Journal of Semiconductors, Volume. 46, Issue 1, 012606(2025)

Direct ink writing of nickel oxide-based thin films for room temperature gas detection

Neha Thakur1、*, Hari Murthy1, Sudha Arumugam2, Neethu Thomas2, Aarju Mathew Koshy2, and Parasuraman Swaminathan2
Author Affiliations
  • 1Department of Electronics and Communication Engineering, School of Engineering and Technology, CHRIST University, Kumbalagodu, Bengaluru, Karnataka, 560074, India
  • 2Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu, 600036, India
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    The rapid industrial growth and increasing population have led to significant pollution and deterioration of the natural atmospheric environment. Major atmospheric pollutants include NO2 and CO2. Hence, it is imperative to develop NO2 and CO2 sensors for ambient conditions, that can be used in indoor air quality monitoring, breath analysis, food spoilage detection, etc. In the present study, two thin film nanocomposite (nickel oxide-graphene and nickel oxide-silver nanowires) gas sensors are fabricated using direct ink writing. The nano-composites are investigated for their structural, optical, and electrical properties. Later the nano-composite is deposited on the interdigitated electrode (IDE) pattern to form NO2 and CO2 sensors. The deposited films are then exposed to NO2 and CO2 gases separately and their response and recovery times are determined using a custom-built gas sensing setup. Nickel oxide-graphene provides a good response time and recovery time of 10 and 9 s, respectively for NO2, due to the higher electron affinity of graphene towards NO2. Nickel oxide-silver nanowire nano-composite is suited for CO2 gas because silver is an excellent electrocatalyst for CO2 by giving response and recovery times of 11 s each. This is the first report showcasing NiO nano-composites for NO2 and CO2 sensing at room temperature.

    Keywords

    1. Introduction

    Air pollution is one of the major health hazards and two of the most common pollutant gases present in the atmosphere and continuously worsening the natural environment and surroundings are NO2 and CO2[1]. NO2 gas, with a yellowish−brown hue and a pungent odor are released during industrial emissions, burning garbage in the open, fuel combustion, and vehicle exhaust[2]. Exposure to NO2 gas can cause serious damage to lung tissues and can threaten the safety of human life. CO2 is an odorless, tasteless, and colorless gas that is not easily detected by human senses. The excess gas release can cause several health issues such as dizziness, increased heart rate, elevated blood pressure, coma, asphyxia, and convulsions[3]. Hence, proper monitoring and regulation of these gases are important. While considering the safety of human health and surroundings, it is imperative to develop high-performance, user and environment-friendly, low-cost (complicated and expensive equipment are not used, a 3 mL syringe is used for printing) sensors for real-time monitoring of these air pollutants.

    Gas sensors operate under various sensing mechanisms such as chemiresistive[4], electrochemical[5], capacitive[6], optical sensing[7], and so on. Among these chemiresistive sensing, i.e., monitoring a change in electrical resistance upon exposure to target gas is the most common method. Existing chemiresistive gas sensors are operational only at temperatures >100 °C, leading to high power consumption and a decreased lifetime[8, 9], a complicated fabrication process[10], are bulky and expensive[11], have shorter life span[12], and longer response and recovery times[13, 14]. Detection limit, working temperature, cost-effective fabrication technique, and calibration transferability are some of the major factors that often limit the usage of these gas sensors in field applications[15]. Due to the potential advantages such as small grain size, controllable morphology, and hetero-junction effect of nano-structured materials/semiconductors, nano-composites are becoming increasingly popular candidates as room temperature chemiresistive gas sensors. In this regard, this work proposes a low-cost simplified sensor fabrication technique suitable for room temperature. The main purpose for developing the room temperature sensor is to make it more useful in indoor air quality monitoring such as homes, offices, buildings, vehicle exhaust systems, etc. to name a few.

