Self-assembled highly porous rugby-like hierarchical ZnO thin film formaldehyde gas sensor

Article abstract of DOI:10.1080/15421406.2019.1693111

A new rugby-like ZnO thin film was self-assembled via a hydrothermal method and the influences of the hydrothermal temperature on crystal growth and gas sensing properties of ZnO thin films were investigated. The results showed that the highly porous rugby-like ZnO crystals were self-assembled via thin fan-shape sheets with abundant nanopores formed by interconnected the smaller nanoplates when the hydrothermal temperature was 140 °C. Its specific surface area value reached 45.0 m² g^?1. The sensor exhibited an excellent gas sensing response, a good selectivity towards formaldehyde and quick response characteristics at the optimum working temperature of 250 °C.

Full text of DOI:10.1080/15421406.2019.1693111

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Self-assembled highly porous rugby-like
hierarchical ZnO thin film formaldehyde gas
sensor
Lei Zhang & Wanli Jiao
To cite this article: Lei Zhang & Wanli Jiao (2019) Self-assembled highly porous rugby-like
hierarchical ZnO thin film formaldehyde gas sensor, Molecular Crystals and Liquid Crystals, 691:1,
15-26, DOI: 10.1080/15421406.2019.1693111
To link to this article: https://doi.org/10.1080/15421406.2019.1693111
Published online: 26 Dec 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
019, VOL. 691, NO. 1, 15–26
2
https://doi.org/10.1080/15421406.2019.1693111
Self-assembled highly porous rugby-like hierarchical ZnO
thin film formaldehyde gas sensor
Lei Zhang and Wanli Jiao
School of Material Science and Engineering, Shandong University of Technology, Zibo, P. R. China
ABSTRACT
KEYWORDS
gas sensing property;
rugby-like hierarchical
nanostructure;
A new rugby-like ZnO thin film was self-assembled via a hydrother-
mal method and the influences of the hydrothermal temperature on
crystal growth and gas sensing properties of ZnO thin films were
investigated. The results showed that the highly porous rugby-like
ZnO crystals were self-assembled via thin fan-shape sheets with
abundant nanopores formed by interconnected the smaller nano-
self-assemble; zinc oxide
ꢀ
plates when the hydrothermal temperature was 140 C. Its specific
2
ꢁ1
surface area value reached 45.0 m
g . The sensor exhibited an
excellent gas sensing response, a good selectivity towards formalde-
hyde and quick response characteristics at the optimum working
ꢀ
temperature of 250 C.
1. Introduction
With the increasing awareness of environmental protection, health and safety, applications
of various sensors for environmental monitoring, such as water, soil, gas, etc., become the
focus of research. Formaldehyde (HCHO) is a colorless and pungent-smelling gas, which
is widely used in the field of chemistry, wood, textile and corrosion protection. However,
long-term exposure to formaldehyde at low concentration (safe exposure limit of 0.08 ppm
and 1 ppm to formaldehyde) can induce heavier hormesis to people’s health [1,2].
ꢁ
3
Generally, the human threshold of smell is 0.06-0.07 mgꢂm , therefore a practical and
effective way to detect formaldehyde is of great significance.
Recently, compared with other detection technology, various metal oxides semicon-
ductor gas sensors, such as SnO , ZnO, In O and WO , have caused widespread con-
2
2
3
3
cern due to the advantages of high sensitivity, quick response-recovery time, lower cost
and simple preparation procedure [3–8]. Among these semiconductor gas sensor, ZnO
is widely used on benefit of its wide band gap of 3.37 eV and good chemical stability.
Doping with various elements, e.g., Ag [9], Y [10], In [11] and Fe [12] have been
thought to enable to modify the surface charge states of ZnO and increase the gas sens-
ing properties. For example, wang reported that the formation of Ag-loaded ZnO heter-
ojunction, due to the continuity of lattice fringes between Ag and ZnO nanoparticle,
enhanced the response [9]. In addition, the sensing properties of 2.5 wt% Fe doping
ZnO to 10 ppm formaldehyde were significantly enhanced to 37.1 at the working
CONTACT Wanli Jiao
jiaowanli1977@163.com
School of Material Science and Engineering, Shandong University
of Technology, Zibo 255049, P. R. China
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
ß 2019 Taylor & Francis Group, LLC

