_applied sciences_  
_Article_  
Optimization of Deposition Parameters of SnO\({}_{2}\) Particles on Tubular Alumina Substrate for H\({}_{2}\) Gas Sensing  
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patients with small bowel pseudo-obstruction and malabsorption [8]. Hence, the reliable detection of H\({}_{2}\) is important from both safety and health perspectives. Gas sensors are sensitive electronic devices which can detect the presence of a gas by generating an electrical signal. There are different gas sensors which work based on different principles. In particular, there are different gas sensors for the detection of H\({}_{2}\) gas, including electrochemical [9], surface acoustic wave [10], optical [11], gasochromic [12], and resistive [13]. Each sensor type has its own unique advantages and disadvantages. Among them, resistive gas sensors are highly popular owing to their unique advantages, such as high sensitivity, high stability, fast dynamics, simple design, simple operation, and low cost [14]. Even though they can be fabricated from different types of semiconducting materials, they are generally fabricated from semiconducting metal oxides, in which their resistances change upon exposure to target gases. Depending on the nature of the target gas and sensing material, the resistance can increase or decrease upon exposure to gas. Generally, n-type metal oxides are preferred to p-type ones, owing to their higher mobility of charge carriers [15]. N-type stannic oxide (SnO\({}_{2}\)) with a band gap of 3.6 eV is widely utilized in gas-sensing applications, because of the high mobility of charge carriers, ease of synthesis, excellent stability, high availability, and low cost [16; 17]. Accordingly, SnO\({}_{2}\)-based gas sensors have been extensively used for the detection of H\({}_{2}\) gas in the literature [18; 19; 20; 21; 22]. There are two general types of sensor configurations for realizing resistive gas sensors. In the planar configuration, the sensing layer is coated on a flat substrate equipped with interdigitated electrodes [23]. In addition, a microheater is generally applied to the back side of the substrate [23]. In tubular gas sensors, the sensing material is coated onto a tubular substrate which is generally alumina equipped with electrodes [24]. In addition, in this configuration, the sensor is heated by applying voltage to a highly resistive nichrome wire which is inserted into an alumina tube [25]. Tubular substrates have advantages such as ease of installation on support, ease of heating by applying voltage on nichrome wires inside them and a simple coating of the sensing layer over them. Generally, in the tubular configuration of gas sensors, alumina [26], and in the planar configuration, alumina or SiO\({}_{2}\)-deposited Si substrates [27], are utilized as substrates. In particular, alumina, with its low price and good electrical insulation, has unique features like good thermal conductivity, low thermal expansion coefficient, high melting point, and high mechanical strength, which are needed to achieve uniform temperature distribution in the sensing area and to prevent mechanical failure by thermal shock, and therefore, it is widely used for the realization of tubular gas sensors [28]. Electrodes in both configurations are often fabricated from Au [29], Pt [30], or Pd-Ag [31] noble metals. Recently, flexible/wearable gas sensors have captured a lot of attention due to their high mechanical flexibility in terms of bending, twisting, stretching, and so on, for new applications such as internet of things [32; 33]. In flexible/wearable gas sensors, generally a planar configuration is used and flexible substrates such as polyethylene terephthalate (PET), polydimethylsiloxane (PDMS), Polyimide (PI), and paper are used as substrates for this purpose [34; 35]. Generally, the effect of deposition parameters on tubular gas sensors is not mentioned in the literature.