    The basic requirements to fabricate high-performance thin-film chemiresistive gas sensors are the proper material choice and film thickness. Recently, several materials such as ZnO[16], CuO[17], SnO2[18], WO3[19], NiO[8], ZnIn2S4[20], AuPt bimetal-decorated SnSe2[21], (FeCoNiCrMn)3O4[22], N-MoS2[23], NbS2[24], MoTe2[25], TiS2[26], Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)[27], bismuth oxide (Bi2O3)[28], TiO2/In2O3[29], etc. have been used for toxic gas detection. NiO (p-type metal oxide) has gained specific attention in room-temperature gas sensing because of its higher oxygen adsorption capability and charge transfer rate[13, 18]. In our previous report, NiO-graphene nano-composite films were investigated for structural, optical, and electrical properties. It was observed that the nano-composite film’s overall performance was improved compared to pristine NiO[30]. In the current work, two NiO nano-composites are formulated, one by adding graphene, a carbon-based material, and the other by adding a one-dimensional metal nanostructure, namely silver nanowires (Ag NWs)[31, 32]. The addition of graphene to NiO offers enhanced electrical conductivity and high adsorption capacity, primarily due to its pore volume, specific surface area, and pore size distribution[33, 34]. Surface functional groups, such as oxygen, can enhance the adsorption of polar gas molecules. Additionally, graphene contains dangling bonds and surface defects, providing additional adsorption sites and increasing sensitivity[35, 36]. Similarly, the addition of noble metal nanostructure in NiO enhances gas response due to more active sites, greater oxygen anion concentration, and reduced adsorption activation energy on interaction with the target gas[37]. Overall, thin film composites’ porous morphology and high surface area are more suitable for gas-sensing applications. Ngo et al. developed a NO2 sensor (3100 ppm) by NiO-rGO (reduced graphene oxide) matrix using a facile solution process to obtain the response and recovery times of 15 and 20 s respectively at 100 °C[38]. Jeong et al. prepared NiO-CNT (carbon nanotubes) structure for NO2 detection at room temperature acquiring response and recovery time of 30 and 20 min, respectively[39]. Zhang et al. developed NiO-graphene film by drop-casting for NO2 detection at 100 °C, providing a response time of 576 s and recovery time of 121 s[40]. Musayeva et al. employed the ultrasonic method to fabricate Ni/NiO/graphene heterostructure for NO2 detection at 150 °C to give excellent response and recovery time of 20 and 10 s respectively[41]. Shanavas et al. dip-coated NiO/rGO on the substrate to develop CO2 (500 ppm) for room temperature yielding a response time of 16 s and recovery time of 22 s[36]. Mitri et al. proposed colloidal quantum dots for NO2 detection (30 ppm) yielding a response time of 12 s and recovery time of 26 min[42]. Recently, Wang et al. synthesized hollow tubular structures constructed by bimetallic ZnIn2S4 using a metal−organic framework (MOF) for NO2 detection (10 ppm) exhibiting a good response and recovery time of 2 and 3.7 s respectively[20]. Li et al. developed a NO2 (5 ppm) sensor using (FeCoNiCrMn)3O4 as a precursor for room temperature sensing. The response and recovery time obtained were 67 and 140 s[22]. Zhao et al. fabricated a NO2 sensor (101000 ppm) with N-MoS2 material providing the reduced band gap from 1.79 to 1.65 eV. The response time obtained was 50 s and the recovery time of 200 s[23]. Kumar et al. used SnO2 nanoparticles for NO2 sensor with an outstanding low-concentration detection capacity of 0.5 ppm, the response and recovery time obtained for 2 ppm was 184 and 432 s[18]. Beniwal et al. developed a NO2 sensor with PEDOT:PSS yielding the response and recovery time of 210 and 60 s respectively at 0.5 ppm[27]. Umar et al. fabricated a Ce doped ZnO nanostructure NO2 sensor (100 ppm, 250 °C) providing the response and recovery time of 11.8 and 56.3 s respectively[43]. Besides a lot of material advancement in the gas sensing field, the authors have focused on NiO-based sensors to make them cost-effective. From the extensive literature survey, only a few reports are available for NiO-graphene-based material delivering a fast response at high temperatures. However, NiO-silver nanowire is still untouched for NO2 and CO2 detection at room temperature which has a vast scope in the future. Table S1 summarizes NiO-based NO2 and CO2 sensors reported in the literature. It is observed from the table that the sensors reported require complex fabrication methods, high operating temperatures, long response time, and recovery time.

    The direct ink writing (DIW) method is increasingly being used to fabricate electronic devices[44]. DIW is preferred due to its large versatility with several materials, easy fabrication of intricate structures, precise control over material properties (porosity, density, and mechanical), simultaneous usage of multiple nozzles, simultaneous printing of different materials, low material wastage, and high scalability[4547]. Typically, the active materials are printed on the interdigitated electrode (IDE) patterns, which provide enhanced sensitivity and precision, enabling the detection of lower concentrations of gases more effectively[48]. IDE-patterned sensors can exhibit robust performance and long-term stability with reduced maintenance.

    This work focuses on developing NO2 and CO2 sensors using NiO-graphene (labeled NG) and NiO-silver nanowires (labeled NA) nano-composite thin films at room temperature. Two nano-composite inks NiO (60 mg/mL)-graphene (20 mg/mL), and NiO (60 mg/mL)-silver nanowires (3 mg/mL) are formulated using sonication and vortex mixing respectively. Ethylene glycol (EG) is used as the solvent for ink preparation. DIW is used to deposit the thin films on top of a silver nanoparticle (NP)–based printed IDE pattern on the glass substrate. The thin films are characterized by their structural, optical, and electrical properties. NO2 and CO2 are used at volumetric flow rates of 5 and 30 SCCM respectively. The current−voltage (I−V) analysis is carried out to confirm the linear response, and the resistance−time (R−t) analysis provides the response and recovery times (defined as the required time for reaching 90% of the maximum response when gas is purged/stopped)[49]. The fabricated sensors have demonstrated a stable response for up to 100 days. In the proposed work, a novel, economical, and simple approach is discussed for fabricating nano-composite thin film gas sensors for NO2 and CO2 detection at room temperature and it has a lot of future scope in the field of emerging flexible and printed electronics.

    2. Materials and methods

    2.1. Materials

    Nickel (Ⅱ) chloride hexahydrate [NiCl2.6H2O, 98%], hydrazine monohydrate (N2H4, 75%), and potassium hydroxide flakes (KOH, 85%) were supplied by Avra Synthesis Pvt. Ltd. and used for NiO synthesis. Graphene powder was procured from Shilpent Enterprises. Silver nitrate (AgNO3, >99.9%, Alfa Aesar), ferric chloride (FeCl3, 97%, Sigma Aldrich), and polyvinylpyrrolidone (PVP 1300 K, Mw = 1 300 000 g/mol and PVP 40 000, Mw = 40 000 g/mol, Sigma Aldrich), were purchased for Ag NW synthesis. Solvents used include ethylene glycol (EMPARTA ACS 99% (EG)), premium grade ethanol (Heyman), and deionized (DI) water (18.2 MΩ·cm). Silver nanoparticle ink from Dycotec Materials (DM SIJ-3200) was used to make an IDE pattern for gas sensing. All the chemicals were used as received without further purification.

