SO2-sensing properties of NiO nanowalls synthesized by the reaction of Ni foil in NH4OH solution

Nickel oxide (NiO) is a p-type metal-oxide semiconductor with wide-ranging applications. Recent studies have focused on the gas-sensing properties of this semiconductor. This study introduces an easy process for growing NiO nanowalls on a glass substrate using Ni foil and aqueous . The morphology and structure of the NiO nanowalls are investigated and confirmed by field-emission scanning electron microscopy and x-ray diffraction (XRD) analyses. The gas-sensing properties of the prepared nanowalls are tested using a dynamic gas-testing system wherein the target gases are and . Gas-sensing data show that the synthesized NiO nanowalls are highly responsive toward . Additionally, a sensing device prepared based on the NiO nanowalls is found to be stable during measurements, exhibiting a linear variation with changes in concentration.

NiO nanomaterials with wall-plate-like morphologies are predominantly synthesized [10][11][12][13] among others (e.g. nanorods, nanowires, nanoflowers, and nanotubes) [14][15][16][17]. Several groups introduce different approaches for the synthesis of NiO nanowalls on Ni metal surfaces. Zhan et al synthesized 3D NiO nanowalls on a Ni foam through a hydrothermal method [10]. Tang et al [13] used electrochemical corrosion approach to prepare NiO nanowalls grown on a Ni foam. Ni et al introduced an easy electrochemical corrosion method to fabricate NiO nanowalls [18]. NiO nanowalls can also be prepared by plasma-assisted oxidation [19]. Recently, our group introduced an easy, low-cost method of synthesizing Ni(OH) 2 /NiO nanowalls through the surface reaction of Ni foil in aqueous NH 4 OH [20]. Nonetheless, the gas-sensing properties of the NiO nanowalls fabricated by these methods remain lacking. The advantage of NiO nanowalls over other morphologies like nanowire, nanoparticles or nanosheets is to provide better film porosity which can enhance the adsorption and desorption processes of the target gases over the film surface.
In this study, Ni(OH) 2 nanowalls were fabricated through the chemical reaction of Ni foil with aqueous NH 4 OH. The NiO nanowalls were obtained by dehydrating the Ni(OH) 2 nanowalls at 500 • C using a hotplate. The SO 2 , NO 2 , NH 3 Advances in Natural Sciences: Nanoscience and Nanotechnology

SO 2 -sensing properties of NiO nanowalls synthesized by the reaction of Ni foil in NH 4 OH solution
and H 2 S sensing properties of the as-prepared NiO nanowalls were investigated and compared at a working temperature of 50 • C − 300 • C.

Experimental details
We used a facile method in our previous study to synthesize the NiO nanowalls using a Ni foil [20]. Figure 1 illustrates our synthesis process. Commercial pure Ni foil (thickness = 0.1 mm; purity = 99.99%; Aldrich) was cut into a 2 cm × 2 cm plate ( figure 1(a)). The plate was immersed in acetic acid for 10 min and then ultrasonically cleaned in a bath sonicator with acetone, ethanol, and distilled water for 5 min. After drying under a flow of N 2 , the plate was folded at the corners to form table-like plate and was added to 25 ml of NH 4 OH solution in a 100 ml Duran laboratory bottle ( figure  1(b)). Meanwhile, a cleaned glass (2 cm × 2 cm) was positioned under the plate. The Duran bottle was kept at 70 • C for 48 h in an oven. After treatment, the glass substrate was covered by a green film (figure 1(c)), dried in an oven at 70 • C for 12 h, and annealed at 500 • C for 1 h using a hotplate. As a result, the green film became a dark brown film (figure 1(d)). A simple parallel Au electrode was patterned on the film by thermal evaporation (figure 1(e)).
The morphologies of samples were characterized by fieldemission scanning electron microscopy (FESEM; JEOL JSM-7610F). The structures of the samples were identified using x-ray diffraction (XRD; XPERT-PRO) with Cu Kα radiation of λ = 1.5418Å. The NiO nanomaterial was extracted from the annealed sample and was dispersed on a molybdenum grid for transmission electron microscopy (TEM; Philips Telnai G2F20 S-TWIN) observations. The gas-sensing properties of the as-synthesized materials were measured using a dynamic gas-testing system. During measurement, the chamber was pumped and was maintained at a vacuum pressure of 10 Torr. The carrier gas was dry air and the total flux rate was 200 sccm. The operating temperature was from 50 • C to 300 • C (300 • C is the maximum value of our gas-testing system). Gas response was defined as S = (Rg−Ra) Ra %, where R g and R a represent sensor resistance in the target gas and air, respectively. Sensor response (τ Response ) and recovery time (τ Recovery ) are determined by the time for sensor resistance to reach 90% of its steady-state value from R a to R g and R g to R a , respectively. Figure 2 shows the XRD patterns of the films before and after annealing at 500 • C. For the pristine film, major diffraction peaks at 11.6 • , 33.6 • , 35.9 • and 60.1 • can be assigned to the   The surface morphologies of the film before and after annealing at 500 • C for 1 h were similar (figures 3(a) and (c)). The films comprise curvy nanowalls with an average thickness of 15 nm. Cross-sectional FESEM images indicated that the thickness of both films is ~1450 nm (figures 3(b) and (d)). The sample before annealing ( figure 3(b)) comprises a film layer (~314 nm) and a nanowall layer (~1122 nm). The sample after annealing is dense with a nanowall thickness of approximately 561 nm ( figure 3(d)). The synthesis of NiO nanowalls, in which Ni can react with NH 4 OH to form Ni(OH) 2 , which was dispersed in the solution, was proposed in our previous study [20]. The nanowalls are probably formed by an oriented assembly of Ni(OH) 2− 4 units onto the Ni(OH) 2 nanoseeds. The transformation from Ni(OH) 2 to NiO without morphology change at 500 • C is well documented [20,22,23]. Figures 4(a) and (b) show the TEM and HR-TEM images of the extracted NiO nanowalls, respectively. The nanowalls were composed of many small crystallites ( figure 4(b)). The dimensions of all crystallites are analyzed in a histogram graph ( figure 4(c)). The average diameter of the crystallites is 2.2 nm, which is similar to the calculated value using the XRD pattern equation above. Figure 5 shows the gas responses of the as-prepared NiO nanowalls toward H 2 S, NO 2 , SO 2 and NH 3 . The device is highly sensitive to SO 2 . The SO 2 response of the device gradually increases from 6.3% at 100 • C to 8.7% at 250 • C and satur ates to 8.8% at 300 • C. The optimal operating temperature of the NiO nanowalls was 200 • C to H 2 S and NO 2 and 100 • C to NH 3 . Figure A1 (appendix) shows the transient curves of the device at different operating temperatures. The positive response of the device to reduced gases (H 2 S, NO 2 and SO 2 ) and to oxidizing gas (NO 2 ) suggests that the NiO nanowalls exhibit a p−type semiconductor sensing characteristic. The gas-sensing mechanisms of NiO to both types of gases are well documented [7,24]. Table 1 shows a comparison of NiO nanowall-based SO 2 sensor with other mat erials. The sensing performance, 'gas response' and 'response/recovery time' of the synthesized NiO nanowalls are not better than those of reported nanomaterials. However, the sensing performance of a gas sensor strongly depends on many factors, such as electrodes (material, gap, size, shape, and position), testing system set-up (gas inlet position and flux rate), and test chamber (chamber volume, medium with/without oxygen, and humidity). Thus, the limit of detection (LOD) to SO 2 should be considered for comparison. For this specification, the NiO nanowalls can detect down to 1 ppm SO 2 gas, which is better than many of the reported SO 2 sensors.

