- Email: [email protected]
Synthesis of WO3 nanoflakes by hydrothermal route and its gas sensing application
Journal Pre-proof Synthesis of WO3 Nanoflakes by Hydrothermal Route and its Gas Sensing Application Pankaj S. Kolhe (Conceptualization) (Methodology) (Data curation) (Formal analysis) (Writing - original draft), Pallavi Mutadak (Formal analysis) (Investigation), Namita Maiti (Supervision) (Funding acquisition), Kishor M. Sonawane (Conceptualization) (Methodology) (Supervision) (Funding acquisition)Writing - review and editing)
PII:
S0924-4247(19)31881-3
DOI:
https://doi.org/10.1016/j.sna.2020.111877
Reference:
SNA 111877
To appear in:
Sensors and Actuators: A. Physical
Received Date:
11 October 2019
Revised Date:
9 January 2020
Accepted Date:
29 January 2020
Please cite this article as: Kolhe PS, Mutadak P, Maiti N, Sonawane KM, Synthesis of WO3 Nanoflakes by Hydrothermal Route and its Gas Sensing Application, Sensors and Actuators: A. Physical (2020), doi: https://doi.org/10.1016/j.sna.2020.111877
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier.
Synthesis of WO3 Nanoflakes by Hydrothermal Route and its Gas Sensing Application
Pankaj S. Kolhea, Pallavi Mutadakb, Namita Maitic, Kishor M. Sonawanea*
a
ro of
Department of Physics, Fergusson College affiliated Savitribai Phule Pune University, Pune 411004, India b Centre for Advanced Studies in Material Science and Condensed Matter Physics, Department of Physics, Savitribai Phule Pune University, Pune 411007, India c Laser & Plasma Technology Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
re
-p
*Corresponding author: Email: [email protected]
Jo
ur
na
lP
Graphical Abstract:
1
ro of -p re
Highlights
WO3 seed layer deposited by spray pyrolysis for growth of WO3 nanoflaked thin film
lP
using hydrothermal method.
Flakes like morphology provide large surface area and plenty of active sites to interact
na
with target gas thereby improving gas sensing. High selectivity towards NH3 gas detection.
Higher sensitivity with fast response and recovery time at comparatively lower
ur
Jo
temperature (150 0C) is obtained.
Abstract:
2
The controlled morphology and size of inorganic materials have attracted intense interest, as these parameters play an important role in determining sensing properties. Herein, WO3 thin film is hydrothermally grown on FTO substrate at 175 ℃ with the assistance of seed layer deposited by spray pyrolysis technique. The WO3 thin film was characterized by X-ray Diffraction (XRD), micro-Raman spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and UV-Visible Spectroscopy for determination of physico-chemical properties. Moreover, X-ray Photoelectron Spectroscopy (XPS) analysis
ro of
is carried out to understand chemical states and boding. Systematic gas sensing studies were performed for NH3, H2S and CO gases under static condition. The sensing study reveals,
WO3 nanoflake exhibits a superior sensor response to NH3 gas. Moreover, it exhibits higher
-p
sensitivity to NH3. Gas sensing properties indicate WO3 nanoflakes holds promise to become
re
a potential candidate for NH3 gas detection at the expense of lower power consumption. Keywords: Pre-seeding WO3; WO3 nanoflakes; hydrothermal method; NH3 gas sensing.
lP
1. Introduction:
Nowadays, a lot of attention has been attracted by the inorganic metal oxide
na
semiconductor nanomaterials owing to ease of synthesis technique and potential applications in diversified domains. Many researchers are focusing on synthesizing metal oxide
ur
nanostructures of various shapes and/or sizes, exhibiting specifically tailored physicochemical properties, which can be molded into devices [1,2]. Much interest, therefore has
Jo
been shown in the study of metal oxide semiconductors such as SnO2, ZnO, In2O3, and WO3 [3-7]. Among all of them, Tungsten oxide (WO3) is n-type semiconductor with direct band gap energy of 2.6 eV [8]. Due to the exotic physico-chemical properties, it shows potential towards technological applications in many fields such as electrochromic devices, solar cells, photo-catalysts and gas sensors [9-12].
3
For the detection of toxic gases, majorly SnO2 and ZnO based gas sensors are commonly used, but these gas sensors exhibit low stability, reproducibility, and selectivity [13]. Therefore, WO3 has been regarded as a promising material for gas sensor due to its low cost, simple synthesis technique and high sensitivity towards toxic and flammable gases/vapors such as SO2, NH3, H2S, H2 and alcohol vapors [14-18]. In recent times, the detection and monitoring of such hazardous gases have become a prime need to protect the environment and human health. Particularly, ammonia (NH3) is very much useful in chemical industries
ro of
and image processing in various fields. It is a colorless, toxic gas with a pungent smell and can cause incurable damage to human health. The minimum permissible limit of NH3 is
tabulated around 50 ppm [19]. Above this limit, a human can feel irritation to the skin, eye
-p
and respiratory system. More attention, therefore has been devoted to the fabrication of ammonia gas detecting sensors.
re
Nanostructured morphology of material plays an important role in the field of gas sensors,
lP
because of their high surface to volume ratio, surface activity, porosity and quantum size effect [20-21]. In addition, it is well known that from application point of view morphology, crystalline size and specific surface area of the sensor material governs its performance [22].
na
In order to achieve better gas sensing performance, many attempts have been made in the synthesis of WO3 nanostructures with different morphologies. To study the effect of
ur
morphology on ethanol vapor sensing, a facile hydrothermal method was employed to obtain
Jo
flower-like morphology of WO3 by varying citric acid concentration, reported by D. Meng et. al. [23]. Z. Liu and co-authors synthesized the 3D structure of WO3 nanorods emanating from a microsphere and studied the effect of annealing temperature on these microspheres structures, as well as NO2 gas sensing behavior [24]. D. Chen et. al. have synthesized singlecrystalline 2D WO3 nameplates via the topochemical transformation process for alcohol sensing study [18]. The effect of operating temperature on the response and recovery time
4
and enhancement in sensitivity due to the ultrathin plate-like morphology was proposed by the authors. Y. Qin and group have reported the synthesis of mesoporous 3D network of WO3 nanowires for NO2 gas sensing and it was observed that WO3 nanowires exhibit high sensitivity and selectivity to NO2 gas along with short response and recovery time at low operating temperatures for ppb level gas [25]. The enhanced or high response of the sensor in a nano-sized regime can be attributed to maximum surface to volume ratio, which makes available large number of active sites on the sensor surface for the interaction with the target
ro of
gas [26]. Zhang et. al. reported the effect of temperature and type of acid on nanoplates like morphology of WO3 nanostructure to investigate gas sensing properties of four different gases [27]. Takacs et. al. have synthesized two samples of WO3 nanoparticles by acidic
-p
precipitation from sodium tungstate solution, one using oxalic acid (type A) and the other by Zocher method (type B) and the layer is deposited using capillary dropping technique for
re
examining sensing of NH3 gas [28]. The authors claimed that the morphology plays an
lP
important role in NH3 sensing, due to which sample B exhibited superior performance. Attempts have been carried out to synthesis of WO3 nanostructures, characterized by unique morphology offering a large surface area, with improved sensing properties for the real-time
na
monitoring application. Also, ammonia gas sensing behavior of nanoconifer like morphology of
[29].
ur
WO3 formed on tungsten substrate using hydrothermal route were stuided previously by our group
Jo
To facilitate a better electron conduction mechanism to improve the sensor characteristics, it is necessary to provide conduction paths for charge transportation. Hence, nanoflowers and nanoflakes like morphologies are reported to exhibit better sensitivity [3033]. In the present work, WO3 thin films possessing nanoflakes morphology were grown on WO3 seeded FTO substrate using a hydrothermal method. The synthesized WO3 film was tested for the detection of toxic gases such as H2S, CO, and NH3. The gas sensing study
5
reveals that WO3 thin film exhibits good gas sensing performance at optimal operating temperature ~150 ℃ to NH3 gas with fairly short response (28 s) and recovery time (68 s). An attempt has been made to provide a plausible gas sensing mechanism, based on the correlation of morphology, effective surface area and interaction of the test gas molecules (adsorption, desorption and/or diffusion) of gas molecules on the surface of the sensor. 2. Experimental Details: 2.1. Synthesis of WO3 Nanoflakes Thin Film:
ro of
Prior to the hydrothermal method, WO3 seeding was carried out on the FTO substrate (commercially available) by spray pyrolysis deposition. The precursor solution for spray
deposition was prepared by dispersing 0.18 g of tungsten oxide (WO3) powder (Alfa Aesar,
-p
99.5% purity) in a 30 ml solution of ammonia solution (Thomas Baker, AR grade) and Deionized (DI) water in 2:1 proportion. This solution was then sprayed on FTO substrate
re
preheated at 400 ℃, with air as a carrier gas. The deposited thin film was annealed at 500 ℃
lP
in air for 2 h. The WO3 seed formation was confirmed using XRD and SEM. The corresponding XRD pattern and SEM image of the WO3 seed layer on the FTO substrate are
na
depicted in Supplementary Fig. (1) and (2) respectively. After confirmation of the seed layer, the synthesis of WO3 thin film was carried out
ur
using the hydrothermal method. First, H2WO4 solution was prepared by dissolving 1.25 g of H2WO4 (Loba Chemie, 99% purity) into 10 ml of H2O2 (Fisher scientific, 30% w/v) and 30
Jo
ml DI water, stirring the solution at 95 ℃. The resultant H2WO4 solution was then diluted to 0.05 M i.e. adding 100 ml DI water. The clear solution thus obtained was further used for the hydrothermal process. The precursor solution for hydrothermal process was prepared by using 12 ml of H2WO4 (0.05 M) solution, 0.08 g of oxalic acid (Thomas Baker, AR grade), 0.08 g of urea (Avra, AR grade), 50 ml of acetonitrile (VETEC, HPLC 99.9 %) and 2 ml HCl (6 M) (Thomas Baker, 35% AR grade) were added into beaker and stirred for 30 min. The 6
WO3 seed coated FTO substrate was placed into 100 ml Teflon lined stainless steel autoclave filled with the as-prepared precursor solution and kept at 180 ℃ for 6 h in an oven. After the completion of the reaction duration, the furnace was allowed to cool down to room temperature naturally. The FTO substrate was removed from the autoclave, washed with ethanol and DI water several times, followed by annealing in air at 500 ℃ for 2 h. At least, four samples were prepared under identical experimental conditions to check the reproducibility.
ro of
2.2. Characterization and Gas Sensing Study:
The XRD (D8 Advance, Bruker AXS) pattern was recorded to confirm the crystal
structure and phase of the WO3 nanostructured thin film using Cu Kα radiation (λ= 1.54 Å).
