- Email: [email protected]
molybdenum disulfide ternary nanocomposite toward carbon monoxide gas sensing
Accepted Manuscript Fabrication cobalt-doped indium oxide/molybdenum disulfide ternary nanocomposite toward carbon monoxide gas sensing Dongzhi Zhang, Junfeng Wu, Yuhua Cao PII:
S0925-8388(18)34064-7
DOI:
https://doi.org/10.1016/j.jallcom.2018.10.365
Reference:
JALCOM 48183
To appear in:
Journal of Alloys and Compounds
Received Date: 27 July 2018 Revised Date:
2 October 2018
Accepted Date: 27 October 2018
Please cite this article as: D. Zhang, J. Wu, Y. Cao, Fabrication cobalt-doped indium oxide/molybdenum disulfide ternary nanocomposite toward carbon monoxide gas sensing, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.10.365. 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.
ACCEPTED MANUSCRIPT
Fabrication cobalt-doped indium oxide/molybdenum disulfide ternary nanocomposite toward carbon monoxide
RI PT
gas sensing
SC
Dongzhi Zhang*, Junfeng Wu, Yuhua Cao
Key Laboratory of Unconventional Oil & Gas Development, Ministry of Education,
China), Qingdao 266580, China
M AN U
College of Information and Control Engineering, China University of Petroleum (East
TE D
*Corresponding authors: Dongzhi Zhang E-mail address: [email protected]
EP
Tel: +86-532-86982928
AC C
Fax: +86-532-86983326
Page 1 of 44
ACCEPTED MANUSCRIPT Abstract This paper demonstrated a high-performance carbon monoxide (CO) gas sensor based on cobalt (Co)-doped indium oxide (In2O3) nanoparticles/molybdenum disulfide
RI PT
(MoS2) nanoflowers nanocomposite. Co-In2O3 nanoparticles were synthesized by a co-precipitation method, and flower-like MoS2 was prepared by one-step hydrothermal route. Layer-by-layer self-assembly technique was employed to
SC
fabricate Co-In2O3/MoS2 film sensor on an epoxy substrate with interdigital electrodes. Scanning electron microscopy (SEM), transmission electron microscope
M AN U
(TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were carried out to fully examine the morphology, microstructure, and elementary composition of the as-prepared samples. The
TE D
CO-sensing characteristics of the Co-In2O3/MoS2 film sensor were systematically investigated under room temperature through exposing the sensor to various concentration of CO gas. The Co-In2O3/MoS2 sensor achieved high sensitivity, fast
EP
response/recovery speed, excellent repeatability and stable long-term stability. An approach of combining gas-sensing experiments with density-functional theory (DFT)
AC C
simulation based on first-principle was used to further explore the CO-sensing mechanism of the Co-In2O3/MoS2 sensor. The Co2+ ion doping, and heterojunctions created at interfaces of Co-In2O3 and MoS2 were attributed to the high-performance CO sensing. Keywords: molybdenum disulfide; Co-doped indium oxide; CO gas sensor; LbL self-assembly; first-principle theory
Page 2 of 44
ACCEPTED MANUSCRIPT 1. Introduction Carbon monoxide (CO) is an odorless, colorless, poisonous and hazardous gas mostly produced from the incomplete combustion of fossil fuels [1]. Owing to the fact
RI PT
that CO molecules can easily bond with the hemoglobin, preventing its combination to oxygen molecules, exposure to an excess amount of CO may result in the acute and chronic injury to human health, and there is even a high possibility of death [2]. US
SC
Occupational Safety and Health Administration (OSHA) suggests that the permissible
M AN U
exposure limit for CO is 35 ppm (10 hour ceiling limit), while the US National Institute for Occupational Safety and Health (NIOSH) recommends a limit of 50 ppm (8 hour ceiling limit) [3]. Hence, a stable and high performance CO sensor for the low concentration is extremely significant and necessary in numerous fields, such as
TE D
vehicle emissions, indoor environment, industrial waste gas discharge, and human breath [4]. Currently, various detection approaches, including resistive [5, 6], potentiometric [7, 8], electrochemical [9], optical fibre [10], and thermoelectric
EP
method [11], have been used to develop CO gas sensor, yet the problems of high-cost,
AC C
operation complexity, and time-consuming limit their extensive application [12]. Fortunately, gas sensors based on nanomaterials can play the roles effectively in gas sensing due to the merits of remarkable physical-chemical stability, low power-consuming, and excellent sensing performance. Thereinto, metal oxides (MOs), such as TiO2 [13], CuO [14], SnO2 [15], ZnO [16], and In2O3 [17], were most commonly applied in CO gas detection. However, most metal oxides based gas sensors exhibit good sensing-characteristics only above 200oC with the side heater, Page 3 of 44
ACCEPTED MANUSCRIPT leading to the high energy consumption and complicated operating condition [18]. Recently, considerable research has been devoted to the development of hand-held and mobile devices, and hence the fabrication of CO sensors working at
RI PT
room-temperature (RT) is highly desirable for these applications. Molybdenum disulfide (MoS2), as a typical two-dimensional (2D) layered transition metal dichalcogenide, have attracted increasing attention due to its physical-chemistry characteristics properties, such as
SC
dramatically fabulous
M AN U
significant direct band gap (1.8 eV) [19], large surface-to-volume ratio [20], outstanding carrier mobility [21], and distinctive electronic properties [22]. Therefore, MoS2 have been focused as an emerging building block for various potential applications, such as supercapacitor, photocatalyst, and especially gas sensors [23-25].
TE D
For example, Wen et al. prepared MoS2/g-C3N4 nanocomposite by a facile ultrasonic dispersion method and exhibited excellent photocatalytic activity, which is much higher than that of pure g-C3N4 nanoparticles [26]. Up to now, multitudinous RT gas
EP
sensors based on MoS2 have been fabricated, such as H2, NO2, NH3, ethanol, and
AC C
humidity sensors [27-31]. Kuru et al. fabricated the H2 gas sensors based on Pd-doped MoS2 and Pd-doped graphene, respectively, and the measurement results shown that the H2 sensing performance of Pd-MoS2 sensor was better than Pd-graphene [32]. Lu et al. synthesized ZnO/MoS2 nanocomposite by hydrothermal route, which shown an excellent sensitivity toward humidity and the sensing properties got a great enhancement compared to the pristine MoS2 and ZnO sensors [33]. Li et al. developed Au-SnO2/MoS2 sensor via a facile hydrothermal method, indicating a better sensing Page 4 of 44
ACCEPTED MANUSCRIPT performance to triethylamine than SnO2/MoS2 and MoS2 sensors, respectively [34]. The previous research achievements illustrate that the transition metal ions doping, and metal oxides decoration with MoS2 can enhance the gas-sensing properties
RI PT
effectively [32, 33]. In this work, a Co-In2O3 nanoparticles/MoS2 nanoflower (Co-In2O3/MoS2) CO gas sensor was developed through layer-by-layer (LbL) self-assembly technique.
SC
XRD, EDS, SEM, TEM, and XPS were conducted to fully inspect the morphology,
M AN U
microstructure, and composition of as-prepared samples. The CO sensing characteristics of the presented Co-In2O3/MoS2 sensor were determined under room temperature. The Co-In2O3/MoS2 film sensor exhibited significantly enhanced sensing performances toward CO, including high sensitivity, fast response/recovery speed,
TE D
distinctive repeatability, and good long-term stability. Finally, a comprehensive research approach supported with gas-sensing experiments and first-principle-based DFT simulations was performed for the exploration of the possible CO-sensing
EP
mechanism of the Co-In2O3/MoS2 sensor.
AC C
2. Experiment
2.1 Materials synthesis
Reagents used in our work, namely In(NO3)3·4.5H2O (>99.5%), NH3·H2O (25%
~28%), Co(NO3)2·6H2O (>98.5%), Na2MoO4·2H2O (>99%), and thioacetamide (>99%), were all supplied from Chinese Medicine Group Chemical Reagent Co. Ltd (Shanghai, China). All reagents were used without further pretreatment. The reagents used for LbL self-assembly, namely 1.5 wt% Poly (diallyldimethylammonium Page 5 of 44
ACCEPTED MANUSCRIPT chloride) (PDDA) and 0.3 wt% poly(styrene sulfonate) (PSS), were blended with NaCl to 0.5 M for increasing the ionic strength. A co-precipitation method was used to synthesize Co-doped In2O3 nanoparticles.
RI PT
7.64 g In(NO3)3·4.5H2O and 0.17 g Co(NO3)2·6H2O were dissolved in 100 mL deionized (DI) water. Next, NH3·H2O was dripped into the above solution to adjust the pH value to the range of 9-10. After washing with ethanol and DI water for
SC
multi-times, the obtained precipitate was dried at 60oC for 8 h. The resulting powder
M AN U
was calcined at 500oC for 3 h to obtain Co-doped In2O3 nanoparticles. Furthermore, pristine In2O3 was synthesized using the similar approach as Co-In2O3 in the absence of Co(NO3)2·6H2O.
To synthesize the flower-like MoS2, an easy-operated one-step hydrothermal
TE D
route was used, which is a frequently-used method for synthesis of nanomaterial, such as ZnO [35-36], SnO2 [37], and TiO2 [38]. 1.21 g Na2MoO4·2H2O and 1.90 g thioacetamide (1.90 g) were dissolved in 60 mL mixed solvent formed from 30 mL DI
EP
water and 30 mL ethanol. Then, the obtained solution was stirred and ultrasonicated
AC C
for 15 min, respectively. The obtained solution was heated at 200oC for 48 h in a 100 mL stainless-steel autoclave with a Teflon liner. The precipitate was washed with ethanol and DI water for multi-times, and it was dried at 60°C for 8 h. Finally, the black powder was collected after calcination under 700°C in nitrogen for 2 h. The growth of MoS2 crystal can be divided into three processes of nucleus growth, MoS2 nano-rods growth and self-assembly. Under hydrothermal conditions, the sulfur source of thioacetamide is facile to coordinate with molybdate since the Page 6 of 44
ACCEPTED MANUSCRIPT reductive functional group (–NH2) in sulfur source can be combined with oxygen atoms of molybdate easily, which is effective to the produce of MoS2 nuclei. Small MoS2 particles are generated by the reaction between the residual molybdenum and
RI PT
sulfur, which can be considered as the formation of nucleus. With the reaction continues, the MoS2 nanorod structures are crookedly grown from the nucleus because of localized stacking and lattice defects. After that, the self-assembly process
SC
is performed by the intrinsic crystal orientation and flower-like porous microsphere
2.2 Sensor Fabrication
M AN U
structure of the MoS2 crystal forms.
A flame resistant (FR-4) material was used as sensor substrate, and a pair of Ni/Cu interdigital electrodes (IDEs) was patterned on the FR-4 substrate. The layer-by-layer (LbL)
TE D
Co-In2O3/MoS2 sensing film was fabricated through
self-assembly method, which is operated via alternative immersion in oppositely charged solutions at room temperature [39]. The typical fabrication process was
EP
shown in Figure 1(a). Firstly, Co-In2O3 and MoS2 suspensions are prepared and
AC C
ultrasonic treated for 15 min, respectively. To enhance the surface electrostatic force, two bi-layers of PDDA/PSS were firstly deposited as precursor layer. After that, five bi-layers of Co-In2O3 and MoS2 were assembled as sensing layer. The deposition time was 10 min for PDDA/PSS layer and 15 min for Co-In2O3/MoS2 layer. Due to the interaction of electrostatic force between two different nanomaterials, each layer mutually have a good connection. After each immersion, the sensor was rinsed with
Page 7 of 44
ACCEPTED MANUSCRIPT DI water and dried with nitrogen to strengthen the interconnection of each layers. The Co-In2O3 and In2O3 gas sensors were fabricated to make a comparison. 2.3 Instrument and analysis
RI PT
The crystallographic structures of In2O3, Co-In2O3, MoS2, and Co-In2O3/MoS2 were inspected by the XRD measurement equipped with an X-ray diffractometer (X’Pert Pro MPD) using Cu Kα radiation (λ = 1.5418 Å). The diffraction angle was
SC
scanned in the range of 10°-70°. The element composition was analyzed by EDS
M AN U
using Hitachi S-4800. The XPS spectra were further measured via an XPS spectrometer (Thermo Scientific Escalab 250Xi). The surface morphologies of Co-In2O3, MoS2, and Co-In2O3/MoS2 were observed using a field-emission scanning electron microscope (Hitachi S-4800). The microstructure was determined via a
TE D
transmission electron microscope (JEOL JEM-2100).
In this work, all gas sensing measurements for as-prepared sensors were carried out at room temperature. The schematic illustration of the experimental setup is
EP
shown in Figure 1(b). The desired concentration of CO was obtained via injecting a
AC C
precalculated quantity of CO gas into the testing chamber using a syringe. Air was used as the baseline gas. The resistance of the sensor was detected by a data logger (Agilent 34970A), which was linked with a PC by RS-232 interface. The normalized response, a parameter to evaluate the gas sensing performance, was determined by R=|△R|/Rair×100% =|Rair-Rgas|/Rair×100%, where Rair and Rgag are the sensor resistance in air and CO gas, respectively. 3. Results and discussion Page 8 of 44
ACCEPTED MANUSCRIPT 3.1 Materials characterizations The XRD spectra of In2O3, Co-In2O3, MoS2, and Co-In2O3/MoS2 are shown in Figure 2(a). No peak of impurity crystal phases is exhibited in the XRD.. Several
RI PT
prominent diffraction peaks at 2θ of 14.4°, 33.1°, 39.7°, and 58.5° are presented in the spectrum of MoS2, attributing to the (002), (100), (103) and (110) planes of hexagonal MoS2 nanocrystals (JCPDS No. 37-1492) [40]. For both undoped and Co-doped In2O3,
SC
all peaks could be well resembled with the single phase of In2O3 (JCPDS 06-416) [41].
