Easy and Green Route towards Nanostructured ZnO as an Active Sensing Material with Unexpected H2S Dosimeter-Type Behaviour

Article abstract of DOI:10.1002/ejic.201801334

Nanostructured ZnO particles were prepared through a straightforward, quick and low-temperature synthesis route involving coprecipitation of the metal precursor salts with oxalic acid, followed by hydrothermal treatment at 135 or 160 °C. The synthesised nanostructured powders were thoroughly characterised by a wide array of analytical techniques from the morphological (Scanning Electron Microscopy -SEM-, Transmission Electron Microscopy -TEM-, Energy-dispersive X-ray Spectroscopy -EDXS-), structural (Powder X-ray Diffraction -PXRD-, Selected Area Electron Diffraction -SAED-), compositional (X-ray Photoelectron Spectroscopy -XPS-) and physical (thermal stability) point of view. As far as functional applications are concerned, the powders were tested as gas sensor materials for H₂S detection. Thereby these ZnO particles showed unexpected gas dosimeter behaviour at 150 °C. Based on these observations and on a comparison with literature a new model for the interaction of ZnO nanostructures with H₂S is proposed.

Full text of DOI:10.1002/ejic.201801334

A Journal of
Accepted Article
Title: Easy and green route for nanostructured ZnO as active sensing
material with unexpected H2S dosimeter-type behaviour
Authors: Stefano Diodati, Jörg Hennemann, Fernando Fresno, Stefano
Gialanella, Paolo Dolcet, Urška Lavrenčič Štangar, Bernd
Smarsly, and Silvia Gross
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201801334
Link to VoR: http://dx.doi.org/10.1002/ejic.201801334

10.1002/ejic.201801334
European Journal of Inorganic Chemistry
FULL PAPER
Easy and green route for nanostructured ZnO as active sensing
material with unexpected H2S dosimeter-type behaviour
Stefano Diodati,[a] Jörg Hennemann,[b,c] Fernando Fresno,[d,e] Stefano Gialanella,[f] Paolo Dolcet,[a,g]
Urška Lavrenčič Štangar,[e,h] Bernd M. Smarsly,*[b] and Silvia Gross*[a]
Dedicated to the memory of Prof. Dieter Kohl (Justus-Liebig-Universität Gießen)
Abstract: Nanostructured ZnO particles were prepared through a
straightforward, quick and low-temperature synthesis route involving
coprecipitation of the metal precursor salts with oxalic acid, followed
by hydrothermal treatment at 135 or 160 °C. The synthesised
nanostructured powders were thoroughly characterised by a wide
array of analytical techniques from the morphological (Scanning
Electron Microscopy –SEM-, Transmission Electron Microscopy -
TEM-, Energy-dispersive X-ray Spectroscopy -EDXS-), structural
(Powder X-Ray Diffraction -PXRD-, Selected Area Electron Diffraction
-SAED-), compositional (X-ray Photoelectron Spectroscopy -XPS-)
and physical (thermal stability) point of view. As far as functional
applications are concerned, the powders were tested as gas sensor
materials for H2S detection. Thereby these ZnO particles show
unexpected gas dosimeter behaviour at 150 °C. Based on these
observations and on a comparison with literature a new model for the
interaction of ZnO nanostructures with H2S is proposed.
[a]
[b]
Dr. S. Diodati, Dr. P.Dolcet, Prof. Dr. S. Gross
Dipartimento di Scienze Chimiche
Università degli Studi di Padova
Via Marzolo 1, 35131- Padova and INSTM, UdR di Padova, Italy
Email: silvia.gross@unipd.it; sterfano.diodati.it@gmail.com;
paolo.dolcet@kit.edu
URL: http://www.chimica.unipd.it/silvia.gross/people.html
Dr. J. Hennemann, Prof. Dr. B. Smarsly
Physikalisch-Chemisches Institut
Justus-Liebig-Universität Gießen
Heinrich-Buff-Ring 17, 35392 Giessen, Germany
Email: bernd.smarsly@phys.chemie.uni-giessen.de
URL: https://www.uni-
giessen.de/faculties/f08/departments/physchem/ag-prof-dr-bernd-
smarsly
[c]
Dr. J. Hennemann
Institut für Angewandte Physik
Justus-Liebig-Universität Gießen
Heinrich-Buff-Ring 16, 35392 Giessen, Germany
Email: Joerg-Hennemann@t-online.de
URL: https://www.uni-
Introduction
giessen.de/faculties/f08/departments/physchem/ag-prof-dr-bernd-
smarsly
Metal oxides are among the most common and most important
binary compounds encountered in materials science and
everyday life; among these, zinc oxide enjoys a widespread use
in a broad variety of fields (from medicine to optics and
electronics).[1-9] ZnO has attracted a great deal of attention due to
its semiconductor properties[10-13]. It has a direct wide band gap
(3.37 eV) and high exciton binding energy (60 meV):[5-6, 13-16] this
property, combined with its availability and relative ease of
preparing large and high-quality crystals, though smaller crystals
can also display interesting and useful functional properties,
makes ZnO a very interesting material for optoelectronics as well
as photocatalysis.[17-19]
In other fields, doped and undoped zinc oxide nanowires[19] find
diverse applications as biosensors, light emitting diodes and
active materials in dye-sensitised solar cells. In addition, ZnO
represents a promising material for gas sensors based on
