Electrodeposition of WO3 nanostructured thin films for electrochromic and H2S gas sensor applications

Journal of Alloys and Compounds 719 (2017) 71e81

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Electrodeposition of WO3 nanostructured thin films for electrochromic and H2S gas sensor applications S. Poongodi a, Palaniswamy Suresh Kumar b, *, D. Mangalaraj a, **, N. Ponpandian a, P. Meena c, Yoshitake Masuda d, Chongmu Lee e a

Department of Nanoscience and Technology, Bharathiar University, Coimbatore, 641 046, India Environmental & Water Technology, Centre of Innovation, Ngee Ann Polytechnic, Singapore 599489, Singapore Department of Physics, PSGR Krishnammal College for Women, Coimbatore 641 004, India d National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan e Department of Materials Science &Engineering, Inha University, 253 Yonghyun-dong, Incheon 402-751, South Korea b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 February 2017 Received in revised form 4 May 2017 Accepted 11 May 2017 Available online 13 May 2017

In this work, Vertically oriented WO3 nanoflakes array films was synthesized via the template free facile electrodeposition method at room temperature. WO3 nanoflakes arrays was adopted as an effective cathode electrode material in the electrochemical devices structure. The WO3 material exhibits superior electrochromic performance shows a larger optical modulation (68.89% at 550 nm), faster response time (tb ¼ 1.93 s, tc ¼ 2.87 s), a higher coloration efficiency of about 154.93 cm2 C1 and with excellent cyclic stability over 2000 cycles without any degradation. Futhermore, WO3 nanoflakes array film was used for the detection of H2S gas that showed excellent response. A considerable increase in porosity and high surface roughness could be conducive for such an excellent and superior electrochromic characteristic as well as gas sensing performances. These results indicates that fabricated WO3 nanoflakes array film by a simple strategy holds a great promise for potential multifunctional applications such as smart windows, gas sensors and optical sensors. © 2017 Elsevier B.V. All rights reserved.

Keywords: WO3 Nanostructure Thin films Electrodeposition Smart windows Gas sensing

1. Introduction Nowadays, many research efforts have been focused on discovering a rational synthesis method for the fabrication of inorganic materials and also on the determination and tailoring of their physical properties [1]. Among these materials, tungsten trioxide (WO3), an n-type semiconductor, has attracted much attention due to its intriguing physiochemical properties and multiple potential applications. Tungsten-trioxide has been intensively investigated for use in many applications such as electrochromic, photocatalysts, gas sensors, smart windows, thermoelectric devices and ferroelectric applications [2]. Many characteristics such as particle size, morphology and structure, strongly influence the performance of WO3. In the past few years, fabrication of vertically aligned nanostructured thin films of WO3

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Poongodi), [email protected] (P.S. Kumar). http://dx.doi.org/10.1016/j.jallcom.2017.05.122 0925-8388/© 2017 Elsevier B.V. All rights reserved.

with porous structure has attracted great interest because of the unique electrochemical and sensing functionalities [3,4]. So far, complex approach such as template directed synthesis, chemical vapour deposition, anodization of W foil, electrodeposition, sol gel method and hydrothermal method have been adopted to obtain the desired porous nanostructured thin films [5,6]. Most widely used method is the hydrothermal process due to its simple operation, low cost and potential for large scale synthesis. However, the main drawback is that the deposition of different nanostructured thin films on various substrates requires additional steps to adhere the well dispersed products onto the substrate. Electrodeposition, on the other hand, is a versatile facile method where the active material is deposited onto the desired conducting substrate under the influence of an electric field at room temperature. This method is efficient in synthesizing the nanostructured thin films with the desired porosity and thickness through the electric field, pH value and deposition duration. Hussain et al. [7] reported the synthesis of tungsten trioxide nanowires by using uniform porous anodized aluminum oxide (AAO) membrane. Deepa et al. [8] have fabricated mesoporous tungsten oxide films with enhanced