M AN U
Owing to the similar size of Co2+ (0.72Å) and In3+ (0.81Å) , Co2+ ions exhibit a high solubility in In2O3 crystalline structure, leading to a slight change of XRD pattern without diffraction peaks of Co compounds after Co2+ doping [42]. Furthermore, the main peaks corresponding to the presence of MoS2 and Co-In2O3 can be clearly
TE D
observed in the XRD of Co-In2O3/MoS2, which testifies the successful preparation of Co-In2O3/MoS2 nanocomposite. The EDS pattern of Co-In2O3/MoS2 is exhibited in Figure 2(b), indicating the element component of the experiment sample. It can be
EP
clearly illustrated the existence of Co, In, O, Mo, and S in the Co-In2O3/MoS2 sample.
AC C
XPS was used to further investigate the elemental compositions and chemical status of Co-In2O3/MoS2 nanocomposite. From the XPS survey spectrum plotted in Figure 3(a), characteristic peaks of all component elements including In, O, Co, Mo, and S can be obviously detected. In Figure 3(b), two peaks at 451.63 eV and 444.09 eV are corresponding to the characteristic spin-orbit split states of In 3d3/2 and In 3d5/2, which are attributed to the In3+ of In2O3 [43]. From XPS pattern of O 1s shown in Figure 3(c), the predominant peak at 529.54 eV is ascribed to the lattice oxygen Page 9 of 44
ACCEPTED MANUSCRIPT species, while the peak at 531.60 eV is associated with the adsorbed oxygen that makes great contribution to the gas-sensing performance [44]. Moreover, the concentrations of Olattice and Oads to O 1s are estimated to be 67.55% and 32.45% via
RI PT
calculating the area proportion of Olattice and Oads. As shown in Figure 3(d), two main peaks at 795.41 eV and 780.38 eV are corresponding to the Co 2p1/2 and Co 2p3/2, and two weak peaks at 802.27 eV and 787.39 are assigned to Co 2p1/2 and Co 2p3/2
SC
satellite structure [45]. From the XPS pattern of Mo 3d shown in Figure 3(e), two
M AN U
main peaks at 232.43 eV and 229.25 eV are attributed to Mo 3d3/2 and Mo 3d5/2 [46]. In Figure 3(f), two peaks of S 2p are located at 163.29 eV and 162.10 eV [47]. Figure 4 shows the micromorphology of MoS2, Co-In2O3, and Co-In2O3/MoS2 inspected via SEM. In Figure 4(a), it can be found that pristine MoS2 exhibits a
TE D
flower-nanosphere morphology composed of numerous wormlike nanorods. Figure 4(b) illustrates the appearance of aggregated Co-In2O3 nanoparticles. SEM image of Co-In2O3/MoS2 nanocomposite is plotted in Figure 4(c) and Figure 4(d), that
Co-In2O3
EP
demonstrating
nanoparticles
are
distributed
on
the
MoS2
AC C
flower-nanospheres. Figure 4(e) shows a cross-sectional image of Co-In2O3/MoS2 film. PDDA/PSS layer is deposited for the surface modification of substrate and facilitate the assembly of Co-In2O3/MoS2. The bi-layer PDDA/PSS is very thin, which is covered by the sensing layer of Co-In2O3/MoS2. Therefore, PDDA/PSS layer contributes little to the gas sensing. Furthermore, the cross-sectional EDS spectrum of Co-In2O3/MoS2 film is plotted in Figure 4(f), indicating that the main elements of Mo, S, In, Co and O, and weak peaks for trace elements of Na and Cl in PDDA/PSS layer. Page 10 of 44
ACCEPTED MANUSCRIPT The TEM images of Co-In2O3/MoS2 nanocomposite are employed to further analyze its nanostructure. Figure 5(a) and 5(b) exhibit the overall perspective of Co-In2O3/MoS2 nanocomposite. It clearly shows that the size of the nanorods is 60-80
RI PT
nm, and the black Co-In2O3 nanoparticles are distributed on the nanorods. Figure 5(c) and 5(d) depict the HRTEM images of Co-In2O3. It can be clearly observed that the lattice fringes of Co-In2O3 are 0.29 and 0.18 nm, which are corresponding well with
SC
(222) and (440) plane of In2O3 due to the high solubility of doping ions caused by the
M AN U
similar size of Co2+ and In3+ [48]. And the lattice spacing of (002) plane for MoS2 is 0.62 nm [49]. 3.2 CO gas sensing properties
To highlight the improvement for the CO sensing performance of
switching
TE D
Co-In2O3/MoS2 nanocomposite sensor, a contrast experiment was operated through Co-In2O3/MoS2,
Co-In2O3,
and
In2O3
sensors
among
different
concentrations of CO gas and air. The time interval of each switching was 120 s. The
EP
resulting time dependent response-recovery curves of Co-In2O3/MoS2, Co-In2O3, and
AC C
In2O3 are shown in Figure 6(a). It can be obviously found that Co-In2O3/MoS2 sensor exhibits the highest sensitivity among these three sensors, and Co-In2O3 sensor shows better sensitivity than pure In2O3 sensor. The experiment result indicates that doping with Co2+ and decoration of MoS2 both lead to an enhancement on the gas sensing property of Co-In2O3/MoS2. Figure 6(b) demonstrates the response values of the Co-In2O3/MoS2, Co-In2O3, and In2O3 as functions of CO concentrations. The fitting equations of the response Y and CO concentration X can be represented as Page 11 of 44
ACCEPTED MANUSCRIPT Y=4.55+2.56lgX, Y=2.21+1.36lgX, and Y=0.501+0.034lgX for Co-In2O3/MoS2, Co-In2O3, and In2O3, respectively, and the regression coefficient is 0.9885, 0.9699, and 0.8246, respectively. The result suggests that Co-In2O3/MoS2 sensor possesses the
RI PT
best sensitivity toward CO gas and the highest linearity with the common logarithm of CO concentration among the three sensors.
Figure 7 exhibits the response/recovery properties of Co-In2O3/MoS2, Co-In2O3,
SC
and In2O3 film sensors toward 10 ppm and 1000 ppm CO, respectively. The time
response/recovery
stage
is
M AN U
taken by a sensor to achieve 90% of the total resistance change in respective defined
as
the
response/recovery
time.
The
response/recovery times for the Co-In2O3/MoS2, Co-In2O3, and In2O3 film sensor are 39 s/81 s, 65 s/90 s, and 70 s/84 s toward 10 ppm CO gas, respectively, and 28 s/88 s,
TE D
62 s/ 106 s, and 77 s/ 123 s toward 1000 ppm CO gas, respectively. The experiment result indicates that the Co-In2O3/MoS2 nanocomposite sensor shows a shorter response/recovery time than Co-In2O3, and In2O3 film sensors, indicating an excellent
EP
response/recovery characteristic for Co-In2O3/MoS2 nanocomposite.
AC C
In conclusion, the results of above contrast experiment testify a fact of the great enhancement of CO sensing properties for Co-In2O3/MoS2 gas sensor through doping modification. Subsequent works are carried out to further investigate other CO-sensing properties of Co-In2O3/MoS2 gas sensor. Figure 8(a) illustrates the repeatability of Co-In2O3/MoS2 gas sensor exposed to 1000 ppm, 100 ppm, and 10 ppm CO gas. The experiment was operated for four response/recovery cycles at room temperature. The measurement results indicate that the Co-In2O3/MoS2 gas sensor can Page 12 of 44
ACCEPTED MANUSCRIPT maintain a remarkable reproducibility and consistency. Figure 8(b) illustrates the long-term stability of Co-In2O3/MoS2 gas sensor exposed to 1000 ppm, 100 ppm, and 10 ppm CO gas. The measurement was carried out every five days over a month. The
outstanding long-term stability of Co-In2O3/MoS2 gas sensor.
RI PT
response of the sensor shows little change with time passing by, verifying an
Table 1 lists the CO sensing properties of Co-In2O3/MoS2 in this work compared
SC
with previous published works [12, 50-53]. The comparison is made with the existing
M AN U
CO gas sensors based on similar state-of-the-art materials, which were fabricated using various methods including hot filament chemical vapor deposition (CVD), ion-induced focusing deposition, hydrothermal and LbL self-assembly technique. The comparison results demonstrate that the presented sensor exhibits a higher CO
TE D
response than the most existing room temperature CO sensors. 3.3 DFT simulation and CO sensing mechanism As above-mentioned results shown, the Co-In2O3/MoS2 film sensor has the
EP
remarkable gas sensing properties toward CO at room temperature. As is known to all,
AC C
the gas-sensing mechanism on the resistance change of semiconductor gas sensors is mainly ascribed to the adsorption-oxidation-desorption process [54]. The illustration of the gas-sensing mechanism for Co-In2O3/MoS2 sensor toward CO gas is shown in Figure 9. In air atmosphere, numerous oxygen molecules are adsorbed on the surface of Co-In2O3/MoS2, and the nanocomposite lost plenty of electrons to promote the ionization of Oads. Since Co-In2O3/MoS2 serves as n-type semiconductor and takes electrons as dominant charge carriers, an electron depletion layer creates on the Page 13 of 44
ACCEPTED MANUSCRIPT surface of Co-In2O3/MoS2, which results in an increase of sensor resistance in air. The reaction in this process can be described as Eq. (1) and (2). When the sensor upon exposure to the CO (reducing gas), a reaction as shown in Eq. (3) is conducted
RI PT
between the CO molecules and the adsorbed O2-(ads), which will make the nanocomposite recapture the electrons trapped by the adsorbed oxygen. As a consequence, the depletion layer will narrow down, inducing a decrease of sensor
SC
resistance.
M AN U
O2(gas) → O2(ads) O2(ads) + e-→O2-(ads)
2CO + O2-(ads) → 2CO2 + e-
(1) (2) (3)
The enhanced sensitivity of Co-In2O3/MoS2 can be attributed to the following
TE D
aspects. Firstly, the flower-like Co-In2O3/MoS2 composed of oriented worm-like nanorods has extremely large surface area yet a 3D co-continuity. This particular structure coupled with oxygen vacancies brings a large amount of active sites for the
EP
adsorption of Oads and CO molecules, which is one important reason why
AC C
Co-In2O3/MoS2 shows enhanced CO-sensing properties. Secondly, the doping of Co2+ ions in In2O3 through replacing In3+ to Co2+ can
generate more oxygen vacancies, which are helpful for the adsorption of oxygen and CO molecules. Moreover, an impurity energy level forms between the top of the valence band and the bottom of the conduction band due to the ion Co2+ doping, which serves as a bridge to promote the electron flow. In other words, more adsorbed
Page 14 of 44
ACCEPTED MANUSCRIPT oxygen ions will be created, leading to an improvement on the sensing performance of the Co-In2O3/MoS2 sensor. Furthermore, a density-functional theory (DFT) simulation using material studio
RI PT
software was carried out to theoretically verify the enhancement of CO sensitivity after Co2+ doping through analysis of the calculated geometry and energy parameters. The In2O3 (110) surface as one of the most stable surface has been frequently
SC
investigated in DFT simulation of gas adsorption on In2O3 [55-57]. Therefore, a gas
M AN U
adsorption model based on In2O3 (110) surface was constructed in this work. The calculated CO adsorption structures of Co-In2O3 and pristine In2O3 are shown in Figure 10(a) and (b), respectively. Table 2 lists the specific DFT calculation results of the adsorption systems including the bond length of C—O bond of CO molecule, the
TE D
shortest atomic distance between adsorbed CO molecule and the substrate, the adsorption energy, the band gap of the substrate, the spin polarization of the substrate, and the charge transfer from adsorbed CO molecule to the substrate, where negative
EP
value presents the electrons flow from CO molecule to the substrate. Thereinto, Ead
AC C
(adsorption energy) was calculated by the Eq. (4) [58, 59]: Ead = Egas/sub - Esub - Egas
(4)
where Egas/sub is the total energy of the whole adsorption system, Esub and Egas are the energies of isolated substrate and adsorbed gas molecule, respectively. From the geometry parameters including bond length and distance, it can be clearly found that CO molecule exhibits a greater change influenced by Co-In2O3 than pristine In2O3. The bond length of CO molecule which changes 0.04 Å after adsorption on Co-In2O3, Page 15 of 44
ACCEPTED MANUSCRIPT but 0.02 Å after adsorption on pristine In2O3. Moreover, the vertical distance between CO molecule and Co-In2O3 (1.799 Å) is shorter than pristine In2O3 (2.540 Å) after adsorption. Because of the more intense interaction of CO molecule and Co-In2O3, the
RI PT
structure of adsorbed CO molecule is changed much more easily, and the CO molecule moves toward the Co-In2O3 more easily than pristine In2O3. The calculated band gap of In2O3 is decreased after doping with Co, which conforms by the above
SC
mentioned theoretical result. And the spin polarization of both substrates are 0,
M AN U
indicating that the material property of non-magnetism have no change after Co doped. Furthermore, the absolute value of Ead and charge transfer (CT) of Co-In2O3 is bigger than pristine In2O3, which directly illustrate an enhanced sensing property toward CO from the view of energy. The diagrams of charge density for CO adsorption system of
TE D
Co-In2O3 and In2O3 are shown in Figure 10(c) and (d), which can visually present the charge distribution of the models. There are much more electrons between the CO molecule and Co-In2O3 than In2O3, illustrating a larger charge transfer of Co-In2O3.