-p
The surface morphology was characterized with the help of SEM (JEOL, JSM-6360A). The
re
micro Raman Spectroscopy (Lab RAM HR) with laser excitation lines of 532 nm at room temperature was employed for further confirmation of the phase structure. The chemical
lP
analysis was performed by X-ray Photoelectron Spectrometer (XPS, Thermo K-Alpha+ Spectrometer) using Al-Kα X -rays, 1486.6 eV). The UV-Visible absorption spectroscopy
na
(JASCO 760) was used to evaluate the energy band gap of WO3 thin film. The as-prepared WO3 nanoflakes thin film was placed in a stainless steel chamber
ur
equipped with multiple feed-through for electrical connections and gas inlet, to examine the gas sensing characteristics towards the toxic gases such as H2S, CO and NH3 (Dry gases,
Jo
commercially available) under static condition. The operating temperature of the thin film was provided using a ceramic infrared heater capable of reaching up to 500 ℃, and the temperature was measured using a K-type thermocouple. For the measurement of resistance of the thin-film sensor, the nickel wire was properly connected to Au plated electrodes which are separated by a distance of 8mm on the sensor surface. The sensor response of the WO3 thin film for fixed concentrations of NH3, CO and H2S (one species injected at a time) at 7
different operating temperatures ranging from 50 to 300 ℃, were recorded to obtain optimal operating temperature. Before the test gas injection, the thin film sensor was kept in an ambient atmosphere for the stabilization of electrical resistance for each measurement. Then, the desired quantity of target gas is purged into the gas chamber through a syringe. Computer-controlled data acquisition system embedded Keithley 2450 source meter was used for recording measurements of change in electrical resistance of WO3 thin film. For the obtained optimum operating temperature, dynamic response towards different concentration
ro of
NH3 gas and response and recovery times were evaluated. The sensor response was calculated using a typical formula given below: Sensor response (S)% =
Ra −Rg Ra
× 100
(1)
-p
where Ra and Rg are the electrical resistance of the thin-film sensor in the air and target gas
re
respectively. The response and recovery time of the sensor is defined as the time taken by the sensor to attain 90 % of the variation in resistance in the presence of target gas and air
3. Results and Discussion:
na
3.1. Structural Analysis:
lP
respectively.
The XRD pattern of WO3 nanostructured grown on WO3 seeded FTO substrate using the
ur
hydrothermal method is shown in Fig. 1. All the observed diffraction peaks are indexed to the monoclinic phase of WO3 (JCPDS Card No. 83-0951) [34]. The observed FTO peaks are
Jo
from the substrate. The observed peaks are sharp enough to reveal the crystallinity of WO3 nanostructured film. No other impurity peaks due to other phases are detected, indicating good purity of the as-synthesized product, under prevailing experimental conditions. The micro Raman spectrum of WO3 nanostructured thin film is shown in Fig. 2. It shows well-defined peaks at 807 and 715 cm-1, corresponding to symmetric and asymmetric vibrations of W6+–O bonds, i.e. O–W–O stretching vibration mode (γ), respectively. Also, the 8
Raman bands observed at 325 and 272 cm-1 are assigned to the bending modes of bridging oxygen ions, i.e. W–O–W bending modes (δ) [35-36]. A peak of relatively weaker intensity observed at 136 cm-1 can be ascribed to the lattice vibration of crystalline WO3 [37]. The Raman spectrum of WO3 does not exhibit any peak related to hydrous or hexagonal phase. Moreover, the peaks at 807, 715 and 272 cm-1 are representative of fundamental modes of the monoclinic phase of WO3, thereby confirming that synthesized nanostructured thin film is indeed of stoichiometric WO3 monoclinic phase.
ro of
3.2. Morphological Analysis: 3.2.1 SEM and TEM Analysis:
The surface morphology of the WO3 nanostructured thin film was examined using SEM
-p
and the corresponding images recorded at different magnifications are shown in Fig. 3 (a,b).
re
The lower magnified SEM image (Fig. 3-a) illustrates that homogeneously distributed, randomly oriented nanoflakes are grown over the entire FTO substrate. A careful observation
lP
of a higher magnified image (Fig. 3-b) reveals that the thickness of these nanoflakes is in the range of 50 to 100 nm. Furthermore, it is observed that the nanoflakes of WO3 are randomly
na
oriented and interconnected, due to which the rough surface and porous morphology is formed. Such a kind of nanostructured morphology is useful for trapping gas molecules that
ur
may interact greatly with sensing material and result in enhancement in sensor response. In addition, the flakes like morphology afford a larger surface area, where a large number of
Jo
active sites are available to interact with a target gas molecule [33]. Moreover, the interconnected nanoflakes provide continuous pathways for the charge transportation, which could be an advantage for gas sensors. The morphology and structural features of the WO3 nanoflakes were further studied by TEM and HRTEM as shown in Fig 4. Fig. 4 (a and b) represents high magnification TEM images of WO3 nanoflakes, which reveals the irregular size of nanoflakes are stacked 9
together. Moreover, the TEM image represents the lateral dimensions of the nanoflakes are in the range of 60- 100 nm. The High-Resolution TEM image of WO3 nanoflakes reveals that the nanoflakes have a single-crystalline structure with resolved lattice fringes. The lattice spacing is found to be 0.36 nm and it is readily indexed to d-spacing of (002) plane which is in accordance with the results of XRD in Fig. 1. Fig. 4(d) depicts selected area electron diffraction (SAED) pattern reveals high crystallinity of WO3 with excellent growth along (002) direction to form nanoflakes like morphology.
ro of
3.3 XPS Analysis:
The gas sensing mechanism can be revealed with knowledge of bonding and valence
states of W and O on the surface of WO3 nanoflakes. In this context, XPS analysis is one of
-p
the fundamental tool. Fig. 5(a) depicts, two spin-orbit doublets corresponding to the different
re
valence states of W. The binding energies 37.8 and 35.6 eV correspond to W4f5/2 and W4f7/2 of W6+ states respectively, and rest of smaller doublets are allocated to W5+ states.
lP
Moreover, there is a weak emission originating from W5p3/2 level can be found at binding energy 41.4 eV. Fig. 5 (b) exhibits O1s spectrum in which peak originates at 530.2 eV
na
corresponds to lattice oxygen O2− bonded with W, the peak of OH- groups observed at 530.9 eV along with smaller H2O peak. The peaks of OH- and H2O are may be attributed to water
ur
vapor from the ambiance [38]. The presence of W5+ states (oxygen vacancies) acts donors in the semiconducting state of WO3, these donors have a significant impact on conductivity and
Jo
gas sensing properties of the film [39]. 3.4 Optical Property Analysis: The absorption spectrum of WO3 nanoflakes thin film, recorded in the wavelength range from 200 to 800 nm, is depicted in Fig. 6. It shows more absorption in the UV range. The absorption edge of the thin film is observed at 470 nm which corresponds to the lower extreme of the visible band. The optical band gap is also obtained from the Tauc plot which is 10
depicted in the inset of Fig. 6. From the Tauc plot, the band gap energy of the WO3 nanoflakes thin film is estimated as ~2.7 eV which agrees very well with previously reported value [40]. 3.5 Gas Sensing Study: The gas sensing study of WO3 nanoflakes thin-film sensor towards the toxic gases such as H2S, CO, and NH3 were carried out in indigenously design gas sensing unit. The conductometric method was employed for the measurements, wherein the thin film sensor is
ro of
exposed to the target gas at a certain temperature and corresponding change in resistances is recorded to evaluate sensor response. As is known, selectivity to a particular target gas holds an important factor for a gas sensor, as in many industrial processes, multiple gases could be
-p
evolved. The response of the sensor, therefore, towards various gas needs to be tested to
re
reveal the selectivity of the sensor towards a particular species. To determine the optimum operating temperature and selectivity towards particular gas, sensor response of WO3 thin
lP
film towards a fixed quantity of H2S, CO, and NH3 gas of concentration 120 ppm, in the temperature range 50–300 ℃ was studied. For metal oxide based gas sensors, the temperature
na
play a crucial role as it has a great effect on the response of the sensor [41]. The plot of sensor response as a function of temperature for H2S, CO, and NH3 gases is shown in Fig. 7.
ur
It clearly shows that the WO3 nanoflakes thin film exhibit higher sensor response to NH3 gas at lower operating temperature than H2S and CO gases. This indicates WO3 sensor exhibits
Jo
good selectivity towards NH3 gas. Hence, in further study, the gas sensing characteristics of WO3 thin film sensor towards NH3 gas have been investigated. As is known, durability of sensor plays a vital role in application point of view. In this regard, the long term stability of WO3 nanoflake thin film sensor for NH3 detection is recorded over the period of 32 days and depicted in Figure 7(b). The stability curve exhibits, the sensor response of WO3 nanoflake thin film is fairly stable over the period of recorded time i.e.32 days. 11
Since, the gas sensing performance of metal oxide is optimized by tailoring the structure and morphology. Also, currently the study of nanostructured WO3 mainly focuses on 1D/2D structures [42-43]. The 2D nanostructures such as nanoflake or nanosheet provides large specific surface area of building blocks and porous nanostructure formed by the arrangement of these building blocks found to be an effective nanostructure towards improving gas sensing properties of WO3 [44]. The improved of gas sensing performance of WO3 nanoflakes may be attributed to its surface morphology. The small thickness and of
ro of
WO3 nanosheets with larger lateral width will lead to higher surface to volume ratio and the evidence of the same can be confirmed from SEM images (Fig. 3). Such nature of WO3 nanoflakes like morphology increases the number of active sites thereby increasing the
increasing the sensitivity of the sensor to NH3 gas.
-p
surface oxygen content, and improves the ability of surface to adsorb reducing gases, thereby
re
It is noticed that the sensor response of the WO3 nanoflaked thin film shows increasing nature with an increase in operating temperature and reaches to a maximum value of ~73 %
lP
at 150 ℃. Further, as this operating temperature increases, the sensor response decreases gradually. Such kind of behavior of WO3 thin film sensor can be explained based on
na
adsorption, desorption and diffusion kinetics of gas molecules on the surface of the sensor. At higher temperature, the oxygen molecules dissociate easily and get adsorbed on the active
ur
sites available on the surface of the sensor which cause an obstruction to adsorbed NH3 gas
Jo
molecules on the active sites of the sensor surface [45]. In addition, the rate of desorption is dominated by the rate of adsorption of the target gas molecule, which reduces the time available for the target gas molecule to interact with active sites. Hence the response at higher temperatures is found to be declined. Hereafter, the optimum operating temperature, i.e. 150 ℃ is chosen for the further investigation of NH3 gas sensing behavior.
12
From the application point of view, the cyclic response to repeated exposure of target gas is an essential characteristic to study. For this purpose, the WO3 nanoflakes thin-film sensor was repeatedly exposed to different concentrations of NH3 gas from 20 to 120 ppm at an optimum operating temperature of 150 ℃ and the change in resistance was recorded. The dynamic sensor response at different concentrations is shown in Fig. 8(a). It is observed that the electrical resistance of the sensor decreases sharply when exposed to NH3 gas and reaches to saturation. The resistance is restored to initial value upon removal of the test gas. Since the
ro of
NH3 is reducing gas and the monotonous decrease of resistance with an increase of NH3 gas concentration proves the basic feature of n-type semiconducting nature of the sensor. The
corresponding plot of gas response as a function of gas concentration is shown in Fig. 8 (b).