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electrochromic performance by the electrodeposition method by using sodium dodecyl sulfate. Vuong et al. [9] fabricated the WO3 nanowire structures with high porosity by using highly porous single-wall carbon nanotubes as the template for efficient detection of CH4 and H2S gas. Similarly, Sun et al. [10] have synthesized mesoporous Fe2O3 with unique magnetic properties and excellent gas sensing and lithium ion storage capacity by using the soft template method. In the present study, the vertically aligned WO3 nanoflakes have been fabricated using peroxotungstic acid solution via a template free, complex agent free electrodeposition method, which has not been reported previously [11]. A survey demonstrates that there are only a few earlier reports on the fabrication of WO3 nanostructured thin film for sensing the toxic H2S gas at different low level concentrations ranging from 0.1 to 10 ppm [12,13]. 2. Experimental 2.1. Synthesis of WO3 precursor solution All the chemicals used in this work were of analytical reagent grade and purchased from Himedia Co., Ltd., India, and used without further purification. Like previous work, the electrolyte solution have been prepared consisting of mixture of peroxo tungstic acid, distilled water and isopropanol [11]. Prior to the formation of WO3 nanostructures by electrodeposition method, WO3 seed layer was coated on fluorine-doped tin oxide (FTO) coated glass substrates by spin coating method. Initially the FTO substrate was washed with acetone, ethanol and distilled water in an ultrasonication bath for 15 min. The WO3 sol was prepared by dissolving 1.25 g of H2WO4 into 20 ml of H2O2 and adding 0.2 g of Poly vinyl alcohol with stirring for 6 h. The prepared sol was spin coated on FTO coated glass substrate at 3000 rpm for 30s and this was repeated 4 times after which it was heated in a muffle furnace at 500  C in air for 2 h. The electrodeposition was carried out at room temperature using a three electrode electrochemical system with a platinum wire as the counter electrode, Ag/AgCl as the reference electrode and FTO coated glass with WO3 seed layer as the working electrode (3  1.5 cm2). The electrodeposition was carried out potentiostatically at 0.45 V against the reference electrode at room temperature without stirring for 30 min. The resulting films were rinsed immediately with distilled water and dried in air. The electrodeposited films were then annealed in a muffle furnace at 450  C for 2 h in air after which structural characterization and electrochromic studies were carried out.

carried out by the Cyclic Voltammetry (CV), Chrono-amperometry (CA) and Chrono-coulometry (CC) techniques using Biologic Science Instruments SP-50 controlled by a personal computer installed with EC-lab software in the three electrode cell configuration with 0.5 M of H2SO4 as the electrolyte, Platinum (Pt) as the counter electrode, Ag/AgCl as the reference electrode and the prepared WO3 thin film as the working electrode. The transmission spectra of WO3 thin films in the fully colored and fully bleached states were measured in the wavelength range between 300 and 900 nm with a UVeVisible spectrophotometer (Jasco V-640) with FTO coated glass substrate as the reference electrode. 2.4. Gas sensing testing For the gas sensing test, multiple networked WO3 thin film sensors were prepared. The WO3 thin film samples were dispersed ultrasonically in a mixture of deionized water (5 ml) and isopropyl alcohol (5 ml). The samples were placed onto 200 nm thick SiO2coated Si (100) substrates equipped with pairs of interdigitated (IDE) Ni (~200 nm)/Au (~50 nm) electrodes with a gap of 20 mm. The flow-through technique was used to test the gas sensing properties. All measurements were performed in a temperature-stabilized sealed chamber with a constant flow rate of 200 cm3/min at 100, 200 and 300  C under 40% RH. The H2S concentration was controlled by mixing H2S gas with synthetic air at different ratios. The electrical resistance values of the gas sensors were determined by measuring the electric current using a Keithley source meter2612 with a source voltage of 1 V. Detailed procedures for sensor fabrication and the sensing tests are described elsewhere [14]. 3. Results and discussion 3.1. Structural analysis The crystal structure of the electrodeposited WO3 nanostructured thin films was investigated by the X-ray diffraction technique. Fig. 1 shows the XRD pattern of the WO3 thin film. The diffraction peaks correspond to (002), (020), (200), (120), (202), (130), (222), (303), (134) and (234) crystal planes with high crystallization, ensuring the formation of the monoclinic phase of WO3 structure (JCPDS NO 89-4476). Most of the major peaks are sharp indicating that the WO3 nanoflakes are well crystallized. No hydrated WO3 peaks are found, indicating the high purity of the

2.2. Characterization techniques The crystal structure and phase of the prepared WO3 nanostructured thin films were characterized by the X-ray diffraction (XRD) pattern obtained using a Panalytical X'pert Pro diffractometer with Cu-Ka radiation (1.5406 Å). FEI (quanta-250) Field Emission Scanning Electron Microscopy (FESEM) was employed to characterize the surface morphology. The phase structure was also confirmed by using Raman spectroscopy (LABRAM-HR) with laser excitation lines of 514 nm at room temperature. The binding energies and composition were determined by X-ray photoelectron spectroscopy (Kratos analytical, ESCA-3400, Shimadzu). UVeVis spectra were recorded using a Jasco V-640 Spectrophotometer. The surface roughness was determined by Atomic force microscopy (Veeco di-caliber). 2.3. Electrochemical characterization The electrochemical measurements of the WO3 thin film were

Fig. 1. XRD pattern of the WO3 nanoflakes array film.