Results and discussion
The SO 2 −sensing behavior of the NiO nanowalls is related to the reaction of the adsorbed oxygen adatoms O − on the NiO surface with the SO 2 molecules, which can be described as equation (1): After the reaction, the reduced number of holes in the accumulation zone results in increased sensor resistance. The sample exhibits higher response toward SO 2 than toward H 2 S although both gases possessed sulfide in the molecules. This behavior can be attributed to the reaction of SO 2 with NiO as mentioned by Tyagi et al [31]. NiO can react with SO 2 gas to produce NiS: The produced SO 3 gas molecules react with the remaining oxygen adatoms on the NiO surface: Under the presence of oxygen, NiS can be transformed to NiO: The released SO 2 in equations (3) and (4) further reacts with oxygen adatoms following equation (1), thereby decreasing the number of holes in the accumulation. Thus, the NiO nanowalls are more sensitive to SO 2 than to H 2 S. Studies on the gas-sensing behavior of Ni 3 O 2 (OH) 4 are rare. However, Ni 3 O 2 (OH) 4 remained in the sample  after annealing. This phase is possibly remained at the bottom of the sample because of the thick/dense film. Ni 3 O 2 (OH) 4 can be easily transformed to NiO by giving off water molecules above 200 • C [32]. The decomposition of Ni 3 O 2 (OH) 4 may be prohibited at the bottom of the film because the film was thick and dense. Thus, the Ni 3 O 2 (OH) 4 layer may not influence the sensor performance during the test.
Device stability is confirmed by the transient curves after exposure to 5 pulses of 20 ppm SO 2 (figure 6(a)). Sensor resistances under 'gas on' and 'gas off' are retained. At 300 • C operating temperature, the response and recovery times of the device to 20 ppm SO 2 are 40 and 57 s, respectively (figure 6(b)). Figure 7 reveals the influence of SO 2 concentration on sensor resistance and sensor response. The minimum SO 2 concentration that the device can detect is 1 ppm (figure 7(a)) with a response of just above 2.5%. The relationship of the increase in sensor response with SO 2 concentration is linear ( figure 7(b)). The linear correlation coefficient R 2 is 99%, indicating the high potential of the as-synthesized nanomaterial as a SO 2 sensor.

Conclusions
Curvy NiO nanowalls were grown on a glass substrate through the reaction of Ni foil in aqueous NH 4 OH. The nanowalls are found to be uniform with an average thickness of 15 nm. The nanowalls are extremely sensitive to SO 2 compared with H 2 S, NO 2 and NH 3 . The highest response of the device to SO 2 is 8.8% at 300 • C, and this value is approximately two, six, and two times higher than the responses of the device to H 2 S, NH 3 and NO 2 , respectively. The as-prepared NiO nanowalls exhibit good stability (in terms of sensor resistance), reasonable response/recovery time (below 1 min), and linear variation of resistance with SO 2 concentration. The LOD of the NiO nanowalls to SO 2 is 1 ppm. Thus, the NiO nanowalls synthesized through our simple method are a high-potential SO 2 sensor.  Adv. Nat. Sci.: Nanosci. Nanotechnol. 9 (2018) 045013