    2.2. Synthesis of nanostructures and inks

    2.2.1. Synthesis of NiO nanopowders

    0.11 M solution of nickel chloride hexahydrate was prepared in ethanol. Hydrazine monohydrate (molar ratio of [Ni+2/N2H4] = 5) was added to the above-prepared solution and stirred for 30 min. 1 M KOH was added to the solution drop-wise to maintain the pH of the solution as 12. The mixture was kept under magnetic stirring for 2 h at room temperature and then left undisturbed overnight. The obtained solution was centrifuged with KOH (once), DI water (three times), and ethanol (once) at 7000 r/min for 15 min. The obtained residue was dried at 80 °C for 5 h in a hot air oven (SIGMA 2000W-230V AC) followed by annealing at 400 °C for 2 h in a muffle furnace to obtain black-colored NiO nanopowder[30].

    2.2.2. Synthesis of silver nanowires

    0.163 g of a binary mixture of PVP of two different molecular weights (1300 K : 40 K in a weight ratio of 1 : 1) was dissolved in 22 mL of EG. 2.5 mL of 0.6 mM FeCl3 in EG and 0.18 g of AgNO3 in 3 mL EG were added subsequently under stirring into the PVP solution. The reaction mixture was transferred to a Teflon-lined stainless steel autoclave reactor after 5 min of mixing and heated at 120 °C for 12 h. The cooled- down mixture was diluted with ethanol in a 5 : 1 volume ratio. Ag NWs were washed by centrifuging (three times) at 1500 r/min for 15 min with ethanol. The purified Ag NWs were dispersed in isopropyl alcohol (IPA) for further processes[50].

    2.2.3. Ink preparation

    NiO ink was prepared by dispersing 60 mg of NiO nanopowder in 1 mL of EG. The ink was prepared by sonicating the mixture in a bath ultrasonicator for 3 h to obtain a uniform dispersion (Fig. 1(a)). The NiO-graphene (NG) composite ink was prepared by adding 20 mg of graphene powder into 1 mL of as-prepared NiO ink under bath sonication for 2 h (Fig. 1(b)). NiO-Ag NW nano-composite ink (NA) was prepared by adding 250 µL of Ag NW dispersion, of concentration 3 mg/mL, into 1 mL of NiO ink and vortex mixing for 3 min (Fig. 1(c)). No stability issues were observed due to particle condensation during and after the ink formulation[30].

    (Color online) (a) NiO ink, (b) Nickel oxide-graphene ink, (c) Nickel oxide-silver nanowire ink.

    Figure 1.(Color online) (a) NiO ink, (b) Nickel oxide-graphene ink, (c) Nickel oxide-silver nanowire ink.

    2.3. Gas sensor fabrication

    An IDE pattern comprised of two comb-like structured electrodes was used for electrical contacts. The IDE pattern was printed on a UV-treated glass slide (2 x 2 cm2) using a commercial silver NP ink (Dycotec Materials DM-SIJ-3200) onto the glass substrate using a custom-built direct writer printer (Avay Biosciences Pvt. Ltd.) (Fig. 2(a)). Fig. 2(b) represents the schematic of the target IDE pattern. Later the IDE pattern was dried at 100 °C for 1 h (Fig. 2(c)). The nano-composite ink was loaded into a commercially available 3 mL syringe and extruded through a 22-gauge needle with an inner diameter of 0.41 mm. The substrate temperature was maintained at 70 °C using the print bed heater. A computer-aided design (CAD) model of 1 x 1 mm2 was loaded onto the D-write slicer software, and the printing parameters were customized to generate a G-code file, which was then loaded into the printer. The NG (Fig. 2(d)) and NA (Fig. 2(e)) square patterns were printed on the IDE patterns. NiO-Ag NWs ink printed pattern is transparent as compared to NiO-graphene ink. The patterns were then annealed at 120 °C for 1 h in a hot air oven to facilitate connectivity of the particles in the printed layer. Post-annealing, contacts were made with copper wires using silver epoxy paste. Further, the contacts were dried at 70 °C for 10 min and then the devices were used for gas sensing. After consecutive heating, it was expected that the solvent (EG) would have completely evaporated, which is in good agreement with the energy-dispersive X-ray spectroscopy (EDS) data obtained later in this work[45]. We ensure excellent consistency between various batch-fabricated gas sensors, by using optimized preparation procedures for all the thin films. The use of high-quality laboratory-grade chemicals further enhances this. Fig. S1 corresponds to the I−V testing results of three different batch-fabricated sensors. It is observed that almost the same I−V plot is obtained for all batches indicating the batch consistency.

    (Color online) (a) DIW printing of IDE. (b) Schematic of IDE pattern. (c) IDE pattern after drying. (d) Printed nickel oxide-graphene sensor. (e) Printed nickel oxide-silver nanowire sensor.