-p
From Fig. 8 (b), it can be concluded that, with the increase in concentration of NH3 gas, the
sensor response of WO3 thin film increases. Also, the cyclic response test confirms that WO3
re
thin-film sensor exhibit the reversible and repeatable electrical response to NH3 gas.
lP
The response and recovery times of the sensor were estimated from a single pulse of the dynamic curve shown in Fig. 8(c), recorded at 120 ppm concentration of NH3 gas and optimum operating temperature of 150 ℃. The values of response and recovery times are
na
estimated as ~28 and 68 s, respectively. The response and recovery time as a function of operating temperature for a fixed concentration of NH3 gas, i.e. 120 ppm is shown in Fig. 8
ur
(d). The plot reveals that operating temperature has a great influence on the response and
Jo
recovery time as well as the overall gas response of the sensor. Also, the recovery time of the sensor is found to be higher compared to the response time. This can be attributed to the operating temperature as well as a slower rate of desorption of reaction product from the sensor surface, then the rate of adsorption of oxygen molecule from the air atmosphere [46]. 4. Gas Sensing Mechanism:
13
The gas sensing characteristics of WO3 nanoflakes thin films reveals the n-type semiconductor behavior of the sensor. Therefore, the gas sensing mechanism is proposed based on the change in electrical resistance of the material when exposed to target gas and air. In the case of metal oxide semiconductor, the gas sensing mechanism strongly depends on the operating temperature as well as adsorption, desorption, and dissociation of oxygen gas molecules on the surface of sensing material. When such n-type WO3 thin film is exposed to an air atmosphere, the oxygen gas molecules are adsorbed on the surface of the sensor.
ro of
These electrophilic adsorbed oxygen gas molecules capture electrons from the conduction band of WO3 thin film, thereby forming chemisorbed oxygen species in the form of O2–, O–
and O2– and this formation depends upon temperature [47]. As a result of this, depletion layer
-p
is formed at the surface of the sensor and consequently, the electron transportation is
hampered thereby giving stable resistance of the WO3 sensor. At lower temperatures, i.e.
re
below 100 ℃, O2– form is chemisorbed as it has lower activation energy, whereas from
lP
temperature 100 – 300 ℃, O– form is chemisorbed and above 300 ℃, O2– form is chemisorbed due to the high activation energy [48]. The reaction of adsorbed oxygen ions on the sensor surface is written as follows:
na
𝑂2 (𝑔𝑎𝑠) ↔ 𝑂2 (𝑎𝑑𝑠)
𝑂2 (𝑎𝑑𝑠) + 𝑒 − ↔ 𝑂2− (𝑎𝑑𝑠)
<100 ℃
(2) (3) (4)
𝑂− (𝑎𝑑𝑠) + 𝑒 − ↔ 𝑂2− (𝑎𝑑𝑠)
(5)
>300 ℃
Jo
ur
𝑂2− (𝑎𝑑𝑠) + 𝑒 − ↔ 2𝑂 − (𝑎𝑑𝑠) 100-300 ℃
When WO3 thin film is exposed to NH3 gas environment, the NH3 gas molecules get
adsorbed on the film surface and interact with chemisorbed oxygen species and some of the NH3 gas molecules diffuse into WO3 thin film. Since NH3 is reducing gas; it releases electrons to the sensor surface, resulting in an increase in electron carrier concentration of the WO3 sensor thereby decreasing the resistance of the film sensor. The magnitude of change in 14
resistance in this process directly portrays into gas response of the WO3 thin-film sensor. In the present study, the operating temperature majorly falls between 100 - 300 ℃, therefore the chemisorbed O– form of oxygen species are dominant. The interaction of NH3 gas with preadsorbed oxygen and WO3 thin-film sensor is written as follows: 2𝑁𝐻3 + 3𝑂− (𝑎𝑑𝑠) ↔ 𝑁2 + 3𝐻2 𝑂 + 3𝑒 −
(6)
2𝑊𝑂3 + 2𝑂− (𝑎𝑑𝑠) ↔ 2[(𝑊𝑂3 )𝑂− ]
(7)
3[(𝑊𝑂3 )𝑂− ] + 2𝑁𝐻3 ↔ 3𝑊𝑂3 + 𝑁2 + 3𝐻2 𝑂 + 3𝑒 −
(8)
ro of
Later, when the WO3 thin film is re-exposed to the air atmosphere, the reversible reaction
takes place which involves simultaneous desorption of NH3 molecules and chemisorptions of oxygen molecules on the sensor surface. As a result of this, the resistance of the sensor is
-p
restored to the original value. Further, It is observed from Fig. 7 the sensor response is
maximum at a particular temperature, such a high response is attributed to higher rate of
re
adsorption and dissociation of adsorbed oxygen or/and target gas. This rate is higher than the
lP
rate of desorption at that temperature. Also, the optimum operating temperature provides a sufficient amount of activation energy to facilitate the interaction of target gas with the
na
adsorbed oxygen species. In addition, nanoflakes like morphology of WO3 thin film also provide porous structure due to the interconnecting nanoflakes which promote the diffusion of NH3 gas through these pores. Therefore target gas molecules have plenty of active sites
ur
avialble for interaction, as result of it the sensor delivers a high sensor response at that
Jo
temperature [49]. All the above-discussed parameters are likely to contribute in improving the gas response of WO3 nanoflakes thin film compared to other morphology illustrated in Table 1.
Table.1 Comparison of gas sensing properties towards of different morphology of WO3 Morphology of WO3
NH3 gas concentration (ppm)
Operating temperature (℃)
Sensor response (S) = (Ra-Rg/Ra) x 100
Ref.
= Ra/Rg 15
Nanowires
300
200
~57 %
2.1
7
Nanoplates
200
300
~35 %
2.5
27
Filamentous
250
225
~13 %
0.10
50
Nanoconifer
300
175
~52 %
2.1
29
Nanoflakes
120
150
~73 %
3.1
Present
network
5. Conclusion: The WO3 thin films characterized by nanoflakes like morphology were grown on FTO
ro of
substrate with a seed layer, under hydrothermal conditions. The XRD pattern and Raman spectroscopy of WO3 thin films confirm the monoclinic structure. The gas sensing
characteristics performed under static condition reveals, WO3 thin film exhibits good
-p
selectivity towards NH3 by giving a maximum response at a temperature of 150 ℃. The improved gas sensing characteristics of WO3 thin film for NH3 gas may be attributed to
re
nanoflakes like morphology, which provides a large surface area and hence, plenty of active
lP
sites are available for adsorption/desorption, and dissociation of oxygen/target gas molecules. In addition, the interconnected nanoflakes form voids between them through which the NH3
na
gas can easily diffuse and interact with the sensing material. Also, these interconnected nanoflakes make the charge carrier transportation easier which indeed helps in improving the response of sensor. The sensor response of WO3 thin film is found to be ~ 73 % towards 120
ur
ppm of NH3 gas with a smaller value of response and recovery times of 28 and 68 s
Jo
respectively as well as excellent repeatability and reproducibility suggesting its potential application as a NH3 gas sensor.
Conflict of interest Authors do not have conflict of interest.
16
Author statement Pankaj S. Kolhe: Conceptualization, Methodology, Data curation, Formal analysis, Writing - Original Draft preparation. Pallavi Mutadak: Formal analysis, Investigation. Namita Maiti: Supervision, Funding acquisition. Kishor M. Sonawane: Conceptualization, Methodology, Supervision, Funding acquisition, Writing- Reviewing and Editing
Acknowledgement:
ro of
Authors, gratefully acknowledges for financial support from BARC, Mumbai, for the SRF
Jo
ur
na
lP
re
-p
award under DAE-BRNS (Sanction No. 34/14/61/2014-BRNS) research project scheme.
17
Reference: [1]
J. Ma, L. Chang, J. Lian, Z. Huang, X. Duan, X. Liu, P. Peng, T. Kim, Z. Liu, W.
Zheng, Ionic liquid-modulated synthesis of ferrimagnetic Fe3S4 hierarchical superstructures, Chem. Commun. 46(27) (2010) 5006-5008. [2]
P.K. Labhane, L.B. Patle, V.R. Huse, G.H. Sonawane, S.H. Sonawane, Synthesis of
reduced graphene oxide sheets decorated by zinc oxide nanoparticles: crystallographic,
[3]
ro of
optical, morphological and photocatalytic study. Chem. Phys. Let. 661 (2016) 13-19. B. Huang, X. Li, Y. Pei, S. Li, X. Cao, R. C. Massé, G. Cao, Novel
Carbon‐Encapsulated Porous SnO2 Anode for Lithium‐Ion Batteries with Much Improved
[4]
-p
Cyclic Stability, Small 12(14) (2016) 1945-1955.
X. Zhang, J. Qin, Y. Xue, P. Yu, B. Zhang, L. Wang, R. Liu, Effect of aspect ratio
re
and surface defects on the photocatalytic activity of ZnO nanorods, Scientific report 4 (2014)
[5]
lP
4596.
L. Gao, Z. Cheng, Q. Xiang, Y. Zhang, J. Xu, Porous corundum-type In2O3
nanosheets: synthesis and NO2 sensing properties, Sens. Actuators. B: Chem. 208 (2015)
[6]
na
436-443.
S. Hilliard, G. Baldinozzi, D. Friedrich, S. Kressman, H. Strub, V. Artero, C. Laberty-
ur
Robert, Mesoporous thin film WO3 photoanode for photoelectrochemical water splitting: a
Jo
sol–gel dip coating approach, Sustain. Energy. Fuels. 1(1) (2017) 145-153. [7]
N.V. Hieu, D.T.T. Le, N.D. Khoang, N.V. Quy, N. Hoa, P. Tam, A.T. Le, T. Trung, A
comparative study on the NH3 gas-sensing properties of ZnO, SnO2, and WO3 nanowires. Inter. J. Nanotech. 8(3-5) (2011) 174-187.
18
[8]
Z. Chen, J. Wang, G. Zhai, W. An, Y. Men, Hierarchical yolk-shell WO3
microspheres with highly enhanced photoactivity for selective alcohol oxidations, Appl. Catal. B: Environ. 218 (2017) 825-832. [9]
E. Khoo, P.S. Lee, J. Ma, Electrophoretic deposition (EPD) of WO3 nanorods for
electrochromic application, J. Euro. Ceram. Soc. 30(5) (2010) 1139-1144. [10]
H. Zheng, Y. Tachibana, K. Kalantar-zadeh, Dye-sensitized solar cells based on
WO3, Langmuir. 26(24) (2010) 19148-19152. F. Amano, E. Ishinaga, A. Yamakata, Effect of particle size on the photocatalytic
ro of
[11]
activity of WO3 particles for water oxidation, J. Phy. Chem. C. 117(44) (2013) 22584-22590. [12]
Z. Wang, P. Sun, T. Yang, Y. Gao, X. Li, G. Lu, Y. Du, Flower-like WO3
Sens. Actuators. B: Chem. 186 (2013) 734-740.
F. Mitsugi, E. Hiraiwa, T. Ikegami, K. Ebihara, Pulsed laser deposited WO3 thin
re
[13]
-p
architectures synthesized via a microwave-assisted method and their gas sensing properties,
[14]
lP
films for gas sensor, Surf. Coat. Tech. 169 (2003) 553-556.
Y. Shimizu, N. Matsunaga, T. Hyodo, M. Egashira, Improvement of SO2 sensing
properties of WO3 by noble metal loading, Sens. Actuators. B: Chem. 77(1-2) (2001) 35-40. T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Gold-loaded tungsten oxide sensor
na
[15]
for detection of ammonia in air, Chem. Lett. 21(4) (1992) 639-642. Y. Shen, B. Zhang, X. Cao, D. Wei, J. Ma, L. Jia, S. Gao, B. Cui, Y. Jin,
ur
[16]
Jo
Microstructure and enhanced H2S sensing properties of Pt-loaded WO3 thin films, Sens. Actuators. B: Chem. 193 (2014) 273-279. [17]
J.Z. Ou, M.H. Yaacob, J.L.Campbell, M. Breedon, K. Kalantar-Zadeh, W. Wlodarski,
H2 sensing performance of optical fiber coated with nano-platelet WO3 film, Sens. Actuators. B: Chem. 166 (2012) 1-6.
19
[18]
D. Chen, X. Hou, H. Wen, Y. Wang, H. Wang, X. Li, R. Zhang, H. Lu, H. Xu, S.
Guan, J. Sun, The enhanced alcohol-sensing response of ultrathin WO3 nanoplates, Nanotechnol. 21(3) (2009) 035501. [19]
B. Timmer, W. Olthuis, A. Van Den Berg, Ammonia sensors and their applications—
a review, Sens. Actuators. B: Chem. 107(2) (2005) 666-677. [20]
J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Hierarchical oxide nanostructures, J.
Mater. Chem. 14(4) (2004) 770-773. D. Xue, Y. Wang, J. Cao, G. Sun, Z. Zhang, Improving methane gas sensing
ro of
[21]
performance of flower-like SnO2 decorated by WO3 nanoplates, Talanta, 199 (2019), 603611.
