LPG and NH<inf>3</inf> sensing characteristics of 1-D interlinked nanowired Co<inf>3</inf>O<inf>4</inf> films deposited by using pulsed D.C. electrochemical deposition method

Abstract

Presently, our environment is polluted by number of gases exhausted from auto and chemical industry. The detection of harmful gases is becoming a need of society. Sensors play an important role in the areas of emissions control, environmental protection, public safety and human health. Over the past decades, several kinds of gas sensors have been developed. Co 3 O 4 is an important p-type semiconductor with a normal spinel structure and it has many commercial or potential applications. However, literature research revealed that thin film gas sensors based on Co 3 O 4 are not reported up to now. In view of this, the LPG and NH 3 sensing characteristics of 1-D interlinked nanowired Co 3 O 4 films deposited by using pulsed D.C. electrochemical deposition method are presented in this paper. The (CH 3 COO) 2 Co.4H 2 O (0.5 M) and H 3 BO 3 (0.15 M) were dissolved one by one in 250 ml of double distilled water and then filtered using Whatman 41 filter paper. The cobalt based thin films were deposited on thoroughly cleaned stainless steel (SS) and copper (CU) substrates (each of size: 2 cm × 2 cm & thickness = 0.5 mm) using pulsed DC electrochemical deposition method. The films were deposited by using following parameters: (1) pH of solution ∼ 4.5 (by adding NaOH / HCl in solution), (2) cathode-anode distance ∼ 2.5 cm, (3) pulsed deposition on time = 1.5 minute, (4) pulsed off time = 20 sec, (5) total deposition time = 11 minute and (6) current density ∼ 8 mA/cm2. All as-deposited films were heated at 350 °C for 2 hr. The films prepared on SS and CU substrates were identified as PESA and PEUA respectively. The resultant films were characterized by using X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The LPG and NH 3 gas sensing properties: sensitivity factor (S.F.), response time, recovery time and repeatability of these films were measured at room temperature (RT) by using home-built static gas sensing system at different concentrations ranging from ∼ 25 to 350 ppm of a given test gas.The XRD and Raman spectroscopy studies clearly indicated the formation of pure Co 3 O 4 phase in these films. The values of lattice parameter (a o ) calculated for PESA and PEUA films are found to be 8.063 A and 8.062 A respectively, which are found to be matching with reported value = 8.084 A for cubic spinel Co 3 O 4 . The morphological studies of films by SEM showed some interesting observations. The surface of each film is found to be covered with the mesh of interlinked wires with more or less flat base. The interlinked wired mesh is noted to be attached firmly to base at different points with the insertion of ends of wires into the surface at those points. The diameters and lengths of 1-D interlinked wires are found to be between 250 – 350 nm and 2 – 10 µm respectively. The densification at the surface of each film is seen to be moderate, however, qualitatively the densification below the interlinked wired mesh structure is found to be good. The LPG and NH 3 gas sensing properties of PESA and PEUA films showed the increase in S.F. with increasing the gas concentration. Both the films are found to be more sensible to LPG gas as compared to the NH 3 gas. In case of NH 3 gas sensing, the maximum values of S.F. are found to be 264 and 232 for the PESA and PEUA films respectively. Further, in case of LPG gas sensing, the maximum values of S.F. are found to be 248 and 230 for the PESA and PEUA films respectively. Hence, PESA film is observed to be more sensible for LPG and NH 3 gases as compared to PEUA film. Further, the measurement of gas sensing properties for number of cycles clearly indicated the repeatability of gas sensing results of these films. For both films, the response time (2 – 3 min.) is found to be much higher than the recovery time (25 – 30 sec). The response time is found to higher for LPG gas as compared to the NH 3 gas for both the films. These results undoubtedly proved the efficacy of 1-D interlinked nanowired Co 3 O 4 films sensors prepared in present work. The observed results indicated the potential use of these 1-D interlinked nanowired Co 3 O 4 films for LPG and NH 3 gas sensing applications in different areas.