    Figure 2.(Color online) (a) DIW printing of IDE. (b) Schematic of IDE pattern. (c) IDE pattern after drying. (d) Printed nickel oxide-graphene sensor. (e) Printed nickel oxide-silver nanowire sensor.

    2.4. Gas sensing setup

    The schematic of the gas sensing setup with all the necessary components is given in Fig. 3. A homemade gas sensing chamber with a single gas inlet and an outlet and a total capacity of 600 mL was used in this study. The sensor was placed inside the gas chamber on a sample holder and the silver epoxy contacts were connected to a Keysight source meter (B2901). Two-probe current−voltage (I−V) measurements measured the resistance (with and without gas). All measurements were carried out at room temperature and constant humidity. NO2 and CO2 gases were sensed separately by filling the chamber with the respective gas for 5 min and then the gas valve was closed. The mass flow controller (MFC) was used to control the gas flow rate of 5 SCCM for NO2 and 30 SCCM for CO2. The sensors were initially tested with different gas concentrations: 2, 5, and 10 SCCM for NO2 gas, with the best response observed at 5 SCCM. In contrast, CO2 gas did not respond quickly at 10, 20, and 40 SCCM. Still, a notable response was achieved at 30 SCCM. For the stability testing of fabricated sensors, the ratio of resistance in gas to air was recorded weekly once for up to 100 days, while the sensor was stored in an ambient atmosphere. The authors ensure adequate ventilation and regularly calibrate and maintain the equipment to prevent leaks and inaccuracies and overcome pollution issues.

    Schematic of gas sensor setup containing gas chamber, mass flow controller, source meter, and data acquisition system.

    Figure 3.Schematic of gas sensor setup containing gas chamber, mass flow controller, source meter, and data acquisition system.

    2.5. Characterization

    Crystal structure and phase characterizations were carried out by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer (Cu anode and Ni filter, wavelength of 0.154 nm) in a Bragg−Brentano geometry. Raman spectra were obtained in a Witec UHTS 300 spectrometer, with an excitation wavelength of 514.5 nm. The morphology of the nanostructures and printed samples were obtained by high-resolution scanning electron microscopy (HRSEM) (Hitachi S-4800, Japan). The surface roughness and thickness of the printed layers were quantified using an optical profilometer (Bruker Contour GT). The surface chemistry was evaluated by an X-ray photoelectron spectrometer (AXIS SUPRA). Viscosity was measured using a rheometer, (Physical MCR 301, Anton Paar), over a shear rate varying from 100 to 1000 s1, using a parallel plate geometry with a plate diameter of 25 mm. Mass density was calculated using a density meter (DMA 4200M). A theta optical tensiometer measured surface tension and contact angles (T200-PD200, Biolin Scientific). The chemical bonding information was obtained from Fourier transform infrared (FTIR) spectroscopy, using an FTIR spectrometer (FTIR-6X). The energy band gap was studied using a UV−Vis (Ultraviolet–visible) spectrophotometer (Jasco, V-650). The current−voltage studies were done using a source meter (Keysight B2901A). The gas flow rate was measured through an Alicat Scientific Inc. mass flow controller (MFC).

    3. Result and discussion

    3.1. Characterization of synthesized nanostructures

    The purity and crystallinity of the as-synthesized NiO nanoparticles, purchased graphene, and synthesized silver nanowires were examined by using powder X-ray diffraction (PXRD), and the data is plotted in Figs. 4(a)−4(c). For NiO, the diffraction peaks can be indexed to the face-centered cubic (FCC) crystalline structure of NiO (ICDD no. 04-0835) as shown in Fig. 4(a)[5153]. Debye Scherrer formula, given in Eq. (1), was used to estimate the crystallite size (L) of NiO as 14.1 nm[54, 55].

    (a) PXRD patterns of synthesized NiO nanoparticles. (b) PXRD patterns of as-purchased graphene powder. (c) PXRD patterns of synthesized silver nanowires. All peaks can be indexed to the respective materials. No impurity peaks are seen in the XRD plots. (d) SEM micrograph of the NiO nanoparticles. (e) SEM micrograph of silver nanowires with a larger magnification image in the inset. These are analyzed to obtain the average particle size, nanowire length, and diameter.

    Figure 4.(a) PXRD patterns of synthesized NiO nanoparticles. (b) PXRD patterns of as-purchased graphene powder. (c) PXRD patterns of synthesized silver nanowires. All peaks can be indexed to the respective materials. No impurity peaks are seen in the XRD plots. (d) SEM micrograph of the NiO nanoparticles. (e) SEM micrograph of silver nanowires with a larger magnification image in the inset. These are analyzed to obtain the average particle size, nanowire length, and diameter.

