A Report on Nio Nanowires in Gas Sensing

Nowadays, there are a lot of concerns about the environment that requires the continuous improvements in gas sensor development. Gas sensors are useful for detecting toxic and inflammable gases in the atmosphere such as hydrogen (H2), ammonia (NH3), carbon monoxide (CO) and many more. However, more emphasis has been focused more on detection of H2 because of its abundance and its usability as a renewable and clean energy source. Gas sensors derived from nanowires of n-type metal oxides have been widely investigated, while p-type metal oxides has just started.

NiO is a p-type metal oxide semiconductor that has a cubic rock salt structure with a wide band gap ranging from 3.6eV to 4.0eV. As for this research, N Kaur et al. investigated NiO nanowires in the field of gas sensing application. There are several ways to synthesize NiO nanowires such as sol gel method, electrospinning method, and for this research, Vapor-liquid-solid method. Vapor-liquid-solid (VLS) is a method that has many advantages compared to other methods such as: lowered reaction energy, no precursor requirement, direct growth on an active substrate, and require only a furnace and pump. Using VLS method to synthesize NiO nanowires also yield low-defect single crystalline nanowires that are preferable in many applications.

The researcher in this academic journal presents the synthesis of NiO nanowires by using VLS method on alumina substrate with different catalysts. The morphology of the synthesized product will be investigated using scanning electron microscopy (SEM). As for the structural characterization, GI-XRD (X-ray diffraction) and Raman spectroscopy were used. To test out the gas sensing ability of the NiO nanowires, conductometric sensing devices were prepared under different gases and working temperature.

To prepare the substrate, first, alumina substrate were ultrasonically cleaned in acetone and dried with synthetic air. After that, catalysts (platinum, palladium, and gold) were deposited onto the substrate using magnetron sputtering technique. The reason for the catalyst in the substrate is to promote nucleation sites during the deposition and growth process of NiO nanowires. To synthesize NiO nanowires using VLS technique, NiO powder were used as source material. NiO powder and substrate are placed into the furnace; however, NiO is placed at a higher temperature and is heated to evaporation. To transfer NiO vapors towards the catalyst deposited alumina substrate, argon gas were used as carrier gas to transport the NiO vapors. Therefore, the higher temperature NiO will condensate onto the substrate because the substrate is at a lower temperature.

To synthesize the gas sensing device, alumina substrates were used to prepare conductometric sensing device. First, soldering pads with titanium tungsten alloy as adhesion layer were deposited onto the substrates. Second, an interdigitated platinum contact was deposited to achieve higher electrical conductance. On the other side of the substrate, a heating element was deposited to study the response at different working temperatures. Finally, the device was attached on the TO packages by using electro-soldered gold wires.

For gas sensing experiment, a homemade test chamber was used to measure the conductometric response of the sensors. To study the behavior of the sensor, the temperature of each sensor can be controlled by varying the power dissipated by each sensor’s heating element. The sensor are tested for different types of gas and concentration of gas and the response were recorded in wide range of temperature (200 to 500oC). The types of gas tested are as follows: ethanol, acetone, H2, and CO. Measurements were obtained by keeping the temperature in the chamber and the humidity constant while applying voltages to the sensor device.

For the result, according to the SEM images, using Platinum and Palladium as the catalyst yielded similar results. Upon using gold as a catalyst, the NiO nanowires yielded better results as compared to other catalyst. A trend of better results was observed when the temperature to induce evaporation was reduced when using gold as the catalyst. A factor that affects the growth of nanowires is the temperature of the substrate. A higher temperature of substrate would result in higher quantity of nanowires and a denser structure. An experiment was also carried out to show that carrier gas flow is a factor that affects the growth of the nanowires. For the morphology, the nanowires produced with the gold catalyst resulted in a more confined shape, whereas the experiment conducted with the nanocatalyst showed no growth. Values obtained from the XRD suggest that NiO is a cubic structure based on the calculated lattice parameter “a”.

For gas sensing result, decrease in conductance is observed when exposed to reducing gases, this is due to the fact that in p-type metal oxide semiconductor, adsorbed molecules of the gases react with the metal oxide surface to form active oxygen ions will form water vapour and inject electron, which in turn reduces the conductance. Increase in conductance is observed when exposed to oxidizing gases, this is due to the fact that oxidizing gases generate deeper acceptor levels therefore increasing the conductance of nanowires. Optimal working temperatures of detecting H2 and CO are at 300oC while optimal working temperature for detecting acetone and ethanol is at 500oC.

In conclusion, the optimum conditions for the best results for NiO nanowires are at induced evaporation temperature of 1400oC, Argon gas flow of 100SCCM and 20SCCM with a pressure of 1mbar using gold as the catalyst. Nanowire growth is affected by factors such as substrate temperature and argon gas flow. The type of catalyst affects the results of the experiment and gold was found to be the best option in producing NiO nanowires. The conductance of the nanowires are affected by oxidizing and reducing gases. Optimal working temperature for the nanowires for acetone and ethanol was found to be 500oC whereas the optimal working temperature for H2 and CO was found to be 300oC.

After reviewing this academic journal, it made us realize the importance of gas sensing in the field of environmental protection. The researchers realized that there are already several studies of n type metal oxides based gas sensing device but not much on the p type counterpart. By being the first few to provide clear analysis of the NiO nanowires using different conditions, they are able to summarize the optimal conditions for gas sensing and provide an argument for NiO nanowires that they may be a suitable option for manufacturers to consider in gas sensing application.