Spray-Pyrolyzed ZnO Thin Film Detects Formaldehyde at Room Temperature

A simple, inexpensive spray-pyrolysis technique has produced a zinc oxide thin film that detects formaldehyde at room temperature, according to a study published in Discover Sensors on 20 September 2026. The sensor, built on a pre-cleaned glass substrate from a zinc acetate precursor solution, responded strongly to just 5 parts per million of the toxic gas, with a measured response of about 24, a response time of 60 seconds and a recovery time of 25 seconds. Because the device operates without heating, the result points toward low-power sensors for monitoring indoor air, where formaldehyde is a persistent concern.

Formaldehyde is a colourless volatile organic compound widely present in building materials and furniture products, including paints, coatings, curtains, fabrics and engineered wood products. The World Health Organization and the National Institute for Occupational Safety and Health have set permissible exposure limits of 0.08 ppm and 1 ppm respectively, and the International Agency for Research on Cancer classifies the gas as a group 1 carcinogen. Long-term exposure, even at low concentrations, is reported to cause respiratory disorders, genetic mutations, nasopharyngeal tumours, aplastic anaemia and leukaemia. These hazards make rapid and accurate detection of the gas a major research focus, and semiconductor-based gas sensors are considered an effective real-time technique for the task.

Zinc oxide has long attracted attention as a sensing material because it is an n-type semiconductor with a wide direct band gap of approximately 3.37 eV and a high exciton binding energy of about 60 meV at room temperature. Its gas sensing performance depends strongly on crystal structure, morphology, surface defects, oxygen vacancies and fabrication technique. However, many ZnO-based sensors reported in the literature require elevated operating temperatures, which increases power consumption and limits practical applications. The new study set out to address this by fabricating a pristine ZnO film under optimized deposition conditions, avoiding the noble-metal decoration, heterostructure engineering or defect modulation that many reported formaldehyde sensors rely on.

The researchers deposited the films by chemical spray pyrolysis using a 0.1 molar zinc acetate dihydrate solution in double-deionized water, with a few drops of acetic acid added for homogeneity. The solution was sprayed with compressed air for 12 minutes at a flow rate of 1 mL per minute, a nozzle-to-substrate distance of 20 cm and a substrate temperature held at 360 degrees Celsius with a precision of plus or minus 3 degrees. Spray pyrolysis was chosen for its simplicity, low cost, compositional control, reproducibility and scalability, and because it requires no expensive vacuum systems while allowing uniform coatings over large-area substrates.

Characterization confirmed the quality of the deposited film. X-ray diffraction showed peaks at 2-theta values of 31.76, 34.43, 36.28, 47.62, 56.62, 62.9 and 67.96 degrees, corresponding to the (100), (002), (101), (102), (110), (103) and (112) planes of hexagonal wurtzite ZnO and matching the standard JCPDS card no. 79-0205, with no secondary phases. The (002) reflection was the most intense, indicating preferential growth along the c-axis. Using Scherrer’s equation on the dominant (002) peak, the team calculated an average crystallite size of 21.9 nm, within the roughly 20 to 50 nm range reported for spray-deposited ZnO films. Calculated structural parameters included a micro strain of 0.001652 and a dislocation density of 0.208 x 10^16 per square metre.

Complementary techniques agreed with the diffraction results. Raman spectroscopy showed a prominent band near 430 cm-1 assigned to the E2 (high) mode of wurtzite ZnO, linked to oxygen vibrations in the lattice and to c-axis growth, along with a band around 1050 cm-1 attributed to second-order Raman scattering. Field-emission scanning electron microscopy revealed a continuous film of densely packed, spherical, porous nanocrystallites with a homogeneous nanoscale surface shape, features that increase the surface area available for gas diffusion and adsorption. Transmission electron microscopy showed aggregated spherical crystallites of about 20 nm, and the selected-area electron diffraction pattern displayed six concentric rings corresponding to the (100), (002), (101), (102), (110) and (103) planes. Energy-dispersive X-ray analysis detected Zn and O peaks, with Si, Mg and Ca signals traced to the glass substrate rather than the film.