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characterized by AFM analysis. Parts of their 2D and 3D surface topographies are shown in Fig. 4(aeb). From Fig. 4(b), it is revealed that the film consists of randomly oriented nanoflakes over the surface with an rms surface roughness of about 47.70 nm [18]. The presence of voids within the film will be favourable for better electrochromic performance [19] and for enhancement of the sensitivity of gas sensors [20]. 3.3. Formation mechanism of WO3 nanoflakes

Fig. 2. Raman spectra of the WO3 nanoflakes array film.

WO3 which could be obtained through the present strategy. The phase structure of the WO3 thin films was further characterized by Raman spectroscopy. Fig. 2 shows the Raman spectrum of the WO3 thin films. The two groups of broad bands such as 500-950 cm1 and 200-400 cm1 are obtained due to the photon activity of the prepared WO3 nanostructured thin film. These four peaks obtained at 807, 718, 272, 559 in the raman spectra of WO3 nanoflakes array film are in good agreement with the monoclinic structure reported earlier [15]. It can be seen from Fig. 2, that the Raman bands of OeWeO stretching mode emerge at 718 and 807 cm1, while the OeWeO) bending mode appears at 278 cm1 [16]. Similarly, the sharp intense peak at 950 cm1 corresponds to the W]O stretching mode of the terminal oxygen atoms [17]. Moreover, the WO3 nanoflakes array film is in monoclinic phase of WO3 which is the same as that depicted in the XRD analysis. 3.2. Morphological analysis Fig. 3 shows the FESEM images of the electrodeposited WO3 thin films obtained using PTA solution. FESEM images show that the homogenously distributed, well aligned nanoflakes array has grown in an almost perpendicular direction over the entire film (Fig. 3(a)). The thickness of these nanoflakes ranges from 10 to 20 nm with a height of about 1 mm. The cross-sectional image clearly reveals that these WO3 nanoflakes have grown vertically and in close contact with the entire substrate. The surface topography of the electrodeposited WO3 thin film was further

Basically, two different mechanisms are available for the cathodic electrodeposition method. They are (i) deposition of metal oxide on the electrodes by the direct reduction of the oxidation state of metals and (ii) precipitation of metal oxide or hydroxide induced by the increase in the interfacial pH value and local supersaturation [21]. The latter mechanism is usually dependent on the local increase of the pH near the electrode surface due to the reduction of O2 or H2O leading to the electrodeposition of metal ions. The reaction mechanism of electrodeposited WO3 can be described by two reaction steps: Equ (1) and Equ (2): þ 2W þ 10 H2 O2 / W2 O2 11 þ 2H þ 9 H2 O

(1)

þ  W2 O2 11 þ ð2 þ nÞH þ ne / 2WO3 þ ð2 þ nÞ= 2H2 O

þ ð 8  nÞ = 4O2

(2)

Fig. 5 shows the schematic diagram of the formation mechanism of electrodeposited WO3 nanostructured thin films. The growth mechanism of electrodeposited WO3 thin film is categorized into four stages: (1) formation of peroxotungstic acid, (2) decomposition of excess hydrogen peroxide, (3) Deposition of WO3 nanostructure through reduction process and (4) calcinations process. Initially tungsten metal powder was dissolved in hydrogen peroxide solution under constant stirring for 24 h in cold bath. The resulting solution was then refluxed at 60  C for 6 h in order to decompose the excess hydrogen peroxide by adding glacial acetic acid. Further, the resultant solution was diluted by adding distilled water and isopropanol in equal ratio. Subsequently, 0.1 M of oxalic acid was added until the pH value of the solution reduced to 0.5. Prior to the electrodeposition, the above solution was heated at 60  C for 15e30 min. This final WO3 precursor sol was used for the electrodeposition process. In the present work, during cathodic electrodeposition, the PTA ions surrounded by oxalate ions and conjugate base moved towards the working electrode via reduction process leading to the formation of WO3 film. Finally, the obtained WO3 film was calcined at 450  C leading to the formation of vertically aligned WO3 nanoflakes array film. In the present work, well aligned WO3

Fig. 3. (aeb) FESEM images of electrodeposited WO3 nanoflakes thin film at pH 0.5. Inset shows the cross sectional images of the WO3 nanoflakes array film.