N. Lu, X. Gao, C. Yang, F. Xiao, J. Wang, X. Su, Enhanced formic acid gas-sensing
-p
[22]
property of WO3 nanorod bundles via hydrothermal method, Sens. Actuators. B: Chem. 223
D. Meng, G. Wang, X. San, Y. Song, Y. Shen, Y. Zhang, K. Wang, F. Meng,
lP
[23]
re
(2016) 743-749.
Synthesis of WO3 flower-like hierarchical architectures and their sensing properties, J. Alloys Compd. 649 (2015) 731-738.
Z. Liu, M. Miyauchi, T. Yamazaki, Y. Shen, Facile synthesis and NO2 gas sensing of
na
[24]
514-519.
Y. Qin, F. Wang, W. Shen, M. Hu, Mesoporous three-dimensional network of
Jo
[25]
ur
tungsten oxide nanorods assembled microspheres Sens. Actuators. B: Chem. 140(2) (2009)
crystalline WO3 nanowires for gas sensing application, J. Alloys Compd. 540 (2012) 21-26. [26]
S. Cao and H. Chen, Hydrothermal synthesis, characterization, and their gas sensing
properties, J. Alloys Compd. 702 (2017) 644-648.
20
[27]
H. Zhang, Z. Liu, J. Yang, W. Guo, L. Zhu, W. Zheng, Temperature and acidity
effects on WO3 nanostructures and gas-sensing properties of WO3 nanoplates. Mater. Research Bull. 57 (2014), 260-267. [28]
M. Takács, C. Dücső, Z. Lábadi, C. Pap, A. Edit, Takács, M., Dücső, C., Lábadi, Z.
and Pap, A.E., 2014. Effect of hexagonal WO3 morphology on NH3 sensing, Procedia Engineering. 87 (2014) 1011-1014. [29]
P.S. Kolhe, P.S. Shirke, N. Maiti, M.A. More, K.M. Sonawane, Facile Hydrothermal
ro of
Synthesis of WO3 Nanoconifer Thin Film: Multifunctional Behavior for Gas Sensing and Field Emission Applications, J. Inorg. Organo. Poly. Mater. 29(1) (2019) 41-48. [30]
W. Zeng, H. Zhang, Z. Wang, Effects of different petal thickness on gas sensing
-p
properties of flower-like WO3• H2O hierarchical architectures, Appl. Surf. Sci. 347 (2015) 73-78.
C. Wang, X. Li, C. Feng, Y. Sun, G. Lu, sheets assembled hierarchical flower-like
re
[31]
lP
WO3 nanostructures: Synthesis, characterization, and their gas sensing properties, Sens. Actuators. B: Chem. 210 (2015) 75-81. [32]
C. Wang, R. Sun, X. Li, Y. Sun, P. Sun, F. Liu, G. Lu, Hierarchical flower-like WO3
na
nanostructures and their gas sensing properties, Sens. Actuators. B: Chem. 204 (2014) 224230.
Z.X. Cai, H.Y. Li, J.C. Ding, X. Guo, Hierarchical flowerlike WO3 nanostructures
ur
[33]
Jo
assembled by porous nanoflakes for enhanced NO gas sensing, Sens. Actuators. B: Chem. 246 (2017) 225-234. [34]
R. Mukherjee, A. Kushwaha, P.P. Sahay, Spray-deposited nanocrystalline WO3 thin
films prepared using tungsten hexachloride dissolved in NN dimethylformamide and influence of in doping on their structural, optical and electrical properties, Electronic. Mater. Lett. 10(2) (2014) 401-410.
21
[35]
H. Habazaki, Y. Hayashi, H. Konno, Characterization of electrodeposited WO3 films
and its application to electrochemical wastewater treatment, Electrochima. Acta. 47(26) (2002) 4181-4188. [36]
L. Zhou, Q. Ren, X. Zhou, J. Tang, Z. Chen, C. Yu, Comprehensive understanding on
the formation of highly ordered mesoporous tungsten oxides by X-ray diffraction and Raman spectroscopy, Microporous. Mesop. Mater. 109(1-3) (2008) 248-257. [37]
T. Siciliano, A. Tepore, G. Micocci, A. Serra, D. Manno, E. Filippo, WO3 gas sensors
326. [38]
Z.X. Cai, H.Y. Li, X.N. Yang, X. Guo, NO sensing by single crystalline WO3
re
D.J. Dwyer, Surface chemistry of gas sensors: H2S on WO3 films, Sens. Actuators.
B: Chem. 5(1-4) (1991) 155-159.
T. Jin, D. Xu, P. Diao, W.P. He, H.W. Wang, S. Z. Liao, Tailored preparation of
lP
[40]
-p
nanowires, Sens. Actuators. B: Chem. 219 (2015) 346-353. [39]
ro of
prepared by thermal oxidization of tungsten, Sens. Actuators B: Chem. 133(1) (2008) 321-
WO3 nano-grassblades on FTO substrate for photoelectrochemical water splitting, CrystEngComm. 18(36) (2016) 6798-6808.
R.F. Garcia-Sanchez, T. Ahmido, D. Casimir, S. Baliga, P. Misra, Thermal effects
na
[41]
associated with the Raman spectroscopy of WO3 gas-sensor materials, J. Phy. Chem. A
C.S. Moon, H.R. Kim, G. Auchterlonie, J. Drennan, J.H. Lee, Highly sensitive and
Jo
[42]
ur
117(50) (2013) 13825-13831.
fast responding CO sensor using SnO2 nanosheets Sens. Actuators. B: Chem. 131(2) (2008), 556-564. [43]
Y. Wang, X.N. Meng, J.L. Cao, Rapid detection of low concentration CO using Pt-
loaded ZnO nanosheets. J. hazardous mater. 381 (2020), 120944.
22
[44]
W. Zeng, H. Zhang, Z. Wang, Effects of different petal thickness on gas sensing
properties of flower-like WO3• H2O hierarchical architectures, Appl. Surf. Sci. 347 (2015), 73-78. [45]
C. Wang, R. Sun, X. Li, Y. Sun, P. Sun, F. Liu, G. Lu, Hierarchical flower-like WO3
nanostructures and their gas sensing properties, Sens. Actuators. B: Chem. 204 (2014) 224230. [46]
S. Poongodi, P.S. Kumar, D. Mangalaraj, N. Ponpandian, P. Meena, Y. Masuda, C.
sensor applications, J. Alloys Compd. 719 (2017) 71-81. [47]
N. Yamazoe, G. Sakai, K. Shimanoe, Oxide semiconductor gas sensors, Catalysis
-p
Surveys from Asia. 7(1) (2003) 63-75. [48]
ro of
Lee, Electrodeposition of WO3 nanostructured thin films for electrochromic and H2S gas
A. Afzal, N. Cioffi, L. Sabbatini, L. Torsi, NOx sensors based on semiconducting
re
metal oxide nanostructures: progress and perspectives, Sens. Actuators. B: Chem. 171 (2012)
[49]
lP
25-42.
P.S. Shewale, G.L. Agawane, S.W. Shin, A.V. Moholkar, J. Lee, J.H. Kim, M.D.
Uplane, Thickness dependent H2S sensing properties of nanocrystalline ZnO thin films
[50]
na
derived by advanced spray pyrolysis, Sens. Actuators. B: Chem. 177 (2013) 695-702. R. Godbole, V.P. Godbole, S. Bhagwat, Surface morphology dependent tungsten
Jo
ur
oxide thin films as toxic gas sensor, Mater. Sci. Semi. Proc. 63 (2017) 212-219.
23
Figure File Figure Captions: Fig. 1 X-ray diffraction pattern of WO3 nanostructured thin film. Fig. 2 Room temperature Raman spectrum of WO3 nanostructured thin film. Fig. 3 SEM micrographs of WO3 nanoflakes thin film a) lower magnified image; b) higher magnified image.
ro of
Fig. 4 TEM micrographs of WO3 nanoflakes thin film a) & b) TEM image; c) HRTEM image; d) SAED pattern. Fig. 5 High resolution XPS peaks of a) W 4f and b) O1s states
Fig. 6 UV-Visible absorption spectra of WO3 nanoflakes thin film with inset showing corresponding Tauc’s plot.
re
-p
Fig. 7 a) Plot of sensor response as a function of temperature (50 - 300 ℃) of WO3 nanoflake thin film towards 120 ppm of H2S, CO and NH3 gas concentration, and b) long-term stability recorded at 150 ℃ for 120 ppm of NH3 gas.
Jo
ur
na
lP
Fig. 8 a) The dynamic transient response of the WO3 nanoflakes film sensor; b) gas response as a function of NH3 gas concentration (20 - 120 ppm); c) enlarged sensing response curve for response and recovery time towards 120 ppm concentration of NH3 gas at 150 ℃; d) Plot of response and recovery time as a function of operating temperature (50-300 ℃) towards 120 ppm of NH3 gas.
24
-p
ro of
Figure File:
Jo
ur
na
lP
re
Fig. 1 X-ray diffraction pattern of WO3 nanostructured thin film.
Fig. 2 Room temperature Raman spectrum of WO3 nanostructured thin film.
25
Jo
ur
na
lP
re
-p
ro of
Fig. 3 SEM images of WO3 nanoflakes thin film a) lower magnified image; b) higher magnified image.
Fig. 4 TEM micrographs of WO3 nanoflakes thin film a) & b) TEM image; c) HRTEM image; and d) corresponding SAED pattern. 26
ro of
ur
na
lP
re
-p
Fig. 5 High resolution XPS peaks of a) W 4f and b) O1s states
Jo
Fig. 6 UV-Visible absorption spectra of WO3 nanoflakes thin film with inset showing corresponding Tauc’s plot.
27
ro of
Jo
ur
na
lP
re
-p
Fig. 7 a) Plot of sensor response as a function of temperature (50 - 300 ℃) of WO3 nanoflake thin film towards 120 ppm of H2S, CO and NH3 gas concentration, and b) long-term stability recorded at 150 ℃ for 120 ppm of NH3 gas.
28
ro of -p re lP
Jo
ur
na
Fig. 8 a) The dynamic transient response of the WO3 nanoflakes film sensor; b) gas response as a function of NH3 gas concentration (20 - 120 ppm); c) enlarged sensing response curve for response and recovery time towards 120 ppm concentration of NH3 gas at 150 ℃; d) Plot of response and recovery time as a function of operating temperature (50-300 ℃) towards 120 ppm of NH3 gas.
29
a
ro of
Author’s Biography Pankaj S Kolhe received Master’s degree in Physics from Fergusson College affiliated to
Savitribai Phule Pune University (SPPU), Pune, India. He received M.Phil degree in 2015
and Ph.D. Degree in 2019 from Savitribai Phule Pune University (SPPU), Pune. His research
Pallavi Mutadak received Master’s degree in Physics from Department of Physics,
re
b
-p
areas include synthesis of Metal Oxide Gas Sensor for Toxic Gases.
Savitribai Phule Pune University (SPPU), Pune, India. Presently, she is perusing Ph.D. in
lP
Department of Physics SPPU. Her research areas include synthesis of heterostructures and
c
na
their work-function and band bending study using XPS and UPS techniques.
Dr. Namita Maiti is Scientific Officer (H) Laser & Plasma Technology Division, Bhabha
ur
Atomic Research Centre Mumbai, India. She received B.E. (Electrical Engineering) from Calcutta University in 1986, M.Tech. (Control Instrumentation) from Indian Institute of Technology, Bombay in 1995 and PhD from HBNI, University of Mumbai in 2013.
Jo
Currently, she is involved in the design of indirectly heated cathode based electron beam welding and melting machines and box coater system for multilayer coating.
a*
Dr. Kishor M. Sonawane is a Associate Professor at Department of Physics, Fergusson
College (Autonomous) affiliated to Savitribai Phule Pune University, India. He received Ph.D. Degree in 2006 from Swami Ramanand Tirth Marathwada University, Nanded, India. 30
He has concurrently served as a reviewer of Journal of alloys and Compounds and Journal of Inorganic and Oraganometallic Polymers and Materials. His current research is focused on
Jo
ur
na
lP
re
-p
ro of
the Metal Oxide Thin Films for Gas Sensing Application.
