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Acetone sensing properties and mechanism of nano-LaFeO3 thick-films
Accepted Manuscript Title: Acetone sensing properties and mechanism of nano-LaFeO3 thick-films Author: Yanping Chen Hongwei Qin Xiaofeng Wang Ling Li Jifan Hu PII: DOI: Reference:
S0925-4005(16)30735-3 http://dx.doi.org/doi:10.1016/j.snb.2016.05.059 SNB 20218
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
25-11-2015 13-4-2016 11-5-2016
Please cite this article as: Yanping Chen, Hongwei Qin, Xiaofeng Wang, Ling Li, Jifan Hu, Acetone sensing properties and mechanism of nano-LaFeO3 thick-films, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.05.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Acetone sensing properties and mechanism of nano-LaFeO3 thick-films Yanping Chen, a Hongwei Qin, *,a Xiaofeng Wang, a,b Ling Li, a and Jifan Hu *,a aSchool
of Physics, State Key Laboratory for Crystal Materials, Shandong University, Jinan 250100, China. bSchool of Science, Dalian University of Technology at Panjin, Panjin 124221, Liaoning, China
* Corresponding authors. E-mail: [email protected] (H. Qin); [email protected] (J. Hu)
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Abstract LaFeO3 nanocrystalline powders prepared by sol–gel method with annealing at 800 °C for 4 h can sensitively detect low concentration of acetone. When exposed to 0.5ppm acetone, the response of LaFeO3 thick film at 260 °C is 2.068 with response time of 62 s and recovery time of 107 s, respectively. The possible acetone sensing mechanisms for LaFeO3 sensor are investigated with first principles calculations. Calculated results demonstrate that acetone could release electrons to the surface of LaFeO3 (010) pre-adsorbed with oxygen species O and O2 . The acetone molecule reacts with oxygen species in the following ways: (1) adsorbs on oxygen species O or (2) replaces of the weakly pre-adsorbed oxygen species
O2 on Fe site, accompanying the formation of oxygen molecule. These above two processes may
play important roles in acetone sensing for LaFeO3. We also find that the acetone molecule can be directly adsorbed on Fe site, transferring some electrons to the LaFeO3 (010) surface. The latter processes may also provide additional contribution to acetone sensing.
Keywords: LaFeO3 thin film; acetone; Sol-gel; Sensing mechanisms; First principles calculations
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1. Introduction Acetone (C3H6O), a kind of volatile gas, is extensively used in chemical industry. However, it is easy to evaporate at room temperature. Inhalation of acetone can cause headache, fatigue and even narcosis and harmfulness to nerve system [1]. In addition, the medical reports show that acetone is one of the products exhaled by diabetic patients who have higher acetone concentrations in their blood and spittle than healthy people [2]. For healthy individuals, the acetone concentration in breath varies from 0.3 to 0.9 ppm. However, in the exhaled air of diabetic patients, the acetone concentration is reported to be more than 1.8 ppm [3]. Such significant difference makes acetone a suitable breath marker in detecting diabetes [2,4]. Different from the clinic detection of blood sugar, breath acetone analysis is noninvasive and can be performed frequently with minimal discomfort to patients. Thus, detecting of low acetone concentrations is essential for our safety and health, especially for developing a breath analysis technique for monitoring diabetes. Recently, there is increasing attention attracted on the extensive investigation of acetone-sensing materials and devices. Semiconductors have been reported as good sensing materials [4]. Various metal oxide semiconductors such as SnO2 [5-11], TiO2 [12-14], ZnO [15-24], Fe2O3 [25-26], NiO [27] and WO3 [29-31] have been developed as acetone sensing materials. Perovskite-type semiconductors such as SmFe1-xMgxO3 [32], LaFeO3 [33-35], La1-xPbxFeO3 [36], LaNi1-xTixO3 [37] and La0.75Ba0.25FeO3 [38] have also been used to detect acetone gas. And it has been found that the response S=Rgas/Rair to 50 ppm acetone is about for 16 for SmFe0.9Mg0.1O3 [32], 7 for La0.68Pb0.32FeO3 [36], 2.6 for SmFeO3 [39], 300 for Pd doped NdFeO3 [40]. The response value is 204 to 80 ppm acetone for LaFeO3 thick film [34], and 2 to 25 ppm acetone for porous LaFeO3 [33]. Among the various semiconductors used for acetone sensors, p-type LaFeO3 with perovskite-type structure (ABO3) has attracted considerable attention in gas sensor due to its
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stability at high temperature and in chemical atmospheres in recent years [33-35, 39]. The resistance of the p-type semiconductor sensor increased abruptly on the injection of acetone, and then decreased rapidly and recovered to its initial value after the test gas was released [32-40]. The most accepted mechanism for gas sensing in semiconductor materials, involves the adsorption of oxygen species on its surface [5-40]. When the sensors were exposed to air, O2 adsorb on the surface and create chemisorbed oxygen species by capturing electrons from the p-type semiconductor surface which increases their conductivity [32-38]. When the reducing gases such as acetone, H2 and CO are introduced at moderate temperature, the reducing gases may react with oxygen species adsorbed on the outer surface of p-type semiconductor and releases the trapped electrons back to the p-type semiconductor surface, which decreases the carrier concentration and increases the resistance of p-type semiconductor. In this work, sensing performances to low concentration acetone for LaFeO3 thick film sensors were investigated. The first principles calculations were performed in order to reveal the sensing mechanism of LaFeO3 in detail.
2. Experimental 2.1. Synthesis and characterization of nanocrystalline LaFeO3 Firstly, stoichiometric ratio of lanthanum nitrate, ferric nitrate and citrate acid (all analytically pure) were completely mixed in deionized water and was heated to 80 °C with continuous stirring. After adding some polyethylene glycol (PEG; molecular weight over 20,000), the solution was well stirred for several hours until the sol was formed. The sol was dried into a gel and then the gel was dried into pieces in a baking box. The pieces were preheated at 400 °C for 2h and ground into a fine powder sample, which was subsequently annealed in an oven at 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C for 4 h, respectively. The structure of resultant powders was characterized by X-ray diffractions using CuKα radiation at room temperature. The
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microstructure of LaFeO3 powders annealed at 800 °C was also observed by a field emission scanning electron microscope (FE-SEM). 2.2. Fabrication and measurements of sensors The powders were mixed with deionized water to form a paste. Then the obtained paste was coated onto a prefabricated alumina tube (4 mm in length and 1.2 mm in diameter, attached with a pair of gold electrodes and platinum wires) by a small brush to form a thick film (film thickness is about 250 μm), and dried at 100 °C for 2h. A Ni-Cr heating wire was inserted into the ceramic tube to form an inside-heated gas sensor. The sensors are annealed at 240 °C for 48 hours on the aging equipment in air. For the sensors based on the LaFeO3 powders annealed at different temperatures, we made three replicates for each chemical analysis, gas concentration, and operating temperature in the experiment. The photograph and schematic image of the as-fabricated sensor are shown in Fig. 1 (a) and (b). Gas sensing properties were measured
using
a
static
system
controlled
by
a
computer.
The gas
with
defined
gas
concentrations controlled by static state method was injected into the testing chamber [7, 9, 41].
3. Results and discussion The X-ray diffraction patterns at room temperature for LaFeO3 powders annealed at 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C are shown in Fig. 2. The pattern indicates that the sample has a single perovskite phase with orthorhombic structure (Pnma). The temperature dependence of the resistance in air for LaFeO3 thick film sensors are shown in Fig. 3. The resistance of all samples decreases with an increase of temperature. However, the resistances of LaFeO3 thick film sensors are very large. For example, resistance at 260°C for the thick film sensor based on LaFeO3 powders annealed at 800°C is about 25.4 M. The gas sensing response in the experiment is defined as S=Rg/Ra, where Ra and Rg represent the resistances of the LaFeO3-based sensor to air and acetone in background of air, respectively. However, in 5
our measurement, we have not measured the resistance of the sensor directly. Instead, a series circuit shown in Fig.1 (c) is used to measure the voltage Uout-put of the known series –resistance R0 directly when the sensor is exposed to air or testing gas in the background of air. The resistance R of the sensor when the sensor is exposed to air or testing gas in the background of air can be calculated based on the following equation:
R
5V U U R
out put
(1)
out put
0
Thus gas sensing response S=Rg/Ra can be obtained.
It is well known that the gas sensitivity is greatly influenced by operating temperature (TO) and annealing temperature (TA). In order to determine the optimum operating temperature and annealing temperature, responses of LaFeO3-based sensors (annealed at temperatures of 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C) to acetone vapor were examined as a function of operating temperature. From Fig. 4, it can be seen that the maximum response S to 5 ppm acetone for LaFeO3 annealed at temperatures of 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C are 1.029, 2.12, 5.195, 2.161 and 1.405 at the optimal operating temperature of 260 °C. The value of response S>1 means that the resistances in acetone gas for LaFeO3 are larger than that in air at the same temperature.