EP
In Figure 11, the density of states (DOSs) of both adsorption systems are fully
AC C
exhibited to further investigate the mechanism in depth. Figure 11 (a) and (b) show the DOSs of Co-In2O3 adsorption system before adsorption and after adsorption, (c) and (d) show the DOSs of In2O3 adsorption system before adsorption and after adsorption. From an overall perspective, the DOS shape of Co-In2O3 have sort of change, yet DOS shape of In2O3 just exhibits little variation, demonstrating a great alteration on electronic structure of Co-In2O3 caused by its strong interaction effect with adsorbed CO molecule, and the electronic structure of In2O3 has hardly any Page 16 of 44
ACCEPTED MANUSCRIPT modulation after adsorption. In Co-In2O3 adsorption system, a peak of Co-In2O3 is created as a bridge at 0 eV after Co doping, which serves as an impurity level generated by Co2+. After CO adsorption, this bridge peak and other two peaks of CO
RI PT
at -3.5 eV and 0 eV are disappeared due to the large amount of charge transfer, and the left peak of CO becomes higher and shifts from -5.2 eV to -6 eV, which resonates to the new energy levels at -6 eV generated in Co-In2O3 and Co 3d by virtue of the
SC
strong interaction between CO molecules and Co-In2O3. The DOS variations of
M AN U
Co-In2O3 adsorption system demonstrate that CO molecule has a strong contact with the Co-In2O3 due to the important role played by doped Co ions that promotes the electron transfer between Co-In2O3 and CO molecules. In pristine In2O3 adsorption system, the similar peaks of CO and In2O3 at -6 eV after adsorption show a lower
TE D
height and a weaker resonance than in Co-In2O3 adsorption system. It can be ascribed to the weaker adsorption strength between In2O3 and CO, indicating that the electrons of In2O3 and CO can hardly distributed on the same energy level without sufficient
EP
interaction of adsorption. And the peak at 2-3.4 eV of CO molecule resonates with
AC C
In2O3 after adsorption, while DOS of In 3d has no peak at the same position. This phenomenon indicates that the CO molecule adsorbed on In2O3 is mainly attributed to the O ions but not In ions. In a word, Co-In2O3 shows an enhanced response toward CO molecule, and its underlying adsorption mechanism is different from pristine In2O3 due to the positive effect made by doping Co2+. Thirdly, another possible reason for the increasing CO sensing performance of Co-In2O3/MoS2 sensor may attribute to n-n junctions formed at the interfaces of Page 17 of 44
ACCEPTED MANUSCRIPT n-type Co-In2O3 and n-type MoS2, leading to a positive impact on the reaction between sensing film and adsorbed oxygen. Figure 12 exhibits the energy band structure of Co-In2O3/MoS2. The work function of Co-In2O3 and MoS2 are 5.44 eV
RI PT
and 5.34 eV, respectively, and the band gap of Co-In2O3 and MoS2 are 1.93 eV and 1.81 eV, respectively, which are computed via DFT method using materials studio software. Owing to the higher Fermi level of MoS2 compared with Co-In2O3,
SC
Co-In2O3 received more electrons from MoS2 until the Fermi level reached an
M AN U
equilibrium, which results in the increasing number of adsorbed oxygen [60]. Hence, a wider depletion layer and a higher barrier formed due to a greater amount of electrons captured by numerous adsorbed oxygen molecules, which cause an increase of the sensor resistance in air [61]. When the sensing film was transferred into
TE D
reductive CO atmosphere, a lot of electrons trapped by oxygen were released back. Therefore, the barrier height and the width of depletion layer sharply decreased, which results in a lower resistance of the gas sensor in CO. In this way,
EP
Co-In2O3/MoS2 sensor reasonably shows an enhanced sensitivity toward CO gas.
AC C
4. Conclusions
A high-performance Co-In2O3/MoS2 CO sensor fabricated by LbL self-assembly
technique was reported in this paper. XRD, EDS, SEM, TEM, and XPS measurements were used to characterize the morphologies, nanostructures, and elementary compositions of as-prepared samples. The CO sensing characteristics of the Co-In2O3/MoS2 sensor were determined under room temperature. The measurement results demonstrated that the Co-In2O3/MoS2 based CO gas sensor has a Page 18 of 44
ACCEPTED MANUSCRIPT high sensitivity, rapid response/recovery speed, and outstanding stability toward CO. An approach of combining experimental investigation with DFT based on first principle was used to further investigate the CO-gas sensing mechanism of the
RI PT
Co-In2O3/MoS2 sensor. The Co-In2O3/MoS2 film sensor was proved to be an extraordinary candidate for real-time detection of CO at room temperature. Acknowledgements
SC
This work was supported by the National Natural Science Foundation of China
M AN U
(No. 51777215), the Key Research & Development Plan Project of Shandong Province (2018GSF117002), the Fundamental Research Funds for the Central Universities of China (18CX07010A), and the Open Funds of National Engineering Laboratory for Mobile Source Emission Control Technology (NELMS2017B03), and
TE D
the Open Fund of Key Laboratory of Marine Spill Oil Identification and Damage
AC C
EP
Assessment Technology, State Oceanic Administration of China (No. 201801).
Page 19 of 44
ACCEPTED MANUSCRIPT Reference [1] S.W. Choi, J. Kim, J.H. Lee, Y.T. Byun, Remarkable improvement of CO-sensing performances in single-walled carbon nanotubes due to modification
RI PT
of the conducting channel by functionalization of Au nanoparticles, Sens. Actuators B 232 (2016) 625-632.
[2] X. Wang, H. Qin, Y. Chen, J. Hu, Sensing mechanism of SnO2 (1 1 0) surface to
SC
CO: density functional theory calculations, J. Phys. Chem. C 118 (2014) 28548–
M AN U
28561.
[3] B.L. Risavi, R.J. Wadas Jr., C. Thomas, D.F. Kupas, A novel method for continuous environmental surveillance for carbon monoxide exposure to protect emergency medical service providers and patients, J. Emerg. Med. 44 (2013)
TE D
637–640.
[4] Y.P. Chen, H.W. Qin, J.F. Hu, CO sensing properties and mechanism of Pd doped SnO2 thick-films, Appl. Surf. Sci. 428 (2018) 207–217.
EP
[5] Q. Wang, C. Wang, H. Sun, P. Sun, Y. Wang, J. Lin, G. Lu, Microwave assisted
AC C
synthesis of hierarchical Pd/SnO2 nanostructures for CO gas sensor, Sens. Actuators B 222 (2016) 257–263.
[6] S. Ghosh, M. Narjinary, A. Sen, R. Bandyopadhyay, S. Roy, Fast detection of low concentration carbon monoxide using calcium-loaded tin oxide sensors, Sens. Actuators B 203 (2014) 490–496.
Page 20 of 44
ACCEPTED MANUSCRIPT [7] T. Hyodo, T. Goto, T. Ueda, K. Kaneyasu, Y. Shimizu, Potentiometric carbon monoxide sensors using an anion-conducting polymer electrolyte and Au-loaded SnO2 electrodes, J. Electrochem. Soc. 163 (2016) B300–B308.
RI PT
[8] L. Zhu, Y. Zheng, J. Jian, Effect of palladium oxide electrode on potentiometric sensor response to carbon monoxide, Ionics 21 (2015) 2919–2926.
[9] S.S.S. Ahmadi, B. Raissi, R. Riahifar, M.S. Yaghmaee, H.R. Saadati, S.
SC
Ghashghaie, M. Javaheri, Fabrication of counter electrode of electrochemical CO
(2015) D3101–D3108.
M AN U
gas sensor by electrophoretic deposition of MWCNT, J. Electrochem. Soc. 162
[10] A. Paliwal, A. Sharma, M. Tomar, V. Gupta, Carbon monoxide (CO) optical gas sensor based on ZnO thin films, Sens. Actuators B 250 (2017) 679–685.
TE D
[11] T. Goto, T. Itoh, T. Akamatsu, N. Izu, W. Shin, CO sensing properties of Au/SnO2-Co3O4 catalysts on a micro thermoelectric gas sensor, Sens. Actuators B 223 (2016) 774–783.
EP
[12] D. Zhang, Y. Sun, C. Jiang, Y. Yao, D. Wang, Room-temperature highly
AC C
sensitive CO gas sensor based on Ag-loaded zinc oxide/molybdenum disulfide ternary nanocomposite and its sensing properties, Sens. Actuators B 253 (2017) 1120–1128.
[13] S. Kim, J. Yoon, J. Park, The effects of excess carbon on the surface area and crystallinity of TiO2 powders prepared by oxidizing mechanically-synthesized TiC and their CO gas-sensing properties, Ceram. Int. 43 (2017) 15306-15315.
Page 21 of 44
ACCEPTED MANUSCRIPT [14] J. Jonca, A. Ryzhikov, S. Palussiere, Organometallic synthesis of CuO nanoparticles: application in low-temperature CO detection, Chem. Phys. Chem. 18 (2017) 2658-2665.
RI PT
[15] Q. Zhou, L.N. Xu, A. Umar, W.G. Chen, R. Kumar, Pt nanoparticles decorated SnO2 nanoneedles for efficient CO gas sensing applications, Sens. Actuators B 256 (2018) 656-664.
SC
[16] S.T. Tan, C.H. Tan, W.Y. Chong, C.C. Yap, A.A. Umar, R.T. Ginting, H.B. Lee,
of
highly
crystalline
M AN U
K.S. Lim, M. Yahaya, M.M. Salleh, Microwave-assisted hydrolysis preparation ZnO
nanorod
array
for
room
temperature
photoluminescence-based CO gas sensor, Sens. Actuators B 227 (2016) 304-312. [17] A. Shanmugasundaram, V. Gundimeda, T.F. Hou, D.W. Lee, Realizing synergy
TE D
between In2O3 nanocubes and nitrogen-doped reduced graphene oxide: an excellent nanocomposite for the selective and sensitive detection of CO at ambient temperatures, ACS Appl. Mater. Inter. 9 (2017) 31728-31740.
EP
[18] D. Zhang, J. Wu, P. Li, Y. Cao, Room-temperature SO2 gas-sensing properties
AC C
based on a metal-doped MoS2 nanoflower: an experimental and density functional theory investigation, ACS Appl. Mater. Inter. 5 (2017) 20666-20677.
[19] Z. Yin, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen, H. Zhang, Single-layer MoS2 phototransistors, ACS Nano 6 (2012) 74-80.
[20] Y. Tong, Y. Liu, Y.S. Zhao, J. Thong, D.S.H. Chan, C.X. Zhu, Selectivity of MoS2 gas sensors based on a time constant spectrum method, Sen. Actuators A 255 (2017) 28-33. Page 22 of 44
ACCEPTED MANUSCRIPT [21] H. Peng, J. Lu, C.X. Wu, Z.X. Yang, H. Chen, W.J. Song, P.Q. Li, H.Z. Yin, Co-doped MoS2 NPs with matched energy band and low overpotential high efficiently convert CO2 to methanol, Appl. Surf. Sci. 353 (2015) 1003-1012.
RI PT
[22] Y.H Zhang, J.L. Chen, L.J. Yue, H.L. Zhang, F. Li, Tuning CO sensing properties and magnetism of MoS2 monolayer through anchoring transition metal dopants, Comput. Theor. Chem. 1104 (2017) 12-17.
SC
[23] M. Acerce, D. Voiry, M. Chhowalla, Metallic 1T phase MoS2 nanosheets as
M AN U
supercapacitor electrode materials, Nat. Nanotechnol. 10 (2015) 313–318. [24] J. Yoon, T. Lee, B.G. Bapurao, J. Jo, B.K. Oh, J.W. Choi, Electrochemical H2O2 biosensor composed of myoglobin on MoS2 nanoparticle-graphene oxide hybrid structure, Biosens. Bioelectron. 93 (2017) 14–20.
TE D
[25] D. Zhang, C. Jiang, Y. Sun, Room-temperature high-performance ammonia gas sensor based on layer-by-layer self-assembled molybdenum disulfide/zinc oxide nanocomposite film, J. Alloys Compd. 698 (2017) 476-483.
EP
[26] M.Q. Wen, T. Xiong, Z.G. Zang, W. Wei, X.T. Tang, F. Dong, Synthesis of
AC C
MoS2/g-C3N4 nanocomposites with enhanced visible-light photocatalytic activity for the removal of nitric oxide (NO), Opt. Express 24 (2016) 10205-10212.
[27] Y.J. Liu, L.Z. Hao, W. Gao, Z.P. Wu, Y.L. Lin, G.X. Li, W.Y. Guo, L.Q. Yu, H.Z. Zeng,J. Zhu, W.L. Zhang, Hydrogen gas sensing properties of MoS2/Si
heterojunction, Sens. Actuators B 211 (2015) 537-543.
Page 23 of 44
ACCEPTED MANUSCRIPT [28] M. Donarelli, S. Prezioso, F. Perrozzi, F. Bisti, M. Nardone, L. Giancaterini, C. Cantalini, L. Ottaviano, Response to NO2 and other gases of resistive chemically exfoliated MoS2-based gas sensors, Sens. Actuators B 207 (2015) 602-613.
RI PT
[29] Y. Niu, R.G. Wang, W.C. Jiao, G.M. Ding, L.F. Hao, F. Yang, X.D. He, MoS2 graphene fiber based gas sensing devices, Sens. Actuators B 211 (2015) 537-543. [30] S.J. Ray, First-principles study of MoS2, phosphorene and graphene based single
SC
electron transistor for gas sensing applications, Sens. Actuators B 222 (2016)
M AN U
492–498.
[31] D. Zhang, Y. Sun, P. Li, Y. Zhang, Facile fabrication of MoS2-modified SnO2 hybrid nanocomposite for ultrasensitive humidity sensing, ACS. Appl. Mater. Interfaces 8 (2016) 14142-14149.
TE D
[32] C. Kuru, C. Choi, A. Kargar, D. Choi, Y.J. Kim, C.H. Liu, S. Yavuz, S. Jin, MoS2 Nanosheet–Pd nanoparticle composite for highly sensitive room tmperature detection of hydrogen, Adv. Sci. 2 (2015) 1500004.
EP
[33] Z. Lu, Y.Q. Gong, X.J. Li, Y. Zhang, MoS2-modified ZnO quantum dots
AC C
nanocomposite: Synthesis and ultrafast humidity response, Appl. Surf. Sci. 399 (2017) 330-336.
[34] W.R. Li, H.H. Xu, T. Zhai, H.Q. Yu, Z.R. Chen, Z.W. Qiu, X.P. Song, J.Q. Wang, B.Q.
Cao,
Enhanced
triethylamine
sensing
properties
by
designing
Au@SnO2/MoS2 nanostructure directly on alumina tubes, Sens. Actuators B 253 (2017): 97-107.
Page 24 of 44
ACCEPTED MANUSCRIPT [35] Z.G. Zang, M.Q. Wen, W.W. Chen, Y.F. Zeng, Z.Q. Zu, X.F. Zeng, X.S. Tang, Strong yellow emission of ZnO hollow nanospheres fabricated using polystyrene spheres as templates, Mater. Design 84 (2015) 418-421.
RI PT
[36] Z. Zang, X. Tang, Enhanced fluorescence imaging performance of hydrophobic colloidal ZnO nanoparticles by a facile method, J. Alloy. Compd. 619 (2015) 98-101.