semiconducting metal oxides.[20-21] The first investigations were
[d]
[e]
Dr. F. Fresno
Photoactivated Processes Unit
IMDEA Energy Institute
Avda. Ramón de la Sagra 3, 28935 Móstoles, Madrid, Spain
Email: fernando.fresno@imdea.org
URL: http://www.energy.imdea.org/people/dr-fernando-fresno
Dr. F. Fresno, Prof. Dr. U. Lavrenčič Štangar
Laboratory for Environmental and Life Sciences
University of Nova Gorica
Vipavska 13, 5000 Nova Gorica, Slovenia
Email: Urska.Lavrencic.Stangar@fkkt.uni-lj.si
URL: http://www.ung.si/en/research/laboratory-for-environmental-
and-life-sciences/staff/urska-lavrencic-stangar/
[f]
Prof. Dr. S. Gialanella
Dipartimento di Ingegneria Industriale
Università degli Studi di Trento
Via Sommarive 9, I-38123, Trento, Italy
Email: stefano.gialanella@ing.unitn.it
URL: https://webapps.unitn.it/du/it/Persona/PER0004889/Curriculum
performed in the 1950’s, examining the influence of hydrogen (H2)
[g]
[h]
Dr. P. Dolcet
[22]
Institut für Technische Chemie und Polymerchemie (ITCP)
Karlsruhe Institute of Technology (KIT)
Engesserstr. 20 76133 Karlsruhe, Germany
on the conductance of ZnO.
In this process, the adsorbed
oxygen on the surface reacts with H2, and electrons in the material
are set free, resulting in a decrease in conductance which can be
used as a sensing signal.[23] Since then, many investigations
concerning the sensing of different gases like ethanol, carbon
monoxide (CO), ammonia (NH3) or nitrogen dioxide (NO2) have
followed.[24-26] Besides these gases, the detection of hydrogen
sulphide (H2S)[27] is quite an important task, especially in the
context of biogas production and use. H2S is a toxic gas which
Prof. Dr. U. Lavrenčič Štangar
Faculty of Chemistry and Chemical Technology
University of Ljubljana
Vecna pot 113, 1000 Ljubljana, Slovenia
Supporting information for this article is given via a link at the end of
the document
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801334
European Journal of Inorganic Chemistry
FULL PAPER
leads, for instance, to the corrosion of compressors, gas storage
tanks, engines and metallic parts of biogas plants.[28-29] Beyond
that, it is also highly harmful to the environment and human
health.[30]
out that all precursors decompose cleanly to non-toxic
compounds (in particular both acetylacetonate and oxalic acid
decompose to CO2), sub-200 °C temperatures are employed and
the only solvent involved is water, thus satisfying many core
principles of the green chemistry approach.[71] As far as the zinc
precursor is concerned, zinc acetylacetonate was additionally
chosen because it is a stable, non-deliquescent compound (unlike
for instance ZnCl2) and because it decomposes cleanly in an
alkaline environment,[40] albeit non-violently. Zn nitrate was
considered less suitable for hydrothermal protocols due to the
potentially hazardous decomposition of nitrates yielding NOx
species which may cause a sudden and uncontrolled increase in
pressure.
Several synthetic parameters, such as nature and nominal ratio
of the employed precursors, the amount of peptising agent and
nature of the base employed in the synthesis, as well as treatment
temperature and time, were explored in order to optimise the
preparation protocol, though only the results of two such
syntheses are reported here.
A complete structural and functional characterisation of the
samples was conducted using several methods such as X-ray
Diffraction (XRD), Scanning Electron Microscopy (SEM),
Transmission Electron Microscopy (TEM) and X-Ray
Photoelectron Spectroscopy (XPS) (see experimental section
below).
In gas-sensing tests, the material showed, besides the “classical”
semiconductor gas sensing behaviour at 450 °C, also a gas
dosimeter behaviour at an operating temperature of 150 °C. This
unexpected behaviour was compared with literature and a new
model for the interaction between ZnO gas sensors and H2S was
proposed.
Recently, ZnO-based semiconductor gas sensors were
introduced,[25, 27] but the underlying physical mechanism seems to
be different from the one described above. Huang et al. showed
that the formation of ZnS on the surface of ZnO nanowires
represents an important step in the sensing of H2S.[31] It is
assumed that ZnS protects the surface of ZnO against oxygen
absorption whereby the conductance increases. The formation of
a ZnS layer on top of ZnO can result in a durable change in
conductance.[25] This behaviour makes it possible to design a H2S
gas dosimeter (a type of sensor which detects the total amount of
gas by accumulation of gas molecules),[32] where the gas is
accumulated either at or in the sensitive layer, consequently
causing the change in the sensor signal to depend on the amount
of adsorbed gas.[33] Regeneration can be achieved by increasing
the temperature,[33] and SEM investigation before gas exposure
and after regeneration shows no morphological changes,
wherefore ZnO dosimeters seem to be suitable for multiple use
(see below).