It indicates that LaFeO3 annealed at 800 °C exhibits the best sensing behavior to acetone vapor. The microstructures observed by FE-SEM for LaFeO3 powders annealed at temperature 800 °C and the density percent versus LaFeO3 grain size are shown in Fig. 5 (a) and (b), respectively. From the FE-SEM picture, we counted one hundred and twenty particles to calculate the density percent. There is a distribution of grain size around 55 nm. 6
The responses S of LaFeO3 thick film sensor annealed at 800 °C to 0.5 ppm, 1 ppm, 5 ppm, 10 ppm acetone at different temperatures (160 °C-420 °C) are shown in Fig. 6. The response increases with an increase of acetone concentration. With increase of operating temperature, the response increases at first, undergoes a maximum, and finally drops. When exposed to 0.5, 1, 5 and 10 ppm acetone, at the optimal operating-temperature (260 °C) the corresponding responses are 2.068, 3.245, 5.195 and 7.925, respectively. It shows that LaFeO3 thick film sensor annealed at 800 °C can sensitively respond to acetone. The relationship between the response and the acetone vapor concentration for LaFeO3 annealed at 800C at the optimal temperature of 260 °C is shown in Fig. 7. The response increases significantly with an increase in acetone vapor concentration. Since the acetone concentration varies from 0.3 to 0.9 ppm for healthy individuals, and is more than 1.8 ppm for diabetic patients [3], our present results show that LaFeO3 thick film sensor annealed at 800 °C can be used to distinguish between diabetic patients and healthy individuals. The dynamic sensing characteristics of LaFeO3 sensor annealed at 800C exposed to 0.5, 1, 5 and 10 ppm acetone gas at the optimal operating temperature of 260 °C are shown in Fig. 8. It can be seen that the resistance of the LaFeO3 thick film sensor annealed at 800 °C increases obviously after the injection of acetone gas, and then decreases after the test gas is blown off, consistent with other p-type semiconductor sensors [33-40]. Such change in the resistance indicates that acetone seems to release electrons to the surface of p-type LaFeO3 [33-35]. The response time has been defined as the time taken to attain 90 % of the final value, and the recovery time as the time taken to regain 10 % of the base value. Both of them are important parameters for one practical sensor. The response time for LaFeO3 sensor to 0.5, 1, 5 and 10 ppm acetone gas are about 62 s, 37 s, 39 s and 51 s, respectively. The recovery times to 0.5, 1, 5 and 10 ppm acetone gas are about 107 s, 82 s, 115 s and 155 s, respectively. When the acetone concentration is higher
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than 1ppm, the response time and recovery time of LaFeO3 sensor become slightly longer with the increase of acetone concentration. Selectivity reflects the ability of a sensor to respond to a certain gas in presence of other gases, which is another important characteristic of one gas sensor. The gas responses of LaFeO3 thick film sensor annealed at 800 °C to various testing gases of 5 ppm are measured at the operating temperature 260 °C, and results are shown in Fig. 9. The LaFeO3 sensor exhibits slow response to HCHO, ammonia and methanol. The highest response of the sensor is 2.44 to 5 ppm ethanol and 1.936 to 5 ppm acetone, which is just next that to ethanol. The selectivity coefficients for LaFeO3 based sensor are 1.495 to HCHO, 1.385 to ammonia, and 1.476 to methanol, indicating the excellent selectivity of the LaFeO3. In order to distinguish these gases, an array of thick film sensors including SnO2 and LaFeO3 thick-film elements could be used. Before introducing the reducing gas, oxygen molecules in air are usually adsorbed on the surfaces of semiconducting LaFeO3, grabbing electrons from its surface and forming the oxygen species. With increasing the temperature in air, the state of oxygen adsorbed on the surface of semiconductor undergoes the following changes:
O2 (gas) → O2- (ads)
(2)
O2 -(ads) →O- (ads)
(3)
O- (ads) → O2- (ads)
(4)
where the ads means the state of adsorption. When the acetone gas is introduced, the reducing gas would interact with the oxygen species O2- and O-, which releases electron back to the surface of LaFeO3, thus increasing the resistance of LaFeO3 sensor. The following reaction has been used in some publications [33, 34, 36, 37]. CH3COCH3 + 8O−ads→ 3CO2 + 3H2O + 8e−
(5) 8
The quantum mechanical approach based on density functional theory (DFT) has shown great promise in providing details of the gas-sensor surface interactions [42-49], such as the structure of adsorbed acetone, adsorption energy and binding mechanism. DFT provides the possible reaction mechanism between acetone and the semiconductor surface. In the following, DFT calculations are performed in order to give more information on the acetone sensing mechanism of LaFeO3. All of the calculations were carried out with the density functional theory (DFT) provided by the program package DMol3 [50, 51]. Calculations were employed using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) method to describe the exchange and correlation energy. Double numerical basis sets with polarization functions (DNP) were utilized. For all calculations, a 3×2×1 Monkhorst-Pack k-point mesh for the Brillouin zone sampling was used. The convergence criterion of optimal geometry for the energy, force, and displacement were 1×10-4 Ha, 2×10-2 Ha, and 5×10-2Å, respectively. Charge transfer was calculated on the basis of the Mulliken Population Analysis (MPA) [52]. The LaFeO3 crystal with an orthorhombic perovskite structure (Pnma (62)) was calculated. The lattice constants were a = 5.535Å, b =7.888Å, and c = 5.599Å, and the atomic fractional coordinates were Fe (0.0000, 0.0000, 0.0000), O(I) (0.2709, 0.0347, 0.2294), O(II) (0.4940, 0.25, 0.5644) and La (0.5184, 0.25, 0.0089). The (010) surface was cleaved from the optimized bulk LaFeO3 and a 10Å vacuum was added to the layers to simulate periodic boundary conditions. There are LaO-terminated and FeO-terminated surfaces for the LaFeO3 (010) surface. We focus our attention upon the FeO-terminated surface, since the LaO-terminated surface is difficult to form from the point of energy [42, 44]. The super cell consisted of two La-O layers and two Fe-O layers for FeO-terminated surface. The optimized structure of LaFeO3 is shown in Fig. 10.
We firstly studied the acetone molecular adsorption directly on clean LaFeO3 (010) surface without the pre-adsorption of oxygen species, the before and after optimized geometry can be seen from Fig. 11. This 9
case may occur only for acetone molecular adsorption on LaFeO3 (010) surface in the high vacuum environment. It seems that the acetone molecule is adsorbed on Fe ion located at the surface lattice of LaFeO3, via the O down mode (labeled as M0). The adsorption energy is a criterion to determine the stability of the adsorption. To evaluate the interaction between the acetone molecule and the LaFeO3 (010) surface, we calculate the adsorption energy as the following equation:
Eads=Esubstrate +Eadsorbate-Esubstrate-adsorbate
(6)
where Esubstrate-adsorbate is the total energy of the adsorbate-substrate system in the equilibrium state, and Esubstrate and Eadsorbate are the total energy of substrate and adsorbate, respectively. The adsorption energy Eads of mode M0 is 0.183eV, which reflects an exothermic process. Such result shows that the adsorption of acetone on LaFeO3 via the O down mode (M0 mode) can occur. LaFeO3 is a p-type semiconductor, the resistances in acetone gas for LaFeO3 are larger than that in air at the same temperature means that acetone releases electrons to the semiconductor surface. The mode M0 is a back donation process, which means that it is not likely responsible for the observed increase of semiconductor resistance directly when exposed to acetone in the background of air. In the latter case, oxygen species are always pre-adsorbed on the semiconductor surface. In the following, the possible interaction modes between acetone and pre-adsorbed oxygen species (
O2 , O ) on LaFeO (010) surface, as well as charges transferring from the acetone molecule to the 3
material surface are investigated. It has shown that Fe ion site is the most favorable for oxygen adsorption, and the side-on adsorption mode is more stable than end-on one [43]. In this work, we concentrated our attention on the adsorption side-on mode of oxygen on Fe site. After optimization of the system, an acetone molecule to the oxygen pre-adsorbed LaFeO3 (010) surface is introduced. In these systems, oxygen species are pre-adsorbed on LaFeO3 (010) surface. However, LaFeO3 (010) surface may not be covered fully by the 10
oxygen species, and there are still some adsorption sites. Four different initial adsorption configurations of acetone molecule on the LaFeO3 (010) surface with pre-adsorption of one oxygen molecule (
O2 ) were considered (see Fig. 12): (M1) on pre-adsorbed oxygen
with two hydrogen atoms of acetone closed to the oxygen atom; (M2) on pre-adsorbed oxygen with carbon bonding to oxygen of acetone closed to the oxygen atom; (M3) on pre-adsorbed oxygen with oxygen (acetone) closed to Fe site with adsorbed oxygen; (M4) on pre-adsorbed oxygen with O-down and two carbons closing to pre-adsorbed oxygen. For M1 and M2 modes, the optimized result shows that the acetone molecule leaves far away from the LaFeO3 (010) surface after optimization. It means that acetone does not prefer to be adsorbed on oxygen pre-adsorbed LaFeO3 (010) surface with two hydrogen atoms of acetone closed to the adsorbed site or with carbon bonding to oxygen of acetone closed to the adsorbed site. For M3 mode, the weakly pre-adsorbed oxygen species leaves away from the adsorption site due to the presence of acetone. For M4 mode, the acetone is adsorbed on Fe site with O-down mode. However, the adsorption energies for M3 and M4 modes are -0.103eV and -0.208eV, respectively. Such results mean that M3 and M4 processes are endothermic, which could not occur spontaneously and need additional energy transferring. Later, we will show that the acetone can be adsorbed on the Fe site of the surface by replacing the weakly pre-adsorbed oxygen species at the optimal operating temperature 533 K for M3 mode. Considering the high amount of oxygen adsorption on the surface of semiconductor, in the following, we consider additional three new modes (shown in Fig. 13), with more oxygen molecules pre-adsorbed on the surface of LaFeO3 (two
O2 ): (N1) on pre-adsorbed oxygen surface with oxygen
(acetone) closed to Fe site; (N2) on pre-adsorbed oxygen surface with carbon bonding to oxygen of acetone closed to the adsorbed oxygen; (N3) on pre-adsorbed oxygen surface with hydrogen (acetone) closed to the adsorbed oxygen. The calculated values of the adsorption energy (Eads) for the N1, N2 and N3 mode are
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0.993eV, 0.057eV and 1.585eV, respectively, which are all exothermic processes. For N2 and N3 modes, the acetone molecule gains charges of 0.107e and 0.082e, which are not likely responsible for the observed increase of semiconductor resistance when exposed to acetone. For N1 mode, the acetone molecule is adsorbed on Fe ion located at the surface lattice of LaFeO3, via the O-down mode. The calculated results show that the transferring charges from the lattice of LaFeO3 into two oxygen molecules are 0.043e and 0.032e, respectively. As shown in Fig. 14 (a), the C atoms in acetone molecule marked 45# and 47# gain charge of 0.592e and 0.221e, the C atom marked 46# loses charge 0.218e. The O atom in acetone lose 0.054e. Meanwhile, the H atoms in the acetone marked 49#, 50#, 51#, 52#, 53# and 54# lose charge of 0.042e, 0.072e, 0.136e, 0.080e, 0.037e and 0.128e, respectively. In other words, there is a net charge of 0.076e transferring from acetone into the lattice of LaFeO3. During the interaction process between acetone and pre-adsorbed oxygen species
O2 on LaFeO (010) surface, our ab initio calculations show that there 3
are charges transferring from acetone to the surface. The electrical negativity of oxygen atoms is larger than that of carbon in carbonyl, leading to the form of C+-O-. A carbonyl group has two centers: a positive center on the carbon atom and a negative electric center on the oxygen atom. When pro-nuclear particles react with the carbonyl compounds, it would firstly attack the carboxyl group and form a bond with carbon atom. Whereas, when electrophilic reagent react with the carbonyl compound, it would firstly attack the oxygen atom of the carboxyl group. Thus the possible acetone sensing mechanism in mode N1 is suggested: An acetone molecule adsorbs on LaFeO3 surface coordinated to Fe site through lone pair electrons, forming η-acetone as shown in Equation (7):
CH3 C=O + O-2 (ads) CH3
-
CH3 C-O CH3
12
+ ads
O-2 (ads)
(7)
Fig. 14 (b) shows the plots of the deformation electron density for N1 mode. The color of blue represents to the trapping of electrons, the color of yellow responds to the electron- release and the white parts represent that the deformation electron density is zero. Fig. 15 (a) shows DOS of the adsorbed acetone for N1 mode and free acetone molecule. The bands of the adsorbed acetone shift toward lower energy as compared to that of free acetone. The density of states (DOS) of acetone adsorbed in N1 mode changes greatly, reflecting that it belongs to chemical adsorption. It can be seen from Fig. 15 (b) that there are overlaps of DOS peaks of O (originating from acetone) and Fe (in the LaFeO3 surface), showing strong orbital hybridizations between their orbits. Four configurations of acetone on the LaFeO3 (010) surfaces with pre-adsorbed oxygen species O
(Fig. 16) were constructed: (S1) on pre-adsorbed oxygen surface with oxygen (acetone) closed to Fe site; (S2) on pre-adsorbed surface oxygen with carbon bonding to oxygen of acetone closed to the adsorbed oxygen; (S3) on pre-adsorbed oxygen surface with hydrogen (acetone) closed to the adsorbed oxygen; (S4) on pre-adsorbed oxygen surface with hydrogen (acetone) closed to O site. The adsorption energy for S1, S2, S3 and S4 modes is 0.935eV, 1.092eV, 1.572eV and 0.914eV, respectively. Both of S1 and S2 show that the acetone molecule could directly interact with O pre-adsorbed LaFeO3 (010) surface. When acetone is introduced, the acetone molecule can adsorb on the lattice Fe site. The acetone molecule may also adsorb
on the pre-adsorbed O forming an acetone-oxygen complex. Our ab initio calculations show that for
these interaction processes between acetone and per-adsorbed of O on LaFeO3 (010) surface, charges mainly transfer from the acetone molecule to the LaFeO3 (010) surface. The numbers of the transferred charges for S1 and S2 mode are 0.004e and 0.212e, respectively. Such calculated results demonstrate that
the interaction of acetone and oxygen species O on LaFeO3 surface brings about the releasing of electrons back to semiconductor LaFeO3. The adsorption of mode S1 is similar to the case of N1, as shown
13
in Equation (8).
