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Nanocomposite NiO:Pd hydrogen sensors with sub-ppm detection limit and low operating temperature

We present results on the fabrication of chemiresistive nanocomposite NiO:Pd hydrogen sensors, which are able to detect hydrogen concentrations as low as 300 ppb in air, operating at temperatures in the 115-145 C range. Thin NiO films were deposited by radio frequency magnetron sputtering on silicon and quartz substrates and their structural and morphological properties were characterized. Pd nanoparticles were added on the NiO surface via pulsed laser deposition and the films were tested as hydrogen sensors before and after Pd deposition. The presence of Pd nanoparticles improved the sensor performance, leading to the detection of hydrogen concentrations at sub-ppm levels. Moreover, the response time of the sensors decreased by a factor between two and four.


Introduction
Hydrogen holds great promise for becoming an alternative fuel for clean energy generation. In this context, hydrogen mass production, distribution, as well as combustion in engines will require advanced control systems to ensure safe storage and usage of the gas, which is colourless and odourless with a lowest explosion limit of 40,000 ppm in air. Therefore, new smart sensors with high sensitivity to hydrogen will be essential. So far many works have been devoted to find suitable materials for hydrogen sensors, in particular metal oxides like SnO 2 [1], WO 3 [2], ZnO [3], and NiO [4,5], and also GaN [6]. The successful development of commercial sensors requires high sensitivity, short response time, good repeatability, and reliability.
In this work, we present results on the fabrication of chemiresistive nanocomposite NiO:Pd hydrogen sensors, which are able to detect hydrogen concentrations as low as 300 ppb in air, operating at temperatures in the 115 -145 o C range. NiO is a widely employed non-toxic metal oxide, with excellent chemical stability [7] and ease of fabrication. The use of NiO for gas sensing applications is limited because native NiO is a p-type semiconductor [8], whereas n-type materials are expected to achieve better gas responses than p-type materials [9]. Among the most sensitive pure NiO hydrogen sensors reported so far, thin NiO films, which are able to detect concentrations as low as 500 ppm, operate in the 300 -650 o C temperature range [10,11]. Pure NiO films have also been shown to operate at temperatures as low as 125 o C for higher hydrogen concentrations of 3000 ppm [12]. NiO films with thin Pt overlayers have been employed as chemiresistive hydrogen sensors, detecting concentrations of 500 ppm while operating in the relatively low temperature range of 150 -420 o C, due to the promoting role of the metallic element [13]. Recently, NiO films with embedded Au nanoparticles achieved sensing of hydrogen concentrations as low as 5 ppm, operating in the 100 -160 o C temperature range [14]. Palladium sensors constitute a separate class of hydrogen sensors, which are based on the unique response of palladium to hydrogen [15]. In this work, we combine for the first time palladium with a metal oxide to develop a high-performance hydrogen sensor. By employing high-quality, conductive NiO films with Pd nanoparticles deposited on the surface, we are able to detect hydrogen concentrations at the sub-ppm level for lower operating temperatures than those commonly published in the literature. Consequently, we improve substantially the detection limit of the sensing device, while simultaneously reducing its power consumption. Thus, NiO performance is now comparable to the performance of n-type metal-oxide sensors, making NiO:Pd an attractive material for gas sensing applications.

Materials and methods
Thin NiO films were deposited by radio frequency magnetron sputtering (RF sputtering) from 3" NiO targets in Ar-O 2 plasma, using an RF power of 200 W at room temperature or 300 o C. High resistivity Si (001) and quartz wafers were used as substrates. The films were not annealed after deposition. The surface morphology and structural properties of the films were characterized using Scanning Electron Microscopy (SEM) and X-Ray diffraction (XRD). The composition of the films was determined by Energy Dispersive Analysis of X-rays (EDS).
The response of the NiO films to low concentrations of hydrogen was investigated, before and after employing pulsed laser deposition (PLD) to deposit Pd nanoparticles on the surface. For pulsed laser deposition, the NiO films were placed in a vacuum chamber, which was evacuated to a pressure of 10 -5 mbar. A Q-switched Nd: was calculated from the hydrogen partial pressure in the chamber, measured by an MKS Baratron gauge. In order to achieve low hydrogen concentrations, hydrogen was mixed with dry nitrogen in a premixing chamber, thus achieving dilution factors below 10 -2 .
The sensor response S was calculated as where R g is the electric resistance of the sample in the presence of hydrogen and R 0 is the resistance of the sample in air.