SC
[37] K. Mahmood, A. Khalid, F. Nawaz, M.T. Mehran, Low-temperature
M AN U
electrospray-processed SnO2 nanosheets as an electron transporting layer for stable and high-efficiency perovskite solar cells, J. Colloid Interf. SCI. 532 (2018) 387-394.
[38] A. Ziarati, A. Badiei, R. Luque, Black hollow TiO2 nanocubes: Advanced
TE D
nanoarchitectures for efficient visible light photocatalytic applications, J. Alloy. Compd. 238 (2018) 177-183.
[39] D. Zhang, C. Jiang, P. Li, Y. Sun, Layer-by-layer self-assembly of Co3O4 MoS2
EP
nanorod-decorated
nanosheet-based
nanocomposite
toward
AC C
high-performance ammonia detection, ACS. Appl. Mater. Inter. 9 (2017) 6462-6471.
[40] X.H. Zhang, X.H. Huang, M.Q. Xue, X. Ye, W.N. Lei, H. Tang, C.S. Li, Hydrothermal
synthesis
and
characterization
microspheres, Mater. Lett. 148 (2015) 67-70.
Page 25 of 44
of 3D flower-like
MoS2
ACCEPTED MANUSCRIPT [41] J.L. Shen, F. Li, B. Yin, L. Sun, C. Chen, S.P. Wen, Y. Chen, S.P. Ruan, Enhanced ethyl acetate sensing performance of Al-doped In2O3 microcubes, Sens. Actuators B 253 (2017) 461-469.
RI PT
[42] Y. Liu, Y.H. Lin, J.L. Lan, Effect of transition-metal cobalt doping on the thermoelectric performance of In2O3 ceramics, J. Am. Cream. Soc. 93 (2010) 2938-2941.
SC
[43] D.D. Wei, Z.S. Huang, L.W. Wang, X.H. Chuai, S.M. Zhang, G.Y. Lu,
M AN U
Hydrothermal synthesis of Ce-doped hierarchical flower-like In2O3 microspheres and their excellent gas-sensing properties, Sens. Actuators B 255 (2018) 1211-1219.
[44] N. Chen, Y.X. Li, D.Y. Deng, X. Liu, X.X. Xing, X.C. Xiao, Y.D. Wang,
TE D
Acetone sensing performances based on nanoporous TiO2 synthesized by a facile hydrothermal method, Sens. Actuators B 238 (2017) 491-500. [45] Z.F. Liu, H.L. Xing, L. Lin, Facial synthesized Co-doped SnO2@multi-walled
EP
carbon nanotubes as an efficient microwave absorber in high frequency range,
AC C
Nano 12 (2017) 9-19.
[46] D. Zhang, H. Chang, Y. Sun, C. Jiang, Y. Yao, Y. Zhang, Fabrication of platinum-loaded cobalt oxide/molybdenum disulfide nanocomposite toward methane gas sensing at low temperature, Sens. Actuators B 252 (2017) 624-632.
[47] X.Q. Qiao, F.C. Hu, D.F. Hou, D.S. Li, PEG assisted hydrothermal synthesis of hierarchical MoS2 microspheres with excellent adsorption behavior, Mater. Lett. 169 (2016) 241-245. Page 26 of 44
ACCEPTED MANUSCRIPT [48] X.J. Liu, L. Jiang, X.M. Jiang, X.Y. Tian, X. Sun, Y.L. Wang, W.D. He, P.Y. Hou, X.L. Deng, X.J. Xu, Synthesis of Ce-doped In2O3 nanostructure for gas sensor applications, Appl. Surf. Sci. 428 (2018) 478-484.
RI PT
[49] A. Shokri, N. Salami, Gas sensor based on MoS2 monolayer, Sens. Actuators B 236 (2016) 378-385
[50] D. Zhang, C. Jiang, J. Liu, Y. Cao, Carbon monoxide gas sensing at room
SC
temperature using copper oxide-decorated graphene hybrid nanocomposite prepared by layer-by-layer self-assembly, Sens. Actuators B 247 (2017) 875-882.
M AN U
[51] R.K. Joshi, J.E. Weber, Q. Hu, B. Johnson, J.W. Zimmer, A. Kumar, Carbon monoxide sensing at room temperature via electron donation in boron doped diamond films, Sens. Actuators B 145 (2010) 527-532.
[52] D. Kim, P.V. Pikhitsa, H. Yang, M. Choi, Room temperature CO and H2 sensing
TE D
with carbon nanoparticles, Nanotechnology 22 (2011) 485501-485507. [53] M. Shojaee, Sh. Nasresfahani, M.H. Sheikhi, Hydrothermally synthesized Pd-loaded SnO2/partially reduced graphene oxide nanocomposite for effective
457-467.
EP
detection of carbon monoxide at room temperature, Sens. Actuators B 254 (2018)
AC C
[54] Y. Liu, Y. Jiao, Z. Zhang, F. Qu, A. Umar, X. Wu, Hierarchical SnO2 nanostructures made of intermingled ultrathin nanosheets for environmental remediation smart gas sensor, and super capacitor applications, ACS Appl. Mater. Interfaces 6 (2014) 2174-2184. [55] J.Y. Ye, C.J. Liu, Q.F. Ge, DFT Study of CO2 Adsorption and hydrogenation on the In2O3 surface, J. Phys. Chem. C 116 (2012) 7817-7825.
Page 27 of 44
ACCEPTED MANUSCRIPT [56] S. Lin, D.Q. Xie, Initial decomposition of methanol and water on In2O3(110): a periodic DFT study, Chin. J. Chem. 30 (2012) 2036-2040. [57] J.Y. Ye, C.J. Liu, D.H. Mei, Q.F. Ge, Active oxygen vacancy site for methanol
RI PT
synthesis from CO2 hydrogenation on In2O3(110): a DFT Study, ACS Catal. 3 (2013) 1296-1306.
[58] K.N. Ding, Y.H. Lin, M.Y. Huang, The enhancement of NO detection by doping
SC
strategies on monolayer MoS2, Vacuum 130 (2016) 146-153.
M AN U
[59] J. Wu, D. Zhang, Y. Cao, Fabrication of iron-doped titanium dio xide quantum dots/molybdenum disulfide nanoflower for ethanol gas sensing, J. Alloy. Compd. 529 (2018) 556-567.
[60] R.Q. Xing, L. Xu, J. Song, C.Y. Zhou, Q.L. Li, D.L. Liu, H.W. Song, Preparation
TE D
and gas sensing properties of In2O3/Au nanorods for detection of volatile organic compounds in exhaled breath, Sci. Rep. 5 (2015) 10717. [61] J. Hu, Y.J. Sun, Y. Xue, M. Zhang, P.W. Li, K. Lian, S. Zhuiykov, W.D. Zhang,
EP
Y. Chen, Highly sensitive and ultra-fast gas sensor based on CeO2-loaded In2O3
AC C
hollow spheres for ppb-level hydrogen detection, Sens. Actuators B 257 (2018) 124-135.
Page 28 of 44
ACCEPTED MANUSCRIPT Table 1. Sensing performance of the CO sensor presented in this work compared with previous works.
Fabrication method
Response (%)
Refs.
Ag-ZnO/MoS2
LbL self-assembly
4.8 (100 ppm)
[12]
CuO/rGO
LbL self-assembly
5.4 (100 ppm)
[50]
B-diamond
Hot filament CVD method
5.5 (100 ppm)
[51]
Acid-SCNPs
Ion-induced focusing deposition
Pd-SnO2/PRGO
Hydrothermal method
Co-In2O3/MoS2
LbL self-assembly
SC
RI PT
Sensing material
[52]
3.8 (400 ppm)
[53]
M AN U
7.8 (100 ppm)
AC C
EP
TE D
8.6 (50 ppm)
Page 29 of 44
This work
ACCEPTED MANUSCRIPT Table 2. The parameters of Co-In2O3 (110) and pristine In2O3 (110) CO adsorption systems. Adsorption
Bond length (Å)
Distance
systems
C—O
(Å)
Co-In2O3
1.150
In2O3 CO
Eg (eV)
Spin (µB)
CT (e)
1.799
-1.34
0.426
0
-0.386
1.144
2.540
-0.61
0.833
0
-0.13
1.146
—
—
—
—
—
AC C
EP
TE D
M AN U
SC
RI PT
Ead (eV)
Page 30 of 44
ACCEPTED MANUSCRIPT
Figure captions Figure 1. The schematic diagram of (a) preparation route for Co-In2O3/MoS2 sensor, and (b) gas-sensing experimental setup.
RI PT
Figure 2. (a) XRD of Co-In2O3/MoS2, Co-In2O3, In2O3, and MoS2 samples. (b) EDS spectrum of Co-In2O3/MoS2.
Figure 3. XPS spectra of Co-In2O3/MoS2: (a) survey spectrum, (b) In 3d, (c) O 1s, (d)
SC
Co 2p, (e) Mo 3d, and (f) S 2p.
M AN U
Figure 4. SEM characterizations of (a) MoS2, (b) Co-In2O3, (c) and (d) Co-In2O3/MoS2. (e) Cross-sectional image of Co-In2O3/MoS2 film on the substrate, (f) cross-sectional EDS spectrum of Co-In2O3/MoS2 film.
Figure 5. (a) and (b) TEM images of Co-In2O3/MoS2. (c) and (d) HRTEM images of
TE D
Co-In2O3/MoS2.
Figure 6. (a) Time-dependent response and (b) fitting curve of Co-In2O3/MoS2, Co-In2O3, and pure In2O3 sensors exposed to different concentrations of CO.
EP
Figure 7. The response-recovery properties of Co-In2O3/MoS2, Co-In2O3, and pure
AC C
In2O3 film sensors toward 10 ppm and 1000 ppm CO. Figure 8. (a) Repeatability, and (b) long term stability of the Co-In2O3/MoS2 gas sensor exposed to 1000 ppm, 100 ppm, and 10 ppm CO. Figure 9. Illustration of sensing-mechanism for the Co-In2O3/MoS2 sensor toward CO gas. Figure 10. CO adsorption models of (a) Co-In2O3 and (b) pristine In2O3, and corresponding charge density diagram of (c) Co-In2O3 and (d) pristine In2O3. Page 31 of 44
ACCEPTED MANUSCRIPT Figure 11. The DOSs for the Co-In2O3 (a) before CO adsorbed, and (b) after CO adsorbed, and pristine In2O3 (c) before CO adsorbed, and (d) after CO adsorbed. Figure 12. The energy-band structure of Co-In2O3/MoS2 sensor (EC1 and EC2, Eg1 and
RI PT
Eg2, Ev1 and Ev2, and Ef are the bottom of conduction band, band gap, top of valence band, and the Fermi-energy level of MoS2 and Co-In2O3, respectively. E1 and E2 are
AC C
EP
TE D
M AN U
Fermi-energy level of Co-In2O3/MoS2 sensor).
SC
potential barrier before and after sensor exposed to CO, respectively. Ef is the
Page 32 of 44
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
EP
Figure 1. The schematic diagram of (a) preparation route for Co-In2O3/MoS2 sensor,
AC C
and (b) gas-sensing experimental setup.
Page 33 of 44
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 2. (a) XRD of Co-In2O3/MoS2, Co-In2O3, In2O3, and MoS2 samples. (b) EDS spectrum of Co-In2O3/MoS2.
Page 34 of 44
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 3. XPS spectra of Co-In2O3/MoS2: (a) survey spectrum, (b) In 3d, (c) O 1s, (d) Co 2p, (e) Mo 3d, and (f) S 2p.
Page 35 of 44
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
Figure 4. SEM characterizations of (a) MoS2, (b) Co-In2O3, (c) and (d) Co-In2O3/MoS2. (e) Cross-sectional image of Co-In2O3/MoS2 film on the substrate, (f) cross-sectional EDS spectrum of Co-In2O3/MoS2 film.
Page 36 of 44
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
Co-In2O3/MoS2.
TE D
Figure 5. (a) and (b) TEM images of Co-In2O3/MoS2. (c) and (d) HRTEM images of
Page 37 of 44
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 6. (a) Time-dependent response and (b) fitting curve of Co-In2O3/MoS2, Co-In2O3, and pure In2O3 sensors exposed to different concentrations of CO.
Page 38 of 44
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 7. The response-recovery properties of Co-In2O3/MoS2, Co-In2O3, and pure
AC C
EP
TE D
In2O3 film sensors toward 10 ppm and 1000 ppm CO.
Page 39 of 44
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 8. (a) Repeatability, and (b) long term stability of the Co-In2O3/MoS2 gas sensor exposed to 1000 ppm, 100 ppm, and 10 ppm CO.
Page 40 of 44
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Figure 9. Illustration of sensing-mechanism for the Co-In2O3/MoS2 sensor toward CO
AC C
EP
TE D
gas.
Page 41 of 44
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 10. CO adsorption models of (a) Co-In2O3 and (b) pristine In2O3, and
AC C
EP
corresponding charge density diagram of (c) Co-In2O3 and (d) pristine In2O3.
Page 42 of 44
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 11. The DOSs for the Co-In2O3 (a) before CO adsorbed, and (b) after CO
AC C
EP
TE D
adsorbed, and pristine In2O3 (c) before CO adsorbed, and (d) after CO adsorbed.
Page 43 of 44
RI PT
ACCEPTED MANUSCRIPT
Figure 12. The energy-band structure of Co-In2O3/MoS2 sensor (EC1 and EC2, Eg1 and
SC
Eg2, Ev1 and Ev2, and Ef are the bottom of conduction band, band gap, top of valence
M AN U
band, and the Fermi-energy level of MoS2 and Co-In2O3, respectively. E1 and E2 are potential barrier before and after sensor exposed to CO, respectively. Ef is the
AC C
EP
TE D
Fermi-energy level of Co-In2O3/MoS2 sensor).
Page 44 of 44
ACCEPTED MANUSCRIPT
Research highlights 1. Co-In2O3/MoS2 nanocomposite was fabricated by layer-by-layer self-assembly
RI PT
technique. 2. CO gas sensing properties of Co-In2O3/MoS2 nanocomposite film sensor was investigated.