    L=Kλβcosθ,

    where λ is the X-ray wavelength in nm, β is the full-width half maximum (FWHM) of the diffraction peak, θ is the scattering angle in radians, and K is the crystallite constant taken according to the shape (0.9) for the Bragg reflection. Fig. 4(b) represents a single prominent peak at 26.3° for the (002) plane corresponding to graphene (ICCD no. 98-008-5640) and the crystallite size was obtained as 17 nm. Fig. 4(c) corresponds to the PXRD spectrum for the synthesized silver nanowires and the peaks correspond to pure Ag (ICDD no. 65-2871). The surface morphology of the NiO NPs was analyzed using SEM and the corresponding image is shown in Fig. 4(d). The average particle size obtained was 51.4 ± 15.4 nm, measured by averaging over 50 particles, indicating some agglomeration of the individual crystallites during ink preparation. The SEM images of silver nanowires are shown in Fig. 4(e) and the magnified view is given in the inset. The average length calculated for silver nanowire was 58.3 ± 9.8 µm, and the diameter was 97.3 ± 10.7 nm. To confirm the printability of the ink, the rheological properties (viscosity, density, and surface tension) of the formulated inks were measured[56]. The viscosity and surface tension values for NG were 17.20 ± 0.02 mPa·s and 57.60±0.05 mN/m respectively and for NA as 20.65 ± 0.07 mPa·s and 44.08 ± 0.21 mN/m indicating printability. The density measured for NG and NA was 1.30 and 1.11 kg/m3 respectively. The contact angle obtained was <90° indicating good wettability.

    3.2. Characterization of the printed thin films

    3.2.1. Structural properties

    XRD analysis of the printed thin film using NG ink is shown in Fig. 5(a). The spectrum has similar NiO peaks as shown in Fig. 4(a) with an additional peak for graphene located at 26.5°[57]. Fig. 5(b) shows the XRD pattern of the as-prepared NA printed film, whereas the silver peaks observed are similar to the peaks shown in Fig. 4(c) while the remaining peaks correspond to NiO[5860]. The Raman spectrum of the printed NG thin film is shown in Fig. 5(c). The following peaks were obtained in the spectra, 1352 cm1 (2 LO mode due to NiO), 1576 cm1 (G band of graphene), and 2713 cm1 (2D band of graphene)[61, 62]. The 2D band represents a second-order Raman process involving two phonons, and its intensity is sensitive to the number of graphene layers[63, 64]. Raman spectrum of NA is shown in Fig. 5(d), and the band at 147 cm1 was observed due to AgO stretching mode (LO). The vibrational peak obtained at 503 cm1 arises from the stretching vibration of 1PLO mode, which also indicates phonon scattering. The band at 1565 cm1 is due to the two-magnon (2M) scattering describing the anti-symmetric stretching[65]. The broadening of the peak can be attributed to the surface roughness in the printed film[66].

    (a) XRD pattern of nickel oxide-graphene nano-composite thin film, "X" represents the NiO peaks, and "O" represents the graphene peaks. (b) XRD of nickel oxide-silver nanowires nano-composite thin film, "X" represents the NiO peaks, and "O" represents the silver peaks. (c) Raman spectrum of nickel oxide-graphene containing NiO (2LO) and graphene (G and 2D bands). (d) Raman spectrum of nickel oxide-silver nanowires corresponding to NiO (1P-LO) and Ag (LO and 2M).

    Figure 5.(a) XRD pattern of nickel oxide-graphene nano-composite thin film, "X" represents the NiO peaks, and "O" represents the graphene peaks. (b) XRD of nickel oxide-silver nanowires nano-composite thin film, "X" represents the NiO peaks, and "O" represents the silver peaks. (c) Raman spectrum of nickel oxide-graphene containing NiO (2LO) and graphene (G and 2D bands). (d) Raman spectrum of nickel oxide-silver nanowires corresponding to NiO (1P-LO) and Ag (LO and 2M).

    The morphology of the printed NG nano-composite films was obtained using a high-resolution scanning electron microscope (HRSEM). The graphene flakes are decorated with NiO nanoparticles as visible in Fig. 6(a), including the lower magnification image in the inset. The morphology of the NA nano-composite is shown in Fig. 6(b), in which Ag NWs are meshing the NiO nanoparticles. The film thickness of NG and NA was measured using surface profilometry as approximately 4.1 ± 0.02 and 1.91 ± 0.21 µm respectively. The film thickness of NG is higher than NA due to the porous nature of graphene powder, which was used in a higher concentration. The surface roughness of NG and NA was 730 ± 0.1 and 294 ± 0.04 nm respectively. EDS data of NG and NA are provided in Figs. 6(c)−6(d) with their respective weight percentages. A minimal amount of carbon (C) is observed in the NA nano-composite, probably due to the presence of ethylene glycol. The small amount of carbon indicates almost complete evaporation of the solvent.

    (Color online) (a) HRSEM images of nickel oxide-graphene morphology showing the graphene flakes decorated with NiO. (b) HRSEM images of nickel oxide-silver nanowires morphology. (c) EDS data for nickel oxide-graphene. (d) EDS data for nickel oxide-silver nanowires.

    Figure 6.(Color online) (a) HRSEM images of nickel oxide-graphene morphology showing the graphene flakes decorated with NiO. (b) HRSEM images of nickel oxide-silver nanowires morphology. (c) EDS data for nickel oxide-graphene. (d) EDS data for nickel oxide-silver nanowires.

    X-ray photoelectron spectroscopy (XPS) was used to analyze the surface chemistry of materials. Fig. 7(a) presents the C1s spectra. The primary peaks of NG and NA were observed at 281 and 282 eV respectively are attributed to CC bonds. A small satellite peak was observed at 284 and 290 eV, indicating C=O bond formation. The Ag 3d5/2 and Ag 3d3/2 binding energies for Ag NWs were acquired at 365 and 371 eV in Fig. 7(b). The O1s spectrum features two main components one at 526 eV, indicative of lattice oxygen in NiO, and a peak at 528 eV associated with metal hydroxyl (OH) bonds. An additional peak at 530 eV for NA corresponds to lattice oxygen as shown in Fig. 7(c)[67, 68]. The Ni 2p3/2 main peak and satellite at 851 and 853 eV. The satellite peaks were observed at 858 and 869 eV. The Ni 2p1/2 main peak at 876 eV was obtained as shown in Fig. 7(d)[6871].