1
6
L. ZHANG AND W. JIAO
ꢀ
temperature of 300 C, together with the short response and recovery time, excellent select-
ivity [12]. On the other hand, surface morphology modification such as the hierarchical
structure of ZnO nanopowders [13,14], especially the flower-like hierarchical structures ZnO
which can be simply and efficiently prepared [15–17], and porous and orderly ZnO thin
films [18–20] have also been concerned because of the high specific surface area and poros-
ity, which should provide more spaces for target gas to diffuse into the inner of the sensing
material film and react with the absorbed oxygen on the surface of sensing material. For
example, Meng et al. [18] reported that the gas responses of flower-like hierarchical struc-
ture films consisting of porous single-crystalline ZnO nanosheets synthesized by a one-pot
wet-chemical method were enhanced greatly and the sensing mechanism of the single-
crystalline structure had been analyzed from the aspect of electronic transport.
In this article, a new rugby-like hierarchical ZnO particles composed of highly porous
nanosheets thin film was self-assembled via a hydrothermal method and the gas-sensing
characteristics of these thin film gas sensors were detected for their excellent gas sensing
selectivity towards formaldehyde.
2. Experimental section
2.1. Preparation of the ZnO thin film
ZnO thin film was deposited using a hydrothermal method on a single crystal silicon
substrate (6 mm ꢃ 3 mm) with Au interdigitated electrode on the surface. The width
and spacing of Au finger electrodes are 20 lm and 30 lm, respectively. Before using, the
substrates were ultrasonic washed with 1 M NaOH solution, 1 M HCl solution and
ethanol for 20 min in succession. Afterward, stored in ethanol for subsequent use.
All reagents were commercially available from Sinopharm Chemical Reagent Co., Ltd.
(China) with analytical grade and were used without further purification. First, a mix-
ture solution containing 50 mL of Zinc acetate ethanol solution (0.01 mol) and 50 mL of
hexamethylenetetramine (HMTA) solution (0.01 mol) were formed under magnetic stir-
ring. Then, a 80 mL of the above mixture solution and the substrates remaining the Au
electrodes side upwards, were transferred into a 100 ml of Teflon-lined stainless steel
autoclave; the autoclave was sealed and kept in an oven at a certain temperature
ꢀ ꢀ ꢀ
120 C, 140 C and 160 C, respectively) for 8 h. After cooled naturally, the substrates
(
were took out and washed with deionized water and the deposition powders was centri-
fuged and washed with deionized water for three times; then all the products were dried
ꢀ
and sintered at 500 C for 2 h to establish the stable microstructures and surface states.
2
.2. Fabrication of ZnO thin film gas sensor
To fabricate the ZnO thin film gas sensor, two Pt wires were welded with silver paste
on the Au electrode at both ends of the interdigitated electrode substrate; and both end
of each Pt wire were welded on a gas sensor pedestal. A Ni-Cr heating wire was pressed
under the interdigitated electrode substrates and also welded on the gas sensor pedestal
to provide necessary working temperature. The diagram of the gas sensing system of
the ZnO thin film gas sensor was shown as Figure 1. The obtained gas sensors were
ꢀ
aged for 72 h at 150 C.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
17
ZnO thin film
Ag paste
Pt wire
Au electrode
Ni-Cr wire
Quartz insulation layer
Si substrate
Figure 1. Diagram of the gas sensing system of the ZnO thin film gas sensor.
2.3. Performance characterization
The crystal structures of ZnO powders were characterized by a polycrystalline X-ray dif-
fractometer (XRD; D8 ADVANCE, Bruker AXS, Germany) with an accelerating voltage
ꢀ
of 40 kV, using Cu Ka radiation (k ¼ 0.154059 nm) and a scan rate of 8 /min. The
1
morphologies of the ZnO thin films on interdigitated electrode substrates were observed
using a scanning electron microscope (SEM; quanta 250, FEI, America) and the elemen-
tal composition of the ZnO powders synthesized together with the ZnO thin film was
examined by energy dispersion spectrum (EDS). Brunauer–Emmett–Teller (BET) surface
area tests (Belsorp Max, Ankersmid Co. Ltd., Holland) were performed at 77 K.
Nitrogen adsorption-desorption isotherm were tested and the pore volumes and pore
size distribution were calculated using the Barret-Joyner-Halenda (BJH) model.
The gas sensing properties of sensors were measured by a gas sensing system (WS-
3
0A, ZhenZhou Winsen Technology Co., Ltd., China). The environment had a pressure
ꢀ
of 101 kPa, a ambient temperature of ꢄ27 C and a relative humidity of ꢄ47%. The
testing voltage was 5.0 V. All these gas sensing experiments were performed in a sealed
chamber (18.0 L in volume) with a gas inlet. The measure procedure was as follows.
Firstly, the gas sensing system began to record the initial resistance of gas sensors in air
(
Ra). Then, the target gas formed by evaporation of the corresponding liquid was
injected onto a flat evaporator in the chamber. The conversion formula is given by Eq.
1). After evaporating and mixing the target gas with a fan in the chamber for 10 s, the
(
response process of gas sensor began and the gas sensing system began to record the
resistance changes of sensors. When the resistance value of the sensor in target gas (Rg)
was in relative stable, the chamber was opened and the target gas was expelled quickly.
The recovery process of gas sensor began and the gas sensing system record the resist-
ance changes of sensors until the resistance reached 90% of its initial value. The gas
sensing response (S) is defined as S ¼ R /R . The response time and recovery time are
a
g
the times required for a change in resistance to reach 90% of its equilibrium value after
injecting and removing the target gas, respectively. The selectivity is the ratio of the
response in the target gas to that in a contrastive gas.
ꢁ
9
ð273 þ T Þ ꢃ V ꢃ C ꢃ 10 ꢃ m
k
U ¼
(1)
ð273 þ T Þ ꢃ 22:4 ꢃ d ꢃ p
B
Where U is the volume of the corresponding liquid (mL), m is the molecular weight
ꢁ
1
ꢁ3
(
(
(
gꢂmol ), d is the density of the tested liquid (gꢂcm ), p is the purity of the liquid
%), V is the volume of the chamber (mL), C is the concentration of the tested gas
ppm), T is the ambient temperature ( C) and T is the chamber temperature ( C).
ꢀ
ꢀ
k
B