CH3
-
CH3
C=O + O-(ads)
C-O
CH3
CH3
+
O-(ads)
(8)
ads
In the following, we paid more attention on the mode S2. In mode S2, the acetone molecule is adsorbed on the pre-adsorbed O
forming a methyl acetate. It can be explained that when acetone
molecules are adsorbed to the surface of the LaFeO3, the pre-adsorbed oxygen species ( O ) on the surface may attack C atom with a positive charge on the carbonyl. The processes can be described by the Equation (9). CH3 C=O +O -
C
ads
CH3
O-
CH3 CH3
O
ads
(9)
This reaction process continuously consumed O-, the trapped electrons on O- return back to the surface of LaFeO3. Such electron transfer is connected with the increase of resistance of LaFeO3 when exposed to acetone. Fig.17 shows the plots of the deformation electron density for S1 and S2 modes. The color of blue represents to the trapping of electrons, the color of yellow responds to the electron- release and the white parts represent that the deformation electron density is zero. Fig. 18 shows DOS of the adsorbed acetone for S1, S2 mode and free acetone. The bands of the adsorbed acetone shift toward lower energy as compared to that of free acetone. The density of states (DOS) of acetone adsorbed in S1 and S2 mode change greatly, reflecting that it belongs to chemical adsorption. It can be seen from Fig. 19 (a) that there are overlaps of DOS peaks of O (originating from acetone) and Fe (in the LaFeO3 surface) in S1 mode, meanwhile there
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are overlaps of DOS peaks of C (originating from acetone) and pre -adsorbed O in S2 mode (see Fig. 19 (b)). It means that there are strong orbital hybridizations between their orbits. In most of previous DFT calculations on gas seining mechanisms, the effect of temperature is not considered due to the lack of appropriate calculation methods and some calculation difficulties. However, it is well-known that the operating temperature has an enormous effect on the sensitivity of the sensor in real experiments. In the following, we try to further consider the effect of the optimal operating temperature (533 K obtained from experiment) using Molecular dynamics (MD) simulation with NVT ensemble for some typical optimized structure. Fig. 20 shows the possible structures of developed modes (M3’, N1’, S1’ and S2’) at 533 K (260 °C) from M3, N1, S1 and S2 mentioned above. For M3’ mode, the acetone molecule was adsorbed with O-down on the Fe site of LaFeO3 (010) surface, through the replacing of the weakly pre-adsorbed oxygen species
O2 and accompanying the formation of oxygen molecule. This
process can be described by Equation (10).
CH3
-
CH3
C=O + O2-(ads)
C-O
CH3
CH3
(10)
ads
For S2’ mode, the acetone molecule is adsorbed on the pre-adsorbed O shown in Equation (8).
O2
+
forming a methyl acetate as
In both M3’ and S2’ modes, acetone molecule reacts directly with the weakly
pre-adsorbed oxygen species (
O2 or O ), where acetone molecule transfer electrons to pre-adsorbed
oxygen species and finally to the surface of LaFeO3. These processes (S2’ and M3’modes) described by Eqs (8) and (9) may play important roles in acetone sensing. Such DFT results are consistent with the conventional sensing explanation or mechanism for semiconductor [33, 34, 36, 37]. However, the
15
suggested formations of CO2 and H2O in the sensing mechanism described by Equation (4) have not been found in our DFT calculation. Perhaps, Equation (4) holds at a much higher operating temperature. For N1’ and S1’ modes, the acetone molecule is adsorbed on Fe site, where acetone molecule directly transfers some electrons to the LaFeO3 (010) surface at 533 K. The later processes (N1’ and S1’ modes) described by Eqs.(6) and (7) may also provide some contribution to acetone sensing.
4. Conclusions LaFeO3 thick film based on nanocrystalline sol-gel powders annealed at 800°C could sensitively detect low concentration acetone. When exposed to 0.5ppm acetone, the response of LaFeO3 thick film at 260 °C is 2.068 with response time of 62 s and recovery time of 107 s, respectively. LaFeO3 thick film may be a promising candidate for developing a breath analysis technique for monitoring diabetes. The DFT calculation results show that there is strong interaction between the acetone with the surface of LaFeO3 (010) pre-adsorbed with oxygen species O and O2 , accompanying with electron transfer from acetone to LaFeO3 (010) surface. The acetone molecule reacts with oxygen species in two ways: (1) adsorb on oxygen species O or (2) replace of the weakly pre-adsorbed oxygen species
O2 on Fe site,
accompanying the formation of oxygen molecule. These processes may play important roles in acetone sensing for LaFeO3. However, the formations of CO2 and H2O at the optimal operating temperature (260 °C) have not been found in our DFT calculation. The formations of CO2 and H2O may occur at much higher temperature. We also find that the acetone molecule can be directly adsorbed on Fe site, transferring some electrons to the LaFeO3 (010) surface. The later processes may also provide addition contribution to acetone sensing.
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Acknowledgements This work was supported by National Natural Science Foundation of China (Nos: 51472145, 51272133 and 51472150)，Shandong Natural Science Foundation (No. ZR2013EMM016), and Foundation of Dalian University of Technology at Panjin (No. DUT14RC-3-113).
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References [1] C.-C. Wang, Y.-C. Weng, T.-C. Chou, Acetone sensor using lead foil as working electrode, Sensors and Actuators B: Chemical 122 (2007) 591-595. [2] S.S. Likhodii, K. Musa, S.C. Cunnane, Breath acetone as a measure of systemic ketosis assessed in a rat model of the ketogenic die, Clinical Chemistry 48 (2002) 115-120. [3] N. Makisimovich, V. Vorottyntsev, N. Nikitina, O. Kaskevich, P. Karabum and F. Martynenko, Adsorption semiconductor sensor for diabetic ketoacidosis diagnosis, Sensors and Actuators B 36 (1996) 419. [4] A.M. Diskin, P. Spanel, D. Smith, Time variation of ammonia, acetone, isopreneand ethanol in breath: a quantitative SIFT-MS study over 30 days, Physiological Measurement 24 (2003) 107–119. [5] S.B. Patil, P.P. Patil, M.A. More, Acetone vapour sensing characteristics of cobalt-doped SnO2 thin films, Sensors and Actuators B 125 (2007) 126–130. [6] J. Zhao, L. Huo, S. Gao, H. Zhao, J. Zhao, Alcohols and acetone sensing properties of SnO2 thin films deposited by dip-coating, Sensors and Actuators B 115 (2006) 460–464. [7] T. Li, W. Zeng, W. Zhao, Gas sensing performance of multiple SnO2 1D nanostructures based on their interconnect manner, Materials Letters 167 (2016) 230-233. [8] S.F. Bamsaoud, S.B. Rane, R.N. Karekar, R.C. Aiyer, Nano particulate SnO2 based resistive films as a hydrogen and acetone vapour sensor, Sensors and Actuators B 153 (2011) 382-391. [9 ] W. Zeng, T. Liu, D. Liu, E. Han, Hydrogen sensing and mechanism of M-doped SnO2 (M= Cr3+, Cu2+ and Pd2+) nanocomposite, Sensors and Actuators B: Chemical, 160 (2011) 455-462. [10] P. Song, Q. Wang, Z. Yang, Preparation, characterization and acetone sensing properties of Ce-doped SnO2 hollow spheres, Sensors and Actuators B 173 (2012) 839-846.
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[11] D. Zhang, A. Liu, H. Chang, B. Xia, Room-temperature high-performance acetone gas sensor based on hydrothermal synthesized SnO2-reduced graphene oxide hybrid composite, RSC Advances 5 (2015) 3016-3022. [12] R. Rella, J. Spadavecchia, M.G. Manera, S. Capone, A. Taurino, M. Martino, A.P. Caricato, T. Tunno, Acetone and ethanol solid-state gas sensors based on TiO2 nanoparticles thin film deposited by matrix assisted pulsed laser evaporation, Sensors and Actuators B 127 (2007) 426-431. [13] M. El-Maazawi, A. N. Finken, A. B. Nair and V. H. Grassian, Adsorption and Photocatalytic Oxidation of Acetone on TiO2: An in Situ Transmission FT-IR Study, Journal of Catalysis 191 (2000)138-146. [14] W. Wang, J. Yu, Q. Xiang and B. Cheng, Enhanced photocatalytic activity of hierarchical macro/mesoporous TiO2–graphene composites for photodegradation of acetone in air, Applied Catalysis B: Environmental 119-120 (2012) 109-116. [15] P. Song, Q. Wang, Z. Yang, Acetone sensing characteristics of ZnO hollow spheres prepared byone-pot hydrothermal reaction, Materials Letters 86 (2012) 168-170. [16] S.S. Nath, M. Choudhury, D. Chakdar, G. Gope, R.K. Nath, Acetone sensing property of ZnO quantum dots embedded on PVP, Sensors and Actuators B 148 (2010) 353-357. [17] X.Wang, W. Wang and Y. Liu, Enhanced acetone sensing performance of Au nanoparticles functionalized flower-like ZnO, Sensors and Actuators B 168 (2012) 39-45. [18] H. Ahn, Y. Wang, S.H. Jee, M.o Park, Y.S. Yoon and D.J. Kim, Enhanced UV activation of electrochemically doped Ni in ZnO nanorods for room temperature acetone sensing, Chemical Physics Letters 511 (2011) 331-335. [19] Y. Zeng, T. Zhang, M. Yuan, M. Kang, G. Lu, R. Wang, H. Fan, Y. He and H.Yang, Growth and selective acetone detection based on ZnO nanorod arrays, Sensors and Actuators B 143 (2009) 93-98.
19
[20] L. Liu, S. Li, J. Zhuang, L. Wang, J. Zhang, H. Li, Z. Liu, Y. Han, X. Jiang and P. Zhang, Improved selective acetone sensing properties of Co-doped ZnO nanofibers by electrospinning, Sensors and Actuators B 155 (2011) 782-788. [21] C.S. Prajapati, Ajay Kushwaha, P.P. Sahay, Influence of Fe doping on the structural, optical and acetone sensing properties of sprayed ZnO thin films, Materials Research Bulletin 48 (2013) 2687-2695. [22] C.S. Prajapati, P.P. Sahay, Influence of In doping on the structural, optical and acetone sensing properties of ZnO nanoparticulate thin films, Materials Science in Semiconductor Processing 16 (2013) 200-210. [23] N.H. Al-Hardan, M.J. Abdullah, A.A. Aziz, Performance of Cr-doped ZnO for acetone sensing, Applied Surface Science 270 (2013) 480-485. [24] P. P. Sahay, Zinc oxide thin film gas sensor for detection of acetone, Jonrnal of Materials Science 4 0 (2005) 4383-4385. [25] P. Gunawan, L. Mei, J. Teo, J. Ma, J. Highfield, Q. Li. Z. Zhong, Ultrahigh Sensitivity of Au/1D α-Fe2O3 to Acetone and the Sensing Mechanism, Langmuir 28 (2012) 14090−14099. [26] H. Shan, C.i Liu, L. Liu, S. Li, L. Wang, X. Zhang, X. Bo and X. Chi, Highly sensitive acetone sensors based on La-doped α-Fe2O3 nanotubes, Sensors and Actuators B 184 (2013) 243-247. [27] N.G. Cho, H. Woo, J. Lee and I.o Kim, Thin-walled NiO tubes functionalized with catalytic Pt for highly selective C2H5OH sensors using electrospun fibers as a sacrificial template, Chemical Communications 47 (2011) 11300–11302. [28] R.S. Khadayate b, J.V. Sali a, P.P. Patil, Acetone vapor sensing properties of screen printed WO3 thick films, Talanta 72 (2007) 1077-1081.