    (Color online) (a) XPS of nickel oxide-graphene (NG) and nickel oxide-silver nanowires (NA) composites showing C1s. (b) XPS of nickel oxide-silver nanowires (NA) composite showing Ag-3d peaks. (c) XPS of nickel oxide-graphene (NG) and nickel oxide-silver nanowires (NA) composite showing O1s. (d) XPS of nickel oxide-graphene (NG) and nickel oxide-silver nanowires (NA) composite showing Ni-2p.

    Figure 7.(Color online) (a) XPS of nickel oxide-graphene (NG) and nickel oxide-silver nanowires (NA) composites showing C1s. (b) XPS of nickel oxide-silver nanowires (NA) composite showing Ag-3d peaks. (c) XPS of nickel oxide-graphene (NG) and nickel oxide-silver nanowires (NA) composite showing O1s. (d) XPS of nickel oxide-graphene (NG) and nickel oxide-silver nanowires (NA) composite showing Ni-2p.

    3.2.2. Optical properties

    FTIR analysis of the printed films provides information regarding the bond formation in the nano-composites. The silicon substrate was used for analysis, and the testing chamber was vacuumed before the measurement for better spectral resolution. For NG (Fig. 8(a)), the formation of carbon−oxygen bonds was indicated at 1040 cm1. A small NiO peak was observed at 1615 cm1. The peak observed at 2360 cm1 was due to CO2 adsorbed from the atmosphere. Vibration bands for hydroxyl groups were observed at 3646 cm1[72]. FTIR spectrum of NA in Fig. 8(b) reported the presence of Ag−O, Ni−O, C=C, O=C=O, and O−H bonds by the peaks observed at 683, 835, 1552, 2359, and 3499 cm1, respectively[7375]. UV−Vis spectroscopy analysis was performed for both NG and NA nano-composites to estimate the band gap (αhv)2 under direct band gap conditions. Pure NiO film reported an optical band gap (Eg) value of 3.8 eV, in line with our previous work[30]. Eg for both NG and NA films was estimated as 2.07 and 2.12 eV respectively by using the Tauc plot as shown in the inset of Figs. 8(c) and 8(d). The change in the optical band gap of NG and NA may be due to the interconnection of graphene and silver nanowires with NiO nanoparticles. The band gap reported in the literature for the NiO-graphene composite was 2.3 eV and for the NiO-silver composite was 1.7 to 2.6 eV which is in line with our report[76, 77].

    (a) FTIR of nickel oxide-graphene representing the associated functional groups. (b) FTIR of nickel oxide-silver nanowires representing the bond formation. (c) UV−Vis absorption spectrum of nickel oxide-graphene, inset representing the band gap information. (d) UV−Vis absorption spectrum of nickel oxide-silver nanowires, with an inset showing the band gap information obtained using the Tauc plot method.

    Figure 8.(a) FTIR of nickel oxide-graphene representing the associated functional groups. (b) FTIR of nickel oxide-silver nanowires representing the bond formation. (c) UV−Vis absorption spectrum of nickel oxide-graphene, inset representing the band gap information. (d) UV−Vis absorption spectrum of nickel oxide-silver nanowires, with an inset showing the band gap information obtained using the Tauc plot method.

    3.2.3. Electrical properties

    The sheet resistance of NG and NA films was determined using four probe techniques as 517 ± 0.43 Ω/sq and 172 ± 0.12 KΩ/sq respectively. Although silver is more conducting than graphene, the NA film has a higher sheet resistance probably due to the lower concentration (one-sixth) of silver nanowires in NiO compared to graphene. Also, the printed NG film is thicker which could lead to a lower resistance when compared to the NA films. The high electrical resistance leads to thermal energy enhancing the gas adsorption, making it suitable for gas sensing applications[78]. NiO is common in both nano-composites which contribute to the sensing and another component (graphene or silver nanowires) responsible for the charge carriers’ electrical conductivity.

    3.3. Gas sensing studies

    3.3.1. Current−voltage (I−V)

    The performance of two nano-composites, NG and NA, as gas sensors were measured by measuring their response to two oxidizing gases: NO2 and CO2. NiO has a wide band gap and at low voltages, carriers (electrons and holes) are not readily available to contribute to the current. As the voltage increases, carriers participate in the conduction process, leading to a nonlinear I−V curve for pure NiO[79]. The I−V characteristics were measured at room temperature with the voltage sweeping from 1 to +1 V. This low voltage range is due to the good electrical conductivity of the nano-composite arising from the graphene to Ag NW network and can also enable the development of low-powered sensors for practical applications. Figs. 9(a) and 9(b) show the linear I−V plot for NG and NA in the presence of NO2 while Figs. 9(c) and 9(d) represent the plots in the presence of CO2[80]. The increase in current upon exposure to gas is due to the rise in the hole concentration at the oxide surface, resulting from the ionosorption of the target oxidizing gas.