31
PII:
S0924-4247(19)31881-3
DOI:
https://doi.org/10.1016/j.sna.2020.111877
Reference:
SNA 111877
To appear in:
Sensors and Actuators: A. Physical
Received Date:
11 October 2019
Revised Date:
9 January 2020
Accepted Date:
29 January 2020
Please cite this article as: Kolhe PS, Mutadak P, Maiti N, Sonawane KM, Synthesis of WO3 Nanoflakes by Hydrothermal Route and its Gas Sensing Application, Sensors and Actuators: A. Physical (2020), doi: https://doi.org/10.1016/j.sna.2020.111877
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier.
Synthesis of WO3 Nanoflakes by Hydrothermal Route and its Gas Sensing Application
Pankaj S. Kolhea, Pallavi Mutadakb, Namita Maitic, Kishor M. Sonawanea*
a
ro of
Department of Physics, Fergusson College affiliated Savitribai Phule Pune University, Pune 411004, India b Centre for Advanced Studies in Material Science and Condensed Matter Physics, Department of Physics, Savitribai Phule Pune University, Pune 411007, India c Laser & Plasma Technology Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
re
-p
*Corresponding author: Email: [email protected]
Jo
ur
na
lP
Graphical Abstract:
1
ro of -p re
Highlights
WO3 seed layer deposited by spray pyrolysis for growth of WO3 nanoflaked thin film
lP
using hydrothermal method.
Flakes like morphology provide large surface area and plenty of active sites to interact
na
with target gas thereby improving gas sensing. High selectivity towards NH3 gas detection.
Higher sensitivity with fast response and recovery time at comparatively lower
ur
Jo
temperature (150 0C) is obtained.
Abstract:
2
The controlled morphology and size of inorganic materials have attracted intense interest, as these parameters play an important role in determining sensing properties. Herein, WO3 thin film is hydrothermally grown on FTO substrate at 175 ℃ with the assistance of seed layer deposited by spray pyrolysis technique. The WO3 thin film was characterized by X-ray Diffraction (XRD), micro-Raman spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and UV-Visible Spectroscopy for determination of physico-chemical properties. Moreover, X-ray Photoelectron Spectroscopy (XPS) analysis
ro of
is carried out to understand chemical states and boding. Systematic gas sensing studies were performed for NH3, H2S and CO gases under static condition. The sensing study reveals,
WO3 nanoflake exhibits a superior sensor response to NH3 gas. Moreover, it exhibits higher
-p
sensitivity to NH3. Gas sensing properties indicate WO3 nanoflakes holds promise to become
re
a potential candidate for NH3 gas detection at the expense of lower power consumption. Keywords: Pre-seeding WO3; WO3 nanoflakes; hydrothermal method; NH3 gas sensing.
lP
1. Introduction:
Nowadays, a lot of attention has been attracted by the inorganic metal oxide
na
semiconductor nanomaterials owing to ease of synthesis technique and potential applications in diversified domains. Many researchers are focusing on synthesizing metal oxide
ur
nanostructures of various shapes and/or sizes, exhibiting specifically tailored physicochemical properties, which can be molded into devices [1,2]. Much interest, therefore has
Jo
been shown in the study of metal oxide semiconductors such as SnO2, ZnO, In2O3, and WO3 [3-7]. Among all of them, Tungsten oxide (WO3) is n-type semiconductor with direct band gap energy of 2.6 eV [8]. Due to the exotic physico-chemical properties, it shows potential towards technological applications in many fields such as electrochromic devices, solar cells, photo-catalysts and gas sensors [9-12].
3
For the detection of toxic gases, majorly SnO2 and ZnO based gas sensors are commonly used, but these gas sensors exhibit low stability, reproducibility, and selectivity [13]. Therefore, WO3 has been regarded as a promising material for gas sensor due to its low cost, simple synthesis technique and high sensitivity towards toxic and flammable gases/vapors such as SO2, NH3, H2S, H2 and alcohol vapors [14-18]. In recent times, the detection and monitoring of such hazardous gases have become a prime need to protect the environment and human health. Particularly, ammonia (NH3) is very much useful in chemical industries
ro of
and image processing in various fields. It is a colorless, toxic gas with a pungent smell and can cause incurable damage to human health. The minimum permissible limit of NH3 is
tabulated around 50 ppm [19]. Above this limit, a human can feel irritation to the skin, eye
-p
and respiratory system. More attention, therefore has been devoted to the fabrication of ammonia gas detecting sensors.
re
Nanostructured morphology of material plays an important role in the field of gas sensors,
lP
because of their high surface to volume ratio, surface activity, porosity and quantum size effect [20-21]. In addition, it is well known that from application point of view morphology, crystalline size and specific surface area of the sensor material governs its performance [22].
na
In order to achieve better gas sensing performance, many attempts have been made in the synthesis of WO3 nanostructures with different morphologies. To study the effect of
ur
morphology on ethanol vapor sensing, a facile hydrothermal method was employed to obtain
Jo
flower-like morphology of WO3 by varying citric acid concentration, reported by D. Meng et. al. [23]. Z. Liu and co-authors synthesized the 3D structure of WO3 nanorods emanating from a microsphere and studied the effect of annealing temperature on these microspheres structures, as well as NO2 gas sensing behavior [24]. D. Chen et. al. have synthesized singlecrystalline 2D WO3 nameplates via the topochemical transformation process for alcohol sensing study [18]. The effect of operating temperature on the response and recovery time
4
and enhancement in sensitivity due to the ultrathin plate-like morphology was proposed by the authors. Y. Qin and group have reported the synthesis of mesoporous 3D network of WO3 nanowires for NO2 gas sensing and it was observed that WO3 nanowires exhibit high sensitivity and selectivity to NO2 gas along with short response and recovery time at low operating temperatures for ppb level gas [25]. The enhanced or high response of the sensor in a nano-sized regime can be attributed to maximum surface to volume ratio, which makes available large number of active sites on the sensor surface for the interaction with the target
ro of
gas [26]. Zhang et. al. reported the effect of temperature and type of acid on nanoplates like morphology of WO3 nanostructure to investigate gas sensing properties of four different gases [27]. Takacs et. al. have synthesized two samples of WO3 nanoparticles by acidic
-p
precipitation from sodium tungstate solution, one using oxalic acid (type A) and the other by Zocher method (type B) and the layer is deposited using capillary dropping technique for
re
examining sensing of NH3 gas [28]. The authors claimed that the morphology plays an
lP
important role in NH3 sensing, due to which sample B exhibited superior performance. Attempts have been carried out to synthesis of WO3 nanostructures, characterized by unique morphology offering a large surface area, with improved sensing properties for the real-time
na
monitoring application. Also, ammonia gas sensing behavior of nanoconifer like morphology of
[29].
ur
WO3 formed on tungsten substrate using hydrothermal route were stuided previously by our group
Jo
To facilitate a better electron conduction mechanism to improve the sensor characteristics, it is necessary to provide conduction paths for charge transportation. Hence, nanoflowers and nanoflakes like morphologies are reported to exhibit better sensitivity [3033]. In the present work, WO3 thin films possessing nanoflakes morphology were grown on WO3 seeded FTO substrate using a hydrothermal method. The synthesized WO3 film was tested for the detection of toxic gases such as H2S, CO, and NH3. The gas sensing study
5
reveals that WO3 thin film exhibits good gas sensing performance at optimal operating temperature ~150 ℃ to NH3 gas with fairly short response (28 s) and recovery time (68 s). An attempt has been made to provide a plausible gas sensing mechanism, based on the correlation of morphology, effective surface area and interaction of the test gas molecules (adsorption, desorption and/or diffusion) of gas molecules on the surface of the sensor. 2. Experimental Details: 2.1. Synthesis of WO3 Nanoflakes Thin Film:
ro of
Prior to the hydrothermal method, WO3 seeding was carried out on the FTO substrate (commercially available) by spray pyrolysis deposition. The precursor solution for spray
deposition was prepared by dispersing 0.18 g of tungsten oxide (WO3) powder (Alfa Aesar,
-p
99.5% purity) in a 30 ml solution of ammonia solution (Thomas Baker, AR grade) and Deionized (DI) water in 2:1 proportion. This solution was then sprayed on FTO substrate
re
preheated at 400 ℃, with air as a carrier gas. The deposited thin film was annealed at 500 ℃
lP
in air for 2 h. The WO3 seed formation was confirmed using XRD and SEM. The corresponding XRD pattern and SEM image of the WO3 seed layer on the FTO substrate are
na
depicted in Supplementary Fig. (1) and (2) respectively. After confirmation of the seed layer, the synthesis of WO3 thin film was carried out
ur
using the hydrothermal method. First, H2WO4 solution was prepared by dissolving 1.25 g of H2WO4 (Loba Chemie, 99% purity) into 10 ml of H2O2 (Fisher scientific, 30% w/v) and 30
Jo
ml DI water, stirring the solution at 95 ℃. The resultant H2WO4 solution was then diluted to 0.05 M i.e. adding 100 ml DI water. The clear solution thus obtained was further used for the hydrothermal process. The precursor solution for hydrothermal process was prepared by using 12 ml of H2WO4 (0.05 M) solution, 0.08 g of oxalic acid (Thomas Baker, AR grade), 0.08 g of urea (Avra, AR grade), 50 ml of acetonitrile (VETEC, HPLC 99.9 %) and 2 ml HCl (6 M) (Thomas Baker, 35% AR grade) were added into beaker and stirred for 30 min. The 6
WO3 seed coated FTO substrate was placed into 100 ml Teflon lined stainless steel autoclave filled with the as-prepared precursor solution and kept at 180 ℃ for 6 h in an oven. After the completion of the reaction duration, the furnace was allowed to cool down to room temperature naturally. The FTO substrate was removed from the autoclave, washed with ethanol and DI water several times, followed by annealing in air at 500 ℃ for 2 h. At least, four samples were prepared under identical experimental conditions to check the reproducibility.
ro of
2.2. Characterization and Gas Sensing Study:
The XRD (D8 Advance, Bruker AXS) pattern was recorded to confirm the crystal
structure and phase of the WO3 nanostructured thin film using Cu Kα radiation (λ= 1.54 Å).
-p
The surface morphology was characterized with the help of SEM (JEOL, JSM-6360A). The
re
micro Raman Spectroscopy (Lab RAM HR) with laser excitation lines of 532 nm at room temperature was employed for further confirmation of the phase structure. The chemical
lP
analysis was performed by X-ray Photoelectron Spectrometer (XPS, Thermo K-Alpha+ Spectrometer) using Al-Kα X -rays, 1486.6 eV). The UV-Visible absorption spectroscopy
na
(JASCO 760) was used to evaluate the energy band gap of WO3 thin film. The as-prepared WO3 nanoflakes thin film was placed in a stainless steel chamber
ur
equipped with multiple feed-through for electrical connections and gas inlet, to examine the gas sensing characteristics towards the toxic gases such as H2S, CO and NH3 (Dry gases,
Jo
commercially available) under static condition. The operating temperature of the thin film was provided using a ceramic infrared heater capable of reaching up to 500 ℃, and the temperature was measured using a K-type thermocouple. For the measurement of resistance of the thin-film sensor, the nickel wire was properly connected to Au plated electrodes which are separated by a distance of 8mm on the sensor surface. The sensor response of the WO3 thin film for fixed concentrations of NH3, CO and H2S (one species injected at a time) at 7
different operating temperatures ranging from 50 to 300 ℃, were recorded to obtain optimal operating temperature. Before the test gas injection, the thin film sensor was kept in an ambient atmosphere for the stabilization of electrical resistance for each measurement. Then, the desired quantity of target gas is purged into the gas chamber through a syringe. Computer-controlled data acquisition system embedded Keithley 2450 source meter was used for recording measurements of change in electrical resistance of WO3 thin film. For the obtained optimum operating temperature, dynamic response towards different concentration
ro of
NH3 gas and response and recovery times were evaluated. The sensor response was calculated using a typical formula given below: Sensor response (S)% =
Ra −Rg Ra
× 100
(1)
-p
where Ra and Rg are the electrical resistance of the thin-film sensor in the air and target gas
re
respectively. The response and recovery time of the sensor is defined as the time taken by the sensor to attain 90 % of the variation in resistance in the presence of target gas and air
3. Results and Discussion:
na
3.1. Structural Analysis:
lP
respectively.