Documented wet chemistry synthetic protocols for zinc oxides
include co-precipitation,[34] miniemulsion synthesis[35] and sol-gel
chemistry.[34, 36-40] However, given the significant influence that the
particle and crystallite size, as well as the composition
(specifically the dopant dispersion and dopant-to-host material
ratio), exert on the final properties of the sample, a wet chemistry
route is generally preferable, allowing low-temperature
crystallisation of the desired product as well as fine control over
the reaction pathways.[41-42] Within this framework, hydrothermal
synthesis, especially of inorganic binary compounds,[15, 18-19, 42-59]
features several advantages over conventional wet-chemistry
methods used in the synthesis of inorganic compounds, such as
co-precipitation followed by calcination,[60-61] or the Pechini
method.[62-65] A further important upside lies within the fact that
synthesis takes place at relatively low temperatures.[66-67] By
adjusting the treatment temperature and the composition of the
reaction medium, the hydrothermal approach furthermore allows
to tune the resulting particle size.[66-68] The relatively high
autogenous pressures employed during synthesis affect several
properties which directly influence solubilisation (such as ionic
product, density, viscosity and dielectric constant). These
conditions can lead to the solubilisation of normally insoluble
precursors under conventional conditions.[41, 69]
Results and Discussion
Structural investigations
Synthesised samples were firstly characterised from a structural
point of view in order to identify the obtained crystalline phases
and to evaluate the effect of the different employed synthesis
parameters. XRD analyses performed on synthesised ZnO
samples (Figure 1) revealed that the compounds were highly
crystalline, with only the ZnO hexagonal (wurtzite) phase (PDF
01-079-2205) being visible and no evidence of amorphous or
spurious phases. Rietveld refinement performed on the patterns
relative to ZnO was only possible through fitting with an
anisotropic model, showing that, for all samples, the crystallites
display preferential growth along certain vectors. Specifically, the
crystallites were larger along the [0002] vector (average of 120
nm) and smaller along the [1011] vector (70 nm), as confirmed by
the rod-like shape of the nanostructures (see SEM micrographs
below).
In this paper we report an easy, fast, reproducible, green and low-
temperature procedure for the hydrothermal synthesis of
nanostructured zinc oxide, based on
a combination of
precipitation of oxalates and hydrothermal treatment, a method
already successfully applied to the synthesis of ferrites (CoFe2O4,
MnFe2O4, NiFe2O4, and ZnFe2O4).[70-71] The nucleation and
crystallisation mechanism of this oxalate-based hydrothermal
synthesis route was recently elucidated for these ferrites by
means of in-situ characterisation.[70] In this study, this synthetic
protocol was adapted to ZnO due to several important advantages
(practical ease, speed, low temperature, aqueous environment
etc.).[71] Among the advantages of this approach, the fact stands
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10.1002/ejic.201801334
European Journal of Inorganic Chemistry
FULL PAPER
Since the ability of a material to function as a sensor or catalyst is
highly influenced by its surface chemistry, surface composition of
the obtained nanostructured materials was investigated by XPS.
Analysis of the XPS spectra for ZnO sample H-ZnO-1 (Figure 2,
table 2) shows no sodium or nitrogen, deriving from the
precursors, thus confirming that the samples are compositionally
pure. Detected binding energies (Table S1 in S.I.) are also in
agreement with values found in literature for ZnO.[43, 46, 72-73]
.
Table 2. Surface concentrations (% at.) for carbon, oxygen, and zinc
Sample
C
O
Zn
H-ZnO-1
16.7
46.0
36.4
Figure 1. XRD pattern of sample H-ZnO-2with reflections indexed. Given the
hexagonal nature of the wurtzite phase, the 4-digit Miller-Bravais notation was
preferred to the more common 3-digit Miller hkl notation.
Morphology studies
A stable crystalline structure in the sensing material is a
necessary feature for reliable gas sensors. Different
morphologies of the same sensing material can lead to completely
different detected signals.[74] If structural rearrangement occurs
during the sensing process, it can impede the gas concentration
determination. This problem also notably applies to copper oxide-
based H2S gas dosimeters, which are prepared from nano-
granular thin films: they show severe morphological changes
during the detection process caused by chemical transformation,
which in turn affects the application. As a consequence, in this
case, only single-use devices are feasible. However, by obtaining
more stable morphologies, reusable gas dosimeters can be
developed.[75-76] Within this framework, to gain further insight into
the morphology of the synthesised materials, TEM micrographs
were acquired (Figure 3). In particular, the bright field image in
Figure 3a), shows the elongated, rod-like, shape of the ZnO
grains, whose crystallographic structure, in agreement with the
XRD data (Fig. 1), is confirmed to be hexagonal (Fig. 3b). The
spot pattern in Figure 3c, obtained after tilting along a near axis
direction the ZnO grains, indicates that the growing direction of
these rod-like structures is [0001].
Surface and bulk quantitative analyses
CHN elemental analysis performed on sample H-ZnO-1 (Table 1)
showed that minimal organic residues from the synthesis are
present in the ZnO sample. This observation can be taken as a
further indication of the effectiveness of the synthesis method in
preparing compositionally pure materials. In essence, oxalic acid
decomposes readily into CO2 and not into carbonaceous residues,
which is a major advantage of this synthetic procedure.