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Fig. 4. (aeb) AFM 2D and 3D images of WO3 nanoflakes array film.

nanoflakes like structure with high crystallinity and porosity have been obtained. 3.4. X-ray photoelectron spectroscopy (XPS) By analyzing the chemical binding states of W (tungsten), the stoichiometry, surface composition, and chemical surface states of the electrodeposited WO3 thin films were evaluated by X-ray photoelectron spectroscopy (XPS). Typical survey spectra of the electrodeposited WO3 thin films are shown in Fig. 6(c). The XPS spectra show tungsten, oxygen and also a trace amount of carbon, owing to the surface contamination. The spectra of W4f core level and O1s are shown in Fig. 6(a, b) for the WO3 nanoflakes. The W4f core level of WO3 nanoflakes film is dominated by the spin orbit doublet with binding energies of 35.0 and 37.1 eV for W4f7/2 and W4f5/2 respectively with a spin-orbit separation (W4f7/2 e W4f5/2) of about 2.1 eV. The energy positon of this doublet corresponds to the 6þ valence state of W [22]. The O1s spectrum of the WO3 nanoflakes film shows a peak located at 530.2 eV (Fig. 6(b)), which

is assigned to the W]O bonding modes in the stoichiometric WO3. These observations reveal that the prepared nanostructured WO3 thin films are fully oxidized and correspond to stoichiometric WO3 which is confirmed by the XRD patterns.

3.5. Optical properties of WO3 thin film The optical characteristics of WO3 nanoflakes array film were analyzed by UVevisible spectroscopy. Fig. 7 shows the transmittance spectra of WO3 array film and it exhibits a lower transmittance of about 70% in the visible-light region due to its high surface roughness and porosity in between the stacking of nanoflakes. Generally, the bandgap was determined using the equation,

ahn ¼ A(hn  Eg)n

(3)

where a the absorption coefficient is calculated using the equation:

Fig. 5. Growth mechanism of the electrodeposited WO3 nanostructured thin films.

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Fig. 6. XPS W4f, O1s and Wide angle spectra of the WO3 nanoflakes array film.

Fig. 7. UVevisible spectra of WO3 nanoflakes array film (a) Transmittance spectra. (b) Reflectance spectra (c) Tauc plot from which bandgap of the WO3 nanoflakes array film is estimated. (d) Refractive index of the WO3 nanoflakes array film.

expðadÞ ¼ T

(4)

where d is the thickness of the film and T is the measured transmittance. As WO3 is an indirect bandgap semiconductor, bandgap energy can be determined by extrapolating the plot of (ahn)1/2 as a function of the photon energy hn. The bandgap energy (Eg) of WO3 nanoflakes array film is found to be 2.37 eV (Fig. 7(c)), which is smaller than that of bulk WO3 (3.27 eV). The smaller bandgap energy might be owing to the presence of oxygen vacancies in the

WO3 lattice. Critical dependence of the bandgap of WO3 on bonding-antibonding interactions leads to smaller bandgap which in turn, might be attributed to the relaxation around oxygen vacancies causing an increase in the WeO bond length and then a decrease in the bonding-antibonding interaction [23]. Fig. 7(b) shows the reflectance spectra of the WO3 nanoflakes array film, from which refractive index can be estimated by using the following formula [24].

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n¼

1 þ R0 0 1R



sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0 4R þ  k2 0 ð1  R Þ2

(5)