20
[29] P. Gao, H. Ji, Y. Zhou and X. Li, Selective acetone gas sensors using porous WO3-Cr2O3 thin films prepared by sol–gel method, Thin Solid Films 520 (2012) 3100-3106. [30] S. Choi, I. Lee, B. Jang, D. Youn, W. Ryu, C.O. Park and I. Kim, Selective Diagnosis of Diabetes Using Pt-Functionalized WO3 Hemitube Networks As a Sensing Layer of Acetone in Exhaled Breath, Analytical Chemistry 85 (2013) 1792−1796. [31] M. Righettoni, A. Tricoli, and S.E. Pratsinis, Thermally Stable, Silica-Doped ε-WO3 for Sensing of Acetone in the Human Breath, Chemistry of materials 22 (2010) 3152-3157. [32] X. Liu, J. Hu, B. Cheng, H. Qin, M. Jiang, Acetone gas sensing properties of SmFe1 xMgxO3 −
perovskite oxides, Sensors and Actuators B 134 (2008) 483-487. [33] P. Song, H. Zhang, D. Han, J. Li, Z. Yang and Q. Wang, Preparation of biomorphic porous LaFeO3 by sorghum straw biotemplate method and its acetone sensing properties, Sensors and Actuators B 196 (2014) 140-146.
[34] X. Liu, H. Ji, Y. Gu, M. Xu, Preparation and acetone sensitive characteristics of nano-LaFeO3 semiconductor thin films by polymerization complex method, Materials Science and Engineering B 133 (2006) 98-101.
[35] P.A. Murade, V.S. Sangawar, G.N. Chaudhari, V.D. Kapse, A.U. Bajpeyee, Acetone gas-sensing performance of Sr-doped nanostructured LaFeO3 semiconductor prepared by citrate solegel route, Current Applied Physics 11 (2011) 451-456. [36] L. Zhang, H. Qin, P. Song, J. Hu, M. Jiang, Electric properties and acetone-sensing characteristics of La1-xPbxFeO3 perovskite system, Materials chemistry and physics 2006 98(2) 358-362. [37] M. Yang, L. Huo, H. Zhao, S. Gao, Z. Rong, Electrical properties and acetone-sensing characteristics
21
of LaNi1-xTixO3 perovskite system prepared by amorphous citrate decomposition. Sensors and actuators B, Chemical 2009 143(1) 111-118. [38] K. Fan, H.i Qin, Z. Zhang, L. Sun, L. Sun and J. Hu, Gas sensing properties of nanocrystalline La0.75Ba0.25FeO3 thick-film sensors, Sensors and Actuators B 171-172 (2012) 302-308. [39] T. Chen, Z. Zhou, Y. Wang, Surfactant CATB-assisted generation and gas-sensing characteristics of LnFeO3 (Ln= La, Sm, Eu) materials, Sensors and Actuators B: Chemical 143 (2009) 124-131. [40] Z.L. Wu, R. Zhang, M. Zhao, S.M. Fang, Z.X. Han, J.F. Hu, K.Y. Wang, Effect of Pd doping on the acetone-sensing properties of NdFeO3, International Journal of Minerals, Metallurgy, and Materials, 19 (2012) 141-145. [41] G.O. Nelson, Controlled test Atmospheres, principles and techniques, Ann Arbor Science Publishers, Ann Arbor, Mich., 1971. [42] L. Sun, J. Hu, H. Qin, M. Zhao, K. Fan, Influences of Ca Doping and Oxygen Vacancy upon Adsorption of CO on the LaFeO3 (010) Surface: A First-Principles Study, The Journal of Physical Chemistry C 115 (2011) 5593-5598. [43] X. Liu, J. Hu, B. Cheng, H. Qin, M. Zhao, C. Yang, First-principles study of O2 adsorption on the LaFeO3 (010) surface. Sensors and Actuators B: Chemical 139(2) (2009) 520-526. [44] Y. L. Lee, J. Kleis, J. Rossmeisl, D. Morgan, Ab initio energetics of LaBO3 (001) (B= Mn, Fe, Co, and Ni) for solid oxide fuel cell cathodes, Physical Review B 80 (2009) 224101. [45] X. Wang, H. Qin, L. Sun, J. Hu, CO2 sensing properties and mechanism of nanocrystalline LaFeO3 sensor, Sensors and Actuators B 188 (2013) 965-971.
[46] Y. Chen, X. Wang, C. Shi, L. Li, H. Qin, J. Hu, Sensing mechanism of SnO2 (1 1 0) surface to H2: Density functional theory calculations, Sensors and Actuators B 220 (2015) 279-287. 22
[47] R. Jinnouchi, K. Kodama, T. Hatanaka and Y. Morimoto, First principles based mean field model for oxygen
reduction reaction, physical chemistry chemical physics 13 (2011) 21070-21083.
[48] Y. Koyama, H. Arai, I. Tanaka, Y. Uchimoto and Z. Ogumi, First principles study of dopant solubility and defect chemistry in LiCoO2, Journal of Materials Chemistry A 2 (2014) 11235-11245. [49] X. Wang, H. Qin, Y. Chen, J. Hu, Sensing Mechanism of SnO2 (110) Surface to CO: Density Functional Theory Calculations, The Journal of Physical Chemistry C 118 (2014) 28548−28561. [50] B. Delley, An All-electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules, The Journal of Chemical Physics 92 (1990) 508-517. [51] B. Delley, From Molecules to Solids with the DMol Approach, The Journal of Chemical Physics 113 (2000) 7756-7764. [52] R. S. Mulliken, Electronic Population Analysis on LCAO-MO Molecular Wave Functions, The Journal of Chemical Physics 23 (1955) 1833-1840.
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Biographies Yanping Chen received her B.S degree from the College of Physics and Electronics, Shandong Normal University, China in 2013. She is currently working toward the PhD degree in the field of condensed matter physics at the School of Physics, Shandong University. The current field of interest is semiconductor gas sensors. Hongwei Qin is currently a professor at the School of Physics, Shandong University. The current fields of interest are magnetism materials, multiferroics materials, functional alloy, rare earth oxide materials, nanomaterials and gas sensors. She has published more than 150 refereed research papers in SCI journals. She has been awarded six invention patents. Xiaofeng Wang received her PhD degree in the field of material physics and chemistry from the School of Physics, Shandong University, China in 2009. She is currently a Lecturer in school of science, Dalian University of Technology at Panjin, China. Ling Li received her B.S degree from the School of Chemistry, Liaocheng University, China in 2008, received her MS degree from the School of Chemistry, University of Jinan, China in 2011. She is currently working toward the PhD degree in the field of material physics and chemistry at the School of Physics, Shandong University. The current field of interest is semiconductor gas sensors. Jifan Hu gained his B.Sc. (July, 1985) in Physics from Shandong University, his M.Sc. (July, 1988) and Ph.D (November, 1993) from Institute of Physics, Chinese Academy of Sciences. He worked as a post doctor in the Institute of Theoretical Physics, Chinese Academy of Sciences from April 1994 to April 1996. He joined the Department of Physics, Shandong University in May 1996, and became a full professor in December 1996. He was appointed as Chang-Jiang Professor (Cheung Kong Scholar) by Chinese National Ministry of Education in the end of 1999. He is currently the director of Institute of Condensed Matter
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Physics, School of Physics at Shandong University. His current fields of interest are magnetic materials, multiferroics materials, functional alloy, rare earth oxide materials, nanomaterials and gas sensors. He has published more than 270 refereed research papers in SCI journals as an author or a coauthor. He has been awarded nine invention patents.
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Figure captions Fig. 1 (a) Fabricated sensor; (b) schematic diagram of the thick film sensor; (c) The electric circuit which was used to test the gas-sensing properties.
Fig. 2 XRD patterns of LaFeO3 powders.
Fig. 3 The operating temperature dependence of the resistances in air for the sensors based on LaFeO3 powders annealed at different temperatures. Fig. 4 Relationship between temperature and the response of LaFeO3-based sensors for 5 ppm acetone in air. Fig. 5 FE-SEM image of LaFeO3 powders annealed at 800 °C and the density percent versus LaFeO3 grain size. Fig. 6 Dependence of the LaFeO3-based sensor response on operating temperature to 0.5, 1, 5 and 10 ppm. Fig. 7 Response of LaFeO3-based sensor at 260 °C versus acetone concentration. Fig. 8 The dynamic sensing characteristics of LaFeO3-based sensor exposed to 0.5, 1, 5 and 10 ppm acetone gas in the background of dry air. Fig. 9 Response of the LaFeO3-based sensor to 5 ppm different gases at 200 °C. Fig. 10 The optimized structure of LaFeO3 (0 1 0) surface. La atoms are shown in blue, Fe atoms in purple and O atoms in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.) Fig. 11 M0 mode of initial adsorption of acetone molecule on the LaFeO3 (0 1 0) surface. The left drawings are the configurations of initial adsorption and the rights are that of optimized adsorption. Fig. 12 Four initial adsorption of acetone molecule, the following modes were considered: (M1) on
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pre-adsorbed oxygen with two hydrogens of acetone closed to the adsorbed site; (M2) on pre-adsorbed oxygen with carbon bonding to oxygen of acetone closed to the adsorbed site; (M3) on pre-adsorbed oxygen with oxygen (acetone) closed to Fe site with adsorbed oxygen; (M4) on pre-adsorbed oxygen with O-down and two carbons closing to pre-adsorbed oxygen. Fig. 13 Three modes of initial adsorption of the acetone molecule on oxygen per-adsorbed LaFeO3(0 1 0) surface. The left drawings are the configurations of initial adsorption and the rights are that of optimized adsorption. (N1) on pre-adsorbed oxygen surface with oxygen (acetone) closed to Fe site with adsorbed oxygen; (N2) on pre-adsorbed oxygen surface with carbon bonding to oxygen of acetone closing to pre-adsorbed oxygen. (N3) on pre-adsorbed oxygen with two hydrogens of acetone closed to the pre-adsorbed oxygen. Fig. 14 (a) Mark numbers of atoms in mode N1; (b) DOS of the adsorbed acetone for N1 mode and free acetone. Fig. 15 The DOS of O (originating from acetone) and Fe (in the LaFeO3 surface). Fig. 16 Four modes of initial adsorption of the acetone molecule on O per-adsorbed LaFeO3 (0 1 0) surface: (S1) on pre-adsorbed oxygen surface with oxygen (acetone) closed to Fe site; (S2) on pre-adsorbed surface oxygen with carbon bonding to oxygen of acetone closed to the adsorbed oxygen; (S3) on pre-adsorbed oxygen surface with hydrogen (acetone) closed to the adsorbed oxygen; (S4) on pre-adsorbed oxygen surface with hydrogen (acetone) closed to O site. Fig. 17 The plots of the deformation electron density for S1 and S2 modes. The color of blue represents to the trapping of electrons, and the color of yellow responds to the electron- release. Fig. 18 DOS of the adsorbed acetone for S1, S2 mode and free acetone. Fig. 19 (a): DOSs of O-p (originating from acetone) and Fe-d (originating from lattice) in mode S1; (b)
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PDOSs of pre-adsorbed O and C (originating from acetone) in mode S2. Fig. 20 Schematic image of M3’, N1’, S1’ and S2’ mode at 773 K.