X-ray photoelectron spectroscopy further confirmed the chemical composition of the film. The survey spectrum showed carbon, zinc and oxygen. The Zn 2p spectrum displayed peaks at binding energies of 1018.66 eV and 1041.73 eV for the 2p3/2 and 2p1/2 levels, with a spin-orbit splitting of 23.07 eV, confirming zinc in the oxidized state. The O 1s peak appeared at approximately 529.79 eV, and its asymmetric shape with a higher binding-energy shoulder suggested multiple oxygen species: lattice oxygen as the main component, plus contributions from oxygen-deficient regions and chemisorbed surface oxygen species associated with oxygen vacancies. Those vacancies, the authors note, provide energetically favourable adsorption sites for oxygen molecules and are a key aspect of the gas sensing mechanism.

The sensing measurements used a static liquid distribution method in a sealed 10-litre chamber at 60 percent relative humidity, with target gas concentrations calculated from the ideal gas law using the analyte’s density, molecular weight and the chamber volume. In a chemiresistive configuration, the sensor’s baseline resistance in air was approximately 2.4 x 10^8 ohms, falling to around 1.0 x 10^7 ohms upon exposure to 5 ppm formaldehyde, which yields the reported response of about 24. In selectivity tests at the same 5 ppm concentration, the response to formaldehyde was markedly higher than the roughly 1 to 1.5 responses recorded for interfering gases, which the authors attribute to the polar C=O group of formaldehyde, its lower effective activation energy for surface reactions, and favourable surface reaction kinetics compared with less reactive gases such as methane and benzene.

Repeatability and stability tests supported the sensor’s reliability. Over four successive exposure and recovery cycles at 5 ppm, the resistance transients were nearly identical, with no significant fluctuations in baseline resistance or response amplitude, indicating reversible adsorption and desorption behaviour. Over an eight-day testing period, the response to 5 ppm formaldehyde fluctuated only slightly between 23.9 and 24.0, with no significant degradation in sensitivity or baseline characteristics, which the authors interpret as evidence that the film maintains its structural integrity and active surface sites over time.

The sensing mechanism follows the standard chemiresistive picture for n-type oxides. In air, oxygen molecules adsorb on the ZnO surface and extract electrons from the conduction band, forming negatively charged species such as O2-, O- and O2-, which creates an electron depletion region and raises the sensor’s resistance. When formaldehyde is introduced, the reducing HCHO molecules react with the adsorbed oxygen species, oxidizing the formaldehyde to carbon dioxide and water and releasing the trapped electrons back into the conduction band. The depletion layer narrows, conductivity rises and the resistance drops, producing the measurable signal. The authors conclude that the spray-pyrolyzed pristine ZnO thin film offers a cost-effective and scalable route to low-power formaldehyde sensors for indoor air quality monitoring, though the reported performance covers the tested 5 ppm concentration and eight-day stability window under ambient conditions.

Subject of Research: Room-temperature formaldehyde gas sensing by a spray-pyrolyzed nanostructured ZnO thin film

Article Title: Formaldehyde gas sensing studies of nanostructured ZnO thin films synthesised using spray pyrolysis

Article References: Begum, S., Nagaraju, P., & Yadav, S. S. (2026). Formaldehyde gas sensing studies of nanostructured ZnO thin films synthesised using spray pyrolysis. Discover Sensors, 2(1), Article 54. https://doi.org/10.1007/s44397-026-00088-4

Image Credits: AI Generated

DOI: 10.1007/s44397-026-00088-4

Keywords: formaldehyde, gas sensor, zinc oxide, thin films, spray pyrolysis, chemiresistive sensing, room temperature detection, nanocrystallites, XRD, XPS, indoor air quality, selectivity