where k is the extinction coefficient and a is the absorption coefficient evaluated from the experimental spectra of transmittance T (l) and reflectance R(l) respectively. Fig. 7 (d) shows the refractive indices of the nanoflakes array like film at different wavelengths. The average refractive index of the electrodeposited WO3 nanoflakes array film at 550 nm is 2.04 in the visible range of about 300e900 nm. A nanoporous WO3 film will enhance the sensitivity of the gas sensor as well as the electrochromic performance of the electrode. In the former case, the presence of porous structures within the film is highly desirable which could be conducive to providing direct conduits for the gas molecules to flow in from the environment [20]. For an electrochromic device, it is highly desirable that the WO3 film has a rough and porous surface, so that it is more beneficial in providing a short diffusion pathway for the counter ions as well as a large active surface, thereby reducing the response time and enhancing the optical modulation. By using the values of refractive index, the porosity value of the nanoflakes array film is found to be 42.85%. The low reflectance of the WO3 nanoflakes array film shown in Fig. 7(d) is attributed to its high porosity [25]. Because of the high porosity, the electrodeposited WO3 nanoflake array film exhibits greater electrochromic performance by minimizing the time of optical modulation and enhancing the electrolyte intercalation/deintercalation of protons during the electrochromic process and also shows better gas sensing performance due to the improvement of the sensitivity of the gas sensor in the interior and on the surface of the porous structures. 3.6. Electrochemical studies In order to evaluate the effect of electrodeposited WO3 nanoflakes film on their electrochromic properties, a typical cyclic voltammogram (CV) technique is employed to examine the cathodic and anodic behavior of electrochromic WO3 thin films. These CV are normalized with respect to the constant geometrical effective area 4.5 cm2 (dimension 3 cm  1.5 cm) of the electrode. Fig. 9(a) shows the CV curves of WO3 nanoflakes array film electrodes obtained at a scan rate of 50 mVs1 using 0.5 M H2SO4 electrolyte and a potential window of - 1.2 V to þ 1.2 V. The mechanism underlying the intercalation/deintercalation of ions and electrons in an electrochromic WO3 film can be described by the equation:

WO3 þ nHþ þ ne 4 Hn WO3

(6)

From the CV plots, by using the values of cathodic and anodic peak currents, the diffusion coefficient (D) of Hþ ions in the electrode during intercalation and deintercalation was calculated using the Randles Sevcik equation,

ip 3

2:72  105  n

=2

 A  C0  y

1

(7)

=

D¼

2

Where D is the diffusion coefficient of Hþ ions, ip is the peak current density, n is the number of electrons involved in the redox process (n ¼ 1 for W5þ/W6þ redox pair), A is the area of the film, Co is the concentration of active ions in the electrolyte, and y is the potential scan rate. The values of diffusion coefficient calculated from CV curves were observed to be 8.955  107cm2s1 (intercalation) and 4.8495  107cm2s1 (deintercalation), which are much higher than those reported earlier. The larger diffusion coefficient also suggests that vertically aligned nanoflakes with voids

paved an easy way for diffusion and charge transfer process of ions in the WO3 films. Fig. 8(b) shows the CV curves for the WO3 nanoflakes electrode at different scan rates from 20 to 100 mV s1. From Fig. 8(b), both the oxidation and reduction peak currents increase upon increasing the scan rate with slight shift in the oxidation peak current, which could be ascribed to the fact that the amount of Hþ ionsand electrons incorporated into the film increases at the larger scan rate due to the increased reaction activity of the films [26]. The switching kinetics is an important criterion to determine the practical applications of electrochromic films. The time required for a 90% change in optical transmittance between the colored and bleached states is termed as switching kinetics. The coloration and bleaching response times of WO3 nanoflakes electrode are estimated from current time transients. Typical chronoamperometry (CA) traces of the WO3 film electrode were recorded by applying an alternative voltage of ±1 V for 10 s as shown in Fig. 8(c). The WO3 electrode gets darkened during negative sweep (-1 V), owing to the intercalation of protons and electrons into the film and gets bleached due to the deintercalation during positive sweep voltage (1 V). For the WO3 nanoflakes array, the estimated switching time for coloration state (tc) and bleaching state (tb) from the current transient plot are 2.87 s and 1.93 s respectively. The two factors, space charge transfer through the electrode and potential barrier at the electrolyte-WO3 interface could be conducive for the faster bleaching kinetics than the coloration kinetics. In order to study the reversibility of the WO3 film, Chronocoulometry measurement was carried out at a voltage sweep ranging from 1.0 to þ1.0 V with steps of 10 s. Fig. 8(d) represents the plot of charge vs. time transient. The reversibility of the films is calculated depending on the ratio of the de-intercalated charge (Qdi) to the intercalated charge (Qi) for coloration/bleaching after 10 s using the following equations:

Reversibility ¼

Qdi Qi

(8)