    (Color online) I−V plots of (a) Nickel oxide-graphene in the presence of air and NO2. (b) Nickel oxide-silver nanowires in the presence of air and NO2. (c) Nickel oxide graphene in the presence of air and CO2. (d) Nickel oxide-silver nanowires in the presence of air and CO2. The I−V plots were measured before (in the air) and after 5 SCCM of NO2 and 30 SCCM of CO2 gas exposure. The current has slightly increased in the presence of gas. These devices exhibit superior gas sensing at room temperature and low applied voltage thereby enabling the development of low-power sensors.

    Figure 9.(Color online) I−V plots of (a) Nickel oxide-graphene in the presence of air and NO2. (b) Nickel oxide-silver nanowires in the presence of air and NO2. (c) Nickel oxide graphene in the presence of air and CO2. (d) Nickel oxide-silver nanowires in the presence of air and CO2. The I−V plots were measured before (in the air) and after 5 SCCM of NO2 and 30 SCCM of CO2 gas exposure. The current has slightly increased in the presence of gas. These devices exhibit superior gas sensing at room temperature and low applied voltage thereby enabling the development of low-power sensors.

    3.3.2. Resistance−time (R−t)

    The gas-sensing response exhibited by the NG nano-composite for repeated flow and purging of the NO2 gas was obtained. The concentration of the gas is maintained at 5 SCCM during flow. Fig. 10(a) is the magnified view of a single cycle of the NG nano-composite. The resistance of the film was decreased by introducing the NO2 gas and increased back to the original stabilized value when the gas was stopped. The response time was 10 s and the recovery time was 9 s. Fig. 10 (b) represents the R−t plot for NA in the presence of NO2 with response and recovery times of 13 and 14 s respectively. Figs. 10(c) and 10(d) show the R−t plot for NA in the presence of NO2 and CO2 with response and recovery time (16 s, 12 s) and (11 s, 11 s) respectively. NG provides a slightly better response for NO2 gas, the probable reason for this is due to NO2 having a higher electron affinity than CO2, and there is electron transfer from the graphene to adsorbed NO2 molecules[7, 8183]. The NA nano-composite provides a better response for CO2 gas because silver is a promising electrocatalyst for CO2[84]. The high selectivity of silver for CO2 is largely due to the weak binding energy with surfaces, though there are minor differences in site composition and coordination[32, 85, 86]. The response and recovery time of NG for NO2 and NA for CO2 obtained are (10 s, 9 s) and (11 s, 11 s) are better than those reported in the literature.

    (Color online) R−t plot curves for (a) Nickel oxide-graphene in the presence of NO2. (b) Nickel oxide-silver nanowires in the presence of NO2. (c) Nickel oxide-graphene in the presence of CO2. (d) Nickel oxide-silver nanowires in the presence of CO2.

    Figure 10.(Color online) R−t plot curves for (a) Nickel oxide-graphene in the presence of NO2. (b) Nickel oxide-silver nanowires in the presence of NO2. (c) Nickel oxide-graphene in the presence of CO2. (d) Nickel oxide-silver nanowires in the presence of CO2.

    3.3.3. Stability

    The sensors’ repeatability and stability were evaluated every 7 to 10 days over 100 days, showing minimal variation in the Rg/Ra values. Rg is the resistance in the presence of gas and Ra is the resistance in ambient conditions. Fig. 11(a) illustrates the stability response of the nano-composites for NO2 gas, showing a nearly linear response for NG. Fig. 11(b) gives the stability response of nano-composites for CO2 gas, showing a nearly linear response for NG. Fig. 11(c) is the histogram comparing the stability response of two gases. The limit of detection (LOD) of the NO2 sensor obtained was 1 SCCM and for CO2 sensor was 10 SCCM.

    (Color online) (a) and (b) Stability of the sensors over 100 days, with measurements once every 7−10 days. The device was tested at 5 SCCM for NO2 and 30 SCCM for CO2 from −1 to 1 V and stored in an ambient atmosphere between tests. (c) Histogram representing nickel oxide-graphene is more stable for NO2 and nickel oxide-silver nanowires are more stable for CO2.

    Figure 11.(Color online) (a) and (b) Stability of the sensors over 100 days, with measurements once every 7−10 days. The device was tested at 5 SCCM for NO2 and 30 SCCM for CO2 from −1 to 1 V and stored in an ambient atmosphere between tests. (c) Histogram representing nickel oxide-graphene is more stable for NO2 and nickel oxide-silver nanowires are more stable for CO2.

    4. Gas sensing mechanism

    The gas-sensing behavior involves the diffusion of the analyte gas to the sensing surface through the porous structure of the thin films and its reaction with negatively charged oxygen on the surface of the metal oxide. At room temperature, oxygen molecules are adsorbed onto the film surface and are ionized into species such as O2 (<100 °C) (Eq. (2)), O (100300 °C), and O2 (>300 °C) by capturing electrons near the surface.

    O2(gas)+eO2(ads).