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S0925-4005(16)30735-3 http://dx.doi.org/doi:10.1016/j.snb.2016.05.059 SNB 20218
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
25-11-2015 13-4-2016 11-5-2016
Please cite this article as: Yanping Chen, Hongwei Qin, Xiaofeng Wang, Ling Li, Jifan Hu, Acetone sensing properties and mechanism of nano-LaFeO3 thick-films, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.05.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Acetone sensing properties and mechanism of nano-LaFeO3 thick-films Yanping Chen, a Hongwei Qin, *,a Xiaofeng Wang, a,b Ling Li, a and Jifan Hu *,a aSchool
of Physics, State Key Laboratory for Crystal Materials, Shandong University, Jinan 250100, China. bSchool of Science, Dalian University of Technology at Panjin, Panjin 124221, Liaoning, China
* Corresponding authors. E-mail: [email protected] (H. Qin); [email protected] (J. Hu)
1
Abstract LaFeO3 nanocrystalline powders prepared by sol–gel method with annealing at 800 °C for 4 h can sensitively detect low concentration of acetone. When exposed to 0.5ppm acetone, the response of LaFeO3 thick film at 260 °C is 2.068 with response time of 62 s and recovery time of 107 s, respectively. The possible acetone sensing mechanisms for LaFeO3 sensor are investigated with first principles calculations. Calculated results demonstrate that acetone could release electrons to the surface of LaFeO3 (010) pre-adsorbed with oxygen species O and O2 . The acetone molecule reacts with oxygen species in the following ways: (1) adsorbs on oxygen species O or (2) replaces of the weakly pre-adsorbed oxygen species
O2 on Fe site, accompanying the formation of oxygen molecule. These above two processes may
play important roles in acetone sensing for LaFeO3. We also find that the acetone molecule can be directly adsorbed on Fe site, transferring some electrons to the LaFeO3 (010) surface. The latter processes may also provide additional contribution to acetone sensing.
Keywords: LaFeO3 thin film; acetone; Sol-gel; Sensing mechanisms; First principles calculations
2
1. Introduction Acetone (C3H6O), a kind of volatile gas, is extensively used in chemical industry. However, it is easy to evaporate at room temperature. Inhalation of acetone can cause headache, fatigue and even narcosis and harmfulness to nerve system [1]. In addition, the medical reports show that acetone is one of the products exhaled by diabetic patients who have higher acetone concentrations in their blood and spittle than healthy people [2]. For healthy individuals, the acetone concentration in breath varies from 0.3 to 0.9 ppm. However, in the exhaled air of diabetic patients, the acetone concentration is reported to be more than 1.8 ppm [3]. Such significant difference makes acetone a suitable breath marker in detecting diabetes [2,4]. Different from the clinic detection of blood sugar, breath acetone analysis is noninvasive and can be performed frequently with minimal discomfort to patients. Thus, detecting of low acetone concentrations is essential for our safety and health, especially for developing a breath analysis technique for monitoring diabetes. Recently, there is increasing attention attracted on the extensive investigation of acetone-sensing materials and devices. Semiconductors have been reported as good sensing materials [4]. Various metal oxide semiconductors such as SnO2 [5-11], TiO2 [12-14], ZnO [15-24], Fe2O3 [25-26], NiO [27] and WO3 [29-31] have been developed as acetone sensing materials. Perovskite-type semiconductors such as SmFe1-xMgxO3 [32], LaFeO3 [33-35], La1-xPbxFeO3 [36], LaNi1-xTixO3 [37] and La0.75Ba0.25FeO3 [38] have also been used to detect acetone gas. And it has been found that the response S=Rgas/Rair to 50 ppm acetone is about for 16 for SmFe0.9Mg0.1O3 [32], 7 for La0.68Pb0.32FeO3 [36], 2.6 for SmFeO3 [39], 300 for Pd doped NdFeO3 [40]. The response value is 204 to 80 ppm acetone for LaFeO3 thick film [34], and 2 to 25 ppm acetone for porous LaFeO3 [33]. Among the various semiconductors used for acetone sensors, p-type LaFeO3 with perovskite-type structure (ABO3) has attracted considerable attention in gas sensor due to its
3
stability at high temperature and in chemical atmospheres in recent years [33-35, 39]. The resistance of the p-type semiconductor sensor increased abruptly on the injection of acetone, and then decreased rapidly and recovered to its initial value after the test gas was released [32-40]. The most accepted mechanism for gas sensing in semiconductor materials, involves the adsorption of oxygen species on its surface [5-40]. When the sensors were exposed to air, O2 adsorb on the surface and create chemisorbed oxygen species by capturing electrons from the p-type semiconductor surface which increases their conductivity [32-38]. When the reducing gases such as acetone, H2 and CO are introduced at moderate temperature, the reducing gases may react with oxygen species adsorbed on the outer surface of p-type semiconductor and releases the trapped electrons back to the p-type semiconductor surface, which decreases the carrier concentration and increases the resistance of p-type semiconductor. In this work, sensing performances to low concentration acetone for LaFeO3 thick film sensors were investigated. The first principles calculations were performed in order to reveal the sensing mechanism of LaFeO3 in detail.
2. Experimental 2.1. Synthesis and characterization of nanocrystalline LaFeO3 Firstly, stoichiometric ratio of lanthanum nitrate, ferric nitrate and citrate acid (all analytically pure) were completely mixed in deionized water and was heated to 80 °C with continuous stirring. After adding some polyethylene glycol (PEG; molecular weight over 20,000), the solution was well stirred for several hours until the sol was formed. The sol was dried into a gel and then the gel was dried into pieces in a baking box. The pieces were preheated at 400 °C for 2h and ground into a fine powder sample, which was subsequently annealed in an oven at 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C for 4 h, respectively. The structure of resultant powders was characterized by X-ray diffractions using CuKα radiation at room temperature. The
4
microstructure of LaFeO3 powders annealed at 800 °C was also observed by a field emission scanning electron microscope (FE-SEM). 2.2. Fabrication and measurements of sensors The powders were mixed with deionized water to form a paste. Then the obtained paste was coated onto a prefabricated alumina tube (4 mm in length and 1.2 mm in diameter, attached with a pair of gold electrodes and platinum wires) by a small brush to form a thick film (film thickness is about 250 μm), and dried at 100 °C for 2h. A Ni-Cr heating wire was inserted into the ceramic tube to form an inside-heated gas sensor. The sensors are annealed at 240 °C for 48 hours on the aging equipment in air. For the sensors based on the LaFeO3 powders annealed at different temperatures, we made three replicates for each chemical analysis, gas concentration, and operating temperature in the experiment. The photograph and schematic image of the as-fabricated sensor are shown in Fig. 1 (a) and (b). Gas sensing properties were measured
using
a
static
system
controlled
by
a
computer.
The gas
with
defined
gas
concentrations controlled by static state method was injected into the testing chamber [7, 9, 41].
3. Results and discussion The X-ray diffraction patterns at room temperature for LaFeO3 powders annealed at 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C are shown in Fig. 2. The pattern indicates that the sample has a single perovskite phase with orthorhombic structure (Pnma). The temperature dependence of the resistance in air for LaFeO3 thick film sensors are shown in Fig. 3. The resistance of all samples decreases with an increase of temperature. However, the resistances of LaFeO3 thick film sensors are very large. For example, resistance at 260°C for the thick film sensor based on LaFeO3 powders annealed at 800°C is about 25.4 M. The gas sensing response in the experiment is defined as S=Rg/Ra, where Ra and Rg represent the resistances of the LaFeO3-based sensor to air and acetone in background of air, respectively. However, in 5
our measurement, we have not measured the resistance of the sensor directly. Instead, a series circuit shown in Fig.1 (c) is used to measure the voltage Uout-put of the known series –resistance R0 directly when the sensor is exposed to air or testing gas in the background of air. The resistance R of the sensor when the sensor is exposed to air or testing gas in the background of air can be calculated based on the following equation:
R
5V U U R
out put
(1)
out put
0
Thus gas sensing response S=Rg/Ra can be obtained.
It is well known that the gas sensitivity is greatly influenced by operating temperature (TO) and annealing temperature (TA). In order to determine the optimum operating temperature and annealing temperature, responses of LaFeO3-based sensors (annealed at temperatures of 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C) to acetone vapor were examined as a function of operating temperature. From Fig. 4, it can be seen that the maximum response S to 5 ppm acetone for LaFeO3 annealed at temperatures of 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C are 1.029, 2.12, 5.195, 2.161 and 1.405 at the optimal operating temperature of 260 °C. The value of response S>1 means that the resistances in acetone gas for LaFeO3 are larger than that in air at the same temperature.