Table 1. Sample composition results (and expected values) from CHN
elemental analysis (weight %)
Sample
Content
C [Exp.]
H [Exp.]
N [Exp.]
H-ZnO-1
ZnO
0.07 [0.00]
0.14 [0.00]
0.00 [0.00]
Figure 2. XPS spectrum of ZnO sample H-ZnO-1 (B.E. corrected for surface
charging)
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10.1002/ejic.201801334
European Journal of Inorganic Chemistry
FULL PAPER
micrographs were collected before and after (Figure 4) heating.
Similarly, XRD analyses were carried out on heat-treated
powders (Figure 5).
Figure 4. SEM micrograph of ZnO sample H-ZnO-1 before (top) and after
treatment at 400°C for 6 h (bottom)
Figure 3. TEM data for sample H-ZnO-1: a) Bright field imaging, showing the
rod-like grains; b) SAED patter of the above grains, confirming the hexagonal
structure; c) SAED spot pattern in axis direction, indicating the [0001] direction
as parallel to the main axis of the grain, parallel to the growth direction
Thermal stability tests
Several practical applications require compounds to operate at
high temperatures, particularly gas sensors, where regeneration
at several hundred degrees is often necessary in order to maintain
a good performance. Within this framework, it was important to
investigate structural and morphological stability of the
synthesised compounds. Therefore, the materials were heated up
to 400 °C (as described in the experimental section), and SEM
Figure 5. XRD patterns of undoped ZnO before and after thermal treatment at
400°C for 6 h, as indicated.
It is evident from both the micrographs and the XRD patterns that
the compound is not subject to any particular degeneration and/or
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10.1002/ejic.201801334
European Journal of Inorganic Chemistry
FULL PAPER
morphological change between room temperature and 400 °C,
proving that a ZnO material with thermal stability was obtained, in
agreement with various literature reports on thermal stability of
ZnO[77-79] in air. Calculation of crystallite size through Rietveld
refinement also evidenced no notable changes: crystallite sizes
remained around 70 nm both before and after treatment. As
reported above, and confirmed by SEM analyses, the ZnO
nanostructures displayed a strong anisotropy, with the crystallites
being larger along the (0002) vector (average of 120 nm) and
smaller along the (1011) vector (70 nm). The SEM micrographs
provided further confirmation on the ZnO crystalline
nanostructures, since the urchin-like morphology evidenced
through microscopy (Figure 4) perfectly fits the information
reported for ZnO particle growth in basic hydrothermal
conditions.[15, 80] It can be argued that the rod-like prismatic grains
visible in the TEM micrographs (Figure 3) result from the
disruption of the urchin-like structures during ultra-sonication
used for sample preparation.
Figure 6. Conductance of the prepared ZnO sensor (powder no. H-ZnO-1) in
dependence of different H2S gas concentrations, measured at an operating
temperature of 450° C. After running-in, the sensor shows a typical behaviour
of an n-type semiconductor gas sensor.
Gas sensing studies
The ZnO sample H-ZnO-1 was also investigated regarding its gas
sensing properties. In literature, numerous investigations
concerning ZnO applied to the detection of H2S gas have already
been reported.[31, 81-82] In these works, different reactions were
observed and relevant mechanisms were proposed, which
depend on the gas concentration, operating temperature and size
of the ZnO structures. In this context, one must distinguish
between surface reactions of H2S with adsorbed oxygen
The sample shows a behaviour typical of a semiconductor-type
gas sensor: at the beginning of the measurement, during warm
up, the conductance decreases in accordance with the typical
behaviour of semiconductors. Upon initial exposure to a low
concentration (only 1 ppm) of H2S, this decrease in conductance
is superimposed with and increase caused by the reaction of ZnO
with H2S, in accordance with equation (1). Afterwards, i.e. when
removing exposure to H2S, the conductance signal decreases
and stabilises to a constant level (approximately 10-8 S). When
the second gas exposure (5 ppm H2S) starts, the conductance
increases markedly and converges to a constant value. Removing
the H2S atmosphere, the conductance again decreases
substantially before rising steeply upon the third exposure (10
ppm H2S), substantially exceeding the value obtained for 5 ppm.
In essence, Figure 6 shows all the features of the typical
behaviour of a semiconductor upon exposure to a reducing gas,
i.e. the warm up period, the reaction with H2S as well as the
regeneration between the gas uptakes.
-
3 O2 (ads) + 2 H2S(g) ⇌ 2 SO2(g) + 2 H2O(g) + 3 e-
(1)
and bulk reactions.
ZnO(s) + H2S(g) ⇌ ZnS(s) + H2O(g)
(2)
Additionally, the following reverse reaction between ZnS and ZnO
also plays an important role.
ZnS(s) + 3/2 O2(g) ⇌ SO2(g) + ZnO(s)
(3)
While these measurements were performed at a temperature of
450 °C, during a second set of experiments the sensor was only
heated up to 150 °C. Using the same three H2S concentrations at
two significantly different temperatures was intended to provide
insight into the underlying chemical process, i.e., if the formation
of ZnS from ZnO actually takes place. Indeed, applying a
temperature of 150 °C results in a different sensing behaviour
(Figure 7).