The XRD pattern of WO3 nanostructured grown on WO3 seeded FTO substrate using the
ur
hydrothermal method is shown in Fig. 1. All the observed diffraction peaks are indexed to the monoclinic phase of WO3 (JCPDS Card No. 83-0951) [34]. The observed FTO peaks are
Jo
from the substrate. The observed peaks are sharp enough to reveal the crystallinity of WO3 nanostructured film. No other impurity peaks due to other phases are detected, indicating good purity of the as-synthesized product, under prevailing experimental conditions. The micro Raman spectrum of WO3 nanostructured thin film is shown in Fig. 2. It shows well-defined peaks at 807 and 715 cm-1, corresponding to symmetric and asymmetric vibrations of W6+–O bonds, i.e. O–W–O stretching vibration mode (γ), respectively. Also, the 8
Raman bands observed at 325 and 272 cm-1 are assigned to the bending modes of bridging oxygen ions, i.e. W–O–W bending modes (δ) [35-36]. A peak of relatively weaker intensity observed at 136 cm-1 can be ascribed to the lattice vibration of crystalline WO3 [37]. The Raman spectrum of WO3 does not exhibit any peak related to hydrous or hexagonal phase. Moreover, the peaks at 807, 715 and 272 cm-1 are representative of fundamental modes of the monoclinic phase of WO3, thereby confirming that synthesized nanostructured thin film is indeed of stoichiometric WO3 monoclinic phase.
ro of
3.2. Morphological Analysis: 3.2.1 SEM and TEM Analysis:
The surface morphology of the WO3 nanostructured thin film was examined using SEM
-p
and the corresponding images recorded at different magnifications are shown in Fig. 3 (a,b).
re
The lower magnified SEM image (Fig. 3-a) illustrates that homogeneously distributed, randomly oriented nanoflakes are grown over the entire FTO substrate. A careful observation
lP
of a higher magnified image (Fig. 3-b) reveals that the thickness of these nanoflakes is in the range of 50 to 100 nm. Furthermore, it is observed that the nanoflakes of WO3 are randomly
na
oriented and interconnected, due to which the rough surface and porous morphology is formed. Such a kind of nanostructured morphology is useful for trapping gas molecules that
ur
may interact greatly with sensing material and result in enhancement in sensor response. In addition, the flakes like morphology afford a larger surface area, where a large number of
Jo
active sites are available to interact with a target gas molecule [33]. Moreover, the interconnected nanoflakes provide continuous pathways for the charge transportation, which could be an advantage for gas sensors. The morphology and structural features of the WO3 nanoflakes were further studied by TEM and HRTEM as shown in Fig 4. Fig. 4 (a and b) represents high magnification TEM images of WO3 nanoflakes, which reveals the irregular size of nanoflakes are stacked 9
together. Moreover, the TEM image represents the lateral dimensions of the nanoflakes are in the range of 60- 100 nm. The High-Resolution TEM image of WO3 nanoflakes reveals that the nanoflakes have a single-crystalline structure with resolved lattice fringes. The lattice spacing is found to be 0.36 nm and it is readily indexed to d-spacing of (002) plane which is in accordance with the results of XRD in Fig. 1. Fig. 4(d) depicts selected area electron diffraction (SAED) pattern reveals high crystallinity of WO3 with excellent growth along (002) direction to form nanoflakes like morphology.
ro of
3.3 XPS Analysis:
The gas sensing mechanism can be revealed with knowledge of bonding and valence
states of W and O on the surface of WO3 nanoflakes. In this context, XPS analysis is one of
-p
the fundamental tool. Fig. 5(a) depicts, two spin-orbit doublets corresponding to the different
re
valence states of W. The binding energies 37.8 and 35.6 eV correspond to W4f5/2 and W4f7/2 of W6+ states respectively, and rest of smaller doublets are allocated to W5+ states.
lP
Moreover, there is a weak emission originating from W5p3/2 level can be found at binding energy 41.4 eV. Fig. 5 (b) exhibits O1s spectrum in which peak originates at 530.2 eV
na
corresponds to lattice oxygen O2− bonded with W, the peak of OH- groups observed at 530.9 eV along with smaller H2O peak. The peaks of OH- and H2O are may be attributed to water
ur
vapor from the ambiance [38]. The presence of W5+ states (oxygen vacancies) acts donors in the semiconducting state of WO3, these donors have a significant impact on conductivity and
Jo
gas sensing properties of the film [39]. 3.4 Optical Property Analysis: The absorption spectrum of WO3 nanoflakes thin film, recorded in the wavelength range from 200 to 800 nm, is depicted in Fig. 6. It shows more absorption in the UV range. The absorption edge of the thin film is observed at 470 nm which corresponds to the lower extreme of the visible band. The optical band gap is also obtained from the Tauc plot which is 10
depicted in the inset of Fig. 6. From the Tauc plot, the band gap energy of the WO3 nanoflakes thin film is estimated as ~2.7 eV which agrees very well with previously reported value [40]. 3.5 Gas Sensing Study: The gas sensing study of WO3 nanoflakes thin-film sensor towards the toxic gases such as H2S, CO, and NH3 were carried out in indigenously design gas sensing unit. The conductometric method was employed for the measurements, wherein the thin film sensor is
ro of
exposed to the target gas at a certain temperature and corresponding change in resistances is recorded to evaluate sensor response. As is known, selectivity to a particular target gas holds an important factor for a gas sensor, as in many industrial processes, multiple gases could be
-p
evolved. The response of the sensor, therefore, towards various gas needs to be tested to
re
reveal the selectivity of the sensor towards a particular species. To determine the optimum operating temperature and selectivity towards particular gas, sensor response of WO3 thin
lP
film towards a fixed quantity of H2S, CO, and NH3 gas of concentration 120 ppm, in the temperature range 50–300 ℃ was studied. For metal oxide based gas sensors, the temperature
na
play a crucial role as it has a great effect on the response of the sensor [41]. The plot of sensor response as a function of temperature for H2S, CO, and NH3 gases is shown in Fig. 7.
ur
It clearly shows that the WO3 nanoflakes thin film exhibit higher sensor response to NH3 gas at lower operating temperature than H2S and CO gases. This indicates WO3 sensor exhibits
Jo
good selectivity towards NH3 gas. Hence, in further study, the gas sensing characteristics of WO3 thin film sensor towards NH3 gas have been investigated. As is known, durability of sensor plays a vital role in application point of view. In this regard, the long term stability of WO3 nanoflake thin film sensor for NH3 detection is recorded over the period of 32 days and depicted in Figure 7(b). The stability curve exhibits, the sensor response of WO3 nanoflake thin film is fairly stable over the period of recorded time i.e.32 days. 11
Since, the gas sensing performance of metal oxide is optimized by tailoring the structure and morphology. Also, currently the study of nanostructured WO3 mainly focuses on 1D/2D structures [42-43]. The 2D nanostructures such as nanoflake or nanosheet provides large specific surface area of building blocks and porous nanostructure formed by the arrangement of these building blocks found to be an effective nanostructure towards improving gas sensing properties of WO3 [44]. The improved of gas sensing performance of WO3 nanoflakes may be attributed to its surface morphology. The small thickness and of
ro of
WO3 nanosheets with larger lateral width will lead to higher surface to volume ratio and the evidence of the same can be confirmed from SEM images (Fig. 3). Such nature of WO3 nanoflakes like morphology increases the number of active sites thereby increasing the
increasing the sensitivity of the sensor to NH3 gas.
-p
surface oxygen content, and improves the ability of surface to adsorb reducing gases, thereby
re
It is noticed that the sensor response of the WO3 nanoflaked thin film shows increasing nature with an increase in operating temperature and reaches to a maximum value of ~73 %
lP
at 150 ℃. Further, as this operating temperature increases, the sensor response decreases gradually. Such kind of behavior of WO3 thin film sensor can be explained based on
na
adsorption, desorption and diffusion kinetics of gas molecules on the surface of the sensor. At higher temperature, the oxygen molecules dissociate easily and get adsorbed on the active
ur
sites available on the surface of the sensor which cause an obstruction to adsorbed NH3 gas
Jo
molecules on the active sites of the sensor surface [45]. In addition, the rate of desorption is dominated by the rate of adsorption of the target gas molecule, which reduces the time available for the target gas molecule to interact with active sites. Hence the response at higher temperatures is found to be declined. Hereafter, the optimum operating temperature, i.e. 150 ℃ is chosen for the further investigation of NH3 gas sensing behavior.
12
From the application point of view, the cyclic response to repeated exposure of target gas is an essential characteristic to study. For this purpose, the WO3 nanoflakes thin-film sensor was repeatedly exposed to different concentrations of NH3 gas from 20 to 120 ppm at an optimum operating temperature of 150 ℃ and the change in resistance was recorded. The dynamic sensor response at different concentrations is shown in Fig. 8(a). It is observed that the electrical resistance of the sensor decreases sharply when exposed to NH3 gas and reaches to saturation. The resistance is restored to initial value upon removal of the test gas. Since the
ro of
NH3 is reducing gas and the monotonous decrease of resistance with an increase of NH3 gas concentration proves the basic feature of n-type semiconducting nature of the sensor. The
corresponding plot of gas response as a function of gas concentration is shown in Fig. 8 (b).