It indicates that LaFeO3 annealed at 800 °C exhibits the best sensing behavior to acetone vapor. The microstructures observed by FE-SEM for LaFeO3 powders annealed at temperature 800 °C and the density percent versus LaFeO3 grain size are shown in Fig. 5 (a) and (b), respectively. From the FE-SEM picture, we counted one hundred and twenty particles to calculate the density percent. There is a distribution of grain size around 55 nm. 6
The responses S of LaFeO3 thick film sensor annealed at 800 °C to 0.5 ppm, 1 ppm, 5 ppm, 10 ppm acetone at different temperatures (160 °C-420 °C) are shown in Fig. 6. The response increases with an increase of acetone concentration. With increase of operating temperature, the response increases at first, undergoes a maximum, and finally drops. When exposed to 0.5, 1, 5 and 10 ppm acetone, at the optimal operating-temperature (260 °C) the corresponding responses are 2.068, 3.245, 5.195 and 7.925, respectively. It shows that LaFeO3 thick film sensor annealed at 800 °C can sensitively respond to acetone. The relationship between the response and the acetone vapor concentration for LaFeO3 annealed at 800C at the optimal temperature of 260 °C is shown in Fig. 7. The response increases significantly with an increase in acetone vapor concentration. Since the acetone concentration varies from 0.3 to 0.9 ppm for healthy individuals, and is more than 1.8 ppm for diabetic patients [3], our present results show that LaFeO3 thick film sensor annealed at 800 °C can be used to distinguish between diabetic patients and healthy individuals. The dynamic sensing characteristics of LaFeO3 sensor annealed at 800C exposed to 0.5, 1, 5 and 10 ppm acetone gas at the optimal operating temperature of 260 °C are shown in Fig. 8. It can be seen that the resistance of the LaFeO3 thick film sensor annealed at 800 °C increases obviously after the injection of acetone gas, and then decreases after the test gas is blown off, consistent with other p-type semiconductor sensors [33-40]. Such change in the resistance indicates that acetone seems to release electrons to the surface of p-type LaFeO3 [33-35]. The response time has been defined as the time taken to attain 90 % of the final value, and the recovery time as the time taken to regain 10 % of the base value. Both of them are important parameters for one practical sensor. The response time for LaFeO3 sensor to 0.5, 1, 5 and 10 ppm acetone gas are about 62 s, 37 s, 39 s and 51 s, respectively. The recovery times to 0.5, 1, 5 and 10 ppm acetone gas are about 107 s, 82 s, 115 s and 155 s, respectively. When the acetone concentration is higher
7
than 1ppm, the response time and recovery time of LaFeO3 sensor become slightly longer with the increase of acetone concentration. Selectivity reflects the ability of a sensor to respond to a certain gas in presence of other gases, which is another important characteristic of one gas sensor. The gas responses of LaFeO3 thick film sensor annealed at 800 °C to various testing gases of 5 ppm are measured at the operating temperature 260 °C, and results are shown in Fig. 9. The LaFeO3 sensor exhibits slow response to HCHO, ammonia and methanol. The highest response of the sensor is 2.44 to 5 ppm ethanol and 1.936 to 5 ppm acetone, which is just next that to ethanol. The selectivity coefficients for LaFeO3 based sensor are 1.495 to HCHO, 1.385 to ammonia, and 1.476 to methanol, indicating the excellent selectivity of the LaFeO3. In order to distinguish these gases, an array of thick film sensors including SnO2 and LaFeO3 thick-film elements could be used. Before introducing the reducing gas, oxygen molecules in air are usually adsorbed on the surfaces of semiconducting LaFeO3, grabbing electrons from its surface and forming the oxygen species. With increasing the temperature in air, the state of oxygen adsorbed on the surface of semiconductor undergoes the following changes:
O2 (gas) → O2- (ads)
(2)
O2 -(ads) →O- (ads)
(3)
O- (ads) → O2- (ads)
(4)
where the ads means the state of adsorption. When the acetone gas is introduced, the reducing gas would interact with the oxygen species O2- and O-, which releases electron back to the surface of LaFeO3, thus increasing the resistance of LaFeO3 sensor. The following reaction has been used in some publications [33, 34, 36, 37]. CH3COCH3 + 8O−ads→ 3CO2 + 3H2O + 8e−
(5) 8
The quantum mechanical approach based on density functional theory (DFT) has shown great promise in providing details of the gas-sensor surface interactions [42-49], such as the structure of adsorbed acetone, adsorption energy and binding mechanism. DFT provides the possible reaction mechanism between acetone and the semiconductor surface. In the following, DFT calculations are performed in order to give more information on the acetone sensing mechanism of LaFeO3. All of the calculations were carried out with the density functional theory (DFT) provided by the program package DMol3 [50, 51]. Calculations were employed using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) method to describe the exchange and correlation energy. Double numerical basis sets with polarization functions (DNP) were utilized. For all calculations, a 3×2×1 Monkhorst-Pack k-point mesh for the Brillouin zone sampling was used. The convergence criterion of optimal geometry for the energy, force, and displacement were 1×10-4 Ha, 2×10-2 Ha, and 5×10-2Å, respectively. Charge transfer was calculated on the basis of the Mulliken Population Analysis (MPA) [52]. The LaFeO3 crystal with an orthorhombic perovskite structure (Pnma (62)) was calculated. The lattice constants were a = 5.535Å, b =7.888Å, and c = 5.599Å, and the atomic fractional coordinates were Fe (0.0000, 0.0000, 0.0000), O(I) (0.2709, 0.0347, 0.2294), O(II) (0.4940, 0.25, 0.5644) and La (0.5184, 0.25, 0.0089). The (010) surface was cleaved from the optimized bulk LaFeO3 and a 10Å vacuum was added to the layers to simulate periodic boundary conditions. There are LaO-terminated and FeO-terminated surfaces for the LaFeO3 (010) surface. We focus our attention upon the FeO-terminated surface, since the LaO-terminated surface is difficult to form from the point of energy [42, 44]. The super cell consisted of two La-O layers and two Fe-O layers for FeO-terminated surface. The optimized structure of LaFeO3 is shown in Fig. 10.
We firstly studied the acetone molecular adsorption directly on clean LaFeO3 (010) surface without the pre-adsorption of oxygen species, the before and after optimized geometry can be seen from Fig. 11. This 9
case may occur only for acetone molecular adsorption on LaFeO3 (010) surface in the high vacuum environment. It seems that the acetone molecule is adsorbed on Fe ion located at the surface lattice of LaFeO3, via the O down mode (labeled as M0). The adsorption energy is a criterion to determine the stability of the adsorption. To evaluate the interaction between the acetone molecule and the LaFeO3 (010) surface, we calculate the adsorption energy as the following equation:
Eads=Esubstrate +Eadsorbate-Esubstrate-adsorbate
(6)
where Esubstrate-adsorbate is the total energy of the adsorbate-substrate system in the equilibrium state, and Esubstrate and Eadsorbate are the total energy of substrate and adsorbate, respectively. The adsorption energy Eads of mode M0 is 0.183eV, which reflects an exothermic process. Such result shows that the adsorption of acetone on LaFeO3 via the O down mode (M0 mode) can occur. LaFeO3 is a p-type semiconductor, the resistances in acetone gas for LaFeO3 are larger than that in air at the same temperature means that acetone releases electrons to the semiconductor surface. The mode M0 is a back donation process, which means that it is not likely responsible for the observed increase of semiconductor resistance directly when exposed to acetone in the background of air. In the latter case, oxygen species are always pre-adsorbed on the semiconductor surface. In the following, the possible interaction modes between acetone and pre-adsorbed oxygen species (
O2 , O ) on LaFeO (010) surface, as well as charges transferring from the acetone molecule to the 3
material surface are investigated. It has shown that Fe ion site is the most favorable for oxygen adsorption, and the side-on adsorption mode is more stable than end-on one [43]. In this work, we concentrated our attention on the adsorption side-on mode of oxygen on Fe site. After optimization of the system, an acetone molecule to the oxygen pre-adsorbed LaFeO3 (010) surface is introduced. In these systems, oxygen species are pre-adsorbed on LaFeO3 (010) surface. However, LaFeO3 (010) surface may not be covered fully by the 10
oxygen species, and there are still some adsorption sites. Four different initial adsorption configurations of acetone molecule on the LaFeO3 (010) surface with pre-adsorption of one oxygen molecule (
O2 ) were considered (see Fig. 12): (M1) on pre-adsorbed oxygen
with two hydrogen atoms of acetone closed to the oxygen atom; (M2) on pre-adsorbed oxygen with carbon bonding to oxygen of acetone closed to the oxygen atom; (M3) on pre-adsorbed oxygen with oxygen (acetone) closed to Fe site with adsorbed oxygen; (M4) on pre-adsorbed oxygen with O-down and two carbons closing to pre-adsorbed oxygen. For M1 and M2 modes, the optimized result shows that the acetone molecule leaves far away from the LaFeO3 (010) surface after optimization. It means that acetone does not prefer to be adsorbed on oxygen pre-adsorbed LaFeO3 (010) surface with two hydrogen atoms of acetone closed to the adsorbed site or with carbon bonding to oxygen of acetone closed to the adsorbed site. For M3 mode, the weakly pre-adsorbed oxygen species leaves away from the adsorption site due to the presence of acetone. For M4 mode, the acetone is adsorbed on Fe site with O-down mode. However, the adsorption energies for M3 and M4 modes are -0.103eV and -0.208eV, respectively. Such results mean that M3 and M4 processes are endothermic, which could not occur spontaneously and need additional energy transferring. Later, we will show that the acetone can be adsorbed on the Fe site of the surface by replacing the weakly pre-adsorbed oxygen species at the optimal operating temperature 533 K for M3 mode. Considering the high amount of oxygen adsorption on the surface of semiconductor, in the following, we consider additional three new modes (shown in Fig. 13), with more oxygen molecules pre-adsorbed on the surface of LaFeO3 (two
O2 ): (N1) on pre-adsorbed oxygen surface with oxygen
(acetone) closed to Fe site; (N2) on pre-adsorbed oxygen surface with carbon bonding to oxygen of acetone closed to the adsorbed oxygen; (N3) on pre-adsorbed oxygen surface with hydrogen (acetone) closed to the adsorbed oxygen. The calculated values of the adsorption energy (Eads) for the N1, N2 and N3 mode are
11
0.993eV, 0.057eV and 1.585eV, respectively, which are all exothermic processes. For N2 and N3 modes, the acetone molecule gains charges of 0.107e and 0.082e, which are not likely responsible for the observed increase of semiconductor resistance when exposed to acetone. For N1 mode, the acetone molecule is adsorbed on Fe ion located at the surface lattice of LaFeO3, via the O-down mode. The calculated results show that the transferring charges from the lattice of LaFeO3 into two oxygen molecules are 0.043e and 0.032e, respectively. As shown in Fig. 14 (a), the C atoms in acetone molecule marked 45# and 47# gain charge of 0.592e and 0.221e, the C atom marked 46# loses charge 0.218e. The O atom in acetone lose 0.054e. Meanwhile, the H atoms in the acetone marked 49#, 50#, 51#, 52#, 53# and 54# lose charge of 0.042e, 0.072e, 0.136e, 0.080e, 0.037e and 0.128e, respectively. In other words, there is a net charge of 0.076e transferring from acetone into the lattice of LaFeO3. During the interaction process between acetone and pre-adsorbed oxygen species
O2 on LaFeO (010) surface, our ab initio calculations show that there 3
are charges transferring from acetone to the surface. The electrical negativity of oxygen atoms is larger than that of carbon in carbonyl, leading to the form of C+-O-. A carbonyl group has two centers: a positive center on the carbon atom and a negative electric center on the oxygen atom. When pro-nuclear particles react with the carbonyl compounds, it would firstly attack the carboxyl group and form a bond with carbon atom. Whereas, when electrophilic reagent react with the carbonyl compound, it would firstly attack the oxygen atom of the carboxyl group. Thus the possible acetone sensing mechanism in mode N1 is suggested: An acetone molecule adsorbs on LaFeO3 surface coordinated to Fe site through lone pair electrons, forming η-acetone as shown in Equation (7):
CH3 C=O + O-2 (ads) CH3
-
CH3 C-O CH3
12
+ ads
O-2 (ads)
(7)
Fig. 14 (b) shows the plots of the deformation electron density for N1 mode. The color of blue represents to the trapping of electrons, the color of yellow responds to the electron- release and the white parts represent that the deformation electron density is zero. Fig. 15 (a) shows DOS of the adsorbed acetone for N1 mode and free acetone molecule. The bands of the adsorbed acetone shift toward lower energy as compared to that of free acetone. The density of states (DOS) of acetone adsorbed in N1 mode changes greatly, reflecting that it belongs to chemical adsorption. It can be seen from Fig. 15 (b) that there are overlaps of DOS peaks of O (originating from acetone) and Fe (in the LaFeO3 surface), showing strong orbital hybridizations between their orbits. Four configurations of acetone on the LaFeO3 (010) surfaces with pre-adsorbed oxygen species O
(Fig. 16) were constructed: (S1) on pre-adsorbed oxygen surface with oxygen (acetone) closed to Fe site; (S2) on pre-adsorbed surface oxygen with carbon bonding to oxygen of acetone closed to the adsorbed oxygen; (S3) on pre-adsorbed oxygen surface with hydrogen (acetone) closed to the adsorbed oxygen; (S4) on pre-adsorbed oxygen surface with hydrogen (acetone) closed to O site. The adsorption energy for S1, S2, S3 and S4 modes is 0.935eV, 1.092eV, 1.572eV and 0.914eV, respectively. Both of S1 and S2 show that the acetone molecule could directly interact with O pre-adsorbed LaFeO3 (010) surface. When acetone is introduced, the acetone molecule can adsorb on the lattice Fe site. The acetone molecule may also adsorb
on the pre-adsorbed O forming an acetone-oxygen complex. Our ab initio calculations show that for
these interaction processes between acetone and per-adsorbed of O on LaFeO3 (010) surface, charges mainly transfer from the acetone molecule to the LaFeO3 (010) surface. The numbers of the transferred charges for S1 and S2 mode are 0.004e and 0.212e, respectively. Such calculated results demonstrate that
the interaction of acetone and oxygen species O on LaFeO3 surface brings about the releasing of electrons back to semiconductor LaFeO3. The adsorption of mode S1 is similar to the case of N1, as shown
13
in Equation (8).