To examine the performance of our material, the powder was
applied to a commercial gas sensor substrate, as described in the
experimental section. After the preparation step, the first gas
measurement started at a temperature of 450 °C (Figure 6).
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10.1002/ejic.201801334
European Journal of Inorganic Chemistry
FULL PAPER
XPS spectra however revealed more interesting data: the sample
used in H2S gas sensing experiments displayed a very low
sulphur content (1.7% atomic– see Table S2 in S.I.).
Whilst the surface oxygen abundance is a poor indicator of the
surface ZnO/ZnS ratio (as surface oxygen may be at least in part
due to other species such as carbonates or hydroxides), a
comparison between zinc and sulphur atomic percentages
reveals that at best only 5.3% of total surface zinc exists as ZnS,
with the remaining 94.7% representing oxidised species.
These results, particularly those concerning surface atomic
abundance indicate that conversion from ZnO to ZnS during H2S
sensing only occurs at a very thin surface layer. This is consistent
with the proposed reaction model (see below).
Figure 7. Gas measurement of the same ZnO sensor as in Figure 6 (above)
(powder no. H-ZnO-1) applying different H2S gas concentrations at an operating
temperature of 150 °C. The sensor shows a dosimeter-type behaviour.
During these measurements, the conductance levels remain
stable during the warm up and prior to the first H2S exposure.
Upon exposure to 1 ppm H2S the conductance continuously
increases following an almost linear behaviour. At the end of the
first exposure, the conductance continues to increase, but with a
lower slope. The onset of the exposure to 5 ppm H2S leads to a
marked increase in the conductance, with a greater slope as
compared to the 1 ppm exposure. Thereafter, the conductance
levels off to a constant level. During the exposure to 10 ppm H2S,
the conductance once again increases almost linearly, exhibiting
an even sharper slope, remaining nearly constant after
interrupting the H2S exposure. This behaviour is typical of a gas
83]
dosimeter[33,
and cannot be attributed to surface reactions
alone. It is noteworthy to mention that CuO, which is also used as
sensor material for the detection of H2S, shows a similar
behaviour at 150 °C. Another interesting and important
characteristic of the material can be seen in SEM images: Figure
8 a) shows another sensor, prepared in the same way, before the
gas measurement began. In Figure 8 b) the same sensor is shown
after sensing measurements performed at 450 °C and
regeneration in ambient air for 30 min at 450 °C. A comparison
between the corresponding SEM images evidences no apparent
alterations to sample structure and morphology. This is a
completely different observation in contrast to CuO thin film
dosimeters for H2S, where the structure is substantially altered by
the measurement activity (vide supra). In this context, stable
structures, which are necessary for a reusable sensor, have so
Figure 8. SEM micrographs of the ZnO on a gas sensor substrate a) before gas
measurements; b) after gas measurements and regeneration at 450 °C,
showing no significant changes and thereby a complete reversibility of the
reactions.
far only been observed with CuO fibres or might be possible with
84-85]
composites.[75,
The surface composition of the ZnO
employed in the dosimeters was also investigated before and
after sensing through XPS measurements. XPS analyses carried
out on the ZnO sample after use as H2S dosimeter (prior to
regeneration at 450°C) revealed no relevant changes in the
binding energies of the Zn2p region (1021.4 eV for Zn2p3/2). This
was expected since a comparison between ZnO and a ZnS
reference revealed very similar Zn2p B.E. values (1021.3-1021.5
eV for Zn2p3/2– see Table S1 in S.I.). Quantitative analysis of the
In comparison to literature, different models have to be combined
to explain the observed conductance behaviour of the ZnO-based
material. In the following, the results of these previous studies are
presented and discussed in comparison to the results of this work.
Thereafter a new model based on all observations will be
discussed.
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10.1002/ejic.201801334
European Journal of Inorganic Chemistry
FULL PAPER
Huang et al. examined two types of ZnO materials. First, ZnO
nanowire arrays with a thickness of about 50 - 60 nm were
investigated at a temperature of 150 °C and exposed to different
H2S concentrations. Second, ZnO microparticles with a structure
size of about 150 - 800 nm were exposed to 2000 ppm H2S in
nitrogen at 150 °C.[31] Based on these studies two models
including surface and bulk reactions are discussed in the following.
The study on nanowires suggests H2S reacts with ZnO, as
described in the above reported equation (2), building up a ZnS
layer on the surface of the ZnO grains, as verified by XRD and
XPS. This layer protects the interior of ZnO against adsorbing
oxygen and avoids binding of electrons. The resulting change in
resistance is used as sensor signal. In this instance, the signal is
proportional to the gas concentration. When the H2S flow is turned
off, the sensor regenerates in ambient air, according to the
chemical reaction (3) and the resistance increases again to its
original value. By contrast, the ZnO microparticles do not show
such behaviour: only surface reactions take place, as described
by equation (1). Due to the greater particle size it was presumed
that no sulfurisation takes place at this temperature. This
assumption is also supported by the results of Neveux et al.
showing that smaller ZnO structures have a higher sulfurisation
Table 3. Summary of the results from the gas measurements obtained in the
present study and from the literature
Temp.
(°C)
Width
(nm)
Sensor
Dosimeter
behaviour
ZnS
Regen.
