Facile synthesis of highly porous CuO nanoplates (NPs) for ultrasensitive and highly selective nitrogen dioxide/nitrite sensing

Copper oxide (CuO) nanoplates (NPs of ∼100 nm width) were successfully synthesized via a chemical method (emulsion method). Superior catalytic activities towards both chemical and electrochemical sensing of nitrite were achieved.

systematically studied and explored. Here we made an effort to representation the probable sensing mechanism of CuO thin films towards NO 2 gas via band structure model. Copper oxide (CuO) NPs is synthesized by cheap emulsion method. In a 250 mL round bottom flask 0.1M Cu(Ac) 2 as a source of 'Cu' and 0.01 M of PVP (Polyvinyl pyrrolidone) were added with 100 mL DI water and the reaction mixture was stirred vigorously at room temperature for 30 min leading to the formation of Cu acetate and PVP mixture. This mixture was transfer in to the oil bath and added drop wise ammonia percentage up to the pH 10-12 & refluxed to obtain brownish black precipitate. The prepared powder was washed several times with DI water and methanol to eliminate the salt and dried in air at 100 o C for 1hr. The prepared powder was then located in alumina boat and calcined at 400 o C for 1hr and consequently cooled to room temperature. For thin film formation, CuO powder was dissolved in polyvinyl alcohol (PVA) solution as a role of binder and constantly stirred for 12 hr to get solution. The obtained uniform solution of CuO was deposited on glass substrate using Doctor Blade method and dried in air atmosphere. This film was then heated at 400 o C for 2 hr to remove PVA binder and then used for gas sensing application.
XRD patterns of CuO samples are recorded by using X-ray diffraction method equipped with strong Cu Kα1 radiation (λ = 1.54 Å), at a scanning rate of 2 degree per min in the scanning range of 2 from 20°-80°. The CuO NPs were drop coated over a carbon tape and further the samples were then sputter coated with Pt prior to their characterization using scanning electron microscopy (FE-SEM, JEOL, and Japan) to avoid charging effect. BET surface area analysis has been performed using AutosorbiQ (Quantachrome Inc., USA) gas sorption system. Low pressure volumetric N 2 adsorption-desorption measurements were performed at 77 K maintained by low temperature liquid N 2 bath with pressure ranging from 0-760 torr. Out gassing route was carried out at 200 °C for 15 h under dynamic vacuum (10-3torr) until a stable weight was achieved. Ultrahigh purity grade (99.999 %) N 2 was used, which is further purified by using calcium aluminosilicate adsorbents to remove the trace amounts of water and other impurities prior to the measurements. For N 2 isotherms, warm and cold free-space correction measurements were performed with ultrahigh pure He gas (99.999% purity). For the measurement, about 200 mg of samples were used and to confirm complete elimination of all guest H 2 O molecules from the samples, weight of the samples was measured before and after out gassing process. Specific surface area was calculated by Brunauer-Emmett-Teller (BET) method. The TGA thermogram was obtained using TG Analyzer (SDT Q600 V20.9 Build 20) in the temperature range from 25 to 600 o C with heating rate of 10 °C/min in N 2 atmosphere.
The thin films of CuO NPs are tested for chemical gas sensing assets. The electrical connections of silver paste separated by 1cm were layered on CuO thin films. The sensor was mounted in a stainless-steel test chamber (volume: 250 cm3). A required concentration of the test gas in the chamber was achieved by injecting a known quantity of gas using a micro syringe. A change in resistance of the film as a function of time (response curve) was recorded at operating temperature of 200 °C for 5, 25, 50, and 100 ppm concentration of NO 2 gas, which was commercially procured. The response data was acquired by using a computer interfaced Keithley 6514 system electrometer. The recovery of the sensor was recorded by exposing the sensor to air. From the response curves, the response % (S) was calculated using the following relation: Where, Rg and Ra are resistances of film in the occurrence of NO 2 and without NO 2 (in air) respectively. Response and recovery times were defined as the time required for 90% of total resistance change on contact to gas and air, respectively.