SC
3. The sensing mechanism of Co-In2O3/MoS2 sensor was discussed by combining
AC C
EP
TE D
M AN U
experiments with DFT simulation.
S0925-8388(18)34064-7
DOI:
https://doi.org/10.1016/j.jallcom.2018.10.365
Reference:
JALCOM 48183
To appear in:
Journal of Alloys and Compounds
Received Date: 27 July 2018 Revised Date:
2 October 2018
Accepted Date: 27 October 2018
Please cite this article as: D. Zhang, J. Wu, Y. Cao, Fabrication cobalt-doped indium oxide/molybdenum disulfide ternary nanocomposite toward carbon monoxide gas sensing, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.10.365. 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.
ACCEPTED MANUSCRIPT
Fabrication cobalt-doped indium oxide/molybdenum disulfide ternary nanocomposite toward carbon monoxide
RI PT
gas sensing
SC
Dongzhi Zhang*, Junfeng Wu, Yuhua Cao
Key Laboratory of Unconventional Oil & Gas Development, Ministry of Education,
China), Qingdao 266580, China
M AN U
College of Information and Control Engineering, China University of Petroleum (East
TE D
*Corresponding authors: Dongzhi Zhang E-mail address: [email protected]
EP
Tel: +86-532-86982928
AC C
Fax: +86-532-86983326
Page 1 of 44
ACCEPTED MANUSCRIPT Abstract This paper demonstrated a high-performance carbon monoxide (CO) gas sensor based on cobalt (Co)-doped indium oxide (In2O3) nanoparticles/molybdenum disulfide
RI PT
(MoS2) nanoflowers nanocomposite. Co-In2O3 nanoparticles were synthesized by a co-precipitation method, and flower-like MoS2 was prepared by one-step hydrothermal route. Layer-by-layer self-assembly technique was employed to
SC
fabricate Co-In2O3/MoS2 film sensor on an epoxy substrate with interdigital electrodes. Scanning electron microscopy (SEM), transmission electron microscope
M AN U
(TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were carried out to fully examine the morphology, microstructure, and elementary composition of the as-prepared samples. The
TE D
CO-sensing characteristics of the Co-In2O3/MoS2 film sensor were systematically investigated under room temperature through exposing the sensor to various concentration of CO gas. The Co-In2O3/MoS2 sensor achieved high sensitivity, fast
EP
response/recovery speed, excellent repeatability and stable long-term stability. An approach of combining gas-sensing experiments with density-functional theory (DFT)
AC C
simulation based on first-principle was used to further explore the CO-sensing mechanism of the Co-In2O3/MoS2 sensor. The Co2+ ion doping, and heterojunctions created at interfaces of Co-In2O3 and MoS2 were attributed to the high-performance CO sensing. Keywords: molybdenum disulfide; Co-doped indium oxide; CO gas sensor; LbL self-assembly; first-principle theory
Page 2 of 44
ACCEPTED MANUSCRIPT 1. Introduction Carbon monoxide (CO) is an odorless, colorless, poisonous and hazardous gas mostly produced from the incomplete combustion of fossil fuels [1]. Owing to the fact
RI PT
that CO molecules can easily bond with the hemoglobin, preventing its combination to oxygen molecules, exposure to an excess amount of CO may result in the acute and chronic injury to human health, and there is even a high possibility of death [2]. US
SC
Occupational Safety and Health Administration (OSHA) suggests that the permissible
M AN U
exposure limit for CO is 35 ppm (10 hour ceiling limit), while the US National Institute for Occupational Safety and Health (NIOSH) recommends a limit of 50 ppm (8 hour ceiling limit) [3]. Hence, a stable and high performance CO sensor for the low concentration is extremely significant and necessary in numerous fields, such as
TE D
vehicle emissions, indoor environment, industrial waste gas discharge, and human breath [4]. Currently, various detection approaches, including resistive [5, 6], potentiometric [7, 8], electrochemical [9], optical fibre [10], and thermoelectric
EP
method [11], have been used to develop CO gas sensor, yet the problems of high-cost,
AC C
operation complexity, and time-consuming limit their extensive application [12]. Fortunately, gas sensors based on nanomaterials can play the roles effectively in gas sensing due to the merits of remarkable physical-chemical stability, low power-consuming, and excellent sensing performance. Thereinto, metal oxides (MOs), such as TiO2 [13], CuO [14], SnO2 [15], ZnO [16], and In2O3 [17], were most commonly applied in CO gas detection. However, most metal oxides based gas sensors exhibit good sensing-characteristics only above 200oC with the side heater, Page 3 of 44
ACCEPTED MANUSCRIPT leading to the high energy consumption and complicated operating condition [18]. Recently, considerable research has been devoted to the development of hand-held and mobile devices, and hence the fabrication of CO sensors working at
RI PT
room-temperature (RT) is highly desirable for these applications. Molybdenum disulfide (MoS2), as a typical two-dimensional (2D) layered transition metal dichalcogenide, have attracted increasing attention due to its physical-chemistry characteristics properties, such as
SC
dramatically fabulous
M AN U
significant direct band gap (1.8 eV) [19], large surface-to-volume ratio [20], outstanding carrier mobility [21], and distinctive electronic properties [22]. Therefore, MoS2 have been focused as an emerging building block for various potential applications, such as supercapacitor, photocatalyst, and especially gas sensors [23-25].
TE D
For example, Wen et al. prepared MoS2/g-C3N4 nanocomposite by a facile ultrasonic dispersion method and exhibited excellent photocatalytic activity, which is much higher than that of pure g-C3N4 nanoparticles [26]. Up to now, multitudinous RT gas
EP
sensors based on MoS2 have been fabricated, such as H2, NO2, NH3, ethanol, and
AC C
humidity sensors [27-31]. Kuru et al. fabricated the H2 gas sensors based on Pd-doped MoS2 and Pd-doped graphene, respectively, and the measurement results shown that the H2 sensing performance of Pd-MoS2 sensor was better than Pd-graphene [32]. Lu et al. synthesized ZnO/MoS2 nanocomposite by hydrothermal route, which shown an excellent sensitivity toward humidity and the sensing properties got a great enhancement compared to the pristine MoS2 and ZnO sensors [33]. Li et al. developed Au-SnO2/MoS2 sensor via a facile hydrothermal method, indicating a better sensing Page 4 of 44
ACCEPTED MANUSCRIPT performance to triethylamine than SnO2/MoS2 and MoS2 sensors, respectively [34]. The previous research achievements illustrate that the transition metal ions doping, and metal oxides decoration with MoS2 can enhance the gas-sensing properties
RI PT
effectively [32, 33]. In this work, a Co-In2O3 nanoparticles/MoS2 nanoflower (Co-In2O3/MoS2) CO gas sensor was developed through layer-by-layer (LbL) self-assembly technique.
SC
XRD, EDS, SEM, TEM, and XPS were conducted to fully inspect the morphology,
M AN U
microstructure, and composition of as-prepared samples. The CO sensing characteristics of the presented Co-In2O3/MoS2 sensor were determined under room temperature. The Co-In2O3/MoS2 film sensor exhibited significantly enhanced sensing performances toward CO, including high sensitivity, fast response/recovery speed,
TE D
distinctive repeatability, and good long-term stability. Finally, a comprehensive research approach supported with gas-sensing experiments and first-principle-based DFT simulations was performed for the exploration of the possible CO-sensing
EP
mechanism of the Co-In2O3/MoS2 sensor.
AC C
2. Experiment
2.1 Materials synthesis
Reagents used in our work, namely In(NO3)3·4.5H2O (>99.5%), NH3·H2O (25%
~28%), Co(NO3)2·6H2O (>98.5%), Na2MoO4·2H2O (>99%), and thioacetamide (>99%), were all supplied from Chinese Medicine Group Chemical Reagent Co. Ltd (Shanghai, China). All reagents were used without further pretreatment. The reagents used for LbL self-assembly, namely 1.5 wt% Poly (diallyldimethylammonium Page 5 of 44
ACCEPTED MANUSCRIPT chloride) (PDDA) and 0.3 wt% poly(styrene sulfonate) (PSS), were blended with NaCl to 0.5 M for increasing the ionic strength. A co-precipitation method was used to synthesize Co-doped In2O3 nanoparticles.
RI PT
7.64 g In(NO3)3·4.5H2O and 0.17 g Co(NO3)2·6H2O were dissolved in 100 mL deionized (DI) water. Next, NH3·H2O was dripped into the above solution to adjust the pH value to the range of 9-10. After washing with ethanol and DI water for
SC
multi-times, the obtained precipitate was dried at 60oC for 8 h. The resulting powder
M AN U
was calcined at 500oC for 3 h to obtain Co-doped In2O3 nanoparticles. Furthermore, pristine In2O3 was synthesized using the similar approach as Co-In2O3 in the absence of Co(NO3)2·6H2O.
To synthesize the flower-like MoS2, an easy-operated one-step hydrothermal
TE D
route was used, which is a frequently-used method for synthesis of nanomaterial, such as ZnO [35-36], SnO2 [37], and TiO2 [38]. 1.21 g Na2MoO4·2H2O and 1.90 g thioacetamide (1.90 g) were dissolved in 60 mL mixed solvent formed from 30 mL DI
EP
water and 30 mL ethanol. Then, the obtained solution was stirred and ultrasonicated
AC C
for 15 min, respectively. The obtained solution was heated at 200oC for 48 h in a 100 mL stainless-steel autoclave with a Teflon liner. The precipitate was washed with ethanol and DI water for multi-times, and it was dried at 60°C for 8 h. Finally, the black powder was collected after calcination under 700°C in nitrogen for 2 h. The growth of MoS2 crystal can be divided into three processes of nucleus growth, MoS2 nano-rods growth and self-assembly. Under hydrothermal conditions, the sulfur source of thioacetamide is facile to coordinate with molybdate since the Page 6 of 44
ACCEPTED MANUSCRIPT reductive functional group (–NH2) in sulfur source can be combined with oxygen atoms of molybdate easily, which is effective to the produce of MoS2 nuclei. Small MoS2 particles are generated by the reaction between the residual molybdenum and
RI PT
sulfur, which can be considered as the formation of nucleus. With the reaction continues, the MoS2 nanorod structures are crookedly grown from the nucleus because of localized stacking and lattice defects. After that, the self-assembly process
SC
is performed by the intrinsic crystal orientation and flower-like porous microsphere
2.2 Sensor Fabrication
M AN U
structure of the MoS2 crystal forms.
A flame resistant (FR-4) material was used as sensor substrate, and a pair of Ni/Cu interdigital electrodes (IDEs) was patterned on the FR-4 substrate. The layer-by-layer (LbL)
TE D
Co-In2O3/MoS2 sensing film was fabricated through
self-assembly method, which is operated via alternative immersion in oppositely charged solutions at room temperature [39]. The typical fabrication process was
EP
shown in Figure 1(a). Firstly, Co-In2O3 and MoS2 suspensions are prepared and
AC C
ultrasonic treated for 15 min, respectively. To enhance the surface electrostatic force, two bi-layers of PDDA/PSS were firstly deposited as precursor layer. After that, five bi-layers of Co-In2O3 and MoS2 were assembled as sensing layer. The deposition time was 10 min for PDDA/PSS layer and 15 min for Co-In2O3/MoS2 layer. Due to the interaction of electrostatic force between two different nanomaterials, each layer mutually have a good connection. After each immersion, the sensor was rinsed with
Page 7 of 44
ACCEPTED MANUSCRIPT DI water and dried with nitrogen to strengthen the interconnection of each layers. The Co-In2O3 and In2O3 gas sensors were fabricated to make a comparison. 2.3 Instrument and analysis
RI PT
The crystallographic structures of In2O3, Co-In2O3, MoS2, and Co-In2O3/MoS2 were inspected by the XRD measurement equipped with an X-ray diffractometer (X’Pert Pro MPD) using Cu Kα radiation (λ = 1.5418 Å). The diffraction angle was
SC
scanned in the range of 10°-70°. The element composition was analyzed by EDS
M AN U
using Hitachi S-4800. The XPS spectra were further measured via an XPS spectrometer (Thermo Scientific Escalab 250Xi). The surface morphologies of Co-In2O3, MoS2, and Co-In2O3/MoS2 were observed using a field-emission scanning electron microscope (Hitachi S-4800). The microstructure was determined via a
TE D
transmission electron microscope (JEOL JEM-2100).
In this work, all gas sensing measurements for as-prepared sensors were carried out at room temperature. The schematic illustration of the experimental setup is
EP
shown in Figure 1(b). The desired concentration of CO was obtained via injecting a
AC C
precalculated quantity of CO gas into the testing chamber using a syringe. Air was used as the baseline gas. The resistance of the sensor was detected by a data logger (Agilent 34970A), which was linked with a PC by RS-232 interface. The normalized response, a parameter to evaluate the gas sensing performance, was determined by R=|△R|/Rair×100% =|Rair-Rgas|/Rair×100%, where Rair and Rgag are the sensor resistance in air and CO gas, respectively. 3. Results and discussion Page 8 of 44
ACCEPTED MANUSCRIPT 3.1 Materials characterizations The XRD spectra of In2O3, Co-In2O3, MoS2, and Co-In2O3/MoS2 are shown in Figure 2(a). No peak of impurity crystal phases is exhibited in the XRD.. Several
RI PT
prominent diffraction peaks at 2θ of 14.4°, 33.1°, 39.7°, and 58.5° are presented in the spectrum of MoS2, attributing to the (002), (100), (103) and (110) planes of hexagonal MoS2 nanocrystals (JCPDS No. 37-1492) [40]. For both undoped and Co-doped In2O3,
SC
all peaks could be well resembled with the single phase of In2O3 (JCPDS 06-416) [41].
M AN U
Owing to the similar size of Co2+ (0.72Å) and In3+ (0.81Å) , Co2+ ions exhibit a high solubility in In2O3 crystalline structure, leading to a slight change of XRD pattern without diffraction peaks of Co compounds after Co2+ doping [42]. Furthermore, the main peaks corresponding to the presence of MoS2 and Co-In2O3 can be clearly
TE D
observed in the XRD of Co-In2O3/MoS2, which testifies the successful preparation of Co-In2O3/MoS2 nanocomposite. The EDS pattern of Co-In2O3/MoS2 is exhibited in Figure 2(b), indicating the element component of the experiment sample. It can be
EP
clearly illustrated the existence of Co, In, O, Mo, and S in the Co-In2O3/MoS2 sample.