Results and discussion
The as-deposited NiO films are polycrystalline with fcc structure, as indicated by XRD measurements (not shown here). The texture of the films depends on the oxygen content in the Ar-O 2 plasma during sputtering. The (111) orientation is observed for deposition in pure Ar or with low oxygen content in the Ar-O 2 plasma and the (200) orientation is observed for deposition in pure O 2 . When a film is deposited in oxygenrich plasma, then both (111) and (200) orientations appear in the XRD spectrum. For NiO films deposited with oxygen content ranging from 9% to 50% in the Ar-O 2 plasma, the lattice parameter increases from 4.194 Å to 4.303 Å, respectively. The composition of the NiO films for the same range of oxygen content in the plasma is calculated from EDS measurements of the film on the silicon substrate and results in an oxygen-tonickel ratio from 1.1 to 1.23, respectively. Deposition at higher oxygen content in the Ar-O 2 plasma does not increase the lattice parameter or the oxygen-to-nickel ratio, but changes the crystal orientation of grains from random to the {200} texture, as well as decreases the lateral size of grains. The attained oxygen-to-nickel ratios result in welldefined p-type conductivity, characteristic of oxygen-rich NiO compounds [5]. In the case of NiO deposition at 300 o C, columnar growth in the <111> direction is preferred, and the oxygen-to-nickel ratio is close to 1.1. The electrical properties of NiO films after sputtering and thermal treatment in Ar or O 2 were previously studied and reported in Refs. [5,16].  between NiO film structure and growth temperature were reported in Refs. [17,18].
The film deposited with 33% O 2 content at 300 o C performed better for hydrogen sensing. This can be explained by the smaller grain size of this film, which results in an increased surface to volume ratio and therefore a bigger active surface area for sensing [12]. Figure 1e shows an EDS spectrum for this film, after the deposition of Pd nanoparticles on the surface. Only oxygen, nickel, silicon, and palladium peaks appear on the spectrum, indicating the film is free from contaminations. The silicon peak originates from the substrate on which the film is deposited. The atomic concentration of palladium in the sensor film is found to be 0.18%. Similar results were obtained for the palladium concentration of the other NiO:Pd films employed in this study, which was expected since palladium was deposited on all films under identical conditions.
Below we present results on hydrogen sensing with the film deposited with 33% O 2 content at 300 o C.

Figure 2
The NiO films were tested as hydrogen sensors before and after deposition of Pd nanoparticles on their surface. Because the films conduct electricity as p-type semiconductors, the electric resistance increases during the interaction with hydrogen, which is a reducing gas [19]. This behaviour is demonstrated in Fig. 2a, which shows the response of the as-deposited NiO film to hydrogen. The sensor response is calculated according to Eq. 1. The measurements in Fig. 2a were taken at an operating temperature of 146 o C. We observe that the film is able to detect low hydrogen concentrations, as low as 8 ppm, at a relatively low operating temperature with very good signal to noise ratio.  hydrogen and atmospheric oxygen, which is adsorbed on the surface of NiO [20].
Additionally, the response of the films increases with increasing operating temperature for similar hydrogen concentrations. This is due to the fact that adsorption-desorption kinetics, which affect the sensor performance, depend on the operating temperature [21,22] and apparently become more efficient near 145 o C. For the operating temperature of 115 o C, the hydrogen detection limit is 6.5 ppm. This is one of the lowest operating temperatures found in the literature for NiO sensors, which typically operate between 300 -650 o C, as mentioned in the Introduction. For the operating temperatures of 132 o C and 145 o C, the hydrogen detection limit is 700 and 300 ppb, respectively.  resulting in highly activated atomic oxygen and hydrogen species, which react more effectively [23]. Additionally, atmospheric oxygen gets adsorbed on the surface of the metallic nanoparticles and reacts with hydrogen. Therefore, the presence of metallic nanoparticles increases the active surface area of the sensor. Comparing the performance of Pd nanoparticles with Au nanoparticles presented in previous works [14,24], we conclude that Pd acts as a better catalyst for the performance of nanocomposite NiO hydrogen sensors.

Conclusions
Thin NiO films were deposited by RF magnetron sputtering and employed as