-p
From Fig. 8 (b), it can be concluded that, with the increase in concentration of NH3 gas, the
sensor response of WO3 thin film increases. Also, the cyclic response test confirms that WO3
re
thin-film sensor exhibit the reversible and repeatable electrical response to NH3 gas.
lP
The response and recovery times of the sensor were estimated from a single pulse of the dynamic curve shown in Fig. 8(c), recorded at 120 ppm concentration of NH3 gas and optimum operating temperature of 150 ℃. The values of response and recovery times are
na
estimated as ~28 and 68 s, respectively. The response and recovery time as a function of operating temperature for a fixed concentration of NH3 gas, i.e. 120 ppm is shown in Fig. 8
ur
(d). The plot reveals that operating temperature has a great influence on the response and
Jo
recovery time as well as the overall gas response of the sensor. Also, the recovery time of the sensor is found to be higher compared to the response time. This can be attributed to the operating temperature as well as a slower rate of desorption of reaction product from the sensor surface, then the rate of adsorption of oxygen molecule from the air atmosphere [46]. 4. Gas Sensing Mechanism:
13
The gas sensing characteristics of WO3 nanoflakes thin films reveals the n-type semiconductor behavior of the sensor. Therefore, the gas sensing mechanism is proposed based on the change in electrical resistance of the material when exposed to target gas and air. In the case of metal oxide semiconductor, the gas sensing mechanism strongly depends on the operating temperature as well as adsorption, desorption, and dissociation of oxygen gas molecules on the surface of sensing material. When such n-type WO3 thin film is exposed to an air atmosphere, the oxygen gas molecules are adsorbed on the surface of the sensor.
ro of
These electrophilic adsorbed oxygen gas molecules capture electrons from the conduction band of WO3 thin film, thereby forming chemisorbed oxygen species in the form of O2–, O–
and O2– and this formation depends upon temperature [47]. As a result of this, depletion layer
-p
is formed at the surface of the sensor and consequently, the electron transportation is
hampered thereby giving stable resistance of the WO3 sensor. At lower temperatures, i.e.
re
below 100 ℃, O2– form is chemisorbed as it has lower activation energy, whereas from
lP
temperature 100 – 300 ℃, O– form is chemisorbed and above 300 ℃, O2– form is chemisorbed due to the high activation energy [48]. The reaction of adsorbed oxygen ions on the sensor surface is written as follows:
na
𝑂2 (𝑔𝑎𝑠) ↔ 𝑂2 (𝑎𝑑𝑠)
𝑂2 (𝑎𝑑𝑠) + 𝑒 − ↔ 𝑂2− (𝑎𝑑𝑠)
<100 ℃
(2) (3) (4)
𝑂− (𝑎𝑑𝑠) + 𝑒 − ↔ 𝑂2− (𝑎𝑑𝑠)
(5)
>300 ℃
Jo
ur
𝑂2− (𝑎𝑑𝑠) + 𝑒 − ↔ 2𝑂 − (𝑎𝑑𝑠) 100-300 ℃
When WO3 thin film is exposed to NH3 gas environment, the NH3 gas molecules get
adsorbed on the film surface and interact with chemisorbed oxygen species and some of the NH3 gas molecules diffuse into WO3 thin film. Since NH3 is reducing gas; it releases electrons to the sensor surface, resulting in an increase in electron carrier concentration of the WO3 sensor thereby decreasing the resistance of the film sensor. The magnitude of change in 14
resistance in this process directly portrays into gas response of the WO3 thin-film sensor. In the present study, the operating temperature majorly falls between 100 - 300 ℃, therefore the chemisorbed O– form of oxygen species are dominant. The interaction of NH3 gas with preadsorbed oxygen and WO3 thin-film sensor is written as follows: 2𝑁𝐻3 + 3𝑂− (𝑎𝑑𝑠) ↔ 𝑁2 + 3𝐻2 𝑂 + 3𝑒 −
(6)
2𝑊𝑂3 + 2𝑂− (𝑎𝑑𝑠) ↔ 2[(𝑊𝑂3 )𝑂− ]
(7)
3[(𝑊𝑂3 )𝑂− ] + 2𝑁𝐻3 ↔ 3𝑊𝑂3 + 𝑁2 + 3𝐻2 𝑂 + 3𝑒 −
(8)
ro of
Later, when the WO3 thin film is re-exposed to the air atmosphere, the reversible reaction
takes place which involves simultaneous desorption of NH3 molecules and chemisorptions of oxygen molecules on the sensor surface. As a result of this, the resistance of the sensor is
-p
restored to the original value. Further, It is observed from Fig. 7 the sensor response is
maximum at a particular temperature, such a high response is attributed to higher rate of
re
adsorption and dissociation of adsorbed oxygen or/and target gas. This rate is higher than the
lP
rate of desorption at that temperature. Also, the optimum operating temperature provides a sufficient amount of activation energy to facilitate the interaction of target gas with the
na
adsorbed oxygen species. In addition, nanoflakes like morphology of WO3 thin film also provide porous structure due to the interconnecting nanoflakes which promote the diffusion of NH3 gas through these pores. Therefore target gas molecules have plenty of active sites
ur
avialble for interaction, as result of it the sensor delivers a high sensor response at that
Jo
temperature [49]. All the above-discussed parameters are likely to contribute in improving the gas response of WO3 nanoflakes thin film compared to other morphology illustrated in Table 1.
Table.1 Comparison of gas sensing properties towards of different morphology of WO3 Morphology of WO3
NH3 gas concentration (ppm)
Operating temperature (℃)
Sensor response (S) = (Ra-Rg/Ra) x 100
Ref.
= Ra/Rg 15
Nanowires
300
200
~57 %
2.1
7
Nanoplates
200
300
~35 %
2.5
27
Filamentous
250
225
~13 %
0.10
50
Nanoconifer
300
175
~52 %
2.1
29
Nanoflakes
120
150
~73 %
3.1
Present
network
5. Conclusion: The WO3 thin films characterized by nanoflakes like morphology were grown on FTO
ro of
substrate with a seed layer, under hydrothermal conditions. The XRD pattern and Raman spectroscopy of WO3 thin films confirm the monoclinic structure. The gas sensing
characteristics performed under static condition reveals, WO3 thin film exhibits good
-p
selectivity towards NH3 by giving a maximum response at a temperature of 150 ℃. The improved gas sensing characteristics of WO3 thin film for NH3 gas may be attributed to
re
nanoflakes like morphology, which provides a large surface area and hence, plenty of active
lP
sites are available for adsorption/desorption, and dissociation of oxygen/target gas molecules. In addition, the interconnected nanoflakes form voids between them through which the NH3
na
gas can easily diffuse and interact with the sensing material. Also, these interconnected nanoflakes make the charge carrier transportation easier which indeed helps in improving the response of sensor. The sensor response of WO3 thin film is found to be ~ 73 % towards 120
ur
ppm of NH3 gas with a smaller value of response and recovery times of 28 and 68 s
Jo
respectively as well as excellent repeatability and reproducibility suggesting its potential application as a NH3 gas sensor.
Conflict of interest Authors do not have conflict of interest.
16
Author statement Pankaj S. Kolhe: Conceptualization, Methodology, Data curation, Formal analysis, Writing - Original Draft preparation. Pallavi Mutadak: Formal analysis, Investigation. Namita Maiti: Supervision, Funding acquisition. Kishor M. Sonawane: Conceptualization, Methodology, Supervision, Funding acquisition, Writing- Reviewing and Editing
Acknowledgement:
ro of
Authors, gratefully acknowledges for financial support from BARC, Mumbai, for the SRF
Jo
ur
na
lP
re
-p
award under DAE-BRNS (Sanction No. 34/14/61/2014-BRNS) research project scheme.
17
Reference: [1]
J. Ma, L. Chang, J. Lian, Z. Huang, X. Duan, X. Liu, P. Peng, T. Kim, Z. Liu, W.
Zheng, Ionic liquid-modulated synthesis of ferrimagnetic Fe3S4 hierarchical superstructures, Chem. Commun. 46(27) (2010) 5006-5008. [2]
P.K. Labhane, L.B. Patle, V.R. Huse, G.H. Sonawane, S.H. Sonawane, Synthesis of
reduced graphene oxide sheets decorated by zinc oxide nanoparticles: crystallographic,
[3]
ro of
optical, morphological and photocatalytic study. Chem. Phys. Let. 661 (2016) 13-19. B. Huang, X. Li, Y. Pei, S. Li, X. Cao, R. C. Massé, G. Cao, Novel
Carbon‐Encapsulated Porous SnO2 Anode for Lithium‐Ion Batteries with Much Improved
[4]
-p
Cyclic Stability, Small 12(14) (2016) 1945-1955.
X. Zhang, J. Qin, Y. Xue, P. Yu, B. Zhang, L. Wang, R. Liu, Effect of aspect ratio
re
and surface defects on the photocatalytic activity of ZnO nanorods, Scientific report 4 (2014)
[5]
lP
4596.
L. Gao, Z. Cheng, Q. Xiang, Y. Zhang, J. Xu, Porous corundum-type In2O3
nanosheets: synthesis and NO2 sensing properties, Sens. Actuators. B: Chem. 208 (2015)
[6]
na
436-443.
S. Hilliard, G. Baldinozzi, D. Friedrich, S. Kressman, H. Strub, V. Artero, C. Laberty-
ur
Robert, Mesoporous thin film WO3 photoanode for photoelectrochemical water splitting: a
Jo
sol–gel dip coating approach, Sustain. Energy. Fuels. 1(1) (2017) 145-153. [7]
N.V. Hieu, D.T.T. Le, N.D. Khoang, N.V. Quy, N. Hoa, P. Tam, A.T. Le, T. Trung, A
comparative study on the NH3 gas-sensing properties of ZnO, SnO2, and WO3 nanowires. Inter. J. Nanotech. 8(3-5) (2011) 174-187.
18
[8]
Z. Chen, J. Wang, G. Zhai, W. An, Y. Men, Hierarchical yolk-shell WO3
microspheres with highly enhanced photoactivity for selective alcohol oxidations, Appl. Catal. B: Environ. 218 (2017) 825-832. [9]
E. Khoo, P.S. Lee, J. Ma, Electrophoretic deposition (EPD) of WO3 nanorods for
electrochromic application, J. Euro. Ceram. Soc. 30(5) (2010) 1139-1144. [10]
H. Zheng, Y. Tachibana, K. Kalantar-zadeh, Dye-sensitized solar cells based on
WO3, Langmuir. 26(24) (2010) 19148-19152. F. Amano, E. Ishinaga, A. Yamakata, Effect of particle size on the photocatalytic
ro of
[11]
activity of WO3 particles for water oxidation, J. Phy. Chem. C. 117(44) (2013) 22584-22590. [12]
Z. Wang, P. Sun, T. Yang, Y. Gao, X. Li, G. Lu, Y. Du, Flower-like WO3
Sens. Actuators. B: Chem. 186 (2013) 734-740.
F. Mitsugi, E. Hiraiwa, T. Ikegami, K. Ebihara, Pulsed laser deposited WO3 thin
re
[13]
-p
architectures synthesized via a microwave-assisted method and their gas sensing properties,
[14]
lP
films for gas sensor, Surf. Coat. Tech. 169 (2003) 553-556.
Y. Shimizu, N. Matsunaga, T. Hyodo, M. Egashira, Improvement of SO2 sensing
properties of WO3 by noble metal loading, Sens. Actuators. B: Chem. 77(1-2) (2001) 35-40. T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Gold-loaded tungsten oxide sensor
na
[15]
for detection of ammonia in air, Chem. Lett. 21(4) (1992) 639-642. Y. Shen, B. Zhang, X. Cao, D. Wei, J. Ma, L. Jia, S. Gao, B. Cui, Y. Jin,
ur
[16]
Jo
Microstructure and enhanced H2S sensing properties of Pt-loaded WO3 thin films, Sens. Actuators. B: Chem. 193 (2014) 273-279. [17]
J.Z. Ou, M.H. Yaacob, J.L.Campbell, M. Breedon, K. Kalantar-Zadeh, W. Wlodarski,
H2 sensing performance of optical fiber coated with nano-platelet WO3 film, Sens. Actuators. B: Chem. 166 (2012) 1-6.
19
[18]
D. Chen, X. Hou, H. Wen, Y. Wang, H. Wang, X. Li, R. Zhang, H. Lu, H. Xu, S.
Guan, J. Sun, The enhanced alcohol-sensing response of ultrathin WO3 nanoplates, Nanotechnol. 21(3) (2009) 035501. [19]
B. Timmer, W. Olthuis, A. Van Den Berg, Ammonia sensors and their applications—
a review, Sens. Actuators. B: Chem. 107(2) (2005) 666-677. [20]
J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Hierarchical oxide nanostructures, J.
Mater. Chem. 14(4) (2004) 770-773. D. Xue, Y. Wang, J. Cao, G. Sun, Z. Zhang, Improving methane gas sensing
ro of
[21]
performance of flower-like SnO2 decorated by WO3 nanoplates, Talanta, 199 (2019), 603611.
N. Lu, X. Gao, C. Yang, F. Xiao, J. Wang, X. Su, Enhanced formic acid gas-sensing
-p
[22]
property of WO3 nanorod bundles via hydrothermal method, Sens. Actuators. B: Chem. 223
D. Meng, G. Wang, X. San, Y. Song, Y. Shen, Y. Zhang, K. Wang, F. Meng,
lP
[23]
re
(2016) 743-749.