AC C
XPS was used to further investigate the elemental compositions and chemical status of Co-In2O3/MoS2 nanocomposite. From the XPS survey spectrum plotted in Figure 3(a), characteristic peaks of all component elements including In, O, Co, Mo, and S can be obviously detected. In Figure 3(b), two peaks at 451.63 eV and 444.09 eV are corresponding to the characteristic spin-orbit split states of In 3d3/2 and In 3d5/2, which are attributed to the In3+ of In2O3 [43]. From XPS pattern of O 1s shown in Figure 3(c), the predominant peak at 529.54 eV is ascribed to the lattice oxygen Page 9 of 44
ACCEPTED MANUSCRIPT species, while the peak at 531.60 eV is associated with the adsorbed oxygen that makes great contribution to the gas-sensing performance [44]. Moreover, the concentrations of Olattice and Oads to O 1s are estimated to be 67.55% and 32.45% via
RI PT
calculating the area proportion of Olattice and Oads. As shown in Figure 3(d), two main peaks at 795.41 eV and 780.38 eV are corresponding to the Co 2p1/2 and Co 2p3/2, and two weak peaks at 802.27 eV and 787.39 are assigned to Co 2p1/2 and Co 2p3/2
SC
satellite structure [45]. From the XPS pattern of Mo 3d shown in Figure 3(e), two
M AN U
main peaks at 232.43 eV and 229.25 eV are attributed to Mo 3d3/2 and Mo 3d5/2 [46]. In Figure 3(f), two peaks of S 2p are located at 163.29 eV and 162.10 eV [47]. Figure 4 shows the micromorphology of MoS2, Co-In2O3, and Co-In2O3/MoS2 inspected via SEM. In Figure 4(a), it can be found that pristine MoS2 exhibits a
TE D
flower-nanosphere morphology composed of numerous wormlike nanorods. Figure 4(b) illustrates the appearance of aggregated Co-In2O3 nanoparticles. SEM image of Co-In2O3/MoS2 nanocomposite is plotted in Figure 4(c) and Figure 4(d), that
Co-In2O3
EP
demonstrating
nanoparticles
are
distributed
on
the
MoS2
AC C
flower-nanospheres. Figure 4(e) shows a cross-sectional image of Co-In2O3/MoS2 film. PDDA/PSS layer is deposited for the surface modification of substrate and facilitate the assembly of Co-In2O3/MoS2. The bi-layer PDDA/PSS is very thin, which is covered by the sensing layer of Co-In2O3/MoS2. Therefore, PDDA/PSS layer contributes little to the gas sensing. Furthermore, the cross-sectional EDS spectrum of Co-In2O3/MoS2 film is plotted in Figure 4(f), indicating that the main elements of Mo, S, In, Co and O, and weak peaks for trace elements of Na and Cl in PDDA/PSS layer. Page 10 of 44
ACCEPTED MANUSCRIPT The TEM images of Co-In2O3/MoS2 nanocomposite are employed to further analyze its nanostructure. Figure 5(a) and 5(b) exhibit the overall perspective of Co-In2O3/MoS2 nanocomposite. It clearly shows that the size of the nanorods is 60-80
RI PT
nm, and the black Co-In2O3 nanoparticles are distributed on the nanorods. Figure 5(c) and 5(d) depict the HRTEM images of Co-In2O3. It can be clearly observed that the lattice fringes of Co-In2O3 are 0.29 and 0.18 nm, which are corresponding well with
SC
(222) and (440) plane of In2O3 due to the high solubility of doping ions caused by the
M AN U
similar size of Co2+ and In3+ [48]. And the lattice spacing of (002) plane for MoS2 is 0.62 nm [49]. 3.2 CO gas sensing properties
To highlight the improvement for the CO sensing performance of
switching
TE D
Co-In2O3/MoS2 nanocomposite sensor, a contrast experiment was operated through Co-In2O3/MoS2,
Co-In2O3,
and
In2O3
sensors
among
different
concentrations of CO gas and air. The time interval of each switching was 120 s. The
EP
resulting time dependent response-recovery curves of Co-In2O3/MoS2, Co-In2O3, and
AC C
In2O3 are shown in Figure 6(a). It can be obviously found that Co-In2O3/MoS2 sensor exhibits the highest sensitivity among these three sensors, and Co-In2O3 sensor shows better sensitivity than pure In2O3 sensor. The experiment result indicates that doping with Co2+ and decoration of MoS2 both lead to an enhancement on the gas sensing property of Co-In2O3/MoS2. Figure 6(b) demonstrates the response values of the Co-In2O3/MoS2, Co-In2O3, and In2O3 as functions of CO concentrations. The fitting equations of the response Y and CO concentration X can be represented as Page 11 of 44
ACCEPTED MANUSCRIPT Y=4.55+2.56lgX, Y=2.21+1.36lgX, and Y=0.501+0.034lgX for Co-In2O3/MoS2, Co-In2O3, and In2O3, respectively, and the regression coefficient is 0.9885, 0.9699, and 0.8246, respectively. The result suggests that Co-In2O3/MoS2 sensor possesses the
RI PT
best sensitivity toward CO gas and the highest linearity with the common logarithm of CO concentration among the three sensors.
Figure 7 exhibits the response/recovery properties of Co-In2O3/MoS2, Co-In2O3,
SC
and In2O3 film sensors toward 10 ppm and 1000 ppm CO, respectively. The time
response/recovery
stage
is
M AN U
taken by a sensor to achieve 90% of the total resistance change in respective defined
as
the
response/recovery
time.
The
response/recovery times for the Co-In2O3/MoS2, Co-In2O3, and In2O3 film sensor are 39 s/81 s, 65 s/90 s, and 70 s/84 s toward 10 ppm CO gas, respectively, and 28 s/88 s,
TE D
62 s/ 106 s, and 77 s/ 123 s toward 1000 ppm CO gas, respectively. The experiment result indicates that the Co-In2O3/MoS2 nanocomposite sensor shows a shorter response/recovery time than Co-In2O3, and In2O3 film sensors, indicating an excellent
EP
response/recovery characteristic for Co-In2O3/MoS2 nanocomposite.
AC C
In conclusion, the results of above contrast experiment testify a fact of the great enhancement of CO sensing properties for Co-In2O3/MoS2 gas sensor through doping modification. Subsequent works are carried out to further investigate other CO-sensing properties of Co-In2O3/MoS2 gas sensor. Figure 8(a) illustrates the repeatability of Co-In2O3/MoS2 gas sensor exposed to 1000 ppm, 100 ppm, and 10 ppm CO gas. The experiment was operated for four response/recovery cycles at room temperature. The measurement results indicate that the Co-In2O3/MoS2 gas sensor can Page 12 of 44
ACCEPTED MANUSCRIPT maintain a remarkable reproducibility and consistency. Figure 8(b) illustrates the long-term stability of Co-In2O3/MoS2 gas sensor exposed to 1000 ppm, 100 ppm, and 10 ppm CO gas. The measurement was carried out every five days over a month. The
outstanding long-term stability of Co-In2O3/MoS2 gas sensor.
RI PT
response of the sensor shows little change with time passing by, verifying an
Table 1 lists the CO sensing properties of Co-In2O3/MoS2 in this work compared
SC
with previous published works [12, 50-53]. The comparison is made with the existing
M AN U
CO gas sensors based on similar state-of-the-art materials, which were fabricated using various methods including hot filament chemical vapor deposition (CVD), ion-induced focusing deposition, hydrothermal and LbL self-assembly technique. The comparison results demonstrate that the presented sensor exhibits a higher CO
TE D
response than the most existing room temperature CO sensors. 3.3 DFT simulation and CO sensing mechanism As above-mentioned results shown, the Co-In2O3/MoS2 film sensor has the
EP
remarkable gas sensing properties toward CO at room temperature. As is known to all,
AC C
the gas-sensing mechanism on the resistance change of semiconductor gas sensors is mainly ascribed to the adsorption-oxidation-desorption process [54]. The illustration of the gas-sensing mechanism for Co-In2O3/MoS2 sensor toward CO gas is shown in Figure 9. In air atmosphere, numerous oxygen molecules are adsorbed on the surface of Co-In2O3/MoS2, and the nanocomposite lost plenty of electrons to promote the ionization of Oads. Since Co-In2O3/MoS2 serves as n-type semiconductor and takes electrons as dominant charge carriers, an electron depletion layer creates on the Page 13 of 44
ACCEPTED MANUSCRIPT surface of Co-In2O3/MoS2, which results in an increase of sensor resistance in air. The reaction in this process can be described as Eq. (1) and (2). When the sensor upon exposure to the CO (reducing gas), a reaction as shown in Eq. (3) is conducted
RI PT
between the CO molecules and the adsorbed O2-(ads), which will make the nanocomposite recapture the electrons trapped by the adsorbed oxygen. As a consequence, the depletion layer will narrow down, inducing a decrease of sensor
SC
resistance.
M AN U
O2(gas) → O2(ads) O2(ads) + e-→O2-(ads)
2CO + O2-(ads) → 2CO2 + e-
(1) (2) (3)
The enhanced sensitivity of Co-In2O3/MoS2 can be attributed to the following
TE D
aspects. Firstly, the flower-like Co-In2O3/MoS2 composed of oriented worm-like nanorods has extremely large surface area yet a 3D co-continuity. This particular structure coupled with oxygen vacancies brings a large amount of active sites for the
EP
adsorption of Oads and CO molecules, which is one important reason why
AC C
Co-In2O3/MoS2 shows enhanced CO-sensing properties. Secondly, the doping of Co2+ ions in In2O3 through replacing In3+ to Co2+ can
generate more oxygen vacancies, which are helpful for the adsorption of oxygen and CO molecules. Moreover, an impurity energy level forms between the top of the valence band and the bottom of the conduction band due to the ion Co2+ doping, which serves as a bridge to promote the electron flow. In other words, more adsorbed
Page 14 of 44
ACCEPTED MANUSCRIPT oxygen ions will be created, leading to an improvement on the sensing performance of the Co-In2O3/MoS2 sensor. Furthermore, a density-functional theory (DFT) simulation using material studio
RI PT
software was carried out to theoretically verify the enhancement of CO sensitivity after Co2+ doping through analysis of the calculated geometry and energy parameters. The In2O3 (110) surface as one of the most stable surface has been frequently
SC
investigated in DFT simulation of gas adsorption on In2O3 [55-57]. Therefore, a gas
M AN U
adsorption model based on In2O3 (110) surface was constructed in this work. The calculated CO adsorption structures of Co-In2O3 and pristine In2O3 are shown in Figure 10(a) and (b), respectively. Table 2 lists the specific DFT calculation results of the adsorption systems including the bond length of C—O bond of CO molecule, the
TE D
shortest atomic distance between adsorbed CO molecule and the substrate, the adsorption energy, the band gap of the substrate, the spin polarization of the substrate, and the charge transfer from adsorbed CO molecule to the substrate, where negative
EP
value presents the electrons flow from CO molecule to the substrate. Thereinto, Ead
AC C
(adsorption energy) was calculated by the Eq. (4) [58, 59]: Ead = Egas/sub - Esub - Egas
(4)
where Egas/sub is the total energy of the whole adsorption system, Esub and Egas are the energies of isolated substrate and adsorbed gas molecule, respectively. From the geometry parameters including bond length and distance, it can be clearly found that CO molecule exhibits a greater change influenced by Co-In2O3 than pristine In2O3. The bond length of CO molecule which changes 0.04 Å after adsorption on Co-In2O3, Page 15 of 44
ACCEPTED MANUSCRIPT but 0.02 Å after adsorption on pristine In2O3. Moreover, the vertical distance between CO molecule and Co-In2O3 (1.799 Å) is shorter than pristine In2O3 (2.540 Å) after adsorption. Because of the more intense interaction of CO molecule and Co-In2O3, the
RI PT
structure of adsorbed CO molecule is changed much more easily, and the CO molecule moves toward the Co-In2O3 more easily than pristine In2O3. The calculated band gap of In2O3 is decreased after doping with Co, which conforms by the above
SC
mentioned theoretical result. And the spin polarization of both substrates are 0,
M AN U
indicating that the material property of non-magnetism have no change after Co doped. Furthermore, the absolute value of Ead and charge transfer (CT) of Co-In2O3 is bigger than pristine In2O3, which directly illustrate an enhanced sensing property toward CO from the view of energy. The diagrams of charge density for CO adsorption system of
TE D
Co-In2O3 and In2O3 are shown in Figure 10(c) and (d), which can visually present the charge distribution of the models. There are much more electrons between the CO molecule and Co-In2O3 than In2O3, illustrating a larger charge transfer of Co-In2O3.