Ref
behaviour
Observed
by: XRD,
XPS
Huang
et
al.[31]
150
150
50-60
Yes
yes
No
No
No
Slow at
150° C/
fast at
450 °C
Wang
et
al.[81]
100-
400
Assumed
Yes
150-
400
At
This
Yes
No
No
No
No
150
150
Assumed
No
450 °C
paper
Huang
et
al.[31]
150-
800
No
No
300-
500
100-
200
Observed
by: XPS
Kim et
al.[82]
Yes
Yes
rate compared to larger ones. Similar results are reported by Kim
Wang
et
al.[81]
86]
and Yong.[82,
However, no dosimeter-type behaviour was
350-
450
100-
400
No
Yes
Yes
Yes
Yes
observed. Kim and Yong investigated 100-200 nm ZnO structures
at temperatures between 300 and 50 0°C under a 50 ppm H2S
gas exposure.[82] They showed via XPS analysis that the
sulfurisation process is even more important at higher
temperatures. At 200 °C and below, no sulfurisation was
observed, in contrast to the results of Huang et al.,[31] who on the
other hand used nanowires with a thinner diameter. In the model
proposed by Kim and Yong, H2S reacts with the adsorbed oxygen
on the surface according to reaction (1). Then electrons are freed
and the depletion layer decreases accompanied by an increase
in the conductance, which is typical of an n-type semiconductor
gas sensor. However, at 300 °C and above, a bulk reaction also
takes place as described in equation (2). The formed ZnS should
behave like a shallow donor and at the same time a diffusion
process should start. Nevertheless, no dosimeter-type behaviour
was observed on this occasion too.
Wang et al. observed both, a typical semiconducting gas sensor
and a dosimeter-type behaviour in ZnO structures measuring
about 100 - 400 nm.[81] At temperatures between 350 and 450 °C
their measurements show the typical sensor behaviour. The
response of the sensor increases directly at the beginning of the
gas exposure and slows down when the exposure ends. However,
the colder the operating temperature, the slower the regeneration
of the sensor. At 150 °C, after an exposure to 20 ppm H2S, the
signal changed only marginally and a regeneration step at 450 °C
was added to bring the signal back to its initial value before the
gas measurement was started. Only at concentrations
considerably below 20 ppm did the sensor signal regenerate at
150 °C. Wang et al. posit that the formation of ZnS is responsible
for the change of the sensor signal. However, it can also be an
indicator of dosimeter behaviour.
150-
400
This
450
Assumed
paper
These results show that the behaviour of the ZnO structures
strongly depends on both operating temperature and particle size
and morphology. Based on previous studies and our own results,
the following conclusions can be drawn:
1. At a low operating temperatures, the sulfurisation rate
seems to decrease with increasing particle size, which
is supported by the work of Neveux et al. and Huang et
al.[31, 86] Note that this effect cannot be simply attributed
to the thermodynamic equilibrium of reaction (2), as the
reaction of ZnO with H2S forming ZnS and H2O is quite
exothermic.
2. With increasing particle size, the sulfurisation rate
seems to increase at higher operating temperatures[82]
.
Gas concentration and diffusion processes should nevertheless
also be taken into consideration.[81, 86]
Based on this data, we developed a new model for the reaction
between H2S and ZnO, accounting for the sensing behaviour. A
schematic overview is given in Figure 9. The model takes into
account the interaction of surface reactions, bulk reactions,
diffusion processes, operating temperature and particle size.
The observations and results of these papers, as well as the
results of our measurements are summarised in table 3.
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10.1002/ejic.201801334
European Journal of Inorganic Chemistry
FULL PAPER
process can take place. Both were observed by Huang et al. with
ZnO material samples possessing smaller sizes than in our
investigation.[31] The sulfurisation during H2S exposure and the
desulfurisation when the H2S gas uptake ends are both observed.
However, for structures larger than the ones considered in our
investigation, Wang et al. assumed a sulfurisation process at this
temperature but only a weak desulfurisation process.[81] We
therefore propose that the particle size of our ZnO material
exhibits an optimal precondition for a sulfurisation under H2S at
150 °C without a desulfurisation process when the gas exposure
ends. However, at a higher temperature of 450 °C it behaves like
the other reported systems. It is possible that sulfurisation takes
place at this temperature, as described by Huang et al., but when
the H2S exposure ends the desulfurisation rate is high enough to
regenerate the system, in accordance with the impact of
temperature on the equilibrium from a thermodynamic point of
view.[31, 82]
In Figure 9 d) the regeneration process is shown: ZnS reacts with
oxygen to form ZnO and SO2 as described in equation (3).[31] This
reaction occurs at higher temperatures when the gas exposure
ends as well as at low temperatures when smaller particles are
involved.[31, 81-82] Since ZnO is once again rendered able to adsorb
oxygen, free electrons in the material become trapped and as a
consequence the conductance decreases.[82]
In addition, diffusion processes can also take place, as depicted
in Figure 9 e). A possible process for this situation is described by
Neveux et al.[86] When ZnO is coated by a ZnS layer, Zn atoms
can diffuse to the surface via zinc vacancies, interstitial positions
and/or grain boundaries. The same can occur with the remaining
oxygen. At the surface, these species can react with H2S and also
form ZnS. As a consequence, vacancies are formed in the interior
of the structure and the original grain morphology is lost. This is a
very important mechanism, which takes place in parallel to the
sulfurisation process and which might lead to an alteration of the
materials’ structure, which in turn can influence sensing
performance, as is the case for CuO thin films interacting with
H2S.[75, 84] This model is a first step to explain the gas sensing
behaviour of ZnO structures with H2S. However, further
investigations are needed as gas concentration as well as the
relative humidity will have to be considered.