CH3
-
CH3
C=O + O-(ads)
C-O
CH3
CH3
+
O-(ads)
(8)
ads
In the following, we paid more attention on the mode S2. In mode S2, the acetone molecule is adsorbed on the pre-adsorbed O
forming a methyl acetate. It can be explained that when acetone
molecules are adsorbed to the surface of the LaFeO3, the pre-adsorbed oxygen species ( O ) on the surface may attack C atom with a positive charge on the carbonyl. The processes can be described by the Equation (9). CH3 C=O +O -
C
ads
CH3
O-
CH3 CH3
O
ads
(9)
This reaction process continuously consumed O-, the trapped electrons on O- return back to the surface of LaFeO3. Such electron transfer is connected with the increase of resistance of LaFeO3 when exposed to acetone. Fig.17 shows the plots of the deformation electron density for S1 and S2 modes. The color of blue represents to the trapping of electrons, the color of yellow responds to the electron- release and the white parts represent that the deformation electron density is zero. Fig. 18 shows DOS of the adsorbed acetone for S1, S2 mode and free acetone. The bands of the adsorbed acetone shift toward lower energy as compared to that of free acetone. The density of states (DOS) of acetone adsorbed in S1 and S2 mode change greatly, reflecting that it belongs to chemical adsorption. It can be seen from Fig. 19 (a) that there are overlaps of DOS peaks of O (originating from acetone) and Fe (in the LaFeO3 surface) in S1 mode, meanwhile there
14
are overlaps of DOS peaks of C (originating from acetone) and pre -adsorbed O in S2 mode (see Fig. 19 (b)). It means that there are strong orbital hybridizations between their orbits. In most of previous DFT calculations on gas seining mechanisms, the effect of temperature is not considered due to the lack of appropriate calculation methods and some calculation difficulties. However, it is well-known that the operating temperature has an enormous effect on the sensitivity of the sensor in real experiments. In the following, we try to further consider the effect of the optimal operating temperature (533 K obtained from experiment) using Molecular dynamics (MD) simulation with NVT ensemble for some typical optimized structure. Fig. 20 shows the possible structures of developed modes (M3’, N1’, S1’ and S2’) at 533 K (260 °C) from M3, N1, S1 and S2 mentioned above. For M3’ mode, the acetone molecule was adsorbed with O-down on the Fe site of LaFeO3 (010) surface, through the replacing of the weakly pre-adsorbed oxygen species
O2 and accompanying the formation of oxygen molecule. This
process can be described by Equation (10).
CH3
-
CH3
C=O + O2-(ads)
C-O
CH3
CH3
(10)
ads
For S2’ mode, the acetone molecule is adsorbed on the pre-adsorbed O shown in Equation (8).
O2
+
forming a methyl acetate as
In both M3’ and S2’ modes, acetone molecule reacts directly with the weakly
pre-adsorbed oxygen species (
O2 or O ), where acetone molecule transfer electrons to pre-adsorbed
oxygen species and finally to the surface of LaFeO3. These processes (S2’ and M3’modes) described by Eqs (8) and (9) may play important roles in acetone sensing. Such DFT results are consistent with the conventional sensing explanation or mechanism for semiconductor [33, 34, 36, 37]. However, the
15
suggested formations of CO2 and H2O in the sensing mechanism described by Equation (4) have not been found in our DFT calculation. Perhaps, Equation (4) holds at a much higher operating temperature. For N1’ and S1’ modes, the acetone molecule is adsorbed on Fe site, where acetone molecule directly transfers some electrons to the LaFeO3 (010) surface at 533 K. The later processes (N1’ and S1’ modes) described by Eqs.(6) and (7) may also provide some contribution to acetone sensing.
4. Conclusions LaFeO3 thick film based on nanocrystalline sol-gel powders annealed at 800°C could sensitively detect low concentration acetone. When exposed to 0.5ppm acetone, the response of LaFeO3 thick film at 260 °C is 2.068 with response time of 62 s and recovery time of 107 s, respectively. LaFeO3 thick film may be a promising candidate for developing a breath analysis technique for monitoring diabetes. The DFT calculation results show that there is strong interaction between the acetone with the surface of LaFeO3 (010) pre-adsorbed with oxygen species O and O2 , accompanying with electron transfer from acetone to LaFeO3 (010) surface. The acetone molecule reacts with oxygen species in two ways: (1) adsorb on oxygen species O or (2) replace of the weakly pre-adsorbed oxygen species
O2 on Fe site,
accompanying the formation of oxygen molecule. These processes may play important roles in acetone sensing for LaFeO3. However, the formations of CO2 and H2O at the optimal operating temperature (260 °C) have not been found in our DFT calculation. The formations of CO2 and H2O may occur at much higher temperature. We also find that the acetone molecule can be directly adsorbed on Fe site, transferring some electrons to the LaFeO3 (010) surface. The later processes may also provide addition contribution to acetone sensing.
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Acknowledgements This work was supported by National Natural Science Foundation of China (Nos: 51472145, 51272133 and 51472150)，Shandong Natural Science Foundation (No. ZR2013EMM016), and Foundation of Dalian University of Technology at Panjin (No. DUT14RC-3-113).
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References [1] C.-C. Wang, Y.-C. Weng, T.-C. Chou, Acetone sensor using lead foil as working electrode, Sensors and Actuators B: Chemical 122 (2007) 591-595. [2] S.S. Likhodii, K. Musa, S.C. Cunnane, Breath acetone as a measure of systemic ketosis assessed in a rat model of the ketogenic die, Clinical Chemistry 48 (2002) 115-120. [3] N. Makisimovich, V. Vorottyntsev, N. Nikitina, O. Kaskevich, P. Karabum and F. Martynenko, Adsorption semiconductor sensor for diabetic ketoacidosis diagnosis, Sensors and Actuators B 36 (1996) 419. [4] A.M. Diskin, P. Spanel, D. Smith, Time variation of ammonia, acetone, isopreneand ethanol in breath: a quantitative SIFT-MS study over 30 days, Physiological Measurement 24 (2003) 107–119. [5] S.B. Patil, P.P. Patil, M.A. More, Acetone vapour sensing characteristics of cobalt-doped SnO2 thin films, Sensors and Actuators B 125 (2007) 126–130. [6] J. Zhao, L. Huo, S. Gao, H. Zhao, J. Zhao, Alcohols and acetone sensing properties of SnO2 thin films deposited by dip-coating, Sensors and Actuators B 115 (2006) 460–464. [7] T. Li, W. Zeng, W. Zhao, Gas sensing performance of multiple SnO2 1D nanostructures based on their interconnect manner, Materials Letters 167 (2016) 230-233. [8] S.F. Bamsaoud, S.B. Rane, R.N. Karekar, R.C. Aiyer, Nano particulate SnO2 based resistive films as a hydrogen and acetone vapour sensor, Sensors and Actuators B 153 (2011) 382-391. [9 ] W. Zeng, T. Liu, D. Liu, E. Han, Hydrogen sensing and mechanism of M-doped SnO2 (M= Cr3+, Cu2+ and Pd2+) nanocomposite, Sensors and Actuators B: Chemical, 160 (2011) 455-462. [10] P. Song, Q. Wang, Z. Yang, Preparation, characterization and acetone sensing properties of Ce-doped SnO2 hollow spheres, Sensors and Actuators B 173 (2012) 839-846.
18
[11] D. Zhang, A. Liu, H. Chang, B. Xia, Room-temperature high-performance acetone gas sensor based on hydrothermal synthesized SnO2-reduced graphene oxide hybrid composite, RSC Advances 5 (2015) 3016-3022. [12] R. Rella, J. Spadavecchia, M.G. Manera, S. Capone, A. Taurino, M. Martino, A.P. Caricato, T. Tunno, Acetone and ethanol solid-state gas sensors based on TiO2 nanoparticles thin film deposited by matrix assisted pulsed laser evaporation, Sensors and Actuators B 127 (2007) 426-431. [13] M. El-Maazawi, A. N. Finken, A. B. Nair and V. H. Grassian, Adsorption and Photocatalytic Oxidation of Acetone on TiO2: An in Situ Transmission FT-IR Study, Journal of Catalysis 191 (2000)138-146. [14] W. Wang, J. Yu, Q. Xiang and B. Cheng, Enhanced photocatalytic activity of hierarchical macro/mesoporous TiO2–graphene composites for photodegradation of acetone in air, Applied Catalysis B: Environmental 119-120 (2012) 109-116. [15] P. Song, Q. Wang, Z. Yang, Acetone sensing characteristics of ZnO hollow spheres prepared byone-pot hydrothermal reaction, Materials Letters 86 (2012) 168-170. [16] S.S. Nath, M. Choudhury, D. Chakdar, G. Gope, R.K. Nath, Acetone sensing property of ZnO quantum dots embedded on PVP, Sensors and Actuators B 148 (2010) 353-357. [17] X.Wang, W. Wang and Y. Liu, Enhanced acetone sensing performance of Au nanoparticles functionalized flower-like ZnO, Sensors and Actuators B 168 (2012) 39-45. [18] H. Ahn, Y. Wang, S.H. Jee, M.o Park, Y.S. Yoon and D.J. Kim, Enhanced UV activation of electrochemically doped Ni in ZnO nanorods for room temperature acetone sensing, Chemical Physics Letters 511 (2011) 331-335. [19] Y. Zeng, T. Zhang, M. Yuan, M. Kang, G. Lu, R. Wang, H. Fan, Y. He and H.Yang, Growth and selective acetone detection based on ZnO nanorod arrays, Sensors and Actuators B 143 (2009) 93-98.
19
[20] L. Liu, S. Li, J. Zhuang, L. Wang, J. Zhang, H. Li, Z. Liu, Y. Han, X. Jiang and P. Zhang, Improved selective acetone sensing properties of Co-doped ZnO nanofibers by electrospinning, Sensors and Actuators B 155 (2011) 782-788. [21] C.S. Prajapati, Ajay Kushwaha, P.P. Sahay, Influence of Fe doping on the structural, optical and acetone sensing properties of sprayed ZnO thin films, Materials Research Bulletin 48 (2013) 2687-2695. [22] C.S. Prajapati, P.P. Sahay, Influence of In doping on the structural, optical and acetone sensing properties of ZnO nanoparticulate thin films, Materials Science in Semiconductor Processing 16 (2013) 200-210. [23] N.H. Al-Hardan, M.J. Abdullah, A.A. Aziz, Performance of Cr-doped ZnO for acetone sensing, Applied Surface Science 270 (2013) 480-485. [24] P. P. Sahay, Zinc oxide thin film gas sensor for detection of acetone, Jonrnal of Materials Science 4 0 (2005) 4383-4385. [25] P. Gunawan, L. Mei, J. Teo, J. Ma, J. Highfield, Q. Li. Z. Zhong, Ultrahigh Sensitivity of Au/1D α-Fe2O3 to Acetone and the Sensing Mechanism, Langmuir 28 (2012) 14090−14099. [26] H. Shan, C.i Liu, L. Liu, S. Li, L. Wang, X. Zhang, X. Bo and X. Chi, Highly sensitive acetone sensors based on La-doped α-Fe2O3 nanotubes, Sensors and Actuators B 184 (2013) 243-247. [27] N.G. Cho, H. Woo, J. Lee and I.o Kim, Thin-walled NiO tubes functionalized with catalytic Pt for highly selective C2H5OH sensors using electrospun fibers as a sacrificial template, Chemical Communications 47 (2011) 11300–11302. [28] R.S. Khadayate b, J.V. Sali a, P.P. Patil, Acetone vapor sensing properties of screen printed WO3 thick films, Talanta 72 (2007) 1077-1081.