EP
In Figure 11, the density of states (DOSs) of both adsorption systems are fully
AC C
exhibited to further investigate the mechanism in depth. Figure 11 (a) and (b) show the DOSs of Co-In2O3 adsorption system before adsorption and after adsorption, (c) and (d) show the DOSs of In2O3 adsorption system before adsorption and after adsorption. From an overall perspective, the DOS shape of Co-In2O3 have sort of change, yet DOS shape of In2O3 just exhibits little variation, demonstrating a great alteration on electronic structure of Co-In2O3 caused by its strong interaction effect with adsorbed CO molecule, and the electronic structure of In2O3 has hardly any Page 16 of 44
ACCEPTED MANUSCRIPT modulation after adsorption. In Co-In2O3 adsorption system, a peak of Co-In2O3 is created as a bridge at 0 eV after Co doping, which serves as an impurity level generated by Co2+. After CO adsorption, this bridge peak and other two peaks of CO
RI PT
at -3.5 eV and 0 eV are disappeared due to the large amount of charge transfer, and the left peak of CO becomes higher and shifts from -5.2 eV to -6 eV, which resonates to the new energy levels at -6 eV generated in Co-In2O3 and Co 3d by virtue of the
SC
strong interaction between CO molecules and Co-In2O3. The DOS variations of
M AN U
Co-In2O3 adsorption system demonstrate that CO molecule has a strong contact with the Co-In2O3 due to the important role played by doped Co ions that promotes the electron transfer between Co-In2O3 and CO molecules. In pristine In2O3 adsorption system, the similar peaks of CO and In2O3 at -6 eV after adsorption show a lower
TE D
height and a weaker resonance than in Co-In2O3 adsorption system. It can be ascribed to the weaker adsorption strength between In2O3 and CO, indicating that the electrons of In2O3 and CO can hardly distributed on the same energy level without sufficient
EP
interaction of adsorption. And the peak at 2-3.4 eV of CO molecule resonates with
AC C
In2O3 after adsorption, while DOS of In 3d has no peak at the same position. This phenomenon indicates that the CO molecule adsorbed on In2O3 is mainly attributed to the O ions but not In ions. In a word, Co-In2O3 shows an enhanced response toward CO molecule, and its underlying adsorption mechanism is different from pristine In2O3 due to the positive effect made by doping Co2+. Thirdly, another possible reason for the increasing CO sensing performance of Co-In2O3/MoS2 sensor may attribute to n-n junctions formed at the interfaces of Page 17 of 44
ACCEPTED MANUSCRIPT n-type Co-In2O3 and n-type MoS2, leading to a positive impact on the reaction between sensing film and adsorbed oxygen. Figure 12 exhibits the energy band structure of Co-In2O3/MoS2. The work function of Co-In2O3 and MoS2 are 5.44 eV
RI PT
and 5.34 eV, respectively, and the band gap of Co-In2O3 and MoS2 are 1.93 eV and 1.81 eV, respectively, which are computed via DFT method using materials studio software. Owing to the higher Fermi level of MoS2 compared with Co-In2O3,
SC
Co-In2O3 received more electrons from MoS2 until the Fermi level reached an
M AN U
equilibrium, which results in the increasing number of adsorbed oxygen [60]. Hence, a wider depletion layer and a higher barrier formed due to a greater amount of electrons captured by numerous adsorbed oxygen molecules, which cause an increase of the sensor resistance in air [61]. When the sensing film was transferred into
TE D
reductive CO atmosphere, a lot of electrons trapped by oxygen were released back. Therefore, the barrier height and the width of depletion layer sharply decreased, which results in a lower resistance of the gas sensor in CO. In this way,
EP
Co-In2O3/MoS2 sensor reasonably shows an enhanced sensitivity toward CO gas.
AC C
4. Conclusions
A high-performance Co-In2O3/MoS2 CO sensor fabricated by LbL self-assembly
technique was reported in this paper. XRD, EDS, SEM, TEM, and XPS measurements were used to characterize the morphologies, nanostructures, and elementary compositions of as-prepared samples. The CO sensing characteristics of the Co-In2O3/MoS2 sensor were determined under room temperature. The measurement results demonstrated that the Co-In2O3/MoS2 based CO gas sensor has a Page 18 of 44
ACCEPTED MANUSCRIPT high sensitivity, rapid response/recovery speed, and outstanding stability toward CO. An approach of combining experimental investigation with DFT based on first principle was used to further investigate the CO-gas sensing mechanism of the
RI PT
Co-In2O3/MoS2 sensor. The Co-In2O3/MoS2 film sensor was proved to be an extraordinary candidate for real-time detection of CO at room temperature. Acknowledgements
SC
This work was supported by the National Natural Science Foundation of China
M AN U
(No. 51777215), the Key Research & Development Plan Project of Shandong Province (2018GSF117002), the Fundamental Research Funds for the Central Universities of China (18CX07010A), and the Open Funds of National Engineering Laboratory for Mobile Source Emission Control Technology (NELMS2017B03), and
TE D
the Open Fund of Key Laboratory of Marine Spill Oil Identification and Damage
AC C
EP
Assessment Technology, State Oceanic Administration of China (No. 201801).
Page 19 of 44
ACCEPTED MANUSCRIPT Reference [1] S.W. Choi, J. Kim, J.H. Lee, Y.T. Byun, Remarkable improvement of CO-sensing performances in single-walled carbon nanotubes due to modification
RI PT
of the conducting channel by functionalization of Au nanoparticles, Sens. Actuators B 232 (2016) 625-632.
[2] X. Wang, H. Qin, Y. Chen, J. Hu, Sensing mechanism of SnO2 (1 1 0) surface to
SC
CO: density functional theory calculations, J. Phys. Chem. C 118 (2014) 28548–
M AN U
28561.
[3] B.L. Risavi, R.J. Wadas Jr., C. Thomas, D.F. Kupas, A novel method for continuous environmental surveillance for carbon monoxide exposure to protect emergency medical service providers and patients, J. Emerg. Med. 44 (2013)
TE D
637–640.
[4] Y.P. Chen, H.W. Qin, J.F. Hu, CO sensing properties and mechanism of Pd doped SnO2 thick-films, Appl. Surf. Sci. 428 (2018) 207–217.
EP
[5] Q. Wang, C. Wang, H. Sun, P. Sun, Y. Wang, J. Lin, G. Lu, Microwave assisted
AC C
synthesis of hierarchical Pd/SnO2 nanostructures for CO gas sensor, Sens. Actuators B 222 (2016) 257–263.
[6] S. Ghosh, M. Narjinary, A. Sen, R. Bandyopadhyay, S. Roy, Fast detection of low concentration carbon monoxide using calcium-loaded tin oxide sensors, Sens. Actuators B 203 (2014) 490–496.
Page 20 of 44
ACCEPTED MANUSCRIPT [7] T. Hyodo, T. Goto, T. Ueda, K. Kaneyasu, Y. Shimizu, Potentiometric carbon monoxide sensors using an anion-conducting polymer electrolyte and Au-loaded SnO2 electrodes, J. Electrochem. Soc. 163 (2016) B300–B308.
RI PT
[8] L. Zhu, Y. Zheng, J. Jian, Effect of palladium oxide electrode on potentiometric sensor response to carbon monoxide, Ionics 21 (2015) 2919–2926.
[9] S.S.S. Ahmadi, B. Raissi, R. Riahifar, M.S. Yaghmaee, H.R. Saadati, S.
SC
Ghashghaie, M. Javaheri, Fabrication of counter electrode of electrochemical CO
(2015) D3101–D3108.
M AN U
gas sensor by electrophoretic deposition of MWCNT, J. Electrochem. Soc. 162
[10] A. Paliwal, A. Sharma, M. Tomar, V. Gupta, Carbon monoxide (CO) optical gas sensor based on ZnO thin films, Sens. Actuators B 250 (2017) 679–685.
TE D
[11] T. Goto, T. Itoh, T. Akamatsu, N. Izu, W. Shin, CO sensing properties of Au/SnO2-Co3O4 catalysts on a micro thermoelectric gas sensor, Sens. Actuators B 223 (2016) 774–783.
EP
[12] D. Zhang, Y. Sun, C. Jiang, Y. Yao, D. Wang, Room-temperature highly
AC C
sensitive CO gas sensor based on Ag-loaded zinc oxide/molybdenum disulfide ternary nanocomposite and its sensing properties, Sens. Actuators B 253 (2017) 1120–1128.
[13] S. Kim, J. Yoon, J. Park, The effects of excess carbon on the surface area and crystallinity of TiO2 powders prepared by oxidizing mechanically-synthesized TiC and their CO gas-sensing properties, Ceram. Int. 43 (2017) 15306-15315.
Page 21 of 44
ACCEPTED MANUSCRIPT [14] J. Jonca, A. Ryzhikov, S. Palussiere, Organometallic synthesis of CuO nanoparticles: application in low-temperature CO detection, Chem. Phys. Chem. 18 (2017) 2658-2665.
RI PT
[15] Q. Zhou, L.N. Xu, A. Umar, W.G. Chen, R. Kumar, Pt nanoparticles decorated SnO2 nanoneedles for efficient CO gas sensing applications, Sens. Actuators B 256 (2018) 656-664.
SC
[16] S.T. Tan, C.H. Tan, W.Y. Chong, C.C. Yap, A.A. Umar, R.T. Ginting, H.B. Lee,
of
highly
crystalline
M AN U
K.S. Lim, M. Yahaya, M.M. Salleh, Microwave-assisted hydrolysis preparation ZnO
nanorod
array
for
room
temperature
photoluminescence-based CO gas sensor, Sens. Actuators B 227 (2016) 304-312. [17] A. Shanmugasundaram, V. Gundimeda, T.F. Hou, D.W. Lee, Realizing synergy
TE D
between In2O3 nanocubes and nitrogen-doped reduced graphene oxide: an excellent nanocomposite for the selective and sensitive detection of CO at ambient temperatures, ACS Appl. Mater. Inter. 9 (2017) 31728-31740.
EP
[18] D. Zhang, J. Wu, P. Li, Y. Cao, Room-temperature SO2 gas-sensing properties
AC C
based on a metal-doped MoS2 nanoflower: an experimental and density functional theory investigation, ACS Appl. Mater. Inter. 5 (2017) 20666-20677.
[19] Z. Yin, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen, H. Zhang, Single-layer MoS2 phototransistors, ACS Nano 6 (2012) 74-80.
[20] Y. Tong, Y. Liu, Y.S. Zhao, J. Thong, D.S.H. Chan, C.X. Zhu, Selectivity of MoS2 gas sensors based on a time constant spectrum method, Sen. Actuators A 255 (2017) 28-33. Page 22 of 44
ACCEPTED MANUSCRIPT [21] H. Peng, J. Lu, C.X. Wu, Z.X. Yang, H. Chen, W.J. Song, P.Q. Li, H.Z. Yin, Co-doped MoS2 NPs with matched energy band and low overpotential high efficiently convert CO2 to methanol, Appl. Surf. Sci. 353 (2015) 1003-1012.
RI PT
[22] Y.H Zhang, J.L. Chen, L.J. Yue, H.L. Zhang, F. Li, Tuning CO sensing properties and magnetism of MoS2 monolayer through anchoring transition metal dopants, Comput. Theor. Chem. 1104 (2017) 12-17.
SC
[23] M. Acerce, D. Voiry, M. Chhowalla, Metallic 1T phase MoS2 nanosheets as
M AN U
supercapacitor electrode materials, Nat. Nanotechnol. 10 (2015) 313–318. [24] J. Yoon, T. Lee, B.G. Bapurao, J. Jo, B.K. Oh, J.W. Choi, Electrochemical H2O2 biosensor composed of myoglobin on MoS2 nanoparticle-graphene oxide hybrid structure, Biosens. Bioelectron. 93 (2017) 14–20.
TE D
[25] D. Zhang, C. Jiang, Y. Sun, Room-temperature high-performance ammonia gas sensor based on layer-by-layer self-assembled molybdenum disulfide/zinc oxide nanocomposite film, J. Alloys Compd. 698 (2017) 476-483.
EP
[26] M.Q. Wen, T. Xiong, Z.G. Zang, W. Wei, X.T. Tang, F. Dong, Synthesis of
AC C
MoS2/g-C3N4 nanocomposites with enhanced visible-light photocatalytic activity for the removal of nitric oxide (NO), Opt. Express 24 (2016) 10205-10212.
[27] Y.J. Liu, L.Z. Hao, W. Gao, Z.P. Wu, Y.L. Lin, G.X. Li, W.Y. Guo, L.Q. Yu, H.Z. Zeng,J. Zhu, W.L. Zhang, Hydrogen gas sensing properties of MoS2/Si
heterojunction, Sens. Actuators B 211 (2015) 537-543.
Page 23 of 44
ACCEPTED MANUSCRIPT [28] M. Donarelli, S. Prezioso, F. Perrozzi, F. Bisti, M. Nardone, L. Giancaterini, C. Cantalini, L. Ottaviano, Response to NO2 and other gases of resistive chemically exfoliated MoS2-based gas sensors, Sens. Actuators B 207 (2015) 602-613.
RI PT
[29] Y. Niu, R.G. Wang, W.C. Jiao, G.M. Ding, L.F. Hao, F. Yang, X.D. He, MoS2 graphene fiber based gas sensing devices, Sens. Actuators B 211 (2015) 537-543. [30] S.J. Ray, First-principles study of MoS2, phosphorene and graphene based single
SC
electron transistor for gas sensing applications, Sens. Actuators B 222 (2016)
M AN U
492–498.
[31] D. Zhang, Y. Sun, P. Li, Y. Zhang, Facile fabrication of MoS2-modified SnO2 hybrid nanocomposite for ultrasensitive humidity sensing, ACS. Appl. Mater. Interfaces 8 (2016) 14142-14149.
TE D
[32] C. Kuru, C. Choi, A. Kargar, D. Choi, Y.J. Kim, C.H. Liu, S. Yavuz, S. Jin, MoS2 Nanosheet–Pd nanoparticle composite for highly sensitive room tmperature detection of hydrogen, Adv. Sci. 2 (2015) 1500004.
EP
[33] Z. Lu, Y.Q. Gong, X.J. Li, Y. Zhang, MoS2-modified ZnO quantum dots
AC C
nanocomposite: Synthesis and ultrafast humidity response, Appl. Surf. Sci. 399 (2017) 330-336.
[34] W.R. Li, H.H. Xu, T. Zhai, H.Q. Yu, Z.R. Chen, Z.W. Qiu, X.P. Song, J.Q. Wang, B.Q.
Cao,
Enhanced
triethylamine
sensing
properties
by
designing
Au@SnO2/MoS2 nanostructure directly on alumina tubes, Sens. Actuators B 253 (2017): 97-107.
Page 24 of 44
ACCEPTED MANUSCRIPT [35] Z.G. Zang, M.Q. Wen, W.W. Chen, Y.F. Zeng, Z.Q. Zu, X.F. Zeng, X.S. Tang, Strong yellow emission of ZnO hollow nanospheres fabricated using polystyrene spheres as templates, Mater. Design 84 (2015) 418-421.
RI PT
[36] Z. Zang, X. Tang, Enhanced fluorescence imaging performance of hydrophobic colloidal ZnO nanoparticles by a facile method, J. Alloy. Compd. 619 (2015) 98-101.