1
8
L. ZHANG AND W. JIAO
3. Results and discussions
3.1. Microstructure of ZnO thin film
Figure 2 shows the XRD patterns of the ZnO nanopowders synthesized at various
hydrothermal temperatures. It can be observed that all diffraction peaks are indexed as
a wurtzite ZnO (JCPDS. 36-1451) which is the most stable and most common form.
The effect of hydrothermal temperature on the morphologies of the ZnO thin films
were investigated by SEM.
Figure 3a shows the low magnification image of ZnO thin film self-assembled at the
ꢀ
hydrothermal temperature of 140 C for 8 h on the interdigitated electrode substrate. It
can be seen that the hierarchical ZnO particles uniform distribute on the whole surface of
the substrate. The high magnification images shown in Figure 3b–d indicate that the ZnO
particles exhibit a rugby-like hierarchical microstructure, which are self-assembled by thin
and uniform fan-shape nanosheets. It is worth noting that every fan-shape nanosheet is
not a whole, but a assembly formed by interconnected smaller nanoplates with an amount
of nanopores when the image is magnified 20000 times (shown in Figure 3d). The max-
imum diameter of a rugby-like ZnO particle is of ꢄ10lm, while the thickness of a piece
of fan-shape nanosheet is in the range of 10nm to 20nm. The morphology of our prod-
uct is different from that of hierarchical flower-like ZnO structure [15,18]. Song reported
that the high annealing temperature is contributed to obtaining the high porosity and
good gas sensing properties [16]. However, we discover that the porosity and morphology
of the ZnO particles also can depend on the hydrothermal temperature.
Thus, the influences of hydrothermal temperature on microstructure and gas sens-
ing properties of ZnO thin film were investigated. Figure 4a–c shows the detailed
ꢀ
images of ZnO thin film self-assembled at the hydrothermal temperature of 120 C,
ꢀ ꢀ
1
40 C and 160 C, respectively. The hydrothermal times are all 8 h. Figure 4a shows
a flower-like hierarchical structure ZnO particle consisting with ZnO nanosheets
with 5 lm in diameter; however, nanopore on the ZnO nanosheets are absent or not
ꢀ
clear due to the lower hydrothermal temperature of 120 C. Most of the ZnO
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
(
c)
(b)
a)
(
20
30
40
50
60
70
80
2
Theta/ degree
Figure 2. XRD patterns of the ZnO powders synthesized at various hydrothermal temperature.
ꢀ ꢀ ꢀ
a) 120 C; (b) 140 C; (c) 160 C.
(