Figure 9. Schematic model for the interaction of ZnO with H2S at different
reaction stages: a) before gas measurements in air; b) surface reactions at the
beginning of the H2S exposure; c) bulk reactions; d) recovery and e) diffusion
processes.
Figure 9 a) shows ZnO in air in the absence of any test gases.
Oxygen adsorbs at the surface of the n-type semiconducting ZnO,
whereby surface acceptor states (binding electrons from bulk) are
created. A depletion layer is thus formed in the near-surface area.
The depletion layer between two particles hinders the electrons
from passing and, as
a consequence, the conductance
decreases.[23] This model is applicable to both measurements and
all regarded systems of Huang et al., Wang et al., and Kim and
Yong as well as particle sizes and temperatures.[31, 81-82]
In Figure 9 b) H2S gas exposure is initiated. The gas reacts with
the adsorbed oxygen, whereby the trapped electrons are set free
and decrease the depletion layer. The resulting increase in
conductance is proportional to the gas concentration.[23] The
reaction mechanism is described by equation (1). Subsequently
H2S reacts with the oxygen to form SO2 and H2O.[82] This reaction
is applicable to all considered systems at all considered
temperatures and both measurements, but can also be just one
part of the whole forthcoming mechanism.
The reactions displayed in Figure 9 c) occur subsequently or in
parallel to the surface reactions depicted in Figure 9 b). Based on
the model by Huang et al. H2S reacts with bulk ZnO, which results
in the formation of a ZnS layer on the top of the structure by the
ongoing gas exposure.[31] The interior of ZnO is consequently
protected against adsorbing oxygen, whereby the electrons in the
material remain free and an increase in conductance is preserved.
The reaction mechanism for this situation is described by equation
(2). This part of the model appears to apply to smaller structures
and can be favoured by an increasing operating temperature.[31,
Conclusions
Nanostructured ZnO was synthesised through a quick, easy, low-
temperature and green hydrothermal route. The samples were
obtained as nanocrystalline powders with high crystallinity and
purity and thoroughly characterised from the structural,
compositional and morphological point of view.
A particular ZnO sample (H-ZnO-1) shows an interesting gas
sensing behaviour. At an operation temperature of 450 °C the
material exhibits a significant response to a H2S gas exposure as
expected for a semiconducting n-type gas sensor. However, at an
operating temperature of 150 °C the material shows gas
dosimeter-type behaviour. Thereby, the slope of the linear
increase of conductance depends directly on the offered gas
concentration. This interesting property is attributed to the particle
82, 86]
With larger structures the sulfurisation rate drops until it is
not recognizable any more at low temperatures.[31, 81]
The dimension of the structures (particle size) in our material is
intermediate relative to the material from Wang et al. and Kim and
Yong.[81-82] This feature might be the reason for the dosimeter-
type behaviour we observed at 150 °C (Figure 7).[33] At this
temperature, the sulfurisation as well as the desulfurisation
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10.1002/ejic.201801334
European Journal of Inorganic Chemistry
FULL PAPER
size (ca. 80 nm in diameter), and our proposed mechanism can
explain the dosimeter behaviour in relation to this particular
particle size. In contrast to CuO thin film H2S gas dosimeters there
are no visible changes in morphology, which can be taken as a
proof of an increased dosimeter longevity.
XPS analysis
Samples were investigated by XPS (X-ray Photoelectron Spectroscopy)
measurements with a Φ 5600ci Perkin-Elmer spectrometer, using a
standard aluminium (Al Kα) source, with an energy of 1486.6 eV operating
at 200 W. The X-ray source employed was located at 54.7° relative to the
analyser axis. The working pressure was < 5·10-8 Pa ~10-11 torr. The
calibration was based on the binding energy (B.E.) of the Au4f7/2 line at
83.9 eV with respect to the Fermi level. The standard deviation for the B.E.
values was 0.15 eV. The reported B.E. were corrected for the B.E.
charging effects, assigning the B.E. value of 284.6 eV to the C1s line of
carbon.[72, 88-89] Survey scans were obtained in the 0-1350 eV range (pass
energy 58.7 eV, 0.5 eV/step, 25 ms/step). Detailed scans (11.75-29.35 eV
pass energy, 0.1 eV/step, 50-150 ms/step) were recorded for relevant
regions (O1s, C1s, Zn2p, ZnLMM). The atomic composition, after a
Shirley-type background subtraction,[90] was evaluated using sensitivity
factors supplied by Perkin-Elmer.[72] Assignment of the peaks was carried
out according to literature data.
