Full Length ArticleCharacterization of PLD grown WO3 thin films for gas sensing
Introduction
Tungsten trioxide (WO3) is one of the most widely studied semiconductor oxides, due to its remarkable physical, optical and optoelectronic properties [1], [2]. WO3 grown as thin film can be used for various applications, such as electrochromic devices (in displays, smart windows and optical switching coatings) [1], [2], [3], sensing numerous toxic and inflammable gases [4], [5], [6], [7], photocatalysis [8], [9], solar energy conversion [2], water splitting [10], and many others.
The quartz crystal microbalance (QCM) is a widely-used technique for detecting the mass of thin films deposited on the crystal surface in the sub-nanogram range, but it could be also applied for monitoring the adsorption of very small amounts of various toxic gases. When the QCM method is used for gas sensing, the surface properties of the sensing material are of higher importance, compared with its bulk. Very thin films are also applicable for gas sensors [11], [12], while their morphology is of high importance. Also, compared with other methods, the advantages of QCM for sensing gases are the high sensitivity, simple technological implementation and easy real-time monitoring, capability of operating at room temperature, relative independence from electromagnetic fields and rapid temperature changes, fast response even at low concentrations, durability, portability, low energy consumption and cost [13], [14].
Although WO3 shows good sensitivity to many gases [4], [5], [6], [7], it is possible to further improve its gas sensing properties by fostering its nanostructured morphology to increase the specific surface area. Therefore, already various techniques have been adopted to grow thin films from oxide materials with higher specific surface area.
Pulsed laser deposition (PLD), also called deposition by laser ablation, is a physical vapor deposition (PVD) process based on the vaporization of condensed matter by means of photons [15], [16], [17]. A highly intense short-pulsed (usually of nano- or pico-seconds) high-power laser beam is focused under vacuum or a working gas atmosphere on a target of the deposited material. If the laser fluence exceeds a specific threshold, the ablated material is directed forward, in form of a plume, towards the deposition substrate, where film is formed by re-condensation [15], [16].
As the species arriving on the substrate are highly energetic, the synthesized films elicit high adherence. Moreover, the plasma plume plays a notable piston-like role, pushing and carrying the removed matter from the ablated target to the substrate [16].
PLD is a versatile PVD method, which presents a distinct set of advantages, that currently increases its appeal in the sensors field. The main ones are: i) congruent target-to-substrate transfer of substances with intricate stoichiometries; ii) control over the growing films morphology by adapting the deposition variables (possibility for growth of rough or nanostructured surfaces); and iii) plasma direct action upon surface can result in a rather large hardness, along with a high adherence of the growing film to the substrate.
- i)
The first feature is particularly important in the case of WO3, which could be chemically modified during transfer. However, as shown in our previous study [18], ns excimer laser pulses can promote the growth of stoichiometric WO3 film after laser transfer.
- ii)
The deposition of substance in form of nanoparticles covering the surface of film is a characteristic of PLD, which can facilitate the natural growth of nanostructured surfaces with an increased active area beneficial for gas sensing [19], [20], [21].
- iii)
The films obtained by PLD after the plasma transfer are usually very adherent to substrate and also have a rather large hardness, which is of key importance for the long term use as gas sensors in specialized devices [17], [22], [23].
Therefore, in the present study PLD was chosen as method allowing the growth of highly pure and adherent thin films with larger surface area, which is more beneficial morphology for gas sensing, compared with other PVD methods.
The usual PLD drawbacks are the rather non-uniform thickness distribution, the possible presence of bigger size (submicronic or even micronic) droplets and the rather limited deposition area of usually only few square centimeters [15], [16]. But when preparing QCM gas sensors none of these limitations are significant for the fabricated device, since of high importance are only the surface properties of the films, and the active sensing area of the QCM sensor is rather small.
Reactive PLD (RPLD) is a strategy to enhance the reactivity between the background gas and the ablated species in order to reconstitute on the substrate the stoichiometry of the likely lost elements in the deposited films. For example, due to its expansion velocity, the lighter oxygen species develop a larger angular distribution in the plume than the heavier elements, such as tungsten, and additional sources to compensate the expected oxygen loss can be necessary [15], [16], [17].
In this study, we prepared QCM gas sensors with RPLD grown WO3 thin films, which were characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and profilometry. The gas sensing was investigated by detecting various concentrations of NO2 in dry air at room temperature.
Section snippets
Preparation of the gas sensors
Previously, prototype QCM gas sensors with several transition metal oxide films (e.g. TiO2, MoO3, ZnO) were already fabricated by our team and tested for sensitivity to NO2 in a specially designed experimental set-up [24]. These films were mostly prepared by reactive magnetron sputtering [25], [26] and atomic layer deposition (ALD) [11], [12]. In the present study, we focused on the sensing behavior of WO3 thin films grown by PLD. Such films were deposited either on quartz resonators (AT-cut 16
Results and discussion
Our previous studies revealed that the roughness of the resonator electrodes influences the gas sensing properties of the WO3 thin films [29]. It was concluded that the optimal conditions can be defined by a compromise between a surface with higher roughness, which has better sorption but slower desorption and decreased quality of the QCM, and smoother one, possessing a decreased sensing ability. Another approach to improve the sorption properties is to design a surface morphology of the
Conclusions
Synthesis by PLD of tetragonal-WO3 films starting from a target with predominantly monoclinic WO3 phase was observed. The PLD method was found to be suitable for fast and cost-effective deposition of WO3 thin films for QCM gas sensors. The films deposited at 300 °C presented a surface topology favorable for the sorption properties, consisting a film matrix with fairly well-distributed protruding craters/cavities. At the same time, it was found that it does not have negative influence on the
Acknowledgements
S.I.B. acknowledges the Postdoctoral Fellowship programme of the Hungarian Academy of Sciences (2013–2015) and the INERA REGPOT Project of Institute of Solid State Physics, Bulgarian Academy of Sciences. N.S., N.M., A.V., and I.N.M. acknowledge the Core Program LAPLACE–IV 2016–2017. G.E.S. and C.B. thank the NIMP Core Programme PN 16 48–3/2016–2017. I.M.S. thanks a János Bolyai Research Fellowship of the Hungarian Academy of Sciences and an OTKA–PD–109129 grant.
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2022, Ceramics InternationalCitation Excerpt :WO3 nanomaterial is a typical n-type semiconductor with a wide bandgap. Therefore, WO3 has gained great interests in various significant applications such as non-emissive displays, smart windows, biosensors, humidity sensors, gas sensors, photonic crystals, air and water treatment from harmful pollutants, water splitting, hydrogen production, optical switching, data storage devices, solar energy conversion and lithium-ion batteries [3–10]. Thin films of WO3 are the most common form in practical applications, which could be easily synthesized through different chemical and physical techniques such as hot filament metal oxide deposition [3], electrodeposition [4], pulsed laser deposition [5], spray [8], RF reactive sputtering [9], chemical bath deposition [11], reactive DC magnetron sputtering [12], ion-beam deposition with post-annealing [13], e-beam evaporation [14], thermal evaporation [15] and sol-gel coating [16].