All electrochemical measurements were carried out with a CHI-660E (CH instruments USA) instrument using a predictable three-electrode system, where a saturated calomel electrode Further the thermogravemetric analysis of as synthesized CuO catalysts is also carried out to understand the process of weight losses throughout calcination. The TGA depicts weight loss percentage with increase in temperature; Fig.S1 is the thermogram of as synthesized samples.
The thermogram depicts the weight loss (w 1 ) of 0.5 to 1 % below 150 °C corresponding to physically absorbed water [1].  Nitrogen adsorption-desorption isotherm of CuO NPs recorded to evaluate the available surface area and the active sites available for chemical and electrochemical interaction of NO 2 and other active gases. Fig. S2 indicates that the CuO NPs displays type II isotherm with a hysteresis loop in the relative pressure (P/P 0 ) range from 0.8 to 0.9. The characteristic hysteresis loop indicates that the CuO is mesoporous in nature and the presence of mesopores could be due to the thermal decomposition of polyvinyl pyrrolidone (PVP) on CuO surface [2]. The BET (Brunauer-Emmett-Teller) surface area calculated from the adsorption branch of the isotherm is found to be 23.60m 2 /gm. The inset in Fig. S2 illustrates the corresponding pore size distribution plot calculated by BJH (Barrett-Joyner-Halenda) from the adsorption data. The average pore radius and pore volume of the CuO NPS is calculated to be 2.3 nm and 2.599e -0.1 cc/gm. The response and recovery time is a significant concern for characterizing a sensor. The response and recovery times for CuO thin films to diverse concentrations of NO 2 were depicted from the electrical response curve Fig S3 (a). Repeatability study of film to 100 ppm NO 2 gas, (c) Stability study of CuO thin film towards 100 ppm NO 2 .
It was observed that as the concentration of NO 2 increases from 5-100 ppm, the response time decreases from 6 sec. to 1sec. and recovery times increases from 18 sec. to 130 sec. The decrease in response time could be due to the availability of vacant sites on thin film of the sensor for adsorption of gas, this may be probably due to, the reaction product's not leaving the oxygen surface immediately after the reaction and therefore the re-adsorption of oxygen is delayed, ensuing in a longer recovery time. NO 2 gas is a heavier gas it's recover longer times. This observed behaviour of increasing recovery time with increasing NO 2 concentration is similar to that of our previous reports on SnO 2 [3] and α-Fe2O3 [4].
Therefore, the repeatability of CuO NPs sensor has been studied further. The plot of repeatability study of CuO NPs sensor to fixed 100 ppm concentration of NO 2 gas is shown in Fig. S3 (b) exhibiting the excellent repeatability in response upon consecutive exposure of NO 2 gas.The stability in response of CuO NPs sensor at 150 °C is calculated for the period of 50 days and the corresponding results are presented in Fig. S3 (c). Results indicate that the response rate of the CuO NPs sensor decreased from 88% to 64% (after the period of 35 days) with 72% stability. Further, it is noted that in case of metal oxide based gas sensors, the decrease in response rate with respect to time is predominantly due to aging effects [5]. The influence of the increase in concentration of NO 2on the electrocatalytic oxidation peak potential and peak current at the CuO NPs electrode in 0.5M KOH was studied using an LSV segment in Fig. S4 (a). The oxidation peak current at -0.30 V vs. RCE shows a linear response with the concentration of NO 2ions in the range of 10-50 µM and this linear range is broader than those of reported comparable electrocatalytic systems.
Furthermore, the influence of the scan rate on the electrocatalytic oxidation peak potential (Epa) and peak current for 20 µM concentration NO 2at the CuO/GCE electrode in 0.5M KOH was studied using LSV, as shown in Fig. S4 (b). The current values were originate to increase with an increase in the scan rate from 10 to 100 mV/s Fig. 4 (b). The linear association between the anodic peak currents and shows that electrooxidation of NO 2is diffusion controlled.