The percentage of electrochromic reversibility for the WO3 nanoflake array film electrodes is 87%. The WO3 nanoflakes array electrode exhibits an excellent reversibility of about 87%, since it offers a well-defined crystalline structure, high porosity of 42.85% and vertically aligned self-assembly of nanoflake like surface morphology. 3.7. Stability and optical studies of WO3 electrodes A typical cyclic voltammogram (CV) is carried out at a potential of ±1.2 V at a scan rate of 50 mVs1using 0.5 M H2SO4 as the working electrolyte in order to study the electrochromic cyclic stability of the WO3 nanoflake array electrode. Fig. 9 (a) represents the CV curves at the 1st, 500th, 1000th and 2000th cycles at room temperature for the WO3 nanoflakes electrodes. From the CV, it is observed that the electrodeposited WO3 nanoflakes electrode exhibits excellent cyclic stability until 2000 cycles with slight reduction in the oxidation peak current, which indicates that the prepared WO3 nanoflakes array film with porous structures will be favourable for the development of high performance EC devices. This enhancement is due to the high crystallinity and porosity of the WO3 electrode. The UVe visible transmittance spectroscopy was performed in the 300e900 nm wavelength range at room temperature for the WO3 thin film electrode in its colored and bleached states in order to determine its optical modulation (Fig. 9(b)). Coloration and bleaching of the WO3 thin film were carried out by applying the potential sweep of ±1.2 V for a fixed time. It is obvious from Fig. 9

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Fig. 8. (a) CV of the WO3 nanoflakes array film at a scan rate of 50 mV s-1. (b) CV of the WO3 nanoflakes array at different scan rate. (c) Chronoamperometric (CA) response of the WO3 thin film. (d) Chronocoulometry CC trace of the WO3 nanoflakes array electrodes in H2SO4 under the potential sweep of 0.7 V to þ 0.7 V for 10 s respectively.

Fig. 9. (a) CV stability of the WO3 nanoflakes array film. (b) Optical transmittance spectra of the colored and bleached states of the WO3 nanoflakes array film with photographic images of the film in its colored and bleached states.

(b) that the optical transparency decreases when the WO3 film is cathodically polarized, and subsequently increases due to bleaching of the electrode. From Fig. 9(b) the transmittance values for the colored and bleached states of WO3 nanoflakes electrode at the visible wavelength of 550 nm were found as Tc ¼ 5.90% and Tb ¼ 68.89%. By using the values of Tb and Tc, the change in optical density (DOD) of WO3 film at 550 nm, is determined by the following equation:




DOD ¼ log

Tb Tc

 (9)

One of the important factors that characterize the electrochromic material is coloration efficiency (CE or h) of the electrode. Coloration efficiency is defined as the change in optical density (DOD) per unit of charge density intercalated into the electrode

material during switching. It can be determined using the following equation (11)

CEor h ¼

DOD Qi =A

(10)

where DOD is the optical density change measured at a wavelength of 550 nm, Qi is the amount of charge intercalated into the WO3 film electrode and A is the area of the film electrode. The calculated coloration efficiency for the electrodeposited WO3 nanoflakes electrode is found to be 154.93 cm2C-1. Furthermore, the estimated CE value of WO3 film electrode is also much higher than those reported earlier [27e29]. According to these results, we can conclude that the large improvement in CE value of WO3 nanoflakes film could be mainly ascribed to an increased porosity due to the

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stacking of nanoflakes and orientation of the morphology. The increased porosity would have paved the way for the larger active surface area for charge transfer reaction, which leads to the excellent electrochromic performance of the electrode. 4. Gas sensing performance Two-dimensional metal oxide nanostructures have been shown to be promising materials for gas sensor applications [4]. As such, it would be interesting to investigate the sensing performance of the as synthesized vertically aligned WO3 nanoflakes towards Volatile Organic Compounds (VOCs) and Volatile Sulphur Compounds. Fuels are used widely in industries as well as transport services all over the world. A variety of pollutant and toxic gases are exhausted out to the environment which leads to serious health hazards due to pollution. Because of the serious health hazards, need for an awareness to monitor hazardous gases has stimulated considerable attention towards the design of sensors for the detection of gases such as CO, CO2, NOx, H2S etc. [30e32] Among the various Volatile Sulphur Compounds gases, Hydrogen sulfide (H2S) is one of the most pungent, toxic, flammable gases which could affect the human nervous system on smelling. Even at low concentrations, it leads to unconsciousness. According to the Occupation Safety and Health Administration, the acceptable exposure levels of H2S gas concentration is about 20 ppme10 ppm [28]. Therefore, in order to target low concentration of H2S, there is a strong demand for the development of cheap, reliable and sensitive gas sensors. On the basis of the above consideration, we have examined the gas sensing performance of the prepared two dimensional WO3 nanoflakes array toward H2S gas at different concentrations ranging from low level of about 0.1e10 ppm. Fig. 10 (a) shows the sensing transients of WO3 thin film sensors to H2S pulses for repeated exposure to H2S gas at different concentrations ranging from 0.1 to 10 ppm at 300  C.The resistance of the sensor decreased on exposure to each H2S pulse and recovered completely to the initial value upon stopping the supply of H2S gas and exposure to air. This is the reliable sensing behavior of an ntype semiconductor sensor. This confirms that the electrical response of the WO3 nanoflakes is quite reversible and repeatable. Fig. 11 (a) shows the response curve of WO3 thin film sensor to H2S gas as a function of H2S gas concentration. Herein, the response was defined as (Rg-Ra)/Ra for acetone gas, where Rg and Ra are the electrical resistances of sensors in H2S gas and air, respectively. It can be seen that the WO3 thin film sensor exhibited responses of