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
19
ꢀ
Figure 3. SEM images of the ZnO thin film self-assembled at the hydrothermal temperature of 140 C
on the interdigitated electrode substrate. (a) ꢃ500; (b) ꢃ2000; (c) ꢃ10000; (d) ꢃ20000.
nanosheets grow vertically on the substrate forming a monolayer structure, so the
thickness of this thin film is about 5 lm. When the hydrothermal temperature
ꢀ
increased to 160 C, the ZnO particle exhibited a spherical-like hierarchical structure,
composing with a large number of ZnO nanosheets, as shown in Figure 4c; however,
the length of the ZnO nanosheets are shorter than those on the rugby-like ZnO par-
ꢀ
ticles synthesized at the hydrothermal temperature of 140 C shown in Figure 4b
and their arrangement is disorganized. Thus, the higher hydrothermal temperature
can strip away the orderly array of fan-like ZnO nanosheets and make the smaller
ZnO nanosheets to shrink and form a spherical-like structure. Compared with the
ꢀ
monolayer structure synthesized at 120 C, Figure 4b and c show multilayer struc-
ture; so the thicknesses of these thin films are about 30 lm.
To confirm there is no residue of carbon or anything, the element composition of the
ZnO powders synthesized together with the ZnO thin film was examined by EDS and
the result shows in Figure 4d. We can see that the sample only has Zn and O elements,
thus there was no a left of some residue or carbon residue.
The porous structure and high specific surface area are conducive to gas adsorption
inward and desorption outward the gas sensing materials. Thus, the BET surface area tests
and Nitrogen adsorption-desorption isotherm were performed at 77 K. Figure 5a–c were
the specific surface areas results of ZnO powders synthesized together with the correspond-
ꢀ
ꢀ
ꢀ
ing ZnO thin film at the hydrothermal temperature of 120 C, 140 C and 160 C, respect-
ively. And the results of samples calculated according to the BET equation are shown in
Table 1.