    At room temperature, the acceptor level of NiO is fully ionized (filled), leaving holes in the valence band. Once the NiO surface is exposed to air, oxygen from the air gets adsorbed on the NiO surface by trapping electrons from surface states. The result is band bending and an increase in hole concentration near the interface, forming a hole-accumulating layer (HAL) at the NiO surface. NO2 is the oxidizing gas or electron acceptor due to its high electron affinity (37.5 KCal/mol)[87]. The molecules react with the surface of NiO by direct adsorption on the surface or by reaction with pre-adsorbed oxygen. As a result of the adsorption of NO2, electrons are released from the surface, leading to an increase in the width of the HAL, as well as increased conductivity (Eqs. (3) and (4)).

    NO2(gas)+eNO2(ads),

    NO2+O2(ads)+2eNO2(ads)+2O(ads).

    The general reaction between CO2 and the surface of metal oxide at room temperature can be described by Eq. (5). In the presence of CO2 gas, CO32 is formed after the reaction with adsorbed O2. This HAL will increase and the recombination decreases leading to the increase in current[88, 89].

    CO2+12O2(ads)CO32(ads).

    Pristine NiO exhibits poor conductivity and lower sensing capability, to improve the conductivity and sensing abilities of NiO, graphene, and silver nanowires were added to form two different nano-composites (NG and NA). Adding graphene to NiO increases the surface area, creating a synergistic effect. The NiO-graphene hetero-junction enhances the electron transfer phenomenon between the adsorbate and adsorbent leading to escalated sensitivity with faster response and better recovery time[90].

    When Ag NWs are assorted to NiO, an interesting spill-over phenomenon was observed. In this phenomenon, the gas molecules first adsorb on the silver active sites and later adsorb to the NiO surface producing more oxygen species. The increased oxygen species enhance the surface reaction by extracting more electrons which leads to the creation of more holes therefore HAL gets broad, and the resistance is reduced. In electronic sensitization, the Fermi level of silver (4.2 eV)[89] is relatively lower than NiO (5 eV)[91]. In this regard, charge carriers are relocated, while the electrons are transported from NiO to Ag until the Fermi level of two nanomaterials reaches equilibrium. The Ag and NiO become negatively and positively charged respectively. The energy bands in NiO are curved and the Schottky barrier is created at the interface. The Schottky barrier can resist the recombination of separated electron−hole pairs and enhance the response to the analyte gas. Electrons from the conduction band of NiO are transported into Ag, due to dissimilarities in work function generating Schottky barriers at the interface between Ag and NiO. Consequently, additional HAL near the NiO surface is formed and hence the rise in current will take place[9296].

    5. Conclusion

    We have successfully fabricated nickel oxide-graphene (NG) and nickel oxide-silver nanowires (NA) nano-composite thin films for NO2 (5 SCCM) and CO2 (30 SCCM) gas detection at room temperature in static mode. Initially, nanostructures (nickel oxide nanopowder and silver nanowires) are synthesized and used to formulate nano-composite inks. The nano-composite inks are printed on thin films with a direct ink writer. The prepared nano-composites are well characterized for structural (XRD, Raman, EDS, HRSEM, surface profilometry, and XPS), optical (FTIR and UV−Vis), and electrical properties (sheet resistance) properties. The XRD pattern confirms NiO, graphene, and silver peaks. The following modes LO, 1P-LO, 2M, G, and 2D modes are observed for nano-composites using the Raman spectrum. FTIR spectrum confirms the presence of O−H, C=C, and O=C=O bonds in the nano-composite films. The optical band gap calculated for NG and NA is 2.09 and 2.12 eV. The sheet resistance was calculated for NG and NA as 517 ± 0.43 Ω/sq and 172 ± 0.12 KΩ/sq, respectively. Both nano-composites are found suitable for NO2 and CO2 gases. However, due to higher electron affinity NG provides a slightly better response and recovery time of 10 and 9 s for NO2. The NA nano-composite has a response and recovery time of 11 s each for CO2 gas due to the catalyst properties of silver. The sensors have shown a stable response for 100 days. Although the sensing responses of NG and NA are similar, suggesting a potential selectivity issue, it is challenging to classify NA as a CO2 sensor. The authors acknowledge this limitation and further optimization and testing are required to ensure the selectivity of the NA sensor for CO2. The inexpensive sensor fabrication with less preparation time and the absence of any toxic and corrosive chemicals during synthesis add to the appeal of this sensor. The proposed work is testing the hypothesis of ink formulation and thin film fabrication for gas sensing and since it gave good results, we would continue in flexible substrates as part of future work.

    Appendix A. Supplementary material

    Supplementary materials to this article can be found online at https://doi.org/10.1088/1674-4926/24080025.

    [61] MM Hossain. Metal oxide semiconductor/graphene heterojunction-based sensors. PhD Dissertation, Clemson University, 1, 1(2017).

    [86] I M Horta, A Jr Godoy, B S Damasceno et al. Development of metal oxide heterostructures for photovoltaic and solar cell applications. Metal Oxide-Based Heterostructures. Amsterdam: Elsevier, 359(2023).

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    Neha Thakur, Hari Murthy, Sudha Arumugam, Neethu Thomas, Aarju Mathew Koshy, Parasuraman Swaminathan. Direct ink writing of nickel oxide-based thin films for room temperature gas detection[J]. Journal of Semiconductors, 2025, 46(1): 012606

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    Paper Information

    Category: Research Articles

    Received: Jul. 17, 2024

    Accepted: --

    Published Online: Mar. 6, 2025

    The Author Email: Neha Thakur (NThakur)

    DOI:10.1088/1674-4926/24080025

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