Experimental Section
Chemicals
Tetraethylammonium hydroxide (20% w/w in water) (TENOH), ammonia
(28-30% in water), zinc acetylacetonate hydrate and sodium hydroxide
were purchased from Sigma Aldrich (Milan, Italy). Oxalic acid dihydrate
(99.8%) was purchased from Carlo Erba (Rodano, Milan, Italy). All
reagents were used without further purification.
Synthesis protocol
In detail, for the hydrothermal synthesis of a generic ZnO sample, a
suspension of zinc oxalate, prepared from an aqueous mixture of zinc
acetylacetonate and oxalic acid, was charged in an autoclave, which was
heated at a set temperature for a set time span (see Table 4). The reaction
scheme, following the protocol explored by Diodati et al.[71] is shown in the
Supporting Information (Figure S1 in S.I.).
Gas measurements
For the gas measurements 3 mm by 3 mm standard gas sensor substrates
from UST GmbH (Geschwenda/Germany) were used. These substrates
contain a 10 Ohm built-in platinum heater and platinum inter digital
structure (IDS) on top with a gap of ca. 25 µm. The electrical read-out and
the heater control were carried out by a homemade electronic setup which
is driven by a Lab-View program. Conductance measurements were
carried out by applying a constant voltage of 1 V on the IDS.
The resulting suspension was sealed in the PTFE cup and placed in a
stainless steel container (4745 General Purpose Acid-Digestion Bomb,
Parr Instrument Company), heated at a set temperature (135 °C or 160 °C
vide infra) for the desired time and then let to cool down by extracting the
autoclaves from the oven and leaving them at room temperature in air. The
obtained solid powders were isolated by centrifugation, washed four times
with deionised water and dried at 90°C in an open air oven. Two different
treatment temperatures were explored (135 °C and 160 °C with yields
equal to respectively 79 and 71%). The prepared specimens with relative
synthesis parameters are listed in Table 4.
For the sensor preparation, a 5% w/w suspension of the powder in distilled
water was prepared. After stirring, 3µL were deposited onto the IDS of a
sensor substrate. Subsequently the substrates were heated up to 400 °C
in 30 minutes. This temperature was held for 6 h to calcinate the sensitive
layer. The test gas was prepared by a homemade gas mixing device using
500 sccm and 20 sccm mass flow controllers from MKS. For all
measurements a gas flow of 200 sccm synthetic air with about 30% relative
humidity was used. After an initial time interval to allow for sensor warm-
up, a controlled amount of H2S was added to the gas stream.
Table 4. Synthesis parameters for the various samples
CHN elemental analysis (microanalysis)
Sample
Target
Zn/Acid
Base
TENOH
(mL)
Treatment
temp.
Treatment
time
Samples were introduced in a quartz tube which was kept at 1020 °C and
through which a constant flow of oxygen-enriched helium was maintained.
The gases resulting from combustion through layers of WO3 and metallic
copper in the primary column were separated by frontal gas-
chromatography through the use of a 2 m Porapak QS chromatographic
column kept at 190 °C. The separated gas components were then
analyzed with a Frison EA 1108 analyzer.
compound
(mol/mol)
H-ZnO-1
H-ZnO-2
ZnO
ZnO
1/1
1/1
NaOH
NaOH
0.2
0.2
135 °C
160 °C
24 hours
24 hours
Powder XRD
Thermal stability
PXRD (Powder X-Ray Diffraction) patterns were collected with a Bruker
D8 Advance diffractometer equipped with a Göbel mirror and employing
the CuKα radiation. The angular accuracy was 0.001° and the angular
resolution was better than 0.01°. All patterns were recorded in the range
10-80° with a scan step 0.03° (2θ) and a 7 second per step acquisition
time. Patterns were analysed through the use of the MAUD[87] program,
based on a Rietveld refinement, to evaluate the crystallite size of the
powders together with other structural and microstructural parameters on
the crystalline phases in the samples.
Compounds were tested for thermal stability by heating 500 mg of the
prepared oxide to 400°C with a 200°C/hour ramp. The compounds were
kept at 400°C for 3 hours and then left to cool down to RT overnight.
Electron Microscopy (SEM, TEM)
SEM (Scanning electron microscopy) measurements were performed
using a Field Emission (FE-SEM) Zeiss SUPRA 40VP, with a primary
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10.1002/ejic.201801334
European Journal of Inorganic Chemistry
FULL PAPER
beam acceleration voltage of 3 kV and a conventional secondary electron
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Financial support was provided by the DFG via the GrK (Research
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10.1002/ejic.201801334
European Journal of Inorganic Chemistry
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Nanostructured ZnO was synthesised
through a quick, easy, low-
ZnO-based H2S dosimeters
Stefano Diodati, Jörg Hennemann,
Fernando Fresno, Stefano Gialanella,
Paolo Dolcet, Urška Lavrenčič Štangar,
Bernd M. Smarsly,* and Silvia Gross*
temperature and green hydrothermal
route, showing an interesting gas
sensing behaviour. At an operation
temperature of 450 °C the material
exhibits a significant response to a
H2S gas exposure as expected for a
semiconducting n-type gas sensor.
However, at an operating temperature
of 150°C the material shows gas
dosimeter-type behaviour.
Page No. – Page No.
Easy and green route to earth-
abundant based functional materials:
ZnO as an effective active material for
H2S dosimeters
This article is protected by copyright. All rights reserved.