SC
[37] K. Mahmood, A. Khalid, F. Nawaz, M.T. Mehran, Low-temperature
M AN U
electrospray-processed SnO2 nanosheets as an electron transporting layer for stable and high-efficiency perovskite solar cells, J. Colloid Interf. SCI. 532 (2018) 387-394.
[38] A. Ziarati, A. Badiei, R. Luque, Black hollow TiO2 nanocubes: Advanced
TE D
nanoarchitectures for efficient visible light photocatalytic applications, J. Alloy. Compd. 238 (2018) 177-183.
[39] D. Zhang, C. Jiang, P. Li, Y. Sun, Layer-by-layer self-assembly of Co3O4 MoS2
EP
nanorod-decorated
nanosheet-based
nanocomposite
toward
AC C
high-performance ammonia detection, ACS. Appl. Mater. Inter. 9 (2017) 6462-6471.
[40] X.H. Zhang, X.H. Huang, M.Q. Xue, X. Ye, W.N. Lei, H. Tang, C.S. Li, Hydrothermal
synthesis
and
characterization
microspheres, Mater. Lett. 148 (2015) 67-70.
Page 25 of 44
of 3D flower-like
MoS2
ACCEPTED MANUSCRIPT [41] J.L. Shen, F. Li, B. Yin, L. Sun, C. Chen, S.P. Wen, Y. Chen, S.P. Ruan, Enhanced ethyl acetate sensing performance of Al-doped In2O3 microcubes, Sens. Actuators B 253 (2017) 461-469.
RI PT
[42] Y. Liu, Y.H. Lin, J.L. Lan, Effect of transition-metal cobalt doping on the thermoelectric performance of In2O3 ceramics, J. Am. Cream. Soc. 93 (2010) 2938-2941.
SC
[43] D.D. Wei, Z.S. Huang, L.W. Wang, X.H. Chuai, S.M. Zhang, G.Y. Lu,
M AN U
Hydrothermal synthesis of Ce-doped hierarchical flower-like In2O3 microspheres and their excellent gas-sensing properties, Sens. Actuators B 255 (2018) 1211-1219.
[44] N. Chen, Y.X. Li, D.Y. Deng, X. Liu, X.X. Xing, X.C. Xiao, Y.D. Wang,
TE D
Acetone sensing performances based on nanoporous TiO2 synthesized by a facile hydrothermal method, Sens. Actuators B 238 (2017) 491-500. [45] Z.F. Liu, H.L. Xing, L. Lin, Facial synthesized Co-doped SnO2@multi-walled
EP
carbon nanotubes as an efficient microwave absorber in high frequency range,
AC C
Nano 12 (2017) 9-19.
[46] D. Zhang, H. Chang, Y. Sun, C. Jiang, Y. Yao, Y. Zhang, Fabrication of platinum-loaded cobalt oxide/molybdenum disulfide nanocomposite toward methane gas sensing at low temperature, Sens. Actuators B 252 (2017) 624-632.
[47] X.Q. Qiao, F.C. Hu, D.F. Hou, D.S. Li, PEG assisted hydrothermal synthesis of hierarchical MoS2 microspheres with excellent adsorption behavior, Mater. Lett. 169 (2016) 241-245. Page 26 of 44
ACCEPTED MANUSCRIPT [48] X.J. Liu, L. Jiang, X.M. Jiang, X.Y. Tian, X. Sun, Y.L. Wang, W.D. He, P.Y. Hou, X.L. Deng, X.J. Xu, Synthesis of Ce-doped In2O3 nanostructure for gas sensor applications, Appl. Surf. Sci. 428 (2018) 478-484.
RI PT
[49] A. Shokri, N. Salami, Gas sensor based on MoS2 monolayer, Sens. Actuators B 236 (2016) 378-385
[50] D. Zhang, C. Jiang, J. Liu, Y. Cao, Carbon monoxide gas sensing at room
SC
temperature using copper oxide-decorated graphene hybrid nanocomposite prepared by layer-by-layer self-assembly, Sens. Actuators B 247 (2017) 875-882.
M AN U
[51] R.K. Joshi, J.E. Weber, Q. Hu, B. Johnson, J.W. Zimmer, A. Kumar, Carbon monoxide sensing at room temperature via electron donation in boron doped diamond films, Sens. Actuators B 145 (2010) 527-532.
[52] D. Kim, P.V. Pikhitsa, H. Yang, M. Choi, Room temperature CO and H2 sensing
TE D
with carbon nanoparticles, Nanotechnology 22 (2011) 485501-485507. [53] M. Shojaee, Sh. Nasresfahani, M.H. Sheikhi, Hydrothermally synthesized Pd-loaded SnO2/partially reduced graphene oxide nanocomposite for effective
457-467.
EP
detection of carbon monoxide at room temperature, Sens. Actuators B 254 (2018)
AC C
[54] Y. Liu, Y. Jiao, Z. Zhang, F. Qu, A. Umar, X. Wu, Hierarchical SnO2 nanostructures made of intermingled ultrathin nanosheets for environmental remediation smart gas sensor, and super capacitor applications, ACS Appl. Mater. Interfaces 6 (2014) 2174-2184. [55] J.Y. Ye, C.J. Liu, Q.F. Ge, DFT Study of CO2 Adsorption and hydrogenation on the In2O3 surface, J. Phys. Chem. C 116 (2012) 7817-7825.
Page 27 of 44
ACCEPTED MANUSCRIPT [56] S. Lin, D.Q. Xie, Initial decomposition of methanol and water on In2O3(110): a periodic DFT study, Chin. J. Chem. 30 (2012) 2036-2040. [57] J.Y. Ye, C.J. Liu, D.H. Mei, Q.F. Ge, Active oxygen vacancy site for methanol
RI PT
synthesis from CO2 hydrogenation on In2O3(110): a DFT Study, ACS Catal. 3 (2013) 1296-1306.
[58] K.N. Ding, Y.H. Lin, M.Y. Huang, The enhancement of NO detection by doping
SC
strategies on monolayer MoS2, Vacuum 130 (2016) 146-153.
M AN U
[59] J. Wu, D. Zhang, Y. Cao, Fabrication of iron-doped titanium dio xide quantum dots/molybdenum disulfide nanoflower for ethanol gas sensing, J. Alloy. Compd. 529 (2018) 556-567.
[60] R.Q. Xing, L. Xu, J. Song, C.Y. Zhou, Q.L. Li, D.L. Liu, H.W. Song, Preparation
TE D
and gas sensing properties of In2O3/Au nanorods for detection of volatile organic compounds in exhaled breath, Sci. Rep. 5 (2015) 10717. [61] J. Hu, Y.J. Sun, Y. Xue, M. Zhang, P.W. Li, K. Lian, S. Zhuiykov, W.D. Zhang,
EP
Y. Chen, Highly sensitive and ultra-fast gas sensor based on CeO2-loaded In2O3
AC C
hollow spheres for ppb-level hydrogen detection, Sens. Actuators B 257 (2018) 124-135.
Page 28 of 44
ACCEPTED MANUSCRIPT Table 1. Sensing performance of the CO sensor presented in this work compared with previous works.
Fabrication method
Response (%)
Refs.
Ag-ZnO/MoS2
LbL self-assembly
4.8 (100 ppm)
[12]
CuO/rGO
LbL self-assembly
5.4 (100 ppm)
[50]
B-diamond
Hot filament CVD method
5.5 (100 ppm)
[51]
Acid-SCNPs
Ion-induced focusing deposition
Pd-SnO2/PRGO
Hydrothermal method
Co-In2O3/MoS2
LbL self-assembly
SC
RI PT
Sensing material
[52]
3.8 (400 ppm)
[53]
M AN U
7.8 (100 ppm)
AC C
EP
TE D
8.6 (50 ppm)
Page 29 of 44
This work
ACCEPTED MANUSCRIPT Table 2. The parameters of Co-In2O3 (110) and pristine In2O3 (110) CO adsorption systems. Adsorption
Bond length (Å)
Distance
systems
C—O
(Å)
Co-In2O3
1.150
In2O3 CO
Eg (eV)
Spin (µB)
CT (e)
1.799
-1.34
0.426
0
-0.386
1.144
2.540
-0.61
0.833
0
-0.13
1.146
—
—
—
—
—
AC C
EP
TE D
M AN U
SC
RI PT
Ead (eV)
Page 30 of 44
ACCEPTED MANUSCRIPT
Figure captions Figure 1. The schematic diagram of (a) preparation route for Co-In2O3/MoS2 sensor, and (b) gas-sensing experimental setup.
RI PT
Figure 2. (a) XRD of Co-In2O3/MoS2, Co-In2O3, In2O3, and MoS2 samples. (b) EDS spectrum of Co-In2O3/MoS2.
Figure 3. XPS spectra of Co-In2O3/MoS2: (a) survey spectrum, (b) In 3d, (c) O 1s, (d)
SC
Co 2p, (e) Mo 3d, and (f) S 2p.
M AN U
Figure 4. SEM characterizations of (a) MoS2, (b) Co-In2O3, (c) and (d) Co-In2O3/MoS2. (e) Cross-sectional image of Co-In2O3/MoS2 film on the substrate, (f) cross-sectional EDS spectrum of Co-In2O3/MoS2 film.
Figure 5. (a) and (b) TEM images of Co-In2O3/MoS2. (c) and (d) HRTEM images of
TE D
Co-In2O3/MoS2.
Figure 6. (a) Time-dependent response and (b) fitting curve of Co-In2O3/MoS2, Co-In2O3, and pure In2O3 sensors exposed to different concentrations of CO.
EP
Figure 7. The response-recovery properties of Co-In2O3/MoS2, Co-In2O3, and pure
AC C
In2O3 film sensors toward 10 ppm and 1000 ppm CO. Figure 8. (a) Repeatability, and (b) long term stability of the Co-In2O3/MoS2 gas sensor exposed to 1000 ppm, 100 ppm, and 10 ppm CO. Figure 9. Illustration of sensing-mechanism for the Co-In2O3/MoS2 sensor toward CO gas. Figure 10. CO adsorption models of (a) Co-In2O3 and (b) pristine In2O3, and corresponding charge density diagram of (c) Co-In2O3 and (d) pristine In2O3. Page 31 of 44
ACCEPTED MANUSCRIPT Figure 11. The DOSs for the Co-In2O3 (a) before CO adsorbed, and (b) after CO adsorbed, and pristine In2O3 (c) before CO adsorbed, and (d) after CO adsorbed. Figure 12. The energy-band structure of Co-In2O3/MoS2 sensor (EC1 and EC2, Eg1 and
RI PT
Eg2, Ev1 and Ev2, and Ef are the bottom of conduction band, band gap, top of valence band, and the Fermi-energy level of MoS2 and Co-In2O3, respectively. E1 and E2 are
AC C
EP
TE D
M AN U
Fermi-energy level of Co-In2O3/MoS2 sensor).
SC
potential barrier before and after sensor exposed to CO, respectively. Ef is the
Page 32 of 44
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
EP
Figure 1. The schematic diagram of (a) preparation route for Co-In2O3/MoS2 sensor,
AC C
and (b) gas-sensing experimental setup.
Page 33 of 44
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 2. (a) XRD of Co-In2O3/MoS2, Co-In2O3, In2O3, and MoS2 samples. (b) EDS spectrum of Co-In2O3/MoS2.
Page 34 of 44
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 3. XPS spectra of Co-In2O3/MoS2: (a) survey spectrum, (b) In 3d, (c) O 1s, (d) Co 2p, (e) Mo 3d, and (f) S 2p.
Page 35 of 44
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
Figure 4. SEM characterizations of (a) MoS2, (b) Co-In2O3, (c) and (d) Co-In2O3/MoS2. (e) Cross-sectional image of Co-In2O3/MoS2 film on the substrate, (f) cross-sectional EDS spectrum of Co-In2O3/MoS2 film.
Page 36 of 44
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
Co-In2O3/MoS2.
TE D
Figure 5. (a) and (b) TEM images of Co-In2O3/MoS2. (c) and (d) HRTEM images of
Page 37 of 44
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 6. (a) Time-dependent response and (b) fitting curve of Co-In2O3/MoS2, Co-In2O3, and pure In2O3 sensors exposed to different concentrations of CO.
Page 38 of 44
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 7. The response-recovery properties of Co-In2O3/MoS2, Co-In2O3, and pure
AC C
EP
TE D
In2O3 film sensors toward 10 ppm and 1000 ppm CO.
Page 39 of 44
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 8. (a) Repeatability, and (b) long term stability of the Co-In2O3/MoS2 gas sensor exposed to 1000 ppm, 100 ppm, and 10 ppm CO.
Page 40 of 44
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Figure 9. Illustration of sensing-mechanism for the Co-In2O3/MoS2 sensor toward CO
AC C
EP
TE D
gas.
Page 41 of 44
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 10. CO adsorption models of (a) Co-In2O3 and (b) pristine In2O3, and
AC C
EP
corresponding charge density diagram of (c) Co-In2O3 and (d) pristine In2O3.
Page 42 of 44
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 11. The DOSs for the Co-In2O3 (a) before CO adsorbed, and (b) after CO
AC C
EP
TE D
adsorbed, and pristine In2O3 (c) before CO adsorbed, and (d) after CO adsorbed.
Page 43 of 44
RI PT
ACCEPTED MANUSCRIPT
Figure 12. The energy-band structure of Co-In2O3/MoS2 sensor (EC1 and EC2, Eg1 and
SC
Eg2, Ev1 and Ev2, and Ef are the bottom of conduction band, band gap, top of valence
M AN U
band, and the Fermi-energy level of MoS2 and Co-In2O3, respectively. E1 and E2 are potential barrier before and after sensor exposed to CO, respectively. Ef is the
AC C
EP
TE D
Fermi-energy level of Co-In2O3/MoS2 sensor).
Page 44 of 44
ACCEPTED MANUSCRIPT
Research highlights 1. Co-In2O3/MoS2 nanocomposite was fabricated by layer-by-layer self-assembly
RI PT
technique. 2. CO gas sensing properties of Co-In2O3/MoS2 nanocomposite film sensor was investigated.
SC
3. The sensing mechanism of Co-In2O3/MoS2 sensor was discussed by combining
AC C
EP
TE D
M AN U
experiments with DFT simulation.