Ultra‑Sensitive, Rapid‑Response Ammonia Gas Sensor Based on Reduced Holey Graphene Oxide Thin Films

Abstract

Gas‑sensing performance of graphene thin films is often limited by nanosheet stacking, which reduces surface accessibility. Here we introduce a novel ammonia (NH₃) sensor that employs holey graphene oxide (HGO) thin films. HGO nanosheets are produced by UV‑assisted photo‑Fenton etching of graphene oxide, creating a porous, single‑layer structure. The sheets are then chemically reduced with pyrrole to form reduced holey graphene oxide (rHGO). By drop‑casting rHGO suspensions onto interdigitated electrodes, we fabricate a chemiresistive sensor that delivers a 2.81 % resistance change at 1 ppm NH₃ and an 11.32 % change at 50 ppm. Remarkably, the sensor self‑reverses to its baseline without external heating or UV/IR illumination and maintains excellent repeatability over multiple cycles. The low‑cost, low‑energy design and outstanding sensitivity make this rHGO thin‑film sensor a compelling platform for portable ammonia detection in environmental, industrial, and medical settings.

Introduction

Chemiresistive sensors are pivotal in environmental monitoring, industrial safety, and healthcare. Conventional solid‑state gas sensors, however, struggle with long‑term stability and sensitivity. Nanostructured materials—nanowires, carbon nanotubes, and graphene—offer superior performance due to high surface‑to‑volume ratios, excellent electrical conductivity, and ease of fabrication. Graphene, a single‑layer carbon lattice, is especially attractive: its carrier density is highly responsive to surface adsorption, enabling detection of gases such as NO₂, NH₃, CO, and H₂O. Reduced graphene oxide (rGO) retains many of graphene’s benefits while being cost‑effective and scalable. Nonetheless, most rGO sensors rely on two‑dimensional films that are prone to restacking, limiting accessible surface area. By converting rGO into a porous, holey structure, we can dramatically enhance gas interaction and sensor response.

Materials and Methods

Materials

Natural graphite powder (Tianyuan, China), pyrrole (Suzhou Chemical Reagents), and ferrous sulfate (Shanghai Chemical Reagents) were used without further purification. All solvents were distilled prior to use.

Preparation of HGO

Graphene oxide (GO) was synthesized via an improved Hummers method. A 57.5 mL 2 M H₂SO₄ solution was combined with 2 g graphite, stirred, and then treated with NaNO₃, KMnO₄, and H₂O₂. After extensive washing with 3 % HCl and water, the dry product was dispersed at 0.5 mg mL^–1 and sonicated. For photo‑Fenton etching, 20 mL H₂O₂ and 100 µL FeSO₄ were added to 5 mL of the GO dispersion, sonicated, and the pH was adjusted to 4 with 1 % HCl. UV irradiation induced the photo‑Fenton reaction, creating nanoscale pores on the GO sheets. The resulting HGO was dialyzed for one week to remove residual ions and reagents.

Preparation of rHGO

HGO (1 mg mL^–1, 50 mL) was sonicated for 1 h, then mixed with 1 mg pyrrole dissolved in 10 mL ethanol. After 20 min of sonication, the mixture was refluxed at 95 °C for 12 h. The product was filtered, washed with DMF and ethanol, and dried, yielding rHGO with reduced oxygen functional groups and attached polypyrrole chains.

Fabrication of Gas Sensor

Interdigitated electrodes (8 pairs, 600 µm finger length, 5 µm gap) were fabricated by sputtering Cr (10 nm) and Au (180 nm) onto a lithographically patterned silicon wafer, followed by lift‑off. A 0.05 µL droplet of 1 mg mL^–1 rHGO ethanol suspension was deposited onto the electrode array and allowed to dry in air, forming a continuous conductive network.

Gas‑Sensing Measurement

The sensor was placed in a custom test chamber. Dry NH₃ was generated by bubbling air through a 4 % NH₃ aqueous solution and passing through a NaOH drying tube. Concentrations were controlled by air dilution and monitored with a mass flow meter; the total flow rate was maintained at 1.0 L min^–1. Resistance changes were recorded at 500 mV using an Agilent 4156C semiconductor tester at 25 °C.

Results and Discussion

Synthesis and Characterization of HGO and rHGO

AFM imaging revealed the formation of holes after 1 h of photo‑Fenton reaction, confirming successful pore generation. The thickness increased from ~1 nm (GO) to ~1.9 nm (HGO), indicative of a single‑layer structure. XPS spectra showed a significant reduction in C–O and C=O peaks after pyrrole reduction, and the appearance of an N 1s peak confirmed polypyrrole attachment. The C/O ratio rose from 2.2 (HGO) to 5.1 (rHGO), underscoring effective deoxygenation.

Raman spectra displayed D and G bands at 1346 cm^–1 and 1597 cm^–1, respectively. The I_D/I_G ratio decreased from 1.29 (HGO) to 1.12 (rHGO), reflecting increased sp² domain size and reduced disorder.

Gas‑Sensing Performance

SEM images confirmed a uniform rHGO network bridging the electrode fingers, resulting in a baseline resistance of ~1 MΩ at 500 mV. Upon exposure to NH₃, the sensor’s resistance increased rapidly, achieving a 2.81 % change at 1 ppm and 11.32 % at 50 ppm. Recovery to within 90 % of baseline occurred within 2 min using only dry air, without UV or IR illumination. The sensor maintained stable performance over several months and across four consecutive 50 ppm exposure cycles, demonstrating excellent repeatability.

Selectivity tests against saturated vapors of xylene, acetone, cyclohexane, chloroform, dichloromethane, and methanol (each diluted to 1 % of saturation) showed that the NH₃ response was more than 2.5 × higher, confirming strong selectivity.

Conclusions

We have developed an NH₃ sensor based on reduced holey graphene oxide thin films that offers high responsivity, rapid response, short recovery, and exceptional selectivity. The facile drop‑casting fabrication route, coupled with low material cost and minimal energy consumption, positions this technology as a strong candidate for next‑generation portable ammonia detectors.