2
0
L. ZHANG AND W. JIAO
Figure 4. SEM images of the ZnO thin films self-assembled at the hydrothermal temperature of
ꢀ ꢀ ꢀ
a) 120 C; (b) 140 C; (c) 160 C and (d) EDS patterns of the ZnO powder synthesized together with
the thin film at the hydrothermal temperature of 140 C for 10 h and annealed at 500 C for 2h.
(
ꢀ
ꢀ
From Table 1, the rugby-like ZnO powders synthesized at the hydrothermal temperature
ꢀ
2
ꢁ1
of 140 C exhibit the largest specific surface area value of 45.0 m g among these three
2
ꢁ1
2
samples, which is higher than those of nanostructure ZnO powders (22.9 m g and 5.7 m
g ) reported by Meng [18] and Guo [13], respectively; but lower than that of hollow, por-
ous yttrium functionalized ZnO nanosphere (89.5 m g )[10]. And its total pore volume is
.224 cm g . The pore size distribution of the rugby-like ZnO powders synthesized at
the hydrothermal temperature of 140 C are calculated using the Nitrogen adsorption-
desorption isotherm and the BJH model shown in Figure 5d. As observed, the N2
ꢁ
1
2
ꢁ1
3
ꢁ1
0
ꢀ

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
21
Figure 5. BET curves of the ZnO nanopowders synthesized at the hydrothermal temperature of
ꢀ
ꢀ
ꢀ
(
a) 120 C; (b) 140 C; (c) 160 C and (d) Nitrogen adsorption-desorption isotherm and BJH pore size
ꢀ
distribution plots (inset) of ZnO nanopowders synthesized at the hydrothermal temperature of 140 C.
Table 1. The BET results of samples synthesized at different hydrothermal temperature.
Hydrothermal
temperature/
BET surface
area/ m g
Average pore
size / nm
Total pore
volume / cm g
ꢀ
2
ꢁ1
3
ꢁ1
C
1
1
1
20
40
60
17.3
45.0
18.5
1.9
19.8
2.0
0.008
0.224
0.009
adsorption-desorption curve exhibits a type IV isotherm, according to the modern IUPAC
classification of adsorption isotherms, with hysteresis loop at the range of 0.8-1 (P/P ), which
0
indicates the sample exhibits mesoporous adsorbent and large porosity. The pore size distri-
bution curve (shown in Figure 5d inset) exhibits a typical narrow distribution characteristics
and the most frequent pore size distribution peak is concentrated at about 18nm in the
range of 1 to 100 nm and another narrow pore size distribution peak is at about 12nm.
3
.2. Gas sensing performances of the ZnO thin film gas sensor
The gas sensing characters of the ZnO thin film sensors had been detected. It is well
known that the adsorption and chemical reaction with gases happened on the surface of
the sensing material are significantly depended on the working temperature. To find the
optimum working temperature, sensing response of sensors to 1000 ppm formaldehyde

2
2
L. ZHANG AND W. JIAO
Figure 6. Response values of the ZnO thin film sensors synthesized at different hydrothermal tem-
perature to 1000 ppm formaldehyde.
ꢀ
gas at the working temperature in the range of 200 to 400 C had been detected,
respectively, and the results show as Figure 6.
We can see that all the responses curves of ZnO thin film sensors exhibited a similar
change regulation with increasing working temperature and they all reach the maximum
ꢀ
value at the working temperature of 250 C. Thus, the optimum working temperature is
ꢀ
determined as 250 C; at this temperature, the response value of the ZnO thin film sen-
ꢀ
sor synthesized at the hydrothermal temperature of 140 C to 1000 ppm formaldehyde is
139.4, which is the highest among those of sensors.
The response and recovery curves of the hierarchical rugby-like ZnO thin film sen-
sors to different formaldehyde concentration (from 10 to 1000 ppm) at the working
ꢀ
temperature of 250 C are shown in Figure 7 a. It is evident that the response value
increases with increasing formaldehyde concentration and the values are 2.0, 5.6, 10.0,
1
1
2.6, 14.9, 47.3 and 139.4 to formaldehyde concentration of 10, 25, 50, 75, 100, 500 and
000 ppm, respectively. Thus, the hierarchical rugby-like ZnO thin film sensor is able to
detect a wide range of formaldehyde concentration, indicating its excellent sensitivity.
Comparison between the sensing performances of the hierarchical ZnO thin film based
sensors and literature is presented in Table 2.
The response and recovery time are also important characteristics to determine the
performance of a gas sensor. In this work, the response times to formaldehyde for dif-
ferent concentration are all less than 10 s, while the recovery times of those are in the
range of 40 to 50 s. It means the quickly response-recovery characteristics.
According to the power laws theory for metal oxide semiconductor sensor, gas adsorp-
tion on the semiconductor surface can be empirically represented as follows [22,23]:
b
S ¼ a½Cꢅ þ 1 ) logðS ꢁ 1Þ ¼ blogðCÞ þ logðaÞ
(2)
Therein, a and b are the constants; S is the gas response; C is the test gas concen-
tration. Thus, according to the experimental data, the straight line of corresponding
log(S-1) vs. log(C) is shown in Figure 7b and the calibration curve at the experimental
data is fitted as:

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
23
Figure 7. (a) Response and recovery curves of the rugby-like ZnO thin film sensor to different concentra-
ꢀ
tion of formaldehyde vapor at the working temperature of 250 C; (b) corresponding log(S-1) vs. log(C).
Table 2. Comparison of formaldehyde gas-sensing characteristics for different ZnO gas sensors.
Working
temperature/ C
Formaldehyde
concentration/ ppm
Response
value
ꢀ
Gas sensor
Synthesis method
Hydrothermal
References
This work
Hierarchical ZnO thin
film sensor
250
50
10
ZnO microflowers sensor
ZnO hollow sphere sensor
ZnO nanoflower sensor
Y functionalized ZnO
nanosphere sensor
flower-like ZnO sensor
ZnO nanorods thin
film sensor
Hydrothermal
Template process
Hydrothermal
Solvothermal
260
385
350
350
50
500
50
5.5
25
insensitive
[16]
[21]
[13]
[10]
50
15
Hydrothermal
chemical spray
pyrolysis
300
Room
temperature
100
100
2.4
2
[17]
[14]
Y ¼ 0:97 ꢃ X ꢁ 0:81
(3)
2
Where Y is log(S-1) and X is log(C). The relative correlation coefficient (R ) is 0.966.
The results highlight the potential application in monitoring formaldehyde gas
concentration.

2
4
L. ZHANG AND W. JIAO
Figure 8. Responses of the hierarchical rugby-like ZnO thin film gas sensor to 100 ppm various target
ꢀ
gas at the working temperature of 250 C.
The selectivity of gas sensor is important in the process of practical application. To
prove the excellent selectivity of the hierarchical rugby-like ZnO thin film gas sensor to
formaldehyde, its gas responses towards 100 ppm various target gas at the working tem-
ꢀ
perature of 250 C were detected, and the results are shown in Figure 8.
Figure 8 shows the response values of the hierarchical rugby-like ZnO thin film gas
sensor to 100 ppm formaldehyde is 14.9, which is higher than those to other gases. The
response to 100 ppm acetone is 6.6. Furthermore, to hydrogen and methylbenzene, the
sensor shows lower sensitivity and the responses are both less than 2. The selectivity of
formaldehyde to ethanol is 2.9, while to methylbenzene reaches 6.5, which mean that
the hierarchical rugby-like ZnO thin film gas sensor has satisfactory selectivity to
formaldehyde.
3.3. Gas sensing mechanism
The excellent gas response to formaldehyde and its quickly gas response-recovery char-
acteristic are owing to the special microstructure of the hierarchical rugby-like ZnO
thin film shown in Figure 3 and the schematic diagram is shown in Figure 9a. The
orderly upright growing fan-shape ZnO nanosheets and an amount of nanopores
increase its specific surface area greatly; at the same time, a large number of intercon-
nection areas as the gas response model of grain-crystal boundary-grain showing
(shown in Figure 9a) are formed and across these area the double-Schottky barrier are
formed. The double-Schottky barrier plays an important role in determining the resist-
ance of the sensor. Figure 9b and c shows the schematic diagram of the gas response
mechanism of the interaction areas in air and in formaldehyde. The interconnection
areas are more active which is favor to oxygen molecules (O ) in air adsorb on the sur-
2
face. Then, as the conditional n-type semiconductor does, the adsorbed oxygen mole-
cules should capture electrons from conductive band of ZnO and ionize into oxygen
-
2-
species, such as O and O . As a result, the depletion region forms and the double-
Schottky barrier potential increased, as shown in Figure 9b. In this work, the double-
Schottky barrier are widely distributed, forming the smaller conduction section area and
the higher Ra. This is the main reason why the gas response of sample synthesized at