Synthesis of WO3 flower-like hierarchical architectures and their sensing properties, J. Alloys Compd. 649 (2015) 731-738.
Z. Liu, M. Miyauchi, T. Yamazaki, Y. Shen, Facile synthesis and NO2 gas sensing of
na
[24]
514-519.
Y. Qin, F. Wang, W. Shen, M. Hu, Mesoporous three-dimensional network of
Jo
[25]
ur
tungsten oxide nanorods assembled microspheres Sens. Actuators. B: Chem. 140(2) (2009)
crystalline WO3 nanowires for gas sensing application, J. Alloys Compd. 540 (2012) 21-26. [26]
S. Cao and H. Chen, Hydrothermal synthesis, characterization, and their gas sensing
properties, J. Alloys Compd. 702 (2017) 644-648.
20
[27]
H. Zhang, Z. Liu, J. Yang, W. Guo, L. Zhu, W. Zheng, Temperature and acidity
effects on WO3 nanostructures and gas-sensing properties of WO3 nanoplates. Mater. Research Bull. 57 (2014), 260-267. [28]
M. Takács, C. Dücső, Z. Lábadi, C. Pap, A. Edit, Takács, M., Dücső, C., Lábadi, Z.
and Pap, A.E., 2014. Effect of hexagonal WO3 morphology on NH3 sensing, Procedia Engineering. 87 (2014) 1011-1014. [29]
P.S. Kolhe, P.S. Shirke, N. Maiti, M.A. More, K.M. Sonawane, Facile Hydrothermal
ro of
Synthesis of WO3 Nanoconifer Thin Film: Multifunctional Behavior for Gas Sensing and Field Emission Applications, J. Inorg. Organo. Poly. Mater. 29(1) (2019) 41-48. [30]
W. Zeng, H. Zhang, Z. Wang, Effects of different petal thickness on gas sensing
-p
properties of flower-like WO3• H2O hierarchical architectures, Appl. Surf. Sci. 347 (2015) 73-78.
C. Wang, X. Li, C. Feng, Y. Sun, G. Lu, sheets assembled hierarchical flower-like
re
[31]
lP
WO3 nanostructures: Synthesis, characterization, and their gas sensing properties, Sens. Actuators. B: Chem. 210 (2015) 75-81. [32]
C. Wang, R. Sun, X. Li, Y. Sun, P. Sun, F. Liu, G. Lu, Hierarchical flower-like WO3
na
nanostructures and their gas sensing properties, Sens. Actuators. B: Chem. 204 (2014) 224230.
Z.X. Cai, H.Y. Li, J.C. Ding, X. Guo, Hierarchical flowerlike WO3 nanostructures
ur
[33]
Jo
assembled by porous nanoflakes for enhanced NO gas sensing, Sens. Actuators. B: Chem. 246 (2017) 225-234. [34]
R. Mukherjee, A. Kushwaha, P.P. Sahay, Spray-deposited nanocrystalline WO3 thin
films prepared using tungsten hexachloride dissolved in NN dimethylformamide and influence of in doping on their structural, optical and electrical properties, Electronic. Mater. Lett. 10(2) (2014) 401-410.
21
[35]
H. Habazaki, Y. Hayashi, H. Konno, Characterization of electrodeposited WO3 films
and its application to electrochemical wastewater treatment, Electrochima. Acta. 47(26) (2002) 4181-4188. [36]
L. Zhou, Q. Ren, X. Zhou, J. Tang, Z. Chen, C. Yu, Comprehensive understanding on
the formation of highly ordered mesoporous tungsten oxides by X-ray diffraction and Raman spectroscopy, Microporous. Mesop. Mater. 109(1-3) (2008) 248-257. [37]
T. Siciliano, A. Tepore, G. Micocci, A. Serra, D. Manno, E. Filippo, WO3 gas sensors
326. [38]
Z.X. Cai, H.Y. Li, X.N. Yang, X. Guo, NO sensing by single crystalline WO3
re
D.J. Dwyer, Surface chemistry of gas sensors: H2S on WO3 films, Sens. Actuators.
B: Chem. 5(1-4) (1991) 155-159.
T. Jin, D. Xu, P. Diao, W.P. He, H.W. Wang, S. Z. Liao, Tailored preparation of
lP
[40]
-p
nanowires, Sens. Actuators. B: Chem. 219 (2015) 346-353. [39]
ro of
prepared by thermal oxidization of tungsten, Sens. Actuators B: Chem. 133(1) (2008) 321-
WO3 nano-grassblades on FTO substrate for photoelectrochemical water splitting, CrystEngComm. 18(36) (2016) 6798-6808.
R.F. Garcia-Sanchez, T. Ahmido, D. Casimir, S. Baliga, P. Misra, Thermal effects
na
[41]
associated with the Raman spectroscopy of WO3 gas-sensor materials, J. Phy. Chem. A
C.S. Moon, H.R. Kim, G. Auchterlonie, J. Drennan, J.H. Lee, Highly sensitive and
Jo
[42]
ur
117(50) (2013) 13825-13831.
fast responding CO sensor using SnO2 nanosheets Sens. Actuators. B: Chem. 131(2) (2008), 556-564. [43]
Y. Wang, X.N. Meng, J.L. Cao, Rapid detection of low concentration CO using Pt-
loaded ZnO nanosheets. J. hazardous mater. 381 (2020), 120944.
22
[44]
W. Zeng, H. Zhang, Z. Wang, Effects of different petal thickness on gas sensing
properties of flower-like WO3• H2O hierarchical architectures, Appl. Surf. Sci. 347 (2015), 73-78. [45]
C. Wang, R. Sun, X. Li, Y. Sun, P. Sun, F. Liu, G. Lu, Hierarchical flower-like WO3
nanostructures and their gas sensing properties, Sens. Actuators. B: Chem. 204 (2014) 224230. [46]
S. Poongodi, P.S. Kumar, D. Mangalaraj, N. Ponpandian, P. Meena, Y. Masuda, C.
sensor applications, J. Alloys Compd. 719 (2017) 71-81. [47]
N. Yamazoe, G. Sakai, K. Shimanoe, Oxide semiconductor gas sensors, Catalysis
-p
Surveys from Asia. 7(1) (2003) 63-75. [48]
ro of
Lee, Electrodeposition of WO3 nanostructured thin films for electrochromic and H2S gas
A. Afzal, N. Cioffi, L. Sabbatini, L. Torsi, NOx sensors based on semiconducting
re
metal oxide nanostructures: progress and perspectives, Sens. Actuators. B: Chem. 171 (2012)
[49]
lP
25-42.
P.S. Shewale, G.L. Agawane, S.W. Shin, A.V. Moholkar, J. Lee, J.H. Kim, M.D.
Uplane, Thickness dependent H2S sensing properties of nanocrystalline ZnO thin films
[50]
na
derived by advanced spray pyrolysis, Sens. Actuators. B: Chem. 177 (2013) 695-702. R. Godbole, V.P. Godbole, S. Bhagwat, Surface morphology dependent tungsten
Jo
ur
oxide thin films as toxic gas sensor, Mater. Sci. Semi. Proc. 63 (2017) 212-219.
23
Figure File Figure Captions: Fig. 1 X-ray diffraction pattern of WO3 nanostructured thin film. Fig. 2 Room temperature Raman spectrum of WO3 nanostructured thin film. Fig. 3 SEM micrographs of WO3 nanoflakes thin film a) lower magnified image; b) higher magnified image.
ro of
Fig. 4 TEM micrographs of WO3 nanoflakes thin film a) & b) TEM image; c) HRTEM image; d) SAED pattern. Fig. 5 High resolution XPS peaks of a) W 4f and b) O1s states
Fig. 6 UV-Visible absorption spectra of WO3 nanoflakes thin film with inset showing corresponding Tauc’s plot.
re
-p
Fig. 7 a) Plot of sensor response as a function of temperature (50 - 300 ℃) of WO3 nanoflake thin film towards 120 ppm of H2S, CO and NH3 gas concentration, and b) long-term stability recorded at 150 ℃ for 120 ppm of NH3 gas.
Jo
ur
na
lP
Fig. 8 a) The dynamic transient response of the WO3 nanoflakes film sensor; b) gas response as a function of NH3 gas concentration (20 - 120 ppm); c) enlarged sensing response curve for response and recovery time towards 120 ppm concentration of NH3 gas at 150 ℃; d) Plot of response and recovery time as a function of operating temperature (50-300 ℃) towards 120 ppm of NH3 gas.
24
-p
ro of
Figure File:
Jo
ur
na
lP
re
Fig. 1 X-ray diffraction pattern of WO3 nanostructured thin film.
Fig. 2 Room temperature Raman spectrum of WO3 nanostructured thin film.
25
Jo
ur
na
lP
re
-p
ro of
Fig. 3 SEM images of WO3 nanoflakes thin film a) lower magnified image; b) higher magnified image.
Fig. 4 TEM micrographs of WO3 nanoflakes thin film a) & b) TEM image; c) HRTEM image; and d) corresponding SAED pattern. 26
ro of
ur
na
lP
re
-p
Fig. 5 High resolution XPS peaks of a) W 4f and b) O1s states
Jo
Fig. 6 UV-Visible absorption spectra of WO3 nanoflakes thin film with inset showing corresponding Tauc’s plot.
27
ro of
Jo
ur
na
lP
re
-p
Fig. 7 a) Plot of sensor response as a function of temperature (50 - 300 ℃) of WO3 nanoflake thin film towards 120 ppm of H2S, CO and NH3 gas concentration, and b) long-term stability recorded at 150 ℃ for 120 ppm of NH3 gas.
28
ro of -p re lP
Jo
ur
na
Fig. 8 a) The dynamic transient response of the WO3 nanoflakes film sensor; b) gas response as a function of NH3 gas concentration (20 - 120 ppm); c) enlarged sensing response curve for response and recovery time towards 120 ppm concentration of NH3 gas at 150 ℃; d) Plot of response and recovery time as a function of operating temperature (50-300 ℃) towards 120 ppm of NH3 gas.
29
a
ro of
Author’s Biography Pankaj S Kolhe received Master’s degree in Physics from Fergusson College affiliated to
Savitribai Phule Pune University (SPPU), Pune, India. He received M.Phil degree in 2015
and Ph.D. Degree in 2019 from Savitribai Phule Pune University (SPPU), Pune. His research
Pallavi Mutadak received Master’s degree in Physics from Department of Physics,
re
b
-p
areas include synthesis of Metal Oxide Gas Sensor for Toxic Gases.
Savitribai Phule Pune University (SPPU), Pune, India. Presently, she is perusing Ph.D. in
lP
Department of Physics SPPU. Her research areas include synthesis of heterostructures and
c
na
their work-function and band bending study using XPS and UPS techniques.
Dr. Namita Maiti is Scientific Officer (H) Laser & Plasma Technology Division, Bhabha
ur
Atomic Research Centre Mumbai, India. She received B.E. (Electrical Engineering) from Calcutta University in 1986, M.Tech. (Control Instrumentation) from Indian Institute of Technology, Bombay in 1995 and PhD from HBNI, University of Mumbai in 2013.
Jo
Currently, she is involved in the design of indirectly heated cathode based electron beam welding and melting machines and box coater system for multilayer coating.
a*
Dr. Kishor M. Sonawane is a Associate Professor at Department of Physics, Fergusson
College (Autonomous) affiliated to Savitribai Phule Pune University, India. He received Ph.D. Degree in 2006 from Swami Ramanand Tirth Marathwada University, Nanded, India. 30
He has concurrently served as a reviewer of Journal of alloys and Compounds and Journal of Inorganic and Oraganometallic Polymers and Materials. His current research is focused on
Jo
ur
na
lP
re
-p
ro of
the Metal Oxide Thin Films for Gas Sensing Application.
31