2.8e9.8%, 12.8e68.9% and 26.2%e85% respectively, to 0.1e10 ppm of H2S concentration at various operating temperature such as 100  C, 200  C and 300  C respectively. The WO3 thin film sensor shows a high response to H2S which increases on increasing the concentration of the H2S gas. This particular response is higher than those reported earlier by using ZnO nanofibres, Cuedoped ZnO nanofibres, SnO2eCuO particles, micron sized commercial powder MoO3, and ZnO-WO3 nanoparticles and WO3 [33e37]. The sensing performance of the WO3 nanoflakes towards 10 ppm of H2S array is compared with that of previously reported sensor material in Table 1. It is well known that the operating temperature will influence the response of a semiconductor sensor. Fig. 11(b) shows the effect of the operating temperatures from 100  C to 300  C on the response of the sensor to H2S gas. It is obvious that the response curve of the WO3 thin film sensors shows a continuous increase on increasing the operating temperature. Therefore, the optimal working temperature for the WO3 thin film sensor is thought to be higher than 300  C. Fig. 11(c) shows the response time of the WO3 thin film sensor to H2S gas as a function of the H2S gas concentration. On the other hand, Fig. 11(d) shows the recovery times of the WO3 thin film sensors to H2S gas as a function of the H2S gas concentration. In this study, response time and recovery time are defined as the times to reach 90% variation in resistance upon exposure to H2S and air, respectively. The WO3 thin film sensor showed faster response than those reported earlier. On the other hand, the recovery time increases with increase in the concentration of H2S gas. The two factors that are mainly conducive for the longer recovery times are, the kinetics of desorption of the reaction product from the oxide surface and subsequent adsorption kinetics of oxygen molecules from the ambient after the sensor is exposed in air. Regarding the effect of working temperature on the sensing speed of the sensors, WO3 thin film sensors showed the fastest response at 200  C and the fastest recovery at 300  C. Overall, the sensors showed the shortest sensing time (sum of the response time and recovery time) at 200  C. On the other hand, little dependence of the response time and recovery time on the H2S concentration was found. The dependence of the response times and recovery times of the sensors on working temperature might also be explained in a similar way to their responses on working temperature. Different oxygen ion species form at different temperatures after the adsorption of oxygen molecules by the sensor surfaces and the reaction rate of H2S with O at 200  C might be 2 higher than that of O at 300  C, leading to the 2 at 100 C or O  fastest sensing at 200 C.The higher gas response towards H2S gas was attributed to well aligned nanostructures for effective diffusion as well as larger effective surface area due to porosity for gas sensing reaction. On exposing tightly packed cluster like microstructures to target gas, the gas cannot spread to particles in the interior of the aggregate, reducing the effective surface area. However, in the case of vertically aligned WO3 nanoflakes array with high porosity, the entire surface area such as both interior and surface of the material will be exposed to the target gas, thereby enhancing the response. 4.1. Possible sensing mechanism of WO3 nanoflakes array towards H2S gas

Fig. 10. (a) Sensing transients of the Wo3 thin film sensor at 300  C.

Based on the above experimental results and reported investigation, the good response of WO3 nanoflakes array films sensors toward H2S, can be elucidated by the following mechanism: as an n-type semiconductor oxide, the operating principle of WO3 is based on change in resistance of the array film owing to adsorption and desorption process of oxygen molecules on the surface of the sensing structure of WO3. When the sensor is exposed to

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Fig. 11. (a) Sensing transients of the WO3 thin film sensor at different temperatures. (b) Response of the WO3 thin film sensor as the function of temperature (c) Response time and (d) Recovery time of the WO3 thin film sensor at different temperature.

atmospheric air, oxygen molecules get absorbed on the surface of the WO3 nanoflaks array film by functioning as electron acceptors to capture electrons from the conduction band of the WO3, thereby forming oxygen species in the form of O, O2- and O 2 at different temperatures (Eqs. (11)e(14)). This will lead to stable resistance by hindering the electron transportation due to formation of a thin layer of depletion region at the surface of WO3 grains (Fig. 12(b)) [38].