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
25
Thin and fan-shaped ZnO sheets
Smaller ZnO nanoplates
Nanopores
Rugby-like ZnO particle
(a)
Adsorbed oxygen ions layer
HCHO
O
2
e-
ZnO
HCHO
e-
O
2
H
2
O
CO
2
-ǃO2-
O
-
e-
e-
e
e-
e-
e-
e-
e- e-
e-
double-Schottky barrier
b)
Depletion layer
(c)
(
Figure 9. (a) Schematic diagram of the rugby-like ZnO particle self-assembled via thin and fan-shape
sheets with abundant nanopores formed by interconnected the smaller nanoplates; (b) gas response
mechanism schematic diagram of the interconnection area in air and (c) in formaldehyde.
ꢀ
the hydrothermal temperature of 140 C is better than others. Another reason is a large
number of open gas channels promote the ability of target gas diffusion and adsorption
into the film and increase the effective reaction area toward target gas.
As far as the ZnO sensing film is exposed to the reductive gas, such as formaldehyde,
-
2-
formaldehyde molecules should adsorb and react with O and O on the surface of the
ZnO particles; resulting in releasing and returning electrons to ZnO conductive band.
The reaction formulas are shown as formula (3) and (4). Thus, the depletion region
decreases, leading to a mitigation in potential barrier and a decreasing in Rg, as shown
in Figure 9c.
ꢁ
ꢁ
HCHO þ 2O ! H O þ CO þ 2e
(3)
(4)
ads
2
2
2
ꢁ
ꢁ
HCHO þ 2O ! H O þ CO þ 4e
ads
2
2
4. Conclusions
Various hierarchical structure ZnO thin films were self-assembled via a hydrothermal
method. Samples are well crystallized in a hexagonal wurtzite structure, however, the
porosity and morphology of the ZnO thin film are largely depended on the hydrother-
ꢀ
mal temperature. When the hydrothermal temperature is 140 C, the morphology of
ZnO thin film exhibited the arranged hierarchical rugby-like structure ZnO particles,
which are self-assembled by the thin and uniform fan-shape ZnO nanosheets with an
amount of nanopores formed by interconnected the smaller ZnO nanoplates. Thus, the
specific surface area value of the hierarchical rugby-like ZnO sample self-assembled at

2
6
L. ZHANG AND W. JIAO
ꢀ
2
ꢁ1
the hydrothermal temperature of 140 C was 45.0 m g , which was higher than those
ꢀ
ꢀ
of samples self-assembled at the hydrothermal temperature of 120 C and 160 C. The
pore size distribution curve exhibits a typical narrow distribution characteristics and the
most frequent pore size distribution peak is concentrated at about 18 nm. Furthermore,
the gas response of the hierarchical rugby-like ZnO thin film gas sensor to 50 ppm for-
ꢀ
maldehyde reached 10 at optimum working temperature of 250 C and it exhibited
excellent gas selectivity towards formaldehyde and quick response characteristics.
Funding
This work was supported by the Natural Science Foundation of Shandong Province (China)
(Grant No. ZR2015EL009).
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