20
[29] P. Gao, H. Ji, Y. Zhou and X. Li, Selective acetone gas sensors using porous WO3-Cr2O3 thin films prepared by sol–gel method, Thin Solid Films 520 (2012) 3100-3106. [30] S. Choi, I. Lee, B. Jang, D. Youn, W. Ryu, C.O. Park and I. Kim, Selective Diagnosis of Diabetes Using Pt-Functionalized WO3 Hemitube Networks As a Sensing Layer of Acetone in Exhaled Breath, Analytical Chemistry 85 (2013) 1792−1796. [31] M. Righettoni, A. Tricoli, and S.E. Pratsinis, Thermally Stable, Silica-Doped ε-WO3 for Sensing of Acetone in the Human Breath, Chemistry of materials 22 (2010) 3152-3157. [32] X. Liu, J. Hu, B. Cheng, H. Qin, M. Jiang, Acetone gas sensing properties of SmFe1 xMgxO3 −
perovskite oxides, Sensors and Actuators B 134 (2008) 483-487. [33] P. Song, H. Zhang, D. Han, J. Li, Z. Yang and Q. Wang, Preparation of biomorphic porous LaFeO3 by sorghum straw biotemplate method and its acetone sensing properties, Sensors and Actuators B 196 (2014) 140-146.
[34] X. Liu, H. Ji, Y. Gu, M. Xu, Preparation and acetone sensitive characteristics of nano-LaFeO3 semiconductor thin films by polymerization complex method, Materials Science and Engineering B 133 (2006) 98-101.
[35] P.A. Murade, V.S. Sangawar, G.N. Chaudhari, V.D. Kapse, A.U. Bajpeyee, Acetone gas-sensing performance of Sr-doped nanostructured LaFeO3 semiconductor prepared by citrate solegel route, Current Applied Physics 11 (2011) 451-456. [36] L. Zhang, H. Qin, P. Song, J. Hu, M. Jiang, Electric properties and acetone-sensing characteristics of La1-xPbxFeO3 perovskite system, Materials chemistry and physics 2006 98(2) 358-362. [37] M. Yang, L. Huo, H. Zhao, S. Gao, Z. Rong, Electrical properties and acetone-sensing characteristics
21
of LaNi1-xTixO3 perovskite system prepared by amorphous citrate decomposition. Sensors and actuators B, Chemical 2009 143(1) 111-118. [38] K. Fan, H.i Qin, Z. Zhang, L. Sun, L. Sun and J. Hu, Gas sensing properties of nanocrystalline La0.75Ba0.25FeO3 thick-film sensors, Sensors and Actuators B 171-172 (2012) 302-308. [39] T. Chen, Z. Zhou, Y. Wang, Surfactant CATB-assisted generation and gas-sensing characteristics of LnFeO3 (Ln= La, Sm, Eu) materials, Sensors and Actuators B: Chemical 143 (2009) 124-131. [40] Z.L. Wu, R. Zhang, M. Zhao, S.M. Fang, Z.X. Han, J.F. Hu, K.Y. Wang, Effect of Pd doping on the acetone-sensing properties of NdFeO3, International Journal of Minerals, Metallurgy, and Materials, 19 (2012) 141-145. [41] G.O. Nelson, Controlled test Atmospheres, principles and techniques, Ann Arbor Science Publishers, Ann Arbor, Mich., 1971. [42] L. Sun, J. Hu, H. Qin, M. Zhao, K. Fan, Influences of Ca Doping and Oxygen Vacancy upon Adsorption of CO on the LaFeO3 (010) Surface: A First-Principles Study, The Journal of Physical Chemistry C 115 (2011) 5593-5598. [43] X. Liu, J. Hu, B. Cheng, H. Qin, M. Zhao, C. Yang, First-principles study of O2 adsorption on the LaFeO3 (010) surface. Sensors and Actuators B: Chemical 139(2) (2009) 520-526. [44] Y. L. Lee, J. Kleis, J. Rossmeisl, D. Morgan, Ab initio energetics of LaBO3 (001) (B= Mn, Fe, Co, and Ni) for solid oxide fuel cell cathodes, Physical Review B 80 (2009) 224101. [45] X. Wang, H. Qin, L. Sun, J. Hu, CO2 sensing properties and mechanism of nanocrystalline LaFeO3 sensor, Sensors and Actuators B 188 (2013) 965-971.
[46] Y. Chen, X. Wang, C. Shi, L. Li, H. Qin, J. Hu, Sensing mechanism of SnO2 (1 1 0) surface to H2: Density functional theory calculations, Sensors and Actuators B 220 (2015) 279-287. 22
[47] R. Jinnouchi, K. Kodama, T. Hatanaka and Y. Morimoto, First principles based mean field model for oxygen
reduction reaction, physical chemistry chemical physics 13 (2011) 21070-21083.
[48] Y. Koyama, H. Arai, I. Tanaka, Y. Uchimoto and Z. Ogumi, First principles study of dopant solubility and defect chemistry in LiCoO2, Journal of Materials Chemistry A 2 (2014) 11235-11245. [49] X. Wang, H. Qin, Y. Chen, J. Hu, Sensing Mechanism of SnO2 (110) Surface to CO: Density Functional Theory Calculations, The Journal of Physical Chemistry C 118 (2014) 28548−28561. [50] B. Delley, An All-electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules, The Journal of Chemical Physics 92 (1990) 508-517. [51] B. Delley, From Molecules to Solids with the DMol Approach, The Journal of Chemical Physics 113 (2000) 7756-7764. [52] R. S. Mulliken, Electronic Population Analysis on LCAO-MO Molecular Wave Functions, The Journal of Chemical Physics 23 (1955) 1833-1840.
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Biographies Yanping Chen received her B.S degree from the College of Physics and Electronics, Shandong Normal University, China in 2013. She is currently working toward the PhD degree in the field of condensed matter physics at the School of Physics, Shandong University. The current field of interest is semiconductor gas sensors. Hongwei Qin is currently a professor at the School of Physics, Shandong University. The current fields of interest are magnetism materials, multiferroics materials, functional alloy, rare earth oxide materials, nanomaterials and gas sensors. She has published more than 150 refereed research papers in SCI journals. She has been awarded six invention patents. Xiaofeng Wang received her PhD degree in the field of material physics and chemistry from the School of Physics, Shandong University, China in 2009. She is currently a Lecturer in school of science, Dalian University of Technology at Panjin, China. Ling Li received her B.S degree from the School of Chemistry, Liaocheng University, China in 2008, received her MS degree from the School of Chemistry, University of Jinan, China in 2011. She is currently working toward the PhD degree in the field of material physics and chemistry at the School of Physics, Shandong University. The current field of interest is semiconductor gas sensors. Jifan Hu gained his B.Sc. (July, 1985) in Physics from Shandong University, his M.Sc. (July, 1988) and Ph.D (November, 1993) from Institute of Physics, Chinese Academy of Sciences. He worked as a post doctor in the Institute of Theoretical Physics, Chinese Academy of Sciences from April 1994 to April 1996. He joined the Department of Physics, Shandong University in May 1996, and became a full professor in December 1996. He was appointed as Chang-Jiang Professor (Cheung Kong Scholar) by Chinese National Ministry of Education in the end of 1999. He is currently the director of Institute of Condensed Matter
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Physics, School of Physics at Shandong University. His current fields of interest are magnetic materials, multiferroics materials, functional alloy, rare earth oxide materials, nanomaterials and gas sensors. He has published more than 270 refereed research papers in SCI journals as an author or a coauthor. He has been awarded nine invention patents.
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Figure captions Fig. 1 (a) Fabricated sensor; (b) schematic diagram of the thick film sensor; (c) The electric circuit which was used to test the gas-sensing properties.
Fig. 2 XRD patterns of LaFeO3 powders.
Fig. 3 The operating temperature dependence of the resistances in air for the sensors based on LaFeO3 powders annealed at different temperatures. Fig. 4 Relationship between temperature and the response of LaFeO3-based sensors for 5 ppm acetone in air. Fig. 5 FE-SEM image of LaFeO3 powders annealed at 800 °C and the density percent versus LaFeO3 grain size. Fig. 6 Dependence of the LaFeO3-based sensor response on operating temperature to 0.5, 1, 5 and 10 ppm. Fig. 7 Response of LaFeO3-based sensor at 260 °C versus acetone concentration. Fig. 8 The dynamic sensing characteristics of LaFeO3-based sensor exposed to 0.5, 1, 5 and 10 ppm acetone gas in the background of dry air. Fig. 9 Response of the LaFeO3-based sensor to 5 ppm different gases at 200 °C. Fig. 10 The optimized structure of LaFeO3 (0 1 0) surface. La atoms are shown in blue, Fe atoms in purple and O atoms in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.) Fig. 11 M0 mode of initial adsorption of acetone molecule on the LaFeO3 (0 1 0) surface. The left drawings are the configurations of initial adsorption and the rights are that of optimized adsorption. Fig. 12 Four initial adsorption of acetone molecule, the following modes were considered: (M1) on
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pre-adsorbed oxygen with two hydrogens of acetone closed to the adsorbed site; (M2) on pre-adsorbed oxygen with carbon bonding to oxygen of acetone closed to the adsorbed site; (M3) on pre-adsorbed oxygen with oxygen (acetone) closed to Fe site with adsorbed oxygen; (M4) on pre-adsorbed oxygen with O-down and two carbons closing to pre-adsorbed oxygen. Fig. 13 Three modes of initial adsorption of the acetone molecule on oxygen per-adsorbed LaFeO3(0 1 0) surface. The left drawings are the configurations of initial adsorption and the rights are that of optimized adsorption. (N1) on pre-adsorbed oxygen surface with oxygen (acetone) closed to Fe site with adsorbed oxygen; (N2) on pre-adsorbed oxygen surface with carbon bonding to oxygen of acetone closing to pre-adsorbed oxygen. (N3) on pre-adsorbed oxygen with two hydrogens of acetone closed to the pre-adsorbed oxygen. Fig. 14 (a) Mark numbers of atoms in mode N1; (b) DOS of the adsorbed acetone for N1 mode and free acetone. Fig. 15 The DOS of O (originating from acetone) and Fe (in the LaFeO3 surface). Fig. 16 Four modes of initial adsorption of the acetone molecule on O per-adsorbed LaFeO3 (0 1 0) surface: (S1) on pre-adsorbed oxygen surface with oxygen (acetone) closed to Fe site; (S2) on pre-adsorbed surface oxygen with carbon bonding to oxygen of acetone closed to the adsorbed oxygen; (S3) on pre-adsorbed oxygen surface with hydrogen (acetone) closed to the adsorbed oxygen; (S4) on pre-adsorbed oxygen surface with hydrogen (acetone) closed to O site. Fig. 17 The plots of the deformation electron density for S1 and S2 modes. The color of blue represents to the trapping of electrons, and the color of yellow responds to the electron- release. Fig. 18 DOS of the adsorbed acetone for S1, S2 mode and free acetone. Fig. 19 (a): DOSs of O-p (originating from acetone) and Fe-d (originating from lattice) in mode S1; (b)
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PDOSs of pre-adsorbed O and C (originating from acetone) in mode S2. Fig. 20 Schematic image of M3’, N1’, S1’ and S2’ mode at 773 K.
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