O ðadsÞ þ e 4O2 ðadsÞ

O2 ðgasÞ4O2 ðadsÞ

The following reaction occurs between H2S and the preadsorbed oxygen species at 300  C, when exposing to low H2S concentrations:

(11)

O2 ðadsÞ þ e 4 O 2 ðadsÞ   O 2 ðadsÞ þ e 4 2O ðadsÞ



< 100 C 

100 < T < 300 C 

> 300 C

 2H2 S þ 3O 2 ðadsÞ42SO2 þ 2H2 O þ 3e

Table 1 Comparison study of different sensor materials towards H2S reducing gas. Materials

T ( C) H2S concentration (ppm)

Response (%) References

ZnO nanofibres Cu-doped ZnO nanofibres SnO2-CuO particles Micron sized commercial powder MoO3 ZnO-WO3 film WO3 powder WO3 nanoflakes array film

10 10 5 20

230 230 300 375

1.57 18.7 0.0006 33

3.5 1 to 5 0.1-10 ppm

260 1.4 350 3.9 100-300oC 2.8 upto 85

[33] [33] [34] [35] [36] [37] Present Work

(12) (13) (14)

(15)

Upon exposure to a reducing gas such as H2S, the reducing gas releases the trapped electrons back to the conduction band by reacting with the oxygen species onto the surface of WO3 nanoflakes array, thereby decreasing the thickness of the depletion layer with reduced resistance state, which results in the increased carrier concentration and electron mobility of the WO3 nanostructure with following mechanism (Fig. 12(c)) of equation (6) [39]:

3WO3 þ 7 H2 S /3 WS2 þ SO2 þ 2 H2 O

(16)

Further the WS2 layer will be oxidized back to WO3, while stopping the flow of H2S by the following reaction: 3WS2 þ 6O (ad) þ 6hþ/ 2WO3 þ 2SO2

(17)

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S. Poongodi et al. / Journal of Alloys and Compounds 719 (2017) 71e81

Fig. 12. (a) WO3 nanoflakes array film, (b)& (c) Sensing mechanism of WO3 in Air and in H2S gas.

Consequently the resistance will return back to its original state. 5. Conclusion In summary, growth of vertically aligned WO3 nanoflakes array was successfully optimized via a simple template and surfactant free electrodeposition method using PTA solution, which has not been reported so far. In addition, the formation mechanism is proposed on the basis of time dependent experiments. The prepared WO3 nanostructured thin films exhibit a high porosity of about 42.85% and high surface roughness 47.70 nm. The fabricated WO3 nanoflakes array have also been demonstrated to exhibits superior electrochromic performance with a larger optical modulation (68.89% at 550 nm), faster response time (tb ¼ 1.93 s, tc ¼ 2.87 s) and superior coloration efficiency of about 154.93 cm2 C1. These advantages together with excellent cyclic stability over 2000 cycles without any degradation makes it very attractive for potential applications in energy saving smart windows. In addition, the H2S gas sensing properties based on the WO3 nanoflakes array film has been studied. The WO3 nanoflakes array also demonstrated excellent sensing properties towards H2S gas. The porous structure between the stacking of nanoflakes and good contact of the array structure on the entire surface of the substrate could be conducive for improvement of electrochromic properties and gas sensing properties by making the diffusion of ions easier and providing a larger active surface area for charge transfer reaction. Acknowledgements One of the authors S. Poongodi gratefully acknowledges the DST PURSE, Government of India for the FESEM facility. The gas sensing studies was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2010-0020163). References [1] J.M. Ma, L. Chang, J.B. Lian, Z. Huang, X.C. Duan, X.D. Liu, P. Peng, T.I. Kim, Z.F. Liu, W.J. Zheng, Chem. Commun. 46 (2010) 5006e5008. [2] L. Santos, C.M. Silveira, E. Elangovand, J.P. Netoa, D. Nunes, L. Pereiraa, R. Martins, J. Viegas, J.J.G. Moura, S. Todorovic, M.G. Almeida, E. Fortunato, Sensors Actuators B 223 (2016) 186e194.

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