β-Ga
2
O
3
nanorod (Figure 3c); no diraction peaks of LSFO
was revealed in the SAED pattern due to its small quantity. The
more detailed distribution of LSFO nanoparticles is provided in
Figure 1. X-ray diraction (XRD) patterns of (a) GaOOH nanorod
arrays, (b) β-Ga
2
O
3
nanorod arrays, (c) β-Ga
2
O
3
/Pt particles nanorod
arrays, (d) β-Ga
2
O
3
/LSFO nanorod arrays.
Figure 2. (a) Top-view and (b) 45° tilt-view SEM images of GaOOH
nanowires grown at 150 °C. (c) Corresponding GaOOH energy-
dispersive X-ray (EDX) spectrum. (d) Cross-sectional view SEM
image of GaOOH nanorod array. (e) Top-view SEM image of the β-
Ga
2
O
3
nanorods from GaOOH nanorods annealed at 1000 °C for 4 h,
and coated with (f) LSFO and (g) Pt particles. All scale bars are 1 μm.

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Hui Chen1, Jiabo Hu1, Guo-Dong Li1, Qian Gao1, Cundi Wei1, Xiaoxin Zou1 
TL;DR: Efficient and selective detection of formaldehyde vapor has been realized by a gas sensor based on porous GaxIn2-xO3 nanofibers assembled by small building blocks and has superior ability to selectively detect formaldehyde against other interfering volatile organic compound gases.
Abstract: The design of appropriate composite materials with unique surface structures is an important strategy to achieve ideal chemical gas sensing. In this paper, efficient and selective detection of formaldehyde vapor has been realized by a gas sensor based on porous GaxIn2-xO3 nanofibers assembled by small building blocks. By tuning the Ga/In atomic ratios in the materials, crystallite phase, nanostructure, and band gap of as-obtained GaxIn2-xO3 nanofibers can be rationally altered. This further offers a good opportunity to optimize the gas sensing performances. In particular, the sensor based on porous Ga0.6In1.4O3 nanofibers assembled by small nanoparticles (∼4.6 nm) exhibits best sensing performances. Toward 100 ppm formaldehyde, its highest response (Ra/Rg = 52.4, at 150 °C) is ∼4 times higher than that of the pure In2O3 (Ra/Rg = 13.0, at 200 °C). Meanwhile, it has superior ability to selectively detect formaldehyde against other interfering volatile organic compound gases. The significantly improved sensi...

90 citations

Journal ArticleDOI
Adeel Afzal1
TL;DR: In this article, the relationship between composition, nanostructure, and gas sensing properties of gallium-containing oxidic nanomaterials such as β-Ga2O3 nanowires, surface-modified Ga 2O3, metal-doped metal oxide composite heterostructures, and Ga2O 3/metal oxide composite structures are studied.

90 citations

References
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Journal ArticleDOI
TL;DR: In the data for the 63 elements, trends that occur simultaneously in both the columns and the rows of the periodic table are shown to be useful in predicting correct values and also for identifying questionable data.
Abstract: A new compilation, based on a literature search for the period 1969–1976, is made of experimental data on the work function. For these 44 elements, preferred values are selected on the basis of valid experimental conditions. Older values, which are widely accepted, are given for 19 other elements on which there is no recent literature, and are so identified. In the data for the 63 elements, trends that occur simultaneously in both the columns and the rows of the periodic table are shown to be useful in predicting correct values and also for identifying questionable data. Several illustrative examples are given, including verifications of predictions published in 1950.

3,569 citations

Journal ArticleDOI
TL;DR: In this article, the authors provide a frame model that deals with all contributions involved in conduction within a real world sensor, and then summarize the contributions together with their interactions in a general applicable model for real world gas sensors.
Abstract: Tin dioxide is a widely used sensitive material for gas sensors. Many research and development groups in academia and industry are contributing to the increase of (basic) knowledge/(applied) know-how. However, from a systematic point of view the knowledge gaining process seems not to be coherent. One reason is the lack of a general applicable model which combines the basic principles with measurable sensor parameters. The approach in the presented work is to provide a frame model that deals with all contributions involved in conduction within a real world sensor. For doing so, one starts with identifying the different building blocks of a sensor. Afterwards their main inputs are analyzed in combination with the gas reaction involved in sensing. At the end, the contributions are summarized together with their interactions. The work presented here is one step towards a general applicable model for real world gas sensors.

2,247 citations

Journal ArticleDOI
TL;DR: Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of the resultant atomic species to the oxide surface.
Abstract: The sensing ability of individual SnO2 nanowires and nanobelts configured as gas sensors was measured before and after functionalization with Pd catalyst particles. In situ deposition of Pd in the same reaction chamber in which the sensing measurements were carried out ensured that the observed modification in behavior was due to the Pd functionalization rather than the variation in properties from one nanowire to another. Changes in the conductance in the early stages of metal deposition (i.e., before metal percolation) indicated that the Pd nanoparticles on the nanowire surface created Schottky barrier-type junctions resulting in the formation of electron depletion regions within the nanowire, constricting the effective conduction channel and reducing the conductance. Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion ...

1,307 citations

Journal ArticleDOI
TL;DR: In this paper, the three key requirements of sensor design are determined by considering each of these three key factors: selection of a base oxide with high mobility of conduction electrons and satisfactory stability (transducer function), selection of foreign receptor which enhances surface reactions or adsorption of target gas (receptor function), and fabrication of a highly porous, thin sensing body (utility factor).
Abstract: Semiconductor gas sensors utilize porous polycrystalline resistors made of semiconducting oxides. The working principle involves the receptor function played by the surface of each oxide grain and the transducer function played by each grain boundary. In addition, the utility factor of the sensing body also takes part in determining the gas response. Therefore, the concepts of sensor design are determined by considering each of these three key factors. The requirements are selection of a base oxide with high mobility of conduction electrons and satisfactory stability (transducer function), selection of a foreign receptor which enhances surface reactions or adsorption of target gas (receptor function), and fabrication of a highly porous, thin sensing body (utility factor). Recent progress in sensor design based on these factors is described.

1,134 citations

Frequently Asked Questions (1)
Q1. What are the contributions in "Perovskite nanoparticle-sensitized ga2o3 nanorod arrays for co detection at high temperature" ?

In this study, the authors employed thermally stable perovskite-type La0. 8Sr0. 2FeO3 ( LSFO ) nanoparticle surface decoration on Ga2O3 nanorod array gas sensors and discovered an order of magnitude enhanced sensitivity to carbon monoxide at 500 °C. The LSFO nanoparticle catalysts was of comparable performance to that achieved by Pt nanoparticles, with a much lower weight loading than Pt. Detailed electron microscopy and X-ray photoelectron spectroscopy studies suggested the LSFO nanoparticle sensitization effect is attributed to a spillover-like effect associated with the gas-LSFO-Ga2O3 tripleinterfaces that spread the negatively charged surface oxygen ions from LSFO nanoparticles surfaces over to β-Ga2O3 nanorod surfaces with faster surface CO oxidation reactions.