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[
Journal of Semiconductors, Volume. 46, Issue 1, 012606(2025)
Direct ink writing of nickel oxide-based thin films for room temperature gas detection
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.
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[
Gas sensors operate under various sensing mechanisms such as chemiresistive[
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[
The direct ink writing (DIW) method is increasingly being used to fabricate electronic devices[
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)[
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[
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[
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 (
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.) (
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
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 s−1, 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
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.
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.
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
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
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.
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 (
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[
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[
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.
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.
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
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)[
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,
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[
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)[
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
[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
Category: Research Articles
Received: Jul. 17, 2024
Accepted: --
Published Online: Mar. 6, 2025
The Author Email: Neha Thakur (NThakur)